CLIMATE CHANGE
A RISK ASSESSMENTDavid King, Daniel Schrag, Zhou Dadi,
Qi Ye and Arunabha Ghosh
Project Manager: Simon Sharpe
Edited by James Hynard and Tom Rodger,
Centre for Science and Policy

NASA Earth Observatory image byJesse Allen and Robert Simmon,using data from NASA/GSFC/METI/ERSDAC/JAROS, and U.S./JapanASTER
Science Team.

Hosts of the project workshops

Sponsors

China National
Expert Committeeon Climate Change

ABOUT THE AUTHORS
Sir David King is the UK Foreign Secretary’s Special Representative for Climate Change, and was formerly the UK
Government’s Chief Scientific Adviser. He has authored over 500 papers on chemical physics and on science and policy,
and his university positions have included Professor of Physical Chemistry at Cambridge University, and Founding
Director of the Smith School of Enterprise and the Environment at Oxford University.
Professor Daniel P. Schrag is Director of the Harvard University Center for the Environment, and serves on the US
President’s Council of Advisors on Science and Technology. He is the Sturgis Hooper Professor of Geology and Professor
of Environmental Science and Engineering, and studies energy technology and policy as well as geochemistry andclimatology. He has received several honors including a MacArthur Fellowship.
Professor Zhou Dadi is a member of the China National Expert Committee on Climate Change, and was formerly
Director General of the Energy Research Institute of the National Development and Reform Commission of theGovernment of China. He is a specialist in energy economics and energy systems analysis, sustainable energy strategy,energy conservation, and climate change policy.
Professor Qi Ye is Director of the Brookings-Tsinghua Centre for Public Policy at the School of Public Policy andManagement, Tsinghua University, and a Senior Fellow of the Brookings Institution. He is an expert on China’s policies
on climate change, the environment, energy, natural resources, biodiversity, and on the theory and practice of sustainable
development.

CLIMATE CHANGE

Dr Arunabha Ghosh is the CEO of the Council on Energy, Environment and Water (CEEW), one of India’s top-ranked
climate think-tanks. He is an expert in climate, energy, water and environment policy, economics and governance, haspresented to parliaments including those of India, Europe and Brazil, and to heads of state, and has previously worked
at Princeton and Oxford Universities, United Nations Development Programme and World Trade Organization. He is afounding Board member of the Clean Energy Access Network and on the Board of the International Centre for Trade andSustainable Development.

A RISK ASSESSMENTDavid King, Daniel Schrag, Zhou Dadi,
Qi Ye and Arunabha GhoshThis report was edited and produced by the Centre for Science and Policy (CSaP) at the University of Cambridge. CSaP’s
mission is to promote the use of expertise and evidence in public policy by convening its unique network of academics
and policy makers.

Project Manager: Simon Sharpe

STATUS OF THIS REPORT

Edited by James Hynard and Tom Rodger,
Centre for Science and Policy

Sir David King led this project in his official capacity as the UK Foreign Secretary’s Special Representative for Climate
Change. The Foreign and Commonwealth Office commissioned this report as an independent contribution to the climatechange debate. Its contents represent the views of the authors, and should not be taken to represent the views of the UK
Government.

ACKNOWLEDGMENTS
The authors wish to thank all of those who contributed their time and expertise to this project, including the contributing
authors to this report, and also those participants in our meetings whose names are not listed here, but whose valuable
insights have contributed to our analysis.
We wish to thank the sponsors of the project, including the UK Foreign and Commonwealth Office, the China National
Expert Committee on Climate Change, the UK Government Office for Science, the Skoll Global Threats Fund, Global
Challenges Foundation, the UK Institute and Faculty of Actuaries, and Willis Research Network, for their generous
support. Special thanks are also due to the UK Department of Energy and Climate Change Science team, China
Meteorological Administration, the CNA Corporation, and the Climate Change Science Institute at Oak Ridge National
Laboratory, for their support to specific aspects of the project.
We also wish to thank all the following individuals for their practical, intellectual and moral support, which has made thisproject possible: James Ballantyne, Oliver Bettis, Steven Bickers, Shourjomoy Chattopadhyay, Vaibhav Chaturvedi, Partha
Dasgupta, Hem Himanshu Dholakia, John Edwards, Vaibhav Gupta, Pradyot C. Haldar, Frances Hooper, Paulette Hunter
Okulo, Anil Jain, Aarti Katyal, Sindhushree Khullar, Anthony W. King, Sylvia Lee, Stephan Lewandowsky, Amy Luers, Luo
Yong, Lisa Matthews, Bessma Mourad, Chris Nicholson, Harriet O’Brien, Dennis Pamlin, Bob Phillipson, Benjamin L.
Preston, Aditya Ramji, Sudatta Ray, Denise Sadler, Sayantan Sarkar, Mihir Shah, Surbhi Singhvi, Morgan Slebos, Wang
Mengni, Nicola Willey, Ken Wright, Zhang Jiansong, Zheng Qi, Zhu Songli.

Hosts of the project workshops

Sponsors

China National
Expert Committeeon Climate Change

CONTENTS
MINISTERIAL FOREWORD

6

16

GLOBAL SEA LEVEL RISE

99

EXECUTIVE SUMMARY		

8

17

LARGE-SCALE ABRUPT OR IRREVERSIBLE CHANGES

103

1

INTRODUCTION 		

14

18

CONCLUDING COMMENTS

107

2

PRINCIPLES AND SCOPE OF THIS RISK ASSESSMENT

18

RISK ASSESSMENT PART 3: SYSTEMIC RISKSRISK ASSESSMENT PART 1: EMISSIONS

28

19

INTRODUCTION 		

3

HISTORY AND PURPOSE OF THE IPCC SCENARIOS

29

20 EXTREME WEATHER AND RESILIENCE OF THE GLOBAL FOOD SYSTEM

4TOWARDS A RISK-BASED PERSPECTIVE ON EMISSIONS SCENARIOS

31

21

5

THE IMPLICATIONS OF CURRENT POLICIES AND PLANS FORSHORT-TERM EMISSIONS

32

6
TECHNOLOGICAL CHALLENGES THAT WILL DETERMINE FUTURE
GLOBAL EMISSIONS

37

7

CONCLUSIONS 		RISK ASSESSMENT PART 2: DIRECT RISKS

CLIMATE CHANGE RISKS TO NATIONAL AND INTERNATIONAL SECURITY

RISK ASSESSMENT PART 4: VALUE

42

44

109
110
113
120

130

22

ECONOMICS 		

131

23

ETHICS 		

134

RECOMMENDATIONS FOR CONTINUING RISKASSESSMENT

137

140

INTRODUCTION: A LONG-TERM RISK ASSESSMENT APPROACH TO
CLIMATE SCIENCE

45

9

GLOBAL TEMPERATURE INCREASE50RISK REDUCTION: ELEMENTS OF A PROPORTIONATERESPONSE

10

THE RISK OF HEAT STRESS TO PEOPLE

57

24

TECHNOLOGY 		

141

11

THE RISKS OF CLIMATE CHANGE FOR CROP PRODUCTION

65

25

FINANCE 		

143

12

THE RISK OF WATER STRESS

74

26

POLITICS 		

146

13

THE RISK OF DROUGHT

84

27

CLOSING THOUGHTS		

148

14

THE RISK OF RIVER FLOODING

88

15

THE RISKS OF SEA LEVEL RISE FOR COASTAL CITIES

94

ANNEXES 		

150

8

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CLIMATE CHANGE: A RISK ASSESSMENT

CLIMATE CHANGE: A RISK ASSESSMENTMINISTERIAL FOREWORD

Just as our assessment of the risk needs to be holistic, so too does our response. Responding to the risk ofclimate change will demand technological innovation, financial investment and political leadership. Each ofthese elements must be brought together to produce a response that is both proportionate and effective.

The Rt Hon. Baroness Anelay of St Johns
Minister of State, UK Foreign and Commonwealth OfficeN

one of us should be in any doubt that climate change poses a great risk. Indeed, it isremarkable that even in the run-up to a general election, the leaders of the UK’s three largest
political parties came together to say that “Climate change is one of the most serious threatsfacing the world today. It is not just a threat to the environment, but also to our national andglobal security, to poverty eradication and economic prosperity.”

In assessing the risk of climate change, the immediate questions for any country anywhere in the world are:How serious is the threat? How urgent is it? How should we prioritise our response, when we have so manyother pressing, national objectives - from encouraging economic recovery to protecting our people aroundthe world? These are all important questions, and we can only truly answer them if we assess the risk in full.
In the past, when assessing the risk of climate change, we have tended to take an approach that is, perhaps,
too narrow - or incomplete. In public debate, we have sometimes treated it as an issue of prediction, as if
it were a long-term weather forecast. Or as purely a question of economics - as if the whole of the threat
could be accurately quantified by putting numbers into a calculator. Often, too, we have not fully assessed
the indirect or systemic risks, such as those affecting international security – even though, as the UK’s firstnational climate change risk assessment found, these could be far greater than the direct risks like coastal
flooding. Assessing the threat of climate change today demands a more coordinated, more sophisticated,
more holistic approach.
Taking a holistic approach to risk management goes to the heart of what the Foreign and Commonwealth
Office does. It is an approach that applies as much to climate change as to, for example, preventing the
spread of nuclear weapons. Earlier this year, I addressed a meeting of the Permanent Five members of the UN
Security Council to discuss nuclear disarmament and non-proliferation. Assessing the risk around this vitalarea of security depends on understanding inter-dependent elements, including: what the science tells us ispossible; what our political analysis tells us a country may intend; and what the systemic factors are, such as
regional power dynamics.
The risk of climate change demands a similarly holistic assessment. So, to understand its full extent, we must:first, take into account countries’ plans and policies, which together affect the future of global emissions;second, understand the science of how our climate may change; third, consider how climate change could
affect the complex systems of the global economy and international security.
Finally, we must also make a judgment about how we value the risk. In other words, how much do we careabout the effects of climate change? How important is it that we act to avoid them? What probability of theiroccurrence can we tolerate? For climate change, as for nuclear proliferation, the answers to these questions
are not easily expressed in economic terms. They depend in part on how we value human life – both now, andin the future. Decisions on national security usually have implications for the budget, but they can rarely be
reduced to simple equations of cost and benefit. That is why it is important that we are open and honest about
any value judgments we make, so that these can be subject to public debate.

We are beginning to see some positive progress. Technological innovation has dramatically cut the costs ofrenewable energy, increasing its share of global energy investment. In turn, more and more countries aretaking policy steps to reduce their emissions and the Paris conference at the end of this year presents anopportunity to scale up our global response.
However, lest we become complacent, we must remember that in one way, climate change differs fromany other subject of diplomatic negotiation: it is governed by a physical process. A process where therisk increases over time, and will continue to do so until we have entirely dealt with its cause. That is whyleadership is so important – to forge ahead, to drive momentum and to show the way for others to follow.
Former British Prime Minister Margaret Thatcher showed just that kind of leadership in her early recognitionof the nature and scale of the risks of climate change. In 1989, she told the UN General Assembly that rather
than being the lords of all we survey, “we are the Lord’s creatures, the trustees of this planet, charged
today with preserving life itself – preserving life with all its mystery and all its wonder. May we all beequal to that task.”
I am delighted that experts from the UK, US, China and India have worked together to produce this report,
which makes those risks even clearer. As we consider its findings, let us remember those words from aquarter-century ago. Let us determine to be both proportionate and effective in our response. Let us showthat we are, indeed, equal to the task before us.

7

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CLIMATE CHANGE: A RISK ASSESSMENTEXECUTIVE SUMMARYT

he most important decision any government has to make about climate change is one ofpriority: how much effort to expend on countering it, relative to the effort that must be spent
on other issues. This risk assessment aims to inform that decision.

CONCLUSIONS OF THE RISK ASSESSMENT
A climate change risk assessment must consider at least three areas: the future pathway of global emissions;the direct risks arising from the climate’s response to those emissions; and the risks arising from theinteraction of climate change with complex human systems. Each of these areas contains large uncertainties.
From our assessment, we draw the following conclusions about the most significant risks.

EMISSIONS: Without increased political commitment and an acceleration of technologicalinnovation, global emissions are likely to follow a medium to high pathway: continuing toincrease for the next few decades, and then levelling off or decreasing gradually.
• Current policies and plans for major countries and regions are, in aggregate, consistent with a medium
to high emissions pathway, with emissions continuing to increase over the next few decades.
• The technological challenges to achieving a low emissions pathway are substantial, and are not being
adequately addressed at present. Without an acceleration of innovation in energy technology and energy
systems – including wind and solar with storage, nuclear, biofuel, petroleum-free passenger transport,
carbon storage, and large-scale energy efficiency – the likelihood of following a pathway in whichemissions fall rapidly and approach zero by late in the century is very low.
• High emissions pathways in which emissions continue to increase throughout the century cannot be
ruled out, given the potential for extraction of large new coal reserves, as well as oil shale and methane
hydrates.
• The climate responds to cumulative emissions, so any pathway that does not bring emissions close tozero will result in risk continually increasing over time.

DIRECT RISKS: The risks of climate change are non-linear: while average conditions maychange gradually, the risks can increase rapidly. On a high emissions pathway, the probability ofcrossing thresholds beyond which the inconvenient may become intolerable will increase overtime.
• For any emissions pathway, a wide range of global temperature increases is possible. On all but
the lowest emissions pathways, a rise of more than 2°C is likely in the latter half of this century. On
a medium-high emissions pathway (RCP61), a rise of more than 4°C appears to be as likely as not by
2150. On the highest emissions pathway (RCP8.5), a rise of 7°C is a very low probability at the end of
this century, but appears to become more likely than not during the course of the 22nd century. A rise of
more than 10°C over the next few centuries cannot be ruled out.

CLIMATE CHANGE: A RISK ASSESSMENT• Humans have limited tolerance for heat stress. In the current climate, safe climatic conditions forwork are already exceeded frequently for short periods in hot countries, and heat waves already cause
fatalities. In future, climatic conditions could exceed potentially lethal limits of heat stress even for
individuals resting in the shade. The probability of exposed individuals experiencing such conditions ina given year starts to become significant for a global temperature rise of around 5°C, and could exceed
50% for a global temperature rise of around 7°C, in hot areas such as northern India, southeastern
China, and southeastern USA.
• Crops have limited tolerance for high temperatures. When critical thresholds are exceeded, yields may
be drastically reduced. The probability of crossing such thresholds in a given year, for studied examplesof maize in the Midwestern US and rice in southern China, appears to rise from near zero at present, tobecome increasingly significant with global temperature rise of more than 2°C, and in the worst cases toreach somewhere in the region of 25% (maize) and 75% (rice) respectively with global temperature riseof around 4-5°C.Biophysical limits on the extent to which such tolerance thresholds can be raised may
be an important constraint on adaptation. This is one reason why high degrees of climate change could
pose very large risks to global food security.
• Thresholds for water stress are largely arbitrary, but thresholds of ‘moderate’, ‘chronic’ and ‘extreme’water shortage are widely used, based on per capita availability. The number of people exposed to
extreme water shortage is projected to double, globally, by mid century due to population growth alone.
Climate change could increase the risk in some regions: for example, on a high emissions pathway,
the probability of the Tigris – Euphrates river basin falling into extreme water shortage could risesignificantly after 2030, reaching close to 100% by 2070.
• In South and East Asia, climate change may slightly offset otherwise increasing risks of water stress,
while increasing the risk of flooding. On a high emissions pathway, what is now a ‘30-year flood’ could
become three times more frequent in the Yellow River and Indus basins, and six times more frequent in
the Ganges basin, over the course of the century, on a central estimate. In the worst case for those threeriver basins, such a flood could be in the region of ten times more frequent by the end of the century.
• On a high emissions pathway, the incidence of extreme drought affecting cropland could increase by
about 50% in the US and South Asia, double globally, and triple in southern Africa, over the course of
the century under central estimates. The uncertainties around these central estimates are large: for the
US and South Asia, in the best case, drought incidence could halve; in the worst case, it could increaseby three or four times.
• With 1m of global sea level rise, the probability of what is now a ‘100-year flood event’ becomes
about 40 times more likely in Shanghai, 200 times more likely in New York, and 1000 times more likely
in Kolkata. Defences can be upgraded to maintain the probability of a flood at a constant level, but thiswill be expensive, and the losses from flooding will still increase, as the floods that do occur will havegreater depth. Thresholds of adaptation beyond which ‘retreat’ from the sea may become more feasible
than further increases in flood protection are not well defined, but the most significant limits may besociopolitical rather than economic or technological.
• Climate models suggest that global sea level rise is unlikely to exceed 1m this century, and that aplausible worst-case scenario could result in an increase of several metres by the end of the 22nd
century. However, due to inertia in the climate system, with a sustained global temperature rise of 2°C
the global sea level may be committed to rise by some 10-15m as ice-sheets gradually melt, but whether
this will take hundreds of years or thousands of years is deeply uncertain.
• Many elements of the climate system are capable of abrupt or irreversible change. Changes tomonsoons or to ocean circulation patterns, die-back of tropical forests, and the release of carbon
from permafrost or sub-sea methane hydrates could all cause large-scale disruption of the climate.
The probabilities of such changes are not well known, but are they expected to increase as the global
temperature rises.

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CLIMATE CHANGE: A RISK ASSESSMENT

CLIMATE CHANGE: A RISK ASSESSMENTSYSTEMIC RISKS: The risks of climate change are systemic. The greatest risks may arise
from the interaction of the climate with complex human systems such as global food markets,
governance arrangements within states, and international security.

RECOMMENDATIONS FOR CONTINUING RISK ASSESSMENT• As climate change increases the frequency of extreme weather events, preliminary analysis suggests
what was a ‘1 in 100 year’ shock to global food production in the latter half of the 20th century may
have become three times more likely by mid-century. If policy and market responses amplify rather than
mitigate the shock, a plausible worst-case scenario in the present day could produce unprecedented
price spikes on the global market, with a trebling of the prices of the worst-affected grains, compared tocurrent levels.

Our recommendations on risk assessment are: apply the right principles; broaden participation in
the process; and report to the highest decision-making authorities.

• Climate change has already increased the probability of extreme events such as the Russian heat waveof 2010, and the Syrian drought of 2007-2011. These events have contributed to unrest and conflict,in combination with other factors such as food export restrictions, existing resource stress, poor
governance and state fragility. At low degrees of climate change, further such risks are most likely toarise in regions where climate change is reducing already stressed resources at the same time as highrates of population growth are increasing demand.

• Identify the biggest risks. Focus on finding out more about worst-case scenarios in relation to longterm changes, as well as short-term events.

• Security risks at high degrees of climate change seem likely to be of a different order of magnitude.
Extreme water stress, and competition for productive land, could both become sources of conflict.
Migration from some regions may become more a necessity than a choice, and could take place on
a historically unprecedented scale. It seems likely that the capacity of the international community
for humanitarian assistance would be overwhelmed. The risks of state failure could rise significantly,
affecting many countries simultaneously, and even threatening those that are currently considereddeveloped and stable. The expansion of ungoverned territories would in turn increase the risksof terrorism. The temptation for states or other actors to take unilateral steps toward climate
geoengineering would be significant, and could become a further source of conflict.

VALUE: Valuing these risks is essentially a subjective exercise.
• Standard economic estimates of the global costs of climate change are wildly sensitive both toassumptions about the science, and to judgments about the value of human life. They are also likely tobe systematically biased towards underestimation of risk, as they tend to omit a wide range of impacts
that are difficult to quantify.
• Even if economic costs could be estimated accurately, their sum total would not be a good measure of
the risks of climate change. Some of the greatest tragedies of the last century had a negligible impact on
global GDP. Some of the greatest risks of climate change may be similarly non-monetary.
• Any valuation of the risks of climate change will involve subjective judgments, most notably with regard
to the importance attached to the wellbeing of future generations. Such judgments should be madetransparently, so that they may be publicly debated.

There is much that we can do to improve our assessment of climate change risk. This is an opportunity, as it
can better inform decisions on risk reduction.

Apply the principles of risk assessment. These include:• Assess risks in relation to objectives, or interests. Start from an understanding of what it is that wewish to avoid; then assess its likelihood.

• Consider the full range of probabilities, bearing in mind that a very low probability may correspondto a very high risk, if the impact is catastrophic.
• Use the best available information, whether this is proven science, or expert judgment. A best
estimate is usually better than no estimate at all.
• Take a holistic view. Assess systemic risks, as well as direct risks. Assess risks across the full range ofspace and time affected by the relevant decisions.
• Be explicit about value judgments. Recognize that they are essentially subjective, and present themtransparently so that they can be subject to public debate.
Risk assessments need to be made on a regular and consistent basis, so that in areas of uncertainty,
any changes or trends in expert judgment are clearly visible over time. This could be facilitated by theidentification and use of a consistent set of indicators in each of the three areas of risk assessment describedabove.
Broaden participation in the risk assessment process. Different participants are important to different
stages of the process:• Defining objectives: Leaders and decision-makers have a role at the beginning, in defining theobjectives and interests against which risks should be assessed.
• Information gathering: Scientists have the lead role in understanding climate change and its direct
impacts. Experts in politics, technology, economics, and other disciplines can provide information
relevant to the future of global emissions, and the indirect impacts of climate change as it interacts withhuman systems.
• Risk assessment: Whereas information gathering may collect whatever is useful or interesting, risk
assessment interrogates that evidence in relation to defined objectives and according to a specific set ofprinciples. Separating these tasks may allow both to be carried out more effectively. Climate change risk
assessments should involve not only scientists, but also experts in risk, who may be drawn from fields
such as defence, intelligence, insurance, and public health.
Report to the highest decision-making authorities. A risk assessment aims to inform those with thepower to reduce or manage the risk. Assessments of specific, local, or sectoral risks of climate change may
be directed at those with specific, local or sectoral responsibility. Assessments of the risk of climate change
as a whole should report directly to those with responsibility for governance as a whole. At the national level,
this means the head of government, the cabinet, or the national security council. At the global level, it means
institutions where heads of government meet to make decisions.

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CLIMATE CHANGE: A RISK ASSESSMENTRISK REDUCTION: ELEMENTS OF A PROPORTIONATE RESPONSE
A risk assessment aims primarily to further our understanding of the problems we face; at the same time, it
may provide some insight into the nature of the solutions.
The greatest risks of climate change arise when thresholds are crossed: what had been gradual becomessudden; what had been inconvenient becomes intolerable. Similarly, the greatest reductions in risk will beachieved by crossing thresholds at which change becomes non-linear.
Political leadership can be a source of non-linear change. With existing technology, there is already theopportunity for political leadership to significantly change the trajectory of any country’s emissions in the
short term.
Technological innovation is a natural source of non-linear change. New technologies can emerge slowly, butthen displace old ones rapidly and suddenly when some invisible threshold is crossed. Accelerating this paceof change, and bringing forward those thresholds, should be a priority in respect of the range of technologies
that are needed to achieve the low carbon transition. The top priority should be to use both technologicalprogress, and policy measures such as carbon pricing, to cross as soon as possible the threshold at whichclean energy becomes cheaper than fossil energy.
In finance, small changes in rules can produce large changes in results. Adjustments to regulations andincentives to incorporate enhanced assessment of long-term risk into the financial system could significantly
increase investment in technologies that serve our long-term economic interests.
The risks of climate change are amplified by feedbacks: rising temperatures melt ice; sea without ice absorbs
more heat; and the temperatures rise faster. Effective risk reduction will also take advantage of positive
feedbacks. Political interventions can change market sentiment, so that the market sends more investment
into clean energy technologies, so that this accelerates technological progress, so that new politicalinterventions become possible.
Just as the risks of climate change are both immediate and long-term, we must act both immediately and witha long-term view. A risk that grows over time will not be managed successfully if our horizons are short-term.
Ultimately, the risks of climate change will only be under control when we have reduced global emissions tonear zero. So while we must do all in our power to reduce emissions now, we must also follow a path that
increases our power to do more in the future.
The risks of climate change may be greater than is commonly realized, but so is our capacity to confront
them. An honest assessment of risk is no reason for fatalism. If we counter inertia with ingenuity, match
feedback with feedback, and find and cross the thresholds of non-linear change, then the goal of preserving asafe climate for the future need not be beyond our reach.

Endnote
1. ‘RCP’ stands for ‘Representative Concentration Pathway’. We refer here to the emissions pathways implicit in
the greenhouse gas concentration scenarios used by the Intergovernmental Panel on Climate Change in its
Fifth Assessment Report.

CLIMATE CHANGE: A RISK ASSESSMENT

CLIMATE CHANGE
A RISK ASSESSMENT

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CLIMATE CHANGE: A RISK ASSESSMENT

115

CLIMATE CHANGE: A RISK ASSESSMENT

INTRODUCTION

Figure 3: The last 100,000 years Temperature change, as measured in Greenland3

T

he Earth’s climate has changed dramatically in the past. It has swung in and out of ice ages,
at whose peak great swathes of North America, Europe and northern Asia were covered
in sheets of ice three kilometres thick. It has been through periods of extreme heat, wheresubtropical climates existed in high northern latitudes. The height of the oceans has changed bymore than a hundred metres.

But human civilization has seen few of those changes. Over the ten thousand years or so in which ourcivilization emerged, the Earth’s climate has been unusually stable. Global temperature and sea levels have
hardly varied. We have taken advantage of this period of stability to grow crops, build cities, and develop a
global economy.

Figure 1: The last 60 million years. Changes in global deep ocean temperature1

Time (Million Years Before Present)

Global deep ocean
temperature,
estimated based on
the oxygen isotope
record in ocean
sediments. Viewed
on this timescale,the long-term trend
has been one ofcooling, since ocean
and atmospherictemperatures peaked
at more than 12°C
above present levels
around 50 million
years ago.

The last time large changes in climate took place, human civilization had not begun. Over the last 10,000 years, theperiod in which humans transitioned from hunter-gathers to an agricultural society, the Earth’s climate experienced an
unusually stable period. This graph shows local temperature change in Greenland. (As noted above, polar temperatureschange more than the global average, and the increase in global average temperature change at the end of the last ice
age, around 10,000 years ago, was about 5°C.)

That period of stability is now ending. The greenhouse gases emitted to the atmosphere by human activities
are trapping heat, adding energy to the Earth’s system. This flow of additional energy is substantial: it isroughly equivalent to adding the energy of four nuclear bombs of the size dropped on Hiroshima, every
second.4 As a result, not surprisingly, the Earth’s climate is warming up.

Figure 4: Energy added to the Earth systemi, 5More than 90% of the energy added to the Earthsystem goes into warming the oceans. Only about
2% goes into warming the atmosphere, and thebalance is taken up by the land and the melting
ice. The total energy added continues to increase
steeply over time.

Figure 2: The last 800,000 years. Changes in Antarctic air temperatures andatmospheric CO2 concentrations2
Antarctic air temperatures andatmospheric CO2 concentrations
over the past 800,000 years,
estimated using data from ice cores.
Viewed on this timescale, the climate
can be seen to have oscillated
between ice ages (troughs) andrelatively warm inter-glacial periods(peaks). The difference between
the ice ages and the interglacials, interms of global average near-surface
air temperature, is around 4-5°C.
The temperature differences shownon this graph are roughly twice aslarge because polar temperatures
vary more than the global average.
The blue star shows the current
CO2 concentration, using data fromatmospheric measurements. As therecord shows, the recent increase inatmospheric CO2 concentration isunprecedented in the past 800,000
years.

i.

Full IPCC caption: Plot of energy accumulation in ZJ (1ZJ = 1021 J) within distinct components of the Earth’s
climate system relative to 1971 and from 1971 to 2010
unless otherwise indicated. See text for data sources.
Ocean warming (heat content change) dominates, withthe upper ocean (light blue, above 700 m) contributing
more than the mid-depth and deep ocean (dark blue,
below 700 m; including below 2000 m estimates startingfrom 1992). Ice melt (light grey; for glaciers and ice caps,
Greenland and Antarctic ice sheet estimates starting from1992, and Arctic sea ice estimate from 1979 to 2008);
continental (land) warming (orange); and atmosphericwarming (purple; estimate starting from 1979) makesmaller contributions. Uncertainty in the ocean estimate
also dominates the total uncertainty (dot-dashedlines about the error from all five components at 90%
confidence intervals).

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CLIMATE CHANGE: A RISK ASSESSMENT

CLIMATE CHANGE: A RISK ASSESSMENT

Small changes in global temperature correspond to large changes in the global climate. If the world were
five degrees cooler than it is now, we would be in an ice age, last experienced some ten thousand years
ago, before the dawn of human civilization. Five degrees warmer, and we would be in a climate of heat lastexperienced by this planet more than ten million years ago, long before the beginning of human existence.

This report does not pretend to give all the answers. Its purpose is to be illustrative: to present a newframework for a climate change risk assessment, and to put forward our best – in some cases rough– estimates of what the findings of such an assessment might be. We hope that these findings will bechallenged, updated, and improved. It is less important that readers should agree with us, than that theyshould understand why we have asked the questions that we have.

That climate five degrees warmer, or more, is a very real possibility. It could occur within the lifetimes ofchildren alive today. Decisions we take now will affect its likelihood, and will continue to influence the climate
for thousands of years and hundreds of human generations into the future.

Why do we need a risk assessment?

We have ended with some thoughts on the question of risk management. A risk assessment is a way to better
understand a problem, not a guide to solving it, and so this is a small part of our report. We provide a few
individual perspectives on how our national and global responses to climate change could be made more
effective, in proportion to the scale of the risk, simply because we would not wish to leave readers with theimpression that the situation is hopeless. That, we believe, is far from the case.

Our starting point is that we have an interest in understanding what the consequences of our decisions
might be. When the consequences could be so far-reaching in space and in time, we have an interest inunderstanding them as fully as possible.
A risk is something bad that might happen. A risk assessment asks the questions: ‘What might happen?’,‘How bad would that be?’ and ‘How likely is that?’ The answers to these questions can inform decisions about
how to respond.
Climate change fits the definition of a risk (more academically described as ‘the effect of uncertainty on
objectives’,6 or ‘an uncertain, generally adverse consequence of an event or activity with respect to somethingthat humans value’7), because it is likely to affect human interests in a negative way, and because many of
its consequences are uncertain. We know that adding energy to the Earth system will warm it up, raisingtemperatures, melting ice, and raising sea levels. But we do not know how fast or how far the climate will
warm, and we cannot predict accurately the multitude of associated changes that will take place. The answer
to the question ‘how bad could it be?’ is far from obvious.
Limiting climate change will take some effort. Although many of the policies that would reduce greenhousegas emissions could also be good for public health, quality of life, and economic growth,8 they will notnecessarily be easy to put in place. They will require the investment of both political and financial capital.
Governments and societies will have to decide how much effort they are prepared to make, and how toprioritize this issue in relation to their other objectives. An assessment of the risks will be a necessary basis
for judging what would be a proportionate response.
It is sometimes argued that a full assessment of the risks of climate change would be counterproductive,
because the risks may be so large and the solutions so difficult that people will be overwhelmed with afeeling of helplessness, and will look the other way. In some cases, this may be true. The anthropologist Jared
Diamond, in addressing the question: ‘Why do some societies make disastrous decisions?’, writes:…consider a narrow river valley below a high dam, such that if the dam burst, theresulting flood of water would drown people for a considerable distance downstream.
When attitude pollsters ask people downstream of the dam how concerned they
are about the dam’s bursting, it’s not surprising that fear of a dam burst is lowest
far downstream, and increases among residents increasingly close to the dam.
Surprisingly, though, after you get just a few miles below the dam, where fear of the
dam’s breaking is found to be highest, concern then falls off to zero as you approachcloser to the dam! That is, the people living immediately under the dam, the ones
most certain to be drowned in a dam burst, profess unconcern. That’s because ofpsychological denial: the only way of preserving one’s sanity while looking up every
day at the dam is to deny the possibility that it could burst. Although psychologicaldenial is a phenomenon well established in individual psychology, it seems likely toapply to group psychology as well.9
Our premise for writing this risk assessment is that we can all choose whether or not to look up at the dam.
Governments can choose either to ignore it, or to send their best experts to inspect it closely. We have taken
the view that it is better to be well informed than not. As the American nuclear strategist Albert Wohlstetter
wrote during the Cold War, “We must contemplate some extremely unpleasant possibilities, just because
we want to avoid them.”10

Endnotes
1. Hansen, J.E. and Sato, M. (2012). Climate Sensitivity Estimated from Earth’s Climate History. Available at http://
www.columbia.edu/~jeh1/mailings/2012/20120508_ClimateSensitivity.pdf
2. Figure by Jeremy Shakun, data from Lüthi et al., 2008 and Jouzel et al., 2007. Source: Figure 3, (2014). Climate
Change: Evidence and Causes. National Academy of Sciences, The Royal Society. https://nas-sites.org/
americasclimatechoices/more-resources-on-climate-change/climate-change-evidence-and-causes/climatechange-evidence-and-causes-figure-gallery/
3. Figure 14.1 from Young, O.R. and Steffen, W. (2009). ‘The Earth System: Sustaining Planetary Life-Support
Systems’. From Folke, C., Kofinas, G. P. and Chapin, F.S. (eds.) (2009). Principles of Ecosystem Stewardship.
Springer New York. pp 295-315. http://www.cs.toronto.edu/~sme/PMU199-climate-computing/pmu199-2012F/
notes/Discovery_of_Global_Warming.html
4. Source: FigBox 3.1-1 from Rhein, M., S.R. Rintoul, S. Aoki, E. Campos, D. Chambers, R.A. Feely, S. Gulev, G.C.
Johnson, S.A. Josey, A. Kostianoy, C. Mauritzen, D. Roemmich, L.D. Talley and F. Wang (2013). ‘Observations:
Ocean’. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth
Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M.
Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press,
Cambridge, United Kingdom and New York, NY, USA.
5. Schellnhuber, H. J. (2013). ‘Avoiding the unmanageable, managing the unavoidable’. From Chatham House
event on ‘Delivering Concrete Climate Change Action’. Available ay http://www.chathamhouse.org/sites/files/
chathamhouse/home/chatham/public_html/sites/default/files/0900%20John%20Schellnhuber.pdf
6. International Organization for Standardization definition
7. International Risk Governance Council (2012). An Introduction to the IRGC Risk Governance Framework.
Available at http://www.irgc.org/wp-content/uploads/2015/04/An_introduction_to_the_IRGC_Risk_
Governance_Framework_final_v2012.pdf
8. See The New Climate Economy online report, available at: http://newclimateeconomy.report/
9. Diamond, J. (2011) ‘Collapse – How Societies Choose to Fail or Survive’. Penguin Books, p436.
10. As quoted in ‘The New Nuclear Age’. The Economist, 6 March 2015.

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CLIMATE CHANGE: A RISK ASSESSMENT

2PRINCIPLES, SCOPE AND APPROACH OF THIS 		RISK ASSESSMENT

Including contributions from Dr David Hare, Trevor Maynard, General Ronald E. Keys & Cherie		
Rosenblum, and Dr Claire Craig

What is new about this assessment?
Many climate science reports and climate change risk assessments have been published by governments,
research institutes and other agencies around the world. Most notably, the reports of the Intergovernmental
Panel on Climate Change (IPCC), which summarize the findings of thousands of scientists, have made anenormous contribution to our collective knowledge. To a very great extent, our assessment relies on thefindings of those earlier assessments.
While some new scientific and other studies have been undertaken to provide content for this report, ourmain aim has been to collate and present existing knowledge in a way that is consistent with the principles
of risk assessment. In particular, we have aimed to put forward a risk assessment that will be relevant togovernments’ decision-making on how to respond to the problem of climate change as a whole, including
through policy on emissions (‘mitigation’, in the climate policy jargon). This sets it apart from those risk
assessments that are primarily intended to inform decision-making on the response to specific impacts ofclimate change (known as ‘adaptation’).
We have aimed to be holistic: to take into account the different factors and the different kinds of knowledge
that are most relevant to an understanding of the risk. The assessment considers four main areas:
1. The future pathway of global greenhouse gas emissions. The future rate and extent of climate change,
and all the risks that flow from it, depend significantly on the future pathway of global emissions. Since nogovernment controls global emissions, from the point of view of any individual government (or person) thisis a variable that depends mainly on the actions of other governments – as well as on non-governmental
forces, such as technological progress, economic trends, investor sentiment, and popular will.
2. The changes in the physical climate, and their direct risks to human interests. Whatever
emissions pathway is eventually followed, the risks will depend on how the physical climate responds:
how much temperature rises in response to emissions; how changes in temperature influence changes
in rainfall; how changes in temperature, rainfall and other factors lead to changes in crop yield, etc. The
risks to human interests will also depend on how successfully we can adapt to these changes. The mostsignificant risks may arise if thresholds are crossed beyond which certain kinds of adaptation are nolonger possible.
3. The systemic risks arising from interactions between changes in the physical climate andhuman systems. In complex systems, small changes can sometimes lead to large divergences in futurestate. The risks of climate change to human interests will depend not only on the direct impacts ofchanges in the physical climate, but also on the response of complex human systems such as the global
economy, food markets, and the system of international security.
4. The value we choose to give to all of the changes that might take place. If risk is defined as‘the effect of uncertainty on objectives’, or as the chance of an adverse impact on something that wevalue, then a risk assessment cannot be complete without some subjective judgment being made about
what one’s objectives are, what it is that one values, and what value one places on avoiding the adverse
impacts. Only once some value judgment has been made can the risk assessment be useful in informing
decisions.

CLIMATE CHANGE: A RISK ASSESSMENT

In each of these four areas, we have drawn on different expertise, and taken different approaches.
Our assessment of the future pathway of global greenhouse gas emissions is a political and a technologicalassessment. With regard to the short-term future, it looks in particular at the policies, plans and targets
that governments are implementing or have announced. With regard to the long-term, it concentrates on
describing the main technological challenges to reducing emissions, and considering their difficulty or thelevel of effort it might take to overcome them. By attempting to make some judgment about the relative
likelihood of different emissions pathways, this assessment differs from others, such as those of the IPCC,
which do not.
The section on changes in the physical climate and their direct risks to human interests is a scientificassessment. It attempts to be consistent with the principles of risk assessment by asking first what it is that
we might wish to avoid, and then how likely that is to occur. Many climate change risk assessments apply
this principle to consideration of the risk of extreme weather events, so as to use this information to inform
decision-making on disaster risk reduction and climate change adaptation. We also apply it to a considerationof long-term changes, and the risk that even the averages of climatic variables eventually reach extreme
values or exceed important thresholds, since this is relevant to decision-making on energy and emissions.
Our consideration of the systemic risks arising from interactions between changes in the physical climate
and complex human systems is in large part a security risk assessment. Recognizing the depth of uncertainty
about the future state of complex systems, it uses the futures tools of scenarios and wargaming to help
us think about what might happen. It differs from much of the published literature on climate change andsecurity by deliberately making explicit distinctions between security risks in the near-term with low degrees
of climate change, and security risks in the long-term with high degrees of climate change.
Finally, our consideration of the value we choose to give to all of the changes that might take place is basedon a recognition of the essential subjectivity of this question. Rather than attempt to quantify this value,
we focus on making clear the limitations of a quantitative approach. We highlight the inescapable ethicalquestions that economics can inform, but not answer. Rather than put forward any valuation as being‘correct’, we invite readers to make up their own minds.
The introductory sections at the beginning of each of the four parts of the risk assessment set out ourapproach in more detail.

Principles of risk assessment
While each stage of our risk assessment has drawn on different kinds of knowledge and applied different
methodologies, we have tried to be consistent in our application of some basic principles. We identified
these principles from literature on risk assessment, and from discussions with expert practitioners in risk
assessment from the fields of finance and national security.1
The principles of risk assessment that we have applied are:1. Identify risks in relation to objectives. As one guide to risk assessment states, ‘Risk assessment
begins and ends with specific objectives.’2 As noted above, our risk assessment ends with theconsideration of value, which we recognize as being essentially subjective. But we must also start withsome objectives, otherwise we cannot identify risks to them. So we have assumed that that our common
objectives are human prosperity and security, and it is risks to those objectives that we consider. It follows
from this principle that in assessing risks, we ask first what might happen that could most affect ourinterests, and then how likely that would be to occur. (We do not ask first what is most likely to happen,
and then how that would affect our interests.)
2. Identify the biggest risks. This follows logically from the first principle. The more a risk could affectour objectives, the more relevance it is likely to have for our decision-making. If risk is defined simply as
the product of likelihood and impact, then the biggest risks may be those which are most likely to occur,or those which would have the greatest impact, or those which fall somewhere in between. Mathematicallyspeaking, this will depend on the shape of the probability distribution function. In practice, the risks ofmost concern are usually those with the greatest impact, especially when there is potential for irreversible
consequences (e.g. death).

