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Geography 600

GEOG/ENVS 600 FINAL EXAM PART 2 – SHORT ESSAYS

INSTRUCTIONS: Choose TWO prompts from the following list and respond to BOTH for the essay portion of your final exam. Aim for about 1 page/500 words for each response. Submit your responses in one document via the Turnitin link in iLearn before Friday, December 18 at 11:59 p.m.

ESSAY OPTIONS (CHOOSE 2):

1. Are recent trends in America’s uses of forms of energy trending towards using more renewables or more non-renewables? Is this trend good or bad, and why? What makes a form of energy renewable, and what makes a form of energy non-renewable? Provide examples of three renewable forms of energy and three kinds of non-renewable forms of energy in your response. (500 words)

2. What is the difference between climate change adaptation and climate change mitigation? Provide an example of each. Then, state briefly how adaptation to climate change relates to human well-being, OR how it relates to environmental well-being (your choice). Finally, provide an example from lecture, reading, or assignment topics of climate adaptation – adapting to what environmental issue? (500 words)

3. We covered a variety of environmental problems and potential solutions over the course of the semester. Choose ONE topic/problem that has been of particular interest to you and explain its importance. Then, discuss solution(s) to this problem, evaluating how realistic/useful stated solution(s) could be. Use examples from readings, lectures, and other course content to support your points. (500 words)

Drawdown

Review

THE

Climate Solutions for a New Decade

Drawdown
Review
THE
Climate Solutions for a New Decade

2020

Project Drawdown is a highly collaborative effort, and the work presented here is
the creation of many, not one. We gratefully acknowledge the many people who
contributed and without whom this work would not have been possible.

The Drawdown Review

Editor-in-Chief: Dr. Katharine Wilkinson

Production Team

Ampersand, Publication Design
Covive, Web Design & Development
Duncan Geere, Data Visualization
Glover Park Group, Media & Public Relations
Christian Leahy, Editorial Consultation & Copyediting
Kit Seeborg, Web & Digital Strategy
Dr. Katharine Wilkinson, Writing & Creative Direction

Project Drawdown Staff*

Crystal Chissell, Vice President of Operations & Engagement
Dr. Jonathan Foley, Executive Director
Catherine Foster, Research Program Coordinator
Chad Frischmann, Vice President of Research
Kit Seeborg, Director of Communication
Dr. Katharine Wilkinson, Vice President of Communication & Engagement

Lead Research Team*

Dr. Ryan Allard, Buildings / Transportation
Kevin Bayuk, Industry / Finance
Dr. Tala Daya, Industry
Dr. Chris Forest, Climate Dynamics
Chad Frischmann, Food Systems /

Health & Education

Denton Gentry, Technology
Dr. João Pedro Gouveia, Electricity
Dr. Mamta Mehra, Land Use & Agriculture
Eric Toensmeier, Land Use & Agriculture

Researchers (2018–2019)

Jimena Alvarez, Land Use & Agriculture / Oceans
Dr. Chirjiv Anand, Buildings
Jay Arehart, Buildings
Beni Bienz, Oceans
Dr. Sarah Eichler Inwood, Land Use & Agriculture
Dr. Stefan Gary, Oceans
Dr. Miranda Gorman, Industry
Dr. Martina Grecequet, Land Use & Agriculture
Dr. Marzieh Jafary, Industry
Ashok Mangotra, Electricity
Dr. Phil Metz, Buildings
Dr. Sarah Myhre, Oceans
Barbara Rodriguez, Buildings
Dr. Ariani Wartenberg, Land Use & Agriculture
Abdulmutalib Yussuff, Electricit

This work also builds upon previous work published in Drawdown in 20

. Many other individuals contributed to that effort, as
named in the book and on Drawdown.org.

* Project Drawdown Staff and Lead Research Team as of March 2020

2020 The Drawdown Review

About Project Drawdown®
The World’s Leading Resource
for Climate

Solutions

Founded in 20

, Project Drawdown
is a nonprofit organization that seeks
to help the world reach “Drawdown”—
the future point in time when levels
of greenhouse gases in the atmo-
sphere stop climbing and start to
steadily decline.

Since the 2017 publication of the New York Times
bestseller Drawdown, the organization has emerged
as a leading resource for information and insight
about climate solutions. We continue to develop
that resource by conducting rigorous review and
assessment of climate solutions, creating compel-
ling and human communication across mediums,
and partnering with efforts to accelerate climate solu-
tions globally.

Cities, universities, corporations, philanthropies,
policymakers, communities, and more turn to Project
Drawdown as they look to advance effective climate
action. We aim to support the growing constellation
of efforts to move climate solutions forward and move
the world toward Drawdown—as quickly, safely, and
equitably as possible.

1(c)(3) nonprofit organization, Project Drawdown is
funded by individual and institutional donations.

Wild honey
harvesting is a
traditional practice
of the Molo
community, West
Timor, Indonesia.

Tweets by ProjectDrawdown

2020 The Drawdown Review

Contents

Foreword

10 Key Insights

Assessing Solutions

Summary of Solutions

Drawdown Solutions
Framework

Reaching Drawdown

p. 7

Forward

p. 14 p. 50 p.

Reduce

Sources

Bringing emissions
to zero

Electricity p.

Food, Agriculture
& Land Use p.

Industry p.

Transportation p.

Buildings p. 42

Other p.

Support
Sin

Uplifting nature’s
carbon cycle

Land Sinks p.

Coastal & Ocean Sinks p.

Engineered Sinks p.

Improve
Society
Fostering equality
for all

Health & Education p.

Top Left: Tokyo, Japan, is home to one of the
best rail transport systems in the world.

Middle Left: Forest restoration in the
Democratic Republic of Congo multi-solves
for climate, livelihoods, and biodiversity.

Bottom Left: India’s Kaas Plateau is a UNESCO
World Natural Heritage Site, celebrated for its
annual wildflower bloom.

2020 The Drawdown Review
2

In the spring of 2017, Project Drawdown
released its inaugural body of work
on climate solutions with the publica-
tion of the best-selling book Drawdown
and open-source digital resources on
Drawdown.org.

That material has influenced university curricula, city
climate plans, commitments by businesses, community
action, philanthropic strategy, and more. This Review rep-
resents the organization’s second seminal publication and
the first major update to our assessment of solutions to move
the world toward “Drawdown”—the future point in time when
levels of greenhouse gases in the atmosphere stop climbing
and start to steadily decline.

Foreword

Science has made clear the wholesale transformation
needed to address the climate crisis. In its 2018 special
report Global Warming of 1.5ºC, the Intergovernmental
Panel on Climate Change (IPCC) calls for “rapid and
far-reaching transitions in energy, land, urban and
infrastructure (including transport and buildings), and
industrial systems.”1 At present, global efforts come
nowhere near the scale, speed, or scope required. Yet
many of the means to achieve the necessary transfor-
mation already exist. Almost daily, there is promising
evolution and acceleration of climate solutions, along-
side growing efforts to sunset fossil fuel infrastructure
and prevent expansion of these antiquated and danger-
ous energy sources.

Project Drawdown conducts an ongoing review and
analysis of climate solutions—the practices and tech-
nologies that can stem and begin to reduce the excess
of greenhouse gases in our atmosphere—to provide the
world with a current and robust resource. (See more
on research methods below.) The Drawdown Review
is core to our efforts to respond nimbly to the rapidly

Foreword

Florida’s coastal
wetlands provide
habitat, flood
control, groundwater
recharge, and storm
protection.

evolving landscape of solutions and the urgency of the
challenge humanity faces. We anticipate regular publi-
cation going forward, including updates as well as new
solutions, scenarios, and insights.

Drawdown is a critical turning point for life on Earth,
and we must strive to reach it quickly, safely, and equi-
tably. What follows is an overview of climate solutions
in hand—now, today—to reach Drawdown and begin
to come back into balance with the planet’s living sys-
tems. These solutions are tools of possibility in the
face of a seemingly impossible challenge. They must
not remain the domain of specialists or select groups.
Widespread awareness and understanding of climate
solutions is vital to kindle agency and effect change
worldwide, across individual, community, organiza-
tional, regional, national, and global scales. People and
institutions of all kinds, in all places, have roles to play
in this great transformation, and the solutions in these
pages are a synthesis of collective wisdom and collec-
tive action unfolding around the globe.

NOTE: All unreferenced numbers are results from Project Drawdown
analysis. All climate solutions are quantified in metric gigatons (Gt) of
carbon dioxide avoided or sequestered. All general references to green-
house gases are expressed in carbon dioxide equivalents (CO2-eq),
using a 100-year global warming potential. All financial results are ex-
pressed in current U.S. dollars.

NOTE: The results we share here represent our best assessment of
climate solutions for the year 2020. Due to changes in methodology
and data, it is not possible to directly compare current results to those
released in 2017 and published in Drawdown. The solutions content in
the original book remains robust and relevant and its broader lessons
still hold.

2020 The Drawdown Review

10 Key
Insights

1 We can reach Drawdown by mid-century if we scale the climate solutions already in hand.Drawdown is a bold goal but an absolutely necessary one,
given that global emissions are still rising each year—not declin-
ing as they need to. Our new analysis shows the world can reach
Drawdown by mid-century, if we make the best use of all existing
climate solutions. Certainly, more solutions are needed and emerg-
ing, but there is no reason—or time—to wait on innovation. Now is
better than new, and society is well equipped to begin that trans-
formation today. If we pursue climate solutions with purpose and
determination, our analysis shows we could reach Drawdown as
early as the mid-2040s—or not until the 2060s, depending on our
level of ambition. (See more on scenarios below.)

Our first body of work in 2017 put a
spotlight on a vast array of climate
solutions, each with its own com-
pelling story and possibility. As the
saying goes, it can sometimes be a
challenge to “see the forest for the
trees,” and that’s certainly true with
climate solutions.

2 Climate solutions are interconnected as a system, and we need all of them. The notion of “silver bullets” has persistent appeal—“what’s
the one big thing we can do?”—but they simply don’t exist for
complex problems such as the climate crisis. A whole system of
solutions is required. Many climate solutions combine and co-
operate, leveraging or enabling others for the greatest impact. For
example, efficient buildings make distributed, renewable electricity
generation more viable. The food system requires interventions on
both supply and demand sides—e.g., better farming practices and
reduced meat consumption. For greatest benefit, electric vehicles
need 100% clean power on which to run. We need many, intercon-
nected solutions for a multi-faceted, systemic challenge.

Throughout this Review, we aim to illuminate what you
might call the “groves” and “forests” beyond the individ-
ual trees, which are sometimes hiding in plain view. Here,
we surface ten key insights to make essential messages of
our work clear, direct, and easy for others to communicate.
Project Drawdown is a living effort and a learning organi-
zation. These insights will continue to deepen, refine, and
expand as the work itself does.

Silvopasture in action at Reserva Natural El Hatico,
a natural reserve near Palmira, Colombia.

10 Key Insights

3 Beyond addressing greenhouse gases, climate solutions can have “co-benefits” that contribute to a better, more equitable world.
Climate solutions are rarely just climate solutions. For example,
those that curb air pollution are also health solutions. Others that
protect and restore ecosystems are also biodiversity solutions.
Many can create jobs, foster resilience to climate impacts such as
storms and droughts, and bring other environmental benefits such
as safeguarding water resources. Climate solutions can advance
social and economic equity if utilized wisely and well—with atten-
tion to who decides, who benefits, and how any drawbacks are
mitigated. The how really matters, as the same practice or technol-
ogy can have very different outcomes depending on implementa-
tion. It takes intention and care to move solutions forward in ways
that heal rather than deepen systemic injustices.

4 The financial case for climate solutions is crystal clear, as savings significantly outweigh costs.Unfounded arguments about the economic inviability of
climate action persist but are patently false. Project Drawdown
analyzes the financial implications of solutions: How much money
will a given solution cost, or save, when compared with the sta-
tus quo technology or practice it replaces? That financial analy-
sis looks at the initial implementation of a solution, as well as the
use or operation of that solution over time. Overall, net operational
savings exceed net implementation costs four to five times over:
an initial cost of $

.5–28.4 trillion versus $95.1–145.5 trillion saved.
If we consider the monetary value of co-benefits (e.g., healthcare
savings from reduced air pollution) and avoided climate damages
(e.g., agricultural losses), the financial case becomes even stronger.
So long as we ensure a just transition for those in sunsetting or
transitioning industries, such as coal, it’s clear that there is no eco-
nomic rationale for stalling on climate solutions—and every reason
to forge boldly ahead.

Left: A woman and
child travel by bicycle
to retrieve water near
Boromo, Burkina Faso.

Right: A Living
Building at the
Georgia Institute of
Technology, designed
to produce more
energy than it uses.

Grasslands are one of
the ecosystems found
within Kilimanjaro
National Park,
Tanzania.

2020 The Drawdown Review
6

5 The majority of climate solutions reduce or replace the use of fossil fuels. We must accelerate these solutions, while actively stopping the use of coal, oil, and gas.
The use of fossil fuels for electricity, transport, and heat currently
drives roughly two-thirds of heat-trapping emissions worldwide.

Of the

solutions included in this Review, roughly 30% reduce the
use of fossil fuels by enhancing efficiency and almost 30% replace
them entirely with alternatives. Together, they can deliver almost
two-thirds of the emissions reductions needed to reach Drawdown.
Alongside accelerating these vital solutions, such as solar and wind
power, retrofitting buildings, and public transit, we must actively
stop fossil fuel production and expansion—including ending billions
of dollars in subsidies and financing and, ideally, directing those
funds to climate solutions instead. Reaching Drawdown depends
on concurrent “stop” and “start” paths of action. A similar stop-
start dynamic exists within food, agriculture, and land use: ending
harmful practices (e.g., deforestation) and advancing helpful ones
(e.g., methods of regenerative agriculture).

6 We cannot reach Drawdown without simultaneously reducing emissions toward zero and supporting nature’s carbon sinks.
Imagine the atmosphere as a bathtub overflowing, as the water
continues to run. The primary intervention is clear: Turn off the tap
of greenhouse gases by bringing emissions to zero. In addition to
curbing the source of the problem, we can also open the drain
somewhat. That’s where nature plays a vital role: absorbing and
storing carbon through biological and chemical processes, effec-
tively draining some of the excess out of the atmosphere. Human
activities can support natural carbon sinks, and many ecosystem-
or agriculture-related climate solutions have the double benefit of
reducing emissions and absorbing carbon simultaneously. It takes
stemming all sources and supporting all sinks to reach Drawdown.
(See further exploration of sources and sinks below.)

Top Left: A woman
examines algae-

based, compostable
bioplastics, designed

for a circular
economy.

Right: Rice is a
key crop of India’s
monsoon season.

Here, a researcher
gathers data during

a farm visit in the
state of Punjab.

Rooftop solar
installation in

upstate New York.

10 Key Insights

7 Some of the most powerful climate solutions receive comparably little attention, reminding us to widen our lens.
Many climate solutions focus on reducing and eliminating fossil
fuel emissions, but others are needed too. Among the top solu-
tions assessed by Project Drawdown, we find some “eye-openers”
that are on par with solutions that often get the spotlight, such as
onshore wind turbines and utility-scale solar photovoltaics:

▶ Food waste reduction and plant-rich diets, which together curb
demand, deforestation, and associated emissions;

▶ Preventing leaks and improving disposal of chemical refriger-
ants, which are potent greenhouse gases, the use of which is
projected to grow significantly;

▶ Restoration of temperate and tropical forests, which are
powerful, vast carbon sinks;

▶ Access to high-quality, voluntary reproductive healthcare and
high-quality, inclusive education, the many ripple effects of
which include climate benefits.

These results are a reminder to look beyond the obvious, to a
broader suite of solutions, and beyond technology, to natural and
social systems.

8 Accelerators are critical to move solutions forward at the scale, speed, and scope required.It goes without saying: Solutions do not scale themselves.
We need means of removing barriers and accelerating their imple-
mentation and expansion. Key “accelerators” can create the condi-
tions for solutions to move forward with greater speed and wider
scope. Some, such as changing policy and shifting capital, are
closer in and have more direct impacts; others, such as shaping
culture and building political power, are further out and more indi-
rect in their effect. Accelerators are heavily dependent on social
and political contexts and work at different scales, from individuals
to larger groups to entire nations. As with solutions, they intersect
and interact; none are singularly effective, and we need them all.
(See more on accelerators below.)

9 Footholds of agency exist at every level, for all individuals and institutions to participate in advanc-ing climate solutions.
The climate crisis requires systemic, structural change across our
global society and economy. The reality of intervening in a complex
system is that no one can do it all, and we all have an opening to
show up as problem-solvers and change-agents and contribute in
significant ways—even when we feel small. The range of climate
solutions illuminates diverse intervention points across individual,
community, organizational, regional, national, and global scales.
The necessary accelerators expand that array of action opportu-
nities even more. It will take a whole ecosystem of activities and
actors to create the transformation that’s required.

10 Immense commitment, collaboration, and ingenu-ity will be necessary to depart the perilous path we are on and realize the path that’s possible. But the
mission is clear: Make possibility reality.

In September 2019, Swedish climate activist Greta Thunberg
testified before the U.S. Congress. “You must unite behind the
science,” she urged. “You must take action. You must do the impos-
sible. Because giving up can never ever be an option.”3 In four short
sentences, she articulated exactly the task and challenge at hand.
Project Drawdown’s mission is to help the world reach Drawdown
as quickly, safely, and equitably as possible. That could also be
humanity’s mission in this pivotal moment for life on Earth. The
current path we are on is beyond dangerous, and it’s easy to be
paralyzed by that perilousness. Yet possibility remains to change
it. Together, we can build a bridge from where we are today to the
world we want for ourselves, for all of life, and, most importantly, for
generations yet to come.

Community health workers in Nepal bring
reproductive healthcare directly to villages.

In Germany, 1.4 million people participated
in the September 2019 climate strike.

2020 The Drawdown Review
8

Drawdown
Solutions
Framework

Drawdown is the future point in time
when levels of greenhouse gases in the
atmosphere stop climbing and start to
steadily decline. This is the point when
we begin the process of stopping further
climate change and averting potentially
catastrophic warming. It is a critical
turning point for life on Earth—one we
must reach as quickly, safely, and equita-
bly as possible.

Atmospheric Greenhouse
Gas Concentrations

M
om

en
t o

f D
ra

w
do

w
n

Cu
rr

en
t T

ra
je

ct
or

Drawdown Solutions Framework

2020 The Drawdown Review

Burning fossil fuels for electricity,
mobility, and heat. Manufacturing
cement and steel. Plowing soils.
Clearing forests and degrading
other ecosystems. All these activities
emit heat-trapping carbon diox-
ide into the air. Cattle, rice fields,
landfills, and fossil fuel operations
release methane—a gas that warms
the planet even more.

Nitrous oxide and fluorinated gases seep
out of agricultural lands, industrial sites,
refrigeration systems, and urban areas,
adding still more heat-trapping pollut-
ants to Earth’s atmosphere. Most of these
greenhouse gases stay airborne, but not
all. Natural biological and chemical pro-
cesses—especially photosynthesis—bring
some of that excess back to plants, soil, or
sea. These “sinks” are nature’s reservoirs for
absorbing and storing carbon.

To understand and advance climate solu-
tions, it ’s important to understand the
sources of emissions and nature’s means
of rebalancing the climate system.

▶ ~25% Electricity

Production

▶ ~24%

Food, Agriculture & Land Use

▶ ~21%

Industry

▶ ~14%

Transportation

▶ ~6%

Buildings

▶ ~10% Other Energy-Related Emissions

Th
e

A
tm

o
s

p
h

e
re

~25% ~24% ~21% ~14% ~6% ~10%

TODAY’S SOURCES

The Challenge Heat-trapping greenhouse gases come from six sectors:2

Greenhouse gas sinks are the counterpoint
to these sources. While ~59% of heat-trap-
ping emissions stay in the atmosphere,
~24% are quickly removed by plants on
land and ~17% are taken up by oceans.4

To reach Drawdown, we must work on all
aspects of the climate equation—stopping
sources and supporting sinks, as well as
helping society achieve broader transfor-
mations. That is, three connected areas call
for action, which we must pursue globally,
simultaneously, and with determination.

1. Reduce Sources,
bringing emissions to zero.
2. Support Sinks,
uplifting nature’s carbon cycle.
3. Improve Society,
fostering equality for all.

Nested within each area of action, there
are sectors and subgroups of diverse solu-
tions—practices and technologies that can
help the world stabilize and then begin
to lower greenhouse gas levels in the
atmosphere. Together, they comprise the
Drawdown Framework for climate solutions.

Th
e
A
tm
o
s
p
h
e
re

~59%

~24% ~17%

Remains in the
Atmosphere

TODAY’S SINKS

NOTE: Land sinks absorb roughly 29% of the carbon
dioxide emissions pumped into the atmosphere each
year, and oceans take up about 23%. When we consid-
er other greenhouse gases, including methane, nitrous
oxide, and fluorinated gases, land absorbs approximately
26% of the total emissions and oceans remove approx-
imately 17%. (Global Carbon Project analysis adjusted
to include all greenhouse gases at 100-year global warm-
ing potential.)

NOTE: These are the sectors where greenhouse gases
are emitted directly into the atmosphere. A sector may
also have indirect impacts on emissions. For example the
6% of emissions attributed to buildings only accounts
for fuels burned on site (e.g., gas for cooking or heating);
power plant emissions tied to buildings’ electricity use are
counted within the electricity sector. (See further explo-
ration below.)

Drawdown Solutions Framework

2020 The Drawdown Review

Reduce Sources

Shift Production

Enhance

Efficiency

Enhance
Efficiency

Electrify
Vehicles

Shift to

Alternatives

Shift
Agriculture

Practices

Address

Refrigerants

Improve
Materials

Use

Waste

Protect

Ecosystems

Food, Agriculture & Land Use
Industry

Electric

ity

Buildings

Transport

Address Waste

& Diets

Shift Energy

Sources

Address Refrigerants

& Improve
the System

T O T A L : M I N 649.2 | M A X 1113.5

Enhance
Efficiency

Minimum CO2-eq (Gt)
reduced/sequestered
(2020-2050)

Maximum CO2-eq (Gt)
reduced/sequestered
(2020-2050)

The Solutions
The Drawdown Solutions Framework organizes climate solutions
by sector and by subgroup, within three overarching areas of
action. Here, you see the potential emissions impact of each sec-
tor, as well as the solution subgroups therein. Using two different
scenarios of solution implementation, we derived the minimum and
maximum impact shown here. (See more on scenarios below.)

X / Y = Min / Max CO2-eq (Gt) reduced/sequestered (2020-2050)

163.8 / 397.0

34.0 / 46.7

1.4 / 185.3

101.3 / 108.3

40.8 / 63.1

46.3 / 108.4

27.4 / 32.9

N/A

11.5 / 26.0

8.9 / 19.9

12.7 / 21.4

19.9 / 33.1

19.3 / 54.8

12.0 / 16.3

Address

Wastes & Diets

T O T A L : M I N 242.3 | M A X 397.8

T O T A L : M I N 85.4 | M A X 85.4

Drawdown Solutions Framework

R
E

D
U

C
E

S
O

U
R

C
E

S
S

U
P

P
O

R
T

S
IN

K
S

IM
P

R
O

V
E

S
O

C
IE

T
Y

Support Sinks

Improve Society

Shift Agriculture

Practices

Health &

Education

Remove &

Store Carbon

Protect & Restore

Ecosystems

Protect & Restore
Ecosystems

Use
Degraded

Land

Engineered Sinks

Health & Education

Land Sinks

Coastal &
Ocean Sinks

Electricity

Food, Agriculture
& Land Use

Industry

Land Sinks

Transport
Engineered Sinks
Buildings
Coastal &
Ocean Sinks
Health &
Education

M I N 85.4 | M A X 85.4

M I N 197.7 | M A X 4 4 4.0

M I N 203.7 | M A X 274.4

M I N 122.9 | M A X 149.6

M I N

.2 | M A X 104.2

M I N 73.6 | M A X 141 .3

M I N 239.0 | M A X 391.9

M I N 2.2 | M A X 4.4

M I N 1 .1 | M A X 1 .5

116.9 / 193.3

78.1 / 120.1

43.0 / 77.6

85.4 / 85.4

1.0 /

1.0

2.2 / 4.4

1.1 / 1.5

2020 The Drawdown Review
1
2020 The Drawdown Review
14

1
Reduce

Sources
bringing emissions to zero

Electricity
Food, Agriculture & Land Use
Industry
Transportation
Buildings
Reduce Sources
15

1.1
Electricity

2020

Since the emergence of electrical systems in the
late 1800s, society has created most of its elec-
tricity by using fossil fuels. The process? Burn coal,
oil, or gas. Heat water to create steam. Steam
turns a turbine. Turbine rotates a generator to
get electrons moving. The locked-up energy of
long-buried plants and animals is transmuted
into electricity, as carbon dioxide spills into the
atmosphere as a byproduct. Today, electric-
ity production gives rise to 25% of heat-trapping
emissions globally.2

How can we generate electricity for the whole world
without burning fossil fuels? How do the means of trans-
mitting, storing, and using electricity need to evolve?

These questions are critical for addressing emissions,
especially given the current push to “electrify every-
thing,” from cars to home heating, needing clean power
on which to run. A mosaic of solutions is required, cen-
tered around electricity efficiency, production, and a
more robust electrical system.

Electricity is particles in motion—a flow
of electrons from one place to another
that keeps air conditioners cooling,
heaters heating, lights illuminating,
computers computing, and all manner
of motors humming. For much of the
world, electricity powers the realities
of daily life, yet 840 million people still
lack access to electricity.5

The Drawdown Review
16

Enhance Efficiency
Electricity efficiency solutions include technologies
and practices that reduce demand for electricity
generation, literally lightening the load. The two
biggest end-users of electricity are buildings and
industry, in roughly equal measure.2 While a home
or factory may be the location of efficiency measures,
these emissions get counted at the power plant where
they are created or avoided, as part of the electric-
ity sector. (See further exploration of buildings and
industry below.)

Shift Production
Production of electricity must move away from fossil
fuels, as quickly as possible. A spectrum of solutions
can help, from small-scale/distributed to large-scale/
centralized. Some solutions harvest photons from the
sun. Others tap nature’s generous kinetic energy—
the movement of wind and water. Still others use
an alternate source of heat, such as geothermal or
nuclear, for the same basic steam-turbine process.

Improve the System

To enable the transition to renewable electricity
production and use, the broader electricity system also
needs to evolve and upgrade. Flexible grids for trans-
mission and effective energy storage make it possible
to better balance electricity supply with demand.

As we look forward, an electricity transformation
is undeniably possible. Already, economics favor
wind and sun over fossil fuels in many places. A
shift away from coal-powered electricity is under-
way in the United States, the United Kingdom, and
much of Europe, albeit not fast or widespread
enough. The speed of transformation is the issue
at hand. We must curtail and supplant 19th and
20th-century forms of production more rapidly—
including the large pipeline of proposed new coal
plants—while ensuring that the future of clean
electricity is equitable and empowering for all.

Windsurfers and
wind turbines catch
the breeze at Brazil’s
Icaraizinho de
Amontada beach.

In the village of
Tinginaput, India,
distributed solar
panels are used for
street lighting.

Overview

1.1 Reduce Sources Electricity

1
Shift Production
2020 The Drawdown Review

Onshore Wind
Turbines

Utility-Scale Solar
Photovoltaics

Distributed Solar
Photovoltaics

Concentrated
Solar Power

Geothermal
Power

Offshore Wind
Turbines

Nuclear
Power

Small
Hydropower

Micro Wind
Turbines

Landfill Methane
Capture

Biomass
Power

Waste-to-
Energy

Methane
Digesters

Ocean
Power18.6 / 24.0

28.0 / 68.6

42.3 / 119.1

0.1 / 0.1

47.2 / 147.7

10.4 / 11.4

6.2 / 9.81.7 / 3.3

1.4 / 1.4

2.5 / 3.6

2.7 / 3.2

0.5 / 0.9

-0.1 / 0.2

2.3 / 3.6

Enhance Efficiency

3
Improve the System
Shift Production
Improve the System
Enhance Efficiency

Overall Impact

Solutions1.1 Reduce Sources Electricity

LED Lighting

District
Heating

Insulation Building
Automation

Systems

Green &
Cool RoofsHigh-Performance Glass

Smart
Thermostats

Solar Hot
Water

Grid Flexibility N/A

Microgrids N/A

Distributed Energy Storage N/A

Utility-Scale Energy Storage N/A

Building Retrofitting N/A

Net-Zero Buildings N/A

High-Efficiency Heat Pumps -3.0 / -1.7

Water Distribution
Efficiency

Dynamic
Glass

Low-Flow
Fixtures

M I N 34.0 | M A X 46.7

N/A

M I N 163.8 | M A X 397.0

NOTE: Where a solution’s impact is N/A, emissions reductions are allocated to other solutions. (See
more below.)

3.1 / 3.3

4.9 / 7.9

16.1 / 17.5

3.8 / 4.3

0.2 / 0.3

2.0 / 2.4

0.7 / 1.3

4.6 / 7.2

0.8 / 3.2

0.2 / 0.4

0.7 / 0.9

Minimum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
Maximum CO2-eq (Gt)
reduced/sequestered
(2020-2050)

X / Y = Min / Max CO2-eq (Gt) reduced/sequestered (2020-2050)

SOLUTIONS

Smart Thermostats*

Enhance Efficiency
LED Lighting

Insulation*

Dynamic Glass*

High-Performance Glass*

Green & Cool Roofs*

District Heating*

High-Efficiency Heat Pumps*

Solar Hot Water*

Building Automation Systems*

Thermostats are mission control for space heating and cooling.
Smart thermostats use algorithms and sensors to become more
energy efficient over time, lowering emissions.

*also in Buildings

LEDs (light emitting diodes) are the most energy-efficient bulbs
available. Unlike older technologies, they transfer most of their
energy use into light, rather than waste heat.

Insulation impedes unwanted airflow in or out of buildings. In new
construction or retrofits, it makes heating and cooling more energy
efficient, with lower emissions.

By responding to sunlight and weather, dynamic glass can reduce
a building’s energy load for heating, cooling, and lighting. More
effective windows lower emissions.

High-performance glass improves window insulation and makes
building heating and cooling more efficient. By minimizing
unnecessary energy use, it curtails emissions.

Green roofs use soil and vegetation as living insulation. Cool roofs
reflect solar energy. Both reduce building energy use for heating
and/or cooling.

District systems heat space and water more efficiently. A central
plant and pipe network channel hot water to many buildings, with
lower emissions than on-site systems.

Heat pumps extract heat from the air and transfer it—from indoors
out for cooling, or from outdoors in for heating. With high efficiency,
they can dramatically lower building energy use.

Solar hot water taps the sun’s radiation, rather than fuel or
electricity. By replacing conventional energy sources with a
clean alternative, it reduces emissions.

These systems can control heating, cooling, lighting, and appli-
ances in commercial buildings. They cut emissions by maximizing
energy efficiency and minimizing waste.

2020 The Drawdown Review
20

Low-Flow Fixtures*

Enhance Efficiency
+ Shift Production

Shift Production

Distributed Solar Photovoltaics

Utility-Scale Solar Photovoltaics

Building Retrofitting*

Net-Zero Buildings*

Concentrated Solar Power

Water Distribution Efficiency

Cleaning, transporting, and heating water requires energy. More
efficient fixtures and appliances can reduce home water use sig-
nificantly, thereby reducing emissions.

Rooftop solar panels are one example of distributed solar photo-
voltaic systems. Whether grid-connected or part of stand-alone
systems, they offer hyper-local, clean electricity generation.

Solar photovoltaics can be used at utility-scale—with hundreds or
thousands of panels—to tap the sun’s clean, free fuel and replace
fossil fuel electricity generation.

Retrofits address electricity and fuel waste with better insulation
and windows, efficient lighting, and advanced heating and cooling
systems. Improved efficiency lowers existing buildings’ emissions.

Buildings with zero net energy consumption combine maximum
efficiency and onsite renewables. They produce as much energy
as they use annually, with low or no emissions.

Concentrated solar power uses sunlight as a heat source. Arrays
of mirrors concentrate incoming rays onto a receiver to heat fluid,
produce steam, and turn turbines.

Pumping water requires enormous amounts of electricity.
Addressing leaks in water-distribution networks, especially in
cities, can curb water loss, energy use, and emissions.

*also in Industry

*also in Buildings
Solutions

1.1 Reduce Sources Electricity

NOTE: This solution represents an integration or system of other solutions.
Emissions reductions associated with building retrofitting are accounted for in those
individual solutions.

NOTE: This solution represents an integration or system of other solutions.
Emissions reductions associated with net-zero buildings are accounted for in those in-
dividual solutions.

Onshore Wind Turbines Onshore wind turbines generate electricity at a utility scale, compa-
rable to power plants. They replace fossil fuels with emissions-free
electricity.

Micro Wind Turbines

Shift Production (cont.)

Geothermal Power

Small Hydropower

Ocean Power

Biomass Power

Nuclear Power

Offshore Wind Turbines

Micro wind turbines can generate clean electricity in diverse
locations, from urban centers to rural areas without access to
centralized grids.

Underground reservoirs of steamy hot water are the fuel for geo-
thermal power. It can be piped to the surface to drive turbines that
produce electricity without pollution.

Small hydropower systems capture the energy of free-flowing water,
without using a dam. They can replace dirty diesel generators with
clean electricity generation.

Wave- and tidal-power systems harness natural oceanic flows—
among the most powerful and constant dynamics on earth—to gen-
erate electricity without pollution.

Biomass feedstock can replace fossil fuels for generating heat and
electricity. Only perennial biomass is advisable, offering a “bridge”
solution to clean, renewable production.

Nuclear power is slow, expensive, risky, and creates radioactive
waste, but it has the potential to avoid emissions from fossil fuel
electricity.

Winds over sea are more consistent than those over land. Offshore
wind turbines tap into that power to generate utility-scale electricity
without emissions.

*also in Industry
SOLUTIONS
2020 The Drawdown Review
22

Methane Digesters*

Improve the System

Microgrids

Distributed Energy Storage

Utility-Scale Energy Storage

Grid Flexibility

Industrial-scale anaerobic digesters control decomposition of
organic waste and convert methane emissions into biogas, an alter-
native fuel, and digestate, a nutrient-rich fertilizer.

A microgrid is a localized grouping of distributed electricity genera-
tion technologies, paired with energy storage or backup generation
and tools to manage demand or “load.”

Standalone batteries and electric vehicles store energy. They can
enable 24/7 electricity supply even when the sun isn’t shining or the
wind isn’t blowing.

Large-scale energy storage ensures electricity supply can match
demand. It enables the shift to variable renewables and curbs
emissions from polluting “peaker” plants.

Smarter, more flexible electric grids can cut energy losses during
distribution. They are critical to enable renewables, which are more
variable than conventional electricity generation.

The emissions reductions enabled by these solutions
are allocated to electricity generation solutions.

Solutions

1.1 Reduce Sources Electricity

Waste-to-Energy*

Landfill Methane Capture*

Waste-to-energy processes (incineration, gasification, pyrolysis)
combust waste and convert it to heat and/or electricity. Emissions
reductions can come with health and environmental risks, however.

Landfills generate methane as organic waste decomposes. Rather
than getting released as emissions, that methane can be captured
and used to produce electricity.

1.2
Food, Agriculture
& Land Use

2020 The Drawdown Review
24

Human activity has transformed a
significant fraction of the planet’s
land, especially for growing food and
harvesting forests. Land is the com-
mon ground of shelter, sustenance,
feed for animals, fiber, timber, and
some sources of energy, as well as the
direct source of livelihood for billions
of people.

Our pursuit of those ends often disrupts or
displaces ecosystems, and the twin forces of a
growing population and rising consumption mean
the challenge of stewarding land in sustainable
ways will only intensify. Today, agriculture and for-
estry activities generate 24% of greenhouse gas
emissions worldwide.2

How can we reduce the pressures on ecosystems and
land, while meeting the growing demands for food and
fiber worldwide? How can we do what we do on land
better, tending it in ways that decrease emissions from
agriculture and forestry?

The answers to these questions are critical for stem-
ming greenhouse gases, sustaining the planet’s liv-
ing systems, addressing food security, and protecting
human health, all inextricably linked. Solutions in this
sector are focused on waste and diets, ecosystem pro-
tection, and better agriculture practices.

Overview

1.2 Reduce Sources Food, Agriculture & Land Use

Address Waste & Diets
By shifting diets and addressing food waste, the global
demand for food can significantly drop. Eating lower on
the food chain and ensuring what’s grown gets eaten
is a powerful combination that lowers farming inputs,
land-clearing, and all associated emissions.

Protect Ecosystems

When land and ecosystems are deliberately protected,
activities that release carbon from vegetation and soil
are stopped before they start. In addition, improving
food production on existing farmland may reduce the
pressure on other, nearby landscapes, thereby sparing
them from clearing.

Shift Agriculture Practices

Better agriculture practices can lower emissions from
cropland and pastures, including methane generated
by growing rice and raising ruminants, nitrous oxide
emitted from manure and overusing fertilizers, and car-
bon dioxide released by disturbing soils.

Farming and forestry practices can also support
the role of land in removing greenhouse gases
from the atmosphere. Many solutions that stop
land-based emissions also enhance carbon sinks
(explored below). Solutions in this sector are signif-
icant for improving food security and agricultural
resilience as well, because many of them contrib-
ute to a more robust food system, better able to
withstand climate impacts.

Left: Central
Kalimantan, Indonesia,
is home to carbon-
rich peatland forests,
which face the
pressures of drainage,
illegal logging,
and fire.

Top Right: At the 2019
Marcha das Mulheres
Indígenas in Brasília,
women lifted up
the importance of
Indigenous peoples’
land rights.

Bottom Right: A
plant-rich dish of
roasted eggplant
with turmeric, yogurt
sauce, roasted
almonds, and
smoked paprika.

Address Waste

& DietsReduced
Food Waste

Plant-Rich Diets

2020 The Drawdown Review

Shift Agriculture Practices
Protect Ecosystems

Address Waste & DietsOverall Impact M I N 151 .4 | M A X 185.3

M I N 40.8 | M A X 63.1

M I N 11 .5 | M A X 26.0

64.8 / 9

1.5

86.7 / 93.8

Here, you see the potential emissions impact of each
subgroup for this sector, as well as the individual
solutions therein. Solutions are scaled relative to one
another within this sector. Each sector is individually
scaled for legibility.
Minimum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
Maximum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
X / Y = Min / Max CO2-eq (Gt) reduced/sequestered (2020-2050)

Improved
Rice

Production

Regenerative
Annual Cropping

System of Rice
Intensification

Farm Irrigation
Efficiency

Conservation
Agriculture

Nutrient
Management

Peatland Protection
& Rewetting

Grassland
Protection

Coastal Wetland
Protection

Indigenous
Peoples’

Forest Tenure

Forest
Protection

Protect

Ecosystems
3
Shift Agriculture
Practices
4
Protect

Ecosystems +

Shift Agriculture
Practices
Solutions

1.2 Reduce Sources Food, Agriculture & Land Use

4.4 / 6.8

7.0 / 10.3

3.2 / 4.0

25.5 / 40.9

0.7 / 1.0

Sustainable Intensification
for Smallholders 0.1 / 0.1

1.1 / 1.5

1.0 / 1.5

2.3 / 12.1

1.1 / 2.1

4.0 / 5.9

2.0 / 3.0

SOLUTIONS
2020 The Drawdown Review

Plant-Rich Diets*

Address Waste & Diets

Coastal Wetland Protection**

Reduced Food Waste*

Consumption of meat and dairy, as well as overall calories, often
exceeds nutritional recommendations. Paring down and favoring
plant-based foods reduces demand, thereby reducing land clearing,
fertilizer use, burping cattle, and greenhouse gas emissions.

Mangroves, salt marshes, and seagrasses sequester huge amounts
of carbon in plants and soil. Protecting them inhibits degradation
and safeguards their carbon sinks.

Roughly a third of the world’s food is never eaten, which means land
and resources used and greenhouse gases emitted in producing it
were unnecessary. Interventions can reduce loss and waste, as food
moves from farm to fork, thereby reducing overall demand.

*also in Land Sinks

Protect Ecosystems

Indigenous Peoples’ Forest Tenure*

Grassland Protection*

Peatland Protection & Rewetting*

Forest Protection*

Secure land tenure protects Indigenous peoples’ rights. With sover-
eignty, traditional practices can continue—in turn protecting ecosys-
tems and carbon sinks and preventing emissions from deforestation.

Grasslands hold large stocks of carbon, largely underground.
Protecting them shields their carbon stores and avoids emissions
from conversion to agricultural land or development.

Forestry, farming, and fuel-extraction are among the threats to car-
bon-rich peatlands. Protection and rewetting can reduce emissions
from degradation, while supporting peatlands’ role as carbon sinks.

In their biomass and soil, forests are powerful carbon storehouses.
Protection prevents emissions from deforestation, shields that car-
bon, and enables ongoing carbon sequestration.

*also in Land Sinks **also in Coastal & Ocean Sinks

Sustainable Intensification for Smallholders*

Protect Ecosystems
+ Shift Agriculture Practices

Sustainable intensification practices can increase smallholder yields,
which, in theory, reduce demand to clear additional land. Practices
include intercropping, ecosystem-based pest management, and
equal resources for women.

*also in Land Sinks
Shift Agriculture Practices

Regenerative Annual Cropping*

Farm Irrigation Efficiency

Nutrient Management

Improved Rice Production*

System of Rice Intensification*

Conservation Agriculture*

Building on conservation agriculture with additional practices, regen-
erative annual cropping can include compost application, green
manure, and organic production. It reduces emissions, increases soil
organic matter, and sequesters carbon.

Overuse of nitrogen fertilizers—a frequent phenomenon in agricul-
ture—creates nitrous oxide. More efficient use can curb these emis-
sions and reduce energy-intensive fertilizer production.

Flooded rice paddies produce large quantities of methane. Improved
production techniques, including alternate wetting and drying, can
reduce methane emissions and sequester carbon.

Pumping and distributing water is energy intensive. Drip and sprin-
kler irrigation, among other practices and technologies, make
farm-water use more precise and efficient.

SRI is a holistic approach to sustainable rice cultivation. By minimiz-
ing water use and alternating wet and dry conditions, it minimizes
methane production and emissions.

Conservation agriculture uses cover crops, crop rotation, and mini-
mal tilling in the production of annual crops. It protects soil, avoids
emissions, and sequesters carbon.

*also in Land Sinks

1.2 Reduce Sources Food, Agriculture & Land Use Solutions

1.3
Industry

2020 The Drawdown Review
30

From concrete to computers, cars
to clothing, industry is the sector of
production that makes them all. It
includes strings of connected activities:
extracting raw materials, manufactur-
ing component parts and complet-
ed goods, provisioning them for use,
dealing with disposal, and (possibly)
putting waste back to work. The domi-
nant mode of operation is take-make-
use-trash—a linear flow of materials
that is inefficient and untenable.

This sector derives its name from the Latin for
“diligence.” Industry’s hard work certainly pro-
pels economic activity but it also creates sub-
stantial emissions—and some of the hardest to
halt. Industry requires the use of energy-hun-
gry machines, furnaces, and boilers, and often
employs polluting processes. Many of its emis-
sions happen on-site—at a plant or factory, for
example—making industry directly responsible
for 21% of all heat-trapping emissions.2 Given its
appetite for electricity, industry also drives almost
half of off-site electricity generation emissions (as
explored above). Within this sector, production of
cement, iron, and steel top the emissions charts.
Aluminum, fertilizers, paper, plastics, processed
foods, textiles, and waste pile up the problem.