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3. Consider the full range of probabilities. This follows from the second principle, since the biggest
risks could lie anywhere in the probability distribution. However, it is worth stating separately, becauseof the particular importance of not ignoring low probability, high impact risks. It is a matter of judgment
how low a probability is worth considering. Insurance firms in Europe are regulated to guard against ‘1
in 200 year’ risks to their solvency (i.e. risks that have a 0.5% probability of occurrence in a given year).
It has been argued that if preserving a stable climate is as important as avoiding the insolvency of an
insurance firm, then we should apply no less a cut-off point to our consideration of climate risks.3 The UK
Government’s National Risk Register of Civil Emergencies gives serious consideration to risks witheven lower probabilities: for example, the risk of major industrial accidents, which is assessed to have alikelihood of between 1 in 20,000 and 1 in 2,000.4 When a probability cannot be meaningfully quantified,
it is usual to consider a ‘plausible worst case’. Again, the question of what is a relevant threshold of‘plausibility’ is a matter of judgment.
4. Use the best available information. This may be quantitative or qualitative, the results of experimentsor the exercise of expert judgment. Even where there is deep uncertainty, a best estimate – based on
the best available information – is usually better than no estimate at all. Where there is no information,
ignorance itself may be a data-point that is relevant to decision-making – as it would be to a man walking
along a cliff-top in a heavy fog.
5. Take a holistic view. This means taking into account all relevant factors, as far as possible – including
human behaviour, and the complex interactions between different parts of a system. While models can be
useful for understanding complex systems, factors that fall outside the consideration of a model should
not be ignored. When a system is impossible to model in a meaningful way, scenarios may be developed
to imagine its possible future states.
6. Be explicit about value judgments. Subjective value judgments are inherent both in identifying what
constitutes a risk (i.e. what it is that we might wish to avoid), and in deciding how much we care about
it. These value judgments need to be clear and explicit, so that readers can easily apply different values ifthey choose. At the same time, once a risk has been identified, the assessment of its likelihood should beentirely objective, based on the best available information.
Finally, as recommended by the International Risk Governance Council,5 we have maintained a clear
separation between risk assessment – analyzing and understanding a risk – and risk management – deciding
what to do about it.
Here we present some brief perspectives from the fields of finance, security and government science advice tofurther illustrate some of these principles and their relevance to understanding the risks of climate change.

21

CLIMATE CHANGE: A RISK ASSESSMENT• Understanding risk drivers and contingent risks is critical in determining potential outcomes. For
climate change, risk drivers include human emissions, climate feedbacks, and human vulnerability.
Contingent risks include ‘second order’ risks, such as political instability arising as a result of climatechange impacts on food and water security.
• Transparency and disclosure of risks are paramount so that markets and decision-makers can respondto risk appropriately.
One of the key risk drivers for climate change is the response of global temperatures to emissions. If
only ‘fast feedbacks’ are taken into account, relatively low temperature increases will be calculated.
But past climate change indicates that in the long term, ‘slow feedbacks’ can lead to much higher
temperature increases (see Figure 5). A risk management approach should examine the risks that these‘slow feedbacks’ are relevant to us and consider the scenarios of most concern. In actuarial work, it is theextreme cases that we consider to be the most important.

Figure 5: Example of a non-modelled risk; alternative estimates of temperaturesensitivity to CO2 (plotted areas include uncertainty of 1 standard deviation, the fullrange of possible outcomes is wider).6

Global Average Temperature Change, °C

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Atmospheric concentration of CO2 parts per million

An actuarial perspective
Dr David Hare, Immediate past-President of the Institute and Faculty of Actuaries
Climate change is primarily a risk management problem – one of the most important goals ofclimate change policy should be to limit the probability of a very bad outcome to an acceptably
small value.
Risk assessment in the actuarial profession is based on understanding scenarios that could have thegreatest impact, even if the probability is low – we are concerned with protecting against the ‘risk ofruin’. To assess and manage the risk of ruin in the insurance industry actuaries rely on three importantelements: models to determine sufficient capital to cover liabilities that could arise from a ‘1 in 200 year
event’; scenario testing to manage risks and assess future risks; and, disclosure and transparency to assist
market forces in imposing discipline on firms.
Modelling alone will not assess or manage risk effectively – all three aspects of risk assessment are
required and are applicable in the case of climate change risk:• Risk models are a representation of the world (albeit imperfect) and should reflect all appropriatequantitative and qualitative data. A factor that is important in determining risk should never be excluded
from consideration simply because it cannot be quantified.

Good decisions are often based on exploring difficult scenarios and then using this information to mitigate
the risk. In the case of climate change, the question remains – are we focusing too much attention on a
2°C world, rather than the risk of a more extreme temperature rise of 4°C or more? These more extreme
scenarios are possible by the end of the century and, due to the uncertainty about the nature and scale ofimpacts, there is no certainty that adaptation will be successful.

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CLIMATE CHANGE: A RISK ASSESSMENT

A reinsurance perspective
Trevor Maynard, Head of Exposure Management and Reinsurance, Lloyds of London
Risk assessments in reinsurance use a mixture of methods including horizon scanning, scenario tests andcatastrophe modeling. The principles of risk assessment we apply include: (i) concentrate your effort on
the largest risks, (ii) base analysis on the best available information, (iii) avoid the dangers of averaging,(iv) carry out continual reassessment of the risk; (v) cater for human factors and (vi) take account of
uncertainty.
Human factors are critically important in assessing the risk of climate change. Just as our catastrophe
models consider the uncertainty in natural hazards, so our climate policy must be based on risk
assessments that consider the possibility that the negotiations fail, that some carbon capture technologies
do not deliver, that policies may be reversed by future administrations, etc. When these are considered wewill see that extreme outcomes are much more likely, and we may decide to strengthen our actions so as to
avoid them.
Avoiding the dangers of averaging is important in identifying the largest risks. My friend Professor
Lenny Smith has an excellent analogy to bring this point home. Imagine three policymakers who likeriver walking; none of whom can swim. They ask their scientific advisor whether the depth of water
ever exceeds head height. The advisor asks three universities to develop models: the first notes thatthe water exceeds head height near to the west shore, the second believes this is not the case but water
exceeds head height in the centre of the river and the third, being very fond of their model, believes theothers are both wrong and the water only exceeds head height near the east shore. The advisor, noting
the uncertainty in the modeling, believes the best approach is to average the three results. The outcome
is regrettable! The fact is that each of the models predict certain death – but the precise location is notknown. By averaging, this crucial information is lost.
In my view, much time is spent worrying about whether a particular climate model is correct regionally.
We cannot predict exactly what temperatures will be in future at different locations, what the sea level rise
will be; how much extreme rainfall or drought will change – but the majority of climate models predict
dire outcomes somewhere – hence that overall prediction, that outcomes will be very serious indeed isvery robust – even if the details are not.
The best available information can take many forms; sometimes, all we have to rely on is expert judgment.
In these cases, it is essential for the expert to communicate without bias. It has always concerned methat our use of the word ‘conservative’ has the opposite meaning in insurance to its meaning in science.
Scientists are ‘conservative’ if they constrain their worst fears, and wait for more evidence before
communicating them; therefore, ‘conservative’ predictions tend to understate risk – they are less than best
estimates. In insurance, ‘conservative’ reserves are higher than would be required by best estimates. In
matters of risk assessment, I feel the insurance point of view is more appropriate.

A security perspective
General Ronald E. Keys, USAF (ret.) Former Commander, U.S. Air Force Air Combat Command,
Chairman CNA Military Advisory Board. Cherie Rosenblum, Executive Director, CNA Military
Advisory Board.
The military and security community is constantly dealing with decision-making under imperfect
information and uncertainty. General Gordon Sullivan, former Chief of Staff of the U.S. Army, stated in thefirst CNA Military Advisory Board report “We never have 100 percent certainty. We never have it. If
you wait until you have 100 percent certainty, something bad is going to happen on the battlefield.”
There is inherent risk in decision making with incomplete information, but as General Sullivan says, thedecision-maker cannot wait. General Sullivan’s comments get to the foundation of why risk assessment,risk management—and the ability to act under uncertainty—is so critical in dealing with the impacts of achanging climate.

CLIMATE CHANGE: A RISK ASSESSMENT

Risk assessment has to pay attention to low probability, high impact risks. As Admiral Frank ‘Skip’ Bowman,
United States Navy (Retired) has said: “Even very low probability events with devastating consequencesmust be considered and mitigation/adaptation schemes developed and employed. We operate our
nuclear submarine fleet in this fashion. Some may argue that this continuing process results inoverdesign and overcautiousness. Maybe so, but our U.S. submarine safety record testifies to thewisdom of this approach. That’s where we should be with climate change knowns and unknowns.”
In a second report on the national security risks of climate change, the CNA Military Advisory Board
warned against a ‘failure of imagination’ with regard to situations of deep uncertainty:

When it comes to thinking about how the world will respond to projected changes in
the climate, we believe it is important to guard against a failure of imagination. For
example, in the summer of 2001, it was, at least partly, stovepipes in the intelligencecommunity and a failure of imagination by security analysts that made it possible forterrorists to use box cutters to hijack commercial planes and turn them into weapons
targeting the World Trade Center and the Pentagon. Regarding these threats, the 9/11
Commission found “The most important failure was one of imagination. We do notbelieve leaders understood the gravity of the threat. The ... danger ... was not a major
topic for policy debate among the public, the media, or in the Congress....” Failure
to think about how climate change might impact globally interrelated systems could
be stovepipe thinking, while failure to consider how climate change might impact allelements of U.S. national power and security is a failure of imagination.
One of the key purposes of risk assessment is to allow decision-makers to weigh choices for action under
uncertainty. To give leaders a process to evaluate threats, probabilities, outcomes, and courses of action
with incomplete information, in this case, divorced from political pain and personal preferences. If policymakers, under the guise of ‘waiting for perfect information’, fail to set strong climate change mitigation
and adaptation policies today, they are ignoring the risks to our economy and our national security forthe future. Risk analysis is pretty simple really: ‘How bad can it be? Can I stand that? And if not, how do I
move the fallout back to something I can live with, and when must I start?’ Two other points are critical:(i) how will I know my plan is working or not in time to change it? And (ii) if we are wrong, what’s thecost and how bad could that be? Not making any decision is actually letting fate decide. The military adageis, ‘Plan for the worst, hope for the best, and accept anything in between’ – and act.
The CNA Corporation’s Military Advisory Board (MAB) is a group of sixteen retired Generals
and Admirals that studies global issues to assess their impact on national and global security.

A government science adviser’s perspective
Dr Claire Craig, Director, UK Government Office for Science
To understand systemic risks we must draw on evidence from all forms of science and scholarship.

The infrastructure created by humans and the natural infrastructure of the planet areboth vital for our survival and wellbeing. It is only possible for more than seven billion
people to inhabit the Earth because of our ability to modify our environment. We
achieved this by creating social and physical structures, and by discovering how toharness the fossil energy sources of the planet to power our modern world. But in spiteof all our innovation and ingenuity we are still critically dependent on our natural
infrastructure, on our interactions with animal and plant health, on weather, climate
and all the other aspects of the physical and biological environment of the planet.7
The UK Government Office for Science provides science advice in situations that range from emergencies
such as the Fukushima Daiichi nuclear incident or the recent Ebola outbreak, to the exploration of thevery long term such as in the future of cities.

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CLIMATE CHANGE: A RISK ASSESSMENT

It is true in all cases that management of any risk (or opportunity) of significance to decision-makers
requires evidence from both the physical and the human sciences. Usually, this is because assessment of
the risk and its responses requires insights into the behaviour of complex systems in which human andphysical behaviours are coupled. Also, because the aim is to change the future the evidence must facilitate
action, and actionable evidence requires insights into human behaviour.
A practical example comes from the UK’s National Risk Register.8 There, the overviews of theconsequences for each risk explicitly include both direct and indirect or systemic impacts. The potential
consequences of pandemic flu, for example, include the direct medical impact of dealing with infections,
together with indirect impacts causing social and economic disruption including potential threats to thecontinuity of essential services, lower production levels, and shortages and distribution difficulties.

CLIMATE CHANGE: A RISK ASSESSMENT

as long as emissions of greenhouse gases are above zero, their concentration in the atmosphere is increasing,and the global temperature is going up. This makes the risks of climate change quite different from the risksof natural hazards such as earthquakes, which tend to be roughly constant over time in a given location, or
the risks from a radioactive waste deposit, which will gradually decrease over time (see Figure 6). For climatechange, if we do not consider the long term, we will not be considering the biggest risks.

Figure 6: A risk roughly constant over time: major earthquakes13

During the recent Ebola emergency, the UK worked with US, French and other partners to manage risks
at source and in the home nations. Getting this right required sophisticated epidemiological modelling.
But decision-makers also drew on the behavioural and social sciences, including anthropology and history,
to help understand human behaviours such as the significance of traditional burial customs. This enabled
them to better assess and anticipate the risks, and to design and monitor interventions to bring the rates
of infection down as rapidly as possible.
What is true about facing up to risk in the short term is also true about major long term risks. It is
certainly true for our understanding of climate risk. We need to consider the role of climate change asrisk multiplier and the interdependencies between different sources of risk.9 GO-Science’s Foresightprogramme has shown how intimately climate change interacts with social, technological and economic
drivers to shape possible futures in the global food and farming system, in patterns of internationalmigration and in flood risk.10 These studies show that we need to get from considering the physics ofclimate change in isolation, to a better understanding of how intimately climate change interacts withsocial, technological and economic drivers.

Scope of the risk assessment, in space and in time
A risk assessment informs decision-making by providing information about the possible consequences ofdecisions. So it is logical that a risk assessment should have a scope in space and time that is wide enough toinclude the most significant consequences of the decisions it is aiming to inform.
As stated above, our risk assessment is intended primarily to inform governments’ decision-making on
emissions policy. The consequences of emissions – the risks of climate change – occur in every part of theworld, and so it follows that our risk assessment should have a global scope. Since we have not attempted
to be comprehensive, we have described a range of risks across the world that we think may be of particularinterest to decision-makers in national governments, particularly those of countries with significant economic
size and political influence.
The logical scope in time of a climate change risk assessment is perhaps not so obvious. Risk assessments
often have a relatively short-term focus: the UK Government’s National Risk Register of Civil Emergencies
considers only the next five years, and its National Security Risk Assessment only the next twenty. The
risks of climate change that could occur over such short time periods are irrelevant to decision-making
on emissions: inertia in the climate system means nothing we do now to reduce emissions will have any
significant effect for at least the next couple of decades.
Decisions relating to emissions have consequences beginning in the medium term, and lasting over a very
long time period. Once a coal-fired power station is built, it is likely to keep operating for several decades(though not inevitably: it could be closed early, if that cost is accepted). Once carbon dioxide has been
emitted to the atmosphere, a substantial fraction of it will still be there, changing the climate, ten thousand
years later.11 So it makes sense for a climate change risk assessment to consider the long term.
A long-term view is not unique to climate change: in assessing the risks arising from the storage of radioactive
nuclear waste, governments have considered timeframes of thousands, hundreds of thousands, and even amillion years.12 But climate change has a particular characteristic that makes consideration of the long termeven more important: the risks of climate change tend to increase over time. This is likely to be true at least for

Figure 7: A risk that decreases over time: radioactivity from nuclear waste14

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CLIMATE CHANGE: A RISK ASSESSMENT

CLIMATE CHANGE: A RISK ASSESSMENT

Figure 8: A risk that increases over time: climate changei, 15
Endnotes

For the purposes of this risk assessment, we have imposed no limit on the time period under consideration.
However, in practice the time period for which a risk assessment can be meaningful depends on the quality of
information available, and the degree of complexity of the risks. For very large, slow-moving components ofthe climate system such as continental ice-sheets, it is both possible and informative to consider what might
happen over hundreds and even thousands of years. The direct risks of climate change – such as the impact
on crop yields – are often assessed out to the year 2100; in some cases we have found it possible to look alittle further. For the systemic risks, such as risks to global security, it is extremely difficult to consider as far
ahead as the end of the century.
In general, we have aimed to look as far ahead as information or reasonable judgment will allow, and we leaveit to readers to decide how much importance they attach to what could occur over different periods of time.

i.

Full IPCC caption: Time series of global annual mean surface air temperature anomalies (relative to 1986–2005) from CMIP5
concentration-driven experiments. Projections are shown for each RCP for the multi-model mean (solid lines) and the 5 to 95% range(±1.64 standard deviation) across the distribution of individual models (shading). Discontinuities at 2100 are due to different numbersof models performing the extension runs beyond the 21st century and have no physical meaning. Only one ensemble member is usedfrom each model and numbers in the figure indicate the number of different models contributing to the different time periods. No
ranges are given for the RCP6.0 projections beyond 2100 as only two models are available.

1. Sources used to identify these principles include:(i) Federation of European Risk Management Associations (2013). A structured approach to enterpriserisk management (ERM) and the requirements of ISO 31000. Retrieved from http://www.ferma.eu/app/
uploads/2011/10/a-structured-approach-to-erm.pdf(ii) International Risk Governance Council (2008). An introduction to the IRGC Risk Governance Framework.
Retrieved from http://www.irgc.org/risk-governance/irgc-risk-governance-framework/(iii) UK Cabinet Office (2012). National Risk Register of Civil Emergencies.
(iv) UK Government. Guidance: Risk assessment: how the risk of emergencies in the UK is assessed . Retrieved
from https://www.gov.uk/risk-assessment-how-the-risk-of-emergencies-in-the-uk-is-assessed(v) UK Government. Fact Sheet 2: National Security Risk Assessment.
(vi) PriceWaterhouseCoopers (2008). A practical guide to risk assessment.
(vii) UK Health and Safety Executive (2001). Reducing Risks, Protecting People: HSE’s decision-making process.
Retrieved from www.hse.gov.uk/risk/theory/r2p2.pdf(viii) Committee of Sponsoring Organizations (2004). Enterprise Risk Management – Integrated Framework.
(ix) US Department of Justice (2005). Assessing and Managing the Terrorism Threat.
(x) Engineering Council (2011). Guidance on risk for the engineering profession .Retrieved from http://www.
engc.org.uk/about-us/guidance-on-risk
2. PriceWaterhouseCoopers (2008). A practical guide to risk assessment. p.15
3. Oliver Bettis (2014). Risk Management and Climate Change: Risk of Ruin. Retrieved from http://www.lse.ac.uk/
GranthamInstitute/wp-content/uploads/2014/01/Oliver-Bettis-Risk-Management-and-Climate-Change-Risk-ofRuin.pdf
4. Cabinet Office (2012). National Risk Register of Civil Emergencies. p.8
5. International Risk Governance Council (2008). An introduction to the IRGC Risk Governance Framework. p.8
6. PALEOSENS project members (20120). ‘Making sense of paleoclimate sensitivity’. Nature 491, 683-691.
doi:10.1038/nature11574. Also see Kirby, A. ‘Earth “may be doubly sensitive” to CO2’. Climate New Network.
http://www.climatenewsnetwork.net/2013/12/earth-may-be-doubly-sensitive-to-co2/
7. Annual Report of the Government Chief Scientific Adviser 2014 (2014). Innovation: Managing Risk, Not Avoiding
It. Available at https://www.gov.uk/government/publications/innovation-managing-risk-not-avoiding-it
8. UK Government (2015). National Risk Register of Civil Emergencies – 2015 Edition. Available at https://www.
gov.uk/government/uploads/system/uploads/attachment_data/file/419549/20150331_2015-NRR-WA_Final.pdf
9. Foresight Report on the “International Dimensions of Climate Change”, July 2011; https://www.gov.uk/
government/publications/international-dimensions-of-climate-change
10. Government Office for Science (2011). The Future of Food and Farming. Available at https://www.gov.uk/
government/publications/future-of-food-and-farming; Government Office for Science (2011). Migration
and Global Environmental Change: Future Challenges and Opportunities. Available at https://www.gov.uk/
government/publications/migration-and-global-environmental-change-future-challenges-and-opportunities;
Government Office for Science (2004). Future Flooding. Available at https://www.gov.uk/government/
publications/future-flooding.
11. Inman, M. (2008) ‘Carbon is Forever’. Nature Reports Climate Change. Retrieved from http://www.nature.com/
climate/2008/0812/full/climate.2008.122.html
12. Blue Ribbon Commission on America’s Nuclear Future (2012), Report to the Secretary of Energy. p.90,
retrieved from http://energy.gov/sites/prod/files/2013/04/f0/brc_finalreport_jan2012.pdf Thanks to Bob Wardfrom the London School of Economics for pointing this out.
13. Data from the National Centres for Environmental Information.
14. Retrieved from http://blogs.egu.eu/network/geosphere/2015/02/02/geopoll-what-should-we-do-withradioactive-waste/
15. Source: Figure 12.5, Collins, M., R. Knutti, J. Arblaster, J.-L. Dufresne, T. Fichefet, P. Friedlingstein, X. Gao, W.J.
Gutowski, T. Johns, G. Krinner, M. Shongwe, C. Tebaldi, A.J. Weaver and M. Wehner (2013). ‘Long-term Climate
Change: Projections, Commitments and Irreversibility.’ Climate Change 2013: The Physical Science Basis.
Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate
Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M.
Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

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RISK ASSESSMENT PART 1:

EMISSIONSWHAT IS THE PROBABILITY OF FOLLOWING A HIGHEMISSIONS PATHWAY?

RISK ASSESSMENT PART 1:

EMISSIONSWHAT IS THE PROBABILITY OF FOLLOWING A HIGH EMISSIONS PATHWAY?

Contributing authors to this section: Professor Daniel Schrag,i Professor Qi Ye,ii Dr Arunabha Ghosh,iii
Mr Anil Jain,iv Professor Zhou Dadi,v Professor Sir David King,vi Professor David MacKay,vii Dr
Elmar Kriegler.viii

In this part of our risk assessment, we attempt to comment on the relative likelihood of different
future pathways of global emissions. This approach differs from climate change assessments
in which emissions pathways are presented as ‘equally plausible’, with no comment on theirprobability. We begin by explaining why, for our purpose, this difference in presentation isnecessary.

3

HISTORY AND PURPOSE OF THE IPCC SCENARIOSIn 1992, the Intergovernmental Panel on Climate Change (IPCC) developed a series of scenarios (IS92) toevaluate future greenhouse gas trajectories and future climate. The purpose of using scenarios was to allow
the climate assessment to compare climate model results based on identical greenhouse gas emissions overtime. There were six scenarios, covering a wide range of trajectories, from low emissions scenarios that had
CO2 emissions peaking by 2020 below 8 billion tons (Gt) of carbon per year, to high emissions scenarios that
had emissions growing steadily through the century, reaching 35 Gt of carbon by 2100.
In 2000, in preparation for the third Assessment Report (TAR), the IPCC published a Special Report on
Emissions Scenarios (SRES), which replaced the IS92 scenarios with 40 different scenarios, grouped intosix ‘families’, each with common themes for the major factors controlling greenhouse gas emissions. For the
SRES scenarios, each family had projections for population, economic growth, economic disparity between
Annex I and non-Annex I countries, and energy technologies. These scenarios covered a slightly narrower
range as the IS92 scenarios, although still including a low-emissions scenario that had emissions decreasing
through most of the century, and several high-emissions scenarios that showed emissions continuing to growthrough 2100.
For the climate science assessment (‘Working Group I’) of the Fifth Assessment Report, the IPCC switched
to using a new set of scenarios – called ‘Representative Concentration Pathways’ (RCPs). RCPs moved
away from explicitly describing the various social factors such as economic or population growth. Instead,the RCPs describe four emissions pathways that lead to four different levels of radiative forcing in 2100(+2.6, +4.5, +6.0 and +8.5 W/m2). The RCPs were the first IPCC scenarios to explicitly consider emissions
past 2100. We know from a variety of modelling studies that peak warming depends primarily on global,
cumulative emissions of CO2, the most important greenhouse gas, a significant portion of which remains
in the atmosphere for tens of thousands of years. Thus, extending the scenarios beyond 2100 is importanti.

Director, Harvard University Center for the Environmentii.

Director, Brookings-Tsinghua Centre for Public Policy, Tsinghua University

iii. CEO, Council on Energy, Environment and Water
iv.

Senior Energy Analyst, National Institute for Transforming India (NITI Aayog). Mr Jain’s views are given in his personal capacity and do
not necessarily represent the official position of NITI Aayog.

v.

Former Director General, Energy Research Institute, National Development and Reform Commission of China

vi. UK Foreign Secretary’s Special Representative for Climate Changevii. Regius Professor of Engineering, University of Cambridge; former Chief Scientific Advisor to the UK Government Department ofEnergy and Climate Change
viii. Vice Chair of Sustainable Solutions, Potsdam Institute for Climate Impact Research

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because it emphasizes that stabilizing radiative forcing requires that emissions must ultimately decrease tonear zero. However, because the social factors are not specified, the RCPs can emerge from a diverse setof possible socio-economic trajectories, as slower growth in energy consumption due to reduced economic
growth, for example, could compensate for a slower shift to non-fossil energy systems, or faster growthin global population.1 The simplicity of the RCP scenarios is an advantage, but it also makes it somewhat
difficult to understand the underlying drivers of the greenhouse gas emissions.
None of the three generations of IPCC scenarios were ever considered to be ‘predictions’ of the future, but
simply different possible futures of greenhouse gas emissions. This allowed the main focus of the climate
science assessment to be the carbon cycle and the climate system, without also needing to confront thehuge range of factors that affect how global greenhouse gas emissions will change over time. One can easily
understand why this decision to avoid any discussion of probability of the different scenarios was made,given the complexities of reaching consensus across all of the participating countries, and given the genuine
uncertainties in all of the social factors. If the objective is simply assessment of climate science, then theapproach of considering a range of different emissions trajectories, and then focusing on how the carbon
cycle and climate system responds to each, is quite reasonable.
At the same time, a much larger range of scenarios, reviewed by the IPCC reports on mitigation (‘Working
Group III’) has looked in depth at how social, political, economic, and technological factors could affectthe future pathway of global emissions. This work helps illustrate what might be a plausible range for
future emissions (see Box: ‘Framing the plausible range’), and supports our understanding of the relative
importance of the different variables.

Framing the plausible range
Models of the global system of energy, land use, population and economy provide a way to project future
emissions on the basis of changes in socioeconomic trends and policy choices. In recent years, suchmodels have been used to produce more than a thousand emissions scenarios. Taken together, these give
us an idea of a plausible range for global emissions over the course of the century (see Figure 1).

Figure 1: Emissions scenarios reviewed in the Fifth Assessment Report of Working
Group 3 of the IPCC. Scenarios are grouped according to their CO2 equivalentconcentrations in the year 2100 (see colour legend).ix Source: IPCC Fifth Assessment
Report Working Group III Figure 6.72

	  ix. Figure caption as used in source: | Emissions pathways for total CO2 and Kyoto gases for the various categories defined in Table
6.2. The bands indicate the 10th to 90th percentile of the scenarios included in the database. The grey bars to the right of the top
panels indicate the 10th to 90th percentile for baseline scenarios (see Section 6.3.1). The bottom panels show for the combined
categories 430–530ppm and 530–650ppm CO2eq the scenarios with and without net negative emissions larger than 20GtCO2eq/
yr. Source: WG III AR5 Scenario Database

At the top end of the range are scenarios where no deliberate action is taken to reduce emissions, andfossil fuel (particularly coal) availability, economic growth and population growth are all assumed
to be high. In these scenarios, emissions can more than triple by the end of the century. In all of the
scenarios reviewed in the IPCC’s Fifth Assessment Report in which no deliberate action is taken to reduce
emissions, emissions continue to increase throughout the century, whatever assumptions are made about
population growth, economic growth, energy intensity of the economy, and fossil fuel availability. Typicallyin these scenarios, emissions by the end of the century are more than double their level in 2010.
In scenarios that incorporate some of the emissions-reducing measures that have already been announced
or implemented by various countries and regions, and extrapolate a similar level of effort into the longer
term future, emissions tend to increase until around the middle of the century, and then slowly return toaround present day levels by the end of the century.
‘Climate stabilization scenarios’ are those in which emissions are calculated backwards from theachievement of a target level of warming or of greenhouse gas concentrations. At the low end of the range
are scenarios designed to be consistent with a good chance of limiting warming to 2°C. These scenarios
typically reach near-zero emissions by the end of the century, and require net negative CO2 emissions
from the energy supply and land use sectors to compensate for remaining positive emissions of othergreenhouse gases from land use and CO2 emissions from transport.

4

TOWARDS A ‘RISK’ PERSPECTIVE ON EMISSIONSSCENARIOS

Our purpose here is to provide an assessment of risk that countries face from climate change. Emissions
trajectories (and cumulative emissions) ultimately control how much climate change the world will
experience, so they must be a central part of that assessment. But if risk is the product of probability andimpact, then with no estimate of probability, there can be no estimate of risk. For this purpose, a neutral
presentation of emissions scenarios is inadequate.
Providing governments with probabilistic assessments of different emissions scenarios therefore seems
fundamental to helping them assess the risks of climate change, and make good decisions about risk
management. If some scenarios in the group are much more likely than others, on the basis of information
available today (even with the enormous uncertainties in factors such as economic growth or technologicalchange), then it is critical for those judgments to be communicated to policy makers around the world. If
those judgments are not communicated, then policymakers may either misinterpret the experts’ selectionof scenarios as representing such judgments, or they may base their decisions on their own estimates of theprobabilities, whether these are explicitly stated or not.3
But placing probabilistic estimates on different emissions scenarios for the world is more easily said thandone. Forecasting the future of global CO2 emissions from fossil fuel consumption alone, leaving aside
other greenhouse gases and emission due to land use, requires predictions of world economic growth andtechnological change over the next two centuries or more, as well as the possibility that climate policies will
significantly influence these. There are so many uncertainties, with a high likelihood of technological, politicaland social surprises of many sorts that could fundamentally change the answer. The task seems Herculean.
As a starting point, we can look at how some probabilistic judgments are already present in the world ofenergy and emissions scenarios. The range of scenarios used and reviewed by the IPCC is not as wide asis physically possible: burning all the accessible fossil fuels in the ground could sustain high emissions
for longer than the highest scenario pathway, and, in theory, an enormous effort to capture carbon andbury it underground could produce emissions lower than the lowest scenario pathway. The range has been
constrained not as much by physics as by a judgment about what is plausible. Similar judgments were madein a scenario-based study4 that took a carbon price of $1000/tonne of CO2 to be the limit of ‘economic
feasibility’, and in another that concluded that although models could compute climate pathways reaching
2 degrees by the end of the century where global emissions began to fall only after 2030, and then fell
extremely rapidly, the difficulty of achieving these reductions ‘make it seem unlikely that such pathways canbe implemented in the real world’.5 In all these cases, a judgment has been made largely on the basis of an

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understanding of politics, informed by knowledge of physical constraints and an assessment of technologicaldifficulty. It is difficult to distinguish any difference in these examples between the concepts of ‘infeasible’,‘implausible’, and ‘very improbable’.

At the state level, California has passed new legislation to achieve a 40% reduction in emissions relative to1990 by 2030. This is by far the most ambitious goal of any U.S. state, but it is too early to tell exactly how
these levels of reduction will be achieved. Other states are also making progress. For example, the state of
Iowa now has more than 30% of its electricity generation coming from wind. And the Regional Greenhouse
Gas Initiative, an electricity sector cap-and-trade regime among several northeast states, is slowly having an
impact on new investments in the electricity sector, after some initial years with a cap set much higher thanactual emissions from the region.

In this report, we attempt to make a judgment based on those same elements. After reviewing what is
generally considered to be the plausible range for global emissions over the course of this century, we provide
two different approaches to estimating the probability of different emissions scenarios within that range,
and then use both approaches to reach a final conclusion. First, we examine the near-term trajectories ofsome of the major countries and regions of the world over the next few decades, based on our knowledge of
those countries’ policies, plans and economic circumstances. Second, we examine a series of technologicalinnovations, some combination of which are required to ultimately displace fossil fuels from our energy mix
and reduce CO2 emissions to near zero. Consideration of the timescale over which those innovations will
occur, as well as the necessary energy infrastructure that must be built, also places some constraints on theprobability of different emissions scenarios coming to pass.

5

THE IMPLICATIONS OF CURRENT POLICIES AND 		PLANS FOR SHORT-TERM EMISSIONSOver the past two years, some of the largest countries and regions have made progress in developing goals
for stabilizing or lowering their greenhouse gas emissions. Some of this progress was made possible byfalling prices for renewable energy technologies, particularly wind and solar photovoltaics. In addition,
economic shifts away from energy-intensive industries are likely to play a major role in the coming years.
In the following section, we review the likely trajectories for the U.S., the European Union, China and India,
providing a brief discussion of the factors that will control their greenhouse gas emissions over the next
few decades. This analysis does not allow for us to evaluate the timescale or probability of deeper emissions
reductions required to ultimately stabilize greenhouse gas levels in the atmosphere, but can provide a
qualitative sense of which of the IPCC emissions scenarios are more likely.

The United States
In the U.S., emissions have fallen more than 10% relative to 2005. Some of this decline comes from sustained
high oil prices until the summer of 2014, which led to a reduction in vehicle miles travelled by passenger
vehicles. The economic impact of the financial crisis of 2008 was also a factor in reducing the growth ofenergy demand. Finally, sustained low prices for natural gas driven by production from shale caused ashift away from coal consumption in the electricity sector, which has also been a major factor in reducing
emissions. Moving forward, the Obama administration has taken steps to achieve much deeper reductions inemissions, proposing a target of 26% to 28% reduction in greenhouse gas emissions relative to 2005 by 2025.
In order to achieve these reductions, two Federal policies have already been created by the Environmental
Protection Agency (EPA), aimed at reducing emissions from transportation and from the electricity sector.
There is also important policy activity at the state level, in particular an economy-wide cap-and-trade regime
in California, and a cap-and-trade market specific to the electricity sector for several northeast states.
The first major Federal action on reducing greenhouse gas emissions was revisions to the Corporate Average
Fuel Economy (CAFE) standards. These standards now require an average performance equivalent of 54.5
miles per US gallon (65.5 miles per imperial gallon) for passenger vehicles by 2025. In addition, in 2011, theObama Administration finalized the first-ever fuel economy standards for heavy-duty trucks, buses, and vans,which applies to model years 2014-2018. In order to reduce emissions from the electricity sector, in April 2012
the EPA proposed a carbon pollution standard for new power plants, prohibiting building new coal-fired power
plants that do not have emissions-reduction technologies (i.e. carbon capture and storage). More recently, theEPA proposed a new rule for existing power plants that specifies emissions reductions for each state basedon the potential to shift from coal to natural gas, improve the efficiency of existing power plants, improve
efficiency in electricity demand, and add renewable generation to the current energy mix. These EPA rules arelikely to be challenged in the courts over the next several years. Their successful implementation will be criticalto reaching the goal of 26% to 28% reduction in emissions relative to 2005 by 2025.

The new efforts by the Obama administration represent an important shift in U.S. policy, and have renewedinterest around the world in achieving more aggressive reductions in greenhouse gas emissions, but real
progress still faces many challenges. First, the substantial drop in the price of oil will make it more difficult
to expand on the reductions in emissions from the transportation sector that the U.S. experienced from2005 until 2014. Second, challenges to the new EPA rules in the courts, as well as the possibility of a new
American president elected in 2016 who may oppose such efforts, means that the U.S. targets for 2025 are
not guaranteed.

The European Union
Europe’s emissions have fallen by nearly 20% since 1990, largely as a result of energy and climate policies,
supported by a decreasing share of energy intensive industry in the economy. At present, this leaves
European per capita emissions above the world average, but still only about half those of the US, Canada andAustralia. The largest emissions reductions initially came from the shift away from coal in the energy mix ofEurope’s largest economies: Germany and the UK both reduced their coal consumption dramatically after
1990 as they shifted to gas, and France continued to reduce its coal consumption as it increased its relianceon nuclear energy for electricity generation.
Climate policy has played an increasingly significant role, particularly in increasing the share of renewables
in electricity generation through both direct subsidies and portfolio standards. Notably, Germany led theway with feed-in tariffs, initiated in 1989, to promote the installation of solar and wind capacity. In 2014,
renewable energy provided 27% of Germany’s electricity generation; solar delivered about 6% and wind 8%.
Spain generated more than 20% of its power from wind in 2013, while it also made significant investments insolar photovoltaic and concentrated solar power. The UK and Denmark have led the world in the installationof offshore wind. Renewable energy subsidies have been complemented by carbon pricing: the European
Union’s Emissions Trading Scheme (ETS) applies to more than 11,000 power stations and industrial plants
in 31 countries; during its first five years of operation it imposed a price in the range of €15–25 per tonneof CO2. As the ETS price has fallen in recent years, countries such as the UK and Sweden have supplemented
it with national carbon taxes of their own. At the same time, regulatory standards have been used toprogressively decrease emissions from vehicles.
Given the progress already made, the EU looks likely to achieve its target of reducing emissions by 20% by
2020 compared to 1990. The EU’s next target – a domestic reduction of at least 40% by 2030 – should also
be achievable, but exceeding it would require overcoming some of the technical and policy challenges thatare already preventing a faster pace of emissions reduction. The expansion of the EU to include eastern
European countries has, on the one hand, slowed down the transition overall, but on the other hand it has
incorporated these countries into the policy process.
The shift of Europe’s major economies away from coal, while not yet complete, has already taken place to an
extent that means it cannot simply be repeated. Further decarbonisation of the power supply will require asignificant growth in low carbon generation. New nuclear power has been ruled out by some countries, suchas Germany, and is included in the plans of others, such as the UK. So the prospects for renewable energy
are critical. It is notable that the countries that have achieved the highest levels of renewable energy as aproportion of power generation are already experiencing some difficulties handling intermittency. Spain and
Ireland have on occasion to cut off their wind generation when it exceeds manageable levels, but Germany
and Denmark increase their electricity exports whenever solar and wind generation peak. It is recognized thatraising the contribution of renewables to power generation from 20% to 40% will require significant advances
in demand management, smart grids and energy storage. Continued reductions in cost can support thisprocess, as can interventions to increase interconnectivity across the European continent. In parallel, Europe
will need to begin to decarbonise its heating and transport – areas where progress to date has been uneven.

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On the policy side, Europe will need to continue the process of reform of the ETS in a way that ensures the
carbon price remains high enough to be effective. In recent years, the low price of around €5 per tonne of
CO2 – in combination with the fall in coal prices – has allowed coal to become more competitive than gas.
This will need to be reversed if coal is to be phased out. Further progress may be made if ETS reform takes
account of its interactions with renewable energy and efficiency policies. Under the cap and trade systemof the ETS, progress on either renewable energy deployment or energy efficiency tends to make the carbon
price fall, reducing the incentive to cut emissions in other sectors. For policies in these three areas to support
each other, the ETS would need either to have a cap that can be lowered as needed to maintain an effective
price, or to be replaced with something more similar to a fixed carbon tax. Similarly, policy on biomass willneed to be reformed to ensure that it does not lead to increased emissions from deforestation and transport.
The creation of the EU Energy Union this year gives some hope that these objectives will be achieved acrossthe EU in coming years.

In the context of this progress, on November 12, 2014, China and the U.S. signed a bilateral agreement on
climate change and clean energy cooperation. Under the joint agreement, “China intends to achieve thepeaking of CO2 emissions around 2030 and to make best efforts to peak early, and intends to increase
the share of non-fossil fuels in primary energy consumption to around 20% by 2030.”6 This is the first timethat China has committed itself to a target for total carbon emissions. Assuming that these goals are achieved,
China’s carbon emissions will continue to increase by roughly one third to one half of the current level in thenext 15 years, reaching per capita carbon emissions of approximately 10 tons, before they level off or decline.