How can we improve industrial processes and materi-
als produced? How can industry make use of waste and
move toward flows of substances that are efficient and
circular?

These questions have implications that reach well
beyond this sector, as it’s fundamentally linked with
mobility, infrastructure, buildings, food, and technolo-
gies of all sorts. Industry solutions cluster around mate-
rials, waste, refrigerants, and energy efficiency.

Overview

1.3 Reduce Sources Industry

Improve Materials
Plastic, metals, and cement are some of the most
ubiquitous materials. They’re also prime candidates
for improvement and replacement with better alter-
natives that can meet the same needs, but with lower
emissions.

Use Waste

Waste can be reclaimed as a resource—something
of value, rather than something to discard—to reduce
the use of raw materials and energy, thereby reducing
emissions. The most advanced approaches move us
toward a circular economy.

Address Refrigerants
The chemicals used in refrigeration are potent green-
house gases, which often leak during use or disposal.
We can better manage and dispose of the fluorinated
gases currently used as refrigerants, and, ultimately,
replace them with benign alternatives.

Enhance Efficiency
Industrial processes can also reduce emissions through
energy-efficiency and using low- and no-carbon energy
sources.

Industry—especially heavy industry—presents
some of the biggest challenges for reducing
emissions to zero. For example, the manufactur-
ing of concrete, a staple of modern construction,
releases a great deal of carbon dioxide. A number
of industrial processes, such as fabricating steel,
require very high temperatures that, for now, rely
on burning fossil fuels. This sector is likely to see
critical new solutions in the years ahead.

NOTE: To date, Project Drawdown has assessed a limited selection of
industry solutions. This solution set will expand in the future (e.g., solu-
tions for production of chemicals, steel, and textiles).

Top Left: CopenHill
is a waste-to-energy
plant that doubles
as an artificial ski
slope in Copenhagen,
Denmark.

Bottom Left: A dairy
farm in Lancaster
County, Pennsylvania,
composts food waste
and cow manure.

Right: Refrigerators
and air conditioners
rely on chemical
refrigerants that
require careful
management
and disposal.

1
Address
Refrigerants
2020 The Drawdown Review

Alternative Refrigerants

Refrigerant Management

Use Waste

Improve Materials

Address RefrigerantsOverall Impact M I N 101 .3 | M A X 108.3

M I N 8.9 | M A X 19.9

M I N 12 .7 | M A X 21.4

57.7 / 57.7

43.5 / 5

0.5

3
Improve Materials
2
Use Waste

Solutions1.3 Reduce Sources Industry

Alternative
Cement

Recycling

Methane
Digesters

Bioplastics

Waste-to-
Energy

Recycled
Paper

Landfill Methane
Capture

Composting

8.0 / 16.1

1.0 / 3.8

2.1 / 3.1

5.5 / 6.0

1.1 / 1.9

1.6 / 2.1

-1.5 /

2.0

3.8 / 6.2

SOLUTIONS
2020 The Drawdown Review

Alternative Cement

Improve Materials
Bioplastics

Cement production requires significant energy and decarbonization
of limestone. Fly ash, a waste product from burning coal, can replace
some of that material and cut emissions.

Most plastics are made from fossil fuels, but bioplastics utilize plants
as an alternative source of carbon. They often have lower emissions
and sometimes biodegrade.

A technician dismantles e-waste for recycling in
Rwanda’s Bugesera District.

Solutions

Refrigerant Management*

Address Refrigerants
Landfill Methane Capture*
Methane Digesters*

Fluorinated gases have a potent greenhouse effect and are widely
used as refrigerants. Managing leaks and disposal of these chemi-
cals can avoid emissions in buildings and landfills.

Landfills generate methane as organic waste decomposes. Rather
than getting released as emissions, that methane can be captured
and used to produce electricity.

Industrial-scale anaerobic digesters control decomposition of
organic waste and convert methane emissions into biogas, an alter-
native fuel, and digestate, a nutrient-rich fertilizer.
*also in Buildings
Use Waste
Recycling

Recycled Paper

Waste-to-Energy*
Composting

To produce new products from recovered materials requires fewer
raw resources and less energy. That’s how recycling household,
commercial, and industrial waste can cut emissions.

Recycled paper takes a circular journey, rather than a linear flow
from logging to landfill. Reprocessing used paper curtails extraction
of virgin feedstock and lowers emissions.

Waste-to-energy processes (incineration, gasification, pyrolysis)
combust waste and convert it to heat and/or electricity. Emissions
reductions can come with health and environmental risks, however.

Composting can range from backyard bins to industrial-scale opera-
tions. Regardless, it converts organic waste into soil carbon, averting
landfill methane emissions in the process.

*also in Electricity

Alternative Refrigerants*

Fluorinated gases are not the only refrigerants available. Alternatives,
such as ammonia or captured carbon dioxide, can replace these
powerful greenhouse gases over time.

1.3 Reduce Sources Industry

1.4
Transportation

2020 The Drawdown Review
36

Getting people or things from point A
to point B, and perhaps back again: In
some ways, transportation is incred-
ibly simple. Human beings would be
stuck at the speed of walk, run, swim,
or horse if it weren’t for planes, trains,
automobiles, buses, bicycles, and
boats. Mobility has played a critical
and complex role in shaping society,
and the demand for it is only growing.

Most of the energy driving mobility has, to date,
been generated by burning liquid hydrocar-
bons, namely gasoline, diesel, and jet fuel. Why?
Because of a formidable combination of energy
density (the energy contained within a liter or
gallon), abundance, and low cost. But account
for what isn’t included in that price, and petro-
leum-powered mobility is expensive indeed.
Particulate matter harms human health. Oil spills
ruin land and water. And then there’s the cost to
the climate system: Transportation is responsible
for 14% of global greenhouse gas emissions.2

How can we support the social good of mobility, but end
its dependence on petroleum? In what ways do vehicles,
infrastructure, and operations need to change to elimi-
nate transportation emissions?

These are the questions society must answer if we
want to keep moving—ourselves or other items—
for reasons of necessity, pleasure, or commerce.
Transportation solutions address alternatives, fuel effi-
ciency, and electrification.

Overview

1.4 Reduce Sources Transportation

The “L” in Chicago
is one of the largest
and busiest public
transit systems in the
United States.

Shift to Alternatives
Alternative modes of mobility reduce demand for fos-
sil-fueled transportation or replace it altogether. With
public and “pooled” transit, we can make the most of
available seats. Compact cities, intentional infrastruc-
ture, and advanced communication technologies make
it possible to walk, cycle, or simply stay put.

Enhance Efficiency
Where combustion engines remain in use, vehicles can
be made far more fuel-efficient through mechanical
improvements, lightweighting, better design, and more
artful operation.

Electrify Vehicles

Electrification of vehicles completely replaces petro-
leum—and has even greater benefits when paired with
renewable electricity generation.

These transportation solutions have the poten-
tial to save money and preempt pollution, but the
transformations required are substantial and the
sector can be slow to move. Vehicles remain in use
for many years. New transportation infrastructure
is expensive and takes time to build. Clean fuels for
airplanes remain distant. But many of the solutions
can, if done intelligently, create more equitable
mobility and livability in our cities and communi-
ties, without forfeiting the stability of our climate.

A cargo ship docks
in Guarujá, a coastal
town near São Paulo,
Brazil.

1
Shift to
Alternatives
2020 The Drawdown Review

Public Transit

Carpooling

Walkable
Cities

Bicycle
Infrastructure

High-speed Rail

Telepresence

Electric Bicycles

Shift to Alternatives

Enhance Efficiency
Electrify Vehicles
Overall Impact

M I N 12 .0 | M A X 16.3

M I N 19.9 | M A X 33.1

M I N 19.3 | M A X 54.8

1.4 / 5.5

2.6 / 6.6

1.3 / 4.1

4.2 / 7.7

7.5 / 23.4

1.3 / 3.8

1.0 / 3.8
Here, you see the potential emissions impact of each
subgroup for this sector, as well as the individual
solutions therein. Solutions are scaled relative to one
another within this sector. Each sector is individually
scaled for legibility.
Minimum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
Maximum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
X / Y = Min / Max CO2-eq (Gt) reduced/sequestered (2020-2050)

2
Enhance Efficiency
3
Electrify Vehicles

Solutions1.4 Reduce Sources Transportation

Hybrid Cars

Efficient Trucks Efficient Ocean
Shipping

Efficient Aviation

Electric Cars

Electric Trains

4.6 / 7.9

4.6 / 9.7

6.3 / 9.2

4.4 / 6.3

11.9 / 15.7

0.1 / 0.6

SOLUTIONS
2020 The Drawdown Review

Walkable Cities

Shift to Alternatives
Telepresence

Bicycle Infrastructure

Walkable cities use planning, design, and density to maximize
walking and minimize driving, especially for commuting. Emissions
decrease as pedestrians take the place of cars.

Telepresence integrates high-performance visual, audio, and network
technologies, so people can interact across geographies. It cuts
down on travel—especially flying—and its emissions.

Bicycles offer an alternative to cars and fossil fuel transport, espe-
cially in cities. Infrastructure is essential for supporting safe and
abundant bicycle use, thereby curbing emissions.

Carpooling
Public Transit

High-Speed Rail

Electric Bicycles

When people share common origins, destinations, or stops en route,
they can ride together. Carpooling uses seats and fuel more effi-
ciently, cutting emissions.

Streetcars, buses, and subways offer alternative, efficient modes of
transport. Public transit can keep car use to a minimum and avert
greenhouse gases.

High-speed rail offers an alternative to trips otherwise made by car
or airplane. It requires special, designated tracks, but can dramati-
cally curtail emissions.

Small battery-powered motors give electric bicycles a boost. It
makes them a more compelling alternative to more polluting forms
of motorized transport, namely cars.

Solutions

Hybrid Cars
Enhance Efficiency

Efficient Trucks

A transitional technology, hybrid cars pair an electric motor and bat-
tery with an internal combustion engine. The combination improves
fuel economy—more miles on a gallon—and lowers emissions.

Fuel-efficiency is critical to reduce road-freight emissions. Existing
fleets can be retrofitted, while new trucks can be built to be more
efficient or fully electric.

Efficient Aviation

Efficient Ocean Shipping

Various technologies and operational practices can lower airplane
emissions to some degree. They include better engines, wingtips,
and light weighting to improve fuel efficiency.

Huge volumes of goods are shipped across oceans. Fuel-saving
ship design, onboard technologies, and operational practices can
improve efficiency and trim emissions.

Electrify Vehicles
Electric Trains
Electric Cars

Rail electrification enables trains to move beyond dirty diesel-
burning engines. When powered by renewables, electric trains can
provide nearly emissions-free transport.

Electric motors supplant gasoline or diesel engines, which are pollut-
ing and less efficient. EVs always reduce car emissions—dramatically
so when powered by renewable electricity.

1.4 Reduce Sources Transportation

1.5
Buildings

2020 The Drawdown Review

4242

Inside is where most people are most
of the time. As central features of
human life, buildings furnish space in
which to dwell, gather, labor, trade,
make, learn, heal, and revel. Of all
the things we create, buildings are
the largest, and they generally persist
for decades, if not centuries. Already
the world has more than 230 billion
square meters of building space. An-
other 65 billion square meters could be
added this decade.6

It’s no surprise that buildings are major drivers
of emissions. Some stem from the materials that
comprise buildings and the process of construc-
tion, renovation, or demolition—what’s known as
“embodied carbon.” Many more emissions are the
result of ongoing use. Fuels are burned on site, pri-
marily to heat space or water or for cooking. The
chemicals used for cooling and refrigeration can
escape as emissions. Through these direct, on-site
sources buildings produce 6% of heat-trapping
emissions worldwide.2 Buildings also use more
than half of all electricity, creating an off-site,
upstream impact on electricity-generation emis-
sions (as explored above).

How can we retrofit existing buildings and create new
buildings to minimize energy use? How can we stop
other, on-site sources of emissions?

These questions are at the heart of making buildings
not only better for the planet, but also more afford-
able to operate and healthier, better places for the
people inside and around them. Building solutions
orient around energy efficiency, energy sources, and
refrigerants.

Overview

1.5 Reduce Sources Buildings

Enhance Efficiency
Whether for building retrofits or brand new construc-
tion, energy-efficiency solutions are largely the same.
Many address the building “envelope” and insulation
—means of keeping conditioned air in and uncondi-
tioned air out—while others use technology to optimize
energy use.

Shift Energy Sources

Clean alternatives can replace more polluting fossil
energy sources typically used to heat space, warm
water, or prepare meals.

Address Refrigerants
The gases used as refrigerants today are potent green-
house gases. We can reduce emissions by managing
leaks that often happen within buildings, as well as
properly disposing of refrigerants (a waste process
that falls under industry, above). Ultimately, these fluo-
rinated gases can be replaced with alternatives that are
not greenhouse gases.

Many building solutions reduce on-site emissions
and enhance electricity efficiency, reducing emis-
sions at the power plant. Taken together, these
solutions can transition buildings from being a
major problem to potentially net-positive, as the
“greenest” buildings can produce more energy
than they consume. These solutions can also
help ease the “energy burden” many low-in-
come households face, as energy bills often eat
up a significant and disproportionate percentage
of income.

A green roof in Leuven,
Belgium, a city that
has invested heavily
in sustainability and
liveability.

Biogas cookstoves
can improve indoor
air quality, protect
forests, and prevent
emissions.

1
Shift Energy
Sources
2020 The Drawdown Review

Solar Hot
Water
District
Heating

Improved Clean
Cookstoves

High-Efficiency
Heat Pumps

Biogas for
Cooking

Address Refrigerants
Shift Energy Sources
Enhance Efficiency
Overall Impact

M I N 27.4 | M A X 32.9

M I N 46.3 | M A X 108.4

N/A

1.7 / 2.7

5.8 / 12.3

2.8 / 11.1

4.6 / 9.7

31.3 / 72.6

Enhance

Efficiency

Solutions1.5 Reduce Sources Buildings

Low-Flow
Fixtures

Dynamic
Glass

Building
Automation

Systems

Insulation

High-Performance
Glass

Smart
Thermostats

Insulation, a key
energy-efficiency
measure, gets
installed in Montreal,
Canada.

3.9 / 4.1

1.6 / 2.6

13.2 / 14.8

0.1 / 0.1

8.1 / 10.3

Building Retrofitting N/A
Net-Zero Buildings N/A

Green & Cool Roofs -0.2 / -0.1

0.7 / 1.2

NOTE: Where a solution’s impact is N/A, emissions
reductions are allocated to other solutions. (See more
below.)

NOTE: All refrigerant-related emissions reductions are
allocated within Industry.

3
Address
Refrigerants

Refrigerant Management N/A

Alternative Refrigerants N/A

SOLUTIONS
2020 The Drawdown Review

Smart Thermostats*
Enhance Efficiency
Building Retrofitting*
Thermostats are mission control for space heating and cooling.
Smart thermostats use algorithms and sensors to become more
energy efficient over time, lowering emissions.
High-Performance Glass*
Green & Cool Roofs*
Low-Flow Fixtures*
Insulation*
Dynamic Glass*
High-performance glass improves window insulation and makes
building heating and cooling more efficient. By minimizing
unnecessary energy use, it curtails emissions.
Green roofs use soil and vegetation as living insulation. Cool roofs
reflect solar energy. Both reduce building energy use for heating
and/or cooling.
Cleaning, transporting, and heating water requires energy. More
efficient fixtures and appliances can reduce home water use sig-
nificantly, thereby reducing emissions.
Insulation impedes unwanted airflow in or out of buildings. In new
construction or retrofits, it makes heating and cooling more energy
efficient, with lower emissions.
By responding to sunlight and weather, dynamic glass can reduce
a building’s energy load for heating, cooling, and lighting. More
effective windows lower emissions.
*also in Electricity
*also in Electricity

Building Automation Systems* These systems can control heating, cooling, lighting, and appli-
ances in commercial buildings. They cut emissions by maximizing
energy efficiency and minimizing waste.

NOTE: These solutions represent an integration or system of other solutions. Emissions
reductions associated with building retrofitting and net-zero buildings are accounted for
in those individual solutions.

Enhance Efficiency +

Shift Energy Sources

Net-Zero Buildings* Buildings with zero net energy consumption combine maximum
efficiency and onsite renewables. They produce as much energy
as they use annually, with low or no emissions.

NOTE: All refrigerant-related emissions reductions are allocated within Industry.

Solutions

1.5 Reduce Sources Buildings
District Heating*
Shift Energy Sources
High-Efficiency Heat Pumps*
District systems heat space and water more efficiently. A central
plant and pipe network channel hot water to many buildings, with
lower emissions than on-site systems.
Heat pumps extract heat from the air and transfer it—from indoors
out for cooling, or from outdoors in for heating. With high efficiency,
they can dramatically lower building energy use.
Solar Hot Water*

Biogas for Cooking

Improved Clean Cookstoves

Solar hot water taps the sun’s radiation, rather than fuel or electricity.
By replacing conventional energy sources with a clean alternative,
they reduce emissions.

Anaerobic digesters process backyard or farmyard organic waste
into biogas and digestate fertilizer. Biogas stoves can reduce emis-
sions when replacing biomass or kerosene for cooking.

Improved clean cookstoves can address the pollution from burning
wood or biomass in traditional stoves. Using various technologies,
they reduce emissions and protect human health.

Address Refrigerants

Enhance Efficiency +
Shift Energy Sources (cont.)

Alternative Refrigerants*

Refrigerant Management*
Fluorinated gases are not the only refrigerants available. Alternatives,
such as ammonia or captured carbon dioxide, can replace these
powerful greenhouse gases over time.
Fluorinated gases have a potent greenhouse effect and are widely
used as refrigerants. Managing leaks and disposal of these chemi-
cals can avoid emissions in buildings and landfills.
*also in Industry
*also in Electricity

1.6
Other

2020 The Drawdown Review
48

Before coal, oil, or gas is burned, there is mining,
extraction, refining, processing, storage, and
transport. All of these processes within the energy
system also generate heat-trapping emissions.
Methane, for example, escapes from gas wells
and pipelines as “fugitive emissions.” As we work
toward a clean energy future, this sector of emis-
sions also requires solutions in the years of transi-
tion, to minimize damage while fossil fuels remain
in the mix. Ending their use, quickly and compre-
hensively, is the true solution.

10% of global greenhouse gas emissions
fall under the category of “other”—
additional emissions mainly related to
the production and use of fossil fuels.2

NOTE: Project Drawdown has not assessed solutions in this sector to date.

Overview1.6 Reduce Sources Other

Gas flaring—burning off methane—is a common practice
in fossil fuel drilling, fracking, refining, and processing, which
generates significant carbon dioxide emissions, along with
other toxic pollutants. Leaks and venting—intentionally

releasing gas directly into the air—are less visible and even
more damaging to the atmosphere, as pure methane is a far
more potent greenhouse gas.

2020 The Drawdown Review
1
2020 The Drawdown Review
50

2
Support
Sinks

uplifting nature’s carbon cycle

Land Sinks

Coastal & Ocean Sinks

Engineered Sinks

Support Sinks

2.1
Land Sinks

2020 The Drawdown Review
52

In addition, soils are, in large part, organic matter—
once-living organisms, now decomposing—mak-
ing them an enormous storehouse of carbon. Land
can therefore be a powerful carbon sink, return-
ing atmospheric carbon to living vegetation and
soils. While the majority of heat-trapping emis-
sions remain in the atmosphere, land sinks cur-
rently return 26% of human-caused emissions to
earth—literally.4

How can we help sequester more carbon in biomass
and soil? What can we do to support and enhance nat-
ural processes, including the capacity of land to renew?

These questions matter not only for emissions but for
a diversity of human needs—and for maintaining a
healthy diversity of flora and fauna. Because soil with
more carbon content can also be more productive and
resilient, these questions are critical for building a thriv-
ing food system, too.

Climate solutions that enhance land-based sinks clus-
ter around waste and diets, ecosystem protection and
restoration, improved agriculture practices, and pru-
dent use of degraded land.

Land is a critical component of the
climate system, actively engaged in
the flows of carbon, nitrogen, water,
and oxygen—essential building blocks
for life. Carbon is the core of trees and
grasses, mammals and birds, lichens
and microbes. Linking one atom to
the next, and to other elements, it’s
the fundamental material of all living
organisms. Plants and healthy ecosys-
tems have an unparalleled capacity to
absorb carbon through photosynthe-
sis and store it in living biomass.

NOTE: Land sinks absorb roughly 29% of the carbon dioxide emissions pumped into the atmosphere each year. When we consider other greenhouse gases, including methane,
nitrous oxide, and fluorinated gases, land absorbs approximately 26% of the total emissions. (Global Carbon Project analysis adjusted to include all greenhouse gases at 100-year
global warming potential.)

There is significant overlap in the solutions that
stop land-based sources of greenhouse emissions
and those that support land-based carbon sinks.
Their unique power is doing both at the same time.
All of them are critical to coming back into balance
with the planet’s living systems.

Overview

2.1 Support Sinks Land Sinks

Address Waste & Diets
Reducing food waste and shifting to plant-rich diets
are two critical interventions to prevent deforestation.
Lower demand for food and farmland spares nature
from additional clearing, indirectly protecting carbon
sinks.

Protect &

Restore Ecosystems

“Let nature be nature” is a powerful principle—let peat-
lands, grasslands, and forests continue to do what they
do best by protecting them from human disturbance.
Where ecosystems have been degraded, restoration
can help them recuperate form and function, including
absorbing and storing more carbon over time.

Shift Agriculture Practices
What and how we grow, graze, or harvest can be a
means to cultivate biomass and regenerate soil car-
bon. An array of “regenerative agriculture” methods
are being rediscovered and developed worldwide, and
show promising results. The integration of trees into
farming through agroforestry practices is particularly
powerful. All solutions that sustainably raise yields on
existing farmland can also reduce the pressure to clear
other areas.

Use Degraded

Land

Lastly, degraded lands can be put to use in ways that
revive productivity, increase biomass, and promote soil
carbon sequestration—all while producing wood, fiber,
or food.

A model farm in
Yangambi, DRC, aims
to improve yields,
food security, and
prevent deforestation
of the country’s vast
tropical forest.

Bamboo can thrive—
and sequester carbon
—on inhospitable
degraded lands.

1
Shift Agriculture
Practices
2020 The Drawdown Review

Perennial
Staple Crops

Silvopasture

Managed
Grazing

Regenerative
Annual Cropping

Multistrata
Agroforestry

Tree
Intercropping

Conservation
Agriculture

Perennial
Biomass Production

System of Rice
Intensification

Improved
Rice Production

Shift Agriculture Practices

Use Degraded Land

Address Waste & Diets

Protect & Restore Ecosystems

Overall Impact

M I N 78.1 | M A X 120.1

M I N 1 .0 | M A X 1 .0

M I N 116.9 | M A X 193.3

M I N 43.0 | M A X 7 7.6

8.3 / 11.9

13.6 / 20.8

16.4 / 26.0

26.6 / 42.3

11.3 / 20.4

15.0 / 24.4

15.5 / 31.3
4.0 / 7.0

5.4 / 8.0

0.8 / 1.2

Protect &

Restore Ecosystems
5
Address Waste
& Diets
4

Protect & Restore

Ecosystems +

Shift Agriculture

Practices
3
Use

Degraded

Land

Solutions2.1 Support Sinks Land Sinks

Tropical Forest

Restoration

Tree Plantations
(on Degraded Land)

Abandoned
Farmland

Restoration

Bamboo Production

Temperate Forest
Restoration

Indigenous Peoples’
Forest Tenure

Forest
Protection

Plant-Rich
Diets

Reduced
Food Waste

Sustainable
Intensification for

Smallholders

Peatland Protection
& Rewetting

Grassland
Protection

0.2 / 0.2

0.8 / 0.8

1.1 / 1.9

1.7 / 2.6

19.4 / 27.8

54.5 / 85.1

0.2 / 0.2

0.6 / 1.0

0.6 / 1.2

12.5 / 20.3

22.2 / 35.9

8.3 / 21.3

SOLUTIONS
2020 The Drawdown Review

Plant-Rich Diets*
Address Waste & Diets
Protect & Restore Ecosystems
Indigenous Peoples’ Forest Tenure*

Tropical Forest Restoration

Grassland Protection*
Peatland Protection & Rewetting*
Consumption of meat and dairy, as well as overall calories, often
exceeds nutritional recommendations. Paring down and favoring
plant-based foods reduces demand, thereby reducing land clearing,
fertilizer use, burping cattle, and greenhouse gas emissions.
Forest Protection*

Temperate Forest Restoration

In their biomass and soil, forests are powerful carbon storehouses.
Protection prevents emissions from deforestation, shields that
carbon, and enables ongoing carbon sequestration.

*also in Food, Agriculture & Land Use

Reduced Food Waste* Roughly a third of the world’s food is never eaten, which means land
and resources used and greenhouse gases emitted in producing it
were unnecessary. Interventions can reduce loss and waste, as food
moves from farm to fork, thereby reducing overall demand.

Secure land tenure protects Indigenous peoples’ rights. With
sovereignty, traditional practices can continue — in turn
protecting ecosystems and carbon sinks and preventing emissions
from deforestation.

Tropical forests have suffered extensive clearing, fragmentation,
degradation, and depletion of biodiversity. Restoring these forests
also restores their function as carbon sinks.

Grasslands hold large stocks of carbon, largely underground.
Protecting them shields their carbon stores and avoids emissions
from conversion to agricultural land or development.
Forestry, farming, and fuel-extraction are among the threats to car-
bon-rich peatlands. Protection and rewetting can reduce emissions
from degradation, while supporting peatlands’ role as carbon sinks.

Almost all temperate forests have been altered in some way—
timbered, converted to agriculture, disrupted by development.
Restoring them sequesters carbon in biomass and soil.

Solutions

2.1 Support Sinks Land Sinks
Sustainable Intensification for Smallholders*
Protect Ecosystems
+ Shift Agriculture Practices
Sustainable intensification practices can increase smallholder yields,
which, in theory, reduce demand to clear additional land. Practices
include intercropping, ecosystem-based pest management, and
equal resources for women.
*also in Food, Agriculture & Land Use
Shift Agriculture Practices
Regenerative Annual Cropping*

Managed Grazing

Silvopasture

Multistrata Agroforestry

Tree Intercropping

Perennial Staple Crops

Conservation Agriculture*
Building on conservation agriculture with additional practices, regen-
erative annual cropping can include compost application, green
manure, and organic production. It reduces emissions, increases soil
organic matter, and sequesters carbon.

Managed grazing involves carefully controlling livestock density, and
timing and intensity of grazing. Compared with conventional pasture
practices, it can improve the health of grassland soils, sequestering
carbon.

An agroforestry practice, silvopasture integrates trees, pasture, and
forage into a single system. Incorporating trees improves land health
and significantly increases carbon sequestration.

Multistrata agroforestry systems mimic natural forests in structure.
Multiple layers of trees and crops achieve high rates of both carbon
sequestration and food production.

Growing trees and annual crops together is a form of agroforestry.
Tree intercropping practices vary, but all increase biomass, soil
organic matter, and carbon sequestration.

Perennial staple crops provide important foods, such as bananas,
avocado, and breadfruit. Compared to annual crops, they have
similar yields but higher rates of carbon sequestration.

Conservation agriculture uses cover crops, crop rotation, and mini-
mal tilling in the production of annual crops. It protects soil, avoids
emissions, and sequesters carbon.
*also in Food, Agriculture & Land Use

2020 The Drawdown Review

A researcher tests soils in western Kenya, to assess
the impact of minimum tillage, integrated soil fertility
management, and other farming practices.

SOLUTIONS
Solutions

2.1 Support Sinks Land Sinks

Perennial Biomass Production

Shift Agriculture Practices (cont.)

Use Degraded Land

Tree Plantations (on Degraded Land)

Bioenergy relies on biomass—often annual crops such as corn.
Perennial plants (e.g., switchgrass, silvergrass, willow, eucalyptus)
are a more sustainable source and sequester modest amounts of
soil carbon.

Abandoned Farmland Restoration

Bamboo Production

Degraded farmland is often abandoned, but need not be. Restoration
can bring these lands back into productivity and sequester carbon
in the process.

*also in Food, Agriculture & Land Use
Improved Rice Production*
System of Rice Intensification*
Flooded rice paddies produce large quantities of methane. Improved
production techniques, including alternate wetting and drying, can
reduce methane emissions and sequester carbon.
SRI is a holistic approach to sustainable rice cultivation. By minimiz-
ing water use and alternating wet and dry conditions, it minimizes
methane production and emissions.

Degraded lands present potential locations for tree plantations.
Managed well, they can restore soil, sequester carbon, and produce
wood resources in a more sustainable way.

Bamboo rapidly sequesters carbon in biomass and soil and can
thrive on degraded lands. Long-lived bamboo products can also
store carbon over time.

2020 The Drawdown Review
60

2.2
Coastal &
Ocean Sinks

While this uptake of heat and carbon has buffered the
planet from more severe climate change, oceans are
paying a steep price. How so? Water temperatures,
marine heat waves, and sea levels are rising. More car-
bon dioxide in seawater makes the ocean more acidic
and less hospitable for shellfish to build shells or coral
to build their skeletons. Oxygen levels in ocean water
have already declined somewhat. In the future, biomass
production through photosynthesis may also drop,
destabilizing the base of the food chain. What’s more,
with fewer organisms alive, fewer would die and sink
into the deep ocean, carrying their carbon with them.

What practices can be used to sequester carbon in
coastal, marine, and open ocean environments? How
can human activity support and enhance natural
processes?

These questions are vital for addressing emissions but
also for shoring up oceans’ life-sustaining role. Even as
oceans suffer, they also are home to significant solu-
tions. Solutions for coastal and ocean sinks center on
ecosystem protection and restoration and improved
agriculture practices.

Ours is a water world. Though Earth
has a land-centric name, oceans cov-
er

% of its surface and make land
livable.7 Some of the planet’s most
critical processes happen where sea
and air meet, as oceans absorb and
redistribute heat and carbon—both
rising due to the glut of emissions in
the atmosphere.

Oceans have absorbed at least 90% of the excess heat gen-
erated by recent climate changes, and, since the 1980s, have
taken up 20-30% of human-created carbon dioxide.7 The latter
happens through the biological processes of photosynthesis
and building calcium carbonate shells, and through simple
chemistry, as carbon dioxide dissolves in seawater. Coastal
and ocean sinks bring 17% of all heat-trapping emissions back
to Earth.4

Overview2.2 Support Sinks Coastal & Ocean Sinks

Protect & Restore Ecosystems
Protecting ecosystems—including mangroves, salt
marshes, and seagrass meadows—supports ongoing
photosynthesis and carbon storage. Because these
“blue carbon” ecosystems have been lost or degraded
in many places, restoration also has a vital role to play.

Shift Agriculture Practices
Along coasts and in the open ocean, select regenera-
tive practices may augment natural carbon sequestra-
tion from seaweed and kelp, while growing fiber and
food from the sea.

Oceans will continue to be on the frontlines of cli-
mate change, as will people who live near them.
Solutions focused on coastal and marine sinks can
provide additional benefits from storm protection
to healthy fisheries. It’s impossible to separate blue
and green, land and sea. They, and we, are funda-
mentally intertwined.

NOTE: Project Drawdown has assessed a very limited selection of coast-
al and ocean solutions to date. This solution set will expand in the future
(e.g., solutions for regenerative ocean farming and marine ecosystem
restoration).

Above: Planting
mangroves as part of
a blue carbon project
on the Persian Gulf.

Left: Kelp forests
along the Southern
California coast
have benefitted from
restoration efforts
but continue to
struggle amidst
warming waters.

2020 The Drawdown Review

1
Protect & Restore
Ecosystems
SOLUTIONS

Coastal Wetland Protection*

Protect & Restore Ecosystems
Mangroves, salt marshes, and seagrasses sequester huge amounts
of carbon in plants and soil. Protecting them inhibits degradation
and safeguards their carbon sinks.
*also in Food, Agriculture & Land Use

Coastal Wetland Restoration Agriculture, development, and natural disasters have degraded many
coastal wetlands. Restoring mangrove forests, salt marshes, and
seagrass beds to health revives carbon sequestration.

Coastal
Wetland

Protection
Coastal Wetland

Restoration

Protect & Restore EcosystemsOverall Impact M I N 1 .1 | M A X 1 .5

0.3 / 0.5 0.8 / 1.0

NOTE: This sector is significantly magnified
for legibility.

Overview2.2 Support Sinks Coastal & Ocean Sinks

Mangrove forest on the island of Nusa
Lembongan, off the coast of Bali.

2020 The Drawdown Review
64

2.3
Engineered
Sinks

Remove carbon. Do something with it. Those are
the central premises of engineered sinks. Remove
can mean pulling carbon from the concentrated
exhaust of a power plant or industrial process,
which falls under the umbrella of “carbon capture.”
Remove can also mean pulling carbon out of the
air, where it’s much less concentrated.

Where carbon goes next is the other critical piece of
the equation. It can be stored or buried—pairing “cap-
ture” and “storage.” Carbon can also be used—cycled
quickly, perhaps for adding bubbles to a beverage or
to make more sustainable jet fuels. Or it can be locked
up for a long while, perhaps in concrete or through the
ancient practice of baking biomass into biochar, then
buried. This so-called “semi-permanent sequestration”
is most powerful.

Could recaptured carbon become a commodity?
Something of value? Perhaps. For now, solutions in
this sector are “coming attractions,” and issues of
cost, scale, and the energy required all remain in the
balance.

Can human engineering play a
supporting role to nature? That’s a
question that grows in relevance and
urgency, given the gap between where
global emissions stand and where
they need to be, posthaste. The sheer
quantity of excess greenhouse gases
means natural processes can’t do it all
when it comes to carbon sequestration.
Select nascent technologies show some
promise to supplement terrestrial,
coastal, and ocean sinks.

NOTE: Project Drawdown has assessed a very limited set of solutions for engineered sinks to date. This solution set will expand in the future (e.g., direct air capture).

Overview

2.3 Support Sinks Engineered Sinks

SOLUTIONS

Biochar Production

Remove & Store Carbon

Biomass slowly baked in the absence of oxygen becomes biochar,
retaining most of the feedstock’s carbon. It can be buried for seques-
tration and potentially enrich soil.

*also in Food, Agriculture & Land Use
1
Remove &
Store Carbon

Biochar
Production

Biochar produced
from forest waste
in Dillard, Oregon,
with the aim of
sequestering carbon
and enhancing soil.

Remove & Store CarbonOverall Impact M I N 2.2 | M A X 4.4

2.2 / 4.4
Here, you see the potential emissions impact of each
subgroup for this sector, as well as the individual
solutions therein. Solutions are scaled relative to one
another within this sector. Each sector is individually
scaled for legibility.
Minimum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
Maximum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
X / Y = Min / Max CO2-eq (Gt) reduced/sequestered (2020-2050)
NOTE: This sector is significantly magnified
for legibility.

2020 The Drawdown Review
1
2020 The Drawdown Review
66

3
Improve
Society

fostering equality for all

Improve Society

Climate solutions are never just climate
solutions. Those that move the world
beyond fossil fuels toward clean energy
also bring down air pollution, perhaps the
world’s worst health crisis.
Many of the agricultural practices that regenerate soil
can be a boon for farmers and ranchers and foster a
more resilient food system. The benefits of protecting and
restoring ecosystems go well beyond carbon sequestra-
tion and storage. Many solutions can be wisely designed
and employed to meet near-term needs—affordable
energy, nutritious food, good jobs, storm protection,
clean water, community, or beauty, for example—while
advancing the long-term aim of reaching Drawdown.
That’s multi-solving.

Other initiatives, designed primarily to ensure rights and
foster equality, can also have cascading benefits to cli-
mate change. For example, where Indigenous peoples’
land rights are protected, so too are culture, traditional
practices, and forest ecosystems. The ripple effects of
Indigenous peoples’ forest tenure are vital to all life on
Earth. Similarly, access to high-quality, voluntary repro-
ductive healthcare and to high-quality, inclusive educa-
tion are fundamental human rights and cornerstones of
gender equality. In more indirect ways, making strides
in health and education can also benefit the climate—
discussed in more detail below. Climate and social
systems are profoundly connected, and those con-
nections open up solutions that are often overlooked.

Health & Education

2020 The Drawdown Review
68

all the rest. Population interacts with the primary
drivers of emissions: production and consumption,
largely fossil-fueled.

It’s critical to note the vast disparities in emissions from
high-income countries compared to low, and between
the wealthiest individuals and those of lesser finan-
cial means. For example, almost half of consumption-
related emissions are generated by just 10% of peo-
ple globally.9 The topic of population also raises the
troubling, often racist, classist, and coercive history of
population control. People’s choices about how many
children to have should be theirs and theirs alone.
And those children should inherit a livable planet. It
is critical that human rights are always centered, that
gender equality is the aim, and that benefits to the
planet are understood as positive ripple effects of
access and agency.

In its most recent report on “world population pros-
pects,” the United Nations notes that the international
community has committed to ensuring that all people
have access to family planning, should they wish to use
it, and the ability to decide how many children to have
and when.10 That can mean changes in everything from
contraception to culture. Living up to those commit-
ments will be a major determinant for which possible
trajectory becomes our path forward.

How many people might call this
planet home in 2050 or 2100? That will
depend, in large part, on fertility rates
and the headway we make on securing
gender equality and advancing human
well-being. When levels of education
rise (in particular for girls and young
women), access to reproductive health-
care improves, and women’s political,
social, and economic empowerment
expand, fertility typically falls.8 Across
the world and over time, this impacts
population.

Currently, we humans number 7.7 billion, and the United
Nations estimates the human family will grow to between
9.4 billion and 10.1 billion in 2050.8 As we consider the future
of climate solutions, it matters how many people will be eat-
ing, moving, plugging in, building, buying, using, wasting, and

3.1
Health &
Education

Overview3.1 Improve Society Health & Education

Health &

Education

Above: A student
attends secondary
school in the
Absheron District
of Azerbaijan.

Health &
Education

Health & EducationOverall Impact M I N 85.4 | M A X 85.4

85.4 / 85.4
Here, you see the potential emissions impact of each
subgroup for this sector, as well as the individual
solutions therein. Solutions are scaled relative to one
another within this sector. Each sector is individually
scaled for legibility.
Minimum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
Maximum CO2-eq (Gt)
reduced/sequestered
(2020-2050)
X / Y = Min / Max CO2-eq (Gt) reduced/sequestered (2020-2050)

Solutions Beyond
the Drawdown List

Project Drawdown has assessed an extensive
but not exhaustive set of global climate solu-
tions, as presented here. We continue to add to it
as we review and quantify the potential of solu-
tions to stop emissions and/or support sinks, as
well as broader societal transformations that also
have climate benefits. Among them are what we
dubbed “coming attractions” in Drawdown—prac-
tices and technologies that are nascent but look to
have promise, pending further development and
investigation. Project Drawdown’s assessment of
solutions will continue to be a living project.

Our analysis depends on the availability of critical
inputs —namely robust data and peer-reviewed
research. Some solutions get outsized attention from
the research community, while others may be under-
valued or passed over. Synthesis is only as inclusive
and robust as the information being synthesized.
We acknowledge those limitations and encourage
research on an increasingly broad solution set, espe-
cially solutions emerging from impacted and frontline
communities.

Other climate solutions are clearly powerful but more
systemic in nature and challenging to quantify, such as
resisting the development of new fossil fuel infrastruc-
ture, increasing overall urban density, or reducing con-
sumption through sharing, repair, and re-use. Project
Drawdown recognizes the limitations of the scope of
our analysis here, too. A broad aperture for solutions
is vital, and we continue to evolve approaches that
support it.

A moose wades in
waters at Denali

National Park and
Preserve, Alaska.

2020 The Drawdown Review

Solutions Beyond Drawdown

72
2020 The Drawdown Review

Assessing Solutions

Project Drawdown’s analysis seeks to determine
whether reaching Drawdown—the future point
in time when levels of greenhouse gases in the
atmosphere stop climbing and start to steadily
decline—is possible using existing, well-proven
climate solutions. To uncover that answer, we
review and evaluate the potential performance
of diverse technologies and practices that reduce
greenhouse gas emissions and/or increase carbon
sequestration from the atmosphere. All of these
climate solutions are financially viable and already
scaling, at least in some places.*

Drawdown fellows analyze solutions, drawing upon years of
advanced study, experience, and a wide range of backgrounds. For
each technology or practice, we review extensive literature and
data describing its potential scale, impact, and cost. We then build
analytical models to estimate how many gigatons of carbon diox-
ide (or equivalent amounts of other greenhouse gases)** a given
solution could avoid and/or remove over time, as well as the cost
of implementing and operating it. We use conservative estimates
of the financial cost and emissions impact for each solution. In
other words, assumptions about costs fall on the high end, while
assumptions about emissions reductions or sequestration rates fall
on the low end.

Throughout our analysis, the total CO2-eq reduced/sequestered
is based on the number of “solution units” (e.g., number of new
wind turbines installed, number of new hectares of forests pro-
tected) active between 2020–2050. “First cost” refers to the cumu-
lative cost to purchase and install those solution units—in other
words, the implementation cost. “Lifetime cost” is the cost to oper-
ate those units throughout a lifetime of use. (For some solutions,
financial data is insufficient or unavailable.)‡

Each solution’s impacts and costs are then compared to the
current practices or technologies it replaces. We call this a base-
line scenario—a world where few or no new climate solutions are

adopted. For example, the potential emissions reductions from
onshore wind turbines are based on comparison to using fossil
fuel power plants for electricity generation. Costs for installing and
operating those turbines are also compared to fossil fuel plants.
The “net” difference results from comparison to the emissions or
costs of the baseline scenario.

To establish a baseline scenario, we use the work of the AMPERE
Project. Their baseline scenario of future energy use, land use, and
greenhouse gas emissions illustrates a possible future where no
new climate action is taken—a future with rising emissions, ele-
vated greenhouse gas levels in the atmosphere, and continued
strong warming for decades. (See more at www.ampere-h2020.eu.)

The individual “bottom-up” solution models can be run in isola-
tion, but we also integrate the models within and across multiple
sectors. This allows us to consider how the ensemble of solutions
might work together, reducing emissions, sequestering carbon, and
moving the world toward Drawdown. Model integration ensures
that resource constraints are accounted for (e.g., available land
for forests or crops), avoids any double-counting of impacts from
overlapping solutions (e.g., different modes of transportation), and
addresses interaction between solutions where possible (e.g.,
increasing demand for electricity from electric vehicles or electric
heat pumps).

After integration, the results are totaled to determine if and
when we reach Drawdown and at what cost (or savings) for
implementation and operation.

* It is important to note that while we evaluate a wide range of solutions, across many
sectors, we do not consider all possible climate solutions. Given the methods used,
we cannot evaluate promising new technologies or emerging solutions where sufficient
data is not yet available.