China
China’s carbon emissions experienced rapid growth driven by the fast-growing economy for more than three
decades. In 1990, energy related carbon emissions from China were 2.27 billion tons CO2, accounting fora little more than a tenth of the world total, while U.S. emissions were more than 5 billion tons, close to aquarter of the world total. In 2014, China’s carbon emissions increased to almost 9 billion tons, nearly fourtimes their 1990 level, while the US added only 5%. In fact, China surpassed the US in 2008, becoming thelargest carbon emitter in the world. In 1990, China’s per-capita carbon emissions were less than half theworld average; in 2007, China’s per capita carbon emissions exceeded the world average and are now quickly
approaching the average level of the EU’s twenty-eight Member States.
In 2009, China set its 2020 carbon management target under the Copenhagen Accord, aiming to reduce its
carbon intensity, measured as carbon emissions per unit of GDP, by 40% to 45% as compared to the 2005
level. By the end of 2014, China’s carbon intensity was 33% lower than the 2005 level, well on track todeliver its Copenhagen pledge. Meanwhile, China’s total carbon emissions continued to grow, from 5.1 billion
tons in 2005 to nearly 9 billion tons in 2014.
Despite the continued increase in total carbon emissions, the growth rate of carbon emissions has been
in a steady decrease since 2005, and was near zero in 2014. Several different government policies haveplayed key roles in bringing down the carbon growth rate. First, energy efficiency in all major sectors has
been improving. By the end of 2014, China’s energy intensity had decreased by about 30% from the 2005
level. Coal fired power plants now use less than 290 grams of coal for generating one kWh of electricity.
The best coal-fired power plants in China are now leading the world in energy efficiency, and the national
average efficiency of all power plants is now rising to among the best in the world. The Top 1000 Enterprises
Program, a nationwide program focused on the greatest energy consumers in China, saved more carbon
emissions in five years than the European Union has saved under the Kyoto Protocol.
A second factor in slowing down the growth rate in emissions is the development of renewable energy. China
is now leading the world in investing in renewable energy, contributing to a quarter of the world total. Morethan 30% of installed wind generation capacity is in China, adding roughly half of the world’s new wind power
development in 2014. The installed capacity of solar power generation in 2005 was 700 MW, and had grown
to more than 28 GW by the end of 2014, a 40-fold increase in less than a decade. It is possible that China will
overtake Germany to become the largest developer of solar power in the world by the end of 2015.
A third factor for reducing the growth in emissions has been a concern for air pollution, which has helped to
set a cap for coal consumption in key regions, which will eventually extend to the whole country. As a result,coal consumption was down by 290 million tons in 2014 compared to the previous year, contributing to astabilization of carbon emissions in China.
Fourth, some provincial and municipal governments have taken leadership to explore low-carbon
development paths. From 2009 to 2012, 42 provinces or cities entered into a national pilot program for lowcarbon development. These pilots seem to be making an impact on other subnational and local governments
on choosing an alternative pathway for addressing economic growth and climate change.
Finally, China has made a deliberate decision to launch a nationwide carbon market in 2016 in order to pricecarbon emissions, based on a pilot program that covers seven provinces or cities. When completed, theChinese carbon market will be the largest one in the world, more than twice the size of the cap-and-trade
program in the E.U.

Much of the progress in achieving a peaking of emissions will come from reductions in coal use in theindustrial sector, outside of electricity generation. But perhaps the most important component of the joint
China-U.S. agreement is the commitment to achieve 20% of non-fossil energy in the overall energy mix,
as this will set the stage for reductions beyond 2030, as non-fossil energy begins to replace fossil capacity.
Reaching this target will not be easy, as it requires 800 to 1000 gigawatts of new electricity generation
capacity to be added, based on wind, water, solar and nuclear, requiring an investment of $1.8 trillion. But
if these goals are achieved, it opens the possibility that economies of scale will bring down the cost of these
non-fossil technologies, enabling them to become more widely used in the rest of the developing world, as
other developing countries make energy choices in the middle of this century.

India
India faces a large developmental challenge of raising the standard of living of its citizens. As per capita
consumption of energy is strongly correlated with quality of life, the above strategy will require an increasein per-capita and national energy consumption. Today, fossil fuels comprise nearly 90% of India’s primarycommercial energy, and this proportion is unlikely to fall in the near future. However, there are vast
opportunities for raising the levels of technology in power generation, lighting and transportation among
others, which could moderate the carbon intensity of the economy by reducing the energy demand. Similarly,
India could leverage its strength in solar energy, as it is endowed with roughly 300 clear, sunny days per year,
as well as a large wind-energy potential. Therefore, actions on both demand and supply sides, by enhancing
energy efficiency and enhancing renewable energy, could moderate the carbon intensity of the economy in the
coming decades.
Climate change impacts developing, tropical countries more than others, both due to their geographic
location and their poor capacity to absorb adverse effects. The dependence of a large share of farmers
in these countries on rain for their livelihood, high density of population, and weak economies further
exacerbate their vulnerability to the impacts of climate change. Being conscious of the above, the Indian
Government has strived to moderate emissions through its developmental agenda. A series of measures havebeen underway as a part of the eight missions under the National Action Plan on Climate Change in the areasof energy efficiency, solar energy, and forestry, among others. The erstwhile Planning Commission (nowreplaced by the NITI Aayog) had also constituted a Committee to suggest a strategy to moderate its carbon
intensity under the Twelfth Five Year Plan (2012-17) and beyond, even while attempting inclusive growth. In
more recent times, the Planning Commission has generated a scenarios based analytical tool – India Energy
Security Scenarios 2047 – along the lines of the UK 2050 calculator, to help policy-makers develop a way
forward in meeting India’s energy challenges. This tool also aims at achieving energy security in terms oflowering import dependence for energy supplies, while ensuring reduction in carbon emissions.
Today, India is the third largest carbon emitter after China and the U.S., or fourth if one considers theEuropean Union as one entity. However, at 1.7 tons per capita CO2 emissions or 2.1 billion tons total in 2010,
it is still far behind the others. It is also evident that the aggregate (and per capita) emissions of India aregoing to keep rising as the standard of living of Indians rises. The per capita emissions of India related toenergy consumption in 2010 were 1.26 tons of CO2, and could grow to between 3.3 tons and 5.1 tons by theyear 2047, depending on the country’s ability to adopt moderate carbon intense pathways.
India’s annual per capita energy use was 614 kgoe (kilogram of oil equivalent) in 2011, while electricity
use on similar parameters was 684 kWh. At this low energy use, a large section of the population is without
access to modern sources of energy, or is served very poorly with large outages. The consumption pattern isfurther skewed towards essential life promoting activities like cooking and keeping warm, with negligible use
in ‘life quality’ enhancing uses such as transport, lighting etc. As stated above, overall consumption of energy
is bound to rise, but energy usage could be made efficient by adoption of technology, behavioural changes
and better planning and infrastructural improvements. India’s demand for building space and consequent

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demand for steel, transport, and energy for cooking and household electricity, are all likely to grow by at leasta multiple of three between now and 2047 (the 100th year of India’s independence). Adoption of efficientbuilding codes, a shift to public transport, and adoption of electric vehicles and efficient devices, could allmoderate India’s emission intensity.

news is the growth of renewable energy installations around the world, and the concomitant reduction ofrenewable energy prices. If India and other developing countries (e.g. those in sub-Saharan Africa) are able
to expand their use of renewables more rapidly than expected, then the high emissions scenarios are unlikely
to occur. But it remains uncertain whether renewables will be able to continue their trajectory as higher levelsof penetration are achieved, when countries face the challenge of intermittency of supply and the difficult
technical challenge of energy storage.

Similarly on the supply side, while coal is expected to remain the major source of energy for heavy industry
and for electricity generation, the share of renewables and gas, and electrification of the energy sector holdlarge promises. Both grid-connected and decentralized renewable energy sources have the potential tocontribute to meeting the electricity and cooking energy needs of the poor (while as a co-benefit, encouraging
rural entrepreneurship).7 In the transportation sector, a combination of demand and supply side interventionsin this area (reduced demand for transport by better urban planning and use of electric vehicles and public
transport) could moderate energy consumption. The share of electricity in primary energy supply, as well as
contribution from renewables, could rise from the present 16% to 22%, and from 4% to 29%, respectively, toachieve a lower carbon intensity by 2047.
Two initiatives are particularly worth highlighting. The first relates to the National Mission on Energy
Efficiency. India has already launched the innovative Perform, Achieve and Trade scheme, mandatingenergy efficiency targets for plants and factories in eight industrial sectors, failing which they would needto purchase additional energy savings certificates from over-performers (2015 will be the first year of
trading). Efficiency will also be a major driver for residential appliances via the Super-Efficient Equipment
Program, which was launched in 2013. Efficiency considerations will also impact the adoption of alternative
chemicals and technologies for air conditioning and refrigeration in residential and commercial buildings. The
government has also developed plans for demand-side management in municipalities to decrease their energy
consumption.
The second policy initiative is the Indian government’s goals for renewable energy. In a total installedcapacity of more than 35 gigawatts of renewable energy (excluding large hydropower), wind power accounts
for nearly 23 gigawatts. The National Solar Mission was launched in 2010 and more than 4 gigawatts
have been deployed. But more recently, the government has announced plans to install 175 gigawatts ofrenewables-based electricity-generating capacity by 2022, including 100 gigawatts of solar power. Meetingthe solar target alone will require a growth rate equivalent to doubling India’s installed solar capacity every
18 months. It will also require a clear understanding of the three factors that drive energy demand in India(access, security, and efficiency); new federal and state policies and incentives; innovative financing for
capital investments estimated at $100 billion or more; and additional funding for manufacturing, training, andjob creation. Project developers will have to grapple with the cost and availability of land, grid connections,and backup power.8
Overall, there is enormous potential for India to reduce the energy intensity of its economy and the emissions
intensity of its energy sector, as energy consumption increases over the next three decades. In particular, thedecreasing costs of solar photovoltaics may be particularly helpful for India to limit the expansion of coalconsumption, given its high solar potential. At the same time, it is important to acknowledge that India’s GDP
had been growing annually at a near 9% growth rate in the past, with a slowing down in recent years. India
aims at growing above 7% annually in the 12th Five Year Plan period (2012-17) and a long term compound
annual growth rate of 7% in the coming decades, too. The power sector is registering a near 10% generation
capacity growth annually, with both fossil fuel and renewable energy based capacities being a part of thisgrowth. India cannot afford to postpone its development for the sake of carbon reduction goals, and thus
overall emissions from India are likely to rise substantially in the coming decades. India can partially temper
this growth in emissions with adoption of efficiency in use of energy, and promote low carbon strategies
even when locking in investments in related infrastructure. A co-benefits approach towards moderate carbon
strategy will also be helpful in curbing energy imports, by replacing imported energy by local resources suchas solar and wind power.9

A Global Perspective on Emissions over the Next Three Decades
What do these different perspectives from these four regions mean for the likelihood of different emissions
scenarios? Current progress in the European Union and in the United States in reducing emissions
is encouraging, but the rate of change is not compatible with low emissions scenarios. Similarly, theannouncement that China’s emissions will peak by 2030 is very important for avoiding the high emissions
scenarios, but still not sufficient for achieving the low emissions scenarios. Perhaps the most important

The low emissions scenarios that have a high probability of limiting warming to less than 2°C will not
be possible unless the EU achieves its goal of an 80% reduction by mid century, the U.S. and China both
accelerate their progress, dramatically reducing their coal consumption in the next two decades, and India
displaces its anticipated increase in coal consumption with an expansion of solar and other renewables.
Other countries and regions must follow suit, with non-fossil technologies ultimately becoming disruptive forsupporting economic development goals.

6

TECHNOLOGICAL CHALLENGES THAT WILL 			
DETERMINE FUTURE GLOBAL EMISSIONS

Another approach to placing constraints on the likelihood of different global emissions trajectories is toevaluate different emissions scenarios through the rate of technological change and energy infrastructure
investment. Quantitative modelling of low-carbon energy systems for different countries allows one toidentify a series of technological innovations or infrastructure investments that must be made to enablereducing emissions to near-zero levels. In the following section, we ask a series of questions that relate toenergy innovation and low-carbon energy systems. For any individual country, there are an infinite number ofpossible combinations of different technologies and approaches, and these are likely to vary across different
countries and regions. Thus, not every one of the questions about technological innovations and investments
discussed below must be answered in the affirmative to achieve near-zero emissions for the world. However,
the modelling makes it clear that many of these innovations will ultimately be required, although the exact
contribution from each one remains uncertain.
One important aspect of this approach is to evaluate a nation’s energy system by sector. For example,
energy for the transportation sector around the world is supplied almost entirely by petroleum. Replacing
the petroleum with non-greenhouse gas emitting alternatives requires not only a technology for passenger
vehicles (e.g. electric vehicles with batteries), but also a technology for replacing diesel fuel for freighttransport, for which batteries are unlikely to be sufficient, and also a technology to replace jet fuel. Rather
than specifying particular technologies in the following discussion, we chose instead to discuss very broad
categories of technological solutions, allowing for the potential for technological surprises. At the same time,
the quantitative modelling of emissions scenarios, both for the world and for individual countries, makes it
clear that we can distinguish between the low and high emissions scenarios in terms of the timescales overwhich energy innovation must occur, and also the timescales of energy infrastructure investment.
1. Can high penetration wind and solar be managed at large scale, using storage, demand
management, backup, and other approaches? Will the cost of renewables decline sufficiently todrive the world’s electricity systems to become dominated by renewable energy?
Over the last decade, there has been enormous progress in reducing the costs of wind and solar power.
Onshore wind, in good locations, is now directly competitive with fossil sources of electricity (i.e. coaland natural gas). Around the world, we have seen the growth of wind as a percentage in overall electricity
generation for countries such as Ireland (19%), Aruba (20%), and Denmark (28%), as well as regions
within countries, such as Iowa (30%). Offshore wind has been deployed in countries like Denmark and theUK, although it remains a much more expensive option relative to onshore wind installations. Progressin reducing the cost of solar photovoltaics has been particularly dramatic, starting with Germany’s
aggressive policies of feed-in tariffs, leading to about 6% of their generating capacity supplied by solar
power in 2014, and then followed by policies around the world that are driving large-scale installationsin China, the U.S. (particularly California), and southern Europe. Coupled with the rapid expansion in
installation have come advances in manufacturing around the world, particularly in China, that have

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driven reductions in prices. Costs for large-scale solar photovoltaic installations in many countries are
now below $2 per watt, encouraging many countries, such as India, to revisit their investment decisions
around electricity generating capacity.
The expansion of renewable electricity in countries and regions around the world has forced a more
intense effort to develop strategies to manage the intermittency of wind and solar energy. Many
approaches have been proposed, but only recently has the extent of renewable penetration in countries
and regions mentioned above created market conditions that allow companies to make money by
deploying energy storage and demand management strategies. The next two decades will be critical indetermining whether countries can surpass canonical limits to intermittent resources of around 20% to30%, and achieve much high levels of renewable penetration, when combined with energy storage, bettertransmission systems, and demand-management strategies that can shift the load to times when renewable
resources are available. There are many theoretical studies that suggest such strategies are possible, but
the details of their implementation, as well as the additional costs, remain uncertain. It is also possiblethat there will be renewed interest in concentrated solar power, and other renewable technologies that
include some storage capacity as an integral component. For some regions with abundant natural gas
resources, such as the United States, expansion of renewables will likely be facilitated by using natural gas
turbines as backup supply. This will not be effective in many regions of the world, such as India. Moreover,using fossil systems as backup does not allow for the deep reductions in carbon emissions required infuture years.
2. Will nuclear power become a serious option for the power sector in terms of cost, safety, andproliferation risk?
For some regions of the world, such as the United Kingdom, nuclear power is a critical component ofplans to achieve low-carbon goals, as their renewable resources are limited. Even in countries with
substantial renewable resources, uncertainty about the costs of storage or demand management, and thus
the potential for deep penetration of renewables, nuclear power may be an attractive option for zerocarbon baseload power because it is highly reliable.
The challenge in the next few decades is to bring down the cost of nuclear power while increasing
the safety and minimizing the risk of proliferation of nuclear weapons, including the risks of nuclear
terrorism, as well as developing better strategies for disposal of nuclear waste. There are many proposals
for new generations of nuclear power plants, including small modular reactors that would bring down
costs through efficiencies of scale in manufacturing, more uniform designs of larger reactors that would
bring down costs by streamlining the engineering and construction, and new reactor types, such as
thorium reactors.
Current deployment efforts are being led by China, which has 24 nuclear power reactors underconstruction, and hopes to increase its nuclear capacity more than threefold to around 60 GW by 20202021, and then to some 150 GW by 2030.19 Other countries, such as Germany and Japan, have moved
away from nuclear power following the tsunami and nuclear accident in Fukushima. But even for countries
with a commitment to nuclear power, the timescale of technological innovation in nuclear power isinherently slow, due to the relatively large scale of capital investments and the risks associated withexperimentation of nuclear design and construction. For many countries that have had substantial nuclear
power programs, such as the U.S., it looks like high costs and concerns about safety will cause a reduction
in nuclear power over the next three decades as nuclear power plants from the 1970s are retired.
3. Can we eliminate the use of petroleum from the passenger vehicle sector (without biofuel thatwill be needed for jet fuel and diesel fuel)?
Use of petroleum in the transport sector is currently one of the largest sources of energy-related
greenhouse gas emissions. Current global petroleum consumption (including natural gas liquids) isroughly 90 million barrels per day, and continues to expand despite reductions in the U.S. and the E.U.
Reaching any of the low-carbon emissions scenarios and stabilizing CO2 at only a modest increase relative
to today will require a massive, global transition over the next few decades to the use of non-fossil
technologies for vehicle transportation.
Current technologies for electric cars are making enormous progress, with several automobile
manufacturers currently producing full electric vehicles for both the luxury (e.g. Tesla, BMW) and entry
levels (Chevrolet, Nissan). At the same time, the cost of these vehicles remains expensive; current prices

are roughly double the cost of equivalent vehicles with internal combustion engines, although progressin reducing costs of batteries is expected in the coming years. Another area of innovation is in smaller
electric vehicles, such as scooters and motorcycles, particularly in China and Southeast Asia. Although theprogress in battery-powered electric cars is encouraging, some automobile manufacturers are investing inhydrogen fuel-cell vehicles as an alternative pathway to non-fossil transportation. Fuel cell vehicles would
require a massive infrastructure investment for transport and distribution of hydrogen, which is currently
made at large scale from natural gas, but could be produced in the future from renewable energy. Thus,
a transition to battery-powered electric vehicles appears to be the most likely technology that can prove
disruptive to internal combustion engines for passenger vehicles, but it remains possible that fuel cells
will also be competitive.
The major challenge that this sector poses for achieving low emissions scenarios is the pace and scale ofthe necessary changes. In 2014, there were more than a billion vehicles in the world, nearly all of themwith internal combustion engines. In order to reach the low emissions targets, non-fossil technologies
would have to become disruptive, dominating sales around the world by mid century, or perhaps even
earlier. Even after electric or fuel cell cars become competitive with internal combustion engine cars, itwill likely take several decades before the technology becomes dominant in the actual vehicles on theroads around the world. Such a large shift in manufacturing and technology is possible, but the pace andscale is daunting without major efforts to bring costs down in the immediate future.
4. Can biomass or alternative technologies be used to displace diesel and jet fuel at a reasonable
cost? How can impacts of biomass through land use be managed?
Replacing petroleum currently used for jet fuel and for diesel fuel for freight transport may be the mostdifficult technological hurdle to reach a non-fossil economy in the future. Current aviation technologies
require hydrocarbon fuels because of the technical requirement for fuel with very high energy densityand very low mass. Some freight transport could be transferred to trains powered with electricity, but it isdifficult to imagine replacing all truck transport with trains. Whatever freight transport will be done withtrucking will be very difficult to electrify, given the power-to-weight challenges of battery technologies in
the foreseeable future. A likely alternative to petroleum-derived diesel and jet fuels is biofuel, produced
through a variety of biochemical and thermochemical processes, including Fischer-Tropsch synthesisthat is used to produce fuels from coal in South Africa. The biomass feedstock requirements of replacing
diesel and jet fuel with biofuel are massive, even with enormous efficiency gains in aviation and trucking.
For example, using current Fischer-Tropsch technologies to produce 10 million barrels of fuel per day– equivalent to roughly 11% of current petroleum demand – using biomass as a feedstock would require
more than 125 million hectares of cropland, even assuming very high biomass yields of 20 dry tons
per hectare. It is not clear whether cultivation of biomass crops for energy use at such a scale can beaccomplished in the context of growing food demand around the world, nor is it clear that 10 million
barrels of jet and diesel fuels per day will be sufficient.
Some work has been done to look at recycling of CO2 from biofuel and fossil fuel combustion into fuels,
first through reduction to carbon monoxide, and then through Fischer-Tropsch synthesis, using hydrogenproduced from renewable sources. In theory, this is possible, but the chemical reduction of CO2 into
carbon monoxide is very energy intensive and remains a challenge to economic competitiveness. Overall,
it is clear that there is no technology currently available that appears to be competitive with petroleum,even at much higher prices and with a very high price on carbon. Thus, relatively little effort has been
dedicated to this problem, and it seems unlikely to emerge as a major priority for several decades. An
appropriate technological replacement for petroleum will eventually emerge as oil supplies dwindle, but
waiting until most of the oil reserves are extracted is simply incompatible with low-emissions scenarios.
5. Is carbon storage feasible at very large scale (i.e., tens of billions of tonnes per year)?
One approach to reducing CO2 emissions involves CO2 capture from emissions sources and then storage in
geological repositories, often referred to as carbon capture and storage (CCS). CCS appears particularly
attractive in reaching a low-carbon economy for several reasons. First, CCS might allow the world totransition to a low-carbon economy without discarding capital investments that have been made inelectricity infrastructure. Currently, there are more than 2,000 power plants that emit at least 1 million tonsof CO2 a year. Together these power plants released more than 10 billion tons of CO2, or roughly one-third

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of global emissions. To the extent that some of these plants can be retrofitted with capture technology andthat appropriate storage locations can be identified, CCS would allow the world to continue to use some ofthese facilities for many decades but dramatically reduce their environmental impact.
Even aside from the use in existing fossil fuel facilities, such as large coal-fired power plants, CCS is likely
to be a critical part of a low-carbon economy in the industrial sector for stationary sources of emissions,such as cement plants, that are difficult to eliminate. One particularly important application of CCS may
be for the biofuels sector, if biofuels become an important source of non-fossil liquid fuel for freighttransport and air transport. Whatever the specific technology used to convert biomass to fuels, whether
through fermentation or through thermochemistry, a biofuel refinery will create a large, concentrated
stream of CO2 that represents a relatively low-cost source of emissions reduction. Such use of CCS can beviewed as ‘air capture’ of CO2 as the life cycle emissions from such a facility would be negative.
Currently, there are a small number of demonstrations of carbon storage in geological repositories aroundthe world. The most successful is the Sleipner Field in the North Sea, operated by StatOil, the Norwegian
Oil Company, and injecting 1 million tons of CO2 per year since 1996. Based on this experience, it seems
very likely that carbon storage in saline formations, whether onshore or offshore, is likely to be effectivein permanently storing CO2, but it remains unknown whether the geologic reservoirs can handle theenormous volumes of CO2 required if CCS ever became a dominant technology, for example in large
biofuel refineries around the world. Estimates of capacity are extremely large, but some studies havequestioned those estimates based on limits to injectivity, potential for induced seismicity, and leakage
potential. Until more commercial-scale demonstration projects are operating in different geologicalsettings around the world, it will be impossible to guarantee that CCS will be feasible at the scale required
to play a major role in a low-carbon world.
6. Will technologies and planning that allow large increases in energy efficiency be deployed atlarge scale?
There are enormous potential gains in energy efficiency across all sectors of our energy system. Buildings
have been a particular emphasis of many studies; new building materials and new building designs can
reduce energy use in buildings by 70% or more relative to current levels, even in countries that haveexperienced relatively high energy prices and are already quite efficient. Similar arguments have been
made for industrial facilities, and also transportation systems, focusing on the potential for use of lightermaterials (e.g. carbon fibre) and more efficient motors and control systems, as well as improvements inurban planning and more efficient systems such as light rail.
Energy efficiency plays a critical role in determining whether the world will follow a low or high-carbon
emissions scenario, primarily by reducing the total amount of industrial capacity required. For example,
if passenger vehicles are powered with electricity, but are lighter and use much less energy than current
vehicles, then less electricity generation will have to be constructed. Similarly, if biofuels become a criticalsource of non-fossil liquid fuels for aviation, then more efficient airplanes with lighter materials will
require fewer biofuel refineries and less land area required for growing biofuel feedstocks.
The critical question is whether these large improvements in energy efficiency that seem to betheoretically possible will actually be implemented across entire countries and regions. Some have argued
that many energy efficiency investments will ultimately save money, and that there are market failuresthat prevent such investments from occurring. On the other hand, progress in energy efficiency has beenslower than expected, leading some economists to question whether the analyses of costs and savings
from energy efficiency are correct. If the world is going to manage to limit emissions and follow one of thelower-emissions scenarios, then it will be critical to achieve as many gains in energy efficiency as possible,
at all scales from appliances and vehicles to factories and large-scale transportation systems.
7. How fast can large-scale energy infrastructure be built?
A transition to a low-carbon or zero-carbon economy at a global scale requires massive new investments
in infrastructure to replace existing systems based on fossil fuels. One large area for new infrastructure
construction will be for production of liquid fuels, whatever the technology. An even larger need will bein the electricity sector, as electrification of transportation, industrial energy demand, and heating forbuildings will dramatically increase overall demand for electricity. If much of this electricity is generatedusing wind or solar systems, then the new capacity requirements will be even larger due to the relatively
low capacity factors for solar and wind (20% to 30%) relative to fossil fuel based generating systems.
Thus, one of the key factors that may limit our ability to lower emissions and achieve a low-carbon
emissions scenario is the rate at which we can build new infrastructure.
In the U.S., for example, the current rate of installation of new generating capacity is roughly 10 GW peryear. At this rate, one would need more than 100 years to rebuild the existing grid, much less the grid
required by a new energy system based largely on renewable sources that would be several times larger.
Over the last decade, China has been building new generating capacity at roughly 100 GW per year, butthat was with overall economic growth rates of more than 10%. Can countries build new infrastructure
quickly enough to lower global emissions, even if their economy is growing more slowly overall? The
required level of industrial activity in terms of materials (e.g. cement, steel, etc.) is daunting, but not
impossible to imagine. In terms of a risk analysis, however, it seems that such a high level of industrial
infrastructure investment is unlikely to occur without extraordinary political will.
8. Will breakthrough technologies such as capture of carbon dioxide from air become feasible?
In this discussion of future energy technologies, it is important to point out the possibility that an
unanticipated technological breakthrough could change our thinking about existing options and futuretechnological choices. One example of such a breakthrough would be the development of capture of
CO2 from air at a low cost, aside from the use of biomass combined with carbon capture and storage(discussed above). Current estimates of cost for air capture are highly variable, with several small
companies focused on building demonstration plants. It seems unlikely that this technology will allow
for a less expensive route to decarbonisation than the replacement of fossil sources of energy, but if air
capture were economically feasible, then it would add another option. In particular, air capture of CO2
might be most important in creating non-fossil liquid fuels without the need for massive land area forenergy crops.
9. How can the use of low-priced fossil resources, such as coal and oil sands, be limited at a global
scale, even if there are large economic incentives for using such resources?
A final question is whether it will be possible to limit the extraction and consumption of those fossil
fuel resources that are abundant and therefore are likely to remain extremely inexpensive throughout
the next century and beyond. Chief among these resources is the world’s coal reserves, which currently
represent almost 70% of the carbon in fossil fuel proved reserves. The total coal resource (i.e., what
could be extracted, not necessarily based on the current prices) is larger still, with vast undiscovered andundeveloped coal deposits across Russia and in Alaska. Some unconventional oil resources can also be
added to this carbon reservoir, such as oil sands and shale oil.
A critical question for the future is whether these fossil resources will be used to create liquid fuel whenpetroleum reserves begin to dwindle sometime over the next century. Ideally, this would be avoided bynew, non-fossil technologies becoming disruptive, making it uneconomical to use coal or unconventionaloil. But if this does not happen, will it be possible to leave these fossil resources in the ground, even when
their extraction and conversion to liquid fuels or to synthetic gas could be highly profitable? This may
depend on whether global concern around climate change is raised to such a high level that it becomespolitically, socially or morally impossible to use these fossil resources, just as certain practices such as
child labour have become socially and politically unacceptable in most countries.
From the carbon cycle perspective, it is clear that the use of coal and unconventional oil in place of
conventional petroleum would be disastrous. If coal-to-liquids or shale oil becomes a dominant part of theworld energy system, this would reverse most of the progress made in emissions reductions over the past
two decades, and would ensure that even the middle emissions scenarios are impossible to achieve.

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Summary of Technological Perspective on Probability of Different Emissions Scenarios
The previous survey gives only a brief overview of the critical technological issues. Indeed, each of thequestions above could constitute a major report by itself. The purpose of this discussion is not to provide acomplete analysis of each of these components of the global energy system, but simply to identify the majorinnovations in energy technology that must be accomplished if the world is to follow a low emissions pathway.
Not every one of these questions must be answered in the affirmative to reduce global CO2 emissions
sufficiently quickly to follow a low emissions trajectory. For example, it is possible to imagine a non-fossil
energy system that does not use nuclear power (although it does make the challenge more difficult). But there
is no question that limiting emissions to levels that are more likely to keep global temperature change below
2°C requires a positive response to nearly all of the questions. And given the current state of technology
development, we do not know any of the answers. Another problem is timescale. Finding technologicalsolutions that will allow us to answer affirmatively these questions with confidence needs to happen quickly,
as a delay will result in more fossil carbon release to the atmosphere.
For the purpose of placing a probability judgement on different emissions scenarios, the scale of thetechnological challenges would support a conclusion that the family of low emissions scenarios seem very
unlikely. Changing this conclusion would require substantial progress on innovation in energy technology
over the next decade or two to allow positive answers to most of these key questions.

7

CONCLUSIONS

Based on an analysis of current policies and plans for major countries and regions, it is very likely that theworld will continue to follow a medium to high emissions pathway for the next few decades. If goals for
reducing emissions in the EU and the U.S. and stabilizing emissions in China are achieved, then the highest
emissions scenarios are less likely to occur, especially if India is able to displace a part of its anticipatedconstruction of new coal-fired power plants with renewable energy capacity. But this will only keep emissions
on a moderate trajectory, still far in excess of what is required to limit the impacts of climate change below a
harmful level.
The technological challenges to achieving the low emissions scenarios are substantial, and are not being
adequately addressed with current policies. An enhanced effort is needed to accelerate innovation in energy
technology. Because much of this innovation occurs through deployment of large-scale energy systems, a
global commitment is needed. Current trends in the costs of renewable energy, particularly solar photovoltaic
systems, are very promising, but still far short of what is required to achieve emissions goals even for the
next few decades.
The climate response to anthropogenic emissions depends on cumulative emissions of CO2. This means thatpartial reduction in emissions is not sufficient, as sustained lower emissions from fossil fuel combustion willcontinue to drive higher levels of atmospheric CO2, and will lead to higher levels of climate risk. Accelerating
the use of fossil fuels, including the use of coal for liquid fuel, the extraction of methane hydrates, and thedevelopment of oil shale, could reverse the current trend towards emissions reductions, and push emissions
even higher than some of the high emissions scenarios. In this case, climate change itself may be the eventual
limiting factor on emissions through a reduction in economic growth and energy demand.

Endnotes
1. This property is exploited in the parallel development of climate and socio-economic scenarios in the new
scenario process (Special Issue: A Framework for the Development of New Socio-economic Scenarios for
Climate Change Research, Climatic Change 122(3), 2014). Currently, the RCPs are augmented by Shared-SocioEconomic Pathways (SSPs) and associated emissions scenarios (https://secure.iiasa.ac.at/web-apps/ene/
SspDb).
2. Clarke L., K. Jiang, K. Akimoto, M. Babiker, G. Blanford, K. Fisher-Vanden, J.-C. Hourcade, V. Krey, E. Kriegler,
A. Löschel, D. McCollum, S. Paltsev, S. Rose, P.R. Shukla, M. Tavoni, B.C.C. van der Zwaan, and D.P. van Vuuren,
2014: Assessing Transformation Pathways. In: Climate Change 2014: Mitigation of Climate Change. Contributionof Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P.
Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge
University Press, Cambridge, United Kingdom and New York, NY, USA, p.432
3. Schneider, S.H. (2002). ‘Can We Estimate the Likelihood of Climatic Changes at 2100?’Climatic Change, 52,
441–451. http://stephenschneider.stanford.edu/Publications/PDF_Papers/SHSClCh2100ed.pdf
4. Rogelj et al. (2013). ‘2020 emissions levels required to limit warming to below 2°C’, Nature Climate Change 3,
405-412. doi:10.1038/nclimate1758.
5. Potsdam Institute for Climate Impact Research (2013). ‘Roadmaps towards sustainable energy futures andclimate protection: a synthesis of results from the RoSE project’. p23 http://www.rose-project.org/Content/
Public/RoSE_REPORT_310513_ES.pdf
6. The White House Office of the Press Secretary (2014). U.S.–China Joint Announcement on Climate Change.
Available at https://www.whitehouse.gov/the-press-office/2014/11/11/us-china-joint-announcement-climatechange
7. Arunabha Ghosh (2015) “The big push for renewable energy in India: What will drive it?” Bulletin of the Atomic
Scientists, in print.
8. Arunabha Ghosh and Karthik Ganesan (2015) Rethink India’s energy strategy. Nature 521(7551): 156-157.
Available at: http://www.nature.com/news/policy-rethink- india-s-energy-strategy-1.17508.
9. World Nuclear Association (2015). Nuclear Power in China. http://www.world-nuclear.org/info/Country-Profiles/
Countries-A-F/China--Nuclear-Power/

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DIRECT RISKSRISK ASSESSMENT PART 2:

DIRECT RISKS

8INTRODUCTION: A LONG-TERM RISK ASSESSMENT
APPROACH TO CLIMATE SCIENCE

Climate risks may be thought of as falling into two categories: the risks of extreme weather events, and therisks due to long-term changes in average conditions. An assessment of climate science will approach thesetwo kinds of risk differently, depending on the nature of the decisions it aims to inform.
An assessment that aims to inform policy on disaster risk reduction will naturally focus on the risk of extreme
weather events. These low probability, high impact events are a form of ‘worst case’ occurrence in any given
climate. Since extreme weather events already pose a danger in the present, and can be forecast a few days
or a season in advance, it may be reasonable for such a risk assessment to have a relatively short-term focus.
An assessment that aims to inform planning for adapting to climate change is also likely to be concerned withextreme events, and with any changes in their frequency or intensity that occur over time, as well as withchanges in average conditions. For some planning purposes, the ‘worst case’ may be important; for others, it
may be the ‘most likely’ case that is most relevant to decision-making. The timescale for such an assessment
may not need to go far beyond the planning horizons in the economic sectors concerned.
A risk assessment that aims to inform our response to climate change as a whole must consider the whole ofthe timescale that is affected by our current decisions on energy and emissions. It has to consider ‘worst case’outcomes not only in terms of individual events, but also in terms of long-term changes: the risk that average
conditions may themselves reach extreme values. That is our aim, in this section of our report.

Knowing the least about what matters mostIf we take global mean temperature increase to be a simple proxy for the extent of climate change, thenthe risks with the largest impacts are likely to occur at the highest degrees of temperature increase. In the
Chapter 9, we see that on a very high emissions pathway, temperature increases of more than 10°C over the
next few centuries cannot be ruled out. The risks at the top end of that range are likely to be those that are
most relevant to our assessment.
However, it appears that most of our scientific knowledge relates to the risks associated with much lower
degrees of temperature increase. Figure 1 shows the number of times each degree of temperature increase ismentioned in the Summary for Policymakers of the IPCC’s report on Impacts, Adaptation and Vulnerability.1
While there are many mentions of impacts at 2°C and 4°C, there is only one mention of 5°C, and no mentionof anything higher.

CLOSEUP OF THE ICE ISLAND FROM PETERMANN GLACIER
NASA Earth Observatory image by Jesse Allen and RobertSimmon, using data from NASA/GSFC/METI/ERSDAC/JAROS,
and U.S./JapanASTER Science Team. Caption by Michon Scott45

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Figure 1: Number of times different degrees of warming are mentioned in WGII SPM

time, a corresponding approach would be to assess that probability as a function of time. Conceptually, this is
the opposite of an approach that asks first what is most likely to happen, and then how that might affect ourinterests.

35
30

For each area of climate science that we consider in this section, we start by asking “What is it that we wish
to avoid?”, and then ask “How likely is that, and how does that likelihood change over time?”

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number of mentions

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Identifying the biggest risks

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15
10
5

6
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1

4

0
2

3

4

5

0

0

0

0

0

6

7

8

9

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Degrees of warming
Times mentioned in WGH SPM

The detailed chapters of the same report suggest that the impacts corresponding to high degrees oftemperature increase are not only relatively unknown, but also relatively unstudied. This is illustrated by thefollowing quotes:• Crops: “Relatively few studies have considered impacts on cropping systems for scenarios where global
mean temperatures increase by 4ºC or more.”2• Ecosystems: “There are few field-scale experiments on ecosystems at the highest CO2 concentrations
projected by RCP8.5 for late in the century, and none of these include the effects of other potential
confounding factors.”3• Health: “Most attempts to quantify health burdens associated with future climate change consider
modest increases in global temperature, typically less than 2ºC.”4• Poverty: “Although there is high agreement about the heterogeneity of future impacts on poverty, few
studies consider more diverse climate change scenarios, or the potential of 4ºC and beyond.”5• Human security: “Much of the current literature on human security and climate change is informedby contemporary relationships and observation and hence is limited in analyzing the human security
implications of rapid or severe climate change.”6• Economics: “Losses accelerate with greater warming, but few quantitative estimates have been
completed for additional warming around 3ºC or above.”7

A simple conclusion is that we need to know more about the impacts associated with higher degrees oftemperature increase. But in many cases this is difficult. For example, it may be close to impossible tosay anything about the changes that could take place in complex dynamic systems, such as ecosystems or
atmospheric circulation patterns, as a result of very large changes very far into the future.

Starting by deciding what we wish to avoid
Rather than attempt to predict the unpredictable, a more manageable approach is to start from our firstprinciple of risk assessment: assess risks in relation to objectives. Or, put another way: focus on what it isthat we wish to avoid. In accordance with this principle, risk assessments usually first identify an impact (orseverity of impact) that one would hope to avoid, and then assess its probability.i If a risk is changing overi.

See, for example, the UK National Risk Register of Civil Emergencies.

Risk assessments, and risk management measures, often focus especially on thresholds at which impacts
become non-linear or irreversible, or beyond which no further severity of impact is possible. For example,
regulations for the structural integrity of buildings in earthquakes, the capital reserve requirements
for insurance firms, and the health and safety standards for people at work, are particularly concernedwith avoiding the non-linear impacts of building collapse, insurance firm insolvency, and worker death,respectively.8,9,10
Where possible, we have identified what it is that we wish to avoid in terms of thresholds or discontinuities inseverity of impact. Where there are no obvious such thresholds, we have attempted to ensure we identify thebiggest risks by simply asking “What is the worst that could happen?”

Using the best available information
Depending on the question we are trying to answer, the best available information may be the laws of physics,
the output of a model, or an expert’s judgment. For the purpose of risk assessment, we may need to use all ofthese – but we need to keep in mind their different levels of reliability.
When a risk assessment is informed by science, as it is here, we also need to bear in mind how culturalpreferences may affect the way expert judgment is presented. In our chapter on principles of risk assessment,
the reinsurance executive Trevor Maynard said it concerned him that the meaning of a ‘conservative’ estimate
appeared to have the opposite meaning in science from its meaning in insurance. Here the scientist Dr Jay
Gulledge explains what might be at the root of this difference, and why it matters for our risk assessment.

ATTITUDES TO ERROR IN SCIENCE AND RISK ASSESSMENT
Dr Jay Gulledge, Director of the Environmental Sciences Division, Oak Ridge National
Laboratory.

Type I error aversion
Scientists who strive to provide useful information about climate change and decision-makers whoseek such information, “are linked by a thin thread of climate information that is relevant totheir respective endeavors, but they are separated by different needs, priorities, processes andcultures.”11 One element that often divides these two communities is the ways in which they characterize
and treat uncertainty about future outcomes.
Scientists are conservative about drawing incorrect conclusions—so much so that they would rather draw
no conclusion than an incorrect one. Consequently, they have developed standard practices and cultural
norms to protect the scientific knowledge pool from being contaminated by falsehoods. For example,
scientists typically apply statistical tests that estimate the probability that a predicted outcome may have
happened purely by chance rather than because of a hypothesized cause. If the probability of the random
outcome is greater than five percent, standard practice is to reject the hypothesis. Ironically, this rigor
often results in the rejection of a correct hypothesis because there was only a small chance—potentiallyless than 6 percent—that the hypothesis was indeed a random outcome.12
Such scenarios involve two types of uncertainty, or ‘error’ in statistical terminology. First is the possibility
that the hypothesized cause is accepted, but is actually wrong. This condition is commonly called a‘false-positive;’ statisticians call it a ‘type I error.’ Conversely, there is the possibility that the hypothesis
is rejected, but is actually correct. This situation presents a false-negative, or ‘type II error.’ Scientists

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are relatively tolerant of false-negatives: in most scientific fields it is not standard practice to estimate theprobability of committing a type II error.
In contrast, professional risk managers are often more concerned about type II errors which could
result in their disregarding a risk with potentially severe consequences.13 For example, even though theprobability of any particular house burning down in a given year is very low, the mortgage lender requires
the homeowner to carry casualty insurance to protect the lender’s investment. The point is that evena very low probability of an outcome may represent a large risk if the outcome would be very severe.
Consequently, when scientists tolerate type II errors, their work may lack rigour from the standpoint of thedecision-makers they seek to inform.