** Carbon dioxide (CO2) is not the only greenhouse gas. Other heat-trapping gases
include methane (CH4), nitrous oxide (N2O), and fluorinated gases (e.g., HFCs). Each has
long-term impacts on climate, depending on how much of it is in the atmosphere, how
long it remains there, and how much heat it traps during its lifetime. Based on these
factors, we can calculate the global warming potential of each greenhouse gas, which
makes it possible to have a “common currency,” translating any given gas into its equiv-
alent in carbon dioxide over a 100-year period.

‡ It is important to note that we do not evaluate additional savings from the climate-
driven damages we might avoid by reaching Drawdown. This could represent extremely
large savings and avoid incalculable non-monetary impacts.

Assessing Solutions

An inspector rappels
down the blades of
a 3 megawatt wind
turbine in Boulder,
Colorado.

http://www.ampere-h2020.eu.

2020 The Drawdown Review
74

Reaching Drawdown

Project Drawdown uses different scenarios to
assess what determined, global efforts to address
climate change might look like. These scenarios
represent various levels of ambition in bringing
the set of climate solutions to scale. All are plau-
sible and economically realistic, but they can vary
significantly in terms of when we might reach
Drawdown, how high atmospheric concentrations
of greenhouse gases might rise before then, and
what the implications for Earth’s climate might
be. Two scenarios are presented in this Review.

Drawdown Scenario 1 is ambitious, at least compared to today’s
political commitments to climate action, but it does not reach
Drawdown within the period of study (2020–2050). Scenario
1 would be on track to reach Drawdown in the mid-2060s.
Drawdown Scenario 2 is bolder, with faster and more pervasive
adoption of climate solutions, reaching the point of Drawdown in
the mid-2040s.

We translate these emissions scenarios into illustrations of future
greenhouse gas concentrations and global temperatures using the
FAIR model—a simple model of Earth’s carbon cycle and climate.
(See more at tiny.cc/FAIRmodel.) The baseline scenario (based on
AMPERE) and the two Drawdown Scenarios are fed into the FAIR
model, which then estimates the resulting CO2-eq concentration
in Earth’s atmosphere (measured in parts per million) and global
mean temperature (measured in degrees Celsius).

As of early 2020, atmospheric carbon dioxide alone is over
410ppm; with other greenhouse gases, we approach 460ppm
CO2-eq. Under Drawdown Scenario 1, CO2-eq concentrations
would rise to ~540ppm in 2050. The resulting global mean tem-
perature would be 1.74˚C above pre-industrial levels in 2050 and
rise to 1.85˚C in 2060—on a path to warm by 2˚C by century’s end.

Under the more ambitious Drawdown Scenario 2, CO2-eq
concentrations would peak at ~490 ppm in the mid-2040s and fall
slightly by 2050 to ~485 ppm. Because there is a time lag between

emissions and planetary warming, global mean temperature would
continue to rise after the point of Drawdown, with peak warming
around 1.52˚C through the 2050s.

The Paris Agreement, drafted in late 2015 and adopted in 2016, set
a global aspiration to keep warming well below 2°C and to pursue
efforts to limit it to 1.5°C. As the IPCC 2018 special report, Global
Warming of 1.5°C, lays out, a 1.5°C world and a 2°C world are dra-
matically different in terms of extreme heat, sea-level rise, species
loss, ecosystem damage, and more. (See more at ipcc.ch/sr15/.)

Interestingly, the Drawdown Scenarios align, respectively, with
meeting a minimum goal of 2°C and a more ambitious goal of 1.5°C.
Drawdown Scenario 1 is roughly in-line with 2˚C temperature rise
by 2100, while Drawdown Scenario 2 is roughly in-line with 1.5˚C
temperature rise at century’s end. In other words, we can avoid
catastrophic warming with climate solutions in hand today. What’s
more, our analysis does not include all possible climate solutions
already available. With other potential solutions, such as those
focused on reducing industrial emissions or capping fugitive meth-
ane, the world might reach Drawdown even more quickly.

We can avoid catastrophic
warming with climate

solutions in hand today.

The Drawdown Scenarios also show that meeting climate targets
can be achieved while ensuring global food security, protecting and
restoring ecosystems, and producing biomass for essential uses—
all without clearing any additional land. That requires bold adop-
tion of solutions to reduce global food, feed, and fiber demands
(mostly by addressing food waste and shifting diets), alongside
multifaceted land-use solutions that produce food and biomass as
well as sequester carbon (including agroforestry, perennial crops,
and restoring degraded forests). In short, this analysis shows we
can meet ambitious climate targets, nourish the world, and restore
healthy ecosystems, without consuming the planet—if we pursue
all possible solutions.

Of course, scenarios are stories of what could be, not what will
be. What will be? That will be decided by our collective ambi-
tion and determined action this decade and beyond.

For more information on the solutions, scenarios, and research methodology, visit Drawdown.org.

http://ipcc.ch/sr15/

NOTE: Total greenhouse gas levels include carbon dioxide, methane, nitrous oxide, and fluorinated gases, expressed in carbon dioxide equivalents (CO2-eq).

Reaching Drawdown

1960

350

0.5

450

1.0

550

1.5

650

2.0

750

2.5

1980

2000

2000
2020
2020

2040

Drawdown
in mid-2060s

Drawdown
in mid-2040s

2060

C
O

2-
eq

C
on

ce
nt

ra
tio

ns
(p

pm
)

C
ha

ng
e

in
T

em
pe

ra
tu

re
(º

C
)

Baseline

Scenario 1

Scenario 2

~2ºC warming

~1.5ºC warming

Project Drawdown’s work points to
two fundamental realities: We can
reach Drawdown by mid-century if
we pursue climate solutions already
in hand; and, doing so will require im-
mense ambition and bold action.

It’s an emotional paradox in some ways, perhaps prompting
a simultaneous sense of hope for what’s possible and over-
whelm about just how much needs to be done. This is espe-
cially true given that, globally, current commitments and
plans for climate action fall far short of what’s required.

The two Drawdown Scenarios may seem unrealistic today—
especially the more ambitious one. (See above.) But it’s important
to note that what may be politically unrealistic at present is physi-
cally and economically realistic, according to our analysis. There is
a path forward for the world. The question is how to bring physical,
economic, and political possibility into alignment.

Forward
2020 The Drawdown Review
76

1 Shape Culture Culture is critical context for climate solutions and action, telling us what’s right or wrong, what’s possible or impossible. Stories, the arts, dialogue, and visioning are some
of the means of (re)shaping culture and collective beliefs about
how the world works, or could. Cultural change can feel diffuse,
but it sets the context for what we do as a society and can foster a
sense of collective courage.

2 Build Power Power is a precondition for creating change. In the past, too much power has been deployed against climate action; too little has been assembled to advance solutions.
We build power by building community, movements, and diverse
leadership. When the concentrated power and entrenched inter-
ests of industry or government work against transformation, people
power offers a corrective.

Accelerating Solutions
Project Drawdown defines “solutions” as practices
and technologies that materially affect the con-
centration of greenhouse gases in the atmosphere.
Their impact is specific and measurable. But solu-
tions do not scale themselves. We need means of
removing barriers and accelerating implementa-
tion and expansion.

Forward

3 Set Goals Goals govern direction. What are we reaching for, and why? On climate but also more broadly, goals can be specific and numeric (e.g., “carbon neutral by 2035”), or they
can be higher-order, more systemic ambitions (e.g., “a climate-just
future”). Sometimes a new goal can dramatically shift where we’re
headed—and the solutions and approaches we bring to bear.

“Accelerators” create the conditions for solutions to move
forward. Some are closer-in and have more direct impacts;
others are further out and more indirect in their effect. They
intersect and interact and, like solutions, are dependent on
social and political context. What might work well in a given
time or place might not work in another. Accelerators also
work at different scales, from individual to larger groups to
entire nations. As with solutions, none are singularly effective,
and we need them all.

4 Alter Rules and Policy Rules create boundaries. They tell us what is desirable and perhaps encouraged, or what is unwanted and perhaps punished. Laws, regulations,
standards, taxes, subsidies, and incentives are means of changing
the state of play on climate, but hinge on who writes the rules.
Policy shifts can advance solutions, while stopping sources of
the problem.

An aerial view of
Drakes Bay in the
Point Reyes Peninsula,
California.

2020 The Drawdown Review

5 Shift Capital Given our economic system, money is neces-sary fuel for making change. Public and private investment and philanthropic giving can stimulate and sustain climate
solutions and efforts to move them forward. Divestment is also
powerful, shifting capital away from sources of the problem, essen-
tially restricting their blood flow.

6 Change Behavior From individuals to corporations and beyond, behavior is what’s done and how. All climate solu-tions have behavioral dimensions, and some hinge almost
entirely on human habit. Knowledge, norms, criteria, and motiva-
tions can shift behavior and create new ways of operating. Where
changes in behavior aggregate, outcomes can shift significantly.

7 Improve Technology To stop the sources of emissions, technology must evolve. “Now is better than new” when it comes to climate solutions, but through innovation, research,
and development, technology may continue to improve and add to
the solutions at hand. This is especially critical for the most intrac-
table sectors, such as heavy industry and aviation.

On both accelerators and solutions, efforts will be aided
by connecting them through communication and collab-
oration; supporting continual learning through education,
knowledge-building, and prototyping; and centering the
experiences, wisdom, and solutions of impacted communi-
ties. We need all of the above—a wide variety of solutions and
accelerators to move the world toward Drawdown, quickly,
safely, and equitably.

Members of a rural
women’s cooperative
on Îles Tristao, Guinea.

Utility-scale solar
photovoltaics in Chile’s
Atacama Desert.

Forward

We are living in a time of dramatic transformation. The basic
physics, chemistry, and biology of this planet make that
non-negotiable; stasis is not an option. Society has a choice
to make about what shape that transformation will take.
Will we employ collective courage and determination and
the legion of existing solutions to move the world away from
widespread climate catastrophe? Will we pursue climate
action in ways that heal systemic injustices and foster resil-
ience, wellbeing, and equality? Who will we choose to be in
this pivotal moment of human history?

A transformation that moves us toward Drawdown is possi-
ble, as demonstrated here, but it will require much more than
the right technologies and practices being available. Genuine
evolution is in order—evolution in what we value, how we treat
one another, who holds the reins of power, the ways institu-
tions operate, and the very contours of our economies. This
time of transformation also asks that we learn from cultures
and communities that have sustained human-nature symbi-
osis for centuries, even millennia.

At times, this can all feel like a draconian assignment. But it’s
also an invitation into deeply meaningful work. Our purpose
as human beings in this moment is to create a livable future,
together—to build a bridge from where we are today to the
world we want for ourselves, for all of life, and for generations
yet to come. With commitment, collaboration, and ingenuity,
we can depart the perilous path we are on and come back
into balance with the planet’s living systems. A better path is
still possible. May we turn that possibility into reality.

Solutions
by Sector

2020 The Drawdown Review
80

SUMMARY OF SOLUTIONS

Some of the results shown here may
surprise you; for example, the solutions
that have a beneficial emissions impact
overall but some detrimental impact
in a given sector (shown as negative
CO2-eq). We invite a deeper dive into the
many particularities and nuances of all
of these solutions, laid out in technical
materials on Drawdown.org.

NOTE:

* Indicates that a solution falls under two sectors; results are apportioned and allocated to each sector.
** Indicates that a solution enables or integrates others; emissions reductions are allocated elsewhere.

The total CO2-eq reduced/sequestered is based on the number of solution units active between 2020–2050, compared to the emissions of a baseline scenario.

“First cost” refers to the cumulative cost to install those solution units. “Lifetime cost” is the cost to operate those units throughout a lifetime of use. The “net” difference results from
comparison to the costs of a baseline scenario. Where a cost is a negative number, it indicates savings.

Summary of Solutions

Reduce Sources bringing emissions to zero

Sector Subgroup Solution

SCENARIO 1
Total CO2-eq (Gt)

Reduced / Sequestered

(2020–2050)

SCENARIO 2
Total CO2 -eq (Gt)

Reduced / Sequestered
(2020–2050)

El
ec

tr
ic

ity

Enhance Efficiency Smart Thermostats * 3.1 3.3

Building Automation Systems * 4.9 7.9

LED Lighting 16.1 17.5

Insulation * 3.8 4.3

Dynamic Glass * 0.2 0.3

High-Performance Glass * 2.0 2.4

Green & Cool Roofs * 0.7 1.3

District Heating * 4.6 7.2

High-Efficiency Heat Pumps * -1.7 -3.0

Solar Hot Water * 0.8 3.2

Low-Flow Fixtures * 0.2 0.4

Water Distribution Efficiency 0.7 0.9

Building Retrofitting * ** N/A N/A

Enhance Efficiency + Shift Production

Net-Zero Buildings * ** N/A N/A

Shift Production Concentrated Solar Power 18.6 24.0

Distributed Solar Photovoltaics 28.0 68.6

Utility-Scale Solar Photovoltaics 42.3 119.1

Micro Wind Turbines

0.1 0.1

Onshore Wind Turbines 47.2 147.7

Offshore Wind Turbines 10.4 11.4

Geothermal Power 6.2 9.8

Small Hydropower 1.7 3.3

Ocean Power 1.4 1.4

Biomass Power 2.5 3.6

Nuclear Power 2.7 3.2

Waste-to-Energy * 0.5 0.9

Landfill Methane Capture * 0.2 -0.1

Methane Digesters * 3.6 2.3

Improve the System Grid Flexibility ** N/A N/A

Microgrids ** N/A N/A

Distributed Energy Storage ** N/A N/A

Utility-Scale Energy Storage ** N/A N/A

E L E C T R I C I T Y T O T A L 200.6 441.1

2020 The Drawdown Review

Reduce Sources bringing emissions to zero
Sector Subgroup Solution
SCENARIO 1
Total CO2-eq (Gt)
Reduced / Sequestered
(2020–2050)
SCENARIO 2
Total CO2 -eq (Gt)
Reduced / Sequestered
(2020–2050)

Fo
od

, A
gr

ic
ul

tu
re

&
L

an
d

U
se

Address Waste & Diets Plant-Rich Diets * 64.8 91.5

Reduced Food Waste * 86.7 93.8

Protect Ecosystems Forest Protection * 4.4 6.8

Indigenous Peoples’ Forest Tenure * 7.0 10.3

Grassland Protection * 3.2 4.0

Peatland Protection & Rewetting * 25.5 40.9

Coastal Wetland Protection * 0.7 1.0

Protect Ecosystems +

Shift Agriculture Practices

Sustainable Intensification
for Smallholders *

0.1 0.1

Shift Agriculture Practices Conservation Agriculture * 1.5 1.1

Regenerative Annual Cropping * 1.0 1.5

Nutrient Management 2.3 12.1

Farm Irrigation Efficiency 1.1 2.1

Improved Rice Production * 4.0 5.9

System of Rice Intensification * 2.0 3.0

F O O D , A G R I C U L T U R E & L A N D U S E T O T A L 204.2 273.9

In
du

st
ry

Improve Materials Alternative Cement 8.0 16.1

Bioplastics 1.0 3.8

Use Waste Composting 2.1 3.1

Recycling 5.5 6.0

Recycled Paper 1.1 1.9

Waste-to-Energy * 1.6 2.1

Landfill Methane Capture * 2.0 -1.5

Methane Digesters * 6.2 3.8

Address Refrigerants Refrigerant Management * 57.7 57.7

Alternative Refrigerants * 43.5 50.5

I N D U S T R Y T O T A L 128.7 143.7

Summary of Solutions

Reduce Sources bringing emissions to zero
Sector Subgroup Solution
SCENARIO 1
Total CO2-eq (Gt)
Reduced / Sequestered
(2020–2050)
SCENARIO 2
Total CO2 -eq (Gt)
Reduced / Sequestered
(2020–2050)

Tr
an

sp
or

ta
tio

n Shift to Alternatives Walkable Cities 1.4 5.5
Bicycle Infrastructure 2.6 6.6

Electric Bicycles 1.3 4.1

Carpooling 7.7 4.2

Public Transit 7.5 23.4

High-Speed Rail 1.3 3.8

Telepresence 1.0 3.8

Enhance Efficiency Hybrid Cars 7.9 4.6

Efficient Trucks 4.6 9.7

Efficient Aviation 6.3 9.2

Efficient Ocean Shipping 4.4 6.3

Electrify Vehicles Electric Cars 11.9 15.7

Electric Trains 0.1 0.6

T R A N S P O R T A T I O N T O T A L 58.0 97.4

Bu
ild

in
gs

Enhance Efficiency Smart Thermostats * 3.9 4.1

Building Automation Systems * 1.6 2.6

Insulation * 13.2 14.8

Dynamic Glass * 0.1 0.1

High-Performance Glass * 8.1 10.3

Green & Cool Roofs * -0.1 -0.2

Low-Flow Fixtures * 0.7 1.2

Enhance Efficiency +
Shift Energy Sources
Building Retrofitting * ** N/A N/A
Net-Zero Buildings * ** N/A N/A

Shift Energy Sources District Heating * 1.7 2.7

High-Efficiency Heat Pumps * 5.8 12.3

Solar Hot Water * 2.8 11.1

Biogas for Cooking 4.6 9.7

Improved Clean Cookstoves 31.3 72.6

Address Refrigerants Refrigerant Management * N/A N/A

Alternative Refrigerants * N/A N/A

B U I L D I N G S T O T A L 73.7 141.2

R E D U C E S O U R C E S T O T A L 665.3 1,097.4

2020 The Drawdown Review

Support Sinks uplifting nature’s carbon cycle

Sector Subgroup Solution
SCENARIO 1
Total CO2-eq (Gt)
Reduced / Sequestered
(2020–2050)
SCENARIO 2
Total CO2 -eq (Gt)
Reduced / Sequestered
(2020–2050)

La
nd

S
in

Address Waste & Diets Plant-Rich Diets * 0.2 0.2

Reduced Food Waste * 0.8 0.8

Protect & Restore Ecosystems Forest Protection * 1.1 1.9

Indigenous Peoples’ Forest Tenure * 1.7 2.6

Temperate Forest Restoration 19.4 27.8

Tropical Forest Restoration 54.5 85.1

Grassland Protection * 0.2 0.2

Peatland Protection & Rewetting * 0.6 1.0

Protect & Restore Ecosystems +

Shift Agriculture Practices
Sustainable Intensification
for Smallholders *

1.2 0.6

Shift Agriculture Practices Conservation Agriculture * 11.9 8.3

Regenerative Annual Cropping * 13.6 20.8

Managed Grazing 16.4 26.0

Silvopasture 26.6 42.3

Multistrata Agroforestry 11.3 20.4

Tree Intercropping 15.0 24.4

Perennial Staple Crops 15.5 31.3

Perennial Biomass Production 4.0 7.0

Improved Rice Production * 5.4 8.0

System of Rice Intensification * 0.8 1.2

Use Degraded Land Abandoned Farmland Restoration 12.5 20.3

Tree Plantations (on Degraded Land) 22.2 35.9

Bamboo Production 8.3 21.3

L A N D S I N K S T O T A L 243.1 387.8

C
oa

st
al

O
ce

an
S

in
ks Protect & Restore Ecosystems Coastal Wetland Protection * 0.3 0.5

Coastal Wetland Restoration 0.8 1.0

C O A S T A L & O C E A N S I N K S T O T A L 1.1 1.5

En
gi

ne
er

Si
nk

s Remove & Store Carbon Biochar Production 2.2 4.4
E N G I N E E R E D S I N K S T O T A L 2.2 4.4

S U P P O R T S I N K S T O T A L 246.4 393.7

Summary of Solutions

Improve Society fostering equality for all

Sector Subgroup Solution
SCENARIO 1
Total CO2-eq (Gt)
Reduced / Sequestered
(2020–2050)
SCENARIO 2
Total CO2 -eq (Gt)
Reduced / Sequestered
(2020–2050)

H
ea

lth
&

uc
at

io
n N/A Health & Education 85.4 85.4

H E A L T H & E D U C A T I O N T O T A L 85.4 85.4

I M P R O V E S O C I E T Y T O T A L 85.4 85.4

Two shamans who live in the forest
community of Cashiboya, Loreto, Perú.

Individual
Solutions
Scenario 1

NOTE: Where a cost is a negative number, it indicates savings. Where a dash is shown, results are not available.

Overall
Ranking Solution

Total CO2 -eq (Gt)
Reduced / Sequestered

(2020–2050)

Net First Cost
to implement solution

(Billions $US)

Net Lifetime Cost
to operate solution

(Billions $US)

Net Lifetime Profit
after implementation

and operation
(Billions $US)

1 Reduced Food Waste 87.4 – – –

2 Health & Education 85.4 – – –

3 Plant-Rich Diets 65.0 – – –

4 Refrigerant Management 57.7 – 600 –

5 Tropical Forest Restoration 54.5 – – –

6 Onshore Wind Turbines 47.2 800 -3,800 –

7 Alternative Refrigerants 43.5 – – –

8 Utility-Scale Solar Photovoltaics 42.3 -200 -12,900 –

9 Improved Clean Cookstoves 31.3 100 1,900 –

10 Distributed Solar Photovoltaics 28.0 400 -7,800 –

11 Silvopasture 26.6 200 2,300 1,700

12 Peatland Protection & Rewetting 26.0 – – –

13 Tree Plantations (on Degraded Land) 22.2 16 100 2,100

14 Temperate Forest Restoration 19.4 – – –

15 Concentrated Solar Power 18.6 400 800 –

16 Insulation 17.0 700 -21,700 –

17 Managed Grazing 16.4 33 -600 2,100

18 LED Lighting 16.1 -1,700 -4,500 –

19 Perennial Staple Crops 15.5 83 800 1,400

20 Tree Intercropping 15.0 100 600 200

21 Regenerative Annual Cropping 14.5 77 -2,300 100

22 Conservation Agriculture 13.4 91 -2,800 100

23 Abandoned Farmland Restoration 12.5 98 3,200 2,600

24 Electric Cars 11.9 4,400 -15,200 –

2020 The Drawdown Review

The rankings shown here are based on
projected emissions impact globally. The
relative importance of a given solution
can differ significantly depending on
context and particular ecological, eco-
nomic, political, or social conditions.

SUMMARY OF SOLUTIONS

25 Multistrata Agroforestry 11.3 54 100 1,700

26 Offshore Wind Turbines 10.4 600 -600 –

27 High-Performance Glass 10.0 9,000 -3,300 –

28 Methane Digesters 9.8 200 2 –

29 Improved Rice Production 9.4 – -400 200

30 Indigenous Peoples’ Forest Tenure 8.7 – – –

31 Bamboo Production 8.3 52 500 1,700

32 Alternative Cement 8.0 -63 – –

33 Hybrid Cars 7.9 3,400 -6,100 –

34 Carpooling 7.7 – -5,300 –

35 Public Transit 7.5 – -2,100 –

36 Smart Thermostats 7.0 100 -1,800 –

37 Building Automation Systems 6.5 200 -1,700 –

38 District Heating 6.3 200 -1,500 –

39 Efficient Aviation 6.3 800 -2,400 –

40 Geothermal Power 6.2 80 -800 –

41 Forest Protection 5.5 – – –

42 Recycling 5.5 10 -200 –

43 Biogas for Cooking 4.6 23 100 –

44 Efficient Trucks 4.6 400 -3,400 –

45 Efficient Ocean Shipping 4.4 500 -600 –

46 High-Efficiency Heat Pumps 4.2 76 -1,000 –

47 Perennial Biomass Production 4.0 200 1,500 900

48 Solar Hot Water 3.6 700 -200 –

49 Grassland Protection 3.3 – – –

50 System of Rice Intensification 2.8 – -14 500

51 Nuclear Power 2.7 100 -300 –

52 Bicycle Infrastructure 2.6 -2,600 -800 –

53 Biomass Power 2.5 51 -200 –

54 Nutrient Management 2.3 – -23 –

55 Biochar Production 2.2 100 700 –

56 Landfill Methane Capture 2.2 -4 6 –

57 Composting 2.1 -60 100 –

58 Waste-to-Energy 2.0 100 96 –

59 Small Hydropower 1.7 49 -300 –

60 Walkable Cities 1.4 – -1,600 –

61 Ocean Power 1.4 200 1,000 –

62 Sustainable Intensification for Smallholders 1.4 – -100 300

and operation
(Billions $US)

Summary of Solutions

63 Electric Bicycles 1.3 -300 -600 –

64 High-Speed Rail 1.3 600 800 –

65 Farm Irrigation Efficiency 1.1 200 -500 –

66 Recycled Paper 1.1 400 – –

67 Telepresence 1.0 86 -1,200 –

68 Coastal Wetland Protection 1.0 – – –

69 Bioplastics 1.0 88 – –

70 Low-Flow Fixtures 0.9 1 -400 –

71 Coastal Wetland Restoration 0.8 – – –

72 Water Distribution Efficiency 0.7 17 -200 –

73 Green & Cool Roofs 0.6 600 -300 –

74 Dynamic Glass 0.3 69 -98 –

75 Electric Trains 0.1 600 -700 –

76 Micro Wind Turbines 0.1 52 19 –

Not
Ranked* Building Retrofitting N/A – – –

Distributed Energy Storage N/A – – –

Grid Flexibility N/A – – –

Microgrids N/A – – –

Net-Zero Buildings N/A – – –

Utility-Scale Energy Storage N/A – – –

SCENARIO 1 TOTAL 997.2 22,479 -95,112 15,600

Overall
Ranking Solution
Total CO2 -eq (Gt)
Reduced / Sequestered
(2020–2050)
Net First Cost
to implement solution
(Billions $US)
Net Lifetime Cost
to operate solution
(Billions $US)
Net Lifetime Profit
after implementation
and operation
(Billions $US)
Overall
Ranking Solution
Total CO2 -eq (Gt)
Reduced / Sequestered
(2020–2050)

Net First Cost
to implement solution

(Billions $US)
Net Lifetime Cost
to operate solution
(Billions $US)
Net Lifetime Profit
after implementation
and operation
(Billions $US)

1 Onshore Wind Turbines 147.7 1,700 -10,200 –

2 Utility-Scale Solar Photovoltaics 119.1 -1,528 -26,500 –

3 Reduced Food Waste 94.6 – – –

4 Plant-Rich Diets 91.7 – – –

5 Health & Education 85.4 – – –

6 Tropical Forest Restoration 85.1 – – –

7 Improved Clean Cookstoves 72.6 300 4,191 –

8 Distributed Solar Photovoltaics 68.6 300 -13,600 –

9 Refrigerant Management 57.7 – 630 –

10 Alternative Refrigerants 50.5 – – –

2020 The Drawdown Review

Scenario 2

* The emissions impacts included in or enabled by these solutions are allocated elsewhere.

Summary of Solutions

11 Silvopasture 42.3 300 3,120 2,400

12 Peatland Protection & Rewetting 41.9 – – –

13 Tree Plantations (on Degraded Land) 35.9 100 260 3,400

14 Perennial Staple Crops 31.3 200 1,922 3,400

15 Temperate Forest Restoration 27.8 – – –

16 Managed Grazing 26.0 100 -1,100 3,500

17 Tree Intercropping 24.4 300 1,080 500

18 Concentrated Solar Power 24.0 600 1,116 –

19 Public Transit 23.4 – -6,600 –

20 Regenerative Annual Cropping 22.3 200 -3,600 300

21 Bamboo Production 21.3 200 1,444 4,400

22 Multistrata Agroforestry 20.4 100 246 3,100

23 Abandoned Farmland Restoration 20.3 200 5,272 4,400

24 Insulation 19.0 900 -24,200 –

25 LED Lighting 17.5 -2,036 -5,000 –

26 Alternative Cement 16.1 -64 – –

27 Electric Cars 15.7 5,800 -21,900 –

28 Solar Hot Water 14.3 2,700 -1,200 –

29 Improved Rice Production 13.8 – -700 400

30 Indigenous Peoples’ Forest Tenure 12.9 – – –

31 High-Performance Glass 12.6 10,800 -4,000 –

32 Nutrient Management 12.1 – -100 –

33 Offshore Wind Turbines 11.4 800 -800 –

34 Building Automation Systems 10.5 300 -3,100 –

35 District Heating 9.9 400 -2,500 –

36 Geothermal Power 9.8 100 -1,300 –

37 Efficient Trucks 9.7 800 -6,100 –

38 Biogas for Cooking 9.7 100 210 –

39 Conservation Agriculture 9.4 100 -2,000 100

40 High-Efficiency Heat Pumps 9.3 200 -2,600 –

41 Efficient Aviation 9.2 900 -3,700 –

42 Forest Protection 8.7 – – –

43 Smart Thermostats 7.4 200 -2,100 –

44 Perennial Biomass Production 7.0 400 2,751 1,700

45 Bicycle Infrastructure 6.6 -7,539 -2,400 –

46 Efficient Ocean Shipping 6.3 800 -900 –

47 Methane Digesters 6.2 200 2 –

48 Recycling 6.0 100 -300 –

49 Walkable Cities 5.5 – -6,500 –

50 Hybrid Cars 4.6 1,700 -3,000 –

51 Biochar Production 4.4 400 1,437 –

52 System of Rice Intensification 4.3 – -100 900

53 Grassland Protection 4.3 – – –

54 Carpooling 4.2 – -2,800 –

55 Electric Bicycles 4.1 -1,155 -1,900 –

56 Telepresence 3.8 400 -4,400 –

57 Bioplastics 3.8 100 – –

58 High-Speed Rail 3.8 1,300 2,164 –

59 Biomass Power 3.6 100 -300 –

60 Small Hydropower 3.3 100 -600 –

61 Nuclear Power 3.2 200 -400 –

62 Composting 3.1 -84 174 –

63 Waste-to-Energy 3.0 200 -1 –

64 Farm Irrigation Efficiency 2.1 400 -1,000 –

65 Recycled Paper 1.9 1,000 – –

66 Low-Flow Fixtures 1.6 100 -800 –

67 Coastal Wetland Protection 1.5 – – –

68 Ocean Power 1.4 300 1,440 –

69 Green & Cool Roofs 1.1 1,000 -600 –

70 Coastal Wetland Restoration 1.0 – – –

71 Water Distribution Efficiency 0.9 100 -400 –

72 Sustainable Intensification for Smallholders 0.7 – -100 200

73 Electric Trains 0.6 2,900 -3,400 –

74 Dynamic Glass 0.5 200 -200 –

75 Micro Wind Turbines 0.1 100 28 –

76 Landfill Methane Capture -1.6 – 22 –

Not
Ranked* Building Retrofitting N/A – – –

Distributed Energy Storage N/A – – –

Grid Flexibility N/A – – –

Microgrids N/A – – –

Net-Zero Buildings N/A – – –

Utility-Scale Energy Storage N/A – – –

SCENARIO 2 TOTAL 1,576.5 28,394 -145,492 28,700

* The emissions impacts included in or enabled by these solutions are allocated elsewhere.

Summary of Solutions

The Indian Ocean meets shore in the Maldives,
an archipelago of low-lying islands and atolls.

It is among the small island nations whose very
existence is threatened by climate change.

1. IPCC (2018). Summary for policymakers. In: Global warming of
1.5°C. An IPCC special report on the impacts of global warming of
1.5°C above pre-industrial levels and related global greenhouse gas
emission pathways, in the context of strengthening the global re-
sponse to the threat of climate change, sustainable development, and
efforts to eradicate poverty. World Meteorological Organization.
https://www.ipcc.ch/sr15/chapter/spm/

2. IPCC (2014). Climate change 2014: Mitigation of climate change.
Contribution of Working Group III to the fifth assessment report of the
Intergovernmental Panel on Climate Change. Cambridge University
Press.
https://www.ipcc.ch/report/ar5/wg3/

3. Thunberg, G. (2019). No one is too small to make a difference.
Penguin Books.

4. Global Carbon Project (2019). Carbon budget and trends 2019.
https://www.globalcarbonproject.org/carbonbudget/

5. IEA, IRENA, UNSD, WB, WHO (2019). Tracking SDG 7: The energy
progress report 2019. The World Bank.
https://trackingsdg7.esmap.org/data/files/download-docu-
ments/2019-Tracking%20SDG7-Full%20Report

REFERENCES

Major Funders

Project Drawdown is deeply grateful to the many individuals and institutions that support our work. Since the
publication of Drawdown in 2017, the generosity of these major funders has allowed us to continue developing a
leading resource for climate solutions:

6. IEA (2017). Energy technology perspectives 2017. International
Energy Agency.
https://www.iea.org/reports/energy-technology-perspectives-2017

7. IPCC (2019). Summary for policymakers. In: IPCC special report
on the ocean and cryosphere in a changing climate. In press.

Summary for Policymakers

8. UN Department of Economic and Social Affairs, Population
Division (2019). World population prospects 2019: Highlights. United
Nations.
https://population.un.org/wpp/Publications/Files/WPP2019_High-
lights

9. Gore, T. (2015, December 22). Extreme carbon inequality: Why the
Paris climate deal must put the poorest, lowest emitting and most
vulnerable people first. Oxfam.
https://www.oxfam.org/en/research/extreme-carbon-inequality

10. UN Department of Economic and Social Affairs, Population
Division (2019). World population prospects 2019: Highlights. United
Nations.
https://population.un.org/wpp/Publications/Files/WPP2019_High-
lights

Additional references for each solution and sector can be found at Drawdown.org.

Ann and Gordon Getty Foundation | Caldera Foundation

Caldwell Fisher Family Foundation | craigslist Charitable Fund

Hopewell Fund | Jamie Wolf | Michael and Jena King Family Fund

Newman’s Own Foundation | Ray C. Anderson Foundation

Rockefeller Brothers Fund | The Heinz Endowments | Trailsend Foundation

Cover Concentrated Solar by Dennis Schroeder/NREL Front Matter Wild Honey by Nanang Sujana/CIFOR • Tokyo Train by Simon
Launay (Unsplash) • Forest Restoration by Axel Fassio/CIFOR • Kaas Plateau by Raju GPK (Unsplash) Foreword Coastal Wetland by
Richard Sagredo (Unsplash) 10 Key Insights Silvopasture by Neil Palmer/CIAT • Retrieving Water by Ollivier Girard/CIFOR • Living
Building by Jonathan Hillyer • Kilimanjaro by Ray in Manila • Bioplastics by Jürgen Grünwald • Rooftop Solar by Stephen Yang/The
Solutions Project • Rice Research by Leo Sebastian/IRRI-CCAFS • Community Health Workers by Rob Tinworth • Climate Strike by
Markus Spiske (Unsplash)

Reduce Sources Powerlines by Charlotte Venema (Unsplash) Electricity Windsurfers & Turbines by Ronaldo Lourenço (Unsplash)
• Distributed Solar by Abbie Trayler-Smith/Panos Pictures/DFID Food, Agriculture & Land Use Peatland Forest by Nanang
Sujana/CIFOR • Marcha das Mulheres by Natalia Gomes/Cobertura Colaborativa • Roasted Eggplant by Stijn Nieuwendijk Industry
CopenHill by Kristoffer Dahl/News Øresund • Compost by Will Parson/Chesapeake Bay Program • Appliances by Janaya Dasiuk
(Unsplash) • Recycling by Rwanda Green Fund Transportation The “L” by Sawyer Bengtson (Unsplash) • Cargo Ship by Sergio Souza
(Unsplash) Buildings Green Roof by Bernard Hermant (Unsplash) • Biogas Cooking by Vidura Jang Bahadur • Insulation by Charles
Deluvio (Unsplash) Other Gas Flaring by WildEarth Guardians

Support Sinks Snoqualmie Pass by Dave Hoefler (Unsplash) Land Sinks Yangambi Farm by Axel Fassio/CIFOR • Bamboo by
kazuend (Unsplash) • Soil Testing by Georgina Smith/CIAT Coastal & Ocean Sinks Planting Mangroves by Rob Barnes/AGEDI/
Blue Forests • Kelp by Shane Stagner (Unsplash) • Mangrove Forest by Joel Vodell (Unsplash) Engineered Sinks Biochar by Tracy
Robillard/NRCS

Improve Society Crosswalk by Ryoji Iwata (Unsplash) Health & Education Student by Allison Kwesell/World Bank

Solutions Beyond Drawdown Moose by Kent Miller/NPS Assessing Solutions Turbine Inspector by Dennis Schroeder/NREL
Forward Drakes Bay by Brian Cluer/NOAA WCR • Solar Farm by Antonio Garcia (Unsplash) • Women in Guinea by Joe Saade/UN
Women Back Matter San Gorgonio Pass by Ian D. Keating • Shamans in Perú by Marlon del Aguila Guerrero/CIFOR • Maldives by
Shifaaz Shamoon (Unsplash)

PHOTO CREDITS

Project Drawdown conducts ongoing review and analysis of climate solutions. Any corrections to the results or content
contained in this publication will be catalogued at www.drawdown.org/errata.

http://www.drawdown.org/errata

The World’s Leading Resource for Climate Solutions

Chapter

Climate Change 201

Synthesis Report

Summary for Policymakers

Summary for Policymaker

SPM

Introductio

This Synthesis Report is based on the reports of the three Working Groups of the Intergovernmental Panel on Climate Change
(IPCC

)

, including relevant Special Reports. It provides an integrated view of climate change as the final part of the IPCC’s
Fifth Assessment Report (AR5).

This summary follows the structure of the longer report which addresses the following topics: Observed changes and thei

causes; Future climate change, risks and impacts; Future pathways for adaptation, mitigation and sustainable development;
Adaptation and mitigatio

In the Synthesis Report, the certainty in key assessment findings is communicated as in the Working Group Reports and
Special Reports. It is based on the author teams’ evaluations of underlying scientific understanding and is expressed as a
qualitative level of confidence (from very low to very high) and, when possible, probabilistically with a quantified likelihood
(from exceptionally unlikely to virtually certain)1. Where appropriate, findings are also formulated as statements of fact with-
out using uncertainty qualifiers.

This report includes information relevant to Article 2 of the United Nations Framework Convention on Climate Change
(UNFCCC).

SPM 1. Observed Changes and their Caus

Human influence on the climate system is clear, and recent anthropogenic emissions of green-
house gases are the highest in history. Recent climate changes have had widespread impacts
on human and natural systems. {1}

SPM 1.1 Observed changes in the climate system

Warming of the climate system is unequivocal, and since the 1950s, many of the observed
changes are unprecedented over decades to millennia. The atmosphere and ocean have
warmed, the amounts of snow and ice have diminished, and sea level has risen. {1.1}

Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. The
period from 1983 to 2012 was likely the warmest 30-year period of the last 1400 years in the Northern Hemisphere, where
such assessment is possible (medium confidence). The globally averaged combined land and ocean surface temperature
data as calculated by a linear trend show a warming of 0.85 [0.65 to 1.06] °C 2 over the period 1880 to 2012, when multiple
independently produced datasets exist (Figure SPM.1a). {1.1.1, Figure 1.1}

In addition to robust multi-decadal warming, the globally averaged surface temperature exhibits substantial decadal and
interannual variability (Figure SPM.1a). Due to this natural variability, trends based on short records are very sensitive to the
beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over

1 Each finding is grounded in an evaluation of underlying evidence and agreement. In many cases, a synthesis of evidence and agreement supports a

assignment of confidence. The summary terms for evidence are: limited, medium or robust. For agreement, they are low, medium or high. A level of
confidence is expressed using five qualifiers: very low, low, medium, high and very high, and typeset in italics, e.g., medium confidence. The follow-
ing terms have been used to indicate the assessed likelihood of an outcome or a result: virtually certain 99–100% probability, very likely 90–100%,
likely 66–100%, about as likely as not 33–66%, unlikely 0–33%, very unlikely 0–10%, exceptionally unlikely 0–1%. Additional terms (extremel

likely 95–100%, more likely than not >50–100%, more unlikely than likely 0–<50%, extremely unlikely 0–5%) may also be used when appropriate. Assessed likelihood is typeset in italics, e.g., very likely. See for more details: Mastrandrea, M.D., C.B. Field, T.F. Stocker, O. Edenhofer, K.L. Ebi, D.J. Frame, H. Held, E. Kriegler, K.J. Mach, P.R. Matschoss, G.-K. Plattner, G.W. Yohe and F.W. Zwiers, 2010: Guidance Note for Lead Authors of the IPCC Fifth Assess- ment Report on Consistent Treatment of Uncertainties, Intergovernmental Panel on Climate Change (IPCC), Geneva, Switzerland, 4 pp.

2 Ranges in square brackets or following ‘±’ are expected to have a 90% likelihood of including the value that is being estimated, unless otherwise
stated.

Summary for Policymakers

SPM

−

−0.

−

0.4

−

0.2

0.2
0.4

(°
C

)

Globally averaged combined land and ocean surface temperature anomal

1850 1900 1950

2000

Year

−0.2

−0.1

−

0.1

−

0.05

0
0.05
0.1

(m
)

Globally averaged sea level change

(b)

1850 1900 1950 2000
Year

(G
tC

O
2/

yr
)

0
5

Fossil fuels, cement and flaring

Forestry and other land us

1850 1900 1950 2000
Year

800

1000

200

400

600

1800

C
H

4
(p

pb
)

270

290

300

310

320

330

N
2O

(p
pb

)
280
300
320

340

380

400

C
O

2
(p

pm
)

1850 1900 1950 2000
Year
0

500

1000

1500

2000

–

1970
1750

–
20

Cumulative CO2
emissions

(G
tC

O
2)

(a)

(c)

(d)

Globally averaged greenhouse gas concentrations

Global anthropogenic CO2 emissions
Quantitative information of CH4 and N2O emission time series from 1850 to 1970 is limited

Figure SPM.1 | The complex relationship between the observations (panels a, b, c, yellow background) and the emissions (panel d,
light blue background) is addressed in Section 1.2 and Topic 1. Observations and other indicators of a changing global climate system. Observa-
tions: (a) Annually and globally averaged combined land and ocean surface temperature anomalies relative to the average over the period 1986 to 2005.
Colours indicate different data sets. (b) Annually and globally averaged sea level change relative to the average over the period 1986 to 2005 in the
longest-running dataset. Colours indicate different data sets. All datasets are aligned to have the same value in 1993, the first year of satellite altimetry
data (red). Where assessed, uncertainties are indicated by coloured shading. (c) Atmospheric concentrations of the greenhouse gases carbon dioxide
(CO2, green), methane (CH4, orange) and nitrous oxide (N2O, red) determined from ice core data (dots) and from direct atmospheric measurements (lines).
Indicators: (d) Global anthropogenic CO2 emissions from forestry and other land use as well as from burning of fossil fuel, cement production and flaring.
Cumulative emissions of CO2 from these sources and their uncertainties are shown as bars and whiskers, respectively, on the right hand side. The global
effects of the accumulation of CH4 and N2O emissions are shown in panel c. Greenhouse gas emission data from 1970 to 2010 are shown in Figure SPM.2.
{Figures 1.1, 1.3, 1.5}

Summary for Policymakers
4
SPM

the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller than the
rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade). {1.1.1, Box 1.1}

Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy
accumulated between 1971 and 2010 (high confidence), with only about 1% stored in the atmosphere. On a global scale,
the ocean warming is largest near the surface, and the upper 75 m warmed by 0.11 [0.09 to 0.13] °C per decade over the
period 1971 to 2010. It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010, and it likely warmed
between the 1870s and 1971. {1.1.2, Figure 1.2}

Averaged over the mid-latitude land areas of the Northern Hemisphere, precipitation has increased since 1901 (medium
confidence before and high confidence after 1951). For other latitudes, area-averaged long-term positive or negative trends
have low confidence. Observations of changes in ocean surface salinity also provide indirect evidence for changes in the
global water cycle over the ocean (medium confidence). It is very likely that regions of high salinity, where evaporation dom-
inates, have become more saline, while regions of low salinity, where precipitation dominates, have become fresher since
the 1950s. {1.1.1, 1.1.2}

Since the beginning of the industrial era, oceanic uptake of CO2 has resulted in acidification of the ocean; the pH of ocean
surface water has decreased by 0.1 (high confidence), corresponding to a 26% increase in acidity, measured as hydrogen ion
concentration. {1.1.2}

Over the period 1992 to 2011, the Greenland and Antarctic ice sheets have been losing mass (high confidence), likely at a
larger rate over 2002 to 2011. Glaciers have continued to shrink almost worldwide (high confidence). Northern Hemisphere
spring snow cover has continued to decrease in extent (high confidence). There is high confidence that permafrost tempera-
tures have increased in most regions since the early 1980s in response to increased surface temperature and changing snow
cover. {1.1.3}

The annual mean Arctic sea-ice extent decreased over the period 1979 to 2012, with a rate that was very likely in the range
3.5 to 4.1% per decade. Arctic sea-ice extent has decreased in every season and in every successive decade since 1979, with
the most rapid decrease in decadal mean extent in summer (high confidence). It is very likely that the annual mean Antarctic
sea-ice extent increased in the range of 1.2 to 1.8% per decade between 1979 and 2012. However, there is high confidence
that there are strong regional differences in Antarctica, with extent increasing in some regions and decreasing in others.
{1.1.3, Figure 1.1}

Over the period 1901 to 2010, global mean sea level rose by 0.19 [0.17 to 0.21] m (Figure SPM.1b). The rate of sea level rise
since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence). {1.1.4,
Figure 1.1}

SPM 1.2 Causes of climate change

Anthropogenic greenhouse gas (GHG) emissions since the pre-industrial era have driven large increases in the atmospheric
concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) (Figure SPM.1c). Between 1750 and 2011,
cumulative anthropogenic CO2 emissions to the atmosphere were 2040 ± 310 GtCO2. About 40% of these emissions have
remained in the atmosphere (880 ± 35 GtCO2); the rest was removed from the atmosphere and stored on land (in plants and
soils) and in the ocean. The ocean has absorbed about 30% of the emitted anthropogenic CO2, causing ocean acidification.
About half of the anthropogenic CO2 emissions between 1750 and 2011 have occurred in the last 40 years (high confidence)
(Figure SPM.1d). {1.2.1, 1.2.2}

Anthropogenic greenhouse gas emissions have increased since the pre-industrial era, driven
largely by economic and population growth, and are now higher than ever. This has led to atmo-
spheric concentrations of carbon dioxide, methane and nitrous oxide that are unprecedented in
at least the last 800,000 years. Their effects, together with those of other anthropogenic driv-
ers, have been detected throughout the climate system and are extremely likely to have been
the dominant cause of the observed warming since the mid-20th century. {1.2, 1.3.1}

Summary for Policymakers

5
SPM

Total anthropogenic GHG emissions have continued to increase over 1970 to 2010 with larger absolute increases between
2000 and 2010, despite a growing number of climate change mitigation policies. Anthropogenic GHG emissions in 2010 have
reached 49 ± 4.5 GtCO2-eq/yr 3. Emissions of CO2 from fossil fuel combustion and industrial processes contributed about 78%
of the total GHG emissions increase from 1970 to 2010, with a similar percentage contribution for the increase during the
period 2000 to 2010 (high confidence) (Figure SPM.2). Globally, economic and population growth continued to be the most
important drivers of increases in CO2 emissions from fossil fuel combustion. The contribution of population growth between
2000 and 2010 remained roughly identical to the previous three decades, while the contribution of economic growth has
risen sharply. Increased use of coal has reversed the long-standing trend of gradual decarbonization (i.e., reducing the carbon
intensity of energy) of the world’s energy supply (high confidence). {1.2.2}

The evidence for human influence on the climate system has grown since the IPCC Fourth Assessment Report (AR4). It is
extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was
caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings together. The best estimate
of the human-induced contribution to warming is similar to the observed warming over this period (Figure SPM.3). Anthro-
pogenic forcings have likely made a substantial contribution to surface temperature increases since the mid-20th century
over every continental region except Antarctica4. Anthropogenic influences have likely affected the global water cycle since
1960 and contributed to the retreat of glaciers since the 1960s and to the increased surface melting of the Greenland ice
sheet since 1993. Anthropogenic influences have very likely contributed to Arctic sea-ice loss since 1979 and have very likely
made a substantial contribution to increases in global upper ocean heat content (0–700 m) and to global mean sea level rise
observed since the 1970s. {1.3, Figure 1.10}

3 Greenhouse gas emissions are quantified as CO2-equivalent (GtCO2-eq) emissions using weightings based on the 100-year Global Warming Potentials,
using IPCC Second Assessment Report values unless otherwise stated. {Box 3.2}

4 For Antarctica, large observational uncertainties result in low confidence that anthropogenic forcings have contributed to the observed warming aver-
aged over available stations.

Gas

CO
2
Fossil fuel and

industrial processes

CO
2
FOLU

CH
4

N
2
O

F-Gases

2010

(GWP

100

SAR)Year (GWP

100
AR5)

Total annual anthropogenic GHG emissions by gases 1970–2010

27 Gt

52 Gt

17%

19%
7.9%

0.44%

2.2%

38 Gt

59%

16%

18%

7.4%
0.81%

49 Gt

11%

16%

6.2%
2.0%

G
H

G
e

m
is

si
on

s
(G

tC
O

2-
eq

0
10
20
30
40
50

201020052000199519901985198019751970

+2.2%/yr

2000–2010

+1.3%/yr
1970–2000

10%

20%

62%

Figure SPM.2 | Total annual anthropogenic greenhouse gas (GHG) emissions (gigatonne of CO2-equivalent per year, GtCO2-eq/yr) for the period 197

to 2010 by gases: CO2 from fossil fuel combustion and industrial processes; CO2 from Forestry and Other Land Use (FOLU); methane (CH4); nitrous oxide
(N2O); fluorinated gases covered under the Kyoto Protocol (F-gases). Right hand side shows 2010 emissions, using alternatively CO2-equivalent emission
weightings based on IPCC Second Assessment Report (SAR) and AR5 values. Unless otherwise stated, CO2-equivalent emissions in this report include the
basket of Kyoto gases (CO2, CH4, N2O as well as F-gases) calculated based on 100-year Global Warming Potential (GWP100) values from the SAR (see Glos-
sary). Using the most recent GWP100 values from the AR5 (right-hand bars) would result in higher total annual GHG emissions (52 GtCO2-eq/yr) from an
increased contribution of methane, but does not change the long-term trend significantly. {Figure 1.6, Box 3.2}

Summary for Policymakers
6
SPM

SPM 1.3 Impacts of climate change

In recent decades, changes in climate have caused impacts on natural and human systems on
all continents and across the oceans. Impacts are due to observed climate change, irrespec-
tive of its cause, indicating the sensitivity of natural and human systems to changing climate.
{1.3.2}

Evidence of observed climate change impacts is strongest and most comprehensive for natural systems. In many regions,
changing precipitation or melting snow and ice are altering hydrological systems, affecting water resources in terms of
quantity and quality (medium confidence). Many terrestrial, freshwater and marine species have shifted their geographic
ranges, seasonal activities, migration patterns, abundances and species interactions in response to ongoing climate change
(high confidence). Some impacts on human systems have also been attributed to climate change, with a major or minor
contribution of climate change distinguishable from other influences (Figure SPM.4). Assessment of many studies covering
a wide range of regions and crops shows that negative impacts of climate change on crop yields have been more common
than positive impacts (high confidence). Some impacts of ocean acidification on marine organisms have been attributed to
human influence (medium confidence). {1.3.2}

Combined anthropogenic forcings

Other anthropogenic forcings

OBSERVED WARMING

Greenhouse gases

Contributions to observed surface temperature change over the period 1951–2010

Natural forcings

Natural internal variabil

ity

–0.5 0.0 0.5 1.0
(°C)

Figure SPM.3 | Assessed likely ranges (whiskers) and their mid-points (bars) for warming trends over the 1951–2010 period from well-mixed greenhouse
gases, other anthropogenic forcings (including the cooling effect of aerosols and the effect of land use change), combined anthropogenic forcings, natural
forcings and natural internal climate variability (which is the element of climate variability that arises spontaneously within the climate system even in the
absence of forcings). The observed surface temperature change is shown in black, with the 5 to 95% uncertainty range due to observational uncertainty.
The attributed warming ranges (colours) are based on observations combined with climate model simulations, in order to estimate the contribution of an
individual external forcing to the observed warming. The contribution from the combined anthropogenic forcings can be estimated with less uncertainty
than the contributions from greenhouse gases and from other anthropogenic forcings separately. This is because these two contributions partially compen-
sate, resulting in a combined signal that is better constrained by observations. {Figure 1.9}

Summary for Policymakers

SPM

SPM 1.4 Extreme even

Changes in many extreme weather and climate events have been observed since about 1950.
Some of these changes have been linked to human influences, including a decrease in cold tem-
perature extremes, an increase in warm temperature extremes, an increase in extreme high sea
levels and an increase in the number of heavy precipitation events in a number of regions. {1.4}

It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased
on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is

Widespread impacts attributed to climate change based on the available scientific literature since the AR4

medlow veryhig

very
low high

Glaciers, snow, ice
and/or permafrost

indicates
confidence range

Rivers, lakes, floo

and/or drought

Terrestrial
ecosystems

Impacts identified
based on availability
of studies acro

a region

Marine ecosystemsCoastal erosion
and/or sea level effects

Wildfire Livelihoods, health and/or economics

Food production

Physical systems Biological systems Human and managed systems

Filled symbols = Major contribution of climate change
Outlined symbols = Minor contribution of climate change

Confidence in attribution
to climate change

Observed impacts attributed to climate change for

932

10544

8101

325529821987

AUSTRALASIA

ASIANORTH AMERICA

CENTRAL AND SOUTH AMERICA

AFRICA

EUROPE

SMALL ISLANDS

POLAR REGIONS (Arctic and Antarctic)

Figure SPM.4 | Based on the available scientific literature since the IPCC Fourth Assessment Report (AR4), there are substantially more impacts in recent
decades now attributed to climate change. Attribution requires defined scientific evidence on the role of climate change. Absence from the map of addi-
tional impacts attributed to climate change does not imply that such impacts have not occurred. The publications supporting attributed impacts reflect a
growing knowledge base, but publications are still limited for many regions, systems and processes, highlighting gaps in data and studies. Symbols indicate
categories of attributed impacts, the relative contribution of climate change (major or minor) to the observed impact and confidence in attribution. Each
symbol refers to one or more entries in WGII Table SPM.A1, grouping related regional-scale impacts. Numbers in ovals indicate regional totals of climate
change publications from 2001 to 2010, based on the Scopus bibliographic database for publications in English with individual countries mentioned in title,
abstract or key words (as of July 2011). These numbers provide an overall measure of the available scientific literature on climate change across regions;
they do not indicate the number of publications supporting attribution of climate change impacts in each region. Studies for polar regions and small islands
are grouped with neighbouring continental regions. The inclusion of publications for assessment of attribution followed IPCC scientific evidence criteria
defined in WGII Chapter 18. Publications considered in the attribution analyses come from a broader range of literature assessed in the WGII AR5. See WGII
Table SPM.A1 for descriptions of the attributed impacts. {Figure 1.11}

Summary for Policymakers
8
SPM

very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of
daily temperature extremes since the mid-20th century. It is likely that human influence has more than doubled the prob-
ability of occurrence of heat waves in some locations. There is medium confidence that the observed warming has increased
heat-related human mortality and decreased cold-related human mortality in some regions. {1.4}

There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased.
Recent detection of increasing trends in extreme precipitation and discharge in some catchments implies greater risks of
flooding at regional scale (medium confidence). It is likely that extreme sea levels (for example, as experienced in storm
surges) have increased since 1970, being mainly a result of rising mean sea level. {1.4}

Impacts from recent climate-related extremes, such as heat waves, droughts, floods, cyclones and wildfires, reveal significant
vulnerability and exposure of some ecosystems and many human systems to current climate variability (very high confi-
dence). {1.4}

SPM 2. Future Climate Changes, Risks and Impacts

Continued emission of greenhouse gases will cause further warming and long-lasting
changes in all components of the climate system, increasing the likelihood of severe,
pervasive and irreversible impacts for people and ecosystems. Limiting climate change would
require substantial and sustained reductions in greenhouse gas emissions which, together
with adaptation, can limit climate change risks. {2}

SPM 2.1 Key drivers of future climate

Cumulative emissions of CO2 largely determine global mean surface warming by the late
21st century and beyond. Projections of greenhouse gas emissions vary over a wide range,
depending on both socio-economic development and climate policy. {2.1}

Anthropogenic GHG emissions are mainly driven by population size, economic activity, lifestyle, energy use, land use patterns,
technology and climate policy. The Representative Concentration Pathways (RCPs), which are used for making projections
based on these factors, describe four different 21st century pathways of GHG emissions and atmospheric concentrations,
air pollutant emissions and land use. The RCPs include a stringent mitigation scenario (RCP2.6), two intermediate scenarios
(RCP4.5 and RCP6.0) and one scenario with very high GHG emissions (RCP8.5). Scenarios without additional efforts to
constrain emissions (’baseline scenarios’) lead to pathways ranging between RCP6.0 and RCP8.5 (Figure SPM.5a). RCP2.6 is
representative of a scenario that aims to keep global warming likely below 2°C above pre-industrial temperatures. The RCPs
are consistent with the wide range of scenarios in the literature as assessed by WGIII5. {2.1, Box 2.2, 4.3}

Multiple lines of evidence indicate a strong, consistent, almost linear relationship between cumulative CO2 emissions and
projected global temperature change to the year 2100 in both the RCPs and the wider set of mitigation scenarios analysed
in WGIII (Figure SPM.5b). Any given level of warming is associated with a range of cumulative CO2 emissions6, and therefore,
e.g., higher emissions in earlier decades imply lower emissions later. {2.2.5, Table 2.2}

5 Roughly 300 baseline scenarios and 900 mitigation scenarios are categorized by CO2-equivalent concentration (CO2-eq) by 2100. The CO2-eq includes
the forcing due to all GHGs (including halogenated gases and tropospheric ozone), aerosols and albedo change.

6 Quantification of this range of CO2 emissions requires taking into account non-CO2 drivers.

Summary for Policymakers
9
SPM
Year

A
nn

l e

m
is
si
on
s
(G
tC
O

2/
yr

)

1950 2000 2050 2100

−100

0
100
200

Historical
emissions

RCP2.6

RCP4.5

RCP6.0
RCP8.5

Fu
ll

ra
ng

e
of

t
he

W
G

III
A

R5
sc

en
ar

io
d

at
ab

as
e

in
2

10
0

Annual anthropogenic CO2 emissions

>1000

720−1000

580−720

530−580

480−530

430−480

(a)
(b)

WGIII scenario categories:

RCP scenarios:

1
2
3
4
5

Te
m

tu
re

c
ha

ng
e

re
la

iv
e

to
1

86
1–

(°
C

)

Warming versus cumulative CO2 emissions

Total human-induced warming

1000 2000 3000 4000 5000 6000 7000 8000 90000

1000 GtC 2000 GtC

Cumulative anthropogenic CO2 emissions from 1870 (GtCO2)

430–480

480–530

530–580

580–720

720–1000

baselines

observed 2000s

Figure SPM.5 | (a) Emissions of carbon dioxide (CO2) alone in the Representative Concentration Pathways (RCPs) (lines) and the associated scenario
categories used in WGIII (coloured areas show 5 to 95% range). The WGIII scenario categories summarize the wide range of emission scenarios published
in the scientific literature and are defined on the basis of CO2-eq concentration levels (in ppm) in 2100. The time series of other greenhouse gas emissions
are shown in Box 2.2, Figure 1. (b) Global mean surface temperature increase at the time global CO2 emissions reach a given net cumulative total, plotted
as a function of that total, from various lines of evidence. Coloured plume shows the spread of past and future projections from a hierarchy of climate-
carbon cycle models driven by historical emissions and the four RCPs over all times out to 2100, and fades with the decreasing number of available models.
Ellipses show total anthropogenic warming in 2100 versus cumulative CO2 emissions from 1870 to 2100 from a simple climate model (median climate
response) under the scenario categories used in WGIII. The width of the ellipses in terms of temperature is caused by the impact of different scenarios for
non-CO2 climate drivers. The filled black ellipse shows observed emissions to 2005 and observed temperatures in the decade 2000–2009 with associated
uncertainties. {Box 2.2, Figure 1; Figure 2.3}

Summary for Policymakers
10
SPM

Multi-model results show that limiting total human-induced warming to less than 2°C relative to the period 1861–1880 with
a probability of >66%7 would require cumulative CO2 emissions from all anthropogenic sources since 1870 to remain below
about 2900 GtCO2 (with a range of 2550 to 3150 GtCO2 depending on non-CO2 drivers). About 1900 GtCO28 had already been
emitted by 2011. For additional context see Table 2.2. {2.2.5}

SPM 2.2 Projected changes in the climate system

Surface temperature is projected to rise over the 21st century under all assessed emission
scenarios. It is very likely that heat waves will occur more often and last longer, and that
extreme precipitation events will become more intense and frequent in many regions. The
ocean will continue to warm and acidify, and global mean sea level to rise. {2.2}

The projected changes in Section SPM 2.2 are for 2081–2100 relative to 1986–2005, unless otherwise indicated.

Future climate will depend on committed warming caused by past anthropogenic emissions, as well as future anthropogenic
emissions and natural climate variability. The global mean surface temperature change for the period 2016–2035 relative to
1986–2005 is similar for the four RCPs and will likely be in the range 0.3°C to 0.7°C (medium confidence). This assumes that
there will be no major volcanic eruptions or changes in some natural sources (e.g., CH4 and N2O), or unexpected changes in
total solar irradiance. By mid-21st century, the magnitude of the projected climate change is substantially affected by the
choice of emissions scenario. {2.2.1, Table 2.1}

Relative to 1850–1900, global surface temperature change for the end of the 21st century (2081–2100) is projected to likely
exceed 1.5°C for RCP4.5, RCP6.0 and RCP8.5 (high confidence). Warming is likely to exceed 2°C for RCP6.0 and

RCP8.5

(high confidence), more likely than not to exceed 2°C for RCP4.5 (medium confidence), but unlikely to exceed 2°C for

RCP2.6

(medium confidence). {2.2.1}

The increase of global mean surface temperature by the end of the 21st century (2081–2100) relative to 1986–2005 is likely
to be 0.3°C to 1.7°C under RCP2.6, 1.1°C to 2.6°C under RCP4.5, 1.4°C to 3.1°C under RCP6.0 and 2.6°C to 4.8°C under
RCP8.59. The Arctic region will continue to warm more rapidly than the global mean (Figure SPM.6a, Figure SPM.7a). {2.2.1,
Figure 2.1, Figure 2.2, Table 2.1}

It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily
and seasonal timescales, as global mean surface temperature increases. It is very likely that heat waves will occur with a
higher frequency and longer duration. Occasional cold winter extremes will continue to occur. {2.2.1}

7 Corresponding figures for limiting warming to 2°C with a probability of >50% and >33% are 3000 GtCO2 (range of 2900 to 3200 GtCO2) and 3300 GtCO2
(range of 2950 to 3800 GtCO2) respectively. Higher or lower temperature limits would imply larger or lower cumulative emissions respectively.

8 This corresponds to about two thirds of the 2900 GtCO2 that would limit warming to less than 2°C with a probability of >66%; to about 63% of the total
amount of 3000 GtCO2 that would limit warming to less than 2°C with a probability of >50%; and to about 58% of the total amount of 3300 GtCO2
that would limit warming to less than 2°C with a probability of >33%.

9 The period 1986–2005 is approximately 0.61 [0.55 to 0.67] °C warmer than 1850–1900. {2.2.1}

Summary for Policymakers
11
SPM

Figure SPM.6 | Global average surface temperature change (a) and global mean sea level rise10 (b) from 2006 to 2100 as determined by multi-model
simulations. All changes are relative to 1986–2005. Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6
(blue) and RCP8.5 (red). The mean and associated uncertainties averaged over 2081–2100 are given for all RCP scenarios as coloured vertical bars at the
right hand side of each panel. The number of Coupled Model Intercomparison Project Phase 5 (CMIP5) models used to calculate the multi-model mean is
indicated. {2.2, Figure 2.1}

Changes in precipitation will not be uniform. The high latitudes and the equatorial Pacific are likely to experience an increase
in annual mean precipitation under the RCP8.5 scenario. In many mid-latitude and subtropical dry regions, mean precipi-
tation will likely decrease, while in many mid-latitude wet regions, mean precipitation will likely increase under the RCP8.5
scenario (Figure SPM.7b). Extreme precipitation events over most of the mid-latitude land masses and over wet tropical
regions will very likely become more intense and more frequent. {2.2.2, Figure 2.2}

The global ocean will continue to warm during the 21st century, with the strongest warming projected for the surface in
tropical and Northern Hemisphere subtropical regions (Figure SPM.7a). {2.2.3, Figure 2.2}

10 Based on current understanding (from observations, physical understanding and modelling), only the collapse of marine-based sectors of the Antarctic
ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century. There is medium confidence
that this additional contribution would not exceed several tenths of a meter of sea level rise during the 21st century.

Global mean sea level rise
(relative to 1986–2005)

RC
P2

RC
P4

RC
P6

RC
P8

Mean over
2081–2100

21
(b)

2000 21002050

Year
1

0.8

0.6

0.4
0.2
0
(m
)
RC
P2
.6

RC
P4
.5

RC
P6

.0
RC

P8
.5

Global average surface temperature change
(relative to 1986–2005) Mean over

2081–2100
(a)

2000 21002050
Year
6
4
2
0

–2

(°
C
)

Summary for Policymakers

SPM

Earth System Models project a global increase in ocean acidification for all RCP scenarios by the end of the 21st century, with
a slow recovery after mid-century under RCP2.6. The decrease in surface ocean pH is in the range of 0.06 to 0.07 (15 to 17%
increase in acidity) for RCP2.6, 0.14 to 0.15 (38 to 41%) for RCP4.5, 0.20 to 0.21 (58 to 62%) for RCP6.0 and 0.30 to 0.32
(100 to 109%) for RCP8.5. {2.2.4, Figure 2.1}

Year-round reductions in Arctic sea ice are projected for all RCP scenarios. A nearly ice-free11 Arctic Ocean in the summer sea-
ice minimum in September before mid-century is likely for RCP8.512 (medium confidence). {2.2.3, Figure 2.1}

It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface
temperature increases, with the area of permafrost near the surface (upper 3.5 m) projected to decrease by 37% (RCP2.6) to
81% (RCP8.5) for the multi-model average (medium confidence). {2.2.3}

The global glacier volume, excluding glaciers on the periphery of Antarctica (and excluding the Greenland and Antarctic ice
sheets), is projected to decrease by 15 to 55% for RCP2.6 and by 35 to 85% for RCP8.5 (medium confidence). {2.2.3}

11 When sea-ice extent is less than one million km2 for at least five consecutive years.
12 Based on an assessment of the subset of models that most closely reproduce the climatological mean state and 1979–2012 trend of the Arctic sea-ice

extent.

RCP2.6 RCP8.5

−20 −10−30−50 −40 0 10 20 30 40 50

(b) Change in average precipitation (1986−2005 to 2081−2100)

3932

(%)

(a) Change in average surface temperature (1986−2005 to 2081−2100)

3932

(°C)
−0.5−1−2 −1.5 0 1 1.5 2 3 4 5 7 9 110.5

Figure SPM.7 | Change in average surface temperature (a) and change in average precipitation (b) based on multi-model mean projections for
2081–2100 relative to 1986–2005 under the RCP2.6 (left) and RCP8.5 (right) scenarios. The number of models used to calculate the multi-model mean
is indicated in the upper right corner of each panel. Stippling (i.e., dots) shows regions where the projected change is large compared to natural internal
variability and where at least 90% of models agree on the sign of change. Hatching (i.e., diagonal lines) shows regions where the projected change is less
than one standard deviation of the natural internal variability. {2.2, Figure 2.2}

Summary for Policymakers

SPM

There has been significant improvement in understanding and projection of sea level change since the AR4. Global mean sea
level rise will continue during the 21st century, very likely at a faster rate than observed from 1971 to 2010. For the period
2081–2100 relative to 1986–2005, the rise will likely be in the ranges of 0.26 to 0.55 m for RCP2.6, and of 0.45 to 0.82 m
for RCP8.5 (medium confidence)10 (Figure SPM.6b). Sea level rise will not be uniform across regions. By the end of the
21st century, it is very likely that sea level will rise in more than about 95% of the ocean area. About 70% of the coastlines
worldwide are projected to experience a sea level change within ±20% of the global mean. {2.2.3}

SPM 2.3 Future risks and impacts caused by a changing climate

Climate change will amplify existing risks and create new risks for natural and human sys-
tems. Risks are unevenly distributed and are generally greater for disadvantaged people and
communities in countries at all levels of development. {2.3}

Risk of climate-related impacts results from the interaction of climate-related hazards (including hazardous events and
trends) with the vulnerability and exposure of human and natural systems, including their ability to adapt. Rising rates and
magnitudes of warming and other changes in the climate system, accompanied by ocean acidification, increase the risk
of severe, pervasive and in some cases irreversible detrimental impacts. Some risks are particularly relevant for individual
regions (Figure SPM.8), while others are global. The overall risks of future climate change impacts can be reduced by limiting
the rate and magnitude of climate change, including ocean acidification. The precise levels of climate change sufficient to
trigger abrupt and irreversible change remain uncertain, but the risk associated with crossing such thresholds increases with
rising temperature (medium confidence). For risk assessment, it is important to evaluate the widest possible range of impacts,
including low-probability outcomes with large consequences. {1.5, 2.3, 2.4, 3.3, Box Introduction.1, Box 2.3, Box 2.4}

A large fraction of species faces increased extinction risk due to climate change during and beyond the 21st century, espe-
cially as climate change interacts with other stressors (high confidence). Most plant species cannot naturally shift their
geographical ranges sufficiently fast to keep up with current and high projected rates of climate change in most landscapes;
most small mammals and freshwater molluscs will not be able to keep up at the rates projected under RCP4.5 and above
in flat landscapes in this century (high confidence). Future risk is indicated to be high by the observation that natural global
climate change at rates lower than current anthropogenic climate change caused significant ecosystem shifts and species
extinctions during the past millions of years. Marine organisms will face progressively lower oxygen levels and high rates and
magnitudes of ocean acidification (high confidence), with associated risks exacerbated by rising ocean temperature extremes
(medium confidence). Coral reefs and polar ecosystems are highly vulnerable. Coastal systems and low-lying areas are at
risk from sea level rise, which will continue for centuries even if the global mean temperature is stabilized (high confidence).
{2.3, 2.4, Figure 2.5}

Climate change is projected to undermine food security (Figure SPM.9). Due to projected climate change by the mid-21st century
and beyond, global marine species redistribution and marine biodiversity reduction in sensitive regions will challenge the sustained
provision of fisheries productivity and other ecosystem services (high confidence). For wheat, rice and maize in tropical and temper-
ate regions, climate change without adaptation is projected to negatively impact production for local temperature increases
of 2°C or more above late 20th century levels, although individual locations may benefit (medium confidence). Global tem-
perature increases of ~4°C or more13 above late 20th century levels, combined with increasing food demand, would pose
large risks to food security globally (high confidence). Climate change is projected to reduce renewable surface water and
groundwater resources in most dry subtropical regions (robust evidence, high agreement), intensifying competition for water
among sectors (limited evidence, medium agreement). {2.3.1, 2.3.2}

13 Projected warming averaged over land is larger than global average warming for all RCP scenarios for the period 2081–2100 relative to 1986–2005.
For regional projections, see Figure SPM.7. {2.2}

Summary for Policymakers

SPM

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tio

Summary for Policymakers
15
SPM

Until mid-century, projected climate change will impact human health mainly by exacerbating health problems that already
exist (very high confidence). Throughout the 21st century, climate change is expected to lead to increases in ill-health in many
regions and especially in developing countries with low income, as compared to a baseline without climate change (high
confidence). By 2100 for RCP8.5, the combination of high temperature and humidity in some areas for parts of the year is
expected to compromise common human activities, including growing food and working outdoors (high confidence). {2.3.2}

In urban areas climate change is projected to increase risks for people, assets, economies and ecosystems, including risks
from heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scar-
city, sea level rise and storm surges (very high confidence). These risks are amplified for those lacking essential infrastructure
and services or living in exposed areas. {2.3.2}

Climate change poses risks for food production

Change in maximum catch potential (2051–2060 compared to 2001–2010, SRES A1B)

Pe
rc

en
ta

ge
o

f y
ie

ld
p

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je

ct
io

ns
–21 to –50%<–50% –6 to –20% –1 to –5% no data 0 to 4% 5 to 19% 20 to 49% 50 to 100% >100 %

(a)
(b)

0 to –5%
–5 to –10%

–10 to –25%

–25 to –50%

–50 to –100%

0 to 5%

5 to 10%

10 to 25%

25 to 50%

50 to 100%

Range of yield change

0
20
40
60
80
100

increase
in yield

decrease
in yield

2010–2029 2030–2049 2050–2069 2070–2089 2090–2109

Figure SPM.9 | (a) Projected global redistribution of maximum catch potential of ~1000 exploited marine fish and invertebrate species. Projections
compare the 10-year averages 2001–2010 and 2051–2060 using ocean conditions based on a single climate model under a moderate to high warming
scenario, without analysis of potential impacts of overfishing or ocean acidification. (b) Summary of projected changes in crop yields (mostly wheat, maize,
rice and soy), due to climate change over the 21st century. Data for each timeframe sum to 100%, indicating the percentage of projections showing yield
increases versus decreases. The figure includes projections (based on 1090 data points) for different emission scenarios, for tropical and temperate regions
and for adaptation and no-adaptation cases combined. Changes in crop yields are relative to late 20th century levels. {Figure 2.6a, Figure 2.7}

Summary for Policymakers

SPM

Rural areas are expected to experience major impacts on water availability and supply, food security, infrastructure and
agricultural incomes, including shifts in the production areas of food and non-food crops around the world (high confidence).
{2.3.2}

Aggregate economic losses accelerate with increasing temperature (limited evidence, high agreement), but global economic
impacts from climate change are currently difficult to estimate. From a poverty perspective, climate change impacts are
projected to slow down economic growth, make poverty reduction more difficult, further erode food security and prolong
existing and create new poverty traps, the latter particularly in urban areas and emerging hotspots of hunger (medium confi-
dence). International dimensions such as trade and relations among states are also important for understanding the risks of
climate change at regional scales. {2.3.2}

Climate change is projected to increase displacement of people (medium evidence, high agreement). Populations that lack
the resources for planned migration experience higher exposure to extreme weather events, particularly in developing coun-
tries with low income. Climate change can indirectly increase risks of violent conflicts by amplifying well-documented drivers
of these conflicts such as poverty and economic shocks (medium confidence). {2.3.2}

SPM 2.4 Climate change beyond 2100, irreversibility and abrupt changes

Many aspects of climate change and associated impacts will continue for centuries, even if
anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible
changes increase as the magnitude of the warming increases. {2.4}

Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately
constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions. A large frac-
tion of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale,
except in the case of a large net removal of CO2 from the atmosphere over a sustained period. {2.4, Figure 2.8}

Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting
biomes, soil carbon, ice sheets, ocean temperatures and associated sea level rise all have their own intrinsic long timescales
which will result in changes lasting hundreds to thousands of years after global surface temperature is stabilized. {2.1, 2.4}

There is high confidence that ocean acidification will increase for centuries if CO2 emissions continue, and will strongly affect
marine ecosystems. {2.4}

It is virtually certain that global mean sea level rise will continue for many centuries beyond 2100, with the amount of rise
dependent on future emissions. The threshold for the loss of the Greenland ice sheet over a millennium or more, and an asso-
ciated sea level rise of up to 7 m, is greater than about 1°C (low confidence) but less than about 4°C (medium confidence)
of global warming with respect to pre-industrial temperatures. Abrupt and irreversible ice loss from the Antarctic ice sheet is
possible, but current evidence and understanding is insufficient to make a quantitative assessment. {2.4}

Magnitudes and rates of climate change associated with medium- to high-emission scenarios pose an increased risk of
abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater
ecosystems, including wetlands (medium confidence). A reduction in permafrost extent is virtually certain with continued rise
in global temperatures. {2.4}

Summary for Policymakers

SPM

SPM 3. Future Pathways for Adaptation, Mitigation and Sustainable Development

Adaptation and mitigation are complementary strategies for reducing and managing the risks
of climate change. Substantial emissions reductions over the next few decades can reduce cli-
mate risks in the 21st century and beyond, increase prospects for effective adaptation, reduce
the costs and challenges of mitigation in the longer term and contribute to climate-resilient
pathways for sustainable development. {3.2, 3.3, 3.4}

SPM 3.1 Foundations of decision-making about climate change

Effective decision-making to limit climate change and its effects can be informed by a wide
range of analytical approaches for evaluating expected risks and benefits, recognizing the
importance of governance, ethical dimensions, equity, value judgments, economic assess-
ments and diverse perceptions and responses to risk and uncertainty. {3.1}

Sustainable development and equity provide a basis for assessing climate policies. Limiting the effects of climate change is
necessary to achieve sustainable development and equity, including poverty eradication. Countries’ past and future contri-
butions to the accumulation of GHGs in the atmosphere are different, and countries also face varying challenges and circum-
stances and have different capacities to address mitigation and adaptation. Mitigation and adaptation raise issues of equity,
justice and fairness. Many of those most vulnerable to climate change have contributed and contribute little to GHG emis-
sions. Delaying mitigation shifts burdens from the present to the future, and insufficient adaptation responses to emerging
impacts are already eroding the basis for sustainable development. Comprehensive strategies in response to climate change
that are consistent with sustainable development take into account the co-benefits, adverse side effects and risks that may
arise from both adaptation and mitigation options. {3.1, 3.5, Box 3.4}

The design of climate policy is influenced by how individuals and organizations perceive risks and uncertainties and take
them into account. Methods of valuation from economic, social and ethical analysis are available to assist decision-making.
These methods can take account of a wide range of possible impacts, including low-probability outcomes with large conse-
quences. But they cannot identify a single best balance between mitigation, adaptation and residual climate impacts. {3.1}

Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate
over time and mix globally, and emissions by any agent (e.g., individual, community, company, country) affect other agents.
Effective mitigation will not be achieved if individual agents advance their own interests independently. Cooperative responses,
including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate
change issues. The effectiveness of adaptation can be enhanced through complementary actions across levels, including
international cooperation. The evidence suggests that outcomes seen as equitable can lead to more effective cooperation.
{3.1}

SPM 3.2 Climate change risks reduced by mitigation and adaptation

Without additional mitigation efforts beyond those in place today, and even with adaptation,
warming by the end of the 21st century will lead to high to very high risk of severe, wide-
spread and irreversible impacts globally (high confidence). Mitigation involves some level
of co-benefits and of risks due to adverse side effects, but these risks do not involve the
same possibility of severe, widespread and irreversible impacts as risks from climate change,
increasing the benefits from near-term mitigation efforts. {3.2, 3.4}

Mitigation and adaptation are complementary approaches for reducing risks of climate change impacts over different time-
scales (high confidence). Mitigation, in the near term and through the century, can substantially reduce climate change

Summary for Policymakers
18
SPM

impacts in the latter decades of the 21st century and beyond. Benefits from adaptation can already be realized in addressing
current risks, and can be realized in the future for addressing emerging risks. {3.2, 4.5}

Five Reasons For Concern (RFCs) aggregate climate change risks and illustrate the implications of warming and of adaptation
limits for people, economies and ecosystems across sectors and regions. The five RFCs are associated with: (1) Unique and
threatened systems, (2) Extreme weather events, (3) Distribution of impacts, (4) Global aggregate impacts, and (5) Large-
scale singular events. In this report, the RFCs provide information relevant to Article 2 of UNFCCC. {Box 2.4}

Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the
21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally (high confidence)
(Figure SPM.10). In most scenarios without additional mitigation efforts (those with 2100 atmospheric concentrations

Un
iqu

e &
th

rea
ten

ed
sy

ste
ms

Gl
ob

al
ag

gr
eg

at
e i

mp
ac

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rg

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le
sin

gu
lar

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en

Ex
tre

me
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ea
the

r e
ve

nts

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trib

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on

of
im

pa
cts

−100

−50

0
50
100

C
ha

ng
e

in
a

nn
ua

l G
H

G
e
m
is
si
on

s
in

2
05

0
(%

r
el

at
iv

e
to

2
01

0
le

ve
ls
)

no change relative to 2010e
m

is
si

on
in

cr
ea

se
em

is
si

n
re

du
ct
io
ns
observed 2000s
1
2
3
4
5

0
Cumulative anthropogenic CO

2
emissions from 1870 (GtCO

2
)

1000 2000 3000 4000 5000 6000 7000 8000

G
lo

ba
l m

ea
n

te
m

pe
ra

tu
re
c
ha
ng
e
(°
C
r
el
at
iv
e
to
p
re
-in
du
st
ria
l l
ev
el
s)

Undetectable

Moderate

High

Very high

Level of additional
risk due to climate

change (see Box 2.4)

(a) Risks from climate change… (b) …depend on cumulative

CO
2

emissions…

baselines
430–480
480–530
530–580
580–720
720–1000
baselines
430–480
480–530
530–580
580–720
720–1000

Figure SPM.10 | The relationship between risks from climate change, temperature change, cumulative carbon dioxide (CO2) emissions and changes in
annual greenhouse gas (GHG) emissions by 2050. Limiting risks across Reasons For Concern (a) would imply a limit for cumulative emissions of CO2 (b)
which would constrain annual GHG emissions over the next few decades (c). Panel a reproduces the five Reasons For Concern {Box 2.4}. Panel b links
temperature changes to cumulative CO2 emissions (in GtCO2) from 1870. They are based on Coupled Model Intercomparison Project Phase 5 (CMIP5)
simulations (pink plume) and on a simple climate model (median climate response in 2100), for the baselines and five mitigation scenario categories (six
ellipses). Details are provided in Figure SPM.5. Panel c shows the relationship between the cumulative CO2 emissions (in GtCO2) of the scenario catego-
ries and their associated change in annual GHG emissions by 2050, expressed in percentage change (in percent GtCO2-eq per year) relative to 2010. The
ellipses correspond to the same scenario categories as in Panel b, and are built with a similar method (see details in Figure SPM.5). {Figure 3.1}

Summary for Policymakers

SPM

>1000 ppm CO2-eq), warming is more likely than not to exceed 4°C above pre-industrial levels by 2100 (Table SPM.1). The
risks associated with temperatures at or above 4°C include substantial species extinction, global and regional food insecurity,
consequential constraints on common human activities and limited potential for adaptation in some cases (high confidence).
Some risks of climate change, such as risks to unique and threatened systems and risks associated with extreme weather events,
are moderate to high at temperatures 1°C to 2°C above pre-industrial levels. {2.3, Figure 2.5, 3.2, 3.4, Box 2.4, Table SPM.1}

Substantial cuts in GHG emissions over the next few decades can substantially reduce risks of climate change by limiting
warming in the second half of the 21st century and beyond. Cumulative emissions of CO2 largely determine global mean
surface warming by the late 21st century and beyond. Limiting risks across RFCs would imply a limit for cumulative emissions
of CO2. Such a limit would require that global net emissions of CO2 eventually decrease to zero and would constrain annual
emissions over the next few decades (Figure SPM.10) (high confidence). But some risks from climate damages are unavoid-
able, even with mitigation and adaptation. {2.2.5, 3.2, 3.4}

Mitigation involves some level of co-benefits and risks, but these risks do not involve the same possibility of severe, wide-
spread and irreversible impacts as risks from climate change. Inertia in the economic and climate system and the possibility
of irreversible impacts from climate change increase the benefits from near-term mitigation efforts (high confidence). Delays
in additional mitigation or constraints on technological options increase the longer-term mitigation costs to hold climate
change risks at a given level (Table SPM.2). {3.2, 3.4}

SPM 3.3 Characteristics of adaptation pathways

Adaptation can reduce the risks of climate change impacts, but there are limits to its effec-
tiveness, especially with greater magnitudes and rates of climate change. Taking a longer-
term perspective, in the context of sustainable development, increases the likelihood that
more immediate adaptation actions will also enhance future options and preparedness. {3.3}

Adaptation can contribute to the well-being of populations, the security of assets and the maintenance of ecosystem goods,
functions and services now and in the future. Adaptation is place- and context-specific (high confidence). A first step towards
adaptation to future climate change is reducing vulnerability and exposure to present climate variability (high confidence).
Integration of adaptation into planning, including policy design, and decision-making can promote synergies with develop-
ment and disaster risk reduction. Building adaptive capacity is crucial for effective selection and implementation of adapta-
tion options (robust evidence, high agreement). {3.3}

Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to
governments (high confidence). National governments can coordinate adaptation efforts of local and sub-national govern-
ments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information,
policy and legal frameworks and financial support (robust evidence, high agreement). Local government and the private
sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of commu-
nities, households and civil society and in managing risk information and financing (medium evidence, high agreement). {3.3}

Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk
perceptions (high confidence). Recognition of diverse interests, circumstances, social-cultural contexts and expectations can
benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous
peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have
not been used consistently in existing adaptation efforts. Integrating such forms of knowledge with existing practices increases
the effectiveness of adaptation. {3.3}

Constraints can interact to impede adaptation planning and implementation (high confidence). Common constraints on
implementation arise from the following: limited financial and human resources; limited integration or coordination of gov-
ernance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adapta-
tion leaders and advocates; and limited tools to monitor adaptation effectiveness. Another constraint includes insufficient
research, monitoring, and observation and the finance to maintain them. {3.3}

Summary for Policymakers
20
SPM

Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits (high confidence).
Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints.
Further, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate conse-
quences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulner-
ability of other people, places or sectors (medium evidence, high agreement). Underestimating the complexity of adaptation
as a social process can create unrealistic expectations about achieving intended adaptation outcomes. {3.3}

Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adap-
tation responses; interactions occur both within and across regions (very high confidence). Increasing efforts to mitigate and
adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water,
energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of
actions with co-benefits include (i) improved energy efficiency and cleaner energy sources, leading to reduced emissions of
health-damaging, climate-altering air pollutants; (ii) reduced energy and water consumption in urban areas through greening
cities and recycling water; (iii) sustainable agriculture and forestry; and (iv) protection of ecosystems for carbon storage and
other ecosystem services. {3.3}

Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote
sustainable development (high confidence). At the national level, transformation is considered most effective when it reflects
a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances
and priorities. Restricting adaptation responses to incremental changes to existing systems and structures, without consider-
ing transformational change, may increase costs and losses and miss opportunities. Planning and implementation of trans-
formational adaptation could reflect strengthened, altered or aligned paradigms and may place new and increased demands
on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical
implications. Adaptation pathways are enhanced by iterative learning, deliberative processes and innovation. {3.3}

SPM 3.4 Characteristics of mitigation pathways

There are multiple mitigation pathways that are likely to limit warming to below 2°C relative
to pre-industrial levels. These pathways would require substantial emissions reductions over
the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases
by the end of the century. Implementing such reductions poses substantial technological, eco-
nomic, social and institutional challenges, which increase with delays in additional mitigation
and if key technologies are not available. Limiting warming to lower or higher levels involves
similar challenges but on different timescales. {3.4}

Without additional efforts to reduce GHG emissions beyond those in place today, global emissions growth is expected to
persist, driven by growth in global population and economic activities. Global mean surface temperature increases in 2

100

in baseline scenarios—those without additional mitigation—range from 3.7°C to 4.8°C above the average for 1850–1900
for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty (5th to 95th percentile
range) (high confidence). {3.4}14

Emissions scenarios leading to CO2-equivalent concentrations in 2100 of about 450 ppm or lower are likely to maintain
warming below 2°C over the 21st century relative to pre-industrial levels15. These scenarios are characterized by 40 to 70%
global anthropogenic GHG emissions reductions by 2050 compared to 201016, and emissions levels near zero or below in
2100. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit
temperature change to less than 2°C, unless they temporarily overshoot concentration levels of roughly 530 ppm CO2-eq

15 For comparison, the CO2-eq concentration in 2011 is estimated to be 430 ppm (uncertainty range 340 to 520 ppm)
16 This range differs from the range provided for a similar concentration category in the AR4 (50 to 85% lower than 2000 for CO2 only). Reasons for this

difference include that this report has assessed a substantially larger number of scenarios than in the AR4 and looks at all GHGs. In addition, a large
proportion of the new scenarios include Carbon Dioxide Removal (CDR) technologies (see below). Other factors include the use of 2100 concentration
levels instead of stabilization levels and the shift in reference year from 2000 to 2010.