Downward bias under uncertainty
Consistent with their aversion to type I error and tolerance of type II error, climate scientists haveoften erred toward underestimating risk when faced with deep uncertainty.14 A stark illustration of thisphenomenon occurred when the IPCC’s ‘Reasons for Concern’ (RFC), first published in 2001,15 were
updated in 2009.16 The RFCs are categories of climate change impacts that IPCC authors deemed ofpotential interest to decision-makers and include risks to unique and threatened ecosystems, extreme
weather events, distribution of impacts geographically and across income classes, aggregate economic
impacts, and sudden dramatic changes in the regulation of the global climate (e.g., a sudden collapse of alarge ice sheet leading to abrupt sea level rise). The RFC assessment evaluated the sensitivity of each RFC
category to global temperature increases between 0 and 5 degrees Celsius.
As governments emphasized climate change research during the 2000s, much more evidence became
available for assessing these risks. After considering the new evidence, the update estimated greater
sensitivity to warming than the original assessment for all five categories of RFC.17 This outcome suggeststhat scientists tend to underestimate risk in the face of incomplete information.

Communication breakdown
There may also be a dangerous interaction between climate scientists’ cultural aversion to type I error anda documented tendency of the public to discount low-probability, high impact outcomes. For example, the
IPCC defines ‘likely’ as 66-90% probability, and ‘unlikely’ as 10-33% probability. When college students
were asked what they thought the term ‘unlikely’ meant for the probability of a land-falling hurricane,
the most common response was 1-10% (i.e. lower than the probability range assigned to the term by the
IPCC).18 If the scientific community tends to underestimate the severity of impacts under uncertainty, andthe public tends to adjust probability of a severe event downward, the net effect may be a serious underappreciation of the potential severity of climate change impacts among the public and decision-makers.

Conclusion of the U.S. National Academy of Sciences
For the reasons described above, among others, the U.S. National Academy of Sciences has stated that
“Scientific priorities and practices need to change so that the scientific community can provide
better support to decision-makers in managing emerging climate risks.”19
Scientists who seek to inform decision-making on climate change need to adopt a more risk-sensitive
analytical approach. In some cases, this adjustment will require more tolerance of type I error and lesstolerance of type II error.

The cultures of science and risk assessment described by Dr Gulledge are not impossible to reconcile. One
might expect them to meet in the middle, resulting in an equal aversion to either kind of error. There aremany fields in which the use of science to support risk assessment has become highly developed – including
the forecasting of extreme weather events.ii
For the purposes of this risk assessment, we have tried to make sure relevant information is not omitted simply
because the uncertainty is high. At the same time, we have aimed to make the uncertainties, and the expertii.

For example, the UK Met Office’s National Severe Weather Warning Service uses a matrix of the probability of an event happening
versus the impact if it does, and on occasion warns of very low probability events that might have huge impacts if they occurred.

judgments, as clearly visible as possible. To do this, we have adapted an old rule of intelligence assessmentiii
and asked scientists to tell us: i) what they know; ii) what they do not know; and iii) what they think.

Illustrative examples
In the pages that follow, we apply this approach to assessing risks associated with global temperature
increase, human heat stress, crop production, water stress, flooding, drought, coastal cities and sea level rise,and large-scale disruption of the climate system. This is a small subset of the risks of climate change, whichleaves out some important issues entirely (such as ocean acidification), and gives relatively brief summaries
of others.
The purpose of this section is not to provide a comprehensive survey of the scientific literature. Indeed, eachof these chapters reflects the perspectives of its individual authors. The purpose is simply to identify some of
the biggest risks, and to illustrate a way in which they may be communicated effectively to policy-makers.

Endnotes
1.

Source: Global Challenges Foundation, analysis of content of IPCC AR5 WG2 SPM http://globalchallenges.org/
wp-content/uploads/Briefing-on-IPCC-AR5-WGII-and-WGIII.pdf
2. See p.506. IPCC (2014). ‘Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and
Sectoral Aspects.’ Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M.
Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea,
and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1132 pp.
3. IPCC (2014), p.276.
4. IPCC (2014), p.735.
5. IPCC (2014), p.816.
6. IPCC (2014), p.779.
7. p.19 of IPCC (2014). ‘Summary for policymakers’. Climate Change 2014: Impacts,Adaptation, and Vulnerability.
Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L.White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, pp. 1-32
8. GFDRR, The World Bank. Knowledge Note 1-2: Building Performance. p.4. Retrieved from http://wbi.worldbank.
org/wbi/Data/wbi/wbicms/files/drupal-acquia/wbi/drm_kn1-2.pdf
9. Bettis, O. (2014). Risk Management and Climate Change: Risk of Ruin. Accessible at http://www.lse.ac.uk/
GranthamInstitute/wp-content/uploads/2014/01/Oliver-Bettis-Risk-Management-and-Climate-Change-Risk-ofRuin.pdf.
10. Health and Safety Executive (2001). Reducing Risks, Protecting People: HSE’s decision-making process. p.46
11. Rogers, W and J. Gulledge (2010). Lost in Translation: Closing the Gap Between Climate Science and National
Security Policy. Center for a New American Security, Washington, D.C.
12. Mabey, N., J. Gulledge, B. Finel and K. Silverthorne (2011). Degrees of Risk: Defining a Risk Management
Framework for Climate Security. E3G, London, UK and Washington, DC (p. 61).
13. Schneider, S.H. and Mastrandrea, M.D., (2009). ‘Risk, uncertainty, and assessing dangerous climate change’.
In: Climate Change Science and Policy, Schneider, S.H., Rosencranz, A., Mastrandrea, M.D., Kuntz-Duriseti, K.
(Eds.), Island Press.
14. Engelhaupt, E. (2007). ‘Models underestimate global warming impacts.’ Environmental Science & Technology,
41, 4488-4489.
15. Smith, J.B. et al. (2001). ‘Vulnerability to climate change and reasons for concern: a synthesis.’ In J.J. McCarthy
et al., eds. Climate change 2001: impacts, adaptation and vulnerability. Contribution of Working Group II to
the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge
University Press.
16. Smith, J.B., et al. (2009). ‘Assessing dangerous climate change through an update of the Intergovernmental
Panel on Climate Change (IPCC) ‘reasons for concern’’. Proceedings of the National Academy of Sciences of
the United States of America, 106, 4133–4137.
17. Mabey, N., J. Gulledge, B. Finel and K. Silverthorne (2011). Degrees of Risk: Defining a Risk Management
Framework for Climate Security. E3G, London, UK and Washington, DC (pp. 65-68).
18. Patt, A.G. and D.P. Schrag (2003)/ ‘Using specific language to describe risk and probability.’ Climatic Change
61: 17–30.
19. National Research Council (2009). Panel on Strategies and Methods for Climate-Related Decision Support.
National Academies Press, Washington, D.C.

iii. Attributed among others to the former US Secretary of State and military General, Colin Powell, who is said to have said: “Look, I havegot a rule. As an intelligence officer, your responsibility is to tell me what you know. Tell me what you don’t know. Then you’re allowed
to tell me what you think. But you always keep those three separated.”

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9

GLOBAL TEMPERATURE INCREASE

Figure 2: Estimate of warming at 2100 from a simple climate model based on emissions
from the RCP pathways. The RCP2.6 pathway represents a world with very rapid
emission reductions. RCP8.5 represents a world with a continued focus on fossil fuels
and significant increases in greenhouse gas use.

Professor Jason Lowe, Head of Mitigation Advice, and Dr Dan Bernie, Senior Scientist, UK Met Office
Hadley Centre

What global temperature increases might we wish to avoid?
Two degrees: The United Nations framework convention on climate change aims to avoid potentially
dangerous climate change and has adopted a long-term goal of keeping global average warming below
2°C above pre-industrial levels.1 The choice of appropriate level is a subjective policy choice informed byestimates of future climate impacts and relative difficulty of adaptation, and the judgment that there is asufficiently high chance of being able to limit warming to this level.
Four degrees: The recent IPCC report concluded that “global climate change risks are high to very highwith global mean temperature increase of 4°C or more above preindustrial levels in all reasons forconcern, and include severe and widespread impacts on unique and threatened systems, substantial
species extinction, large risks to global and regional food security, and the combination of hightemperature and humidity compromising normal human activities, including growing food or
working outdoors in some areas for parts of the year”.2
Seven degrees: There has been much less research focusing on the impacts at higher temperatures but
limited studies suggest the possibility of even greater impacts, with a rise in temperature of around 7°C
potentially giving rise to extreme heat events in excess of human physiological tolerance in some regions.3
It is important to note that, whilst global average warming is convenient to use as a simple metric, many
specific risks depend on local warming and heat extremes (as discussed in the next two sections). Land
warms faster than the oceans, so warming in most land areas will exceed the global average, in some places
by a significant amount. In addition, there will be changes in extremes, such as the hottest day of the year.

How likely are we to exceed the temperature thresholds we’ve identified?
The climate response for any future emissions or concentrations scenario must be expressed as a range,
frequency distribution or probability distribution because of uncertainty in the relationship between changes
in atmospheric greenhouse gas emissions and concentrations and the climate response. The IPCC fifth
assessment estimated a likely range of warming by 2081-2100 relative to a near present day period to
be in the range of 0.3°C to 4.8°C (equivalent to 0.9°C to 5.4°C relative to pre-industrial) for the range ofconcentration scenarios considered.4
This set of IPCC simulations does not sample all of the known uncertainties in the climate systems, and theexperimental set-up over-constrains the spread in atmospheric concentration of greenhouse gases for a given
emission pathway. Like the real climate, the climate model used in the AVOID2 programme takes emissions
as its input and includes the dependence on CO2 and the uncertainty in carbon cycle-climate feedback5 – theway in which the amount of carbon absorbed and emitted by soil vegetation and the ocean may change in
response to climate change, and in turn accelerate climate change. It shows the probability of exceedinga range of different warming levels in 2100 relative to pre-industrial level when this additional factor is
included in the climate simulations. Even the lowest emissions scenario (RCP2.6) has a more than a 33%
chance of exceeding 2ºC. The probability of warming beyond 4°C is significant in the middle two pathways,
and in the highest emissions pathway, RCP8.5, is somewhere in the region of 90% (Figure 2) in thesesimulations. In the highest pathway, there is also a small probability of exceeding 7°C.

It is important to recognize how these probabilities change over time, and to look beyond 2100, especiallyfor the higher emission scenarios, because in some cases impacts and the probability of major climate system
disruption will still be increasing. Figure 3 shows the probability of exceeding 2°C, 4°C and 7°C respectively
as a function of time, for the four emissions pathways.
• The probability of exceeding 2°C can be seen to exceed 50% within the first half of the century for thehighest emissions pathway, and to exceed 80% late in the century for all except the lowest emissions
pathway.
• The probability of exceeding 4°C by 2150 appears to be somewhere in the region of 100%, 50%, and20%, for the highest and middle two post-2100 continuation pathways respectively, while remaining at
only a few percent for the lowest pathway.
• On the highest emissions pathway (the RCP8.5 extension scenario, where beyond 2100 emissions areheld constant for 50 years, and then sharply reduced), the probability of exceeding 7°C appears to
exceed 50% during the 22nd century, before peaking at around 65% in the following century.
Alternative model set-ups may show small differences in these probabilities, but the conclusions will be
qualitatively the same.

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Figure 3: Probability of exceeding: (a) 2°C; (b) 4°C; and (c) 7°C based on the projectedwarming to 2500 from a simple climate model set up to cover the range of climate
sensitivity from the more complex general circulation models, and sampling uncertainty
in climate-carbon cycle feedback.

Could the planet warm even further in the next century and beyond?

(a)

RCP likelihood of T>2.0°C

As shown above, for the highest emission pathway considered in Working Group I of the IPCC Fifth
Assessment there is a sizeable possibility of more than 7°C warming above pre-industrial levels in the period
after 2100.
It is difficult, if not impossible, to provide a robust estimate of the maximum possible warming. First, it isunclear how to define an absolute upper emissions scenario. For instance, it might include known reserves offossil fuels, or perhaps projected increases in reserves, which are very uncertain. It may or may not include
economic constraints on extracting and using the fossil fuels. For this study we do not look beyond RCP8.5
and its time-extension. The transient evolution of the IPCC model simulations is shown in Figure 4 below.
By 2300 a small subset of the climate models reach global average temperature increases in excess of 10ºC
above pre-industrial levels.

Figure 4: Time series of global annual mean surface air temperature anomalies (relative
to 1986–2005) from IPCC 5th assessment climate models. (Add 0.6°C to these numbers
to compare to a pre-industrial baselinei, 6). Projections are shown for each RCP for
the multi-model mean (solid lines) and the 5% to 95% range across the distribution ofindividual models (shading).

(b)

RCP likelihood of T>4.0°C(c)

RCP likelihood of T>7.0°C

A second reason that we cannot easily estimate the maximum warming is due to the remaining uncertainty inthe climate system response. The upper values of our estimates of climate sensitivity are not well bounded.
Additionally, there are a range of missing processes that might change the level of warming by, for instance,
contributing additional greenhouse gases. Using the published studies from the IPCC 5th assessment of thepossible extra forcing provided by known earth system feedbacks as an extra component in a simple climate
model we estimated this could add around a further degree of warming on to our median estimate of warmingin RCP8.5 by 2100 (Figure 5).7 Put another way, this could bring forward the time at which the probabilityof exceeding 4°C on RCP8.5 reaches 50%, by a handful of years in the central estimate, or by more than adecade in the more extreme but unlikely case.

i.

Full IPCC caption: Time series of global annual mean surface air temperature anomalies (relative to 1986–2005) from CMIP5
concentration-driven experiments. Projections are shown for each RCP for the multi-model mean (solid lines) and the 5 to 95% range(±1.64 standard deviation) across the distribution of individual models (shading). Discontinuities at 2100 are due to different numbersof models performing the extension runs beyond the 21st century and have no physical meaning. Only one ensemble member is usedfrom each model and numbers in the figure indicate the number of different models contributing to the different time periods. No
ranges are given for the RCP6.0 projections beyond 2100 as only two models are available.

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If climate sensitivity turns out to be at the higher end of our estimated range the effect of other earth systemprocesses that could amplify warming may become greater, but additional modelling is needed to quantify
reliably how much. However, we do know that some of these amplifying factors may take a significant time tobe fully realized and so could become larger beyond 2100.

The fraction of greenhouse gas emissions that remain in the atmosphere for a significant time after
production or release can be constrained to a likely range but a single value is not known. We are awareof feedbacks between changes in climate and the carbon cycle, and between the climate and atmosphericchemistry. All of the above have been factored into our quantitative view of the future by one means oranother, but again precise values are unknown.

Figure 5: Estimate of added warming from earth system processes considered in the
IPCC assessment but not typically included in climate change estimates.

We know that in the distant past there have been large-scale disruptions to the climate system, similar to what
we would consider today as tipping points. We think that the chances of these events occurring in the future
are more likely at greater levels of warming but we do not know the precise conditions needed to trigger
these events (see chapter 17).
We know that there are a range of earth system processes, and that the majority of those considered in the
IPCC assessment, such as thawing permafrost, might accelerate warming and climate disruption acrossthe planet. But, while we are starting to make estimates of these effects, they are rarely included in current
climate models or climate risk assessments (see Figure 6 as an exception). Additionally there are processes,
such as the potential release of methane from hydrate stores in and under ocean sediments, that could
contribute significant extra warming.13 We know that these stores are very large, and have contributed
significantly to warming over long time periods in the distant past. Our best estimate is that they will only
have a very small effect over the next century or so, but we have very limited understanding of what might
happen in the longer-term future.
Production of this chapter was supported by the AVOID2 programme (funded by the UK
Government and Natural Environment Research Council)’ with ‘Production of this chapter was
supported by the AVOID 2 programme (DECC) under contract reference 1104872. and the EU
HELIX programme (funded by European Union Seventh Framework Programme FP7/2007-2013
under grant agreement no 603864)

Increases in warming such as we project for the RCP8.5 emissions scenarios are unprecedented in theobservational record, and even (using proxy measurements) to around 1000AD.8 Looking over a longer
period the IPCC Fifth Assessment Report concludes that during the mid-Pliocene (3.3 to 3.0 million yearsago), temperatures were 1.9 to 3.6°C above pre-industrial levels. During the early Eocene (52 to 48 million
years before present) global mean temperature were 9 to 14°C higher then pre-industrial levels, for anatmospheric CO2 concentration of around 1000ppm, which is slightly higher than the 2100 concentration
expected in RCP8.5.9 When considering these distant historical periods it is important to keep in mind
both uncertainty in the records and whether the past period really represents a suitable analogue to theanthropocene.

What do we know, what do we not know and what do we think?
Climate projections of the future need to be placed in the context of our understanding of the climate system.
We have a clear and longstanding knowledge of the basic physics that tell us increases in atmosphericgreenhouse gases are expected to warm the planet.10 We know that the planet has warmed over recent
decades and that this warming is unusual compared to the expected natural variations.11 This can be
explained by the extra energy accumulated in the climate system.
We now have a range of estimates of how sensitive the climate system is to changes in radiative forcing,but we do not know a single precise value. We also understand many of the processes that determine thissensitivity. Whilst there is evidence that complex climate models can simulate skillfully many aspects of pastand present climate, expert judgement in the IPCC assessment considers that the 5th to 95th percentile rangeof 21st century warming by the current generation of complex climate models is too narrow for the range of
future greenhouse gas concentration increases.12

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Endnotes
1. United Nations (1992). United Nations Framework Convention on Climate Change. Available at http://unfccc.
int/resource/docs/convkp/conveng.pdf
2. IPCC (2014). ‘Summary for policymakers’. Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part
A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, pp. 1-32.
3. Sherwood, S.C. and Huber, M. (2010). ‘An adaptability limit to climate change due to heat stress’. PNAS 107
4. Table SPM.2 in IPCC (2013). ‘Summary for Policymakers’. Climate Change 2013: The Physical Science Basis.
Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate
Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M.
Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA
5. Lowe, J., Huntingford, C., Raper, S.C.B., Jones, C.D., Liddicoat, C.D. and Gohar, L.K. (2009). ‘How difficult is it
to recover from dangerous levels of global warming?’. Environmental Research Letters 4 doi:10.1088/17489326/4/1/014012
6. Figure 12.5, Collins, M., R. Knutti, J. Arblaster, J.-L. Dufresne, T. Fichefet, P. Friedlingstein, X. Gao, W.J. Gutowski,
T. Johns, G. Krinner, M. Shongwe, C. Tebaldi, A.J. Weaver and M. Wehner (2013). ‘Long-term Climate Change:
Projections, Commitments and Irreversibility.’ Climate Change 2013: The Physical Science Basis. Contribution ofWorking Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
7. Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C.
Jones, C. Le Quéré, R.B. Myneni, S. Piao and P. Thornton (2013). ‘Carbon and Other Biogeochemical Cycles’.
Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K.
Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge,
United Kingdom and New York, NY, USA, pp. 465–570, doi:10.1017/CBO9781107415324.015.
8. Masson-Delmotte, V., M. Schulz, A. Abe-Ouchi, J. Beer, A. Ganopolski, J.F. González Rouco, E. Jansen, K.
Lambeck, J. Luterbacher, T. Naish, T. Osborn, B. Otto-Bliesner, T. Quinn, R. Ramesh, M. Rojas, X. Shao and A.
Timmermann (2013). ‘Information from Paleoclimate Archives’. Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex
and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp.
383–464, doi:10.1017/CBO9781107415324.013.
9. Masson-Delmotte, V. et al (2013).
10. e.g. Houghton, J. (2009) Global Warming: The Complete Briefing. Cambridge University Press, Cambridge.
11. Bindoff, N.L., P.A. Stott, K.M. AchutaRao, M.R. Allen, N. Gillett, D. Gutzler, K. Hansingo, G. Hegerl, Y. Hu, S. Jain,
I.I. Mokhov, J. Overland, J. Perlwitz, R. Sebbari and X. Zhang (2013). ‘Detection and Attribution of Climate
Change: from Global to Regional’. Climate Change 2013: The Physical Science Basis. Contribution of Working
Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F.,
D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 867–952, doi:10.1017/
CBO9781107415324.022..
12. Table SPM.2 in IPCC (2013). ‘Summary for Policymakers’.
13. Masson-Delmotte, V. et al (2013).

10

THE RISK OF HEAT STRESS TO PEOPLE

Dr. Tord Kjellstrom, Professor Alistair Woodward, Dr. Laila Gohar, Professor Jason Lowe, Dr. Bruno
Lemke, Lauren Lines, Dr. David Briggs, Dr. Chris Freyberg, Dr. Matthias Otto, Dr. Olivia Hyatti

What global temperature increases might we wish to avoid?
The human body has behavioural and physical mechanisms that work to maintain its core temperature
at about 37°C. If the body’s internal temperature rises above this level, then body systems and vital
physiological functions are compromised, and in severe cases, death can result. The climatic conditions
relevant to such heat stress may be measured in terms of the Wet Bulb Globe Temperature (WBGT), whichtakes account of temperature, humidity, wind speed, and solar radiation.1 We calculate WBGT for in-shade (nosolar heat addition) or indoor (no air conditioning) conditions from climate data using methods described by
Lemke and Kjellstrom.2 We have considered heat stress thresholds relevant to four human interests: survival,
sleep, work and sport.
Survival
The threshold for survivability is defined as climatic conditions so extreme that if a person is exposed to them(i.e. not protected by air-conditioning), core body temperature rises to potentially fatal levels while sleeping
or carrying out low energy daily tasks.
For day-time heat we set the threshold for survivability according to the WBGT that causes core bodytemperature to rise to 42°C, for an average individual at rest,ii in the shade, for four hours. We estimate thisoccurs when the daily maximum WBGT is ≥ 40°C. We have set the threshold at the situation when 10% of
the days in the hottest month are projected to exceed this threshold (‘the three hottest days in the hottest
month’), since at this point exceeding it at least once becomes almost certain.
For night-time heat we define the threshold for survivability as conditions which prevent a reduction in core
body temperature overnight, so that the heat exposure of the following day adds directly onto the high heat
exposure of the previous day.3 We estimate this occurs in most people when the average minimum WBGT is≥36°C. (We use the same ‘three night’ assumption as used above for days).
Sleep
Our threshold for ‘sleepability’ is defined in terms of conditions that permit some reduction in core bodytemperature, but not to the full extent necessary for normal sleep:4 specifically, when the core bodytemperature remains above 37°C during eight hours of rest, at night. We estimate this applies for most people
when the average minimum WBGT is ≥ 30°C. (Again, we use the same ‘three night’ assumption.)
It is important to note that individual susceptibility to heat varies widely. Relevant factors include age, gender,
health status, and past exposures to heat.5 In relation to the survival and sleep thresholds described above,
we have sought to identify a plausible mid-range temperature, i.e. one at which roughly 50% of the population
cannot stay in heat balance.
Work
As human muscle activities create important intra-body heat production, working people are at particular risk
as climate change increases ambient heat levels.6 We have defined the limits to work according to time-based
threshold limit values published by the United States Occupational Safety and Health Administration7 and the‘no work at all’ ceiling recommended by the US National Institute of Occupational Safety and Health.8 On thebasis of these guidelines, which are summarized in Figure 1, we conclude that when WBGT reaches 36°C it is
not safe for medium/heavy labour,iii even with rest breaks. We define ‘too hot to work’ as conditions in which
the average daily maximum WBGT is 36°C or more for a month.

i.

Dr. Tord Kjellstrom, Department of Public Health and Clinical Medicine, Umeå University; Professor Alistair Woodward, Universityof Auckland; Dr Laila Gohar, Met Office Hadley Centre; Professor Jason Lowe, UK Met Office Hadley Centre; Dr. Bruno Lemke,
Nelson Marlborough Institute of Technology; Dr. David Briggs, Imperial College, London; Dr. Chris Freyburg, Massey University;
Dr. Matthias Otto, Nelson Marlborough Institute of Technology; Olivia Hyatt, Health and Environment International Trust.

ii.

Emitting heat at a rate of 120W

iii. Equivalent roughly to working at a rate of 400W

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Figure 1: The relationship between heat and work - a function of ideal humanphysiology and a pointer to fundamental temperature thresholds.9

Figure 2:

Percentage of probability

Survivability (day): Probability (%) that a person in a region is exposed to heat thatcauses core body temperature to rise to 42°C, for an average individual at rest in theshade for 4 hours. Defined as WBGTmax ≥ 40°C, for 10% of the days in the hottest
calendar month of the year. Temperature increase is relative to present day.iv
100

N India80

SE China

60

SE USA

40
20
0
0

1

2

3

4

5

6

7

8

9

Global average temperature increase, degr. C

Sport
The limit in this case is based on a guidance note from sports medicine experts in the USA.10 This statesthat competitive outdoor sports activities should cease when WBGT reaches 28°C. We define the thresholdfor ‘sportability’ as a situation in which all daylight hours in the three hottest months exceed 28°C WBGT.
Given what is known about the 24 hour distribution of temperatures and humidity, we infer that the thresholdconditions are met when the average WBGT is ≥ 28°C, for three months. We note that this level includes a
substantial ‘safety margin’ to protect the most sensitive individuals.

Survivability (night): Probability (%) that a person in a region encounters conditions
that prevent any fall in core body temperature at night. Defined as WBGTmin ≥ 36°C,
for 10% of the nights in the hottest calendar month of the year. Temperature increase isrelative to present day.
100

How close are we to these thresholds in the current climate?
Heat stress already causes many deaths and a great deal of illness each year, especially in low income tropicalcountries.11 However, even in the hottest parts of the world, temperatures in populated areas seldom if everapproach the thresholds of survivability described above.
The OSHA threshold limit values are frequently exceeded for short periods, in hot countries, but not for theextended periods described by our threshold.12 Similarly the 28°C WBGT sportability limit is crossed every
year in many places, for short periods. (For instance, games at the Australian Open Tennis championship inJanuary 2014 were cancelled when ambient (dry bulb) temperatures exceeded 40°C.13 However, these hightemperatures are seldom sustained, at present.

N India

90
Percentage of probability

58

80

SE China

70

SE USA

60
50
40
30
20
10

Estimates of the likelihood of exceeding these thresholds as a function of global
average temperature increase, for selected locations
Figure 2 shows how the probability of crossing these thresholds could increase in three regions: North India,
Southeast China, and Southeast USA, as global temperatures rise. On these graphs, ‘probability’ represents
the proportion of each region’s population estimated to be in areas where climatic conditions cross therelevant threshold. The relationship between global temperature increase and local climatic conditions has
been estimated using climate models, and the spatial distribution of population has been estimated basedon UN projections. There are some rough approximations in this calculation, but it serves to provide anillustration of the risk. A full description of the methods is located in the Annex.

0
0

1

2

3

4

5

6

7

8

9

Global average temperature increase, degr. C

iv.

Global average temperature increase is shown here as relative to a ‘present day’ baseline, as defined by the 30-year average centredon 1995 (i.e. from 1980 to 2009). This is about 0.6 degrees C higher than the ‘pre-industrial’ baseline used in the temperature increase
chapter, and referred to generally throughout this report.

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Sleepability: Probability (%) that a person in a region encounters heat that prevents
core body temperature from falling down to normal (37°C) during eight hours of rest atnight. Defined as WBGTmin ≥ 30°C, for three nights in a month. Temperature increase isrelative to present day.

Sportability: Probability (%) that a person in a region encounters conditions in which
daylight hours in the hottest three months of the year exceed 28°C WBGT. Definedas daily WBGTmean ≥ 28°C during three months. Temperature increase is relative topresent day.

100

N India80

SE China

60

SE USA

40
20
0

Percentage of probability

CLIMATE CHANGE: A RISK ASSESSMENT

Percentage of probability

CLIMATE CHANGE: A RISK ASSESSMENT

100

N India80

SE China

60

SE USA

40
20
0

0

1

2

3

4

5

6

7

8

9

0

Global average temperature increase, degr. C

100

N India80

SE China

60

SE USA

40
20
0
0

1

2

3

4

5

6

7

Global average temperature increase, degr. C

1

2

3

4

5

6

7

8

9

Global average temperature increase, degr. C

Workability: Probability (%) that a person in a region cannot work at 400W. Defined as
average daily WBGTmax ≥ 36°C for a month. Temperature increase is relative to present
day.

Percentage of probability

60

8

9

The projections shown here suggest the first limit to be crossed will be that related to sport. Using thedefinition of dangerous heat that is applied today in many countries, there is about a 40% chance that
individuals in northern India will not be able to participate in competitive outdoor activities in summertime
when global average temperatures have risen on average by one degree compared to the present. With four
degrees of warming this probability will have risen to around 80-90% in northern India and southeastern
USA, and there is a 50:50 chance the population of southeastern China will be affected similarly.
According to these estimates, the limits on work, as defined above, will emerge before the world warms byfour degrees on average compared to the present. At this point, in northern India there is a probability of
about 30% that temperatures will be so high that moderate/heavy outdoor work cannot be carried out in thehottest month. The probability of exceeding the threshold is close to 80-100% in all regions when the global
average temperature rises by 7-8°C.
Heat so severe that it is not possible to reduce core body temperature while sleeping, as defined above, will
be encountered once the global average temperature increases by more than 5°C. In northern India andsoutheastern China the probability of being exposed to heat that makes healthy sleep impossible rises rapidly
when global warming exceeds 6°C. At +8°C global warming we estimate a probability of 50-90% in the study
regions that individuals will encounter conditions so hostile that normal sleep becomes impossible.
The daytime survivability threshold that we have defined is not crossed until global warming exceeds 4°C.
But if warming continues, we estimate that populations in all study regions will be at risk: the probabilityof encountering conditions that cannot be tolerated, even in the shade and at rest, at +6°C global warming
range from about 50% (southeastern China) to 80% (northern India).
We can understand how these risks vary over time by comparing these results with the findings of theprevious chapter. For example, it is notable that the probability of passing several of our heat stressthresholds rises steeply when global temperature increases by around 4°C compared to the present (around
4.8°C relative to pre-industrial). As the previous chapter showed, on the highest emissions pathway such anincrease becomes more likely than not by the end of this century.

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What is a plausible worst case for heat stress due to climate change this century andbeyond?

What do we know, what do we not know, and what do we think?

An even more extreme threshold than any we have used here has been defined on thermodynamic grounds:
when wet bulb temperature exceeds skin temperature, since it is impossible at this point for the body to shed
heat. This would occur when wet bulb temperature exceeds 35°C.v, 14 In practical terms, the limit for survival
will be reached at lower ambient temperatures, due to the necessity in all populations for some moderate light activity.
Sherwood & Huber (2010) suggested that these conditions would first be experienced in small areas when
global average temperature rose 7°C above the current level, and that large populated areas of the globe
would experience these conditions once global warming reached 10-11°C on average above the current
level. Elsewhere in this report we conclude there is a ‘sizable possibility’ (probability about 60%) that global
average temperatures will rise by more than 7°C above pre-industrial in the 22nd century, under the highemissions scenario RCP8.5. It is difficult to estimate the risks of warming greater than this. However, by
2300 a small subset of the IPCC climate models reach global average temperature increases in excess of 10°C
above pre-industrial levels.
To provide more information on the projections used here, Figure 3 shows three maps that display average
daily peak WBGT during the hottest month of the year, at three different points of global temperature
increase.15 The Annex also includes estimates of the proportion of work hours lost due to heat stress, atdifferent levels of physical activity, in relation to global average temperature increase.

What we know: The response of the human body to heat is well-understood, and the limits that extreme
temperatures impose on functioning and well-being are clearly demarcated. We also understand the manyfactors that influence vulnerability to heat stress. Surveys in many parts of the world find that heat is alreadya significant constraint on work and sport. Globally temperatures are rising, and it is expected, with highconfidence, that episodes of extreme heat will occur more frequently in the future.16
What we do not know: There are many uncertainties in these climate model estimates particularly withregard to regional variations. Temperatures are projected with greater confidence than humidity and rainfall.
An even greater uncertainty concerns social adaptation: undoubtedly it will occur, but the speed of change,
its inclusiveness or otherwise, and the costs and acceptability of responses such as 24/7 air-conditioning andtotally indoor lifestyles are uncertain.
What we think: It is important to recognize that the probabilities shown here understate the risks that apply
in many locations. This results from the wide variations in temperatures due to local meteorological and otherenvironmental factors (the urban heat island effect, for instance, may increase night-time temperatures by
as much as 10°C). How much people are exposed to outdoor conditions will also vary greatly. Those without
access to artificial cooling, and people who must work outdoors, unprotected, to maintain their livelihoods
will obviously be more severely affected than those who can live and work away from the heat. The effect of
being in afternoon sunlight, rather than in the shade, adds an extra 3-4°C on to WBGT. The old, the young
and those with chronic poor health are especially vulnerable to heat-related illness.
When it is too hot to sleep comfortably, the stressful effects of exposure to high temperatures during the
day are likely to be magnified. Similarly, we would expect productivity at work to be reduced significantly
by persistent high night-time temperatures. For these and other reasons we suggest that beyond a certain
point, the heat stress implications of rising global temperatures could threaten the habitability of low-income
regions in which people rely on local agriculture for their livelihoods. Throughout the hottest parts of theworld, heat will threaten the viability of industries and activities whose environments cannot be artificiallycooled. This may include some utilities, construction, and emergency response services such as ambulancecrews and fire-fighters.

Figure 3:

Zero global temp increase

Production of this chapter was supported by the AVOID 2 programme (DECC) under contractreference 1104872. Elements of the production of this chapter were supported by the EU HELIX
programme (funded by European Union Seventh Framework Programme FP7/2007-2013 under
grant agreement no 603864)

Endnotes

4.4°C global temp increase

7.7°C global temp increasev.

E.g. temperature is 35 degrees C with 100% humidity, or other equivalent combinations

1. Parsons K. (2014). Human Thermal Environments. The Effects of Hot, Moderate and Cold Environments on
Human Health, Comfort and Performance. Third Edition. Boca Raton, CRC Press.
2. Lemke B, Kjellstrom T (2012). ‘Calculating workplace WBGT from meteorological data’. Industrial Health, 50,
267-278.
3. Krauchi K. (2007). ‘The human sleep-wake cycle reconsidered from a thermoregulatory point of view.’
Physiology & Behavior 90, 236-245
4. Krauchi K. (2007).
5. Parsons (2014).
6. Kjellstrom T, Holmer I, Lemke B (2009). ’Workplace heat stress, health and productivity - an increasing
challenge for low and middle-income countries during climate change’. Global Health Action 2: Special Volume,
pp. 46-51.
7. OSHA (1999). Section III Chapter 4: Heat Stress. OSHA Technical Manual.Occupational Safety and HealthAdministration [Internet]; [Accessed on 27 Apr 2015]. Washington DC. Available at: http://www.osha.gov/dts/
osta/otm/otm_iii/otm_iii_4.html.
8. NIOSH (1986). Criteria for a Recommended Standard. Occupational Exposure to Hot Environments. DHHS
(NIOSH) Publication No.86-113. Department of Health and Human Services April 1986.
9. Heat at work safety guidelines based on ISO (1989). Hot environments - Estimation of the heat stresson working man, based on the WBGT-index (wet bulb globe temperature). ISO Standard 7243. Geneva:
International Standards Organization and NIOSH (1986) recommendations, and adapted from graph in
Kjellstrom, T, Kovats S, Lloyd SJ, Holt T, Tol RSJ (2009). ‘The direct impact of climate change on regional labour
productivity.’ Int Archives of Environmental & Occupational Health 64, 217-227)

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10. Binkley H, Beckett J, Casa DJ, Kleiner DM, Plummer PE (2002). ‘National Athletic Trainers’ Association Position
Statement: Exertional Heat Illnesses’. J Athletic Training 2002; 37(3): 329-343; American College of Sports
Medicine (2007). ‘Exertional Heat Illness During Training and Competition’. Medicine & Science in Sports &Exercise 39(3): 556-572
11. Hyatt OM, Lemke B, Kjellstrom T. (2010). ‘Regional maps of occupational heat exposure: past, present andpotential future.’ Global Health Action 3: 1-11; Kjellstrom T, Lemke B, Otto M (2013). ‘Mapping occupational heat
exposure and effects in South-East Asia: Ongoing time trends 1980-2009 and future estimates to 2050’. Indust
Health 51: 56-67.
12. Crowe J, Wesseling C, Solano BR, Umana M, Ramirez AR, Kjellstrom T, Morales D, Nilsson M. (2013). ‘Heat
exposure in sugarcane harvesters in Costa Rica’. Am J Industrial Medicine 56: 1157-1164; Sheffield PE, Herrera
JGR, Lemke B, Kjellstrom T, Romero L. (2013). ‘Current and future heat stress in Nicaraguan work places undera changing climate’. Industrial Health 51: 123-127
13. See http://www.bbc.com/news/world-asia-25743438)
14. Sherwood S, Huber M. (2010). ‘An adaptability limit to climate change due to heat stress.’ PNAS 107: 9552-4
15. Generated using the HadGEM climate model, with the RCP8.5 emissions pathway.
16. IPCC (2014). Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth
Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and
L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.

11

THE RISKS OF CLIMATE CHANGE FOR CROP 			PRODUCTION

Professor John R. Porter,i Dr Manuel Montesinoii and Dr Mikhail Semenov.iii

For this assessment of climate change risks to crop production, we draw heavily on the chapter ‘Food
Security and Food Production Systems’1 of the IPCC’s Fifth Assessment Report. We add to this somediscussion of important thresholds.

What do we wish to avoid, and how likely is it?
In terms of the risk of climate change to the production of individual crops, one thing we wish to avoid
is crop failure. This may be defined as: “Reduction in crop yield to a level that there is no marketablesurplus or the nutritional needs of the community cannot be met.”2 Since this level is not easily defined,
this chapter considers two cases:
i. Plausible worst case reductions in average yield.
ii. The possibility of near-complete loss of yield in a given year.
In terms of the risk of climate change to global crop production, what we wish to avoid is the failure of
production to keep pace with growing demand.

THE RISK TO INDIVIDUAL CROPS: RISK OF CROP FAILURE
Plausible worst case reductions in average yield
As climate change progresses over time, its effect on crop yields is projected to become increasingly negative.
The magnitude of this effect is highly uncertain. This progression, and its uncertainty, are illustrated by figure 1.

Figure 1: Summary of projected changes in crop yields due to climate change over the
21st century.iv From IPCC AR5 WG2 Summary for Policymakers.3

i.

Professor John R. Porter is Professor of Climate and Food Security at the University of Copenhagen.

ii.

Dr Manuel Montesino is a member of the University of Copenhagen.

iii. Dr Mikhail Semenov is a member of Rothamstead Research.
iv.

The figure includes projections for different emission scenarios, for tropical and temperate regions, and for adaptation and noadaptation cases combined. Relatively few studies have considered impacts on cropping systems for scenarios where global mean
temperatures increase by 4°C or more. For five timeframes in the near term and long term, data (n=1090) are plotted in the 20-year
period on the horizontal axis that includes the midpoint of each future projection period. Changes in crop yields are relative to late20th-century levels. Data for each timeframe sum to 100%.

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Figure 1 shows an aggregation of projections for change in the yield of different crops, in different regions,at different degrees of climate change, and under different assumptions including with regard to whether
adaptive measures have been taken. From this high-level overview, some initial ‘worst case’ information may
be inferred: by 2030-2049, the lowest tenth of projections give yield decreases of 25% - 50%. By 2090-2109,
the lowest fifth of projections give yield decreases of 50% - 100%.

From the underlying data, we can see how wide is the range of projections for a given crop, in a given region,
for a given temperature increase. For example, of two studies considering the impact on wheat productionin Pakistan with a temperature change of 3°C, one estimates a 23% increase, and the other a 24% reduction.
Similarly one study estimates the impact of a 3°C change on rice production in China to be anywhere between
a reduction of 40% and an increase of 0.2%.6 The low end of these ranges gives a rough idea of a plausible
worst case.