Summary for Policymakers
21
SPM

before 2100, in which case they are about as likely as not to achieve that goal. In these 500 ppm CO2-eq scenarios, global 20

emissions levels are 25 to 55% lower than in 2010. Scenarios with higher emissions in 2050 are characterized by a greater
reliance on Carbon Dioxide Removal (CDR) technologies beyond mid-century (and vice versa). Trajectories that are likely to
limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. A lim-
ited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are
characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70% and 95% below
2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and
their likelihood to keep warming to below a range of temperature levels, see Figure SPM.11 and Table SPM.1. {3.4}

21002000 2020 2040 2060 2080 2100
–

0
20

100

120

140

B
as

el
in

e
RCP8.5

RCP6.0

RCP4.5

RCP2.6

Associated upscaling of low-carbon energy supply

0
20
40
60
80
100

2030 2050 2100 2030 2050 2100 2030 2050 2100 2030 2050 2100

Lo
w

-c
ar

bo
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s

ha
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o
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(%
)

A
nn

ua
l G

H
G

e
m
is
si

on
s

(G
tC

O
2-

eq
/y

r)
Year

>1000
720–1000
580–720
530–580
480–530
430–480
Full AR5 database range

ppm CO

2
-eq

ppm CO
2
-eq
ppm CO
2
-eq
ppm CO
2
-eq
ppm CO
2
-eq
ppm CO
2
-eq

GHG emission pathways 2000–2100: All AR5 scenarios

90th

Percentile

Median

10th Percentile

Min

75th
Max

Median
25th

Percentile

+
18

0%
+
18
5%

+
95

+
13

5%
+
13
5%

+
14

5%
2010

580–720 ppm CO
2
-eq 530–580 ppm CO

2
-eq 480–530 ppm CO

2
-eq 430–480 ppm CO

2
-eq
(a)
(b)

Figure SPM.11 | Global greenhouse gas emissions (gigatonne of CO2-equivalent per year, GtCO2-eq/yr) in baseline and mitigation scenarios for different
long-term concentration levels (a) and associated upscaling requirements of low-carbon energy (% of primary energy) for 2030, 2050 and 2100 compared
to 2010 levels in mitigation scenarios (b). {Figure 3.2}

Summary for Policymakers

SPM

Table SPM.1 | Key characteristics of the scenarios collected and assessed for WGIII AR5. For all parameters the 10th to 90th percentile of the scenarios
is shown a. {Table 3.1}

CO2-eq Con-
centrations in

2100
(ppm CO2-eq) f

Category label
(conc. range)

Subcategories

Relative
position
of the
RCPs d

Change in CO2-eq
emissions compared

to 2010 (in %) c

Likelihood of staying below a specific
temperature level over the 21st cen-

tury (relative to 1850–1900) d, e

2050 2100 1.5ºC 2ºC 3ºC 4ºC

<430 Only a limited number of individual model studies have explored levels below 430 ppm CO2-eq j

450
(430 to 480)

Total range a, g RCP2.6 –72 to –41 –118 to –78
More unlikely

than likely

Likely

Likely
Likely

500
(480 to 530)

No overshoot of
530 ppm CO2-eq

–57 to –42 –107 to –73

Unlikely

More likely
than not

Overshoot of 5

ppm CO2-eq

–55 to –25 –114 to –90
About as

likely as not

550
(530 to 580)

No overshoot of
580 ppm CO2-eq

–47 to –19 –81 to –59

More unlikely
than likely iOvershoot of 580

ppm CO2-eq
–16 to 7 –183 to –86

(580 to 650) Total range

RCP4.5

–38 to 24 –134 to –50

(650 to 720) Total range –11 to 17 –54 to –21
Unlikely

More likely
than not

(720 to 1000) b Total range RCP6.0 18 to 54 –7 to 72

Unlikely h

More unlikely
than likely

>1000 b Total range RCP8.5 52 to 95 74 to 178 Unlikely h Unlikely
More unlikely

than likely

Notes:
a The ‘total range’ for the 430 to 480 ppm CO2-eq concentrations scenarios corresponds to the range of the 10th to 90th percentile of the subcategory of
these scenarios shown in Table 6.3 of the Working Group III Report.
b Baseline scenarios fall into the >1000 and 720 to 1000 ppm CO2-eq categories. The latter category also includes mitigation scenarios. The baseline sce-
narios in the latter category reach a temperature change of 2.5°C to 5.8°C above the average for 1850–1900 in 2100. Together with the baseline scenarios
in the >1000 ppm CO2-eq category, this leads to an overall 2100 temperature range of 2.5°C to 7.8°C (range based on median climate response: 3.7°C
to 4.8°C) for baseline scenarios across both concentration categories.
c The global 2010 emissions are 31% above the 1990 emissions (consistent with the historic greenhouse gas emission estimates presented in this report).
CO2-eq emissions include the basket of Kyoto gases (carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) as well as fluorinated gases).
d The assessment here involves a large number of scenarios published in the scientific literature and is thus not limited to the Representative Concentration
Pathways (RCPs). To evaluate the CO2-eq concentration and climate implications of these scenarios, the Model for the Assessment of Greenhouse Gas
Induced Climate Change (MAGICC) was used in a probabilistic mode. For a comparison between MAGICC model results and the outcomes of the models
used in WGI, see WGI 12.4.1.2, 12.4.8 and WGIII 6.3.2.6.
e The assessment in this table is based on the probabilities calculated for the full ensemble of scenarios in WGIII AR5 using MAGICC and the assessment in
WGI of the uncertainty of the temperature projections not covered by climate models. The statements are therefore consistent with the statements in WGI,
which are based on the Coupled Model Intercomparison Project Phase 5 (CMIP5) runs of the RCPs and the assessed uncertainties. Hence, the likelihood
statements reflect different lines of evidence from both WGs. This WGI method was also applied for scenarios with intermediate concentration levels where
no CMIP5 runs are available. The likelihood statements are indicative only {WGIII 6.3} and follow broadly the terms used by the WGI SPM for temperature
projections: likely 66–100%, more likely than not >50–100%, about as likely as not 33–66%, and unlikely 0–33%. In addition the term more unlikely
than likely 0–<50% is used. f The CO2-equivalent concentration (see Glossary) is calculated on the basis of the total forcing from a simple carbon cycle/climate model, MAGICC. The CO2- equivalent concentration in 2011 is estimated to be 430 ppm (uncertainty range 340 to 520 ppm). This is based on the assessment of total anthropogenic radiative forcing for 2011 relative to 1750 in WGI, i.e., 2.3 W/m2, uncertainty range 1.1 to 3.3 W/m2. g The vast majority of scenarios in this category overshoot the category boundary of 480 ppm CO2-eq concentration. h For scenarios in this category, no CMIP5 run or MAGICC realization stays below the respective temperature level. Still, an unlikely assignment is given to reflect uncertainties that may not be reflected by the current climate models. i Scenarios in the 580 to 650 ppm CO2-eq category include both overshoot scenarios and scenarios that do not exceed the concentration level at the high end of the category (e.g., RCP4.5). The latter type of scenarios, in general, have an assessed probability of more unlikely than likely to stay below the 2°C temperature level, while the former are mostly assessed to have an unlikely probability of staying below this level. j In these scenarios, global CO2-eq emissions in 2050 are between 70 to 95% below 2010 emissions, and they are between 110 to 120% below 2010 emissions in 2100.

Summary for Policymakers

SPM

Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 (consistent with a likely chance to keep warming below 2°C
relative to pre-industrial levels) typically involve temporary overshoot17 of atmospheric concentrations, as do many scenarios
reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq in 2100 (Table SPM.1). Depending on the level of overshoot,
overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture
and storage (BECCS) and afforestation in the second half of the century. The availability and scale of these and other CDR
technologies and methods are uncertain and CDR technologies are, to varying degrees, associated with challenges and
risks18. CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where
mitigation is more expensive (high confidence). {3.4, Box 3.3}

Reducing emissions of non-CO2 agents can be an important element of mitigation strategies. All current GHG emissions
and other forcing agents affect the rate and magnitude of climate change over the next few decades, although long-term
warming is mainly driven by CO2 emissions. Emissions of non-CO2 forcers are often expressed as ‘CO2-equivalent emissions’,
but the choice of metric to calculate these emissions, and the implications for the emphasis and timing of abatement of the
various climate forcers, depends on application and policy context and contains value judgments. {3.4, Box 3.2}

17 In concentration ‘overshoot’ scenarios, concentrations peak during the century and then decline.
18 CDR methods have biogeochemical and technological limitations to their potential on the global scale. There is insufficient knowledge to quantify how

much CO2 emissions could be partially offset by CDR on a century timescale. CDR methods may carry side effects and long-term consequences on a
global scale.

Before 2030 After 2030

–12

–9

–6

–3

0
3
6

Past 1900–2010

2000–2010

AR5 scenario range

Interquartile range and median
of model comparisons with
2030 targets

Cancún
Pledges

<50 GtCO 2 -eq

Annual GHG
emissions in 2030

>55 GtCO
2
-eq

Future 2030–2050

+
90

0%
2010
(G
tC
O
2-
eq
/y
r)
Year

(%
/y

r)
(%
)

Annual GHG emissions Rate of CO
2
emissions change Share of zero and low-carbon energy

2005 2010 2015 2020 2025 2030

2030 2030 2050 2100 2100 2050

20
40
60
80
100

Figure SPM.12 | The implications of different 2030 greenhouse gas (GHG) emissions levels for the rate of carbon dioxide (CO2) emissions reductions
and low-carbon energy upscaling in mitigation scenarios that are at least about as likely as not to keep warming throughout the 21st century below 2°C
relative to pre-industrial levels (2100 CO2-equivalent concentrations of 430 to 530 ppm). The scenarios are grouped according to different emissions levels
by 2030 (coloured in different shades of green). The left panel shows the pathways of GHG emissions (gigatonne of CO2-equivalent per year, GtCO2-eq/
yr) leading to these 2030 levels. The black dot with whiskers gives historic GHG emission levels and associated uncertainties in 2010 as reported in Figure
SPM.2. The black bar shows the estimated uncertainty range of GHG emissions implied by the Cancún Pledges. The middle panel denotes the average
annual CO2 emissions reduction rates for the period 2030–2050. It compares the median and interquartile range across scenarios from recent inter-model
comparisons with explicit 2030 interim goals to the range of scenarios in the Scenario Database for WGIII AR5. Annual rates of historical emissions change
(sustained over a period of 20 years) and the average annual CO2 emission change between 2000 and 2010 are shown as well. The arrows in the right
panel show the magnitude of zero and low-carbon energy supply upscaling from 2030 to 2050 subject to different 2030 GHG emissions levels. Zero- and
low-carbon energy supply includes renewables, nuclear energy and fossil energy with carbon dioxide capture and storage (CCS) or bioenergy with CCS
(BECCS). [Note: Only scenarios that apply the full, unconstrained mitigation technology portfolio of the underlying models (default technology assumption)
are shown. Scenarios with large net negative global emissions (>20 GtCO2-eq/yr), scenarios with exogenous carbon price assumptions and scenarios with
2010 emissions significantly outside the historical range are excluded.] {Figure 3.3}

Summary for Policymakers
24
SPM

Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the
21st century to below 2°C relative to pre-industrial levels. It will require substantially higher rates of emissions reductions
from 2030 to 2050; a much more rapid scale-up of low-carbon energy over this period; a larger reliance on CDR in the long
term; and higher transitional and long-term economic impacts. Estimated global emissions levels in 2020 based on the
Cancún Pledges are not consistent with cost-effective mitigation trajectories that are at least about as likely as not to limit
warming to below 2°C relative to pre-industrial levels, but they do not preclude the option to meet this goal (high confidence)
(Figure SPM.12, Table SPM.2). {3.4}

Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions, but
increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in
which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective
benchmark for estimating macro-economic mitigation costs (Figure SPM.13). Under these assumptions mitigation scenarios
that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global
consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitiga-
tion—of 1 to 4% (median: 1.7%) in 2030, 2 to 6% (median: 3.4%) in 2050 and 3 to 11% (median: 4.8%) in 2100 relative to
consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century (Figure SPM.13).
These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 (median: 0.06) percentage
points over the century relative to annualized consumption growth in the baseline that is between 1.6 and 3% per year (high
confidence). {3.4}

In the absence or under limited availability of mitigation technologies (such as bioenergy, CCS and their combination BECCS,
nuclear, wind/solar), mitigation costs can increase substantially depending on the technology considered. Delaying additional
mitigation increases mitigation costs in the medium to long term. Many models could not limit likely warming to below 2°C
over the 21st century relative to pre-industrial levels if additional mitigation is considerably delayed. Many models could
not limit likely warming to below 2°C if bioenergy, CCS and their combination (BECCS) are limited (high confidence)
(Table SPM.2). {3.4}

0
2
4
6
8
10
12
0
200
400
600
800
1000
Re
du
ct
io

n
in

c
on
su
m

pt
io

n
re

la
tiv

e
to

b
as

el
in

e
(%

)

Global mitigation costs and consumption growth in baseline scenarios
C

on
su

m
pt

io
n

in
c

or
re

sp
on

di
ng

b
as
el
in

e
sc

en
ar

io
s

(%
in

cr
ea

se
fr

om
2

01
0)

CO
2
-eq concentrations in 2100 (ppm CO

2
-eq)
450 (430–480)

0.06
(0.04 to 0.14)

500 (480–530)

0.06
(0.03 to 0.13)

550 (530–580)

0.04
(0.01 to 0.09)

580–650

0.03
(0.01 to 0.05)

Percentage point reduction in annualized consumption growth rate over 21st century (%-point)

84th Percentile

Median

16th Percentile

Corresponding
baseline scenarios

20
30

20
50

20
30

20
50

21
00

Figure SPM.13 | Global mitigation costs in cost-effective scenarios at different atmospheric concentrations levels in 2100. Cost-effective scenarios
assume immediate mitigation in all countries and a single global carbon price, and impose no additional limitations on technology relative to the models’
default technology assumptions. Consumption losses are shown relative to a baseline development without climate policy (left panel). The table at the top
shows percentage points of annualized consumption growth reductions relative to consumption growth in the baseline of 1.6 to 3% per year (e.g., if the
reduction is 0.06 percentage points per year due to mitigation, and baseline growth is 2.0% per year, then the growth rate with mitigation would be 1.94%
per year). Cost estimates shown in this table do not consider the benefits of reduced climate change or co-benefits and adverse side effects of mitigation.
Estimates at the high end of these cost ranges are from models that are relatively inflexible to achieve the deep emissions reductions required in the long
run to meet these goals and/or include assumptions about market imperfections that would raise costs. {Figure 3.4}

Summary for Policymakers
25
SPM

Mitigation scenarios reaching about 450 or 500 ppm CO2-eq by 2100 show reduced costs for achieving air quality and energy
security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience
of the energy system. {4.4.2.2}

Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions
and fuels exist (high confidence). Most mitigation scenarios are associated with reduced revenues from coal and oil trade for
major exporters (high confidence). The availability of CCS would reduce the adverse effects of mitigation on the value of fossil
fuel assets (medium confidence). {4.4.2.2}

Solar Radiation Management (SRM) involves large-scale methods that seek to reduce the amount of absorbed solar energy
in the climate system. SRM is untested and is not included in any of the mitigation scenarios. If it were deployed, SRM would

Table SPM.2 | Increase in global mitigation costs due to either limited availability of specific technologies or delays in additional mitigation a relative to
cost-effective scenarios b. The increase in costs is given for the median estimate and the 16th to 84th percentile range of the scenarios (in parentheses) c. In
addition, the sample size of each scenario set is provided in the coloured symbols. The colours of the symbols indicate the fraction of models from systematic
model comparison exercises that could successfully reach the targeted concentration level. {Table 3.2}

Mitigation cost increases in scenarios with
limited availability of technologies d

[% increase in total discounted e mitigation costs
(2015–2100) relative to default technology assumptions]

Mitigation cost increases
due to delayed additional

mitigation until 2030

[% increase in mitigation costs
relative to immediate mitigation]

2100
concentrations
(ppm CO2-eq)

no CCS nuclear phase out limited solar/wind limited bioenergy
medium term costs

(2030–2050)

long term
costs

(2050–2100)

450
(430 to 480)

138%
(29 to 297%)

7%
(4 to 18%)

6%
(2 to 29%)

64%
(44 to 78%)

}
44%

(2 to 78%)
37%

(16 to 82%)
500

(480 to 530)
not available

(n.a.)
n.a. n.a. n.a.

550
(530 to 580)

39%
(18 to 78%)

13%
(2 to 23%)

8%
(5 to 15%)

18%
(4 to 66%)

}
15%

(3 to 32%)
16%

(5 to 24%)

580 to 650 n.a. n.a. n.a. n.a.

Symbol legend—fraction of models successful in producing scenarios (numbers indicate the number of successful models)

: all models successful

: between 80 and 100% of models successful

: between 50 and 80% of models successful

: less than 50% of models successful

Notes:
a Delayed mitigation scenarios are associated with greenhouse gas emission of more than 55 GtCO2-eq in 2030, and the increase in mitigation costs is mea-
sured relative to cost-effective mitigation scenarios for the same long-term concentration level.
b Cost-effective scenarios assume immediate mitigation in all countries and a single global carbon price, and impose no additional limitations on technology
relative to the models’ default technology assumptions.
c The range is determined by the central scenarios encompassing the 16th to 84th percentile range of the scenario set. Only scenarios with a time horizon
until 2100 are included. Some models that are included in the cost ranges for concentration levels above 530 ppm CO2-eq in 2100 could not produce associ-
ated scenarios for concentration levels below 530 ppm CO2-eq in 2100 with assumptions about limited availability of technologies and/or delayed additional
mitigation.
d No CCS: carbon dioxide capture and storage is not included in these scenarios. Nuclear phase out: no addition of nuclear power plants beyond those under
construction, and operation of existing plants until the end of their lifetime. Limited Solar/Wind: a maximum of 20% global electricity generation from solar
and wind power in any year of these scenarios. Limited Bioenergy: a maximum of 100 EJ/yr modern bioenergy supply globally (modern bioenergy used for
heat, power, combinations and industry was around 18 EJ/yr in 2008). EJ = Exajoule = 1018 Joule.
e Percentage increase of net present value of consumption losses in percent of baseline consumption (for scenarios from general equilibrium models) and
abatement costs in percent of baseline gross domestic product (GDP, for scenarios from partial equilibrium models) for the period 2015–2100, discounted
at 5% per year.

Summary for Policymakers

SPM

entail numerous uncertainties, side effects, risks and shortcomings and has particular governance and ethical implications.
SRM would not reduce ocean acidification. If it were terminated, there is high confidence that surface temperatures would
rise very rapidly impacting ecosystems susceptible to rapid rates of change. {Box 3.3}

SPM 4. Adaptation and Mitigation

Many adaptation and mitigation options can help address climate change, but no single
option is sufficient by itself. Effective implementation depends on policies and cooperation at
all scales and can be enhanced through integrated responses that link adaptation and mitiga-
tion with other societal objectives. {4}

SPM 4.1 Common enabling factors and constraints for adaptation and mitigation responses

Adaptation and mitigation responses are underpinned by common enabling factors. These
include effective institutions and governance, innovation and investments in environmentally
sound technologies and infrastructure, sustainable livelihoods and behavioural and lifestyle
choices. {4.1}

Inertia in many aspects of the socio-economic system constrains adaptation and mitigation options (medium evidence, high
agreement). Innovation and investments in environmentally sound infrastructure and technologies can reduce GHG emis-
sions and enhance resilience to climate change (very high confidence). {4.1}

Vulnerability to climate change, GHG emissions and the capacity for adaptation and mitigation are strongly influenced by
livelihoods, lifestyles, behaviour and culture (medium evidence, medium agreement). Also, the social acceptability and/or
effectiveness of climate policies are influenced by the extent to which they incentivize or depend on regionally appropriate
changes in lifestyles or behaviours. {4.1}

For many regions and sectors, enhanced capacities to mitigate and adapt are part of the foundation essential for managing
climate change risks (high confidence). Improving institutions as well as coordination and cooperation in governance can help
overcome regional constraints associated with mitigation, adaptation and disaster risk reduction (very high confidence). {4.1}

SPM 4.2 Response options for adaptation

Adaptation options exist in all sectors, but their context for implementation and potential to
reduce climate-related risks differs across sectors and regions. Some adaptation responses
involve significant co-benefits, synergies and trade-offs. Increasing climate change will
increase challenges for many adaptation options. {4.2}

Adaptation experience is accumulating across regions in the public and private sectors and within communities. There is
increasing recognition of the value of social (including local and indigenous), institutional, and ecosystem-based measures
and of the extent of constraints to adaptation. Adaptation is becoming embedded in some planning processes, with more
limited implementation of responses (high confidence). {1.6, 4.2, 4.4.2.1}

The need for adaptation along with associated challenges is expected to increase with climate change (very high confidence).
Adaptation options exist in all sectors and regions, with diverse potential and approaches depending on their context in
vulnerability reduction, disaster risk management or proactive adaptation planning (Table SPM.3). Effective strategies and
actions consider the potential for co-benefits and opportunities within wider strategic goals and development plans. {4.2}

Summary for Policymakers
27
SPM

Table SPM.3 | Approaches for managing the risks of climate change through adaptation. These approaches should be considered overlapping rather than
discrete, and they are often pursued simultaneously. Examples are presented in no specific order and can be relevant to more than one category. {Table 4.2}

V
u

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&
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&
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ju

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m

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ts

Tr
an

sf
o
rm
at
io
n

Overlapping
Approaches

Category Examples

Human
development

Improved access to education, nutrition, health facilities, energy, safe housing & settlement structures,
& social support structures; Reduced gender inequality & marginalization in other forms.

Poverty alleviation
Improved access to & control of local resources; Land tenure; Disaster risk reduction; Social safety nets
& social protection; Insurance schemes.

Livelihood security
Income, asset & livelihood diversification; Improved infrastructure; Access to technology & decision-
making fora; Increased decision-making power; Changed cropping, livestock & aquaculture practices;
Reliance on social networks.

Disaster risk
management

Early warning systems; Hazard & vulnerability mapping; Diversifying water resources; Improved
drainage; Flood & cyclone shelters; Building codes & practices; Storm & wastewater management;
Transport & road infrastructure improvements.

Ecosystem
management

Maintaining wetlands & urban green spaces; Coastal afforestation; Watershed & reservoir
management; Reduction of other stressors on ecosystems & of habitat fragmentation; Maintenance
of genetic diversity; Manipulation of disturbance regimes; Community-based natural resource
management.

Spatial or land-use
planning

Provisioning of adequate housing, infrastructure & services; Managing development in flood prone &
other high risk areas; Urban planning & upgrading programs; Land zoning laws; Easements; Protected
areas.

Structural/physical

Engineered & built-environment options: Sea walls & coastal protection structures; Flood levees;
Water storage; Improved drainage; Flood & cyclone shelters; Building codes & practices; Storm &
wastewater management; Transport & road infrastructure improvements; Floating houses; Power plant
& electricity grid adjustments.

Technological options: New crop & animal varieties; Indigenous, traditional & local knowledge,
technologies & methods; Efficient irrigation; Water-saving technologies; Desalinisation; Conservation
agriculture; Food storage & preservation facilities; Hazard & vulnerability mapping & monitoring; Early
warning systems; Building insulation; Mechanical & passive cooling; Technology development, transfer
& diffusion.

Ecosystem-based options: Ecological restoration; Soil conservation; Afforestation & reforestation;
Mangrove conservation & replanting; Green infrastructure (e.g., shade trees, green roofs); Controlling
overfishing; Fisheries co-management; Assisted species migration & dispersal; Ecological corridors;
Seed banks, gene banks & other ex situ conservation; Community-based natural resource management.

Services: Social safety nets & social protection; Food banks & distribution of food surplus; Municipal
services including water & sanitation; Vaccination programs; Essential public health services; Enhanced
emergency medical services.

Institutional

Economic options: Financial incentives; Insurance; Catastrophe bonds; Payments for ecosystem
services; Pricing water to encourage universal provision and careful use; Microfinance; Disaster
contingency funds; Cash transfers; Public-private partnerships.

Laws & regulations: Land zoning laws; Building standards & practices; Easements; Water regulations
& agreements; Laws to support disaster risk reduction; Laws to encourage insurance purchasing;
Defined property rights & land tenure security; Protected areas; Fishing quotas; Patent pools &
technology transfer.

National & government policies & programs: National & regional adaptation plans including
mainstreaming; Sub-national & local adaptation plans; Economic diversification; Urban upgrading
programs; Municipal water management programs; Disaster planning & preparedness; Integrated
water resource management; Integrated coastal zone management; Ecosystem-based management;
Community-based adaptation.

Social

Educational options: Awareness raising & integrating into education; Gender equity in education;
Extension services; Sharing indigenous, traditional & local knowledge; Participatory action research &
social learning; Knowledge-sharing & learning platforms.

Informational options: Hazard & vulnerability mapping; Early warning & response systems;
Systematic monitoring & remote sensing; Climate services; Use of indigenous climate observations;
Participatory scenario development; Integrated assessments.

Behavioural options: Household preparation & evacuation planning; Migration; Soil & water
conservation; Storm drain clearance; Livelihood diversification; Changed cropping, livestock &
aquaculture practices; Reliance on social networks.

Spheres of change

Practical: Social & technical innovations, behavioural shifts, or institutional & managerial changes that
produce substantial shifts in outcomes.

Political: Political, social, cultural & ecological decisions & actions consistent with reducing
vulnerability & risk & supporting adaptation, mitigation & sustainable development.

Personal: Individual & collective assumptions, beliefs, values & worldviews influencing climate-change
responses.

Summary for Policymakers

SPM

SPM 4.3 Response options for mitigation

Mitigation options are available in every major sector. Mitigation can be more cost-effective
if using an integrated approach that combines measures to reduce energy use and the green-
house gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions and
enhance carbon sinks in land-based sectors. {4.3}

Well-designed systemic and cross-sectoral mitigation strategies are more cost-effective in cutting emissions than a focus
on individual technologies and sectors, with efforts in one sector affecting the need for mitigation in others (medium confi-
dence). Mitigation measures intersect with other societal goals, creating the possibility of co-benefits or adverse side effects.
These intersections, if well-managed, can strengthen the basis for undertaking climate action. {4.3}

Emissions ranges for baseline scenarios and mitigation scenarios that limit CO2-equivalent concentrations to low levels
(about 450 ppm CO2-eq, likely to limit warming to 2°C above pre-industrial levels) are shown for different sectors and gases
in Figure SPM.14. Key measures to achieve such mitigation goals include decarbonizing (i.e., reducing the carbon intensity of)
electricity generation (medium evidence, high agreement) as well as efficiency enhancements and behavioural changes, in
order to reduce energy demand compared to baseline scenarios without compromising development (robust evidence, high
agreement). In scenarios reaching 450 ppm CO2-eq concentrations by 2100, global CO2 emissions from the energy supply
sector are projected to decline over the next decade and are characterized by reductions of 90% or more below 2010 levels
between 2040 and 2070. In the majority of low-concentration stabilization scenarios (about 450 to about 500 ppm CO2-eq,
at least about as likely as not to limit warming to 2°C above pre-industrial levels), the share of low-carbon electricity supply
(comprising renewable energy (RE), nuclear and carbon dioxide capture and storage (CCS) including bioenergy with carbon
dioxide capture and storage (BECCS)) increases from the current share of approximately 30% to more than 80% by 2050,
and fossil fuel power generation without CCS is phased out almost entirely by 2100. {4.3}

CO
2

min

75th
max

25th

Percentile

Baselines

Scenarios

430–480 ppm CO
2
-eq median

30
20

–20

–10

0
40
50

D
ire

ct
e

m
is
si
on
s
(G
tC
O
2-
eq

/y
r)

Direct CO
2
emissions by major sectors, and non-CO

2
emissions, for baseline and mitigation scenarios

Transport Buildings Industry Electricity Net AFOLU Non-CO
2

2010
20
30
20
50
21
00

80 GtCO
2
/yr

93
29

78
29

80
22

65
22

80
22
80
22
65
22

147
36

127
36

131
32

118
32

121
36

107
36

Figure SPM.14 | Carbon dioxide (CO2) emissions by sector and total non-CO2 greenhouse gases (Kyoto gases) across sectors in baseline (faded bars) and
mitigation scenarios (solid colour bars) that reach about 450 (430 to 480) ppm CO2-eq concentrations in 2100 (likely to limit warming to 2°C above pre-
industrial levels). Mitigation in the end-use sectors leads also to indirect emissions reductions in the upstream energy supply sector. Direct emissions of the
end-use sectors thus do not include the emission reduction potential at the supply-side due to, for example, reduced electricity demand. The numbers at the
bottom of the graphs refer to the number of scenarios included in the range (upper row: baseline scenarios; lower row: mitigation scenarios), which differs
across sectors and time due to different sectoral resolution and time horizon of models. Emissions ranges for mitigation scenarios include the full portfolio
of mitigation options; many models cannot reach 450 ppm CO2-eq concentration by 2100 in the absence of carbon dioxide capture and storage (CCS).
Negative emissions in the electricity sector are due to the application of bioenergy with carbon dioxide capture and storage (BECCS). ‘Net’ agriculture,
forestry and other land use (AFOLU) emissions consider afforestation, reforestation as well as deforestation activities. {4.3, Figure 4.1}

Summary for Policymakers
29
SPM

Near-term reductions in energy demand are an important element of cost-effective mitigation strategies, provide more
flexibility for reducing carbon intensity in the energy supply sector, hedge against related supply-side risks, avoid lock-in to
carbon-intensive infrastructures, and are associated with important co-benefits. The most cost-effective mitigation options in
forestry are afforestation, sustainable forest management and reducing deforestation, with large differences in their relative
importance across regions; and in agriculture, cropland management, grazing land management and restoration of organic
soils (medium evidence, high agreement). {4.3, Figures 4.1, 4.2, Table 4.3}

Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions, with high mitigation
potential in some sectors, in particular when complementing technological and structural change (medium evidence, medium
agreement). Emissions can be substantially lowered through changes in consumption patterns, adoption of energy savings
measures, dietary change and reduction in food wastes. {4.1, 4.3}

SPM 4.4 Policy approaches for adaptation and mitigation, technology and finance

Effective adaptation and mitigation responses will depend on policies and measures across
multiple scales: international, regional, national and sub-national. Policies across all scales
supporting technology development, diffusion and transfer, as well as finance for responses
to climate change, can complement and enhance the effectiveness of policies that directly
promote adaptation and mitigation. {4.4}

International cooperation is critical for effective mitigation, even though mitigation can also have local co-benefits. Adapta-
tion focuses primarily on local to national scale outcomes, but its effectiveness can be enhanced through coordination across
governance scales, including international cooperation: {3.1, 4.4.1}

• The United Nations Framework Convention on Climate Change (UNFCCC) is the main multilateral forum focused on
addressing climate change, with nearly universal participation. Other institutions organized at different levels of gover-
nance have resulted in diversifying international climate change cooperation. {4.4.1}

• The Kyoto Protocol offers lessons towards achieving the ultimate objective of the UNFCCC, particularly with respect to
participation, implementation, flexibility mechanisms and environmental effectiveness (medium evidence, low agree-
ment). {4.4.1}

• Policy linkages among regional, national and sub-national climate policies offer potential climate change mitigation ben-
efits (medium evidence, medium agreement). Potential advantages include lower mitigation costs, decreased emission
leakage and increased market liquidity. {4.4.1}

• International cooperation for supporting adaptation planning and implementation has received less attention histori-
cally than mitigation but is increasing and has assisted in the creation of adaptation strategies, plans and actions at the
national, sub-national and local level (high confidence). {4.4.1}

There has been a considerable increase in national and sub-national plans and strategies on both adaptation and mitigation
since the AR4, with an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce
adverse side effects (high confidence): {4.4.2.1, 4.4.2.2}

• National governments play key roles in adaptation planning and implementation (robust evidence, high agreement)
through coordinating actions and providing frameworks and support. While local government and the private sector
have different functions, which vary regionally, they are increasingly recognized as critical to progress in adaptation,
given their roles in scaling up adaptation of communities, households and civil society and in managing risk information
and financing (medium evidence, high agreement). {4.4.2.1}

• Institutional dimensions of adaptation governance, including the integration of adaptation into planning and decision-
making, play a key role in promoting the transition from planning to implementation of adaptation (robust evidence,

Summary for Policymakers
30
SPM

high agreement). Examples of institutional approaches to adaptation involving multiple actors include economic options
(e.g., insurance, public-private partnerships), laws and regulations (e.g., land-zoning laws) and national and government
policies and programmes (e.g., economic diversification). {4.2, 4.4.2.1, Table SPM.3}

• In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitiga-
tion in a cost-effective way but have been implemented with diverse effects due in part to national circumstances as
well as policy design. The short-run effects of cap and trade systems have been limited as a result of loose caps or caps
that have not proved to be constraining (limited evidence, medium agreement). In some countries, tax-based policies
specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the
link between GHG emissions and GDP (high confidence). In addition, in a large group of countries, fuel taxes (although
not necessarily designed for the purpose of mitigation) have had effects that are akin to sectoral carbon taxes. {4.4.2.2}

• Regulatory approaches and information measures are widely used and are often environmentally effective (medium evi-
dence, medium agreement). Examples of regulatory approaches include energy efficiency standards; examples of infor-
mation programmes include labelling programmes that can help consumers make better-informed decisions. {4.4.2.2}

• Sector-specific mitigation policies have been more widely used than economy-wide policies (medium evidence, high
agreement). Sector-specific policies may be better suited to address sector-specific barriers or market failures and may be
bundled in packages of complementary policies. Although theoretically more cost-effective, administrative and political
barriers may make economy-wide policies harder to implement. Interactions between or among mitigation policies may
be synergistic or may have no additive effect on reducing emissions. {4.4.2.2}

• Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such
as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of renewable energy (RE)
policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent
years. At the same time, reducing subsidies for GHG-related activities in various sectors can achieve emission reductions,
depending on the social and economic context (high confidence). {4.4.2.2}

Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to
human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable
development. The potential for co-benefits for energy end-use measures outweighs the potential for adverse side effects
whereas the evidence suggests this may not be the case for all energy supply and agriculture, forestry and other land use
(AFOLU) measures. Some mitigation policies raise the prices for some energy services and could hamper the ability of socie-
ties to expand access to modern energy services to underserved populations (low confidence). These potential adverse side
effects on energy access can be avoided with the adoption of complementary policies such as income tax rebates or other
benefit transfer mechanisms (medium confidence). Whether or not side effects materialize, and to what extent side effects
materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementa-
tion. Many co-benefits and adverse side effects have not been well-quantified. {4.3, 4.4.2.2, Box 3.4}

Technology policy (development, diffusion and transfer) complements other mitigation policies across all scales, from interna-
tional to sub-national; many adaptation efforts also critically rely on diffusion and transfer of technologies and management
practices (high confidence). Policies exist to address market failures in R&D, but the effective use of technologies can also
depend on capacities to adopt technologies appropriate to local circumstances. {4.4.3}

Substantial reductions in emissions would require large changes in investment patterns (high confidence). For mitigation
scenarios that stabilize concentrations (without overshoot) in the range of 430 to 530 ppm CO2-eq by 210019, annual invest-
ments in low carbon electricity supply and energy efficiency in key sectors (transport, industry and buildings) are projected
in the scenarios to rise by several hundred billion dollars per year before 2030. Within appropriate enabling environments,
the private sector, along with the public sector, can play important roles in financing mitigation and adaptation (medium
evidence, high agreement). {4.4.4}

19 This range comprises scenarios that reach 430 to 480 ppm CO2-eq by 2100 (likely to limit warming to 2°C above pre-industrial levels) and scenarios
that reach 480 to 530 ppm CO2-eq by 2100 (without overshoot: more likely than not to limit warming to 2°C above pre-industrial levels).

Summary for Policymakers
31
SPM

Financial resources for adaptation have become available more slowly than for mitigation in both developed and developing
countries. Limited evidence indicates that there is a gap between global adaptation needs and the funds available for adapta-
tion (medium confidence). There is a need for better assessment of global adaptation costs, funding and investment. Potential
synergies between international finance for disaster risk management and adaptation have not yet been fully realized (high
confidence). {4.4.4}

SPM 4.5 Trade-offs, synergies and interactions with sustainable development

Climate change is a threat to sustainable development. Nonetheless, there are many opportu-
nities to link mitigation, adaptation and the pursuit of other societal objectives through inte-
grated responses (high confidence). Successful implementation relies on relevant tools, suit-
able governance structures and enhanced capacity to respond (medium confidence). {3.5, 4.5}

Climate change exacerbates other threats to social and natural systems, placing additional burdens particularly on the poor
(high confidence). Aligning climate policy with sustainable development requires attention to both adaptation and mitigation
(high confidence). Delaying global mitigation actions may reduce options for climate-resilient pathways and adaptation in
the future. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time,
particularly if limits to adaptation are exceeded. Increasing efforts to mitigate and adapt to climate change imply an increas-
ing complexity of interactions, encompassing connections among human health, water, energy, land use and biodiversity
(medium evidence, high agreement). {3.1, 3.5, 4.5}

Strategies and actions can be pursued now which will move towards climate-resilient pathways for sustainable development,
while at the same time helping to improve livelihoods, social and economic well-being and effective environmental manage-
ment. In some cases, economic diversification can be an important element of such strategies. The effectiveness of integrated
responses can be enhanced by relevant tools, suitable governance structures and adequate institutional and human capacity
(medium confidence). Integrated responses are especially relevant to energy planning and implementation; interactions
among water, food, energy and biological carbon sequestration; and urban planning, which provides substantial opportu-
nities for enhanced resilience, reduced emissions and more sustainable development (medium confidence). {3.5, 4.4, 4.5}

Climate

For Complete Set of Factsheets visit css.umich.edu

Climate Change: Policy and Mitigation
The Challenge
Climate change is a global problem that will require global cooperation to address.
The objective of the United Nations Framework Convention on Climate Change
(UNFCCC), which virtually all nations, including the U.S., have ratified, is
to stabilize greenhouse gas (GHG) concentrations at a level that will not cause
“dangerous anthropogenic (human-induced) interference with the climate system.”1
Due to the persistence of some GHGs in the atmosphere, significant emissions
reductions must be achieved in coming decades to meet the UNFCCC objective.
In 2018, the Intergovernmental Panel on Climate Change (IPCC) published the
Special Report on Global Warming of 1.5oC. The report details the impacts of a 1.5oC
temperature rise and proposes mitigation strategies to remain below the 1.5oC target. It
will require lowering global carbon dioxide (CO2) emissions in 2030 by 45% compared
to 2010 and will require net zero emissions around 2050. Current national targets
under the Paris Agreement would lead to 52–58 gigatons (Gt) CO2-equivalents (CO2e)
per year by 2030 — not enough to meet the 1.5oC target. 2018 GHG emissions were
approximately 42 GtCO2 and would need to drop to between 25-30 GtCO2 per year
by 2030 to remain on target.2 In 2018, U.S. GHG emissions were 6.7 GtCO2e.3

General Policies
Market-Based Instruments
• Market-based approaches include carbon taxes, subsidies, and cap-and-trade programs.4
• In a tradable carbon permit system, permits equal to an allowed level of emissions are distributed or auctioned. Parties with emissions below

their allowance are able to sell their excess permits to other parties that have exceeded their emissions allowance.4
• Market-based instruments are recognized for their potential to reduce emissions by allowing for flexibility and ingenuity in the private sector.4

Regulatory Instruments
• Regulatory approaches include non-tradable permits, technology and emissions standards, product bans, and government investment.4
• In 2007, the U.S. Supreme Court ruled that CO2 and other GHG emissions meet the Clean Air Act’s defition of air pollutants, which are

regulated by the U.S. Environmental Protection Agency (EPA).5 After several appeals, the U.S. Court of Appeals upheld the ruling in 2012.6
• In the U.S., the Safer Affordable Fuel-Efficient (SAFE) vehicles rule, administered by NHTSA, was implemented in 2020.7 In comparison to

the 2012 Corporate Average Fuel Economy (CAFE) standards, the SAFE rule has lower efficiency improvement targets of 1.5% per year and
will result in 867-923 million metric tons more CO2 emissions compared to CAFE standards.7,8

Voluntary Agreements
• Voluntary agreements are generally made between a government agency and one or more private parties to “achieve environmental objectives

or to improve environmental performance beyond compliance.”9 EPA partners with the public and private sectors to oversee a variety of
voluntary programs aimed at reducing GHG emissions, increasing clean energy adoption, and adapting to climate change.10

The Kyoto Protocol
• The Kyoto Protocol came into force on February 16, 2005, and established mandatory, enforceable targets for GHG emissions. Initial emissions

reductions for participating countries ranged from –8% to +10% of 1990 levels, while the overall reduction goal was 5% below the 1990 level
from 2008 to 2012. When the first commitment period ended in 2012, the Protocol was amended for a second commitment period; the new
overall reduction goal would be 18% below 1990 levels by 2020.11

The Paris Agreement
• In December of 2015, all Parties of the UNFCCC reached a climate change mitigation and adaptation agreement, called The Paris Agreement,

in order to keep global temperatures below a 2°C increase above pre-industrial temperatures.12
• The Paris Agreement entered into force on November 4, 2016. As of June 2020, The Paris Agreement had 197 signatories of which 189 parties

accounting for at least 55% of total global emissions have ratified the agreement.13 In June 2017, President Trump announced that the U.S.
would withdraw from the Paris Agreement. The withdrawal is scheduled to take place on November 4, 2020.14

Government Action in the U.S.
Federal Policy
• According to the U.S. Senate, “…Congress should enact a comprehensive and effective national program of mandatory, market-based limits

and incentives on emissions of greenhouse gases that slow, stop, and reverse the growth of such emissions at a rate and in a manner that will
not significantly harm the United States economy and will encourage comparable action by other nations…”15

Carbon Emission Pathways to Achieve 1.5C Target2

Cite as: Center for Sustainable Systems, University of Michigan. 2020. “Climate Change: Policy and Mitigation Factsheet.” Pub. No. CSS05-20. October 2020

• In 2015, the proposed Clean Power Plan set a national limit for CO2 emissions from power plants. In early 2016, the plan was stayed by
the Supreme Court due to several lawsuits against it.16 In October 2017, the EPA proposed to repeal the Clean Power Plan.17 The repeal was
finalized in 2019 and was replaced by the Affordable Clean Energy Rule.18

• Due to the Consolidated Appropriations Act of 2008, large emitters of GHGs in the U.S. must report emissions to the EPA.19
• In 2019, a Green New Deal resolution was introduced in the U.S. House. It proposes at 10-year mobilization effort to focus on goals such as

net-zero GHG emissions, economic security, infrastructure investment, clean air and water, and promoting justice and equality.20

State Policy
• Climate change action plans have been enacted by 34 states and D.C.21
• 23 states and D.C. have GHG emission reduction targets. For example, California

is targeting emissions 40% below 1990 levels by 2030 and economy-wide carbon-
neutrality by 2045.22

• 29 states, D.C., and three U.S. territories have Renewable Portfolio Standards,
which specify the percentage of electricity to be generated from renewable sources
by a certain date. Three states have Clean Energy Standards, which specify the
percentage of electricity to be generated from low-to-no carbon sources and can
include renewables, nuclear, and advanced fossil fuel plants with carbon capture
and sequestration.23 25 governors have joined the US Climate Alliance, to uphold
the GHG reductions outlined in the Paris Agreement. The alliance represents 55%
of the U.S. population and more economic activity than all other Paris signatories
other than the U.S. and China.24

Mitigation Strategies
Stabilizing atmospheric CO2 concentrations cannot be accomplished without
changes in energy production and use. Effective mitigation cannot be achieved
without individual agencies working collectively towards reduction goals.9
Stabilization wedges are one display of GHG reduction strategies; each wedge
represents 1 billion tons of carbon avoided per year over 50 years.25
• Energy Savings: Many energy efficiency efforts require an initial capital

investment, but the payback period is often only a few years. In 2016, the
Minneapolis Clean Energy Partnership planned to retrofit 75% of Minneapolis
homes for efficiency and allocated resources to buy down the cost of energy
audits and provide no-interest financing for energy efficiency upgrades.26

• Fuel Switching: Switching power plants and vehicles to less carbon-intensive
fuels can achieve emission reductions quickly. For instance, switching from an average coal plant to a natural gas combined cycle plant can
reduce CO2 emissions by approximately 50%.9

• Capturing and Storing Emissions: CO2 can be captured from large point sources both pre- and post-combustion of fossil fuels. Once CO2
is separated, it can be stored underground depending on the geology of a site. Currently, CO2 is used in enhanced oil recovery (EOR), but
long-term storage technologies remain expensive.27 Alternatively, existing CO2 can be removed from the atmosphere through Negative
Emissions Technologies and approaches such as direct air capture and sequestration, bioenergy with carbon capture and sequestration, and
land management strategies.28

Individual Action
• There are many actions that individuals can take to reduce their GHG emissions; many involve energy conservation and also save money.
• Choose a fuel-efficient or electric vehicle and keep your car well maintained, including properly inflated tires.29
• Decrease the amount you drive by using public transportation, riding a bike, walking, or telecommuting. For a 20-mile round trip commute,

switching to public transit can prevent 4,800 lbs of CO2 emissions per year.29
• Ask your electricity supplier about options for purchasing energy from renewable sources.
• When purchasing appliances, look for the Energy Star label and choose the most energy efficient model.
• Energy Star light bulbs use ~90% less energy than a standard bulb, last 15 times longer, and save ~$55 in electricity costs over their lifetimes.30
• Space heating is the largest use of household energy (34%).31 Ensure that your house is properly sealed by reducing air leaks, installing the

recommended level of insulation, and choosing Energy Star windows.32,33

1. United Nations (UN) (1992) United Nations Framework Convention on Climate Change (UNFCCC).
2. Intergovernmental Panel on Climate Change (IPCC) (2018) Special Report: Global Warming of 1.5C
3. U.S. Environmental Protection Agency (EPA) (2020) Inventory of U.S. Greenhouse Gas Emissions and

Sinks 1990 – 2018.
4. U.S. EPA (2001) The United States Experience with Economic Incentives for Protecting the

Environment.
5. Massachusetts, et al. v. EPA, et al. (2007) Supreme Court of the United States. Case No. 05-1120.
6. U.S. EPA (2018) “U.S. Court of Appeals – D.C. Circuit Upholds EPA’s Actions to Reduce Greenhouse

Gases under the Clean Air Act.”
7. National Highway Traffic Safety Administration (NHTSA) and U.S. EPA (2020) “The Safer Affordable

Fuel-Efficient (SAFE) Vehicles Rule for Model Years 2021–2026 Passenger Cars and Light Trucks,
Final Rule.” Federal Register, 85:84.

8. Federal Register (2012) Rules and Regulations, Vol. 77, No. 199, Monday, October 15, 2012.
9. IPCC (2014) Climate Change 2014: Mitigation of Climate Change.
10. U.S. EPA (2018) “Clean Energy Programs.”
11. UNFCCC (2020) “What is the Kyoto Protocol.”
12. UNFCCC (2016) Summary of the Paris Agreement.
13. UNFCCC (2019) Paris Agreement Status of Ratification.
14. U.S. Department of State (2019) On the U.S. Withdrawal from the Paris Agreement.
15. U.S. Congress (2005) Energy Policy Act of 2005. 109th Congress.

16. U.S. EPA (2019) Fact Sheet: Repeal of the Clean Power Plan.
17. U.S. EPA (2017) Fact Sheet: Proposal to Repeal the Clean Power Plan.
18. U.S. EPA (2019) Electric Utility Generating Units: Repealing the Clean Power Plant
19. U.S. EPA (2012) “Greenhouse Gas Reporting Program.”
20. The Library of Congress (2019) Bill Summary and Status 116th Congress, HR 109.
21. Center for Climate and Energy Solutions (2019) “U.S. State Climate Action Plans.”
22. Center for Climate and Energy Solutions (2019) U.S. State Greenhouse Gas Emissions Targets.
23. DSIRE (2019) U.S. Summary Maps: Renewable and Clean Energy Standards.
24. United States Climate Alliance (2020) U.S. Climate Alliance Fact Sheet.
25. Pacala, S. and R. Socolow (2004) Stabilization Wedges: Solving the Climate Problem for the Next 50

Years with Current Technologies. Science, 305: 968-972.
26. U.S. EPA (2018) “2016 Climate Leadership Award Winners.”
27. Kleinman Center for Energy Policy (2020) The Challenge of Scaling Negative Emissions.
28. The National Academies of Sciences, Engineering, and Medicine (2018) Negative Emissions

Technologies and Reliable Sequestration: A Research Agenda.
29. Center for Climate and Energy Solutions (2020) “Reducing Your Transportation Footprint.”
30. Energy Star (2020) “Light Bulbs.”
31. U.S. Energy Information Administration (2020) Annual Energy Outlook 2020.
32. U.S. DOE (2020) “Do-It-Yourself Home Energy Audits.”
33. U.S. DOE (2020) “Update or Replace Windows.”

Stabilization Wedges25

States with Renewable and/or Clean Energy Standards23

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Annals of the American Association of Geographers

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Climate, Capital, Conflict: Geographies of Success
or Failure in the Twenty-First Century

Glen MacDonald

To cite this article: Glen MacDonald (2020) Climate, Capital, Conflict: Geographies of Success
or Failure in the Twenty-First Century, Annals of the American Association of Geographers, 110:6,
2011-2031, DOI: 10.1080/24694452.2020.1800300

To link to this article: https://doi.org/10.1080/24694452.2020.1800300

Published online: 03 Sep 2020.

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P R E S I D E N T I A L A D D R E S S

Climate, Capital, Conflict: Geographies of Success
or Failure in the Twenty-First Century
Glen MacDonald

Department of Geography, UCLA, and School of Geography and Sustainable Development, University of St. Andrews

Anthropogenic climate change will disproportionately affect equatorial regions and closely adjacent areas, referred
to here as the Fateful Ellipse. The vulnerability of these regions is exacerbated by a lack of capital for adaptive
measures against the impacts of climate change. The increasing transference of capital from governmental control
to private hands, and the increasing concentration of such capital into the hands of fewer individuals raises further
concerns about capacity to mitigate or adapt to climate change. In addition, conflicts arise regarding the choice of
climate change solutions. Ironically, the people of the Fateful Ellipse, who are most vulnerable to climate change,
produce the lowest amount of carbon per capita. As a result of the colonial enterprise, including slavery, they also
paid a heavy price toward the economic ascendency of Europe and North America and the Industrial Revolution
that fueled the rise in greenhouse gas production. The discipline of geography itself owes some measure of its
development and ascendency to colonialism and the exploitation of the Fateful Ellipse. As geographers we have
the capacity, and a special responsibility, to contribute to the development of climate change solutions and global
environmental justice. Key Words: capital, climate change, conflict, Fateful Ellipse, geography, inequality.

人为的气候变化，会不成比例地影响赤道及其相邻区域（即，命运椭圆）。这些地区的脆弱性，由
于缺乏应对气候变化影响所需的资金而加剧。资本持续地从政府转移到私人、持续地聚集在少数人
手中，进一步增加了我们对减轻和适应气候变化的能力上的顾虑。此外，气候变化解决方案的不同
选择也带来了冲突。具有讽刺意味的是，命运椭圆的脆弱性最强，但那里的人均碳排放最少。由于
殖民经营（包括奴隶制），命运椭圆为欧美经济优势和（产生温室气体的）工业革命付出了沉重的
代价。地理学的发展和优势也得益于对命运椭圆的殖民主义和掠夺。做为地理学者，我们有能力和
责任为解决气候变化、实现全球环境正义而贡献力量。关键词：资本，气候变化，冲突，命运椭
圆，地理学，不平等。

El cambio clim�atico antropog�enico afectar�a de manera desproporcionada a las regiones ecuatoriales y �areas
adyacentes, referidas aqu�ı como la Elipse Funesta. La vulnerabilidad de estas regiones se ve exacerbada por la
falta de capital para medidas adaptativas contra los impactos del cambio clim�atico. La creciente transferencia
de capital desde el control gubernamental a manos privadas, y la creciente concentraci�on de tal capital en
manos de unos pocos individuos levanta mayores preocupaciones sobre la capacidad de mitigar o adaptarse al
cambio clim�atico. Adem�as, se presentan conflictos en lo que concierne a la escogencia de soluciones al
cambio del clima. Ir�onicamente, la gente de la Elipse Funesta, que son los m�as vulnerables al cambio
clim�atico, producen las m�ınimas cantidades de carb�on per c�apita. Como resultado de la empresa colonial,
incluida la esclavitud, ellos pagan tambi�en un alto precio hacia la ascendencia econ�omica de Europa y
Norteam�erica, y la Revoluci�on Industrial que aliment�o el alza en la producci�on de gases de invernadero. La
propia disciplina de la geograf�ıa, por su desarrollo y ascendencia, est�a en deuda en cierta medida con el
colonialismo y la explotaci�on de la Elipse Funesta. Como ge�ografos, tenemos la capacidad, y una
responsabilidad especial, para contribuir al desarrollo de soluciones sobre el cambio clim�atico y la justicia
ambiental global. Palabras clave: cambio clim�atico, capital, conflicto, desigualdad, Elipse Funesta, geograf�ıa.

What is geography? This question has beena frequent focus of American Associationof Geographers Presidential Addresses.
Many thoughtful discourses have ensued. As all
geographers know, though, we are the products of

the times and places in which we live and work.
We comprehend the geographies we encounter and
our discipline through the prism of our personal
experiences and interests. My own perspective is
largely centered on climate change and its impacts.

Annals of the American Association of Geographers, 110(6) 2020, pp. 2011–2031 # 2020 by American Association of Geographers
Published by Taylor & Francis, LLC.

http://crossmark.crossref.org/dialog/?doi=10.1080/24694452.2020.1800300&domain=pdf&date_stamp=2020-09-22

With this in mind, I focus on the global challenge
of climate change, because that is my context for
defining geography. I also believe that climate
change must have a central place in defining our
wider discipline and our efforts in the twenty-first
century. I am not the first president to focus on this
topic (e.g., Winkler 2016) and will certainly not be
the last. Here I will consider anthropogenic climate
change broadly in relationship to the global distribu-
tion of capital. By capital I mean the national finan-
cial resources available to pay for climate change
mitigation or adaptation. I then consider conflicts

related to climate change. Such conflicts include not
only violent physical altercations but also the con-
flicts of perspectives and prescriptions that swirl
around the topic of climate change, the latter being
my main focus. These are massive and complex
issues. The constraints of space and the limitations
of my own knowledge mean that these remarks will
be a hurried sketch of a massive and unfolding land-
scape. At the conclusion I will turn again to our
eternal question of “What is geography?” and the
significance of anthropogenic climate change to
defining our discipline.

Figure 1. Top: Representative Concentration Pathway 8.5 projected CO2 emissions, projected mean global temperature increase, and
projected sea level rise. Bottom: Per capita gross domestic product percentage change, ratio of wealth held by top 1 percent and bottom
50 percent. Note: GDP¼ gross domestic product; PPM¼ parts per million. Data from Meinshausen et al. (2011), IPCC (2014), Burke,
Hsiang, and Miguel (2015), Jevrejeva et al. (2016), and Alverado et al. (2017).

2012 MacDonald

The Steepening Slope: Trajectories of
Global Anthropogenic Climate Change

Let’s start by taking an ageographical global view
of data and model projections for anthropogenic
greenhouse gas (GHG) emissions and related climatic
changes. For the past two centuries, human popula-
tion has grown at an incredible rate, climbing from
approximately 1 billion people in 1800 to more than
7.6 billion today. Human population is projected to
reach 9.7 billion people in 2050 and 10.9 billion by
2100 (Roser, Ritchie, and Ortiz-Ospina 2019; United
Nations 2019; U.S. Census Bureau 2019). At the
same time, atmospheric concentrations of CO2 have
grown in an exponential manner from 280 ppm in
1800 to approximately 408 ppm in 2019 (Institute for
Atmosphere and Climate 2019; National Oceanic
and Atmospheric Administration, Earth System
Research Laboratories 2019). The “business as usual”
GHG scenario developed by the Intergovernmental
Panel on Climate Change (IPCC 2014) is called
Representative Concentration Pathway 8.5 (RCP 8.5;
Figure 1). According to RCP 8.5, atmospheric con-
centrations of CO2 will reach levels of 540 ppm by
2050 and 935 ppm by 2100 (Meinshausen et al.
2011). Other GHGs, such as CH4 and N2O, are also
increasing. By 2100 the concentrations of CO2 and
these other GHGs would be the equivalent of CO2
reaching 1,231 ppm (Meinshausen et al. 2011).

The IPCC has produced more optimistic projec-
tions of GHG emissions for the twenty-first century.
These are based on possible changes in behavior and
technologies. Although the first IPCC report was
issued in 1990, and there were alarms sounded about
GHGs, climatic change, and associated dangers well
before that (e.g., Revelle et al. 1965; Matthews,
Kellogg, and Robinson 1971; Mercer 1978), the
annual rate of emissions of CO2 has continued to rise
in recent decades. This trajectory of increasing GHGs
has in fact accelerated despite growing concern and
attendant international efforts such as the Kyoto
Protocol (United Nations Framework Convention on
Climate Change [UNFCCC] 1997) and the Paris
Agreement (United Nations Climate Change 2016).
Since 1970, emissions of CO2 have increased by 90
percent, largely due to the burning of fossil fuels
(Boden, Marland, and Andres 2017). After three
years of little to no growth, CO2 emissions are pro-
jected to have increased by approximately 2 percent
in 2018 and reach record highs (Figueres et al. 2018;

Global Carbon Project 2018). In 2019 the global
GHG trajectory remains close to RCP 8.5.

That the increasing GHG concentrations are pro-
ducing climatic changes consistent with long pro-
jected outcomes (e.g., Revelle et al. 1965; Manabe
and Wetherald 1975) is clearly evident. Manabe and
Wetherald (1975) displayed great prescience when
they wrote several decades ago:

It is shown that the CO2 increase raises the temperature
of the model troposphere, whereas it lowers that of
the model stratosphere. The tropospheric warming is
somewhat larger than that expected from a radiative-
convective equilibrium model. In particular, the increase
of surface temperature in higher latitudes is magnified
due to the recession of the snow boundary and the
thermal stability of the lower troposphere which limits
convective beating to the lowest layer. It is also shown
that the doubling of carbon dioxide significantly increases
the intensity of the hydrologic cycle of the model. (3)

Since the beginning of the twentieth century, global
average surface temperature has increased by about
1.2 �C (Copernicus 2019). Approximately 80 percent
of this warming has occurred since the 1970s. The
trajectory of warming has steepened in recent years.
This, along with the accumulated warming of past
decades, has led to 2014, 2015, 2016, 2017, 2018,
and 2019 being the warmest years in the period of
instrumental climate records (National Oceanic and
Atmospheric Administration 2020). Although varying
in magnitude, overall surface warming has been
almost universal in terms of geography. Warming has
indeed been amplified at higher northern latitudes as
predicted by Manabe and Wetherald (1975). As a
consequence of the warming, many of the world’s gla-
ciers have been declining in mass and extent at an
unprecedented rate (Zemp et al. 2015; IPCC 2019b).
Perhaps the most worrying aspect of this decline is
the situation in Greenland. Analysis of Greenland’s
ice cover indicates that beginning in the 1980s the
glacial mass balance there began to deviate negatively
from natural variability. Since that time, the loss of
ice has increased sixfold (Mouginot et al. 2019). A
recent report from the IPCC (2019b) concludes that
ice loss from Greenland doubled between 2007 and
2016 and the loss from Antarctica tripled. As the
planet has warmed, the world’s oceans have absorbed
much heat, and this has led to thermal expansion of
the sea at the same time that melting glaciers have
contributed additional water to ocean basins (IPCC
2019b). Accordingly, global mean sea level has risen

Climate, Capital, Conflict 2013

by 16 to 21 cm since the start of the twentieth cen-
tury. The rate of sea-level rise has been accelerating
in recent decades, and approximately 7 cm of the rise
has occurred since 1993 (Wuebbles et al. 2017). The
rate of sea level rise between 2006 and 2017 was
about two and a half times as fast as that for the
period from 1900 to 1990 (IPCC 2019b).

Although the long-term trajectories and recent
accelerations of changes in GHGs, temperature, gla-
cial mass balance, and sea level are clear, many other
important aspects of climatic change remain less well
resolved. Debate continues around drought frequency
and intensity (Milly and Dunne 2016; Mukherjee,
Mishra, and Trenberth 2018) or whether there have
been significant changes in episodic extreme events
such as superstorms, floods, and large hurricanes
(Trenberth, Fasullo, and Shepherd 2015; Klotzbach
et al. 2018). A recent reanalysis by Dai and Zhao
(2017), however, presents compelling evidence that
accelerating surface warming and associated enhanced
evaporation rates since the 1980s have become an
increasingly important cause of widespread drying on
the continents. Despite present uncertainties regard-
ing some aspects of climate change, it is clear that
more than temperature is changing.

Up to now, the world is likely to have experienced
only the relatively gentle piedmont of the mountain-
ous landscape of climate change that lies ahead.
What are some of the projections for the rest of this
century? The 2014 IPCC report and subsequent
assessments (e.g., Raftery et al. 2017) suggest that
global mean annual temperature will likely rise by
3.2 �C to 5.4 �C above the average of the early indus-
trial period (1850–1899; Figure 1). These projections
might well be understatements, though. A recent run
of two French general circulation models that will be
used in the next IPCC assessment suggest the planet
might experience a 6 �C to 7 �C increase in tempera-
ture by the end of the century (CNRS 2019).
Estimated changes in global mean sea level (Figure 1)
have also increased in magnitude since the time of
the 2014 IPCC report and now range from 90 cm to
over 200 cm by 2100 (e.g., Jevrejeva et al. 2016). For
some perspective, the streets of the French Quarter of
New Orleans are only about 91 cm above sea level
today and vulnerable to even lower estimates of sea-
level rise. The higher estimate would put streets in
Miami and lower Manhattan under water by the end
of the century. Low-lying island nations such as
Tuvalu and Kiribati in the South Pacific will likely

become uninhabitable by the end of the century
(Batur and Weber 2017). A rise of 200 cm could dis-
place an estimated 187 million people (Willis and
Church 2012).

The effects of the climatic changes projected
under the RCP 8.5 scenario extend well beyond
changes in climate and coastlines. Two important
concerns for humans are health and nutrition. For
about 30 percent of the world’s population, heat
stroke is an important health threat and potential
cause of mortality (Mora et al. 2017). Under RCP
8.5, it is possible that by 2100 approximately 74 per-
cent of the world’s population will be exposed to the
dangers of lethal heat stroke (Mora et al. 2017).
Health risks could increase from a number of tropi-
cal and temperate infectious diseases. Malaria- and
dengue-carrying mosquitoes or ticks that carry Lyme
disease could spread to higher latitudes and eleva-
tions. In addition, warmer temperatures might accel-
erate pathogen and vector life cycles and increase
transmission rates in areas where these diseases
already exist (Wu et al. 2016; Andersen and Davis
2017). Warming in some hot tropical regions, how-
ever, might also decrease diseases such as malaria
because of increased mortality in mosquito popula-
tions due to the higher temperatures (Murdock,
Sternberg, and Thomas 2016). Of course, increased
human population size will in itself create a higher
number of humans at risk of disease. For example,
due to climate change coupled with population
growth, the number of people exposed to dengue
fever could increase to more than 5 billion by 2100
(Hales et al. 2002).

There has been much active research on estimat-
ing the impacts of climate change on the world’s
agriculture. For example, under the RCP 8.5 climatic
scenario there could be a 40 percent decline in
global maize production and a 20 percent decline in
global soybean production. These effects could lead
to 175 million more people being undernourished
(Fischer et al. 2005; Iizumi et al. 2017). The world
need not wait until the end of the century to feel
the impacts of climate change on disease and nutri-
tion. According to the World Health Organization
(2014), “Compared with a future without climate
change, the following additional deaths are projected
for the year 2030: 38 000 due to heat exposure in
elderly people, 48 000 due to diarrhoea, 60 000 due
to malaria, and 95 000 due to childhood under-
nutrition” (1).

2014 MacDonald

The points just raised barely scratch the surface of
the climatic changes and their impacts that lie ahead
along the current GHG trajectory. The potential dev-
astating effects on the other species that inhabit the
Earth, overall biodiversity, and ecosystems services are
not even considered here. The observations and pro-
jections just outlined are being updated continu-
ously—and projected changes often become worse
with each iteration (e.g., IPCC 2019a, 2019b). This
indicates that not only does humankind face tough
slopes ahead in terms of mitigating or adapting to cli-
mate change but those slopes are steepening.

Trajectories of Capital and Inequality

Efforts to counteract the effects of twenty-first-
century climatic change will require massive
amounts of capital to cover everything from
increased cooling costs for our buildings, to con-
structing sea walls for low-lying cities, to offsetting
the financial tolls from disease and heat-related
deaths. History has shown that economists and
financial managers find it impossible to accurately
estimate the timing or magnitude of the next reces-
sion under normal market conditions. It is not sur-
prising that projecting the costs of climate change
over the decades ahead is difficult and has generated
a wide range of estimates. The figures presented here
are a sampler of such analyses but are in no sense
meant to be considered definitive.

An analysis of global demand for increased resi-
dential air conditioning due to climate change sug-
gests that by 2100 more than 40,000 additional
petajoules of energy could be required annually for
this purpose (Isaac and Van Vuuren 2009). This
might be crudely approximated as an additional
annual cost of $2.9 trillion (values given hereafter
are roughly equivalent to costs in 2018 U.S. dollars).
Additional annual costs for coastal infrastructure to
protect against higher sea level could range from $12
billion to $71 billion (Hinkel et al. 2014).
According to the World Health Organization
(2014), the direct costs of climate change on human
health could be $2 to $4 billion per year by 2030.
These represent only a tiny subsampling of costs,
and all climate change–related costs would increase
as the century continues and climate change contin-
ues unabated. Estimating the total costs of the con-
tinued growth of GHGs is difficult. There is a wide
range of values estimated for the cost of losses of

privately manageable financial assets, from $2.5 tril-
lion to more than $13 trillion (Economist
Intelligence Unit 2015; Dietz et al. 2016). The
upper figure represents just a bit less than 10 percent
of the world’s total stock of financial assets held by
financial institutions other than banks (Economist
Intelligence Unit 2015). When these losses are cal-
culated from the perspective of public-sector finan-
ces, the losses could be on the order of $43 trillion
(Economist Intelligence Unit 2015). That would be
roughly 61 percent of the value of all of the world’s
stock markets today (Economist Intelligence
Unit 2015).

Climate change will also affect the generation of
new capital. This will be reflected in declines in
annual gross domestic product (GDP) in many coun-
tries and globally (Figure 1). Again, such estimates
vary widely and have often been in the range of 7
percent to 10 percent (Stern 2008). A recent analy-
sis, based on nonlinear relationships between tem-
perature increase and declines in economic
productivity, suggests that the decline in global GDP
could more likely be 23 percent under RCP 8.5
(Burke, Hsiang, and Miguel 2015). Any of these pro-
jected declines would be a serious economic contrac-
tion in a world where global GDP has typically
grown annually at a rate of between 2 percent and 3
percent. The dependence on endless compound
growth and the limitations placed on such growth by
the environment have been identified as key contra-
dictions in the capitalist system (Harvey 2014) that
will certainly be exacerbated by climate change.

Who controls capital will undoubtedly influence
who benefits most from climate change adaptation
efforts. There has, in recent decades, been a marked
concentration of capital in the hands of the wealthi-
est people and wealthiest nations in the world. This
group typically represents the top 1 percent of the
population. Data from the World Economic Forum
(2016) indicate that by 2015 the economic top 1
percent controlled as much wealth as the remaining
99 percent of the world’s population. Today the
world’s top 1 percent control about twice the
amount of capital as the entire bottom 50 percent. If
the global trajectory of capital concentration into
the hands of the few follows the recent trajectory of
the United States (Alvaredo et al. 2017), then by
the close of the twenty-first century the top 1 per-
cent could control about eight times as much of the
world’s wealth as the bottom 50 percent (Figure 1).

Climate, Capital, Conflict 2015

As the control of capital concentrates into a pro-
portionally smaller number of private hands, how
much will be available in public coffers for govern-
ments to use in funding climate change mitigation
and adaptation efforts? The trajectory toward impov-
erishment of public treasuries relative to the private
sector has been striking. Since the 1970s there has
been a marked decline in the ratio of public wealth
relative to private wealth in many of the world’s
developed national economies, as well as in China
(Piketty, Yang, and Zucman 2017). In the United
States and in the United Kingdom, the percentage
of national wealth in the public sector has declined
from more than 10 percent and more than 20 per-
cent, respectively, to slightly negative values as the
sum of the national debt has grown (Piketty, Yang,
and Zucman 2017). In June 2019 the Financial Times
reported that the level of global national debt had
risen to its highest level since World War II—
although not yet nearly as high as during that time
(Stubbington 2019). If such trends continue, public-
sector efforts to adapt to or mitigate climate change
and its impacts will need to rely on increasing levels
of governmental deficit spending that are potentially
unsustainable economically.

The increase in costs and decrease in income
caused by climate change will accelerate going for-
ward into the twenty-first century. Given current
trends in wealth concentration and inequality, the

capital to adapt to or mitigate climate change will
be concentrated in a smaller proportion of the
world’s population. At the same time, a smaller pro-
portion of the world’s capital will be controlled
directly by governments. As a result, a lower propor-
tion of the world’s capital will be available to gov-
ernments to fund society-wide goals of climate
change mitigation and adaptation. Funding such
goals would likely entail significant governmental
debt and the possibility of associated financial
repression and societal costs that are often necessi-
tated to resolve such debts (Reinhart and Rogoff
2015). Governments might simply not have the cap-
ital necessary to confront climate change at the scale
required. Can we count on wealthy individuals and
the private sector to fill this gap and allocate capital
in a manner that reflects the greater global good
rather than self-interest? The future of capital and
its distribution can further steepen the global gradi-
ent of successfully addressing climate change.

Geographies of Climate Change
and Capital

Up to now, the discussion has been ageographical,
focusing on global trajectories of climate change and
capital. Success tackling the challenges ahead, how-
ever, requires an explicitly geographical perspective.
Climate change is without doubt a global issue, but

Figure 2. University of Notre Dame Global Adaptation Initiative (2019) map of climate change vulnerability by country and the
general location and the Fateful Ellipse discussed in text. Map reproduced by permission of the University of Notre Dame Global
Adaptation Initiative.

2016 MacDonald

any effort to adapt to or mitigate climate change
must be predicated on understanding the importance
of space, place, and associated regional dimensions.
The Notre Dame Global Adaptation Initiative
(2019) provides a useful mapped summary of
national vulnerability to climate change (Figure 2).
The analysis is based on each nation’s biophysical
exposure to climate change, the sensitivity of eco-
nomic sectors and the population to climate change,
and the adaptive capacity in terms of sector-specific
resources for adaptation. Examination of the geo-
graphic distribution of RCP 8.5 climatic changes,
their effects, and economic capacity to respond
shows that a number of nations in the equatorial
regions and adjacent areas are particularly at risk
(Figure 2). This geographic area, defined by height-
ened human vulnerability to climate change, lies
within a delineable multicontinental region that
extends from South and Central America, across
Africa, and through Southwest and Southeast Asia. I
refer to this high-risk region as the Fateful Ellipse
(Figure 2). I use the term fateful because through fail-
ure of humankind to act on climate change this
region could be fated to some of the most extreme
negative impacts. Alternatively, through coordinated
global efforts the region could succeed in averting
extreme calamities and establish new mechanisms
and positive benchmarks for human cooperation and
achievement. Tackling climate change calls for coor-
dinated action and moving beyond the constraints of
tropicality, whereby the equatorial regions have often
been viewed as separate and inferior to the temperate
zones—and fated to remain so (Arnold 1996;
Clayton and Bowd 2006). The broader reality of
shared humanity and shared planet must be
embraced. What happens in the Fateful Ellipse in the
face of climate change will, for good or bad, define
the success or failure of humanity in the twenty-first
century. It is the most fateful of geographies.

If we map the distribution of maximum daily tem-
peratures—the ones that are likely to cause heat
stroke, for example—we find it is in the already hot
tropical and subtropical regions where such extreme
temperatures are concentrated. In RCP 8.5 projec-
tions, average maximum daily temperatures of more
than 35 �C will be widespread in the Fateful Ellipse.
In large areas of Africa and Southwest Asia, temper-
atures higher than 40 �C are projected. Although the
impacts of anthropogenic climate change on precipi-
tation remain difficult to estimate, the high

temperatures and associated high potential evapo-
transpiration rates will likely result in net increases in
aridity in some regions of the Fateful Ellipse (Dai and
Zhao 2017).

Geographic projections of heat-related deaths
clearly show that they will be concentrated in the
tropical and subtropical areas delineated by the
Fateful Ellipse (Mora et al. 2017). Some portions of
southwest Asia are projected to have maximum daily
temperatures higher than 50 �C to 60 �C (Pal and
Eltahir 2016). Prolonged exposure to temperatures
greater than 35 �C can generate heatstroke, and tem-
peratures higher than 60 �C in humid conditions can
quickly be lethal. Even physical infrastructure and
machinery can be compromised by prolonged tem-
peratures of 40 �C and above (Pal and Eltahir 2016).
The high daily maximum temperatures projected for
Southwest Asia in 2100 have led Pal and Eltahir
(2016) to conclude that for humans some areas of
Southwest Asia might simply become uninhabitable.

A global mapping of vulnerability to rising sea
levels, due to factors of topography, population size,
infrastructure, and economic capacity, shows that it
is the coastlines and the islands of the Fateful Ellipse
in Africa, southern Asia, the Caribbean Sea, the
Indian Ocean, and the tropical Pacific Ocean that
are at the most risk (Nicholls and Cazenave 2010).
A recent modeling of increased storm surge exposure
for the end of the twenty-first century similarly indi-
cates that the greatest risks are found in southern
Asia, the Caribbean, Pacific Oceania, and sub-
Saharan Africa (Lloyd et al. 2016). Based on these
projections, 8.2 million people in Bangladesh and
2.7 million people in Mozambique will be exposed
to storm surge–related mortality (Lloyd et al. 2016).

Temperatures within portions of the Fateful
Ellipse might indeed become so hot as to be lethal
to pathogens and insect vectors of diseases like
malaria (Paaijmans, Read, and Thomas 2009; Lyons
et al. 2012; Mordecai et al. 2013). It is likely, how-
ever, that increased human population density, cou-
pled with faster pathogen generation times and
pathogen and vector survival at higher latitudes and
altitudes, will increase disease load in many portions
of the Fateful Ellipse (Hales et al. 2002; World
Health Organization 2014; Wu et al. 2016). High
temperatures, coupled with coastal flooding and
increased inland flooding and extreme climate
events, will increase incidences of disease such as
cholera in the Fateful Ellipse (Wendel 2015).

Climate, Capital, Conflict 2017

Declines in crop yields due to climatic changes by
the end of the twenty-first century also provide evi-
dence of disproportionate impacts within the Fateful
Ellipse. It is possible that warmer temperatures, if
coupled with sufficient moisture, will increase yields
of rice under the RCP 8.5 scenario (Iizumi et al.
2017). Yields of wheat, maize, and soybeans, how-
ever, are likely to experience declines within the
Ellipse under an RCP 8.5 climate (Iizumi et al.
2017). In taking a broad view of crops and potential
productivity at the close of the twenty-first century,
Cline (2007) concluded that there will be a concen-
tration of crop yield declines within countries of the
Fateful Ellipse relative to other parts of the globe.
Agricultural productivity declines of 15 percent to
25 percent are estimated for many Ellipse countries.
The most recent IPCC (2019a) report highlights the
exposure of Africa and southern portions of Asia to
increased food vulnerability.

What about future available capital and the finan-
cial capacity of nations to mitigate the impacts of the
changing climate? Here again, we see a disproportion-
ate share of the burden is likely to be borne within the
Ellipse. The GDP of most countries within the Fateful
Ellipse is already well below that of more developed
nations (International Monetary Fund 2019). The
highest proportions of people living in extreme poverty
are similarly found concentrated in nations of the
Fateful Ellipse (Roser and Ortiz-Ospina 2017).
Alarmingly, analysis by Burke, Hsiang, and Miguel
(2015) suggests that GDP for many countries within
the Fateful Ellipse could decline by 50 percent or more
by the end of the century due to climate change.

There is a bitter irony in the coalescence of some
of the most egregious impacts of climate change on
some of the most vulnerable peoples of the world, as
represented by the Fateful Ellipse. This irony reflects
both recent conditions and deeper history. If we look
at the per capita production of the GHGs that are
driving climate change, we see that the countries
within the Fateful Ellipse are the world’s lowest pro-
ducers by far (Muntean et al. 2018; Ritchie and Roser
2018). For example, in 2017 the annual per capita
production of CO2 by the United States was 15.74
tons. Many countries within the Fateful Ellipse pro-
duced less than 2.0 tons per capita, and some nations
in Africa produced less than 1.0 ton per capita.

The irony of the Fateful Ellipse has an even
deeper and more troubling context. The countries
within this region were at the heart of Europe’s

colonial empires (Lehning 2013). The unequal trade
relations between these regions and colonial powers
provided resources and contributed capital that
helped to drive the Industrial Revolution and associ-
ated economic growth and prosperity of Europe
(Habib 1984; Sheppard 2015). Commodities such as
gold, silver, cotton, sugar, coffee, and tea flowed
from the Fateful Ellipse. A part of that enterprise
and associated trade network was the odious subjec-
tion and commerce in human beings—slave labor
and the slave trade. This human enslavement and
trafficking contributed to European economies into
the early nineteenth century and underlays part of
U.S. prosperity into the mid-nineteenth century
(Habib 1984; Baptist 2014; Olmstead and Rhode
2018). Fanon (1963) famously stated, “Europe is lit-
erally the creation of the Third World. The wealth
which smothers her is that which was stolen from
the underdeveloped peoples” (102). In short, the
people of the Fateful Ellipse paid a high price for the
Industrial Revolution, which has led to the climate
crisis, yet they reaped far fewer economic benefits
than the former colonial powers and the United
States. They are now set to pay a disproportionately
high price for the effects of climate change.

The Paris Climate Agreement to cut carbon emis-
sions is a laudable step in addressing the climate
change challenge created by the developed world
and foisted onto the entire planet. Importantly, the
Paris Agreement also recognizes the obligation of
the wealthy countries of the world to assist the
countries within the Fateful Ellipse. The Agreement
pledges to mobilize $100 billion a year of public and
private funds for transfer to developing countries by
2020. Although these levels have not yet been fully
met, this is also a step in the right direction. It is
particularly troubling, though, that the United
States, which is one of the largest emitters of GHGs
and whose economic development benefited from
the African slave trade into the 1860s, has indicated
its withdrawal from the Paris Agreement.

Climate, Capital, and Conflict: The
Battle of Ideas, Policy, and Ideology

It has been suggested by a number of researchers
that climate change can or will lead to increased
human conflict (e.g., Hsiang, Meng, and Cane 2011;
Hsiang, Burke, and Miguel 2013; Bollfrass and
Shaver 2015; Mach et al. 2019). The proposition

2018 MacDonald

that climate change can instigate violent conflict
has also been taken up in a somewhat sensationalist
manner by the popular press (e.g., Parenti 2011).
Much of this focus has been on the countries of the
Fateful Ellipse. The factors presumed to drive this
increase in violent conflict include drought and agri-
cultural failure, mass migrations driven by climate
change, inundation by the sea of low-lying areas,
and so forth. Some studies suggest that increased
temperatures in and of themselves influence human
cognition and might induce a greater propensity to
violence (Gamble and Hess 2012; Halali, Meiran,
and Shalev 2017). After a large review of relevant
studies, Hsiang, Burke, and Miguel (2013) concluded
that the occurrence of climatic departures, measured
as standard deviations from average conditions, pro-
vided the best guide to an increased propensity for
violence driven by climate. Their mapped summary
of climatic potential for conflict shows that these
climatic departures were among their greatest magni-
tudes in the Fateful Ellipse. Troublingly, the region
has already experienced high levels of state fragility
and warfare relative to the rest of the world in the
post–World War II period (Marshall 2019). The
view that climate, rather than other causes is, or will
be, a principal driver of violent conflict in the
Fateful Ellipse or elsewhere remains a matter of
strong debate (Buhaug et al. 2014; Hsiang et al.
2014; Koubi 2019), however. It is hard to imagine
that in some locations and in some circumstances
the pressures of climate change would not increase
the likelihood of violent conflict, but it also does
not seem accepted that this will be a universal
response either. A recent analysis based on expert
consensus concluded that although climate has influ-
enced armed conflict within countries, and climate
change is estimated to increase future risk of armed
conflict, other drivers including poor socioeconomic
conditions and weak governance capacity are sub-
stantially more influential (Mach et al. 2019). In
addition, the analysis concluded that the specific
mechanisms by which climate influences conflict
linkages remains unclear.

I wish to shift focus to the intellectual and policy
conflicts that arise over climate change, and particu-
larly the conflicting views on how to best mitigate it.
First, let’s dismiss the chimera that there is legitimate
scientific conflict over the basic fact that human
activity has dramatically increased levels of atmo-
spheric GHGs and this is altering the climate.

Observed levels of atmospheric GHG increases,
record-breaking temperatures, and climate model pro-
jections have now convinced 97 percent of climate
scientists that human activity is altering the climate
(Cook et al. 2016). There clearly remain uncertainties
in projections of future climate (Knutti and Sedl�a�cek
2013; Woldemeskel et al. 2016; Soden, Collins, and
Feldman 2018), but the basic outlines of the phenom-
enon and its challenges are clear. Even the well-
known climate change skeptic, Patrick Michaels,
agrees that humans are changing the climate, although
he disputes the potential magnitude of future changes
(Michaels and Knappenberger 2016). There are some
scholars who argue that the benefits of significantly
decreasing CO2 emissions are outweighed by the eco-
nomic costs or that the warming associated with cli-
mate change and the effects of increased CO2 levels
might actually be a net benefit (Lomborg 2007;
Goklany 2015; Lindzen 2017), but they are a small
minority, often with very few to no peer-reviewed cli-
mate change publications to substantiate their views. I
accept the 2009 statement by the American
Association for the Advancement of Science (AAAS)
and eighteen other scientific organizations who have
reached the conclusion that “based on multiple lines
of scientific evidence [… ] global climate change
caused by human activities is now underway, and it is
a growing threat to society” (AAAS 2009). Therefore,
the discussion that follows will focus on the conflicts
that arise due to the differing potential pathways for
mitigating climate change. By mitigation I mean signifi-
cantly and quickly decreasing GHG concentrations in
the atmosphere or significantly attenuating the cli-
matic impacts of these gases. The GHG target levels
required for this effort are massive. According to the
UN Secretary-General, at least seventy-seven nations
have agreed that a goal of net-zero carbon emissions
by 2050 is required (Guterres 2019). The current
potential strategies for mitigating anthropogenic cli-
mate change can be broadly grouped into five
approaches: geoengineering approaches, technology
substitution approaches, consumer behavior
approaches, economic reform approaches, and societal
change approaches.

Geoengineering Approaches

Geoengineering approaches largely address climate
change through one of two means—removing GHGs
from the atmosphere or decreasing the radiative

Climate, Capital, Conflict 2019

impact of the GHGs. There are a wide variety of
potential means of removing GHGs. Major ones
include forestation and vegetation restoration, ocean
fertilization and alkalinity enhancement, increasing
soil carbon sequestration, enhancement of rock
weathering and mineral carbonation, and direct air
capture of CO2 and storing or recycling it (Royal
Society 2018). The necessary scale and potential
costs of such an enterprise are immense. In 2018
global CO2 emissions reached a record high of 37.1
gigatons (Global Carbon Project 2018). This amount
of CO2 is difficult to visualize, but one metric ton is
about the same volume as a two-story detached
home. Potential costs for carbon capture are declin-
ing as technologies improve (Keith et al. 2018;
Tollefson 2018). A recent report by the Royal
Society (2018), however, concluded that pricing for
the emissions of CO2 would still need to be around
$100 per ton to make a GHG removal (GGR) strat-
egy economically feasible. Given present technolo-
gies and costs, the first recommendation from the
report is “Continue and increase global efforts to
reduce emissions of greenhouse gases. Largescale
GGR is challenging and expensive and not a
replacement for reducing emissions” (Royal Society
2018, 10).