There are, however, dangers in averaging. When the data for different studies are disaggregated, as in figure
2, the wide range of possible outcomes is more clearly visible.

Figure 2: Percentage simulated yield change as a function of local temperature change
for the three major crops and for temperate and tropical regions.v, 4 From IPCC AR5.5

What can we say about the likelihood of the low end projections versus that of the high end projections?
There are many uncertain factors (discussed below), and the projections are not probabilistic. To a first
approximation, we may assume that the worst case and the best case are equally likely, but that the overall
trend is clearly for a reduction in crop yields for increases in local temperatures. (This has to be seen in thecontext of an increasing human population, as discussed below).
While the data in the IPCC figures show a wide range of impacts on crop production for a certain range oftemperature increases, it is notable that the range of temperature increases considered is relatively narrow.
As can be seen from Figure 2, all the projections relate to local temperature increases of 1-5°C, and most ofthem are at the lower end of this scale. Since land temperatures increase more than the global average, mostof these results may be considered to fall into a range of global mean temperature increase of roughly 1-3°C.
As noted in the IPCC AR5 WG2 Summary for Policymakers, ‘relatively few studies have considered impacts
on cropping systems for scenarios where global mean temperatures increase by 4°C or more’.
Chapter 9 observes that in the worst case, global mean temperature could increase by more than 7°C thiscentury, and more than 10°C over the next few centuries. The impact of climate change on crop production
for the upper half of this range is relatively unstudied. In this sense, our knowledge of worst case scenarios
for climate change impacts on crop production is severely lacking.
This lack of information is particularly important given that the relationship between temperature increase
and impact on crop yield is not expected to be linear. The World Bank’s ‘Turn Down the Heat’ report on
the impacts of climate change at 4°C stated: “Recent research also indicates that there may be larger
negative effects at higher and more extreme temperatures… In particular, there is an emerging riskof nonlinear effects on crop yields because of the damaging effect of temperature extremes. Field
experiments have shown that crops are highly sensitive to temperatures above certain thresholds. Thiseffect is expected to be highly relevant in a 4°C world. Most current crop models do not account for thiseffect, leading to recent calls for an ‘overhaul’ of current crop-climate models.”7

Near-complete loss of yield in a given year
The decline in crop yields shown above mainly considers the shortening of the growing season caused byraised average temperatures. A shorter time-period in the field translates into less time to settle, grow andproduce dry matter. At the same time, it has long been known that crops can also be severely damagedby short and extreme heat events. Temperatures exceeding critical thresholds, especially during sensitive
periods, may cause drastic drops of yield.8 Temperatures equal to or higher than 30-34°C at the time offlowering may inhibit pollen production and grain setting giving unstable yields from year-to-year. Figure
3 shows a range of thresholds for wheat, maize and rice, including the lethal limits – in the range of 45–
47°C – beyond which the plant dies. While some crop models incorporate this non-linear response to hightemperatures,9 the majority do not.

v.

Percentage simulated yield change as a function of local temperature change for the three major crops and for temperate and tropicalregions. Dots indicate where a known change in atmospheric CO2 was used in the study; remaining data are indicated by x. Notethat differences in yield value between these symbols do not measure the CO2 fertilization effect, as changes in other factors suchas precipitation may be different between studies. Non-parametric regressions (LOESS, span = 1 and degree = 1) of subsets of thesedata were made 500 times. These bootstrap samples are indicated by shaded bands at the 95% confidence interval. Regressions are
separated according to the presence (blue) or absence (red) of simple agronomic adaptation (Table 7-2). In the case of tropical maize,
the central regression for absence of adaptation is slightly higher than that with adaptation. This is due to asymmetry in the data—not
all studies compare adaptated and non-adapted crops. Figure 7-8 presents a pairwise adaptation comparison. Note that four of the1048 data points across all six panels are outside the yield change range shown. These were omitted for clarity. Some of the studies
have associated temporal baselines, with center points typically between 1970 and 2005. Note that local warming in cropping regions
generally exceeds global mean warming (Figure 21-4).

67

Mean optimum te

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20
RISK ASSESSMENT
PART 2: DIRECT RISKSRISK ASSESSMENT PART 2: DIRECT RISKS

10

Figure 3: Rice, wheat and maize - Mean maximum temperature for leaf initiation, shoot0 root growth and lethality.10
growth,Leaf initiation

Shoot growth

Root growth

Figure 4: Probability of exceeding threshold temperature for flowering for rice in
Jiangsu, as a function of (a) temperature increase, and (b) time. Results are shown for
two climate models (GISS and MIROC) and for three varieties of rice.

(c)
Mean maximum temperature (ºC)

68

30

Rice

50

Wheat

Maize

40
30
20
10
0

Leaf
initiation

Shoot
growthRoot
growthLethallimitsA new study undertaken for this risk assessment by Dr Manuel Montesino, Dr Mikhail Semenov and Professor
Dr John R Porter investigated how the likelihood of crossing the threshold temperature for flowering could
increase over time. The study looked at the highest emissions scenario (RCP8.5) to identify maximum
probability boundaries for three major crops in three growing areas: wheat in the Punjab, India, rice in
Jiangsu, China, and maize in Illinois, USA.vi The study considered several varieties and managementsvii foreach crop.
The results supported earlier findings that crop failure due to high temperature stress at flowering is an
important issue to consider, especially for maize and rice. For the crop-location combinations examined, thelargest risk was for rice in Jiangsu. The probability of exceeding the threshold temperature at least once forflowering during the time when the crop would be at that stage of the growth cycle increased from close tozero in the present day, to above 25% for two varieties (early and late rice) and 80% for another (single rice),for a global temperature increase of 4.7°C (local temperature increase of 7°C), reached by the high sensitivity
model in 2090 (see Figure 4). This result could also be interpreted as a decrease in the return time from 1 in
100 years at present, to around 1 in 8, or 1 in 1.25 years (depending on variety) by the end of the century.

vi. The precise locations used were: Punjab (31.01ºN, 75.4ºE); Jiangsu (32.9ºN,119.16ºW); Illinois (39.7ºN,-89.5ºW), USAvii. ‘Management’ in this context refers mainly to planting and sowing dates. In some cases planting/sowing dates were assumed equal fordifferent cultivars and for some others, when data was available, each cultivar implied a particular planting/sowing date.

For maize in Illinois, the return time for exceeding the threshold temperature for flowering reduced fromaround 1 in 100 now to 1 in 50 ( probability of 1-3%) for a global temperature increase of 2-3°C, and 1 in
6 (probability between 6-40% ) for 4.7°C (with local temperature increase of 8°C). For wheat in the Punjab
there was a less significant risk of acute effects, since it remained possible for flowering to take place earlyenough in the year to avoid the hottest temperatures.

Figures 5 and 6: Probability of exceeding threshold temperature for flowering formaize in Illinois (5), and wheat in Punjab (6) as a function of (a) temperature increase,and (b) time. Results are shown for two climate models (MRI and MIROC for maize, andGISS and HadG for wheat) and for several varieties of each crop.

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CLIMATE CHANGE: A RISK ASSESSMENTRISK ASSESSMENT PART 2: DIRECT RISKSRISK ASSESSMENT PART 2: DIRECT RISKSRISK TO GLOBAL CROP PRODUCTION: RISK OF FAILING TO KEEP UP WITH DEMAND
The world already has several hundred million undernourished people, but not because there is not enoughfood. Food security depends not only on production, but also on the availability and affordability of food topeople, on the systems of storage, transport and trade, and on patterns of consumption and nutrition. Climatechange is likely to affect all the components of food security, and food security will be affected by non-climate
factors too. As the IPCC stated, “The overall impact of climate change on food security is considerably
more complex and potentially greater than projected impacts on agricultural productivity alone.”13This more complex risk is considered in Part III of our risk assessment: Systemic Risks.
Here we focus on the narrower question of whether global crop production will be able to keep up withgrowing demand. Global demand for crops is projected to grow by around 60 – 100% between 2005 and
2050, due to population and economic growth.14 The FAO estimates that meeting this demand will requirecrop production to grow by about 14% per decade.15 Rapid growth in agricultural output has been achieved in
the past, through combinations of better crop varieties and management, in almost equal proportion. There is
scope for further growth, notably from the intensification of agriculture within developing countries mainly by
improvements in infrastructure and crop management, especially fertilizer use and more effective irrigation.
The question is whether such high rates of growth can be achieved, and high levels of output maintained,
under any degree of climate change.
Ultimately, this must depend largely on our capability for what is known as adaptation. Understanding
whether there are limits to adaptation, constraints on it, or thresholds beyond which it becomes significantly
more difficult, is therefore a critical question for a risk assessment.
The main adaptive responses to reduce the risk of climate change to crop production are:
A caveat of the study is that the results presented here do not compute the effect of climate change on crops.
Results refer only to the probability of the temperatures exceeding thresholds. We know that temperatures
above flowering thresholds will have an acute effect on yield, but we have not quantified that effect. This
would require experimental and further modelling studies, but the balance of probability is that the risk toyield from short-term intense periods of high temperatures at sensitive stages of crop development can give
dramatic decreases in crop yields for two of the three major global crops.

i. growing the crop at a different time of year;
ii. increasing the crop’s tolerance for extreme conditions;iii. growing the crop in a different place (migration of production zones);
iv. growing a different crop altogether.

Risk to production of individual crops: what we know, what we do not know, and what
we think

All of these responses may be subject to some limitations or constraints. For example:

What we know:
Research on crop physiology over the past 30-40 years has enabled us to understand, quantify and thus
to some extent predict the effects of environmental factors such as temperature, CO2 level and water andnutrient supply on the major crops.11 This understanding has been derived from hundreds of experiments inlaboratories, growth chambers and fields.

i. The timing of crop development – i.e. the window for growth - depends on day-length (’photoperiod’) as
well as temperature. There are limits to what can be achieved by shifting the time and place of planting,since a favourable shift with respect to temperature could correspond to an unfavourable shift withrespect to photoperiod.

What we do not know:
A main source of uncertainty in crop responses to climate change is how combinations of growth anddevelopment controlling factors affect yield. This is particularly the case with non-major but important crops
such as millet, sorghum, and vegetables. Combinations of effects such as changes in CO2 level, temperature,and nutrition etc. have been studied less commonly than single factors. A second major uncertainty is theeffects of biotic stresses from diseases, pests and weeds. Taking these factors into account could mean thereal range of uncertainty is even wider than the range of projections given by crop models.
What we think:
Most of the factors not taken into account in the models – and the projections – are likely on balance to havea negative effect. Invasive weeds are expected to spread further and become more competitive as a resultof climate change; studies suggest a tendency for the risk of insect damage to plants to increase; and more
frequent intense precipitation, flooding and drought would all be expected to further reduce average yields.12

ii. Crops’ tolerance for high temperatures may be raised either by breeding or by genetic modification, but
the extent to which this is possible is ultimately subject to biophysical limits. The evidence is that there
is little genetic variation either between varieties within a crop or between crops themselves in thesesensitivities.16
iii. There is a finite supply of unused land, and not all of it is suitable for the crops we might wish to grow.
For example, the potential for wheat production in Russia to be shifted northwards is limited by the poor
nutritional quality of soils in that region. (Those soils also contain large amounts of carbon; their tillage
could release huge amounts of CO2 and methane into the atmosphere, further exacerbating warming).
iv. Over-reliance on a few major crops means alternatives are currently under-utilized and under-researched.

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What do we know, what do we not know, and what do we think?
We know that food security, the balance between the demand for and the supply of food, is a matter ofmore than just crop production. It is essential to look at where in the food chain increases can be achieved,
efficiencies gained and losses reduced (these issues are discussed further in Part III). At the same time, weknow that crop production is vitally important.
We know that climate change poses a risk to crop production, as described above, and that there are potential
constraints on our ability to adapt. We do not know enough about where and when those constraints might beencountered.
What we think may be summarized by the IPCC’s headline conclusion that: “Global temperature increasesof ~4°C or more above late 20th century levels, combined with increasing food demand, would
pose large risks to food security globally and regionally’17 and its more detailed statement that: ‘The
existence of critical climatic thresholds and evidence of non-linear responses of staple crop yields totemperature and rainfall thus suggest that there may be a threshold of global warming beyond whichcurrent agricultural practices can no longer support large human civilizations, and the impacts on
malnourishment and under-nutrition… will become much more severe. However, current models toestimate the human health consequences of climate-impaired food yields at higher global temperaturesgenerally incorporate neither critical thresholds nor nonlinear response functions, reflecting
uncertainties about exposure-response relations, future extreme events, the scale and feasibilityof adaptation, and climatic thresholds for other influences such as infestations and plant diseases.
Extrapolation from current models nevertheless suggests that the global risk to food security becomesvery severe under an increase of 4°C to 6°C or higher in global mean temperature (medium evidence,high agreement).”

Endnotes
1.

2.
3.

4.

5.
6.

Porter, J.R., L. Xie, A.J. Challinor, K. Cochrane, S.M. Howden, M.M. Iqbal, D.B. Lobell, and M.I. Travasso (2014).
‘Food security and food production systems’, Climate Change 2014: Impacts, Adaptation, and Vulnerability.
Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, pp. 485-533.
As defined by businessdictionary.com
Figure SMP.7, p.18 from IPCC (2014). ‘Summary for policymakers’. Climate Change 2014: Impacts, Adaptation,
and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth
Assessment Report of the Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken,
K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel,
A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge,
United Kingdom and New York, NY, USA, pp. 1-32.
Data are taken from a review of literature: Rosenzweig and Parry, 1994; Karim et al., 1996; El-Shaher et al., 1997;
Kapetanaki and Rosenzweig, 1997;Lal et al., 1998; Moya et al., 1998;Winters et al., 1998; Yates and Strzepek,
1998;Alexandrov, 1999; Kaiser, 1999; Reyenga et al., 1999;Alexandrov and Hoogenboom, 2000; Southworth
et al., 2000; Tubiello et al., 2000; DeJong et al., 2001; Izaurralde et al., 2001;Aggarwal and Mall, 2002;AbouHadid, 2006;Alexandrov et al., 2002; Corobov, 2002; Chipanshi et al., 2003; Easterling et al., 2003; Jonesand Thornton, 2003;Luo et al., 2003; Matthews and Wassmann, 2003; Droogers, 2004; Howden and Jones,
2004; Butt et al., 2005; Erda et al., 2005; Ewert et al., 2005; Gbetibouo and Hassan, 2005; Izaurralde et al.,2005; Porter and Semenov, 2005; Sands and Edmonds, 2005; Thomson et al., 2005; Xiao et al., 2005; Zhang
and Liu, 2005; Zhao et al., 2005;Abraha and Savage, 2006; Brassard and Singh, 2007, 2008; Krishnan et al.,2007;Lobell and Ortiz-Monasterio, 2007; Xiong et al., 2007; Tingem et al., 2008;Walker and Schulze, 2008; El
Maayar et al., 2009; Schlenker and Roberts, 2009; Thornton et al., 2009a, 2010, 2011; Tingem and Rivington,
2009; Byjesh et al., 2010; Chhetri et al., 2010;Liu et al., 2010; Piao et al., 2010; Tan et al., 2010; Tao and Zhang,2010, 2011a,b;Arndt et al., 2011; Deryng et al., 2011; Iqbal et al., 2011;Lal, 2011;Li et al., 2011; Rowhanji et al., 2011;
Shuang-He et al., 2011; Osborne et al., 2013.
Figure 7-4, p. 498 from Porter et al (2014).
See Box 7-1, pp. 509-512 in IPCC (2014). Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part
A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, 1132 pp.

7.

World Bank (2012). Turn down the heat : why a 4°C warmer world must be avoided. Washington DC : World
Bank. p46. http://documents.worldbank.org/curated/en/2012/11/17097815/turn-down-heat-4°c-warmer-worldmust-avoided
8. Negative impacts of high temperature stress at the time of flowering on the yield of grain crops have been
documented in plant experiments with wheat (FERRIS, RACHEL, et al. “Effect of high temperature stress atanthesis on grain yield and biomass of field-grown crops of wheat.” Annals of Botany 82.5 (1998): 631-639);
rice (Jagadish, S. V. K., P. Q. Craufurd, and T. R. Wheeler. “High temperature stress and spikelet fertility in rice
(Oryza sativa L.).” Journal of experimental botany 58.7 (2007): 1627-1635) and peanut (Prasad, Pagadala V.
Vara, et al. “Effects of short episodes of heat stress on flower production and fruit‐set of groundnut (Arachis
hypogaea L.).” Journal of Experimental Botany 51.345 (2000): 777-784).
9. Challinor, A. J., et al. “Simulation of the impact of high temperature stress on annual crop yields.” Agriculturaland Forest Meteorology 135.1 (2005): 180-189
10. Sanchez, B., Rasmussen, A. and Porter, J. (2014) ‘Temperatures and the growth and development of maize andrice: a review’. Global Change Biology 20, 408–417, doi: 10.1111/gcb.12389
11. Hay, R.K.M. and Porter, J. R. (2006). The Physiology of Crop Yield. Blackwells Scientific Publications).
12. Porter et al (2014).
13. Porter et al (2014), p502
14. World Bank (2012), p44
15. Porter et al (2014), p506
16. See Sanchez et al (2014) and Porter JR and Gawith M (1999). ‘Temperatures and the growth and developmentof wheat: a review.’ European Journal of Agronomy, 10, 23–36.
17. IPCC (2014). ‘Summary for Policymakers’.

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12

THE RISK OF WATER STRESS

Table W1: Numbers of people (millions) living in water-stressed watersheds. The
figures for 2050 are based on a medium population growth assumption, and the ranges
represent the effects of low and high growth assumptions.

Professor Nigel Arnell, Director of the Walker Institute for Climate System Research.

<500m3/capita/year

Population<1700m3/capita/year<1000m3/capita/year<500m3/capita/year

Population

N. Africa

162

150

94

209

254 (226-326)

244 (213-282)

216 (153-248)

329 (292-383)

W. Africa

54

17

4

309

484 (373-610)

367 (185-489)

37 (20-117)

756 (616-926)

C. Africa

3

0

0

110

11 (10-14)

8 (6-9)6 (0-8)239 (202-277)

E. Africa

94

10

2

193

326 (272-441)

299 (159-381)

103 (13-221)

418 (349-496)

Sn Africa

47

20

0

210

186 (133-282)

101 (45-177)

24 (4-35)

488 (396-609)

S. Asia

1,394

1,172

199

1,706

2121 (1,906-2,526)

1,802 (1,512-2,183)

746 (628-1,003)

2,390 (2,151-2,722)

SE Asia

7

0

0

605

27 (25-31)

0 (0-0)0 (0-0)791 (728-889)

E Asia

1,202

691

386

1,546

1084 (1,035-1,184)

643 (613-660)

359 (340-388)

1,434 (1,375-1,510)

Central
Asia

1

0

0

46

65 (57-80)

2 (1-2)0 (0-0)70 (62-84)

Middle
East

166

93

71

214

356 (295-397)

310 (222-344)

190 (164-209)

379 (339-420)

Australasia

0

0

0

35

0 (0-0)0 (0-0)0 (0-0)50 (50-45)

W. Europe

220

123

20

411

220 (239-165)

138 (166-55)

23 (24-15)

425 (441-344)

C. Europe

51

8

0

118

23 (24-20)

1 (8-1)0 (0-0)102 (103-96)

E. Europe

20

4

3

221

9 (8-18)

4 (3-4)4 (3-4)186 (178-196)

Canada

6

6

0

35

7 (8-5)7 (8-5)7 (8-0)44 (45-31)

US

78

54

27

312

99 (102-75)

74 (76-56)

38 (39-26)

390 (402-303)

MesoAmerica

58

26

0

197

124 (112-154)

61 (53-101)

31 (28-37)

279 (250-346)

Brasil

0

0

0

195

0 (0-0)0 (0-0)0 (0-0)237 (218-269)

South
America

15

4

4

198

46 (29-56)

19 (17-25)

6 (5-7)278 (251-329)

Water resources are under stress in many regions due to increasing demands and, in places, falling quality.
Climate change has the potential to change the risks of water stress.1 The focus in this section is on strategic
definitions of water stress, which are based on generalized indicators of the amount of water that is available
and the demands on that resource. Operational definitions, on the other hand, are typically based on thereliability of the supply of appropriate quality water and are strongly determined by local conditions.

What do we want to avoid?
The most widely used sets of indicators of high-level water resources stress are based on the ratio of total
resources to population (‘resources per capita’) and the proportion of resources that are withdrawn forhuman use.2 The first is simpler but does not reflect stresses introduced by high per-capita water use, forexample where there is significant irrigation for agriculture; on the other hand, data on current and futurewater withdrawals can be highly uncertain.
There are three widely used thresholds for defining levels of water stress on the basis of per capita availability.
Basins or countries with average annual resources between 1000 and 1700 m3 per capita per year are
typically classed as having ‘moderate water shortage’, and if resources are below 1000 m3 per capita per year
then the region is classed as having ‘chronic water shortage’. If resources are below 500 m3 per capita peryear then the shortage is ‘extreme’.3 The thresholds are essentially arbitrary, although derive ultimately froman assessment of exposure to water resources stress in Africa.4
In 2010, almost 3.6 billion people, out of a global population of around 6.9 billion, were living in watershedswith less than 1700 m3 per capita per year (Table W1), and almost 2.4 billion were living in watersheds withless than 1000 m3 per capita per year (chronic water shortage).i Approximately 800 million people were
living in watersheds with less than 500 m3 per capita per year (extreme water shortage).

i.

The methods used here to estimate future risks to water resources are summarised in the Annex.

2050<1000m3/capita/year2010<1700m3/capita/year

74

Global
3,576
2,376
809
6,868 5449 (4,853-6,382) 4,079 (3,286-4,774) 1,789 (1,430-2,317)								9,283 (8,44410,273)

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How will exposure to water stress change in the future?

Middle East: water stressed population

Figure W1: The number of people living in water-stressed watersheds (<1000m3/
capita/year), with and without climate change. The plots show two climate pathways(RCP2.6 and RCP8.5). The solid line represents the median estimate of impact foreach pathway, and the shaded areas show the 10% to 90% range. A medium growth
population projection is assumed.
Global: water stressed population

200

4000

160

350

Millions of people

300
200
100

300
250
200
150
100
50

21
0
0

20
90

20
80

20
70

20
60

20
50

20
40

20
30

20
20

21
0
0

20
90

20
80

20
70

20
60

20
50

20
40

20
30

20
20

No climate change

20
10

0

0

No climate change

RCP2.6

RCP2.6

RCP8.5

RCP8.5In the absence of future climate change, the numbers of people living in watersheds with chronic water
shortage decreases in East Asia from 2030, and in South Asia from 2060. If climate change is included,
it results in more rainfall and river runoff in East Asia. This combines with the reduction in population to
reduce apparent water shortage – but there is a chance (shown by the shaded area) that climate change
would slow the reduction in exposure to water shortage. Similarly, Figure W1 shows that climate change
could substantially increase the number of people living in watersheds with chronic water shortage in south
Asia – or reduce them. In the US and the Middle East climate change is very likely to increase exposure
throughout the century, and in North Africa is more likely than not to produce an increase in exposure.

US: water stressed population

5000

400

400

20
10

Climate change will also affect the number of people living in water-stressed conditions. Figure W1 shows
the change in numbers of people living in watersheds with chronic water shortage (less than 1000 m3 per
capita per year) through the 21st century, for the globe as a whole and for five regions, under no climatechange and under two different climate change pathways (RCP2.6 and RCP8.5, low and high greenhouse gas
emission scenarios respectively). The shaded areas show the range of potential numbers due to uncertainty
in the pattern of resource change due to climate change. The plots all assume the medium population growth
projection.

North Africa: water stressed population

500

Millions of people

Through the 21st century, changes in total population will result in changes in exposure to water resources
stress. By 2050, under a medium population growth assumption,5 the number of people living in watershedswith less than 1000 m3 per capita per year will increase to around four billion – a bigger proportion of
the global population than in 2010. The effects of population growth on the number of people living inwatersheds with less than 500 m3 per capita per year are even more pronounced: it will double by 2050 toaround 1.8 billion people. The magnitudes of the changes are influenced to a certain extent by the assumed
changes in population (as shown in Table W16), but even under low-growth assumptions there are significant
increases in exposure to water scarcity at global and regional scales.

Millions of people

Millions of people

180

3000
2000
1000

Calculating risk

140

Figure W2 shows the risk by region that climate change increases by more than 10% the numbers of people
living in watersheds with chronic water shortage under the two climate pathways. By 2050, the largest ofthese probabilities are in central Asia, Europe, the USA, Central America and southern America. By 2100, theprobabilities that exposure to shortage will increase are considerably greater in most regions than in 2050.
Under the low emissions climate pathway the probabilities are smaller in most regions than under the highemissions pathway, particularly by 2100.

120
100
80
60
40
20

0

90

80

0
21

20

20

20
70

50

60
20

40

20

20

20
30

20
10

0
0
21

90

80

20

20

60

20
70

20

50

40
20

20
30

20
20
RCP2.6

RCP2.6

RCP8.5

RCP8.5

East Asia: water stressed population

South Asia: water stressed population
2000

1000
900

1800

No climate change

No climate change

RCP2.6

RCP2.6

RCP8.5

RCP8.5

0

90

0
21

20

80
20

20
70

20
10

0
0
21

90
20

80
20

20
70

60
20

50
20

40
20

20
30

0

20
20

200

0

60

400

100

50

200

20

300

800
600

20

400

1200

40

500

1400

20

700
600

1600

20
30

Millions of people

800

20
20

20
10

20

No climate change

No climate change

20
10

20
20

0

0

Millions of people

76

79

RCP2.6

RCP2.6

RCP8.5

RCP8.5

0
21
0

90
20

80
20

60

50

20
70

20

20

20
10

0
21
0

80

90
20

20

20
70

50

60
20

40

20

20

20
30

20
20

0

20
10

10

0

40

20

10

90

RCP2.6

RCP2.6

RCP8.5

RCP8.590

RCP2.6

RCP2.6

RCP8.5

RCP8.5

21
0
0

20
90

0
0
21

90
20

0
21
0

90
20

20

20
10

0
21
0

90
20

20

80

0

20
70

10

0

80

20

10

60

20
80

30

20
70

20

60

30

50
40

20

50
40

70
60

50

60

80

20

70

40

80

20
20

Percentage of riskof crossing threshold

100

90

20

80

Karun: risk of falling below 1000m3/
capita/year

100

50

20

20
10

0
0
21

90
20

80
20

20
70

20

20

60

0

50

10

0

20
70

20

10

20

20
70

30

60

20

20

30

50
40

50

50
40

70
60

20

60

80

40

70

20

80

20
20

Percentage of riskof crossing threshold

100

90

40

20
60

Indus: risk of falling below 500m3/
capita/year

100

20

20
50

20
10

RCP2.6
RCP8.520
40

Percentage of riskof crossing threshold
21
0
0

20
90

20
80

20
70

20
60

20
50

20
40

20
30

RCP2.6

20
10

30

20

20

50
40

20
30

30

70
60

20
20

50

80

20

RCP8.5

40

90

30

0

20

90

50
40

10

20
30

100

Percentage of riskof crossing threshold

100

70
60

0

Ganges: risk of falling below 1700m3/
capita/year

Percentage of riskof crossing threshold

Rio Grande: risk of falling below
1000m3/capita/year

80

10

Tigris-Euphrates: risk of falling below
500m3/capita/year

Figure W3: Risk of resources per capita falling below specified thresholds for nineillustrative watersheds under two climate pathways (note that the thresholds vary
between watersheds). The dotted line shows risk with current (2010) population, andthe solid line shows risk under the medium population growth projection

40

20

20
30

BrazilSouth America

USA

Central America

CanadaC. Europe

E. Europe and Russia

W. Europe

Australasia

East Asia

Central Asia

South Asia

South East Asia

N. Africa

The risks posed by climate change to water scarcity can also be assessed at the basin scale. Figure W3 shows
the probability that resources per capita falls below defined thresholds in nine major basins that are, or likely
will be, exposed to water resources stresses (note that the thresholds vary between basins). The dotted lines
show probability of falling below the thresholds under the two climate pathways assuming population remains
at 2010 levels, and the solid lines show probability under the medium growth population projection. The
difference between the solid and dotted lines represents the effect of population change on probability (andof course this difference varies with population projection).

70

30

20
20

RCP2.6

60

50
40

20
10

RCP2.6

80

60

20
10

RCP8.5

Middle East

Sn Africa

Brazil0

RCP8.5

Sacramento: risk of falling below
1700m3/capita/year70

20South America

USA

Central America

CanadaC. Europe

E. Europe and Russia

W. Europe

Australasia

East Asia

Central Asia

South Asia

South East Asia

N. Africa

Middle East

Sn Africa

East Africa

Cen. Africa

0

90

80

40

East Africa

20

60

Cen. Africa

40

80West Africa

Percentage of probability

60West Africa

Percentage of probability

80

100

90

20
20

100

100

100

20
20

2100: Probability number of people withchronic water shortage increasing by
more than 10%

Percentage of riskof crossing threshold

2050: Probability number of people
with chronic water shortage increasing
by more than 10%

Huang He: risk of falling below 500m3/
capita/year

20

Yangtse: risk of falling below 1700m3/
capita/year

20
30

Figure W2: The risk that climate change increases by more than 10% the numbers ofpeople living in water-stressed watersheds, relative to the situation with no climatechange, under the two climate pathways. A medium growth population projection isassumed.

20
30

RISK ASSESSMENT PART 2: DIRECT RISKS

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RISK ASSESSMENT PART 2: DIRECT RISKS

2020

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CLIMATE CHANGE: A RISK ASSESSMENT

Percentage of riskof crossing threshold78

CLIMATE CHANGE: A RISK ASSESSMENT

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Moulouya: risk of falling below 500m3/
capita/year

The assessment in Figure W4 is based on the assumption that all the climate models used to estimate impacts
are equally plausible, and that they span the range of potential regional climate change impacts. This, ofcourse, is not necessarily the case. The global-scale impacts are largely dominated by impacts in south and, toa lesser extent, east Asia, and are therefore very sensitive to projections of how the south Asian monsoon may
change (see box on Variation in the Indian monsoon).

100

Percentage of riskof crossing threshold

90
80
70
60
50

Variation in the Indian monsoon

40
30
20

Professor Brian Hoskins, FRS

10

21
0
0

20
90

20
80

20
70

20
60

20
50

20
40

20
30

20
20

20
10

0

RCP2.6
RCP8.5What is a plausible worst case for water stress due to climate change?
There are considerable uncertainties in the projected impacts of climate change on water stress, evenassuming a single projection for changes in population. The chance of impacts exceeding some defined
threshold – as shown in the previous section – represents one aspect of this uncertainty, but anotherassessment of risk can be based on a plausible ‘worst case’.
By 2050, up to 620 million people may be added to the 4 billion people (Table W1) living in watersheds withchronic water shortage, under high emissions and the most extreme climate scenario. Figure W4 shows the‘worst case’ by region in 2050, along with the 10th percentile from the distribution of impacts (the upper partof the shaded region in Figure W1). In some regions the worst case is little different to the 10th percentile,
but in others is considerably larger reflecting greater uncertainty in projected impacts. However, the worst
cases shown in Figure W4 do not occur simultaneously: the global ‘worst case’ is not equal to the sum of theregional worst cases. Under no one plausible pattern of climate change does every water-stressed region see
the maximum reduction in runoff.

Figure W4: Plausible ‘worst case’ impacts of climate change in 2050 on water stress.
The graph shows the increase in numbers of people living under chronic water shortage
under the RCP8.5 climate pathway and the medium growth population projection. Thereis a 10% probability that the impact is greater than that shown by the blue dots, and thered dots show the maximum calculated impact

In impact studies, it is assumed that the set of available projections of future climate from the state-ofthe-art climate models spans the space of possible climates. In particular, in determining the risk posedby increasing greenhouse gases it is assumed that the worst case scenarios in any region are included
amongst the model projections. However, climate models developed and tested in the context of theclimate of the 20th century have known deficiencies, some of which are common to most models. Someof these deficiencies are likely to influence the simulation of future extreme climate changes. Further, it is
not clear that the climate models are able to simulate any major climate change due to the crossing of athreshold in the climate system that could occur. This could, for example, be of the nature of the onset ofsignificant greenhouse gas emissions from melting permafrost, the destabilization of the West Antarctic Ice
Sheet or a drastic reduction in the overturning circulation in the Atlantic Ocean. Palaeo-climate runs withthe models show that they are unable to simulate just how different the monsoon systems of the world
have been in the past. For example the Sahara was green with vegetation some seven thousand years ago.
However, climate models do not produce this big change in rainfall. One recent study concluded, “Stateof-the-art climate models are largely untested against actual occurrences of abrupt change. It is ahuge leap of faith to assume that simulations of the coming century with these models will provide
reliable warning of sudden, catastrophic events.”7
The amazing thing about the Indian summer monsoon is the large effect of a small variation from one year
to year: 10% more rainfall and there are floods, 10% less and there are huge problems for farmers. In any
year monsoon active and break periods occur. Breaks that last more than a couple of weeks also cause
major agricultural problems. Climate models are in general projecting a slight strengthening of monsoon
rainfall, but it should be recognized that the changes in particular monsoons, such as that in India, could
be much more significant than this suggests. Given the large perturbation of the climate system due togreenhouse gas emissions, we should be prepared that the future Indian Monsoon could have average
rainfall outside the current normal range, and the variability between one year and another and in theactive-break cycle could be very different.

What do we know, what do we not know, and what do we think?

300

Our estimates of future risks are based on (i) projections of regional future climate change, (ii) projections ofhydrological consequences of climate change and (iii) projections of future population and exposure to water
resources stress. What happens in practice also depends on future adaptation.

250
200
150
100
50

Worst case10th percentile

BrazilSouth America

USA

Central America

CanadaE. Europe and Russia

C. Europe

W. Europe

Australasia

Central Asia

East Asia

South Asia

South East Asia

N. Africa

Middle East

Sn Africa

East Africa

Cen. Africa

0West Africa

Millions of people

80

Projections of regional future change depend partly on the assumed rate of growth in emissions and partly
on the projected patterns of changes in regional and seasonal climate – particularly precipitation. Whilst
the broad patterns of precipitation changes are reasonably consistent between models, the details and theprecise magnitudes of change differ. The quantitative estimates of impacts on water stress therefore vary, andthese tend to be larger than the apparent differences in precipitation change between climate models. This isbecause exposure to water shortage is concentrated in particular regions of the globe, and it is at the local toregional scale that the differences between climate models are greatest.

81

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CLIMATE CHANGE: A RISK ASSESSMENTRISK ASSESSMENT PART 2: DIRECT RISKSRISK ASSESSMENT PART 2: DIRECT RISKSAs climate models improve their representation of atmospheric dynamics and the distribution of precipitation
then the precise quantitative estimates of impacts on water stress will change, but for years to comedifferences between models will remain and there will therefore be a distribution of potential climate change
impacts for any one place.
The impacts of climate change on water shortage are assessed by using a hydrological model to translate
climatic changes to changes in runoff. As with climate models, different hydrological models can givedifferent responses to the same input data. Comparisons of the effects of hydrological model uncertainty
are still in their infancy,8 but early indications are that adding impact model uncertainty adds to the rangeof potential impacts. Moreover, it is likely that hydrological models have similar biases (they all tend tooverestimate river flows in dry regions, for example) and none yet explicitly incorporate the effects ofchanges in glacier volumes which may affect future resources in some regions.
Projections of future population are also uncertain (as shown in Table W1), because they are based on
different assumptions about changes in fertility rates, mortality rates and migration. It is not possible toassign likelihoods to different population projections, so it is necessary to estimate risks separately under
different plausible population narratives and projections.
Finally, the actual effects of climate change on ‘real’ water shortages will depend on the management
infrastructure and institutions which are put in place to cope with water shortage. There is already a very
considerable difference between developed and developing countries. Some management interventions
will offset the effects of climate change, but others may not. The effects of future adaptation on the ‘real’
consequences of future water shortages will therefore depend on (i) the extent to which adaptation takes
place (limited by a number of factors including finance and institutional capacity, alongside potential physicalconstraints such as the availability of feasible locations for storage reservoirs) and (ii) how effective theadaptation measures are in practice.

Lessons from risk assessment
The key conclusions from this section are therefore:• Climate change alters substantially the future risk of exposure to water shortages, but the effects arestrongly exaggerated or reduced by changes in population. Put the other way, the pressures on water
resources posed by increasing populations are substantially altered – exaggerated or reduced – byclimate change.
• Climate change reduces the probability of exposure to water shortages in some regions – particularly
in parts of east and south Asia - but this may be associated with substantial changes in flood risk (seechapter 14).
• The risks posed by climate change are typically less under low emissions than high emissions, but thedifference varies from place to place depending on how close watersheds in a region are to the water
shortage threshold. In some cases, risks are less under high emissions than low emissions, because
larger increases in runoff are enough to push watersheds out of the water shortage category.

Production of this chapter was supported by the AVOID 2 programme (DECC) under contractreference 1104872.

Endnotes
1. Jimenez-Cisneros, B.E., Oki, T., Arnell, N.W., Benito, G., Cogley, J.G., Doll, P., Jiang, T. & Mwakalila, S.S. (2014)‘Freshwater resources. In Field’, C.B., Barros, V. et al. (eds) Climate Change 2014: Impacts, Adaptation and
Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change. Cambridge University Press: Cambridge. 229-269.
2. Falkenmark, M., Berntell, A., Jagerskog, A., Lundqvist, J., Matz, M. & Tropp, H. (2007) On the verge of a newwater scarcity: a call for good governance and human ingenuity. SIWI Policy Brief. Stockholm International
Water Institute. Rijsberman, F. (2006) Water scarcity: fact or fiction? Agricultural Water Management 80 (1-3)
5-22.
3. Falkenmark, M., Berntell, A., Jagerskog, A., Lundqvist, J., Matz, M. & Tropp, H. (2007) On the verge of a newwater scarcity: a call for good governance and human ingenuity. SIWI Policy Brief. Stockholm International
Water Institute. Kummu, M., Ward, P.J., de Moel, H & Varis, O. (2010) Is physical water scarcity a newphenomenon? Global assessment of water shortage over the last two millennia. Environ. Res. Lett. 5 (JulySeptember 2010) 034006 doi:10.1088/1748-9326/5/3/034006
4. Falkenmark, M. (1989) ‘The massive water scarcity now threatening Africa: why isn’t it being addressed?’ Ambio
18 (2) 112-118
5. Shared Socio-economic Pathway (SSP) 2. https://secure.iiasa.ac.at/web-apps/ene/SspDb/. This is similar to the
UN 2012 medium population projections.
6. SSP1 and SSP3 are used to represent the extremes: these are very similar to the UN 2012 low and highpopulation projections.
7. Valdes, P. (2011). ‘Built for stability’. Nature Geoscience 4, 414-416. doi:10.1038/ngeo1200
8. Schewe, J., Heinke, J., Gerten, D., Haddeland, I., Arnell, N.W., Clark, D.B., Dankers, R., Eisner, S., Fekete, B., ColonGonzalez, F.J., Gosling, S.N., Kim, H., Liu, X., Masaki, Y., Portmann, F.T., Satoh, Y., Stacke, T., Tang, Q., Wada, Y.,
Wisser, D., Albrecht, T., Frieler, K., Piontek, F., Warszawski, L. & Kabat, P. (2014) Multi-model assessment of water
scarcity under climate change. Proceedings of the National Academy of Sciences 111, 3245-3250 doi.10.1073/
pnas.1222460110

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13

THE RISK OF DROUGHT

85

Figure D1: The proportion of regional cropland affected by drought with and without
climate change. The plots show two climate pathways (RCP2.6 and RCP8.5). The solidline represents the median estimate of impact for each pathway, and the shaded areas
show the 10% to 90% range.

Professor Nigel Arnell, Director of the Walker Institute for Climate System Research
Global: drought affected cropland

US: drought affected cropland

6
4
2

RCP2.6

RCP2.6

RCP8.5

RCP8.5Percentage of cropland

6
4
2

0
0
21

90
20

80
20

60

2

RCP2.6

RCP2.6

RCP8.5

RCP8.510
8
6
4
2

RCP2.6
RCP8.5

0
0
21

90
20

80
20

20
70

50

60
20

20

40
20

20
30

20
20

No climate change

0
0
21

90
20

80
20

60

20
70

20

50
20

40
20

20
30

20
20

20
10

0
0
21

90
20

80
20

20
70

50

60
20

20

20

No climate changeSouthern Africa: drought affectedcropland

20
10

20
70

4

No climate change0

1. See Annex’
2. (assuming no change in total cropland area)

50

6

0

40

0

8

How does the likelihood of drought change over time?

For the high emissions pathway, the proportion of cropland exposed to drought increases by around 75%
by 2050 and more than doubles by the end of the century, under the median estimate. On this pathway, theincidence of drought roughly triples in Southern Africa, and increases by about 50% in the US and South Asia,
over the course of the century. For the low emissions pathway, the increases are much less. At the same time,
what is very clear is that there is there is considerable uncertainty in the projections. The uncertainty ranges
suggest that while in the best case, drought incidence could halve in some regions, in the worst case, it could
increase by a factor of three or four.

20

10

8

Figure D1 shows the proportion of croplandii affected by drought under high (red) and low (green) emissions
pathways, for the globe as a whole and for four major regions, as a function of time. This is equivalent to theprobability that a given part of that cropland is in drought, in any given year.

20

40
20

South Asia: drought affected cropland

10

20
10

20
30

No climate change

East Asia: drought affected cropland
Percentage of cropland

20
20

20
10

0
0
21

90
20

80
20

20
70

50

60
20

40

20

20

No climate change• A water resources drought – a deficit in the amount of water available for distribution to consumers(such as irrigators).

This section focuses on meteorological drought, and characterises drought on the basis of an indicator ofaccumulated deficits of precipitation. More particularly, it looks at the average proportion of cropland at any
given time experiencing ‘extreme’ drought. We have defined ‘extreme drought’ for this purpose as being thelevel of precipitation deficit that occurs approximately 2% of the time in the current climate.i It is important
to note that this measure does not take into account the effect of raised temperatures, which, by increasing
rates of evaporation, will further increase the risk.

8

0

20
30

0

• A hydrological drought – a deficit in river flows and groundwater levels

One type of drought does not necessarily map directly onto another. Droughts – of whatever type – also
vary in their duration, intensity (amount of deficit) and spatial extent. It is therefore much more difficult tocharacterize both the ‘impact’ and the ‘risk’ of drought than the impact and risk of flooding. There are also
many different indicators of drought, tailored to different characterizations of drought.

Percentage of cropland

2

20
30

• An agricultural drought – a lack of water in the soil

4

20
20

• A meteorological drought – a lack of precipitation

6

20
20

Drought is a major challenge to people, agriculture and economies across the world. Broadly speaking, there
are four types of ‘drought’.

10

8

20
10

What do we want to avoid?

Percentage of cropland

10

Percentage of cropland

84

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Figure D2: The risk that climate change increases by more than 50% the average annual area ofcropland affected by ‘drought’, relative to the situation with no climate change, under the two
climate pathways.

Figure D3: Plausible ‘worst case’ impacts of climate change in 2050 on exposure to
drought. The graph shows the additional annual proportion of cropland affected bydrought under the RCP8.5 climate pathway, relative to the proportion affected with noclimate change. There is a 10% probability that the impact is greater than that shownby the blue dots, and the red dots show the maximum calculated impact. The averageannual proportion of cropland affected by drought in the absence of climate change isjust under 2%.

2050: probability of cropland area
affected by drought increasing by >50%

2100: probability of cropland area
affected by drought increasing by >50%
100

Percentage of cropland
RCP8.5
RCP2.6

14
12
10
8
6
4
2
BrazilSouth America

USA

Central America

CanadaC. Europe

W. Europe

Australasia

East Asia

Central Asia

South Asia

South East Asia

N. Africa

Middle East

Sn Africa

East Africa

Cen. Africa

0West Africa

BrazilSouth America

USA

Central America

CanadaC. Europe

E. Europe and Russia

W. Europe

Australasia

East Asia

Central Asia

South Asia

18
16

E. Europe and Russia

RCP8.5
RCP2.6South East Asia

0
N. Africa

BrazilSouth America

USA

Central America

CanadaC. Europe

E. Europe and Russia

W. Europe

Australasia

East Asia

Central Asia

South Asia

South East Asia

N. Africa

Middle East

Sn Africa

East Africa

Cen. Africa

West Africa

020

Middle East

20

40

Sn Africa

40

60

East Africa

60

20
80

Cen. Africa

80

West Africa

Percentage of probability

100

Percentage of probability

86

Worst case10th percentile

This uncertainty is illustrated in Figure D2, which shows the risk that climate change increases the averageannual area of cropland in a region affected by drought by more than 50% in 2050 and 2100. By 2050, theprobability is greater than 70% - under high emissions – in southern and northern Africa, the Middle East,
Australasia, western Europe and central America. In contrast, there is a relatively low probability that climatechange would increase drought extent in East Asia and Canada. Lower emissions reduce the risk, particularly
by 2100.

What is a plausible worst case for changes in drought due to climate change?
At the global scale, there is a 10% probability that by 2050 the incidence of drought would have increased
by 150%, and the plausible worst case would be an increase of 300% by the latter half of the century. The
proportion of cropland affected by drought under the plausible worst case scenario more than doubles inevery region (Figure D3) and in all except Canada and East Asia increases by a factor of at least three. The
global worst case is not equivalent to the sum of the regional worst cases, because no one plausible climate
scenario produces the biggest impact everywhere.
Figure D3 is based on the assumption that all the climate models used to estimate impacts are equallyplausible and that they span the range of potential regional climate changes. This is not necessarily the case,so the numbers are to be regarded as indicative.

What do we know, what do we not know, and what do we think?
Estimates of how drought risk will change in the future are based on (i) projections of future regional climatechange, (ii) projections of how these translate into changes in drought characteristics, and (iii) projections of
future exposed land and people. There are uncertainties in all of these.
Projections of future regional climate change depend partly on the assumed rate of growth in emissions,and partly on projected changes in regional and seasonal precipitation. Climate model simulations typicallyshow that whilst global precipitation goes up with climate change, there is strong variability across space. In
general, wet regions get wetter, but dry regions get drier. However, the magnitude of the change in a regionis uncertain, as are the boundaries of regions which see increases or decreases in rainfall. Temperature
increases across all land areas with climate change, and this will exaggerate the effect of rainfall deficits byincreasing evaporation.
The way in which changes in climate translate into changes in drought depends on local conditions. Most
agricultural systems are tuned to local climatic conditions, so it is departures from those conditions thatprove challenging. That is why our drought indicator is defined in relation to average local conditions, rather
than being defined in absolute terms. However, different measures of drought are also possible to define, andthese would give different indications of both current exposure and future risk.
Finally, the estimated future impacts on agriculture and society depend on changes in exposure to droughts
and vulnerability to their effects. This will depend not only on population change, economic growth and theextent of croplands, but also on the degree to which drought mitigation measures (such as forecasting andwarning, provision of supplementary water supplies or market interventions) are developed.

Production of this chapter was supported by the AVOID 2 programme (DECC) under contractreference 1104872.

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Global: flooded population
200

What do we want to avoid?

120
100
80
60
40

RCP8.5

0
0
21

90
20

80
20

60

50

20
70

20

20

40
20

20
30

RCP8.5South East Asia: flooded population
30

Millions of people

0.8
0.6
0.4
0.2

25
20
15
10
5

No climate change

0
0
21

90
20

80
20

60

20
70

20

50
20

40
20

20
30

0
0
21

90
20

80
20

20
70

50

60
20

20

40
20

20
30

0

20
20

0

20
10

20
20

0
0
21

90
20

80
20

20
70

50

60
20

40

20

20

20
30

No climate change
RCP2.6

No climate change

RCP2.6

RCP2.6

RCP8.5

RCP8.5

East Asia: flooded population
The global total increases very substantially – byaround five or six times, over the course of the
century for the high emissions pathway. This islargely due to increases in South, southeast andEast Asia. There is a clear difference between
the high and low emissions pathways, but there
is also very high uncertainty in the numbers ofpeople affected by flooding in the future due touncertainty in changes in precipitation.

20

15

10

5

The methods used here are summarised in the annex
No climate change
RCP2.6
RCP8.5

0
0
21

90
20

80
20

20
70

50

60
20

20

40
20

20
30

20
20

20
10

0

i.

20

No climate change

1

How could the impact of flooding change over time?
Population change alone will increase the numbers of people affected by flooding in the future. Climatechange could increase the number further, in some regions. Figure F1 shows the numbers of people affectedby floods greater than the current ‘30-year flood’ globally and for four major world regions, for high (red) andlow (green) emissions pathways, as a function of time.

40

RCP2.6

US: flooded population

Millions of people

For the purposes of this assessment we shall take as our threshold floods of the magnitude of current 1 in
30 year flood events. In 2010 just over 700 million people were living in major floodplains2 and – on average– over 20 million of these were affected by floods with a return period of greater than once every 30 years.i
Almost half of these people live in South Asia. Some of the flood-prone populations are protected by flood
defences so do not actually see their properties flooded, although they are likely to be indirectly affected
through impacts on their communities and infrastructure.3

60

0

20
20

0

• Floods along major rivers with extensive floodplains typically occur following prolonged periods ofheavy rainfall or snowmelt, and flood waters may persist for weeks.

The relative contribution of these three broad scales of flooding to the overall flood threat varies from country
to country. At the global scale, there is little information on the numbers of people exposed to flash flooding,because the hazard is highly localised. Most information at the global scale therefore relates to flooding along
major rivers and floodplains with catchments of several thousand square kilometres.

80

20• Flash floods occur when the volume of water produced by intense heavy rainfall generates significant
overland flow, and are typically localized and small-scale.

• Between these two extremes are floods that are locally generated by rainfall and snowmelt within acatchment area.

Millions of people

160
140

20
10

River floods are generated through intense or prolonged rainfall or through snowmelt. There are three main
scales of river flooding:

100

180

Millions of people

River flooding is the most serious and widespread weather hazard affecting the world. According to theMunich Re natural hazards catalogue, between 1980 and 2014 river floods accounted for 41% of all loss
events, 27% of fatalities and 32% of losses.1 By changing the timing and amount of precipitation, climatechange has the potential to substantially alter flood regimes and therefore future flood losses.

South Asia: flooded population

20
20

Professor Nigel Arnell, Director of the Walker Institute for Climate System Research

Figure F1: The average annual number of people affected by river flooding with andwithout climate change. The plots show two climate pathways (RCP2.6 and RCP8.5).
The solid line represents the median estimate of impact for each pathway, and theshaded areas show the 10% to 90% range. A medium growth population projection isassumed.

20
10

THE RISK OF RIVER FLOODING

89

20
10

14

Millions of people

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How could the likelihood of flooding change over time?

is very large uncertainty in the change in future flood probability. In the best case, some regions could see asmall reduction in probability. In the worst case, flooding on the Ganges, Indus and Huang He could be in theregion of ten times more frequent by the end of the century.

Figure F2: The probability that flood magnitude in a given year exceeds the magnitudeof the current 30-year return period flood in five illustrative catchments, under two
climate pathways.
Yangtse: annual probability ofexceeding current 30-year flood

Huang Ho: annual probability ofexceeding current 30-year flood

Exceedance probability

0.2
0.15
0.1
0.05

0.1
0.05

No climate change
RCP2.6
RCP8.5

0
0
21

90
20

80
20

60

50

20
70

20

20

40
20

20
30

20
20

0

BrazilSouth America

USA

Central America

CanadaC. Europe

E. Europe and Russia

W. Europe

Australasia

East Asia

Central Asia

South Asia

South East Asia

N. Africa

Middle East

Sn Africa

East Africa

Cen. Africa

West Africa

Percentage of probability
BrazilSouth America

USA

Central America

CanadaC. Europe

E. Europe and Russia

W. Europe

0.2
0.15
0.1
0.05

0
0
21

20
90

80
20

20
70

20
60

50
20

40
20

20
30

20
20

RCP2.6

0.15

Australasia

0.25

RCP8.50.2

0RCP8.5

RCP2.60.25

20

RCP2.6

RCP8.50.3

40

RCP8.5

No climate change

Mississippi: annual probability ofexceeding current 30-year flood

60

RCP2.6

20
10

0
0
21

20
90

80
20

20
70

50

20
60

20

40
20

20
30

No climate change

80

0.3

0

20
20

0

East Asia

0

90

80

0
21

20

20

60

20
70

20

50

Indus: annual probability of exceeding
current 30-year flood

0.25

0Central Asia

RCP8.520

South Asia

RCP8.540

South East Asia

RCP2.660

N. Africa

RCP2.680

West Africa

No climate change

20

40
20

20
30

No climate change

0.3

20
10

20
20

20
10

0
0
21

90
20

80
20

60

50

20
70

20

40

20

20

20
30

20
20

20
10

0

Ganges: annual probability of exceeding
current 30-year flood

20
10

0.05

Middle East

0.05

0.1

100

Sn Africa

0.1

100

0.15

East Africa

0.15

2100: probability of number of people
affected by flooding increasing by >50%

0.2

Cen. Africa

0.2

2050: probability of number of people
affected by flooding increasing by >50%

0.25

Percentage of probability

0.25

0

Exceedance probability

Figure F3: The risk that climate change increases by more than 50% the numbers ofpeople affected by the current 30-year flood, relative to the situation with no climatechange, under the two climate pathways. A medium growth population projection isassumed.

0.3

Exceedance probability

Exceedance probability

0.3

Exceedance probability

90

Figure F2 shows change through the 21st
century in the probability of experiencing aflood greater than the baseline ‘30-year flood’.
In the Asian examples, the probability offlooding increases very substantially under thehigh emissions pathway: tripling in the Huang
He and Indus, and multiplying by six in theGanges (becoming a 1 in 5 year event), overthe course of the century, according to thecentral estimate. The increase in probability
is considerably lower under the low emissions
pathway. The figures show, however, that there

Figure F3 shows the risk by region that climate change increases by more than 50% the numbers of people
affected by the current 30-year flood, relative to the situation without climate change. By the 2050s, there
is at least a 50% chance that climate change alone would lead to a 50% increase in flooded people acrosssub-Saharan Africa, and a 30-70% chance that such an increase would be seen in Asia. By 2100 the risks
are greater. Under the low emissions pathway the probabilities are lower in all regions than under the highemissions pathway, particularly in 2100.

What is a plausible worst case for changes in river flooding due to climate change?
It is clear from Figure F1 that there is considerable uncertainty in projected impacts of climate change. By
2050, under the ‘worst case’ climate scenario (the climate model pattern that projects the greatest increasein rainfall in the regions with the greatest flood-prone population), approximately 115 million extra people
would be flooded in each year (relative to the situation with no climate change). Figure F4 shows the ‘worst
case’ by region. In most regions the ‘worst case’ has approximately twice the impact of the 10th percentile
impact. However, the worst cases shown in each region do not occur under the same plausible climate
scenario: the global worst case is not the sum of the regional worst cases.
Figure F4 is based on the assumption that all the climate models used to estimate impacts are equallyplausible and that they span the range of potential regional climate changes. This is not necessarily the case,so the numbers are to be regarded as indicative. Changes in south Asia (and therefore the global total) arestrongly dependent on projected changes in the south Asian monsoon (see the previous chapter on water
stress).

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Figure F4: Plausible ‘worst case’ impacts of climate change in 2050 on exposure toriver flooding. The graph shows the increase in numbers of people affected by flooding
under the RCP8.5 climate pathway and the medium growth population assumption.
There is a 10% probability that the impact is greater than that shown by the blue dots,
and the red dots show the maximum calculated impact. Note that the impacts in south
Asia are separately indicated, as they are far larger than those in other regions.

Finally, the estimated future impacts on human society depend on changes in exposure to floods andvulnerability to their effects. This will depend not only on population and economic growth, but also on
the extent to which physical flood defences are developed, buildings and infrastructure are sited to reduce
exposure, and measures are implemented to help individuals and communities respond to and recover fromfloods and loss.

25

Worst caseMillions of people

20

53 to 101

Production of this chapter was supported by the AVOID 2 programme (DECC) under contractreference 1104872.

10th percentile

15

Endnotes10

1. MunichRe (2015) NatCatSERVICE: Loss events worldwide 1980-2014.
2. As defined in the UNISDR PREVIEW data base.
3. The UNDP estimates that around 50-60 million people are affected by river flooding each year, but thisincludes people affected by smaller magnitude floods. UNDP (2009) Risk and poverty in a changing climate:2009 Global Assessment Report on Disaster Risk Reduction. UNDP: New York.

5

South America

BrazilCentral America

USA

CanadaC. Europe

E. Europe and Russia

W. Europe

Australasia

Central Asia

East Asia

South East Asia

South Asia

Middle East

N. Africa

Sn Africa

East Africa

Cen. Africa

0West Africa

92

What do we know, what do we not know, and what do we think?
Estimates of how flood risk will change in the future are based on (i) projections of future regional climatechange, (ii) projections of how these translate into changes in flood characteristics and (iii) projections of
future exposed population and the implementation of flood defences.
Projections of future regional climate change depend partly on the assumed rate of growth in emissions,and partly on projected changes in regional and seasonal precipitation. From meteorological first principles,
we would expect that – other things being equal – the frequency of high rainfall events would increase in a
warmer world, simply because the hydrological cycle is enhanced and warmer air can hold more water. The
frequency of flash flooding can therefore be expected to increase.
However, changes in atmospheric circulation patterns potentially have a greater impact on the magnitudesof persistent or prolonged heavy rainfall that have the greatest influence on flooding in most river basins,
and these changes are currently uncertain. Wet regions are likely to get wetter, but the precise magnitudeof change is uncertain, and the extent to which climate change alters the relative variability in rainfall fromday to day and year to year is uncertain too. Higher temperatures would also in general mean that lessprecipitation would fall as snow during winter so there would be less snow to melt during the melt season –but this will vary from place to place depending on temperature regime, and may be offset or exaggerated bycirculation changes generating more or less precipitation during winter.
The effects of changes in precipitation on river flood characteristics are typically estimated using ahydrological model, perhaps combined with a hydraulic model to simulate the routing of flood flows along
the river network and through floodplains. Flood frequencies are estimated by fitting a statistical frequency
distribution to time series of flood flows. All of these stages introduce uncertainty in the projected effect of agiven change in precipitation regime.

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CITIES

15

Hallegatte et al (2013) came to the following conclusions concerning the future of coastal flooding in the 136
largest coastal cities (in 2005) over the next 50 years or so:6
• Damage could rise from US$6 billion/year to US$52 billion/year solely due to increase in population,
property and its value.

Robert J. Nicholls,i Tim Reeder,ii Sally Brown,iii and Ivan D. Haigh.iv• With additional climate change and subsidence, global losses could approach US$1 trillion or more per
year if flood defences are not upgraded.

Understanding the consequence of sea-level rise for coastal cities has long lead times and huge political
implications. Civilization has emerged and developed during a period of several thousand years over which
sea level has been unusually stable in geological terms. We have now moved out of this period and thechallenge will be to develop a long-term proactive assessment approach to manage this challenge.
In 2005 there were 136 coastal cities with a population exceeding one million people and a collective
population of 400 million people.1 All these coastal cities are threatened by flooding from the sea to varying
degrees2 and these risks are increasing due to growing exposure (people and assets), rising sea levels due
to climate change, and in some cities, significant coastal subsidence due to human agency (drainage andgroundwater withdrawals from susceptible soils).

What is it that we wish to avoid?
The population of New Orleans peaked at about 625,000, before being extensively flooded by Hurricane
Betsy in 1965.3 After that event the city’s flood defences were upgraded and the city population recovered toabout 500,000. In 2005, 80 percent of the city was flooded by Hurricane Katrina, with 800 deaths and morethan US$40 billion damages.4 Subsequently, improved defences have been provided costing US$15 billion,5but much of the city’s economy has relocated and the current population is only about 300,000. Is thisprocess the progressive abandonment of New Orleans due to repeated flooding and indicative of how coastal
cities might be abandoned due to flooding and sea-level rise?
This example contains many of the things we wish to avoid in respect of sea-level rise and coastal cities:
major flood events with associated deaths and damage, high costs of protection, and the potential for sudden
abandonment or gradual decline.

How do risks grow with sea-level rise?
Flood risks grow with sea-level rise as it raises the likelihood of extreme sea levels. Figure 1 shows theincrease in the frequency of current 100 year events in New York, Shanghai and Kolkata, as sea levels rise.
Taking one example, a 1m rise in relative sea-level rise increases the frequency of current 100 year flood
events by about 40 times in Shanghai, about 200 times in New York, and about 1000 times in Kolkata.

• Even if protection levels are maintained (i.e. flood probability is kept the same thanks to upgradeddefences), annual losses will grow as individual floods become more severe due to flood depths
increasing with relative sea-level rise. To maintain present levels of flood risk (average losses per year),
protection will need to be upgraded to reduce flood probabilities below present values.
• Even with upgraded protection, the magnitude of losses when flood events do occur would increase forthe reasons stated above, making it critical to also prepare for larger disasters than we experience today.
Beyond a 50 year time frame, sea levels will continue to rise and protection will have to be progressively
upgraded into the future with uncertain consequences. This raises the question of whether there are potential
thresholds (as discussed below) which, if passed, could reverse the current and forecast trends of growth forcoastal cities.

What are the potential limits to cities’ adaptation to sea level rise and when might they
be reached?
Climate mitigation can stabilise the rate of sea-level rise, which makes adaptation more feasible. However,even if global temperature is stabilized, sea level will continue to rise for many centuries as the deep ocean
slowly warms and the large ice sheets reach a new equilibrium: this has been termed the commitment tosea-level rise. This suggests that in coastal areas mitigation and adaptation must be considered together as
the committed sea-level rise necessitates an adaptation response.7 This perspective changes the mitigation
discussion towards avoiding high end changes in climate over longer time spans than are typicallyconsidered.
There is not extensive literature or significant empirical information on the limits to adaptation in coastal
cities. These limits are not predictable in a formal sense – while the rise in mean sea level raises the likelihoodof a catastrophic flood, extreme events are what cause damage and trigger a response, be it abandonment,
defence upgrade or something else.
Generalising the discussion, there are several types of potential limits that could be grouped into three broad
categories, as discussed below:• Physical/engineering limits: Adaptation and improving protection involve a number of steps as sea
levels rise and increasing efforts are required to keep coastal areas dry. Each step involves higher costs.
However, given the high value of cities, there is a strong incentive to mobilise large amounts of capital
to fund adaptation. Important issues are rates of sea-level rise and maintenance. Accelerated rates ofsea-level rise will present greater demands and hence challenges, although a rate of rise of a metre per
century should present no major technical challenges. Maintenance requires annual investment of 1 to
2 percent of the capital value to maintain the defences. It is possible that there are other limits related
to other issues such as ecology or related effects on local water supplies that will be reached at lower
levels of sea-level rise.

Figure 1: The increase of frequency of present 100-year events (in the base year) as
relative sea levels rise in three major coastal cities.
1000

Number of events
per 100 years

94

100

10

1
0

0.2

0.4

0.6

Relative sea-level rise
New York
Shanghai
Kolkata

0.8

1

i.

Robert J. Nicholls is Professor of Coastal Engineering at the University of Southampton.

ii.

Tim Reeder is a Senior Climate Change Adviser for the Environment Agency.

iii. Sally Brown is a Research Fellow at the University of Southampton.
iv.

Ivan D. Haigh is a Lecturer in Coastal Oceanography at the University of Southampton.

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CLIMATE CHANGE: A RISK ASSESSMENT

CLIMATE CHANGE: A RISK ASSESSMENTRISK ASSESSMENT PART 2: DIRECT RISKSRISK ASSESSMENT PART 2: DIRECT RISKS• Economic/financial limits: At smaller levels of sea-level rise there will be a wide range of choices
between retreat and continued protection. As sea levels rise higher, these adaptation choices will
become starker, and the high costs of protection may lead to managed or unmanaged retreat if that
becomes a less costly alternative. However, while relocation of major cities might be considered, theassociated high costs can also provide a strong incentive to upgrade protection. In cost-benefit terms,
protection is favoured,8 but this of course depends on funds being available: poorer cities will be mostchallenged, with uncertain consequences. Further, a loss of confidence in a city due to a major flood
could undermine its economic base, and hence the resources available for improving protection. If thisoccurs, it is probably a multi-stage process of decline over several events. However, once decline starts,
it may be a self-reinforcing process.
• Socio-political limits: Thresholds and limits will vary dependent on the prevailing base conditions,confidence and attitudes to risk. Good governance will deliver effective and well-maintained protection
systems. Cities with a strong background in flood protection will be better prepared and more likely tohave a planned long-term approach to the radical actions that will be needed as sea level continues torise. Those that have not prepared and have an unrealistic expectation of continuation of the present
paradigm may well reach limits earlier. In many ways, this is the hardest limit to characterise andquantify, and yet it might be the most influential limit for coastal cities in practice.
If protection limits are reached, a few cities may have to be largely abandoned as most of their land area is in
the flood plain. However, most coastal cities have large areas outside the coastal flood plain. Hence, in thesecases, retreat would be about reconfiguring the city to the new land-water interface.9
There are very few studies which quantify the sea level rise threshold at which cities will have to beabandoned, and in particular there has been little analysis of the art of the possible or feasible for adaptation
beyond about 4 metres for any city. Exceptions include the Thames Estuary where a key threshold foradaptation of 5 metres of mean sea-level rise was identified related to limits of sea wall raising and tidal
barrier construction in a closed system scenario. For sea levels higher than this, the entire discharge of theriver Thames would need to be pumped to the sea.10 In the Netherlands, the Delta Programme considered asea level rise of 4 metres by 2200.11 It was concluded that continuation of dyke-raising and beach and dune
nourishment with sand could still be effective up to this level.

Conclusion
The lack of knowledge on sea level rise thresholds for coastal cities is of real concern. London’s Thames
Estuary 2100 (TE2100) plan12 (illustrated in Figure 2), the Dutch Delta Programme13 and New York14
have defined adaptive pathways into the future. These define a portfolio of adaptation measures which canbe progressively and flexibility applied to manage flood risk as sea level rises: more slowly or rapidly as
circumstances demand. This is a best practice proactive approach for planning to address the issue. Suchanalysis illuminates where thresholds will be met in each city.
This approach could be applied widely to the coastal cities around the world to identify the adaptation
choices.15 This would include recognizing where protection might not be viable and a retreat approach may
be needed. By taking a proactive approach to adaptation assessment the full scale of the long term challenge
can be illuminated and the long term adaptive actions planned.

Figure 2: TE2100 Project Plan High-level options and pathways developed by TE2100(on the y-axis) shown relative to threshold levels increase in extreme water level (onthe x-axis). For example, the blue line illustrates a possible ‘route’ where a decision
maker would initially follow HLO2 then switch to HLO4 if sea level was found to increase
faster than predicted. The sea level rise shown incorporates all components of sea levelrise, not just mean sea level.16

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Endnotes
Hanson, S, Nicholls, R. J., Patmore, N, Hallegatte, S, Corfee-Morlot, J, Herweijer, C and Chateau, J (2011)
A global ranking of port cities with high exposure to climate extremes. Climatic Change, 140, (1), 89-111.
(doi:10.1007/s10584-010-9977-4)
2. Hanson et al., 2011; Hallegatte, Stephane, Green, Colin, Nicholls, Robert J. and Corfee-Morlot, Jan (2013) Future
flood losses in major coastal cities. Nature Climate Change, 3, 802-806. (doi:10.1038/nclimate1979).
3. Grossi, P. and Muir-Wood, R. (2006) Flood Risk in New Orleans: Implications for Future Management andInsurability, Risk Management Solutions, Newark, USA, 31pp.
4. Kates, R.W., C. E. Colten, S. Laska, and S. P. Leatherman (2006) Reconstruction of New Orleans after Hurricane
Katrina: A research perspective. PNAS, 103 (40) 14653-14660; doi:10.1073/pnas.0605726103
5. Link, L.E. (2010) The anatomy of a disaster, an overview of Hurricane Katrina and New Orleans. Ocean
Engineering 37, 4–12.
6. Hallegatte et al (2013)
7. Nicholls, R.J., P.P. Wong, V.R. Burkett, J.O. Codignotto, J.E. et al. (2007). “Coastal systems and low-lying areas.”
In: Parry, M.L. et al. (eds.), Climate Change 2007: Impacts, Adaptation and Vulnerability. Fourth Assessment
Report, Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, pp. 315356.; Wong, P.P., Losada, I.P., Gattuso, J.-P., Hinkel, J. et al. (2014). “Coastal systems and low-lying areas.” In:
Field, C.B. et al (eds.) Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral
Aspects. Fifth Assessment Report, Intergovernmental Panel on Climate Change Cambridge University Press,
Cambridge, UK, pp. 361-409.
8. e.g. Nicholls, Robert J, Tol, Richard S J and Vafeidis, Athanasios T (2008) Global estimates of the impactof a collapse of the West Antarctic ice sheet: an application of FUND. Climatic Change, 91, (1-2), 171-191.
(doi:10.1007/s10584-008-9424-y).
9. Lonsdale, K.G, Downing, T.E, Nicholls, R. J, Parker, D, Vafeidis, A.T, Dawson, R and Hall, J (2008) Plausible
responses to the threat of rapid sea-level rise in the Thames Estuary. Climatic Change, 91, (1-2), 145-169.
(doi:10.1007/s10584-008-9483-0).
10. Reeder T, Wicks J, Lovell L, Tarrant T. (2009) Protecting London from tidal flooding: limits to engineering
adaptation. In: Adger, N.W., Lorenzoni, I and O’Brien, K. (eds.) Adapting to climate change: thresholds, values,
governance. Cambridge University Press, Cambridge, pp 54-63.
11. Deltacommissie (2008). Working together with Water. A living land builds for its future. Findings of the
Deltacommissie. The Netherlands: Delta Commissie (Delta Commission).
12. Environment Agency (2012a) Thames Estuary 2100: managing risks through London and the Thames Estuary.
TE2100 Plan; 2012. https://brand.environment-agency.gov.uk/mb/CtyxlR
13. Delta Commission, (2008) and the implemented programme described at http://www.government.nl/issues/delta-programme
14. New York City Panel on Climate Change (2010) Climate change adaptation in New York city: building a risk
management response. Annals of the New York Academy of Sciences, 1196: 1–354.
15. Reeder, T. and Ranger N. (2011) “How do you adapt in an uncertain world? Lessons from the Thames Estuary
2100 project.” World Resources Report, Washington DC 2011. Available online at http://www.wri.org/sites/
default/files/uploads/wrr_reeder_and_ranger_uncertainty.pdf
16. Reeder, T. and Ranger N. (2011)16

GLOBAL SEA-LEVEL RISEI

1.

Professor Daniel Schrag, Director, Harvard University Center for the Environment.

What we wish to avoid
As the previous chapter discussed, it is difficult to define thresholds in the height or rate of sea level rise thatwill cause significant problems for coastal cities. But it is clear that the faster sea level rises, the more difficultand expensive adapting to it will become. It may already be impossible to avoid long-term sea level rise of
more than 10 metres from melting of polar ice sheets, but it may be possible to limit the rate of melting. So
what we wish to avoid is any change that causes a significant acceleration in the rate of sea level rise.

What we know
Over 90% of the energy imbalance caused by increased greenhouse gas concentrations in the atmosphereis absorbed by the oceans (compared to roughly 3% that goes into warming the Earth’s surface). This extra
energy is raising the heat content of the ocean, and correspondingly the volume, due to thermal expansion.
In addition, ice is melting from the polar ice sheets and also from mountain glaciers, raising the volume ofthe oceans. Finally, mostly independent of climate change, groundwater is being pumped from aquifers onland, and then released to streams and rivers, also contributing to global sea level rise. Since pre-industrial
times, the global average sea level has risen by about 20cm.1 Based on precise measurements from satellite
altimetry, we know that the rate of rise has averaged 3.4mm/year since the 1990s, and we know that this rateis accelerating.
Thermal expansion is the main contributor to sea level rise at present, but the main contributor in the future
will be melting of polar ice sheets. These contain water roughly equivalent to a globally averaged sea level
rise of:• Greenland ice-sheet: 7m• West Antarctic ice-sheet: 6m• East Antarctic ice-sheet: more than 50m

What we do not know
We do not know how quickly the melting of these ice-sheets could occur. This is deeply uncertain: neither
observations nor models provide enough information. The timescale for the Greenland and West Antarctic
ice-sheets could be hundreds of years, or thousands of years. The timescale for parts of the East Antarctic icesheet is likely to be longer because it is so cold and also less vulnerable to rapid collapse due to topography
and glacial structure.

What we think
Paleoclimate data suggests that the Greenland ice-sheet probably cannot survive in a world where
atmospheric carbon concentrations are above 400ppm (their current level), and almost certainly not in a
550ppm world – equivalent to a level in the lowest emissions scenarios that we have earlier in this reportcharacterized as very unlikely. The same is probably true for the West Antarctic ice-sheet, and for small
parts of the East Antarctic ice-sheet. This implies that we may already be committed to some 10-15m of sea
level rise in the long-term future. Figure 1, taken from the IPCC, shows a central estimate of around 12m
long-term committed sea level rise, if temperature rise is held steady at 2°C. But the rate is wildly uncertain:
whether this will take 500 years or 10,000 years is really unknown.
i.

This contribution is intended as a brief summary of the author’s opinion on the state of knowledge of risks of sea level rise, and not
a comprehensive review of the existing literature. For additional information and references, readers should consult Chapter 13 ofWorking Group 1 from the IPCC AR5.

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Figure 1: committed sea level rise as a function of long-term global temperature
increase.2 Committed sea level is the amount that will occur regardless of global efforts
to stabilize global greenhouse gas emissions.

For this century, sea level rise of at least 40cm looks likely, as anything less would require a slowing in thecontribution of ice-sheets, which would be the opposite of what is being observed. More than 1m appears
unlikely, but there is much more uncertainty over the upper end of the range. Figure 2 shows the extent towhich expert opinion varies.

Full IPCC caption: (Left column)
Multi-millennial sea level commitment
per degree Celsius of warming
as obtained from physical model
simulations of (a) ocean warming,(b) mountain glaciers and (c) theGreenland and (d) the Antarctic ice
sheets. (e) The corresponding total
sea level commitment, comparedto paleo estimates from past warm
periods (PI = pre-industrial, LIG =last interglacial period, M11 = Marine
Isotope Stage 11, Plio = Mid-Pliocene).
Temperatures are relative to preindustrial. Dashed lines provide linear
approximations in (d) and (e) withconstant slopes of 1.2, 1.8 and 2.3 m°C–1. Shading as well as the verticalline represents the uncertainty range
as detailed in the text. (Right column)
2000-year-sea level commitment.
The difference in total sea levelcommitment (j) compared to thefully equilibrated situation (e) arises
from the Greenland ice sheet which
equilibrates on tens of thousands ofyears. After 2000 years one finds
a nonlinear dependence on thetemperature increase (h) consistent
with coupled climate–ice sheet
simulations by Huybrechts et al.
(2011) (black dot). The total sea levelcommitment after 2000 years is
quasi-linear with a slope of 2.3 m °C–1.

Figure 2: Expert opinion on likely extent of sea level rise in 2100 and 2300, for low andhigh emissions pathways (from Horton et al4).5

A plausible worst-case scenario
The range of uncertainty is much smaller in the relatively short timescale of this century, but is still significant
from a human perspective. IPCC projections for sea level rise have tended to increase over time. The Third
Assessment Report estimated a range of 0.2 – 0.6m over the century; the Fifth Assessment Report estimates
1m as the upper end of the ‘likely range’, plus a few additional tenths of a metre in the event of a collapse ofparts of the West Antarctic ice-sheet. Some more recent scientific findings have suggested that such a collapse
has already begun.3
At the time of the IPCC’s Third Assessment Report, it was predicted that the ice-sheets on Antarctica would
grow, contributing a net decrease to the change in sea level. Satellite measurements of ice-sheet mass now
show the opposite to be occurring. Antarctica as a whole is losing mass, although there are parts of the
Antarctic ice sheet that are gaining mass from increased precipitation. The data for Greenland show anacceleration in mass loss, but as the observations only exist for slightly more than a decade, it is impossible
to say whether this acceleration is part of a multi-decadal oscillation, or the beginning of a long-term trend.

A plausible worst-case scenario would be a significant acceleration of sea level rise, as well as a commitment
to even more rapid sea level rise of several times the current rate at the end of this century and throughout
the next century. This could only occur if one of two things happens:
i. A rapid acceleration of summer surface-melting on Greenland. In the few locations on Greenland
where there are outlet glaciers that bring ice directly into contact with the ocean, Greenland’s glaciers
are already moving at close to the maximum speed physically possible, which is constrained by friction.
Some acceleration in the rate of mass loss is possible from an increase in the number of such locations,particularly around northern Greenland, but more substantial acceleration is likely to come from enhanced
surface melting in summer. Satellite images show that the area of melting of the surface of the Greenland ice
sheet has been expanding rapidly. The formation of melt-pools acts as a positive feedback. These processes
are not yet well understood: deposition of black carbon may be important, as well as temperature rise. In
addition, it is not known what effect the continuing retreat of summer sea-ice will have.

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ii. Collapse of the Ross ice shelf and other large ice shelves from Antarctica. Temperatures on
Antarctica are low enough that surface melting is not the major factor in creating ice loss, but rather ice
discharge to the ocean. Currently, the discharge of glaciers on West and East Antarctica into the ocean
are being slowed down by large ice-shelves – floating land ice that has been pushed out into the ocean –including the Ross ice shelf covering the Ross Sea. The collapse of the Ross ice shelf or other critical ice
shelves would allow the rate of ice discharge to accelerate, adding several metres to sea level rise over a
century or two. We do not understand exactly how this would occur, but some work has shown that ice
shelves disintegrate when slightly warmer (0 - 2°C) water flows over the continental shelf beneath theice shelf and melts it from below. Current observations are so limited that we do not know whether theseprocesses are already taking place.

17

LARGE-SCALE ABRUPT OR IRREVERSIBLE 			CHANGES

Professor Jason Lowe, Head of Mitigation Advice, UK Met Office Hadley Centre.

What we wish to avoid
It is also important to emphasize that the two possibilities described above have different implications for the
possibility of stabilizing or reversing sea level rise. The melting on Greenland could be stopped if warming
was reduced, for example through proposals to cool the Earth through solar radiation management (or‘geoengineering’). But if a major ice shelf disintegrates, it will be much more difficult to stop or slow theglacial discharge, as the flow depends on gravitational instability of the ice sheet, and is very insensitive tosurface temperature, at least over fairly long timescales. Thus, the consequent sea level rise would likely be
unstoppable and irreversible.

Endnotes
1. IPCC (2013): ‘Summary for Policymakers’. Climate Change 2013: The Physical Science Basis. Contribution ofWorking Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA
2. Fig. 13.14 from IPCC (2013). Climate Change 2013: The Physical Science Basis. Contribution of Working GroupI to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin,
G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge
University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp, doi:10.1017/CBO9781107415324.
3. http://www.sciencemag.org/content/348/6237/899 / http://www.the-cryosphere.net/9/1005/2015/tc-9-10052015.html
4. Horton et al (2014). ‘Expert assessment of sea-level rise by AD2100 and AD2300’. Quaternary Science
Reviewshttp://www.sciencedirect.com/science/article/pii/S0277379113004381
5. Box plots of survey results from all experts who provided at least partial responses to questions. The numberof respondents for each of the four questions is shown in the top left corner; it is lower than the total of
90 participants since not all answered each question. Participants were asked to estimate likely (17 the 83rd
percentiles) and very likely (5th - 95th percentiles) sea-level rise under two temperature scenarios and at
two time points (AD 2100 and AD 2300), resulting in four sets of responses. Shaded boxes represent therange between the first and third quartiles of responses. Dashed horizontal line within the box is the median
response. Whiskers (solid lines) represent two standard deviations of the responses. Filled circles show
individual responses that are beyond two standard deviations of the median.