Reducing the radiative effect of GHGs through
the application of aerosols such as sulfates to the
lower stratosphere would cost relatively little. A
recent estimate places this annual cost at US$2.25
billion (Smith and Wagner 2018). This approach,
although potentially cost-effective in lowering tem-
peratures, is controversial for a number of reasons
(Keith, Parson, and Morgan 2010; Williamson and
Turley 2012; Dalby 2013, 2015; Parson and Keith
2013; Surprise 2018; Trisos et al. 2018). These
include questions about how the process would be
governed, concerns about the effects of the resulting
unchecked increases in CO2 on vegetation function-
ing and on ocean acidification, unintended conse-
quences of how the geoengineered climate would
function, and how to deal with the fact that some
regions would benefit by the climatic manipulations
but others might not. In addition, if the aerosol
enrichment process is stopped, temperatures would
accelerate with unprecedented rapidity, and poten-
tially catastrophically, toward an equilibrium with
the high levels of GHGs then in the atmosphere.
Finally, atmospheric geoengineering has also been
criticized from the Marxist perspective insofar as it

would serve as an artificial prop to a capitalistic
hegemony, masking capitalism’s contradictions and
its inability to respond adequately to climate change
(Surprise 2018). Although Parson (2017) argued
that some form of atmospheric geoengineering is
essential to meeting global temperature targets pro-
posed under the Paris Climate Agreement, he also
acknowledged that it cannot replace efforts to miti-
gate GHG emissions.

Technological Substitution Approaches

Technological substitution approaches use low to
zero carbon producing technologies to reduce CO2
emissions while maintaining economic health. This is
a path that is being widely prescribed today and
includes efforts such as wind, solar, and tidal genera-
tion of electricity; the use of biofuels; and the replace-
ment of high-carbon-emitting equipment such as
internal combustion engines by equipment powered
by electrical energy. The amount of global electricity
demand met by renewable energy is predicted to
reach 29.4 percent by 2023 (International Energy
Agency 2018). This is a positive step. Unless that
growth rate increases significantly by midcentury,
however, a large proportion of energy demands will
still rely on fossil fuels. Research on past technology
transitions, such as the shift from coal-based thermal
power to electricity, indicates that such transitions
tend to be slow (Fouquet 2010). Some studies suggest
that 100 percent renewable energy is an achievable
goal for the twenty-first century but will require
exceptionally careful planning and implementation
road mapping (Jacobson et al. 2017). Other recent
analyses of the potential rates of global transition to
100 percent renewable energy, though, indicate that
many previous estimates have been overly optimistic
and underestimate the challenges (Heard et al. 2017).
Some analyses have concluded that renewable energy
alone will likely never meet all global energy require-
ments and completely displace GHG-producing
energy sources (Moriarty and Honnery 2016).

Consumer Behavior Approaches

Anthropogenic GHG emissions are largely driven
by consumer demand for services, such as transporta-
tion, light, heating, cooling, communication and
entertainment, and computing, and for goods,
including food. As population increases and

2020 MacDonald

economies grow, such consumer demands are
increasing. It has been projected that a growing
global middle class could increase annual consumer
spending from $37 trillion in 2017 to $64 trillion by
2030 (European Commission 2020). If consumer
behavior is voluntarily modified so that people
demand less per capita in terms of GHG-dependent
services and goods, then emissions will decline pro-
portionally. It has been argued that there is poten-
tial for significant progress to be made in mitigation
of GHG emissions through voluntary changes in
consumer choices for services and products (Girod,
van Vuuren, and Hertwich 2014). Consumer
actions such as no-fly movements in Sweden and
other parts of Europe are an example of consumer
demand modification. It is reported that the Flygfritt
movement has convinced 14,500 Swedes to forsake
air travel in 2019 and has a target of 100,000 no-fly
advocates for 2020 (Abend 2019). The desire to do
what is environmentally correct, to be seen as doing
so, and the positive feelings that come from this
have also been identified as motivators in electric
vehicle adoption by consumers in Sweden (Rezvani,
Jansson, and Bengtsson 2018). Although the drivers
of such reductions are complex and extend beyond
individual consumer choice, per capita GHG emis-
sions from Sweden fell from 6.86 metric tons in
1997 to 4.54 metric tons in 2016. In contrast, rap-
idly growing production, economies, middle classes,
and consumption in China and India have driven
per capita emissions upward as more consumers are
now able to afford increased services and goods
(Wang, Su, and Li 2018). That demand is often
not for products and services that could be classified
as luxuries in more developed economies. Is it real-
istic to expect consumers in growing economies
such as China and India to forsake improvements
in heating, cooling, transportation, and other
aspects of life that many other parts of the world
take for granted? Even in relatively affluent devel-
oped economies, there are also limitations to the
impact that green marketing and voluntary con-
sumer choice can have in achieving environmental
goals and GHG reductions (Wymer and Polonsky
2015; Fuchs et al. 2016). There might be a move
toward reduced flying by some Europeans, but as a
recent survey of 2,066 British consumers indicated,
people are less likely to sacrifice more basic goods
and services for the sake of the environment
(Kantenbacher et al. 2019).

Economic Reform Approaches

It is widely agreed that economic reform approaches,
as represented by economic and regulatory incentives,
are required to make any significant gains in GHG
mitigation (Carley et al. 2017; Heard et al. 2017;
International Energy Agency 2018). Carbon taxes and
cap and trade systems are the two main approaches
that are being applied today. In essence these
approaches operate via the profit motivation within
the capitalist system by placing a governmentally deter-
mined per ton price on the production of CO2. This
then drives technological and operational innovation
to reduce the amount of GHGs produced by a given
service or product and increase the profitability to the
producer. The taxes or charges that accrue to the gov-
ernment can be used to offset the costs to the con-
sumer and to fund clean technology development and
climate change adaptation. There is some conflict
among economists as to which approach is most effec-
tive (Goulder and Schein 2013; Kosnik 2018)—a sim-
ple tax on GHG emissions that can escalate in per ton
charge over time versus a set of escalating emissions
caps and charges for additional emissions and a provi-
sion for firms to sell surplus emissions allocations. As
of April 2019, some forty-six nations and twenty-eight
subnational jurisdictions have placed, or were in the
process of placing, some price on carbon emissions
(Carbon Leadership Pricing Coalition 2019). The
identification of target industries, price levels, proposed
uses of accrued funds, and mechanisms for global appli-
cation, however, remains complex and uncertain.
Large emitters such as the United States and India
have no national policy in place or on the horizon.
Despite acknowledgment of its potential power as a
market force, carbon pricing via direct taxation or cap
and trade has not thus far produced the decreases in
GHG emissions that are required (Climate Action
Tracker 2019). Nor is it likely that this mechanism
alone will produce those reductions in the time frame
required to avert continued high emissions and climate
change (Tvinnereim and Mehling 2018).

Societal Change Approaches

It has been argued by some, most notably eco-
Marxists, that the present capitalist world system is
incapable of dealing with the global challenge of cli-
mate change. Deeper and broader societal changes
are required. Wainwright and Mann (2015) stated,
“If the character of political life prevents a radical

Climate, Capital, Conflict 2021

response to crisis, then it is the political that must
change” (313). From the eco-Marxist perspective,
climate change can be seen as reflecting the second
contradiction of capitalism (Harvey 2014), wherein
capitalism’s need for infinite economic growth
bumps up against environmental limitations to such
growth (O’Connor 1988; Surprise 2018). Thus, it is
argued, green capitalism, as represented by geoengin-
eering approaches, technology substitution
approaches, consumer behavior approaches, and eco-
nomic reform approaches, works to maintain unjust
capitalistic hegemonies of power that will likely be
ineffective in mitigating climate change. In this
view, capitalism is both the root cause of anthropo-
genic climate change and other environmental crises
and a barrier to addressing these crises. Various ver-
sions of these arguments for the need for deeper
sociopolitical change have been championed in criti-
cal environmental literature of both the academic
and popular press (e.g., B€ohm, Misoczky, and Moog
2012; Dalby 2013, 2015; Swyngedouw 2013; Foster
2015; Klein 2015; Wainwright and Mann 2015;
Surprise 2018; Ghotge 2018a, 2018b). It is often the
case, though, that the fight against climate change
has also become a proposed mechanism to introduce
a wide variety of socioeconomic changes that can
appear at best tangential to combating climate
change or addressing social and environmental
impacts directly related to climate change. This is
illustrated in a quote from Klein (2015) writing
about climate change:

[I]t could be the best argument progressives have ever
had to demand the rebuilding and reviving of local
economies; to reclaim our democracies from corrosive
corporate influence; to block harmful new free trade
deals and rewrite old ones; to invest in starving public
infrastructure like mass transit and affordable housing;
to take back ownership of essential services like energy
and water; to remake our sick agricultural system. (7)

A similar shopping list of assorted socioeconomic
and political goals is contained in the Green New
Deal being proposed by progressive politicians in the
United States. These broad agendas can fuel push-
back against action on climate change. Some conser-
vatives argue that the climate crisis is simply a cover
being used by progressives to push broader political
agendas or by Marxists to overturn the capitalist
world order. This is often used by conservatives as a
justification against making significant efforts to
address climate change. In this sense, climate change

is dismissed as a socialist Trojan horse being
employed as a ruse to restructure the world’s econ-
omy (Varney 2019). On the other hand, radical pro-
gressives might argue that such broad reconfiguring
of the political-economic system is the only way to
significantly tackle climate change and related envi-
ronmental justice inequalities. Environmental justice
is the principal that everyone, regardless of income,
race, nationality, or religion, is entitled to equal pro-
tection from environmental harm. Many of the calls
for radical societal restructuring as a climate change
solution, however, are largely aspirational and lack
solid guidance and details on specific steps to be
taken. Climate change is a problem requiring a
global response. How will such global decisions be
prioritized and taken? Who specifically will be
empowered to make the very hard choices on whom
to disadvantage and by how much to achieve the
goals? Quis custodiet ipsos custodes? It should be
acknowledged that some of these same issues con-
front neoliberal market-based solutions that rely on
guiding the market-based systems through regulation.

Those in favor of market-based approaches point
out that the environmental records of the Soviet
Union and communist China were often as abysmal
as anything in the capitalist world (Hamilton 2012).
In addition, capital will be required for efforts to
mitigate GHG emissions and adapt to unavoidable
near-term climate change impacts. This additional
capital could be obtained by governments through
increased taxation, but that has limitations, particu-
larly if GDPs and tax bases are declining due to cli-
mate change. The neoliberal position would be that
additional capital can be created by market-driven
economic growth. Proponents of green capitalism
support this latter approach and also point out that
the past comparative economic performance of
tightly controlled economies in Marxist-based social-
ist regimes does not engender confidence in that
path for producing economic growth and additional
economic resources for climate change mitigation
and adaptation. The increase in GDP and decrease
in poverty rates in China following the opening of
the economy to some privatization and a more mar-
ket-based system in 1997 and 1998 stand in contrast
to the precipitous declines witnessed in Venezuela
under the Maduro socialist regime and associated
large-scale nationalization program. In 1997 the per
capita GDP in China was $750 and by 2018 it stood
at $9,470 (World Bank 2019). Over the same period

2022 MacDonald

the percentage of Chinese living in extreme poverty
declined from 42.0 percent to 0.7 percent (World
Bank 2019). In contrast, beginning in the latter
years of President Chavez’s administration and accel-
erating under President Maduro, poverty, malnutri-
tion, infant mortality, and many other indexes
indicate a human catastrophe has evolved disas-
trously (Lynch and Hickey 2019). It must be borne
in mind, however, that China, with its strong cen-
tral control of many macroeconomic and social deci-
sions, and Venezuela, with its economic dependence
on oil and pervasive corruption in the current
regime, are not clear examples of either a pure mar-
ket economy or a functional socialist state, respec-
tively (Maya 2018; Miranda 2018).

Although much more effort is needed, it can be
argued that the largely capitalistic global system has
made some progress over the past decades in address-
ing poverty and related ills within the region of the
Fateful Ellipse (Roser and Ortiz-Ospina 2017; World
Bank 2018). Between 1990 and 2015 the proportion
of the world’s population living in extreme poverty
dropped from 35.9 percent to 10.0 percent (World
Bank 2018). Although extreme poverty rates in
Africa in particular remain unacceptably high, these
have dropped from 58.9 percent in 1993 to 41.1 per-
cent in 2015 (Roser and Ortiz-Ospina 2017). In sub-
Saharan Africa, 33 percent of the population were
undernourished in 1900 to 1992, and this had
declined to 23 percent in 2015 and 2016 (Food and
Agriculture Organization of the United Nations, the
International Fund for Agricultural Development,
and the World Food Programme 2015). Progress has
been made under the current world order, but for
many millions it is still not enough.

The current neoliberal approaches to the global
economy and their potential to confront climate
change and associated economic inequality do not
invite sanguinity. The apparent inability of many
governments to meet emissions targets or enact
meaningful economic and regulatory means to do so
is a troubling indictment of current green capitalism
approaches. The remarkable concentration of wealth
in the hands of private individuals and corporations
and accelerating trend toward economic inequality
discussed earlier removes much of the power of capi-
tal from most citizens and from their governments.
Traditional capitalist economists, weighing in via
Lomborg’s (2010) book Smart Solutions to Climate
Change, have concluded that only geoengineering

and technology transfer provide “very good” and
“good” solutions to climate change. Depending solely
on such solutions as a road map to navigate climate
change might well be considered a course to com-
plete disaster.

Looking beyond competing ideologies, it can be
argued that it is not capitalism in and of itself that
has created anthropogenic climate change and other
current environmental challenges; rather, it is the
drive to increase the growth of techno-industrial
economies. This goal has been embraced by both cap-
italist and Marxist governments and societies over the
twentieth century (Hamilton 2012). The solution to
climate change, and the myriad environmental chal-
lenges facing humanity in the twenty-first century,
might well need to transcend the capitalist–socialist–
Marxist discourse (Wainwright and Mann 2015).
Nevertheless, the arguments about neoliberal green
capitalism versus a socialist or more radical eco-
Marxist approaches to climate change are one of the
most heated ideological conflicts facing us.

Conclusion

It can be concluded that there is no single simple
solution to mitigation of GHG emissions and their
climatic effects. In addition, any meaningful path
will not be cost-free to governments, nor to consum-
ers. Conflicts will continue over the best path(s) for-
ward. As illustrated here, though, we do have at our
disposal a number of tools that might be used in
concert to achieve success (see, e.g., Hawken 2017),
if there is will and international cooperation rather
than debilitating conflict and inaction. Recent anal-
ysis by Rogelj et al. (2018) concluded that tempera-
ture increases could be kept within �1.5 �C by
rapidly shifting away from fossil fuel in favor of
large-scale low-carbon energy supplies, reducing con-
sumer energy use, and applying geoengineering in
the form of CO2 capture and removal. They also
concluded that any such pathways were not possible
under conditions of internationally uncoordinated
and short-term climate policies or high socioeco-
nomic inequalities. Beyond being internationally
coordinated, our way forward must include attention
to social and environmental justice to achieve broad
support. This is required not just to ensure success in
the fight against climate change but also because it
is our moral obligation to the peoples of the Fateful
Ellipse. Calling attention to the ethical onus to

Climate, Capital, Conflict 2023

counter climate change is nothing new, but the
delay in doing so meaningfully is both complex in
its causes and troubling in its magnified impact on
future generations (Gardiner 2006). We cannot ever
forget history and the unequal geographies of the cli-
mate change challenge if we hope to succeed in
tackling this ethically and effectively. This means
that the more affluent nations of the world cannot
forsake their moral obligations to the larger world
population. The importance of basic morality in
framing and building consensus for climate change
policy should not be dismissed or discounted in
terms of being important compelling action. As
Adger, Butler, and Walker-Springett (2017) argued,
“Moral dimensions of public discourse about climate
change give salience and political legitimacy to pol-
icy interventions as well as their processes and out-
comes. Moreover, there is evidence that moral
framings of public policy issues affect engagement
according to political orientation” (372).

The time for action is now. Much of the responsi-
bility to act lies with the peoples of the countries
and regions outside the Fateful Ellipse that have pro-
duced the GHG crisis. We who occupy those regions
have been the largest beneficiaries of the economic
fruits of the GHG world. It has to be accepted that
no realistic path forward is possible without all of us
accepting personal responsibility and some personal
sacrifice. When we take to the streets and demand
governments take action on climate change, we
have to understand and accept that any meaningful
governmental actions will have some impact on us
individually. It is unclear, however, whether people
recognize and are willing to make the personal sacri-
fices required. Kuper (2019), writing in the Financial
Times, put this pointedly: “The only way to prevent
climate catastrophe is ‘degrowth’ now, not 2050:
stop most flying, meat-eating, clothes-buying until
we have green alternatives, ban privately owned cars
and abandon sprawling suburbs.” How many of even
the most ardent climate change activists are willing
to embrace all of these prescriptions for themselves?
If voluntary consumer behavioral change is not a
complete solution, who will people empower to legis-
late and impose required changes in personal lifestyle
at regional to global levels? These questions arise
whether the path ahead lies with effective green
capitalism, environmental democratic socialism, or
eco-Marxism. The most difficult climate change con-
flicts might not be choices between technical

solutions, carbon pricing strategies, or political ideol-
ogies but between our own individual wants and
needs and the needs of the wider planet, including
the Fateful Ellipse. One thing is clear, though: The
longer we wait to resolve these conflicts, the deeper
the required sacrifices are likely to be.

What Is Geography: What Should It Be
in Light of Climate Change?

Let us now turn back to the old question of what
is geography. I believe that our discipline must have
climate change and associated issues of environmen-
tal justice as a central focus. In no other discipline
do we see such breadth of physical sciences, life sci-
ence, social sciences, humanities, and geospatial tech-
niques. The solution to climate change will be
multifaceted, and all of these perspectives are
required. As geographers, we are also well aware of
the complexity and uncertainty such an effort
engenders (Winkler 2016). The spirited engagement
of critical social geographers with climate change,
however, provides one example of taking up the task
(Darly 2013, 2015; Swyngedouw 2013; Wainwright
and Mann 2015; Surprise 2018). To succeed, our
efforts must be predicated on pointing out and
addressing the unequal geographies of climate
change and capital (Sheppard 2015). This is the
time for geographers to be at the forefront and to
make good on our long-held claims that the multi-
faceted geographical perspective is of fundamental
importance. In doing so we can also create a nexus
that draws together the diverse elements of geogra-
phy into a more cogent whole.

Climate change also presents a moral obligation
that is particularly incumbent on geography. The
growth of the modern discipline of geography in the
eighteenth through early twentieth centuries did so
in collaboration with the colonial enterprise (Driver
1992; Clayton 2020). The age of explorations, map-
making, and the florescence of geography was part
and parcel of colonial expansion. The development
of the modern academic world, and our discipline
along with it, was supported by the financial fruits of
the Industrial Revolution and the GHG world it has
created. Colonial exploitation and GHG effects that
are so severely afflicting that Fateful Ellipse today
are part of our legacy as geographers. We as geogra-
phers have a debt to repay.

2024 MacDonald

The threats of climate, capital, and conflict are
pressing; the trajectories frightening. Time is short.
Geography is a key element for success in tackling
these challenges. It is often stated that “geography is
what geographers do.” Given the importance of
geography to combating climate change, and our
obligation as geographers to do so, we might also
define our discipline and organize research, educa-
tional, and advocacy efforts around what we can and
must do as geographers in the face of this global
threat: Geography is what must be done.

Coda: May 2020

When I delivered my address at New Orleans and
wrote the subsequent article at St. Andrews, I could
not have imagined the world upended by the
COVID-19 pandemic. I now find it impossible to
consider climate change without reference to what
has been revealed by the pandemic. Like climate
change, the pandemic is a global crisis. At this time
more than 180 countries and territories have been
afflicted, with almost 3 million confirmed cases and
more than 200,000 known deaths. Factory produc-
tion, shop sales, food and service industries, and the
transportation sector are declining precipitously
under stay-at-home orders. In response, unemploy-
ment rates are skyrocketing. The World Trade
Organization projects a global trade contraction of
13 percent to 32 percent. All of this is reflected in
declining carbon emissions. Some early estimates
provided by Carbon Brief (Evans 2020) suggest the
equivalent of a 5.5 percent decline in CO2 emis-
sions, the largest emissions decline in modern times.
These are all early data and estimates and will be
revised as the pandemic runs its course. It is sober-
ing, but important, to realize that at this point the
decrease in carbon emissions produced by the cur-
rent economic pain and surrendering of personal
freedoms is likely still less than the annual reduc-
tions needed by 2030 to avoid a warming of 1.5 �C
or greater based on recent IPCC assessments. This
underscores how steep the road ahead is in the bat-
tle against climate change.

Consider human responses to the pandemic and
the insights they provide. There have been many
denials of the threat posed by COVID-19. These
denials of science are distressingly similar to what cli-
mate scientists often face. Unlike climate change,
though, which is comparatively slow and against

which wealthy nations have a degree of economic
insulation, the mounting COVID-19 cases, hospital-
izations, and deaths have provided inarguable evi-
dence of an immediate crisis. Throughout the world
many countries have responded responsibly and fol-
lowed scientifically informed guidance on limiting the
spread of the virus. Elsewhere, though, including in
the United States, there is also much disarray.
Testing has been uneven. The U.S. government
has withdrawn funding from the World Health
Organization and, similar to climate change, has abro-
gated its leadership role nationally and internationally
in combating the pandemic. Some jurisdictions are
competing aggressively with others to secure scarce
medical supplies. There have been increasing numbers
of street protests against the measures enacted to curb
the spread of the virus. In the United States some
protestors and politicians have made it clear that
they are less concerned about other people’s health
and well-being than they are about their own per-
sonal indulgences and economic position. If denial,
disarray, and selfishness become the overwhelming
responses to the jarringly evident COVID-19 pan-
demic, what hope is there for any significant actions
to avoid the seemingly less proximal climate change
catastrophe? On the other hand, there has also been
a great focusing of scientific effort and many altruistic
acts by the medical community, essential workers,
volunteers, and others who have stepped up, and in
cases sacrificed personally, to end or blunt the
impacts of the pandemic. It is hoped that these ulti-
mately will be the role models from whom humanity
takes it lead and by which we move forward in
response to climate change.

Acknowledgments

I express my profound gratitude to the member-
ship of the American Association of Geographers for
electing me president and providing me the opportu-
nity of presenting this address. The resulting article
was written during my tenure as a Global Visiting
Fellow at the University of St. Andrews, Scotland. I
thank the members of the University, my colleagues
in the School of Geography and Sustainable
Development, and the Head of the School, Professor
Keith Bennett MRIA, for support and hospitality. I
thank Daniel Clayton of St. Andrews and Eric
Sheppard of UCLA for their generous time and
thoughtful comments on an earlier draft of the

Climate, Capital, Conflict 2025

article. I also thank the Reverend Dr. Donald
MacEwan, Chaplain to the University, and the
Turning Pages study group at St. Andrews for fellow-
ship and much useful discussion of an earlier draft of
the article. David Butler provided a careful reading
and many appreciated edits.

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Climate, Capital, Conflict 2031

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https://doi.org/10.3189/2015JoG15J017

Abstract

The Steepening Slope: Trajectories of Global Anthropogenic Climate Change
Trajectories of Capital and Inequality
Geographies of Climate Change and Capital
Climate, Capital, and Conflict: The Battle of Ideas, Policy, and Ideology
Geoengineering Approaches
Technological Substitution Approaches
Consumer Behavior Approaches
Economic Reform Approaches
Societal Change Approaches
Conclusion
What Is Geography: What Should It Be in Light of Climate Change?
Coda: May 2020
Acknowledgments
References

Summary Findings

Fourth National Climate AssessmentU.S. Global Change Research Program 25

NCA4 Summary Findings
These Summary Findings represent a high-level synthesis of the material in the underlying
report. The findings consolidate Key Messages and supporting evidence from 16 national-level
topic chapters, 10 regional chapters, and 2 chapters that focus on societal response strategies
(mitigation and adaptation). Unless otherwise noted, qualitative statements regarding future
conditions in these Summary Findings are broadly applicable across the range of different
levels of future climate change and associated impacts considered in this report.

1. Communities

Climate change creates new risks and exacerbates existing vulnerabilities in communities across
the United States, presenting growing challenges to human health and safety, quality of life, and
the rate of economic growth.

The impacts of climate change are already
being felt in communities across the country.
More frequent and intense extreme weather
and climate-related events, as well as changes
in average climate conditions, are expected to
continue to damage infrastructure, ecosystems,
and social systems that provide essential ben-
efits to communities. Future climate change
is expected to further disrupt many areas of
life, exacerbating existing challenges to pros-
perity posed by aging and deteriorating infra-
structure, stressed ecosystems, and economic
inequality. Impacts within and across regions

will not be distributed equally. People who are
already vulnerable, including lower-income and
other marginalized communities, have lower
capacity to prepare for and cope with extreme
weather and climate-related events and are ex-
pected to experience greater impacts. Prioritiz-
ing adaptation actions for the most vulnerable
populations would contribute to a more equi-
table future within and across communities.
Global action to significantly cut greenhouse
gas emissions can substantially reduce cli-
mate-related risks and increase opportunities
for these populations in the longer term.

2. Economy

Without substantial and sustained global mitigation and regional adaptation efforts, climate
change is expected to cause growing losses to American infrastructure and property and impede
the rate of economic growth over this century.

In the absence of significant global mitigation
action and regional adaptation efforts, rising
temperatures, sea level rise, and changes in
extreme events are expected to increasingly
disrupt and damage critical infrastructure and
property, labor productivity, and the vitality
of our communities. Regional economies and
industries that depend on natural resourc-
es and favorable climate conditions, such as

agriculture, tourism, and fisheries, are vulner-
able to the growing impacts of climate change.
Rising temperatures are projected to reduce
the efficiency of power generation while in-
creasing energy demands, resulting in higher
electricity costs. The impacts of climate change
beyond our borders are expected to increas-
ingly affect our trade and economy, including
import and export prices and U.S. businesses

Summary Findings

Fourth National Climate AssessmentU.S. Global Change Research Program 26

with overseas operations and supply chains.
Some aspects of our economy may see slight
near-term improvements in a modestly warmer
world. However, the continued warming that
is projected to occur without substantial and
sustained reductions in global greenhouse gas
emissions is expected to cause substantial net
damage to the U.S. economy throughout this

century, especially in the absence of increased
adaptation efforts. With continued growth in
emissions at historic rates, annual losses in
some economic sectors are projected to reach
hundreds of billions of dollars by the end of the
century—more than the current gross domestic
product (GDP) of many U.S. states.

3. Interconnected Impacts

Climate change affects the natural, built, and social systems we rely on individually and through
their connections to one another. These interconnected systems are increasingly vulnerable to
cascading impacts that are often difficult to predict, threatening essential services within and
beyond the Nation’s borders.

Climate change presents added risks to inter-
connected systems that are already exposed
to a range of stressors such as aging and de-
teriorating infrastructure, land-use changes,
and population growth. Extreme weather and
climate-related impacts on one system can re-
sult in increased risks or failures in other crit-
ical systems, including water resources, food
production and distribution, energy and trans-
portation, public health, international trade,
and national security. The full extent of climate
change risks to interconnected systems, many

of which span regional and national boundaries,
is often greater than the sum of risks to individ-
ual sectors. Failure to anticipate interconnected
impacts can lead to missed opportunities for
effectively managing the risks of climate change
and can also lead to management responses
that increase risks to other sectors and regions.
Joint planning with stakeholders across sec-
tors, regions, and jurisdictions can help identify
critical risks arising from interaction among
systems ahead of time.

4. Actions to Reduce Risks

Communities, governments, and businesses are working to reduce risks from and costs asso-
ciated with climate change by taking action to lower greenhouse gas emissions and implement
adaptation strategies. While mitigation and adaptation efforts have expanded substantially in
the last four years, they do not yet approach the scale considered necessary to avoid substantial
damages to the economy, environment, and human health over the coming decades.

Future risks from climate change depend
primarily on decisions made today. The inte-
gration of climate risk into decision-making
and the implementation of adaptation activities
have significantly increased since the Third
National Climate Assessment in 2014, including

in areas of financial risk reporting, capital in-
vestment planning, development of engineering
standards, military planning, and disaster risk
management. Transformations in the ener-
gy sector—including the displacement of coal
by natural gas and increased deployment of

Summary Findings

Fourth National Climate AssessmentU.S. Global Change Research Program 27

renewable energy—along with policy actions
at the national, regional, state, and local lev-
els are reducing greenhouse gas emissions in
the United States. While these adaptation and
mitigation measures can help reduce damages
in a number of sectors, this assessment shows
that more immediate and substantial global
greenhouse gas emissions reductions, as well as
regional adaptation efforts, would be needed to

avoid the most severe consequences in the long
term. Mitigation and adaptation actions also
present opportunities for additional benefits
that are often more immediate and localized,
such as improving local air quality and econ-
omies through investments in infrastructure.
Some benefits, such as restoring ecosystems
and increasing community vitality, may be
harder to quantify.

5. Water

The quality and quantity of water available for use by people and ecosystems across the country
are being affected by climate change, increasing risks and costs to agriculture, energy production,
industry, recreation, and the environment.

Rising air and water temperatures and chang-
es in precipitation are intensifying droughts,
increasing heavy downpours, reducing snow-
pack, and causing declines in surface water
quality, with varying impacts across regions.
Future warming will add to the stress on water
supplies and adversely impact the availability
of water in parts of the United States. Changes
in the relative amounts and timing of snow and
rainfall are leading to mismatches between wa-
ter availability and needs in some regions, pos-
ing threats to, for example, the future reliability
of hydropower production in the Southwest
and the Northwest. Groundwater depletion is
exacerbating drought risk in many parts of the
United States, particularly in the Southwest and

Southern Great Plains. Dependable and safe
water supplies for U.S. Caribbean, Hawai‘i, and
U.S.-Affiliated Pacific Island communities are
threatened by drought, flooding, and saltwater
contamination due to sea level rise. Most U.S.
power plants rely on a steady supply of water
for cooling, and operations are expected to be
affected by changes in water availability and
temperature increases. Aging and deteriorating
water infrastructure, typically designed for past
environmental conditions, compounds the cli-
mate risk faced by society. Water management
strategies that account for changing climate
conditions can help reduce present and future
risks to water security, but implementation of
such practices remains limited.

6. Health

Impacts from climate change on extreme weather and climate-related events, air quality, and the
transmission of disease through insects and pests, food, and water increasingly threaten the
health and well-being of the American people, particularly populations that are already vulnerable.

Changes in temperature and precipitation are
increasing air quality and health risks from
wildfire and ground-level ozone pollution.
Rising air and water temperatures and more

intense extreme events are expected to in-
crease exposure to waterborne and foodborne
diseases, affecting food and water safety. With
continued warming, cold-related deaths are

Summary Findings

Fourth National Climate AssessmentU.S. Global Change Research Program 28

projected to decrease and heat-related deaths
are projected to increase; in most regions,
increases in heat-related deaths are expected
to outpace reductions in cold-related deaths.
The frequency and severity of allergic ill-
nesses, including asthma and hay fever, are
expected to increase as a result of a changing
climate. Climate change is also projected to
alter the geographic range and distribution of
disease-carrying insects and pests, exposing
more people to ticks that carry Lyme disease
and mosquitoes that transmit viruses such
as Zika, West Nile, and dengue, with varying
impacts across regions. Communities in the
Southeast, for example, are particularly vul-
nerable to the combined health impacts from

vector-borne disease, heat, and flooding. Ex-
treme weather and climate-related events can
have lasting mental health consequences in af-
fected communities, particularly if they result
in degradation of livelihoods or community
relocation. Populations including older adults,
children, low-income communities, and some
communities of color are often dispropor-
tionately affected by, and less resilient to, the
health impacts of climate change. Adaptation
and mitigation policies and programs that help
individuals, communities, and states prepare
for the risks of a changing climate reduce the
number of injuries, illnesses, and deaths from
climate-related health outcomes.

7. Indigenous Peoples

Climate change increasingly threatens Indigenous communities’ livelihoods, economies, health,
and cultural identities by disrupting interconnected social, physical, and ecological systems.

Many Indigenous peoples are reliant on nat-
ural resources for their economic, cultural,
and physical well-being and are often unique-
ly affected by climate change. The impacts of
climate change on water, land, coastal areas,
and other natural resources, as well as infra-
structure and related services, are expected to
increasingly disrupt Indigenous peoples’ liveli-
hoods and economies, including agriculture and
agroforestry, fishing, recreation, and tourism.
Adverse impacts on subsistence activities have
already been observed. As climate changes con-
tinue, adverse impacts on culturally significant
species and resources are expected to result
in negative physical and mental health effects.
Throughout the United States, climate-related

impacts are causing some Indigenous peoples
to consider or actively pursue community re-
location as an adaptation strategy, presenting
challenges associated with maintaining cultural
and community continuity. While economic,
political, and infrastructure limitations may
affect these communities’ ability to adapt,
tightly knit social and cultural networks present
opportunities to build community capacity and
increase resilience. Many Indigenous peoples
are taking steps to adapt to climate change
impacts structured around self-determination
and traditional knowledge, and some tribes are
pursuing mitigation actions through develop-
ment of renewable energy on tribal lands.

Summary Findings

Fourth National Climate AssessmentU.S. Global Change Research Program 29

8. Ecosystems and Ecosystem Services

Ecosystems and the benefits they provide to society are being altered by climate change, and
these impacts are projected to continue. Without substantial and sustained reductions in global
greenhouse gas emissions, transformative impacts on some ecosystems will occur; some coral
reef and sea ice ecosystems are already experiencing such transformational changes.

Many benefits provided by ecosystems and the
environment, such as clean air and water, pro-
tection from coastal flooding, wood and fiber,
crop pollination, hunting and fishing, tourism,
cultural identities, and more will continue to
be degraded by the impacts of climate change.
Increasing wildfire frequency, changes in insect
and disease outbreaks, and other stressors are
expected to decrease the ability of U.S. for-
ests to support economic activity, recreation,
and subsistence activities. Climate change has
already had observable impacts on biodiversity,
ecosystems, and the benefits they provide to
society. These impacts include the migration
of native species to new areas and the spread
of invasive species. Such changes are project-
ed to continue, and without substantial and
sustained reductions in global greenhouse
gas emissions, extinctions and transformative

impacts on some ecosystems cannot be avoid-
ed in the long term. Valued aspects of regional
heritage and quality of life tied to ecosystems,
wildlife, and outdoor recreation will change
with the climate, and as a result, future gener-
ations can expect to experience and interact
with the natural environment in ways that are
different from today. Adaptation strategies,
including prescribed burning to reduce fuel for
wildfire, creation of safe havens for important
species, and control of invasive species, are
being implemented to address emerging im-
pacts of climate change. While some targeted
response actions are underway, many impacts,
including losses of unique coral reef and sea ice
ecosystems, can only be avoided by significant-
ly reducing global emissions of carbon dioxide
and other greenhouse gases.

9. Agriculture and Food

Rising temperatures, extreme heat, drought, wildfire on rangelands, and heavy downpours are
expected to increasingly disrupt agricultural productivity in the United States. Expected increas-
es in challenges to livestock health, declines in crop yields and quality, and changes in extreme
events in the United States and abroad threaten rural livelihoods, sustainable food security, and
price stability.

Climate change presents numerous challenges
to sustaining and enhancing crop productivity,
livestock health, and the economic vitality of
rural communities. While some regions (such
as the Northern Great Plains) may see con-
ditions conducive to expanded or alternative
crop productivity over the next few decades,
overall, yields from major U.S. crops are expect-
ed to decline as a consequence of increases in

temperatures and possibly changes in water
availability, soil erosion, and disease and pest
outbreaks. Increases in temperatures during
the growing season in the Midwest are pro-
jected to be the largest contributing factor to
declines in the productivity of U.S. agriculture.
Projected increases in extreme heat conditions
are expected to lead to further heat stress for
livestock, which can result in large economic

Summary Findings

Fourth National Climate AssessmentU.S. Global Change Research Program 30

losses for producers. Climate change is also ex-
pected to lead to large-scale shifts in the avail-
ability and prices of many agricultural products
across the world, with corresponding impacts
on U.S. agricultural producers and the U.S.
economy. These changes threaten future gains
in commodity crop production and put rural
livelihoods at risk. Numerous adaptation strate-
gies are available to cope with adverse impacts

of climate variability and change on agricultural
production. These include altering what is pro-
duced, modifying the inputs used for produc-
tion, adopting new technologies, and adjusting
management strategies. However, these strat-
egies have limits under severe climate change
impacts and would require sufficient long- and
short-term investment in changing practices.

10. Infrastructure

Our Nation’s aging and deteriorating infrastructure is further stressed by increases in heavy pre-
cipitation events, coastal flooding, heat, wildfires, and other extreme events, as well as changes
to average precipitation and temperature. Without adaptation, climate change will continue to de-
grade infrastructure performance over the rest of the century, with the potential for cascading im-
pacts that threaten our economy, national security, essential services, and health and well-being.

Climate change and extreme weather events
are expected to increasingly disrupt our Na-
tion’s energy and transportation systems,
threatening more frequent and longer-lasting
power outages, fuel shortages, and service
disruptions, with cascading impacts on oth-
er critical sectors. Infrastructure currently
designed for historical climate conditions is
more vulnerable to future weather extremes
and climate change. The continued increase in
the frequency and extent of high-tide flooding
due to sea level rise threatens America’s tril-
lion-dollar coastal property market and public
infrastructure, with cascading impacts to the
larger economy. In Alaska, rising temperatures
and erosion are causing damage to buildings
and coastal infrastructure that will be costly
to repair or replace, particularly in rural areas;
these impacts are expected to grow without

adaptation. Expected increases in the severity
and frequency of heavy precipitation events
will affect inland infrastructure in every region,
including access to roads, the viability of bridg-
es, and the safety of pipelines. Flooding from
heavy rainfall, storm surge, and rising high tides
is expected to compound existing issues with
aging infrastructure in the Northeast. Increased
drought risk will threaten oil and gas drilling
and refining, as well as electricity generation
from power plants that rely on surface water
for cooling. Forward-looking infrastructure
design, planning, and operational measures and
standards can reduce exposure and vulnerabil-
ity to the impacts of climate change and reduce
energy use while providing additional near-
term benefits, including reductions in green-
house gas emissions.

Summary Findings

Fourth National Climate AssessmentU.S. Global Change Research Program 31

11. Oceans and Coasts

Coastal communities and the ecosystems that support them are increasingly threatened by the
impacts of climate change. Without significant reductions in global greenhouse gas emissions
and regional adaptation measures, many coastal regions will be transformed by the latter part of
this century, with impacts affecting other regions and sectors. Even in a future with lower green-
house gas emissions, many communities are expected to suffer financial impacts as chronic
high-tide flooding leads to higher costs and lower property values.

Rising water temperatures, ocean acidification,
retreating arctic sea ice, sea level rise, high-tide
flooding, coastal erosion, higher storm surge,
and heavier precipitation events threaten our
oceans and coasts. These effects are projected
to continue, putting ocean and marine species
at risk, decreasing the productivity of certain
fisheries, and threatening communities that
rely on marine ecosystems for livelihoods and
recreation, with particular impacts on fishing
communities in Hawai‘i and the U.S.-Affiliated
Pacific Islands, the U.S. Caribbean, and the Gulf
of Mexico. Lasting damage to coastal property
and infrastructure driven by sea level rise and
storm surge is expected to lead to financial
losses for individuals, businesses, and commu-
nities, with the Atlantic and Gulf Coasts facing
above-average risks. Impacts on coastal energy
and transportation infrastructure driven by sea
level rise and storm surge have the potential

for cascading costs and disruptions across the
country. Even if significant emissions reduc-
tions occur, many of the effects from sea level
rise over this century—and particularly through
mid-century—are already locked in due to his-
torical emissions, and many communities are
already dealing with the consequences. Actions
to plan for and adapt to more frequent, wide-
spread, and severe coastal flooding, such as
shoreline protection and conservation of coast-
al ecosystems, would decrease direct losses and
cascading impacts on other sectors and parts
of the country. More than half of the damages
to coastal property are estimated to be avoid-
able through well-timed adaptation measures.
Substantial and sustained reductions in global
greenhouse gas emissions would also signifi-
cantly reduce projected risks to fisheries and
communities that rely on them.

12. Tourism and Recreation

Outdoor recreation, tourist economies, and quality of life are reliant on benefits provided by our
natural environment that will be degraded by the impacts of climate change in many ways.

Climate change poses risks to seasonal and
outdoor economies in communities across the
United States, including impacts on economies
centered around coral reef-based recreation,
winter recreation, and inland water-based
recreation. In turn, this affects the well-being
of the people who make their living supporting
these economies, including rural, coastal, and
Indigenous communities. Projected increases

in wildfire smoke events are expected to impair
outdoor recreational activities and visibility
in wilderness areas. Declines in snow and ice
cover caused by warmer winter temperatures
are expected to negatively impact the winter
recreation industry in the Northwest, North-
ern Great Plains, and the Northeast. Some
fish, birds, and mammals are expected to shift
where they live as a result of climate change,

Summary Findings

Fourth National Climate AssessmentU.S. Global Change Research Program 32

with implications for hunting, fishing, and other
wildlife-related activities. These and other cli-
mate-related impacts are expected to result in
decreased tourism revenue in some places and,
for some communities, loss of identity. While
some new opportunities may emerge from
these ecosystem changes, cultural identities
and economic and recreational opportunities

based around historical use of and interaction
with species or natural resources in many areas
are at risk. Proactive management strategies,
such as the use of projected stream tempera-
tures to set priorities for fish conservation, can
help reduce disruptions to tourist economies
and recreation.

Unit 12 : Earth’s Changing Climate -1- www.learner.org

Unit 12 : Earth’s Changing Climate

Glaciologists. Courtesy of Lonnie Thompson

Overview
Earth’s climate is a sensitive system that is subject to
dramatic shifts over varying time scales. Today human
activities are altering the climate system by increasing
concentrations of heat-trapping greenhouse gases in the
atmosphere, which raises global temperatures. In this unit,
examine the science behind global climate change and
explore its potential impacts on natural ecosystems and
human societies.

Sections:

1. Introduction

2. Tipping Earth’s Energy Balance

3. Climate Change: What the Past Tells Us

4. Past Warming: The Eocene Epoch

5. Global Cooling: The Pleistocene Epoch

6. Present Warming and the Role of CO2

7. Observed Impacts of Climate Change

8. Other Potential Near-Term Impacts

9. Major Laws and Treaties

10. Further Reading

Unit 12 : Earth’s Changing Climate -2- www.learner.org

1. Introduction

For the past 150 years, humans have been performing an unprecedented experiment on Earth’s
climate. Human activities, mainly fossil fuel combustion, are increasing concentrations of greenhouse
gases (GHGs) in the atmosphere. These gases are trapping infrared radiation emitted from the
planet’s surface and warming the Earth. Global average surface temperatures have risen about 0.7°C
(1.4°F) since the early 20th century.

Earth’s climate is a complex system that is constantly changing, but the planet is warmer today than
it has been for thousands of years, and current atmospheric carbon dioxide (CO2) levels have not
been equaled for millions of years. As we will see below, ancient climate records offer some clues
about how a warming world may behave. They show that climate shifts may not be slow and steady;
rather, temperatures may change by many degrees within a few decades, with drastic impacts on
plant and animal life and natural systems. And if CO2 levels continue to rise at projected rates, history
suggests that the world will become drastically hotter than it is today, possibly hot enough to melt
much of Earth’s existing ice cover. Figure 1 depicts projected surface temperature changes through
2060 as estimated by NASA’s Global Climate Model.