Many elements of the climate system are capable not only of steady, gradual change over long time-periods,
but also of non-linear change when critical thresholds are passed. Some of these may result in relatively
abrupt change and some may be irreversible. Both types may have large-scale consequences for the climate,both directly and indirectly, often with disproportionate impacts in some regions of the world.
As discussed in the previous chapter, an acceleration in the melt rate of the Greenland or Antarctic ice-sheets
could lead to a significant acceleration in global sea level rise; on time scales of several centuries or more thiswould very likely be irreversible. Similarly, an acceleration of temperature rise could result from large-scale
thawing of permafrost and the release of extra carbon dioxide or methane into the atmosphere, or from therelease of sub-sea methane hydrates (as discussed in the chapter on global temperature increase).
Changes in atmospheric circulation patterns are very difficult to predict, and could potentially be abrupt. As
noted in the chapter on water stress, large-scale changes in monsoons cannot be ruled out. Changes to the El
Niño phenomenon, linked to extreme weather in many parts of the world, are also possible. Ocean circulation
patterns could also change: and in particular, the Atlantic meridional overturning circulation could weaken or
collapse. This would affect temperature, rainfall, and some extreme weather over large parts of the northernhemisphere. On a regional scale, changes in rainfall and temperature in combination with other factors could
cause large-scale die-back of tropical forests, such as the Amazon, which in turn would weaken the natural
sink for atmospheric carbon dioxide and produce a further amplification of warming.1

When could these changes occur?
There is great uncertainty over when, or at what degree of global temperature rise, thresholds associated withthese large-scale changes might be passed. However, the evidence suggests that the probability of crossing
such thresholds increases with global temperature rise, and if temperature rise continues, there is potential
for crossing several of them during the 21st century. Improving our understanding of these trigger points is apriority for climate research.
The IPCC’s Fifth Assessment Report reviewed recent evidence of both the likelihood and the consequencesof experiencing these large-scale changes in the current century.2 These findings are summarised in the tableshown below, which is adapted from the recent IPCC report (‘table 12.4’).3 More recent published research
has reinforced these findings.4
From a risk assessment perspective, it is important to understand what this evidence-based expert judgement
is saying with respect to magnitude of impact, probability, and time.
Magnitude of impact: For some of these changes, the magnitude of impact being considered is a very
high one. For example, for a methane hydrate (clathrate) release, the magnitude being considered is‘catastrophic’. For ice-sheet collapse, what is being considered is ‘near-complete disintegration’, which would
result in sea level rise of many metres – considered exceptionally unlikely this century. In some cases, a lower
impact threshold may also be significant: for example, the likelihood of partial ice-sheet collapse causing an
acceleration of sea level rise in the short-term, and commitment to higher sea level rise in the long-term, may
be much higher. The IPCC’s Summary for Policymakers estimates that ‘Risks [of large-scale singular events]
increase disproportionately as temperature increases between 1–2°C additional warming and become highabove 3°C, due to the potential for a large and irreversible sea level rise from ice sheet loss.’5

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Table: Components in the Earth system that have been proposed in the literature
as potentially being susceptible to abrupt or irreversible change. Column 2 defines
whether or not a potential change can be considered to be abrupt under the AR5
definition. Column 3 states whether or not the process is irreversible in the context
of abrupt change, and also gives the typical recovery time scales. Column 4 provides
an assessment, if possible, of the likelihood of occurrence of abrupt change in the 21st
century for the respective components or phenomena within the Earth system, for the
scenarios considered in this chapter.6

Time: The probability estimates noted above are given only with respect to the current century. As noted
above, these probabilities are expected to increase with global temperature rise, and as the chapter on that
subject showed, under medium and high emissions scenarios global temperature rise is projected to continue
beyond the end of the century. The consequences of these changes are also expected to continue beyond thecentury time-scale.

Change in climate
system component

Potentiallyabrupt(AR5definition

Irreversibility if
forcing reversed

Projected likelihood of 21st century
change in scenarios considered

Systemic interactions

Atlantic MOC collapse

Yes

Unknown

Very unlikely that the AMOC will undergo arapid transition (high confidence)Ice sheet collapse

No

Irreversible formillennia

Exceptionally unlikely that either Greenlandor West Antarctic Ice sheets will suffer nearcomplete disintegration (high confidence)

Permafrost carbon

No

Irreversible formillennia

Possible that permafrost will become a net
source of atmospheric greenhouse gases(low confidence)

Clathrate methane
releaseYes

Irreversible formillennia

Very unlikely that methane from clathrates
will undergo catastrophic release (highconfidence)

Tropical forests
dieback

Yes

Reversible withincenturies

Low confidence in projections of thecollapse of large areas of tropical forest

Boreal forests dieback

Yes

Reversible withincenturies

Low confidence in projections of thecollapse of large areas of boreal forest

Disappearance ofsummer Arctic sea ice

Yes

Reversible within
years to decades

Likely that the Arctic Ocean becomes
nearly ice-free in September before midcentury under high forcing scenarios suchas RCP8.5 (medium confidence)Long-term droughts

Yes

Reversible within
years to decades

Low confidence in projections ofchanges in the frequency and duration of
megadroughts

Monsoonal circulation

Yes

Reversible within
years to decades

Low confidence in projections of a collapse
in monsoon circulations

Probability: The estimates of likelihood given above may be read with reference to the IPCC’s standard
approach to expressing estimated ranges of likelihood in qualitative terms. The ranges corresponding to
the terms used here are: ‘Exceptionally unlikely’: 0-1%; ‘Very unlikely’: 0-10%; ‘Likely’: 66-100%.7 Although
in the IPCC report many of these changes are reported as being “very unlikely” in the IPCC’s calibrated
terminology, the probability estimates are what many risk adverse stakeholders might still consider
unacceptably high when viewed in parallel with the magnitude. It is notable that for several of the large-scalechanges, the uncertainty around their likelihood is so great that no estimate is given.

All three of these factors – magnitude of impact, probability, and time – have to be taken into account inreaching a view on the scale of the risks presented by these possible large-scale changes.

An additional aspect to consider is the potential for interaction between these large-scale climate systemchanges, with one change leading to a cascade of other major events. For instance, we can speculate thatenhanced melting from the Greenland ice-sheet could not only raise sea level but also slow the Atlantic
over-turning circulation. One consequence of changes in Atlantic circulation and sea surface temperatures
is expected to be a shift in atmospheric circulation, which could have negative impacts on the health of the
Amazon forest and its ability to take up atmospheric carbon. At the same time, the rise in sea level fromGreenland might also affect the stability of the ice shelves and ice sheets in the southern Hemisphere, leading
to further sea level rise.
Figure 2 captures some of these effects (this is based on an earlier figure by Kriegler et al.)8 It effectively
highlights the fact that the concomitant effects of climate change can cause widespread impacts in a numberof areas, presenting significant systemic risks. This is an area explored in greater detail in the next section ofthis report. It is clear that if we are to have a full understanding of climate change risks, we must assess thelikelihood of large-scale climate changes, the interactions between them, and the critical thresholds at which
they could be triggered. The recent emergence of complex earth system models is finally providing a tool thatwill allow climate scientists to start to explore these interactions.

Figure 2: Possible interaction of large-scale climate disruption.9

Production of this chapter was supported by the AVOID 2 programme (DECC) under contractreference 1104872.

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Endnotes
1. Kriegler, E., Hall, J.W., Held, H., Dawson, R., and Schellnhuber, H.J. (2009). ‘Imprecise probability assessment oftipping points in the climate system’. PNAS 106, 5041-5046. doi: 10.1073/pnas.0809117106.
2. Collins, M., R. Knutti, J. Arblaster, J.-L. Dufresne, T. Fichefet, P. Friedlingstein, X. Gao, W.J. Gutowski, T. Johns, G.
Krinner, M. Shongwe, C. Tebaldi, A.J. Weaver and M. Wehner (2013). ‘Long-term Climate Change: Projections,
Commitments and Irreversibility’. Climate Change 2013: The Physical Science Basis. Contribution of Working
Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin,
G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge
University Press, Cambridge, United Kingdom and New York, NY, USA.
3. Table 12.4, p. 1115 in IPCC, (2013). Climate Change 2013: The Physical Science Basis. Contribution of Working
Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin,
G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge
University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp, doi:10.1017/CBO9781107415324.
IPCC AR5 WG1 Table 12.4
4. Good, P. et al. (2014). An updated view of tipping points and the relevance for long-term climate goals.
Available at http://avoid-net-uk.cc.ic.ac.uk/wp-content/uploads/delightful-downloads/2015/02/AVOID2_
WPA5_v2_final.pdf.
5. IPCC (2014). ‘Summary for policymakers’. Climate Change 2014: Impacts, Adaptation, and Vulnerability.Part
A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, p. 12.
6. Table from Collins, M., R. Knutti, J. Arblaster, J.-L. Dufresne, T. Fichefet, P. Friedlingstein, X. Gao, W.J. Gutowski,
T. Johns, G. Krinner, M. Shongwe, C. Tebaldi, A.J. Weaver and M. Wehner (2013). ‘Long-term Climate Change:
Projections, Commitments and Irreversibility.’ Climate Change 2013: The Physical Science Basis. Contribution ofWorking Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
7. IPCC (2013). ‘Summary for Policymakers’. Climate Change 2013: The Physical Science Basis. Contribution ofWorking Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. p.4
8. Kriegler et al. (2009).
9. Adapted from Kriegler et al. (2009).

18

CONCLUDING COMMENTS

In this chapter, we have considered various forms of threshold in the severity of climate change and its
impacts. These include:• Thresholds in the physical climate system itself: for example, the degree of warming at which an icesheet may be committed to collapse;• Biophysical thresholds, such as the degree of heat and humidity that is potentially fatal to humans, orthe temperature that exceeds the tolerance of a crop;• Socioeconomic thresholds, such as the quantity of per capita water resources required to meet basic
human needs, or the point at which it becomes less costly to retreat from coastal areas than to protect
them against flooding;• Thresholds defined by political decisions, such as the 2°C temperature target.
It is clear that in many cases, when a threshold is passed, there is a non-linear increase in the severity of
impact. For example: a significant acceleration in the rate of sea level rise; a very severe decrease in crop
yield instead of a moderate decrease; the death of a person from heat stress instead of the experience of nonfatal heat stress. Avoiding these impacts is likely to be a high priority for a decision-maker, so assessing theirlikelihood should be a high priority for a risk assessment.
Our assessment looked, where possible, at how the probability of crossing these thresholds might increaseover time, or as a function of global temperature rise. In several cases, it appeared that there could be a nonlinear increase in probability, once a certain point of time or temperature was passed.
The idea of there being thresholds of impact, or limits to adaptation that might be important, is not new. The
chapter on ‘Adaptation opportunities, constraints and limits’1 in the IPCC’s Fifth Assessment Report provides
an authoritative overview of this subject. However, the approach we have taken – presenting the probabilityof crossing these thresholds as a function of time (or of global temperature increase) – does not seem to be
the generally preferred method. The IPCC Fifth Assessment Report’s Working Group II report included only
one graph in this format (showing the risk of mass coral mortality),2 compared to some twenty graphs thatshowed severity of impact as a function of time.3
As we discussed at the beginning of this section, an approach that first defines what it is we wish to avoid,and then assesses its likelihood as a function of time, seems consistent with the principles of risk assessment.
If results presented it this way make it plain that under a certain course of action (e.g. a high emissions
pathway), that likelihood could become very high, then this will be useful information for a decision-maker.
There seems to be considerable scope for developing this approach further. For example, more locally
relevant socioeconomic thresholds could be defined, and better ways to estimate probabilities could be
developed.
A limitation of this approach may be that it will tend to focus on individual impacts of climate change,rather than the effect of many impacts in combination. In the real world, different impacts of climate change
will frequently overlap and interact. The difficulty of anticipating such interactions – especially if humanactions are also taken into account – may mean that the risks arising from them are easily overlooked. The
IPCC found that “Interactions of climate change impacts on one sector with changes in exposure andvulnerability, or with adaptation and mitigation actions affecting the same or a different sector, aregenerally not included or well integrated into projections of risk. However, their consideration leads tothe identification of a variety of emergent risks that were not previously assessed or recognized.”4This most difficult area of risk assessment – the interactions between the impacts of climate change andcomplex human systems – is the subject of the next part of our report.

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Endnotes

RISK ASSESSMENT PART 3:

1. Klein, R.J.T., G.F. Midgley, B.L. Preston, M. Alam, F.G.H. Berkhout, K. Dow, and M.R. Shaw (2014). ‘Adaptation
opportunities, constraints, and limits. Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part
A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, pp. 899-943.
2. Fig 30-10 in Hoegh-Guldberg, O., R. Cai, E.S. Poloczanska, P.G. Brewer, S. Sundby, K. Hilmi, V.J. Fabry, and
S. Jung (2014). ‘The Ocean’. Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional
Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L.
Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L.White(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1655-1731.
3. See graphs at: https://ipcc-wg2.gov/AR5/report/full-report/
4. Oppenheimer, M., M. Campos, R. Warren, J. Birkmann, G. Luber, B. O’Neill, and K. Takahashi (2014). ‘Emergent
risks and key vulnerabilities’. Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and
Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M.
Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea,
and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, p.
1053.

SYSTEMIC RISKS

Martin Bureau/AFP/Getty Images

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SYSTEMIC RISKS19

INTRODUCTION

Our human civilization and the natural environment on which it depends are both examples of complex,
dynamic systems. The defining characteristic of such systems is that they are made up of large numbers ofinteracting, interdependent components. These interactions are often non-linear: their causes and effects are
not proportional to each other. Small changes can sometimes have very large effects. For this reason, thebehaviour of these systems is very difficult to predict.1
A high degree of uncertainty, if it affects our objectives or things we value, can correspond to a high degreeof risk. Systemic risks may be simply defined as ‘risks that can trigger unexpected large-scale changes of asystem, or imply uncontrollable large-scale threats to it’.2
In this part of our risk assessment, we are concerned with the risks that could arise from the interaction ofclimate change with human social and economic systems. As we noted at the beginning of this report, humancivilization developed during a period of several millennia of unusual climatic stability. We are now, throughour emissions of greenhouses gases, applying a forcing to the climate system – disturbing its stability. As
described in previous chapters, this could lead to large changes in the natural world, far outside the range ofhuman experience. Intuitively, we might guess that if small changes can sometimes produce very large effects
in complex systems, then very large changes could be quite likely to produce very large effects.
The systemic risks of climate change may be very large, but they are much less straightforward to assessthan the direct risks considered in earlier chapters. In this section, we begin with an example of the systemicrisks experienced by a city in relation to a current climate event. We then consider the observed changes andfuture risks to systems in a region – the Arctic – and the way in which these changes could affect the restof the world. Finally, we explore in depth two categories of risk that could be significant at the international
scale: climate change risks to global food security, and climate change risks to national and international
security.

An example of systemic risks at the city level: drought in São PauloDr Jose Marengo is Research Director at the National Center for Monitoring and Early Warningof Natural Disasters in São Paulo, Brazil. Since October 2014 he has served on a crisis task force
set up to advise the Brazilian government on its handling of the drought in the southeast of thecountry. Here he describes some of its effects.
The São Paulo Metropolitan Area is the largest metropolis in South America, and accounts for a thirdof Brazil’s national GDP. For the last year, it has been affected by an exceptional drought. Rainfall over
the reservoir system surrounding the city fell in 2014 to the lowest level recorded since 1940, and thiscoincided with a period of high temperatures. The extent to which climate change affected the likelihoodof such low rainfall is difficult to assess, but it may well have played a role in increasing the likelihood of
the high temperatures.3,4 (As chapter 13 discussed, in future, climate change could increase the risks ofdrought in many parts of the world.)
While the low rainfall decreased water availability, high temperatures led to increased water use – inagriculture, industry, and people’s homes. The city’s rapid development over recent decades had increased
its vulnerability: population growth increased water demand; forests and wetlands that had historicallysoaked up rainwater and released it slowly into reservoirs were destroyed; and poor-quality infrastructure
continued to leak some 30-40% of its water.

The most direct impact, obviously, has been a lack of water. The state-run utility company has reduced its
extraction from reservoirs by a third, cut its pump pressure at night, and offered discounts to customers
who reduce their consumption. Parts of the city now rely on water-trucks for their supply.
Lack of water has hampered the functioning of schools, hospitals and businesses. Agriculture has suffered:
the price of products such as tomatoes and lettuce increased by around 30% at the height of the drought;
other affected crops included sugar cane, oranges and beans.5 Meanwhile, the number of forest fires in theregion increased by 150% from 2013 to 2014.6 Electricity generation has also been affected. More than
70% of Brazil’s electricity supply comes from hydroelectric power, and around 70% of this is generated in
the São Paulo region. By the end of 2014, the reservoirs supplying this power generation were almost dry.
As a result, energy tariffs were predicted to increase by 20-25% in 2015.7
The effects of the water shortage, including water rationing, increased water bills, and otherinconveniences have even led to protests and social unrest in some parts of the city. The overall loss to theeconomy from the drought so far has been estimated at US$5 billion, making it the fifth most expensive
natural disaster in the world in 2014.8

An example of systemic risks at the regional level: climate change in the Arctic
Dr Tero Mustonen works for the Snowchange Cooperative and the University of Eastern Finland.
His work gives him contact with scientists and indigenous peoples from around the Arctic who
are observing and documenting the changing climate and its effects. Here he describes a few ofthese observed changes, and some future risks.
The Artic is warming at twice the rate of anywhere else on Earth,9 so the changes we observe there atpresent give us some idea of the scale of the changes we could witness elsewhere in future. The changes
already seen in the Arctic are profound. Rising average temperatures have led to heat extremes that werepreviously unheard of: a peak temperature of 37.2°C has been recorded in boreal Finland.10 Melting of theGreenland ice-sheet has accelerated, while the extent of summer sea-ice has reduced by some 40%, at a
rate faster than most scientific models predicted.11 In many areas the permafrost – previously permanently
frozen ground – is beginning to thaw. The whole region is under-going a system-shift, with potentiallymajor consequences for ecosystems and human societies.12
The indigenous peoples of the Arctic are directly affected by these changes in ecosystems. Unpredictable
weather patterns have already disrupted the traditional calendar of the Kola Sámi in northwestern
Russia.13 In the Eurasian North, the Skolt Sámi people, anticipating the severe impacts of climate change
on reindeer herding habitats, are beginning to adapt their culture to rely instead upon in-land fishing.14
The Inuit’s hunting and food-sharing culture is under threat, as reduced sea ice results in the decline of
populations of the animals they hunt. In the future, the Arctic Council assessed that reductions in species’
ranges and availability, less predictable weather, and threats to safe travel caused by the changing ice
conditions would ‘present serious challenges to human health and food security, and possibly even thesurvival of some cultures.’15
Industrialised societies of the region face disruption too. While the retreat of the sea ice may enable theexpansion of shipping and offshore oil extraction, the thawing of the permafrost threatens to destabilizebuildings, roads, pipelines, and airports.16 Communities and industrial facilities may need to be relocated.
Transportation on ice roads and across tundra, as well as oil and gas extraction in terrestrial locations
will be more difficult as the frozen periods of the year become shorter and less predictable.17 The unstableground even poses a serious risk to the safe operation of nuclear facilities, such as the Bilibino Nuclear
Power Plant in northeastern Siberia.18
These changes in the Arctic have the potential to increase the risks in the rest of the world. As noted inearlier chapters, the thawing of the permafrost could accelerate the rise in global temperatures, and themelting of the Greenland ice-sheet could add significantly to global sea level rise.

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Systemic risks at the international level

Climate change risks to global food security

As part of the UK government’s first national climate change risk assessment, a study19 was commissioned tolook into the ‘indirect’ risks of climate change to the UK: those which arose not from climate change impacts
within the country’s borders, such as flooding, but which affected the country indirectly as a result of its
interactions with other countries.

To better understand the systemic risks of climate change to global food security, the UK Foreign andCommonwealth Office and the UK Government Science and Innovation Network jointly commissioned the UK’s
Global Food Security programme to bring together a cross-disciplinary task-force of academics, industry andpolicy experts from the UK and US to make an assessment. The task-force considered the risks of extreme
weather to the global food system in the current climate, and how these risks have already increased and could
increase further due to climate change. The task-force’s full report Extreme weather and resilience of the
global food system, which includes recommendations for how these risks can be managed and reduced, canbe found at http://www.foodsecurity.ac.uk/assets/pdfs/extreme-weather-resilience-of-global-food-system.pdf.1

The study reached a striking conclusion: ‘Climate change impacts around the world multiply existing threats
to the UK, and some of these could be an order of magnitude greater than threats from domestic climate
impacts’ (emphasis added). While the study only considered a scenario in which global emissions werealigned with the target of limiting global temperature increase to 2°C, it noted that higher emissions scenarios
could lead to much more severe impacts, particularly over the longer term.

The following section presents a summary of the report’s risk assessment.

Most of the largest risks identified in that study fell into two categories: risks to global food security, and risks
to national and international security. For this risk assessment, we considered those two categories of risk insome depth. The next two sections summarise our findings.

Endnotes
1. Helbing, D. (2010). ‘Systemic Risks in Society and Economics’. Paper prepared for IRGC Workshop on Emerging
Risks, December 2009. Available at http://irgc.org/IMG/pdf/ Systemic_Risks_Helbing2.pdf .
2. Helbing, D. (2010). Page 2
3. Nobre, C.A., J. A. Marengo, M. E. Seluchi, A. Cuartas, L. M. Alves, H. Camargo (2015). ‘The Record Drought andWater Crisis of 2014 in Southeastern Brazil.’ Submitted to Bull. Amer. Met. Soc., 2015
4. For more details on the underlying climatic patterns and similar droughts, see Cavalcanti IFA, Kouksy VE(2001) ‘Drought in Brazil during summer and fall of 2001 and associated circulation features’, Climanalise,
1: 1-10 (available from: http://climanalise.cptec.inpe.br/~rclimanl/revista/pdf/criseing.pdf); DAEE (2013)
Macrometrópole. ‘Sumario Executivo-Plano Diretor de Aproveitamento de Recursos Hídricos para a
Macrometrópole Paulista’. Departamento de Aguas e Energia Elétrica (DAEE). Secretaria de Planejamentoe Desenvolvimento Regional, Secretaria do Meio Ambiente, Secretaria de Saneamento e Recursos Hídricos.
Governo do Estado de São Paulo, São Paulo, Brasil, pp. 44; and Obregón, G. O., J. A Marengo, C. A. Nobre(2014), ‘Rainfall and climate variability: long-term trends in the Metropolitan Area of São Paulo in the 20th
century’, Climate Research, 61: 93–107, 2014, doi: 10.3354/cr01241
5. According to the Centre for Food Storage and Market Distribution of the State of São Paulo (CEAGESP),
the Brazilian Citrus Association, the Cereal Stock Market of São Paulo and the Brazilian Union of Sugar Cane
Producers (UNICA).
6. According to INPE (www.inpe.br)
7. Electric Energy Commercialization Chamber of Brazil
8. Munich Re (2015) Natural catastrophes 2014. Analyses, assessments, positions. TOPICS-GEO 2014. Munich,Germany. 67 pp.
9. Overland, J., Hanna, E., Hansses-Bauer, I., Kim, S.-J., Walsh,J, Wang, M., Bhatt, U.S. (2014) ‘Air Temperature’,
Arctic Report Card: Update for 2014, available at http://www.arctic.noaa.gov/reportcard/air_temperature.html
10. According to the Finnish Meteorologial Institute
11. See page 9 of IPCC (2013). ‘Summary for Policymakers’. Climate Change 2013: The Physical Science Basis.
Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate
Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and
P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. andLarsen, J.N., O.A. Anisimov, A. Constable, A.B. Hollowed, N. Maynard, P. Prestrud, T.D. Prowse, and J.M.R. Stone(2014). ‘Polar regions’. Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects.
Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate
Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O.
Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L.White (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1567-1612.
12. Arctic Council (2013). Arctic Resilience Interim Report 2013. Stockholm Environment Institute and Stockholm
Resilience Centre, Stockholm.
13. Helbing, D. (2010). ‘Systemic Risks in Society and Economics’ Paper prepared for IRGC Workshop on Emerging
Risks, December, 2009, http://irgc.org/IMG/pdf/ Systemic_Risks_Helbing2.pdf
14. Feodoroff, P and Mustonen, T. Näätämö and Ponoi River Collaborative Management Plan. Kontiolahti:
Snowchange Cooperative, 2013.
15. Arctic Council (2005). Arctic Climate Impact Assessment. Cambridge University Press, Cambridge. p. 6
16. Arctic Council (2005). p. 6
17. Helbing, D. (2009)18. Bellona 2015a. Glitching safety system at Russia’s aged Bilibino NPP causes emergency reactor shutdown
http://bellona.org/news/nuclear-issues/nuclear-issues-in-ex-soviet-republics/2010-11-glitching-safety-systemat-russias-aged-bilibino-npp-causes-emergency-reactor-shutdown. Cited 13th April, 2015.
19. PwC (2013). International threats and opportunities of climate change for the UK http://www.pwc.co.uk/
sustainability-climate-change/publications/international-threats-and-opportunities-of-climate-change-to-theuk.jhtml

20 EXTREME WEATHER AND RESILIENCE OF THE 		GLOBAL FOOD SYSTEM
Prepared for the UK-US Taskforce on Extreme Weather and Global Food System Resilience
Lead Authors in alphabetical order (*=coordinating lead authors)
Rob Bailey*,i Tim G. Benton*,ii Andy Challinor,iii Joshua Elliott*,iv David Gustafson*,v Bradley
Hiller,vi Aled Jones*,vii Chris Kent,viii Kirsty Lewis*,ix Theresa Meacham,x Mike Rivington,xi Richard
Tiffin*xii & Donald J. Wuebbles.xiii

Food demand and supply and the impact of weather in a changing world
Demand for food, at a global level, is increasing faster than yields are growing, leading to increasing pressure
on land.2 By 2050, the FAO estimates that demand will increase over 60% above the current situation.
Demand growth is driven by population and demographic change, and increasing global wealth. This, inturn, leads to greater per capita food demand, often associated with demand for more livestock produce.
In 2007/8, a small weather-related production shock, coupled with historically low stock-to-use levels,
led to rapid food price inflation, as measured by the FAO Food Price Index and associated with the maininternationally traded grains.3 This increase was compounded by some countries imposing barriers to local
export, to ensure their own food security, leading to an FAO price spike of over 100%. A similar price spike
occurred in 2010/11, partly influenced by weather in Eastern Europe and Russia.4
These spikes created a number of significant impacts around the world. In rich countries, where food is freely
available, food price inflation was marked and the poorest suffered, resulting in people trading down on foodquality or quantity, and in the process spending significantly more. In poorer countries, especially those withfragile governance, rapid food price inflation undermined civil order, and, in part was a spark for the ArabSpring and the consequences that have followed.5 In 2012, the worst drought to hit the American Midwest forhalf a century triggered comparable spikes in international maize and soybean prices.
This sequence of price spikes, and their consequences, re-alerted the world to the need to focus on global
food availability and the volatility in its supply. In 2012, Sir John Beddington, then UK Government Chief
Science Advisor, commissioned a report on food system resilience from the UK’s Food Research Partnership.
That report concluded: “The evidence is not available properly to describe with any certainty howi.

Chatham House, UK.

vii. Global Sustainability Institute, Anglia Ruskin University, UK.

ii.

UK Global Food Security Programme. University of Leeds, UK.

viii. Met Office, UK.

iii. University of Leeds. UK.

ix. Met Office, UK.

iv.

NASA Goddard Institute for Space Studies, Columbia
University, USA.

x.

ILSI Research Foundation, Washington, USA.

xii. The James Hutton Institute, Aberdeen, Scotland, UK.

v.

vi. Global Sustainability Institute, Anglia Ruskin University, UK.

UK Global Food Security Programme.

xi. Met Office, UK.
xiii. Department of Atmospheric Sciences, University of Illinois,
USA.

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variable weather will impact on food production systems and worldwide trade, but our contention
is that we need greater investigation of what they could be, with perhaps greater consideration
being given to reasonable ‘worst case scenarios’. … Given that the frequency of weather extremes isincreasing, the potential for large impacts, and unprecedented ones, is growing.”6

By examining production shocks in the recent past, we show that weather events, particularly drought, are amajor driver of these shocks. Using the example of these past events we generated a set of scenarios, in thepresent or near-future, of weather-driven production shocks for each of the four crops. These we combined tocreate a plausible worst case scenario.8

Here we summarise the outputs from the Taskforce into three areas: (i) how the changing weather may createshocks to the global food system, and, from this the development of a ‘plausible’ worst case scenario for ashock; (ii) plausible market and policy responses to the worst case scenario; and (iii) how the combination of
scenario and responses may impact upon different societies, economies and the environment.

Weather and shocks to the global food system
Food production of the globally most important commodity crops (maize, soybean, wheat and rice) comesfrom a small number of major producing countries. The exposure of a large proportion of global productionof the major crops is therefore concentrated in particular parts of the globe (Fig 1), and so extreme weather
events in these regions have the largest impact on global food production. Simultaneous extreme weather
events in two or more of these regions – creating a multiple bread basket failure – would represent a serious
production shock, however understanding the covariance of extreme weather events in different production
regions is currently under-researched. There is an urgent need to understand the driving dynamics ofmeteorological teleconnections, such as the El Niño – which may be becoming more extreme7 - in order toquantify the likelihoods of coincident production shocks in major food-producing regions.

Figure 1: Proportion of the total calories coming from the main four commodity crops
per country. Within each country, agricultural production is also typically concentrated(see fig S7a in Foley 2011 for a spatially resolved map). For example, the bulk of calories
produced in the US come from soy and maize in the Midwest, in Brazil agricultural
production, mainly soy, is concentrated inland from the SE coast; rice predominantly
comes from the Indo-Gangetic plain, SE China and SE Asia; and wheat production isconcentrated in NW Europe and around the Black Sea.

Plausible scenario for extreme weather’s impacts on crop production
Analysis of the historical records indicated that in 1988/89 there was a significant drought-related impact
on the yields of maize and soybean, with global production decreasing by 12% and 8.5% respectively.
In 2002/3, drought impacted on wheat in Europe, Russia, India and China and rice in India, with global
production of wheat decreasing by 6%, and of rice by 4%. Our plausible worst case scenario is that thetwo patterns of weather - that resulted in maize and soy, and wheat and rice being significantly affected occur simultaneously. Without further work we cannot quantify the risk of this scenario, but we consider
that a significant impact on all four crops of these magnitudes is plausible.

Changing profiles of risks over time: a first analysis
Through the use of climate models coupled to crop models, we can explore the changing risk of major shocks
to the food system. To gauge whether we should be concerned about changing risks, we undertook an initial
exploration using an ensemble of agriculture and climate impact models.9 In terms of global calorie-weighted
yields of maize, soy, wheat and rice produced, the ensemble produces a distribution of yields in response tomodelled weather (shown in Fig 2 as histograms for ‘historical’ and ‘near-term’ future, and as box plots for 4
time periods). Comparing the histograms, there are changes in the shape of the distribution in future relative
to the last decades: they ‘flatten out’. This change in shape represents a significant increase in variance
between the modelled historical and the near term future, and this increasing variability continues to increase
throughout the century (Fig 2, top right hand panel).
This ensemble analysis suggests that what we would call an extreme food production shock in the late 20th
century will become more common in the future (Fig 2). These data indicate that a 1-in-200 year event forthe climate in the late 20th century equates to a loss of approximately 8.5% (Fig 2 top), and over the next
decades (2011-2040), a 1-in-200 year event is about 15% larger in magnitude and equivalent to the loss of
9.8% of calorie production. Furthermore, according to the ensemble, an event that we would have called 1-in100 years over the period 1951-2010 may become as frequent as a 1-in-30 year event before the middle ofthe century.

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Figure 2: Model-based distributions of global calorie-weighted yield of maize, soy,wheat, and rice for the historical (1951-2010) and near-future (2011-2040) periodwith (top row) and without (bottom row) the effects of fertilization from increasing
atmospheric CO2 included. The estimated magnitude of a 1-in-200 year event ineach period is indicated by arrows on the histograms. The box plots summarise the
distributions and show the likely increasing variability continuing throughout thecentury.

…the global food system does exhibit characteristics consistent with a fragile one thatis vulnerable to self-propagating disruptions. That is, in a setting where countries
are increasingly interconnected and more food is traded globally over the [last two
decades], a significant majority of countries are either dependent on imports for theirstaple food supply or would look to imports to meet any supply shortfalls.12
In essence, through deeper trade food importing countries have reduced costs and vulnerability to localized
production shocks, but at the expense of increased exposure to systemic risks such as a shock in a major
production hub or a sudden deterioration in connectivity. Recent price spikes illustrate clearly the systemicrisks associated with disruptions in major production regions and/or outbreaks of trade restrictions. Other
factors thought to have amplified these price spikes include biofuel mandates, low ratios of stocks relative todemand and depreciation of the US dollar.13
Historically, following past production shocks, individual grain prices have more than doubled in a shortspace of time.14 The food system’s resilience to a weather-related shock can be defined by how much foodprices, access and availability are affected by it. Resilience therefore depends on the magnitude of thephysical shock, and on the policy and market responses that may amplify or buffer its effects as it propagates
through the system.
In response to the last decade’s food price spikes, many governments have developed strategic responses
to better manage food production. However, other key problems pertaining to demand and trade responses
remain unaddressed. If we are to avoid the worst impacts of future production shocks, we need to develop
greater understanding of how responses may amplify, or mitigate, the price impact of production shocks.
These responses are determined by the actions of agents mediated through markets. Governments are
significant players in this, both through their direct influence on markets and their indirect influence on theother agents including farmers, food manufacturers and retailers, consumers and relief agencies.

The analysis shown in the top row of Fig 2, assumes full effectiveness of CO2 fertilization. Recently questions
have been raised about the magnitude of this beneficial effect.10 If we assume instead that there is no CO2
fertilization at this large scale, we find similar but even more severe effects in later decades (Fig 2 bottom):a 1-in-200 year event in the near-term future is ~25% greater magnitude and the extreme left tail indicatesthe potential for historically unprecedented events. Without CO2 effects, a historically 1-in-100 year event isestimated to occur more than once every 10 years by the second half of the 21st century.
These results are a preliminary analysis and are limited by the availability of high-resolution global climate
model runs. Significant work is needed to reduce the uncertainty and better understand the way extreme
weather may change. Nonetheless, the indications are clear that the global food system is facing increasing
risks due to more frequent extreme weather.

Policy and market responses to weather-influenced production shocks
Global food trade has increased in recent years, bringing well understood benefits. Trade allows countries
with limited productive potential to meet domestic demand; it facilitates specialisation and efficiency; andit generally increases resilience by smoothing local disruptions. However, not all disruptions are equal. As
Section 3 highlighted, the system is not robust to a shock in one or more major production regions, pointing
to inherent systemic risk in the geographical concentration of global food production.
As noted by May (regarding financial and ecosystem networks, but similarly applies to the global food
network which shares some network properties) there is a complex interplay between robustness andvulnerability. Greater interconnectedness reduces countries’ vulnerability11 to local production shocks, but
may increase vulnerability to shocks in distant breadbasket regions. It also means the food system is more
vulnerable to a sudden reversal in connectivity, for example due to an outbreak of trade restrictions. A recent
study examining the evolution of trade networks over the period 1992-2009 concluded:

To capture the potential market and government responses to food production shocks in wheat, maize,soybean and rice, we conducted a literature review, undertook a historical data analysis and completed~50 interviews with experts from industry and policy around the world. Taking the plausible worst caseproduction shocks set out in Box 1 as a starting point, we developed a detailed scenario of how weather
and responses may interact on a global scale to produce a significant market shock. This scenario for 2016
involves export restrictions being put in place by major food producing countries, and large-scale purchases
and consumption subsidies being used by major importers or highly import-dependent countries. The
scenario is described in detail in the full version of this report [www.foodsecurity.ac.uk/assets/pdfs/extremeweather-resilience-of-global-food-system.pdf], together with a scenario for 2026 that assumes a plausible
deterioration in global food system resilience to have occurred in the intervening period.
Economic modelling of the price impacts of these scenarios has not been possible, and in any case, typicaleconomic simulation models are poorly suited to modelling short-run prices during periods where markets
are in disequilibrium and the magnitude of the shock is significantly ‘out-of-sample’. Nevertheless, it is ourjudgement that the combined production shock and responses outlined below in the 2016 plausible worst
case scenario could see the FAO food price index reach record highs, surpassing 250 compared to around
170 at the time of writing, with a likely trebling in the price of individual grains. By way of comparison,
the index reached 226 in 2008 and 238 in 2011. All other things being equal, the 2026 scenario would be
expected to result in an even higher price spike. Economic modelling of these scenarios would be one subject
for subsequent research.

Factors that may amplify the impact of future production shocks in 2026
The consequences for global food security of any production shock depends not only on the responses ofkey actors, but critically also on the overall resilience of the food system and prevailing macroeconomic
conditions. It is far from difficult to develop a plausible worst case scenario for 2026 in which system
resilience is lowered over the next decade and macroeconomic conditions are unfavourable, making the
global food system considerably more vulnerable to the same shocks.
Factors that would cumulatively reduce the resilience of the global food system to supply shocks andincrease the likelihood of a price crisis include: low stock-to-use ratios; the reduced self-sufficiency of China;
increasingly inelastic demand; the recovery of oil prices; cumulative underinvestment in infrastructure in

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key exporting regions; and the depreciation of the US dollar. Under this set of preconditions the production
shocks considered here would almost certainly result in a more dramatic price response. Consequently, theresponses of societies and governments would likely be more extreme. A larger number of countries would
probably experience civil unrest. This would raise the stakes for governments, and result in more statesintervening.

How would a plausible worst case scenario impact on societies, economies and theenvironment?
The preceding section set out a plausible worst case scenario in 2016, comprising a weather-related global
production shock amplified through the responses of market actors that could plausibly result in a spike of
the FAO food price index to over 250 in 2016. Based on this scenario, it is possible to consider the potential
consequences for human populations and national economies. Information on possible country level impacts
was collected through an expert interview process. An ‘Interview Questionnaire’ was developed and a panelof experts from academia/research institutions, government and the private sector were interviewed about thelikely impacts in Brazil, China, Egypt, Ethiopia, Europe, India, Russia, Saudi Arabia, and the United States.
This analysis revealed the following broad expectations of how the plausible worst case scenario might unfold
at the national and societal level. These are highly consistent with the impacts observed during the 2007/8
and 2010/11 price spikes.
• The hardest impacts would be felt by import dependent developing countries, particular in
Sub-Saharan Africa. These countries would be expected to experience the most pronounced short-term
deteriorations in poverty rates and nutrition security. At the economy level, impacts would likely include
inflation, deteriorations in the balance of payments and budgetary pressures arising from higher foodsubsidies and social transfers.
• Other import dependent countries could experience social unrest. In particular, in the wake of
the Arab Spring and ongoing instability in the region, the highly import dependent countries of the
Middle East and North Africa region could be particularly vulnerable.
• Impacts on major economies would be muted. Consumers in large industrialised countries suchas the US and EU, where food represents a small share of household expenditures, would be relatively
unaffected.15 The crop sectors of these economies, and other major agricultural producers, would likely
benefit from higher prices, though other sectors could suffer. Poor food consumers in China would likely
be relatively unaffected due to government intervention to buffer these households from food priceinflation through the use of strategic reserves and price controls.
• The supply response may have negative consequences in the longer term. In response to theprice spike, agricultural output would likely increase through a combination of extensification andintensification. In the short-term, this would increase supply and help stocks to recover, facilitating adecline in food prices. However, if extensification occurred at the expense of high carbon value and/or high biodiversity value land such as forest, this could have long-term environmental costs. Similarly,
unsustainable intensification could degrade soils, deplete freshwater supplies and increase greenhousegas emissions and eutrophication. The risk of unsustainable production responses is likely to be higher
in the event of a dramatic price spike, with potential long-term consequences for the resilience of foodproduction.

Conclusion
We have argued that the risk of a serious weather-related shock to global food production appears to be
increasing rapidly due to climate change. Such an event could have serious implications for the stability of
global grain markets and human security in vulnerable countries. In our full report, we set out five broad
areas where action can begin to be taken in order to address this.16 First, and perhaps foremost, is to better
understand the risks. In particular, we need to better understand the evolving risk of weather-related shocks.
In addition, there is also a need to better understand the ways a shock propagates through the trade network,and the immediate actions that can mitigate its effects. Finally, there are strategic actions that can be taken to
create structural changes that build the resilience of the global and local food systems.

Endnotes1.