Figure 1. Surface air temperature increase, 1960 to 2060

Past climate changes were driven by many different types of naturally-occurring events, from
variations in Earth’s orbit to volcanic eruptions. Since the start of the industrial age, human activities

Unit 12 : Earth’s Changing Climate -3- www.learner.org

have become a larger influence on Earth’s climate than other natural factors. High CO2 levels
(whether caused by natural phenomena or human activities) are a common factor between many past
climate shifts and the warming we see today.

Many aspects of climate change, such as exactly how quickly and steadily it will progress, remain
uncertain. However, there is strong scientific consensus that current trends in GHG emissions will
cause substantial warming by the year 2100, and that this warming will have widespread impacts
on human life and natural ecosystems. Many impacts have already been observed, including higher
global average temperatures, rising sea levels (water expands as it warms), and changes in snow
cover and growing seasons in many areas.

A significant level of warming is inevitable due to GHG emissions that have already been released,
but we have options to limit the scope of future climate change—most importantly, by reducing fossil
fuel consumption (for more details, see Unit 10, “Energy Challenges”). Other important steps to
mitigate global warming include reducing the rate of global deforestation to preserve forest carbon
sinks and finding ways to capture and sequester carbon dioxide emissions instead of releasing
them to the atmosphere. (These responses are discussed in Unit 13, “Looking Forward: Our Global
Experiment.”)

2. Tipping Earth’s Energy Balance

Earth’s climate is a dynamic system that is driven by energy from the sun and constantly impacted by
physical, biological, and chemical interactions between the atmosphere, global water supplies, and
ecosystems (Fig. 2).

Unit 12 : Earth’s Changing Climate -4- www.learner.org

Figure 2. Components and interactions of the global climate system

As discussed in Unit 2, “Atmosphere,” energy reaches Earth in the form of solar radiation from the
sun. Water vapor, clouds, and other heat-trapping gases create a natural greenhouse effect by
holding heat in the atmosphere and preventing its release back to space. In response, the planet’s
surface warms, increasing the heat emitted so that the energy released back from Earth into space
balances what the Earth receives as visible light from the sun (Fig. 3). Today, with human activities
boosting atmospheric GHG concentrations, the atmosphere is retaining an increasing fraction of
energy from the sun, raising earth’s surface temperature. This extra impact from human activities is
referred to as anthropogenic climate change.

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Figure 3. Earth’s energy balance

Many GHGs, including water vapor, ozone, CO2, methane (CH4), and nitrous oxide (N2O), are
present naturally. Others are synthetic chemicals that are emitted only as a result of human activity,
such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and
sulfur hexafluoride (SF6). Important human activities that are raising atmospheric GHG concentrations
include:

• fossil fuel combustion (CO and small quantities of methane and NO);

• deforestation (CO releases from forest burning, plus lower forest carbon uptake);

• landfills (methane) and wastewater treatment (methane, NO);

Unit 12 : Earth’s Changing Climate -6- www.learner.org

• livestock production (methane, NO);

• rice cultivation (methane);

• fertilizer use (NO); and

• industrial processes (HFCs, PFCs, SF).

Measuring CO2 levels at Mauna Loa, Hawaii, and other pristine air locations, climate scientist Charles
David Keeling traced a steady rise in CO2 concentrations from less than 320 parts per million (ppm)
in the late 1950s to 380 ppm in 2005 (Fig. 4). Yearly oscillations in the curve reflect seasonal cycles
in the northern hemisphere, which contains most of Earth’s land area. Plants take up CO2 during the
growing season in spring and summer and then release it as they decay in fall and winter.

Figure 4. Atmospheric CO2 concentrations, 1958–2005

Global CO2 concentrations have increased by one-third from their pre-industrial levels, rising from
280 parts per million before the year 1750 to 377 ppm today. Levels of methane and N2O, the most
influential GHGs after CO2, also increased sharply in the same time period (see Table 1 below).

If there are so many GHGs, why does CO2 get most of the attention? The answer is a combination
of CO2’s abundance and its residence time in the atmosphere. CO2 accounts for about 0.1 percent of

Unit 12 : Earth’s Changing Climate -7- www.learner.org

the atmosphere, substantially more than all other GHGs except for water vapor, which may comprise
up to 7 percent depending on local conditions. However, water vapor levels vary constantly because
so much of the Earth’s surface is covered by water and water vapor cycles into and out of the
atmosphere very quickly—usually in less than 10 days. Therefore, water vapor can be considered a
feedback that responds to the levels of other greenhouse gases, rather than an independent climate
forcing (footnote 1).

Other GHGs contribute more to global climate change than CO2 on a per-unit basis, although their
relative impacts vary with time. The global warming potential (GWP) of a given GHG expresses
its estimated climate impact over a specific period of time compared to an equivalent amount by
weight of carbon dioxide. For example, the current 100-year GWP for N2O is 296, which indicates
that one ton of N2O will have the same global warming effect over 100 years as 296 tons of CO2.
Internationally-agreed GWP values are periodically adjusted to reflect current research on GHGs’
behavior and impacts in the atmosphere.

However, CO2 is still the most important greenhouse gas because it is emitted in far larger quantities
than other GHGs. Atmospheric concentrations of CO2 are measured in parts per million, compared
to parts per billion or per trillion of other gases, and CO2’s atmospheric lifetime is 50 to 200 years,
significantly longer than most GHGs. As illustrated in Table 1, the total extent to which CO2 has
raised global temperature (referred to as radiative forcing and measured in watts per square meter)
since 1750 is significantly larger than forcing from other gases.

Table 1. Current greenhouse gas concentrations.

Gas Pre-1750
concentration

Current
tropospheric
concentration

100-year GWP Atmospheric
lifetime (years)

Increased
radiative
forcing

(watts/meter)

Carbon dioxide 280 parts per
million

377.3 parts per
million

1 Variable (up to
200 years)

1.66

Methane 688-730 parts
per billion

1,730-1,847
parts per billion

23 12 0.5

Nitrous oxide 270 parts per
billion

318-319 parts
per billion

296 114 0.16

Tropospheric
ozone

25 34 Not applicable
due to short
residence time

Hours to days 0.35

Unit 12 : Earth’s Changing Climate -8- www.learner.org

Gas Pre-1750
concentration
Current
tropospheric
concentration
100-year GWP Atmospheric
lifetime (years)
Increased
radiative
forcing
(watts/meter)

Industrial
gases (HFCs,
PFCs, halons)

0 Up to 545 parts
per trillion

Ranges from
140 to 12,000

Primarily
between 5 and
260 years

0.34 for all
halocarbons
collectively

Sulfur
hexafluoride

0 5.22 parts per
trillion

22,200 3,200 0.002

A look at current emissions underlines the importance of CO2. In 2003 developed countries emitted
11.6 billion metric tons of CO2, nearly 83 percent of their total GHG emissions. Developing countries’
reported emissions were smaller in absolute terms, but CO2 accounted for a similarly large share
of their total GHG output (footnote 2). In 2004, CO2 accounted for 85 percent of total U.S. GHG
emissions, compared to 7.8 percent from methane, 5.4 percent from N2O, and 2 percent from
industrial GHGs (footnote 3).

These emissions from human activities may reshape the global carbon cycle. As discussed in Units
2 (“Atmosphere”) and 3 (“Oceans”), roughly 60 percent of CO2 emissions from fossil fuel burning
remain in the atmosphere, with about half of the remaining 40 percent absorbed by the oceans and
half by terrestrial ecosystems. However, there are limits to the amount of anthropogenic carbon that
these sinks can take up. Oceans are constrained by the rate of mixing between upper and lower
layers, and there are physical bounds on plants’ ability to increase their photosynthesis rates as
atmospheric CO2 levels rise and the world warms.

Scientists are still trying to estimate how much carbon these sinks can absorb, but it appears clear
that oceans and land sinks cannot be relied on to absorb all of the extra CO2 emissions that are
projected in the coming century. This issue is central to projecting future impacts of climate change
because emissions that end up in the atmosphere, rather than being absorbed by land or ocean
sinks, warm the earth.

3. Climate Change: What the Past Tells Us

Throughout much of its 4.5 billion year history, Earth’s climate has alternated between periods of
warmth and relative cold, each lasting for tens to hundreds of millions of years. During the warmest
periods, the polar regions of the world were completely free of ice. Earth also has experienced
repeated ice ages—periods lasting for millions of years, during which ice sheets advanced and
retreated many times over portions of the globe. During the most extreme cold phases, snow and ice
covered the entire globe (for more details, see Unit 1, “Many Planets, One Earth”).

Unit 12 : Earth’s Changing Climate -9- www.learner.org

From the perspective of geological time our planet is currently passing through a relatively cold phase
in its history and has been cooling for the past 35 million years, a trend that is only one of many
swings between hot and cold states over the last 500 million years. During cold phases glaciers and
snow cover have covered much of the mid-latitudes; in warm phases, forests extended all the way to
the poles (Fig. 5).

Figure 5. Ice sheet advance during the most recent ice age

Courtesy National Oceanic and Atmospheric Administration Paleoclimatology Program.

Scientists have analyzed paleoclimate records from many regions of the world to document Earth’s
climate history. Important sources of information about past climate shifts include:

• Mineral deposits in deep sea beds. Over time, dissolved shells of microscopic marine
organisms create layers of chalk and limestone on sea beds. Analyzing the ratio of
oxygen-18 (a rare isotope) to oxygen-16 (the common form) indicates whether the shells
were formed during glacial periods, when more of the light isotope evaporated and rained
down, or during warm periods.

• Pollen grains trapped in terrestrial soils. Scientists use radiocarbon dating to determine
what types of plants lived in the sampled region at the time each layer was formed. Changes
in vegetation reflect surface temperature changes.

Unit 12 : Earth’s Changing Climate -10- www.learner.org

• Chemical variations in coral reefs. Coral reefs grow very slowly over hundreds or
thousands of years. Analyzing their chemical composition and determining the time at which
variations in corals’ makeup occurred allows scientists to create records of past ocean
temperatures and climate cycles.

• Core samples from polar ice fields and high-altitude glaciers. The layers created in ice
cores by individual years of snowfall, which alternate with dry-season deposits of pollen and
dust, provide physical timelines of glacial cycles. Air bubbles in the ice can be analyzed to
measure atmospheric CO levels at the time the ice was laid down.

Understanding the geological past is key to today’s climate change research for several reasons.
First, as the next sections will show, Earth’s climate history illustrates how changing GHG levels
and temperatures in the past shaped climate systems and affected conditions for life. Second,
researchers use past records to tune climate models and see whether they are accurately estimating
dynamics like temperature increase and climate feedbacks. The more closely a model can replicate
past climate conditions, the more accurate its future predictions are likely to be.

4. Past Warming: The Eocene Epoch

Scientists have looked far back in time to find a period when atmospheric GHG concentrations were
as high as they could rise in coming decades if current emission trends continue. The Eocene epoch,
which lasted from 55 million to 38 million years ago, was the most recent time when scientists think
that CO2 was higher than 500 parts per million.

Fossil evidence shows that Earth was far warmer during the Eocene than it is now. Tropical trees
grew over much larger ranges to the north and south than they occupy today. Palm trees grew as far
north as Wyoming and crocodiles swam in warm ocean water off Greenland. Early forms of modern
mammals appeared, including small creatures such as cat-sized horses whose size made them well
adapted to a warm climate (Fig. 6). Without ice cover at the poles, sea levels were nearly 100 meters
higher than today. The deep ocean, which today is near freezing, warmed to over 12°C.

Unit 12 : Earth’s Changing Climate -11- www.learner.org

Figure 6. Phenacodus, a sheep-sized herbivore found in the Eocene era

Courtesy Wikimedia Commons. Public Domain.

Scientists cannot measure CO2 levels during the Eocene—there are no ice cores because there is
no ice this old—but from indirect measurements of ocean chemistry they estimate that atmospheric
CO2 levels were three to ten times higher than pre-industrial levels (280 ppm). These concentrations
were probably related to a sustained increase in CO2 released from volcanoes over tens of millions
of years. Because this climate persisted for tens of millions of years, living species and the climate
system had time to adapt to warm, moist conditions. If humans release enough GHGs into the
atmosphere to create Eocene-like conditions in the next several centuries, the transition will be much
more abrupt, and many living organisms—especially those that thrive in cold conditions—will have
trouble surviving the shift.

A troubling lesson from the Eocene is that scientists are unable to simulate Eocene climate conditions
using climate models designed for the modern climate. When CO2 levels are raised in the computer
models to levels appropriate for what scientists think existed during the Eocene, global temperatures
rise but high latitude temperatures do not warm as much as what scientists measure, particularly in
winter. Some scientists believe that this is because there are unrecognized feedbacks in the climate
system involving types of clouds that only form when CO2 levels are very high. If this theory is correct,
future climate could warm even more in response to anthropogenic release of CO2 than most models
predict.

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The beginning of the Eocene also hosted a shorter event that may be the best natural analogue for
what humans are doing to the climate system today. Fifty-five million years ago a rapid warming
episode called the Paleocene-Eocene Thermal Maximum (PETM) occurred, in which Earth’s
temperature rose by 5 to 6°C on average within 10,000 to 30,000 years. Several explanations have
been proposed for this large, abrupt warming, all of which involve a massive infusion of GHGs into
the atmosphere, resulting in a trebling or perhaps a quadrupling of CO2 concentrations, not unlike
what is predicted for CO2 levels by 2100 (footnote 4).

5. Global Cooling: The Pleistocene Epoch

During the Pleistocene epoch, which began about 2 million years ago, Earth’s average temperature
has always been cold enough to maintain ice at high latitudes. But Pleistocene climate has not
been constant: ice coverage has fluctuated dramatically, with continental ice sheets advancing
and retreating over large parts of North America and Europe. These peak glacial periods are often
referred to as “Ice Ages” or “Glacial Maxima.” During the Pleistocene, Earth has experienced more
than 30 swings between prolonged glacial periods and brief warmer interglacial phases like the one
we live in today.

Glacial advances and retreats shaped Earth’s topography, soils, flora, and fauna (Fig. 7). During
glaciation events, huge volumes of water were trapped in continental ice sheets, lowering sea levels
as much as 130 meters and exposing land between islands and across continents. These swings
often changed ocean circulation patterns. During the most extreme cold phases, ice covered up to 30
percent of the Earth’s surface.

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Figure 7. Pleistocene glacial deposits in Illinois

Courtesy Illinois State Geological Survey.

As glaciers advanced and retreated at high latitudes, ecosystems at lower latitudes evolved to adapt
to prevailing climate conditions. In North America, just south of the advancing glaciers, a unique type
of grass steppe supported distinctive cold-adapted fauna dominated by large mammals such as the
mammoth, woolly rhinoceros, and dire wolf.

Why did Pleistocene temperatures swing back and forth so dramatically? Scientists point to a
combination of factors. One main cause is variations in Earth’s orbit around the sun. These variations,
which involve the tilt of the Earth’s pole of rotation and the ellipticity of the Earth’s orbit, have regular
timescales of 23,000, 41,000, and 100,000 years and cause small changes in the distribution of solar
radiation received on the Earth (footnote 5). The possibility that these subtle variations could drive
changes in climate was first proposed by Scottish scientist James Croll in the 1860s. In the 1930s,

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Serbian astronomer Milutin Milankovitch developed this idea further. Milankovitch theorized that
variations in summer temperature at high latitudes were what drove ice ages—specifically, that cool
summers kept snow from melting and allowed glaciers to grow.

However, changes in summer temperature due to orbital variations are too small to cause large
climate changes by themselves. Positive feedbacks are required to amplify the small changes in solar
radiation. The two principal feedbacks are changes in Earth’s albedo (the amount of light reflected
from the Earth’s surface) from snow and ice buildup, and in the amount of CO2 in the atmosphere.

Ice core samples from the Vostok station and the European Project for Ice Coring in Antarctica
(EPICA) document that CO2 levels have varied over glacial cycles. From bubbles trapped in the ice,
scientists can measure past concentrations of atmospheric CO2. The ice’s chemical composition
can also be used to measure past surface temperatures. Taken together, these records show that
temperature fluctuations through glacial cycles over the past 650,000 years have been accompanied
by shifts in atmospheric CO2. GHG concentrations are high during warm interglacial periods and
are low during glacial maxima. The ice cores also show that atmospheric CO2 concentrations never
exceeded 300 parts per million—and therefore that today’s concentration is far higher than what has
existed for the last 650,000 years (Fig. 8).

Figure 8. Vostok ice-core CO2 record

One important lesson from ice cores is that climate change is not always slow or steady. Records
from Greenland show that throughout the last glacial period, from about 60,000 to 20,000 years ago,

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abrupt warming and cooling swings called Dansgaard-Oeschger, or D-O, events took place in the
North Atlantic. In each cycle temperatures on ice sheets gradually cooled, then abruptly warmed by
as much as 20°C, sometimes within less than a decade. Temperatures would then decline gradually
over a few hundred to a few thousand years before abruptly cooling back to full glacial conditions.
Similar climate fluctuations have been identified in paleoclimate records from as far away as China.

These sharp flips in the climate system have yet to be explained. Possible causes include changes
in solar output or in sea ice levels around Greenland. But they are powerful evidence that when the
climate system reaches certain thresholds, it can jump very quickly from one state to another. At the
end of the Younger Dryas—a near-glacial phase that started about 12,800 years ago and lasted for
about 1,200 years—annual mean temperatures increased by as much as 10°C in ten years (footnote
6).

6. Present Warming and the Role of CO2

There is clear evidence from many sources that the planet is heating up today and that the pace
of warming may be increasing. Earth has been in a relatively warm interglacial phase, called the
Holocene Period, since the last ice age ended roughly 10,000 years ago. Over the past thousand
years average global temperatures have varied by less than one degree—even during the so-called
“Little Ice Age,” a cool phase from the mid-fourteenth through the mid-nineteenth centuries, during
which Europe and North America experienced bitterly cold winters and widespread crop failures.

Over the past 150 years, however, global average surface temperatures have risen, increasing by
0.6°C +/- 0.2°C during the 20th century. This increase is unusual because of its magnitude and the
rate at which it has taken place. Nearly every region of the globe has experienced some degree of
warming in recent decades, with the largest effects at high latitudes in the Northern Hemisphere. In
Alaska, for example, temperatures have risen three times faster than the global average over the
past 30 years. The 1990s were the warmest decade of the 20th century, with 1998 the hottest year
since instrumental record-keeping began a century ago, and the ten warmest years on record have all
occurred since 1990 (Fig. 9).

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Figure 9. Global temperature record

As temperatures rise, snow cover, sea ice, and mountain glaciers are melting. One piece of evidence
for a warming world is the fact that tropical glaciers are melting around the globe. Temperatures at
high altitudes near the equator are very stable and do not usually fluctuate much between summer
and winter, so the fact that glaciers are retreating in areas like Tanzania, Peru, Bolivia, and Tibet
indicates that temperatures are rising worldwide. Ice core samples from these glaciers show that
this level of melting has not occurred for thousands of years and therefore is not part of any natural
cycle of climate variability. Paleoclimatologist Lonnie Thompson of Ohio State University, who has
studied tropical glaciers in South America, Asia, and Africa, predicts that glaciers will disappear from
Kilimanjaro in Tanzania and Quelccaya in Peru by 2020.

“The fact that every tropical glacier is retreating is our warning that the
system is changing.”

Lonnie Thompson, Ohio State University

Rising global temperatures are raising sea levels due to melting ice and thermal expansion of
warming ocean waters. Global average sea levels rose between 0.12 and 0.22 meters during
the 20th century, and global ocean heat content increased. Scientists also believe that rising
temperatures are altering precipitation patterns in many parts of the Northern Hemisphere (footnote
7).

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Because the climate system involves complex interactions between oceans, ecosystems, and
the atmosphere, scientists have been working for several decades to develop and refine General
Circulation Models (also known as Global Climate Models), or GCMs, highly detailed models typically
run on supercomputers that simulate how changes in specific parameters alter larger climate
patterns. The largest and most complex type of GCMs are coupled atmosphere-ocean models,
which link together three-dimensional models of the atmosphere and the ocean to study how these
systems impact each other. Organizations operating GCMs include the National Aeronautic and
Space Administration (NASA)’s Goddard Institute for Space Studies and the United Kingdom’s
Hadley Centre for Climate Prediction and Research (Fig. 10).

Figure 10. Hadley Centre GCM projection

Researchers constantly refine GCMs as they learn more about specific components that feed into the
models, such as conditions under which clouds form or how various types of aerosols scatter light.
However, predictions of future climate change by existing models have a high degree of uncertainty
because no scientists have ever observed atmospheric CO2 concentrations at today’s levels.

Modeling climate trends is complicated because the climate system contains numerous feedbacks
that can either magnify or constrain trends. For example, frozen tundra contains ancient carbon and
methane deposits; warmer temperatures may create a positive feedback by melting frozen ground

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and releasing CO2 and methane, which cause further warming. Conversely, rising temperatures that
increase cloud formation and thereby reduce the amount of incoming solar radiation represent a
negative feedback. One source of uncertainty in climate modeling is the possibility that the climate
system may contain feedbacks that have not yet been observed and therefore are not represented in
existing GCMs.

Scientific evidence, including modeling results, indicates that rising atmospheric concentrations of
CO2 and other GHGs from human activity are driving the current warming trend. As the previous
sections showed, prior to the industrial era atmospheric CO2 concentrations had not risen above 300
parts per million for several hundred thousand years. But since the mid-18th century CO2 levels have
risen steadily.

In 2007 the Intergovernmental Panel on Climate Change (IPCC), an international organization of
climate experts created in 1988 to assess evidence of climate change and make recommendations
to national governments, reported that CO2 levels had increased from about 280 ppm before the
industrial era to 379 ppm in 2005. The present CO2 concentration is higher than any levels over at
least the past 420,000 years and is likely the highest level in the past 20 million years. During the
same time span, atmospheric methane concentrations rose from 715 parts per billion (ppb) to 1,774
ppb and N2O concentrations increased from 270 ppb to 319 ppb (footnote 8).

Do these rising GHG concentrations explain the unprecedented warming that has taken place over
the past century? To answer this question scientists have used climate models to simulate climate
responses to natural and anthropogenic forcings. The best matches between predicted and observed
temperature trends occur when these studies simulate both natural forcings (such as variations
in solar radiation levels and volcanic eruptions) and anthropogenic forcings (GHG and aerosol
emissions) (Fig. 11). Taking these findings and the strength of various forcings into account, the
IPCC stated in 2007 that Earth’s climate was unequivocally warming and that most of the warming
observed since the mid-20th century was “very likely” (meaning a probability of more than 90 percent)
due to the observed increase in anthropogenic GHG emissions (footnote 9).

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Figure 11. Comparison between modeled and observations of temperature rise since the
year 1860

© Intergovernmental Panel on Climate Change, Third Assessment Report, 2001.
Working Group 1: The Scientific Basis, Figure 1.1.

Aerosol pollutants complicate climate analyses because they make both positive and negative
contributions to climate forcing. As discussed in Unit 11, “Atmospheric Pollution,” some aerosols such
as sulfates and organic carbon reflect solar energy back from the atmosphere into space, causing
negative forcing. Others, like black carbon, absorb energy and warm the atmosphere. Aerosols also
impact climate indirectly by changing the properties of clouds—for example, serving as nuclei for
condensation of cloud particles or making clouds more reflective.

Researchers had trouble explaining why global temperatures cooled for several decades in the
mid-20th century until positive and negative forcings from aerosols were integrated into climate
models. These calculations and observation of natural events showed that aerosols do offset some
fraction of GHG emissions. For example, the 1991 eruption of Mount Pinatubo in the Philippines,
which injected 20 million tons of SO2 into the stratosphere, reduced Earth’s average surface
temperature by up to 1.3°F annually for the following three years (footnote 10).

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But cooling from aerosols is temporary because they have short atmospheric residence times.
Moreover, aerosol concentrations vary widely by region and sulfate emissions are being reduced
in most industrialized countries to address air pollution. Although many questions remain to be
answered about how various aerosols are formed and contribute to radiative forcing, they cannot be
relied on to offset CO2 emissions in the future.

7. Observed Impacts of Climate Change

Human-induced climate change has already had many impacts. As noted above, global average
surface temperatures rose by 0.6°C +/- 0.2°C and sea levels rose by 0.12 to 0.22 meters during
the 20th century. Other observed changes in Earth systems that are consistent with anthropogenic
climate change include:

• Decreases by about two weeks in the duration of ice cover on rivers and lakes in the mid-
and high latitudes of the Northern Hemisphere over the 20th century;

• Decreases by 10 percent in the area of snow cover since satellite images became available
in the 1960s;

• Thinning by 40 percent of Arctic sea ice in late summer to early autumn in recent decades,
and decrease by 10 to 15 percent in extent in spring and summer since the 1950s (Fig. 12);

• Widespread retreat of non-polar glaciers;

• Increases by about 1 to 4 days per decade in growing seasons in the Northern Hemisphere,
especially at higher latitudes, during the last 40 years; and

• Thawing, warming, and degrading of permafrost in some regions .

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Figure 12. Arctic sea ice coverage, 1979 and 2003

The Earth is not warming uniformly. Notably, climate change is expected to affect the polar regions
more severely. Melting snow and ice expose darker land and ocean surfaces to the sun, and
retreating sea ice increases the release of solar heat from oceans to the atmosphere in winter.
Trends have been mixed in Antarctica, but the Arctic is warming nearly twice as rapidly as the rest of
the world; winter temperatures in Alaska and western Canada have risen by up to 3–4°C in the past
50 years, and Arctic precipitation has increased by about 8 percent over the past century (mostly as
rain) (footnote 12).

Observed climate change impacts are already affecting Earth’s physical and biological systems. Many
natural ecosystems are vulnerable to climate change impacts, especially systems that grow and
adapt slowly. For example, coral reefs are under serious stress from rapid ocean warming. Recent
coral bleaching events in the Caribbean and Pacific oceans have been correlated with rising sea
surface temperatures over the past century (footnote 13). Some natural systems are more mobile.
For example, tree species in New England such as hemlock, white pine, maple, beech, and hickory
have migrated hundreds of meters per year in response to warming and cooling phases over the
past 8,000 years (footnote 14). But species may not survive simply by changing their ranges if other
important factors such as soil conditions are unsuitable in their new locations.

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Insects, plants, and animals may respond to climate change in many ways, including shifts in range,
alterations of their hibernation, migrating, or breeding cycles, and changes in physical structure and
behavior as temperature and moisture conditions alter their immediate environments. A recent review
of more than 40 studies that assessed the impacts of climate change on U.S. ecosystems found
broad impacts on plants, animals, and natural ecosystem processes. Important trends included:

• Earlier spring events (emergence from hibernation, plant blooming, and onset of bird and
amphibian breeding cycles);

• Insect, bird, and mammal range shifts northward and to higher elevations; and

• Changes in the composition of local plant and animal communities favoring species that
are better adapted to warming conditions (higher temperatures, more available water, and
higher CO levels).

Because many natural ecosystems are smaller, more isolated, and less genetically diverse today
than in the past, it may be increasingly difficult for them to adapt to climate change by migrating or
evolving, the review’s authors concluded (footnote 15). This is especially true if climate shifts happen
abruptly so that species have less response time, or if species are adapted to unique environments
(Fig. 13).

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Figure 13. Polar bear hunting on Arctic sea ice

8. Other Potential Near-Term Impacts

In its 2007 assessment report the IPCC projected that global average surface temperatures for the
years 2090 to 2099 will rise by 1.1 to 6.4°C over values in 2001 to 2010. The greatest temperature
increases will occur over land and at high northern latitudes, with less warming over the southern
oceans and the North Atlantic (footnote 16).

This rate of warming, driven primarily by fossil fuel consumption, would be much higher than the
changes that were observed in the 20th century and probably unprecedented over at least the past
10,000 years. Based on projections like this, along with field studies of current impacts, scientists
forecast many significant effects from global climate change in the next several decades, although
much uncertainty remains about where these impacts will be felt worldwide and how severe they will
be.

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Climate change is likely to alter hydrologic cycles and weather patterns in many ways, such as
shifting storm tracks, increasing or reducing annual rainfall from region to region, and producing more
extreme weather events such as storms and droughts (Fig. 14). While precipitation trends vary widely
over time and area, total precipitation increased during the 20th century over land in high-latitude
regions of the Northern Hemisphere and decreased in tropical and subtropical regions (footnote 17).

Figure 14. Flooding in New Orleans after Hurricane Katrina, 2005

Rising temperatures and changing hydrological cycles are likely to have many impacts, although
it is hard to predict changes in specific regions—some areas will become wetter and some dryer.
Storm tracks may shift, causing accustomed weather patterns to change. These changes may
upset natural ecosystems, potentially leading to species losses. They also could reduce agricultural
productivity if new temperature and precipitation patterns are less than optimal for major farmed
crops (for example, if rainfall drops in the U.S. corn belt). Some plant species may migrate north
to more suitable ecosystems—for example, a growing fraction of the sugar maple industry in the
northeastern United States is already moving into Canada—but soils and other conditions may not be
as appropriate in these new zones.

Some natural systems could benefit from climate change at the same time that others are harmed.
Crop yields could increase in mid-latitude regions where temperatures rise moderately, and winter
conditions may become more moderate in middle and high latitudes. A few observers argue that
rising CO2 levels will produce a beneficial global “greening,” but climate change is unlikely to increase
overall global productivity. Research by Stanford University ecologist Chris Field indicates that

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elevated CO2 prevents plants from increasing their growth rates, perhaps by limiting their ability to
utilize other components that are essential for growth such as nutrients. This finding suggests that
terrestrial ecosystems may take up less carbon in a warming world than they do today, not more.

Undesirable species may also benefit from climate change. Rising temperatures promote the spread
of mosquitoes and other infectious disease carriers that flourish in warmer environments or are
typically limited by cold winters (Fig. 15). Extreme weather events can create conditions that are
favorable for disease outbreaks, such as loss of clean drinking water and sanitation systems. Some
vectors are likely to threaten human health, while others can damage forests and agricultural crops.

Figure 15. Infectious diseases affected by climate change

© Climate change 1995, Impacts, adaptations and mitigation of climate change:
scientific-technical analyses, working group 2 to the second assessment report of the
IPCC, UNEP, and WMO, Cambridge Press University, 1996.

Melting of polar ice caps and glaciers is already widespread and is expected to continue throughout
this century. Since the late 1970s Arctic sea ice has decreased by about 20 percent; in the past
several years, this ice cover has begun to melt in winter as well as in summer, and some experts
predict that the Arctic could be ice-free by 2100. Ice caps and glaciers contain some 30 million cubic
kilometers of water, equal to about 2 percent of the volume of the oceans. Further melting of sea ice
will drive continued sea-level rise and increase flooding and storm surge levels in coastal regions.

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Warmer tropical sea surface temperatures are already increasing the intensity of hurricanes, and this
trend may accelerate as ocean temperatures rise (footnote 18). Stronger storms coupled with rising
sea levels are expected to increase flooding damage in coastal areas worldwide. Some scientists
predict that extreme weather events, such as storms and droughts, may become more pronounced,
although this view is controversial. In general, however, shifting atmospheric circulation patterns
may deliver “surprises” as weather patterns migrate and people experience types of weather that fall
outside their range of experience, such as flooding at a level formerly experienced only every 50 or
100 years.

Human societies may already be suffering harmful impacts from global climate change, although it is
important to distinguish climate influences from other socioeconomic factors. For example, financial
damages from storms in the United States have risen sharply over the past several decades, a
trend that reflects both intensive development in coastal areas and the impact of severe tropical
storms in those densely populated regions. Human communities clearly are vulnerable to climate
change, especially societies that are heavily dependent on natural resources such as forests,
agriculture, and fishing; low-lying regions subject to flooding; water-scarce areas in the subtropics;
and communities in areas that are subject to extreme events such as heat episodes and droughts.
In general, developed nations have more adaptive capacity than developing countries because
wealthier countries have greater economic and technical resources and are less dependent on
natural resources for income.

And more drastic changes may lie in store. As discussed above, climate records show that the
climate can swing suddenly from one state to another within periods as short as a decade. A 2002
report by the National Research Council warned that as atmospheric GHG concentrations rise, the
climate system could reach thresholds that trigger sudden drastic shifts, such as changes in ocean
currents or a major increase in floods or hurricanes (footnote 19).

“Just as the slowly increasing pressure of a finger eventually flips a switch
and turns on a light, the slow effects of drifting continents or wobbling
orbits or changing atmospheric composition may ‘switch’ the climate to a
new state.”

Richard B. Alley, ChairCommittee on Abrupt Climate Change,National
Research Council

How much the planet will warm in the next century, and what kind of impacts will result, depends
on how high CO2 concentrations rise. In turn, this depends largely on human choices about fossil
fuel consumption. Because fossil fuel accounts for 80 percent of global energy use, CO2 levels will
continue to rise for at least the next 30 or 40 years, so additional impacts are certain to be felt. This
means that it is essential both to mitigate global climate change by reducing CO2 emissions and to
adapt to the changes that have already been set in motion. (For more on options for mitigating and
adapting to climate change, see Unit 13, “Looking Forward: Our Global Experiment.”)

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9. Major Laws and Treaties

Science plays a central role in international negotiations to address global climate change. In 1988,
the World Meteorological Organization and the United Nations Environment Programme established
the Intergovernmental Panel on Climate Change (IPCC), an organization composed of official
government representatives that is charged with assessing scientific, technical, and socio-economic
information relevant to understanding climate change risks, potential impacts, and mitigation and
adaptation options (footnote 20). The IPCC meets regularly to review and assess current scientific
literature and issues “assessment reports” at approximately five-year intervals (most recently in
2007). IPCC reports are adopted by consensus and represent a broad cross-section of opinion from
many nations and disciplines regarding current understanding of global climate change science.
The panel’s recommendations are not binding on governments, but its models and estimates are
important starting points for international climate change negotiations.

The most broadly-supported international agreement on climate change, the United Nations
Framework Convention on Climate Change (FCCC), was opened for signature in 1992 and entered
into force in 1994 (footnote 21). To date it has been ratified by 189 countries, including the United
States. FCCC signatories pledge to work toward stabilizing atmospheric GHG concentrations “at
a level that would prevent dangerous anthropogenic interference with the climate system,” but
the Convention does not define that level. As a result, it has not been a significant curb on GHG
emissions, although it creates a system for nations to report emissions and share other relevant
information and for developed countries to provide financial and technical support for climate change
initiatives to developing countries.

Recognizing that the FCCC commitments were not sufficient to prevent serious climate change,
governments negotiated the Kyoto Protocol, which commits industrialized countries to binding
GHG emission reductions of at least 5 percent below their 1990 levels by the period of 2008–2012
(footnote 22). The Protocol focuses on developed countries in reflection of the fact that they are the
source of most GHGs emitted to date, although it allows developed countries to fulfill their reduction
commitments partially through projects to reduce or avoid GHG reductions in developing countries.

The Kyoto Protocol entered into force in 2005 and has been ratified to date by 163 countries,
representing 61.6 percent of developed countries’ GHG emissions. The United States signed the
Protocol but has not ratified it. President George W. Bush argued that the economic impact of its
assigned reductions (7 percent below 1990 levels) would be too severe and instead emphasized
voluntary domestic reduction commitments.

For all of the controversy that it has generated, the Kyoto Protocol alone will not reduce the threat
of major climate change because it covers only 40 percent of global GHG emissions without U.S.
participation, does not require emission reductions from rapidly developing countries, like India and
China, that are major fossil fuel consumers, and only covers emission through the year 2012. No
single option has emerged yet as a follow-on, but analysts widely agree that the next phase of global

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action against climate change will have to take a longer-term approach, address the costs of reducing
GHG emissions, and find ways to help developing countries reap the benefits of economic growth
on a lower-carbon pathway than that which industrialized countries followed over the past 150 years.
Continually improving our scientific understanding of climate change and its impacts will help nations
to identify options for action.

10. Further Reading

Center for Health and the Global Environment, Harvard Medical School, Climate Change Futures:
Health, Ecological, and Economic Dimensions (Boston, MA: Harvard Medical School, November
2005).

Climate Timeline (http://www.ngdc.noaa.gov/paleo/ctl/index.html). This web site, maintained by the
National Geophysical Data Center at the National Oceanographic and Atmospheric Administration,
links changes in weather and climate through history to specific historic events.

“The Discovery of Global Warming,” (http://www.aip.org/history/climate). Created by Spencer Weart,
author of the book of the same title, this site includes detailed essays on the history of climate change
science, case studies, and links to relevant scientific and historical publications.

James E. Hansen, “Can We Still Avoid Dangerous Human-Made Climate Change?” February
10, 2006, http://www.columbia.edu/~jeh1/newschool_text_and_slides . In this speech and
accompanying slides, a leading U.S. climate scientist makes the case for action to slow global climate
change.

Footnotes

1. Water vapor contributes to climate change through an important positive feedback loop: as the
atmosphere warms, evaporation from Earth’s surface increases and the atmosphere becomes able to
hold more water vapor, which in turn traps more thermal energy and warms the atmosphere further.
It also can cause a negative feedback when water in the atmosphere condenses into clouds that
reflect solar radiation back into space, reducing the total amount of energy that reaches Earth. For
more details, see National Oceanic and Atmospheric Administration, “Greenhouse Gases: Frequently
Asked Questions,” http://lwf.ncdc.noaa.gov/oa/climate/gases.html.

2. Key GHG Data: Greenhouse Gas Emissions Data for 1990–2003 Submitted To the United
Nations Framework Convention on Climate Change (Bonn: United Nations Framework
Convention on Climate Change, November 2005), pp. 16, 28.

3. U.S. Environmental Protection Agency, “The U.S. Inventory of Greenhouse Gas Emissions
and Sinks: Fast Facts,” April 2006, http://yosemite.epa.gov/oar/globalwarming.nsf/content/
ResourceCenterPublicationsGHGEmissions.html.

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4. John A. Higgins and Daniel P. Schrag, “Beyond Methane: Towards a Theory for the Paleocene-
Eocene Thermal Maximum,” Earth and Planetary Science Letters, vol. 245 (2006), pp. 523–537.

5. National Oceanographic and Atmospheric Administration, Paleoclimatology Branch, “Astronomical
Theory of Climate Change,” http://www.ncdc.noaa.gov/paleo/milankovitch.html; Spencer R. Weart,
The Discovery of Global Warming (Cambridge, MA: Harvard University Press, 2003), pp. 74–77.

6. “Abrupt Climate Change,” Lamont-Doherty Earth Observatory, Columbia University, http://
www.ldeo.columbia.edu/res/pi/arch/examples.shtml.

7. Intergovernmental Panel on Climate Change, Climate Change 2007: The Scientific Basis,
Summary for Policymakers (Cambridge, UK: Cambridge University Press, 2007), pp. 4–6.

8. Ibid., pp. 2–3.

9. Ibid., p. 8.

10. U.S. Geological Survey, “Impacts of Volcanic Gases on Climate, The Environment, and People,”
May 1997, http://pubs.usgs.gov/of/1997/of97-262/of97-262.html.

11. IPCC, Climate Change 2001: Synthesis Reports, Summary for Policymakers (Cambridge, UK:
Cambridge University Press, 2001), p. 6.

12. ACIA, Impacts of a Warming Arctic: Arctic Climate Impact Assessment (Cambridge, UK:
Cambridge University Press, 2004), p. 12.

13. J.E. Weddell, ed., The State of Coral Reef Ecosystems of the United States and Pacific
Freely Associated States, 2005, NOAA Technical memorandum NOS NCCOS 11 (Silver Spring,
MD: NOAA/NCCOS Center for Coastal Monitoring and Assessment’s Biogeography Team, 2005), pp.
13–15, http://ccma.nos.noaa.gov/ecosystems/coralreef/coral_report_2005/.

14. David R. Foster and John D. Aber, eds., Forests in Time: The Environmental Consequences
of 1,000 Years of Change in New England (New Haven: Yale University Press, 2004), pp. 45–46.

15. Camille Parmesan and Hector Galbraith, Observed Impacts of Global Climate Change in the
U.S. (Arlington, VA: Pew Center on Global Climate Change, 2004), http://www.pewclimate.org/global-
warming-in-depth/all_reports/observedimpacts/index.cfm.

16. IPCC, Climate Change 2007: The Scientific Basis, p. 749.

17. United Nations Environment Programme, “Observed Climate Trends,” http://www.grida.no/
climate/vital/trends.htm.

18. Kerry Emanuel, “Increasing Destructiveness of Tropical Cyclones Over the Past 30 Years,”
Nature, vol. 436, August 4, 2005, pp. 686–88, and “Anthropogenic Effects on Tropical Cyclone
Activity,” http://wind.mit.edu/~emanuel/anthro2.htm.

19. National Research Council, Abrupt Climate Change: Inevitable Surprises (Washington, DC:
National Academy Press, 2002).

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20. http://www.ipcc.ch/.

21. http://unfccc.int/essential_background/convention/items/2627.php.

22. http://unfccc.int/essential_background/kyoto_protocol/items/2830.php.

Glossary

aerosols : Liquid or solid particles that are suspended in air or a gas. Also referred to as particulate
matter.

albedo : The fraction of electromagnetic radiation reflected after striking a surface.

anthropogenic : Describing effects or processes that are derived from human activities, as opposed to
effects or processes that occur in the natural environment without human influences.

coral bleaching : Refers to the loss of color of corals due to stress-induced expulsion of symbiotic,
unicellular algae called zooxanthellae that live within their tissues. Stress can be induced by:
increased water temperatures (often attributed to global warming), starvation caused by a decline in
zooplankton levels as a result of overfishing, solar irradiance (photosynthetically active radiation and
ultraviolet band light), changes in water chemistry, silt runoff, or pathogen infections.

deforestation : Removal of trees and other vegetation on a large scale, usually to expand agricultural
or grazing lands.

global warming potential : A measure of how much a given mass of greenhouse gas is estimated
to contribute to global warming. Compares the gas in question to that of the same mass of carbon
dioxide.

Intergovernmental Panel on Climate Change (IPCC) : Established in 1988 by two United Nations
organizations to assess the risk of human-induced climate change.

Kyoto Protocol : An amendment to the international treaty on climate change, assigning mandatory
targets for the reduction of greenhouse gas emissions to signatory nations.

paleoclimate : Referring to past climates of the Earth.

permafrost : Soil that stays in a frozen state for more than two years in a row.

radiocarbon dating : A radiometric dating method that uses the naturally occurring isotope carbon-14
to determine the age of carbonaceous materials up to about 60,000 years.

residence time : A broadly useful concept that expresses how fast something moves through a
system in equilibrium; the average time a substance spends within a specified region of space, such
as a reservoir. For example, the residence time of water stored in deep groundwater, as part of the
water cycle, is about 10.000 years.

Unit 12 : Earth’s Changing Climate -31- www.learner.org

sinks : Habitats that serve to trap or otherwise remove chemicals such as plant nutrients, organic
pollutants, or metal ions through natural processes.

United Nations Framework Convention on Climate Change : A treaty signed by nations at the Earth
Summit in 1992 to stabilize and reduce greenhouse gas emissions. In 1997 the Kyoto Protocol, an
agreement among 150 nations, was added, setting specific reduction levels.

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