See also http://www.foodsecurity.ac.uk/assets/pdfs/climate-and-global-crop-production-shocks.pdf, http://
www.foodsecurity.ac.uk/assets/pdfs/review-of-responses-to-food-production-shocks.pdf and http://www.
foodsecurity.ac.uk/assets/pdfs/country-impacts-of-global-grain-production-shocks.pdf.
2. Bajzelj, B.et al. (2014). ‘Importance of food-demand management for climate mitigation’. Nature Clim. Change,
4, 924-929.
3. Global Food Security (2014). ‘Food price spikes and global markets’. Insight 3, pp. 1-3 http://www.foodsecurity.
ac.uk/assets/pdfs/1404-gfs-insight-food-price-spikes.pdf
4. Coulibaly, A.L. (2013) The Food Price Increase of 2010–2011: Causes and Impacts Background Paper No. 201302-E. Library of Parliament. http://www.parl.gc.ca/Content/LOP/ResearchPublications/2013-02-e.htm
5. Lagi, M., Bertrand, K.Z., and Bar-Yam, Y. ( 2011). ‘The food crises and political instability in North Africa and the
Middle East.’ arXiv preprint arXiv:1108.2455; Bellemare, M. F. (2015). ‘Rising Food Prices, Food Price Volatility,
and Social Unrest’. Am. J. Agricultural Economics, 97, 1-21; Natalini, D.,,et al.(2015) , ‘Quantitative assessment ofapproaches to measuring likelihood of food riots in countries’, Sustainability, 7 (4), 4360-4385
6. Benton, T. et al. (2012). Sever weather and UK Food Chain Resilience: Detailed appendix to synthesis report.
http://www.foodsecurity.ac.uk/assets/pdfs/frp-severe-weather-uk-food-chain-resilience.pdf
7. Cai, W., et al. (2014). ‘Increasing frequency of extreme El Nino events due to greenhouse warming’. Nature
Clim. Change, 4, 111-116.
8. The global production loss estimated from the historical and simulated anomalies using the Global Gridded
Crop Model Intercomparison (GGCMI) data for the four crops in 1988/9 and 2002/3.
9. The AgMIP/ISI-MIP model ensemble https://www.agmip.org/ag-grid/ggcmi/
10. Foley, J. A. et al. (2011). ‘Solutions for a cultivated planet’. Nature, 478, 337-342.
11. O’Leary, G. J., et al. (2015). ‘Response of wheat growth, grain yield and water use to elevated CO2 under a FreeAir CO2 Enrichment (FACE) experiment and modelling in a semi-arid environment’. Global Change Biology.
doi: 10.1111/gcb.12830; Feng, Z., Rütting, T., Pleijel, H., Wallin, G., Reich, P.B., Kammann, C.I., Newton, P.C.D.,
Kobayashi, K., Luo, Y., Uddling, J., 2015. Constraints to nitrogen acquisition of terrestrial plants under elevatedCO2. Global Change Biol. (in press); Myers, S.S., Zanobetti, A., Kloog, I., Huybers, P., Leakey, A.D., Bloom, A.J.,
Carlisle, E., Dietterich, L.H., Fitzgerald, G., Hasegawa, T., 2014. Increasing CO2 threatens human nutrition. Nature
510, 139-142.
12. May, R.M., 2013. Networks and webs in ecosystems and financial systems. Philosophical Transactions of theRoyal Society a-Mathematical Physical and Engineering Sciences 371, 20120376.
13. World commodity prices are priced in dollars. Depreciation of the dollar leads to cheaper prices in othercurrencies. This leads to either reduction in supply or an increase in demand, which then drives up world
prices.
14. Page 11 in Puma, M. J. et al. (2015). Assessing the evolving fragility of the global food system. Environmental
Research Letters,10(2), 024007.
15. Charlebois, P. and Hamann, N. (2010) The consequences of a strong depreciation of the USD on agricultural
markets Catalogue A38-4/21-2010E-PDF Project 10-055-r http://www.agr.gc.ca/pol/index_e.php
16. This is on average. Analysis of UK purchases following the 2007/8 food price spike show a complex pattern ofchanges in response to food price increases: as prices increased UK households bought 4.2% less. They also
traded down to save money, by buying cheaper alternatives. Nonetheless, trading down didn’t compensate
prices rises and overall the food bill for an average family rose and was 12% more in 2011 than 2007. The
poorest spent 17% more in 2011 compared to 2007, so their relative food bill increased by 40% more than
average (https://www.gov.uk/government/publications/family-food-2011).
17. Global Food Security (2012). Global Food Systems and UK Food Imports: Resilience, Safety and Security.
Available at http://www.foodsecurity.ac.uk/assets/pdfs/gfs-and-uk-food-imports.pdf.

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CLIMATE CHANGE RISKS TO NATIONAL AND 			
INTERNATIONAL SECURITY

Contributors to this section: Vice Admiral Pradeep Chauhan (Retd),i Vice Admiral Lee Gunn (Retd),ii
Major General A N M Muniruzzaman (Retd),iii Professor Shi Yinhong,iv Sarang Shidore,v
Dr Arunabha Ghosh,vi Catherine Trentacoste,vii ED McGrady,viii Francesco Femia,ix Caitlin E.
Werrell,x Nadia Schweimler.xi
The security risks of climate change may be the hardest of all climate risks to assess, because they involve thelongest chains of causation or influence, and the most unpredictable factors. However, since they may be thebiggest risks of all,1 assessing them to the fullest extent possible is essential.
There is potential for confusion and underestimation of risk if assessments of climate security risks do
not make clear the degree of climate change they are considering. A focus on risks in the current climate
may well be enough to inform policy on adaptation and resilience, but to inform decisions with long-term
implications (such as those relating to global emissions), a longer-term view is also necessary. Here wedeliberately make clear distinctions wherever possible between risks in the present climate, risks in the future
under low degrees of climate change, and risks in the future under high degrees of climate change.

Climate change risks to security in the present
A growing body of credible, empirical evidence has emerged over the past decade to show that the climatechange that has occurred thus far – involving an increase of 0.8°C in global average temperatures – is alreadyinfluencing dynamics associated with human, sub-national, national and international security. This evidence
does not generally attempt to pinpoint precise causal relationships, but instead considers how climate change
may have altered probabilities and interacted with other factors to increase the risks. Here we give three
examples.
Drought, displacement and conflict in Syria
The Middle East, North Africa and Mediterranean region has experienced a drying trend over the last few
decades, with a notable decline in winter precipitation. Climate change is thought to have played a significant
role in this trend, as was forecast by previous climate modelling,2 and to have made the extreme drought
suffered by Syria between 2007 and 2011 some two to three times more likely.3 During the drought, crop
failure and the loss of livestock were severe and widespread. This contributed to a mass internal displacementof farmers and herders – around two million people – many of whom fled to urban areas which werealready stressed with Iraqi and Palestinian refugees.4 By 2009, more than 800,000 Syrians had lost their
entire livelihood as a result of the droughts; by 2011, around 1 million were extremely food insecure, and2-3 million had been driven into extreme poverty.5 While many other factors were important in driving thepolitical unrest and conflict that followed, it is difficult to imagine that this widespread impoverishment andlarge-scale displacement did not play a role.

i.

Vice Admiral Pradeep Chauhan, AVSM & Bar, VSM, I.N. (Retd),
Former Commandant of Indian Navy, and Assistant Chiefof Naval Staff, Foreign Cooperation, Strategic Threats andTransformation

v.

ii.

Vice-Admiral Lee F Gunn, US Navy (Retd.) Former Inspector
General of the Department of the Navy (Navy and Marine
Corps), formerly Commander of Expeditionary Strike Group
Three, and most recently President of the Institute for Public
Research at CNA

vii. Catherine Trentacoste, Research Scientist, CNA Corporationiii. Major General A N M Muniruzzaman, Chairman, Global Military
Advisory Council on Climate Change, President, Bangladesh
Institute of Peace and Security Studies
iv.

Professor Shi Yinhong, Professor of International Relations,
Chairman of the Academic Committee of the School ofInternational Studies, Director of the Centre on American
Studies, Renmin University of China

Sarang Shidore, Visiting Scholar, University of Texas at Austin,
formerly co-leader of strategic futures project at the Institutefor Defense Studies and Analyses, New Delhi

vi. CEO, Council on Energy, Environment and Water
viii. Ed McGrady, Director of Gaming, CNA Corporation
ix. Francesco Femia, Co-Founder and Director, The Center for
Climate and Securityx.

Caitlin E. Werrell, Co-Founder and Director, The Center for
Climate and Security

xi. Nadia Schweimler, Climate and Development Knowledge
Network, PwC

Heat waves, food price spikes, and civil unrest
In the summer of 2010, Russia suffered an extreme heat wave. Climate change is estimated to have madethis event approximately three times more likely to occur than it would have been otherwise.6 The heat wave
combined with and contributed to drought and fire, and reduced Russia’s wheat production that year bymore than 30%.7 At the same time, related droughts affected wheat harvests in Ukraine and China. Reduced
production, protectionist measures, commodity speculation, and large-scale purchases on the global market
all contributed to a more than doubling of the global wheat price in the second half of 2010.8, 9 In highly
import-dependent countries such as Egypt, the price of wheat rose by 300% in late 2010 and early 2011.10
The top nine wheat-importing countries in the world, on a per capita basis, are all in the Middle East andNorth Africa. Seven of these countries – Libya, Jordan, Algeria, Tunisia, Yemen, Egypt and Iraq – are ranked
lower than ‘very high’ on the Human Development Index, and spend between 35% and 45% of their average
household income on food. All seven experienced political protests resulting in civilian deaths in 2011.11 In
many of these countries, food prices are recognized to have been one of the factors that led to the unrest– notably in Tunisia, Jordan and Yemen, where demonstrators waved baguettes on the streets.12 In Egypt,although urban protests primarily focused on other social and economic concerns, bread protests occurred inrural areas across the country in parallel to the events in Tahrir Square, and may have broadened the appealof the revolution to rural communities.13
Clearly, climate change did not on its own cause any of these events. But it appears to have played a role,
combining with other stresses and weaknesses to destabilize environmental, economic, social and politicalsystems.
Climate variability, change, and conflict
Evidence of the influence of climate change on sub-national conflict in Africa has materialized over the past
five years. Across the continent, rainfall variability has been found to affect ‘both large-scale and smaller-scaleinstances of political conflict’.14 A strong correlation has been detected between climate change, resource
stress and range wars among pastoralists in East Africa.15
Recent research has investigated climate change and conflict across wider ranges of temporal andgeographical scales, and found evidence of similar relationships.16 One evaluation of sixty primary studies
and forty-five conflict data sets found that the likelihood of violence at a number of levels, from interpersonal
to international, increases significantly with climatological changes.17 These studies have deliberately
excluded consideration of mitigating circumstances, such as governance and wealth, in order to attempt todetect a trend in the changing likelihood of conflict. This is a weakness, in terms of the practicality of theresults. However, it is a strength in terms of identifying a statistically-significant correlation between climaticchanges (particularly precipitation), and the likelihood of conflict, across time and space, and across a rangeof scales.

Climate change risks to security in the future
In the near term future, population and economic growth are expected to significantly increase pressure on
resources. Global demand for food, water and energy is projected to increase by approximately 35%, 40% and
50% respectively, by 2030 as compared to 2012.18 At the same time, climate change could negatively affectthe availability of these resources. National security and intelligence assessments of several governments
have recognised the potential for this confluence of trends to contribute to security risks.19
With regard to security risks in the long term, relatively little analysis is available. The IPCC found that:“Much of the current literature on human security and climate change is informed by contemporary
relationships and observation and hence is limited in analyzing the human security implications ofrapid or severe climate change.” 20
To support our assessment of how security risks could vary between low degrees of climate change in thenear term, and potentially high degrees of climate change in the long term, we commissioned the CNA
Corporation, experts in futures analysis and wargaming, to design and conduct a wargame and scenarios
exercise. This was held in Delhi in March 2015, hosted by the Council on Energy, Environment and Water.
The twenty-four participants included senior scientists, security experts, diplomats, and retired military
personnel from countries including India, China, the US, the UK, Bangladesh, Germany, the Netherlands,
and Finland. The game investigated the decisions made by participants as they played the roles of leadersof major countries and regions, aiming to further national economic and security interests in the context

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of a changing climate over the next half-century or more. The scenarios exercise consisted of round-tablediscussions to identify the most significant near-term and long-term security risks in a scenario where climatechange progressed at a rate close to the upper end of what is currently assessed as the likely range.21 Bothexercises were conducted by four independent groups of participants operating in parallel. The assumptions
used in both exercises were reviewed for reasonableness and plausibility by the Climate Change Science
Institute of Oak Ridge National Laboratory.
Here we discuss some of the biggest security risks identified, grouping them by theme. Analysis from thewargame and scenarios exercise conducted for this assessment is presented together with some more detailed
comments from individual participants, and with relevant findings from a few other published studies thathave explicitly considered the security risks of high degrees of climate change. References note where otherassessments have reached similar conclusions.

State failure
Our scenarios exercise found that in the near-term future, climate change would be most likely to increase
the risks of state failure in states that are already highly water stressed or food insecure, at the same time
as suffering from poverty, social tensions, and poor governance. We considered that countries in the Middle
East and North Africa region may be at particular risk: most are already water-stressed, many of them to an
extreme degree.22 The large population increases projected for many countries in the region – in the rangeof 50% for Egypt, 70% for Syria, 90% for Yemen, and 130% for Iraq, between 2010 and 2050, will further
decrease per capita water resources. At the same time, climate models predict a drying trend for the region.
One study projects a reduction in streamflow of 10-30% in large parts of the region, and of 30-50% in theworst affected areas, with a global temperature rise of 2.7°C.23
We also considered that countries where a high proportion of the population relies on subsistence farming
may be at particular risk of instability due to climate change impacts on agriculture. Sub-Saharan Africa
already has the highest proportion of food insecure people in the world, with more than a quarter of the
population undernourished in 2010-2012, and more than half in some areas.24 Many of the countries in
the region – more than 30 in the continent as a whole – are projected to double their populations by midcentury,25 and for a significant number, this will reduce arable land per capita to below a threshold of extreme
stress.26 At the same time, land temperatures in Africa are projected to rise faster than the global land
average, and it is thought very likely that climate change will reduce cereal crop productivity, with strong
adverse effects on food security.27 Clearly, economic development and adaptation to climate change will be
critical, and the risks will be greatest where these efforts are less successful.
Climate change is likely to increase environmental stresses on many countries at the same time. A report by
the German Advisory Council on Global Change suggested that this, in combination with the tendency offailed states to destabilize their neighbours, could lead to the emergence of ‘failing sub-regions’ in parts of
the world where climate change impacts are particularly severe.28 Vice-Admiral Chauhan (retd.) of the Indian
Navy gives an example of how stresses affecting the countries of South Asia could interact with each other.

A perspective on climate change and the risk of state failure
Vice Admiral Pradeep Chauhan, AVSM & Bar, VSM, I.N. (Retd)
Nation-states are far from being inherently stable. Many have suffered near-continuous internal tensions
throughout their histories, arising from ethno-religious differences and socio-economic inequalities. The
extent to which the writ of nation states runs is often quite limited, both in terms of its robustness, andin terms of geography. Historically, the resilience of government structures in the face of unexpected
and large-scale crises has frequently been found to be severely wanting. State failure is not a precisely
defined term, but it may be characterised by: (a) an inability to provide security to the population resulting
from failure to retain a monopoly on the legitimate use of force; (b) an inability to provide and equitably
distribute essential goods and services; (c) a serious erosion of the power to make and enforce collective
decisions; and (d) the involuntary movement of populations including refugees.

High degrees of climate change could increase the risks of state failure in countries that are economicallyunderdeveloped, resource stressed, or already unstable for other reasons. In South Asia, drought in
Afghanistan and Pakistan, and incessant flooding and loss of land to the sea in Bangladesh, could put
those countries’ governments under great stress, and precipitate large-scale migration into India. In
India, this would combine with an internal population shift from rural to urban areas, further increasing
demographic pressure in cities – many of the largest of which – including Kolkata, Chennai and Mumbai –are coastal, and will be increasingly vulnerable to flooding both from sea level rise and from more intense
rainfall. At the same time, both the influx of internal and external migrants, and the increasing variabilityof the monsoon, could further destabilise the ‘Red Corridor’, a swathe of economic deprivation andmisgovernance that cuts through almost all the eastern states of India, in which Marxist-Leninist rebels
are waging a campaign of violence against the state. The temptation to solve this problem through militaryintervention could become overwhelming.
At the high degrees of climate change possible in the long-term future, participants in our scenarios exercise
considered that there could even be risks to the political integrity of states that are currently considereddeveloped and stable. These could arise from the combined effects of food and water insecurity, social
stresses caused by inequality and large-scale internal migration, the increasing expense and difficulty ofprotecting coastal cities, and the breakdown of infrastructure systems subject to multiple stresses.xii
Terrorism
Participants in our exercise saw the risk of terrorism as closely linked to the risk of state failure. Whileterrorism has complex causes, the power vacuum left by a failing or collapsed state provides conditions inwhich terrorist groups can become established and grow stronger. Participants considered that the inequalityof climate change impacts between countries, and the potential for large-scale displacement of people, could
further increase the risk. Here Vice Admiral Lee Gunn (Retd.) describes how climate change could increase
the appeal of terrorism, at the same time as terrorism itself is becoming more dangerous.

A perspective on climate change and terrorism
Vice-Admiral Lee Gunn, US Navy (Retd.) Former Inspector General of the Department of the Navy
(Navy and Marine Corps), formerly Commander of Expeditionary Strike Group Three, and mostrecently President of the Institute for Public Research at CNA
No society, however prosperous overall it may be, appears to be entirely immune to terrorists’ recruiting.
Terrorism can arise when two essential conditions are met: the presence of an appealing, unifying, or
disruptive idea, and the social disenfranchisement of a section of society. The more the members ofa segment of society feel themselves to be economically, culturally, or politically disenfranchised or
marginalised, and the more difficult or distasteful their circumstances, the more fertile their community
may become for terrorists’ recruiting.
Even in the current climate, some nations already struggle to provide for the basic needs of their
populations (security, health, employment, freedom from want); while other nations have failed to do soentirely. As a result there are already marginalised populations where the appeal of terrorism is strong,
and territories that are effectively ungoverned where terrorist groups are left to operate with little
constraint. Climate change will disproportionately affect the countries that are already the weakest, andthe people within them who are already the most vulnerable. It has the potential to significantly increase
the ranks of disenfranchised populations within countries, as well as to increase the extent of ungoverned
spaces.29 At the same time, terrorism is becoming more dangerous as some of these groups take advantageof new technologies and globalisation, and we can expect this trend to continue.

xii. A similar conclusion was reached by the ‘Age of Consequences’ scenarios study undertaken by a group of scientists, security experts
and historians in the US in 2007. This study suggested that in a ‘severe’ climate change scenario (defined by a temperature increaseof 2.6°C by 2040), ‘massive nonlinear events in the global environment give rise to massive nonlinear social events… The internalcohesion of nations will be under great stress, including in the United States’; and that in a ‘catastrophic’ scenario (5.6°C by 2100), ‘The
collapse and chaos associated with extreme climate change futures would destabilize virtually every aspect of modern life’ and therange of problems could ‘overwhelm the traditional instruments of national security (the military in particular) and other elements ofstate power and authority’.

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Migration and displacement
There are many ways in which climate change could lead to migration and displacement, with attendantsecurity risks, as described here by Major General A N M Muniruzzaman (Retd).

Studies that have considered migration under high degrees of climate change have judged that the complexityof causes seen at present could be reduced to a more simple equation. The UK Government’s Foresightreport37 found that: ‘some impacts of environmental change… may give rise to significant permanent
displacement of whole populations as a result of existing settlements being, in effect, uninhabitable’. Similarly,
a study cited by the IPCC argues that ‘the most significant difference between the nature of human migrationin response to 4°C of warming relative to 2°C would be to remove many people’s ability to choose whether tostay or leave when confronted with environmental changes.’38 In this context, the number of people forced tomove could be of an order of magnitude greater than anything experienced in human history.39

A perspective on climate change and security risks from migration and displacement
Major General A N M Muniruzzaman, ndc, psc (Retd), President of Bangladesh Institute of Peaceand Security Studies, Chairman of Global Military Advisory Council on Climate Change
Historically, people have moved from place to place in search of a better life and to escape danger. Usually,
their decision to migrate has a number of influences, and cannot be attributed to a single cause. In the
coming century, climate change could emerge as an increasingly powerful influence. The ways in which
this could happen include:• Sea level rise, with attendant flooding and coastal erosion, is likely to displace populations from lowlying coastal areas, and small island states. Millions of people in Bangladesh could be displaced, andaround 40 small island states could face partial or complete submergence.
• River flooding can displace people both directly, and indirectly through disruption of agricultural
livelihoods. Flooding is projected to increase in many regions, but it could be a particular problem inSouth Asia due to the contribution of melting glaciers.
• Desertification and drought are both projected to increase with climate change. Both can be drivers of
migration, especially of pastoral societies or those depending on rain-fed agriculture.
• Natural disasters tend to lead to short-term displacement, but persistent extreme events in a region canlead to migration. In future, this could include persistent dangerous heat extremes.
It has been speculated that the number of people displaced or migrating as a result of future climate change
could run into the hundreds of millions,30 but it is impossible to make an estimate with any confidence, andit will depend greatly on the rate and extent of climate change that is experienced. The security risks that
could arise from large-scale migration have been widely recognized,31, 32, 33, 34 and include:• Destabilized borders: Migrants and refugees may be forcibly resisted by local populations or by
governments. This can lead to conflict between groups, and potentially between states.
• Conflict over resources: Environmental migration has been found to be more likely to lead toconflict when the destination country is already resource stressed.35 With climate change, this could
often be the case, as countries within a region are affected in similar ways.
• Ethnic and cultural conflict: Migrants and displaced people often have to endure difficult living
conditions and discrimination, which can lead to social division and tensions. Historically, conflictappears to have been more likely when migrants and destination country residents are from different
ethnic groups, or when there is already distrust between their respective nations or social groups.36• Disease: Displaced populations often lack appropriate sanitary and medical facilities. This cancontribute to the spread of disease across borders.

All four groups of participants in our scenarios exercise identified migration – both within and between
countries – as a significant security risk. Concerns were as much about the management of political andsocial tensions as about economic costs and pressure on resources. Participants from large countries wereparticularly concerned about how governance structures could cope, and social cohesion be maintained,
in situations where the differing local severity of climate impacts led to large-scale internal migration. At
the same time, it was felt that the pressure of increasing numbers of international migrants and refugees
could result in a rise of xenophobia and nationalism. In the game, it was notable that increasing numbers ofrefugees contributed to several large countries becoming more isolationist in their foreign policies.

As was noted in our scenarios exercise, if a high degree of climate change is reached, then by that time, much
more will be known about the actual emissions path that the world has followed, about climate sensitivity,
and about the impacts of climate change. It will then be fairly clear whether or not further climate change,
with even more adverse consequences, is to be expected. The incentive for populations – and even states – toattempt to move to more favourable territory would then be correspondingly higher.
Humanitarian crises, nationalism, and global governanceParticipants in our exercise considered it extremely likely that climate change would exacerbate humanitariancrises over the coming decades.40 The greater uncertainty was around the extent to which the international
community would have the capability and willingness to respond to these crises in the future. In the scenarios
exercise, participants suggested that the multiple pressures could contribute to a shift towards nationalism,
and away from values associated with human rights, democracy, and cooperative global governance.
Post-game analysis found that as climate conditions had worsened, and the number of regions requiring aid
and humanitarian assistance had increased, in all four instances of the game at least one major developed
country reduced its assistance so as to concentrate on solving its own internal problems, and in someinstances a majority of countries did so. In such cases, countries only turned their gaze outward again when
prompted by refugees or terrorism. Countries that persisted with an internationalist approach suffered anincreasingly unsupportable burden.
A similar risk was identified by the German Advisory Council on Global Change, which argued that, given
how difficult the international community finds dealing with a few failed states in the current climate, theconsequences of high degrees of climate change could over-stretch conventional security policy, and pose arisk to the global governance system as a whole.41
Resource competition and inter-state conflict
We noted above how resource stress, intensified by climate change, could increase the risks of state failure.
Here we consider whether the same stresses could be a factor in inter-state conflict. First, Sarang Shidore
addresses the case of water stress.

A perspective on transboundary water resources and conflict risks
Sarang Shidore, Visiting Scholar, University of Texas at Austin, formerly co-leader of strategic
futures project at the Institute for Defense Studies and Analyses, New Delhi
While many research studies have considered the links between climate change and sub-national
conflict, relatively few have taken on the question of climate-influenced inter-state conflict. Those that
have, have tended to focus primarily on transboundary water resources, drawing on a long history ofinterdependencies and disputes.42 In general, there is little support for the hypothesis of ‘water wars’ - theidea that scarcity necessarily leads to increased armed warfare between states. In fact, a number of studies
have dissected the many cooperative mechanisms that states have voluntarily put in place, even underfraught conditions. Some of these examples are the Indus Water Treaty between India and Pakistan, theMekong River Commission, and treaties and consultations on the use of the Nile river in Africa.
Nevertheless, there is evidence that water scarcity and variability can increase political tensions between
states sharing a common water resource, especially if their relations are poor due to other reasons,and can lead to diplomatic, trade, and other forms of non-military conflict.43 Political tensions overwater have arisen in South Asia with respect to the Indus, Ganges, and Brahmaputra rivers (India,
Pakistan, Bangladesh, and China), in Central Asia with the Syr Darya river (Kyrgyzstan, Uzbekistan, andKazakhstan), and in Africa with the Nile (Egypt, Ethiopia, and Sudan). Other potential sites for water
scarcities enhancing latent interstate tensions include the Jordan river (Israel-Palestine and Jordan),

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the Tigris-Euphrates (Turkey, Syria and Iraq), and a number of rivers in the water-stressed regionsof northern, eastern and southern sub-Saharan Africa such as the Kuito river (Namibia, Angola, andBotswana).
An important caveat to bear in mind is that most of the existing body of research relates to the current
climate. The variabilities, scarcities, and (in some cases) surpluses induced by climate change in, forexample, a 3°C world are likely to be much greater than any recorded in modern history, and could act as
major destabilizing factors at a range well beyond the ambit of existing studies of past resource-conflict
events.

Climate geo-engineering
In two of the four instances of the game, participants decided to invest in climate geoengineering (in the formof solar radiation management) in order to limit global temperature rise. While this was widely perceived ashaving significant risks of its own, participants were balancing these against the increasing risks of loss ofgovernance, national isolation, and resource depletion (food, fuel, and water). Participants considered that
security risks could arise from the fact that there was no recognised authority for decision-making on climate
geoengineering, and no means of preventing unilateral action by a country, region, corporation, or even an
individual.44

Conclusions
Post-game analysis found that in all four games, meeting national requirements for resources including
food, fuel and water became an increasingly high priority — in some cases rising above traditional national
security priorities — as time went on. One action that was noticeably absent from the game was the decision
to use military force to invade a region to gain control of the region’s resources. This may have been related
to the fact that climate change tended to have the most severe impact on the resources of countries thatwere already relatively weak in terms of both military and economic power, reinforcing inequality between
countries.
The game was not resolved in enough detail to investigate the tensions that could arise over specifictransboundary rivers; however, in the scenarios exercise it was considered to be a significant risk that overthe long term, water stress in parts of the Middle East, Central Asia and South Asia could become so direthat the historical trend of water insecurity driving cooperation between conflicting parties could be broken.
It was recognised that desalination could be an important technology, but its high energy cost could prove a
constraint for some developing countries, especially for regions far from the coast.
Participants in the scenarios exercise highlighted the risks linked to the stability or otherwise of global foodmarkets, as Professor Shi Yinhong describes here.

A perspective on global food security and conflict risks
Professor Shi Yinhong, Professor of International Relations, Renmin University of China
The severity of the impact of high degrees of climate change on food production is not well known; neither
is our ability to adapt to it. In a plausible worst case scenario where production does not keep up withgrowing demand, food could become the single most sought-after resource globally. Global markets could
be destabilised, with prices high and volatile. Large fluctuations in price, or constraints on availability,
could contribute to state failure in highly import-dependent countries.
In developed, high-consuming countries, pressure for secure, affordable supply, together with a loss ofconfidence in the markets, would result in a high priority being placed on the security of imports. This
could lead to competition for the leasing or acquisition of arable land in developing countries, with
contracts being enforced, where necessary, with both soft and hard national power. The risk of conflict
would be significant in situations where the developing countries themselves faced shortfalls. At thesame time, the importance of overseas assets to food security would lead great powers to invest more in
defending strategic trade routes, which could themselves become subject to military confrontation.
None of these events are inevitable. While history contains many examples of hardship leading toaggression, examples of the contrary may be found as well – particularly in Asian nations, where
Buddhism and Daoism are widely practiced. It is possible that the hardship of future climate change could
lead to greater international cooperation in addressing common problems, and a positive transformationof the global political culture. However, from today’s perspective, few would have firm confidence in suchan outcome.

Participants in our exercise acknowledged the deep uncertainty involved in any attempt to consider how
human society and civilization might develop even a few decades into the future. Technological development,
and the future of governance at the national and global levels, were both identified as particularly importantunknowns.
Certain areas of technology were identified that could have a direct bearing on some of the risks: rates ofprogress in desalination of water, breeding or modification of crops, and renewable energy with storage
technologies would all be likely to affect relative levels of resource stress, and the risks that could arise
from such stress. In addition, there were the ‘unknown unknowns’ of future technologies that have not yet
been invented. The contribution of any technologies to mitigate the risks would depend not only on rates
of progress, but also on the equity or otherwise of their availability for use. This would depend in turn on
governance.
Governance would play a critical role in determining whether systems broke down or remained resilient
under stress. Participants in our exercise felt that beyond the familiar distinctions between democracy anddictatorship, centralised or decentralised, nationalist or internationalist, there were possibilities for future
models of governance to emerge which, like unknown technologies, have yet to be imagined. The relative
importance of markets, militaries, religions, states, alliances, regional associations, and global structures
could all change.
Despite the depth of these uncertainties, it was recognised that the human economy existed within the natural
physical environment, and could not be separated from it. Climate change would subject many parts of thatenvironment to intense pressure, and create stresses that would be difficult for any system of governance tomanage. As levels of stress increased, so would the scale of the systems at risk – from city infrastructure, tostate governments, to international systems of transport and trade. The risk of disruption was considered
likely to be very significant even at low degrees of climate change, and likely to increase in a non-linear
manner as climate change progressed to higher degrees, or at a faster rate.

Endnotes
1.

2.

3.

4.

5.
6.
7.

As suggested by PwC (2013). International threats and opportunities of climate change for the UK http://www.
pwc.co.uk/sustainability-climate-change/publications/international-threats-and-opportunities-of-climatechange-to-the-uk.jhtml
Hoerling et al. (2012), ‘On the Increased Frequency of Mediterranean Drought’ Journal of the American
Meteorological Society – see also NOAA press release: http://www.noaanews.noaa.gov/stories2011/20111027_
drought.html
Kelley, C.P., Mohtadi, Shahrzad M., Cane, M. A., Seager, R. and Kushnir,Y. (2015). ‘Climate change in the Fertile
Crescent and implications of the recent Syrian drought’. Proceedings of the National Academy of Sciences 11,
pp. 3241 - 3246
Femia, F. and Werrell, C. (2012). ‘Syria: Climate Change, Drought and Social Unrest’. The Center for Climate
and Security. http://climateandsecurity.org/2012/02/29/syria-climate-change-drought-and-social-unrest/ ;Femia, F. and Werrell, C. (eds.) (2013). The Arab Spring and Climate Change. Center for American Progress,
The Center for Climate and Security, Stimson. https://climateandsecurity.files.wordpress.com/2012/04/
climatechangearabspring-ccs-cap-stimson.pdf ; Gleick, P. (2014) Water, Drought, Climate Change, and Conflictin Syria Weather, Climate and Society 6, 331 – 340, available at http://journals.ametsoc.org/doi/abs/10.1175/
WCAS-D-13-00059.1
Femia, F. and Werrell, C. (eds.) (2013). pp25-27
Otto, F. E. L., N. Massey, G. J. vanOldenborgh, R. G. Jones, and M. R. Allen (2012), Reconciling two approaches
to attribution of the 2010 Russian heat wave, Geophys. Res. Lett.,39, L04702, doi:10.1029/2011GL050422.
Femia, F. and Werrell, C. (eds.) (2013). p7

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8. Porter, J.R., L. Xie, A.J. Challinor, K. Cochrane, S.M. Howden, M.M. Iqbal, D.B. Lobell, and M.I. Travasso (2014).
‘Food security and food production systems’. Climate Change 2014: Impacts, Adaptation, and Vulnerability.
Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, pp. 485-533.
9. Sternberg, T. (2011) ‘Regional drought has a global impact‘. Nature 472, 169. doi:10.1038/472169d
10. Femia, F. and Werrell, C. (eds.) (2013).
11. Femia, F. and Werrell, C. (eds.) (2013). pp12-13
12. Femia, F. and Werrell, C. (eds.) (2013). pp17-19
13. Mahr, K. (2011). ‘Bread is life: food and protest in Europe’. Time. http://science.time.com/2011/01/31/bread-islife-food-and-protest-in-egypt/
14. Hendrix, S. and Saleyhan, I. (2012) ‘Climate change, rainfall, and social conflict in Africa’. Journal of Peace
Research 49, pp. 35-50
15. Butler, C.K and Gates, S. (2012). ‘African range wars: climate, conflict, and property rights’. Journal of Peace
Research 49, pp. 23-34.
16. Hsiang, S. M. and Burke, M. (2013). ‘Climate, conflict, and social stability: what does the evidence say?’ Climatic
Change 123, pp. 39-55
17. Hsiang, S. M., Burke, M. and Miguel, E. (2013). ‘Quantifying the influence of climate on human conflict’. Science
341, pp. 1212 - 1226
18. National Intelligence Council (2012). Global Trends 2030: Alternative Worlds. https://globaltrends2030.files.
wordpress.com/2012/11/global-trends-2030-november2012.pdf
19. For example: The U.S. Department of Defense’s Quadrennial Defense Reviews in 2010 and 2014 identify
climate change as an “accelerant of instability” and a “threat multiplier,” respectively (http://www.defense.
gov/QDR/QDR%20as%20of%2029JAN10%201600.pdf and http://www.defense.gov/pubs/2014_Quadrennial_
Defense_Review.pdf). The UK’s 2010 Strategic Defense and Security Review (HM Government (2010). Securing
Britain in an Age of Uncertainty: The Strategic Defence and Security Review https://www.sis.gov.uk/media/
pdfs/strategic-defence.pdf) acknowledged the security risks of a changing climate, specifically tasking the
FCO with addressing the issue. Australia’s 2013 National Security Strategy (Australian Government (2013).
Strong and Secure: A Strategy for Australia’s National Security http://apo.org.au/files/Resource/dpmc_
nationalsecuritystrategy_jan2013.pdf ) stated that: “The more severe effects of climate change, in particular
the increase in frequency and severity of natural disasters, compounded by competition over scarce natural
resources, may contribute to instability and tension around the globe, especially in fragile states.”
20. IPCC (2014). ‘Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral
Aspects.’ Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L.
Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, p. 779
21. The aim being to assess something close to a ‘plausible worst case’ rather than the most likely case.
22. Based on data from UN Statistics Division (2011) – Environmental Indicators: Inland Water Resources. http://
unstats.un.org/unsd/ENVIRONMENT/waterresources.htm The thresholds of 1,700 and 1000 m3 of freshwater
resources per capita per year are often used to indicate water stress and water scarcity respectively, as
proposed by Malin Falkenmark in 1989. They are based on estimated minimum requirements for household,agricultural and industrial use.
23. See Figure 3-4, IPCC, (2014). Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and
Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M.
Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea,
and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1132 pp.
24. See section 7.1.2 of Porter, J.R., L. Xie, A.J. Challinor, K. Cochrane, S.M. Howden, M.M. Iqbal, D.B. Lobell, and M.I.
Travasso (2014). ‘Food security and food production systems. Climate Change 2014: Impacts, Adaptation, andVulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach,
M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy,
S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United
Kingdom and New York, NY, USA, pp. 485-533.
25. United Nations, Department of Economic and Social Affairs, Population Division (2013). World Population
Prospects: The 2012 Revision. Available at http://esa.un.org/wpp/Documentation/publications.htm
26. Combing population data and projections from UN Population Division – World Population Prospects, the2012 Revision, with arable land per capita data from the Food and Agriculture Organization as shown at http://
data.worldbank.org/indicator/AG.LND.ARBL.HA.PC, and using 0.07 Ha per capita as the definition of extreme
land stress, as used by UN organizations – see http://www.un.org/en/events/environmentconflictday/pdf/
GN_Renewable_Consultation.pdf and http://www.fao.org/fileadmin/templates/solaw/files/thematic_reports/
TR_02_light.pdf
27. See Executive Summary of Niang, I., O.C. Ruppel, M.A. Abdrabo, A. Essel, C. Lennard, J. Padgham, and P.
Urquhart, (2014). ‘Africa’. Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional
Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L.
Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1199-1265.

28. German Advisory Council on Global Change (2007). World in transition: climate change as a security risk.
http://www.wbgu.de/fileadmin/templates/dateien/veroeffentlichungen/hauptgutachten/jg2007/wbgu_
jg2007_kurz_engl.pdf
29. US Department of Defense - Quadrennial Defense Review 2014 – as cited in CNA Military Advisory Board(2014). National Security and the Accelerating Risks of Climate Change, p2 https://www.cna.org/sites/default/
files/MAB_2014.pdf
30. Myers, N. (2005). “Environmental Refugees: An Emergent Security Issue.” Presented at the 13th Economic
Forum, Prague, 23-27 May; and Christian Aid (2007). Human tide: the real migration crisis. London: ChristianAid.
31. Population Action International (2010). Climate Change, Migration, and Population Growth. http://pai.org/wpcontent/uploads/2012/01/climatemigration.pdf
32. The CNA Corporation (2007). National Security and the Threat of Climate Change. Alexandria, Virginia: The
CNA Corporation.
33. High Representative and the European Commission (HREC) (2008). Climate Change and International
Security, Paper to the European Council, S113/08, p. 4.
34. Guterres, A. (2009). ‘Climate change, natural disasters and human displacement: a UNHCR perspective.’ The
United Nations Refugee Agency.
35. Reuveny, R. (2007). ‘Climate Change-Induced Migration and Violent Conflict’. Political Geography 26, 656-673.
36. Reuveny, R. (2007). ‘Climate Change-Induced Migration and Violent Conflict’. Political Geography 26, 656-673.
37. Government Office for Science (2011). Migration and Global Environmental Chaneg: Future Challenges andOpportunities. The Government Office for Science, London. Retrieved from http://www.bis.gov.uk/assets/
foresight/docs/migration/11-1116-migration-and-global-environmental-change.pdf
38. Gemenne, F. (2011). ‘Why the numbers don’t add up: a review of estimates and predictions of people displacedby environmental changes’. Global Environmental Change 21, S41-S49. Cited in Smith, K.R., A. Woodward, D.
Campbell-Lendrum, D.D. Chadee, Y. Honda, Q. Liu, J.M. Olwoch, B. Revich, and R. Sauerborn (2014). ‘Human
health: impacts, adaptation, and co-benefits’. Climate Change 2014: Impacts, Adaptation, and Vulnerability.
Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, pp. 709-754.
39. Campbell, K. M., Gulledge, J. McNeill, J. R., Podesta, J. Ogden, P., Fuerth, L. Woolsey, J., Lennon, A. T. J., Smith,J., Weitz, R., Mix, D. (2007). The Age of Consequences: The Foreign Policy and National Security Implicationsof Global Climate Change. Center for Strategic and International Studies, Center for a New American Security.
http://csis.org/files/media/csis/pubs/071105_ageofconsequences.pdf
40. Ministry of Defence (2014). Strategic Trends Programme: Global Strategic Trends – Out to 2045. https://www.
gov.uk/government/uploads/system/uploads/attachment_data/file/348164/20140821_DCDC_GST_5_Web_
Secured.pdf
41. German Advisory Council on Global Change (2007). World in transition: climate change as a security risk.
http://www.wbgu.de/fileadmin/templates/dateien/veroeffentlichungen/hauptgutachten/jg2007/wbgu_
jg2007_kurz_engl.pdf
42. Bernauer, T. and Kalbhenn, A. (2010). ‘The Politics of International Freshwater Resources,’ in Denemark, R. A.
(ed.) International Studies Encyclopedia (Hoboken, NJ: Wiley-Blackwell), 5800-5821.
43. Devlin, C. and Hendrix, C. S. (2014). ‘Trends and Triggers Redux: Climate Change, Rainfall, and Interstate
Conflict’. Political Geography 43, 27-39. Also see Bernauer, T. and Siegried, T. (2012). ‘Climate Change andInternational Water Conflict in Central Asia’. Journal of Peace Research 49, 227-239.
44. Ghosh, A. (2014) “Environmental Institutions, International Research Programmes, and Lessons for
Geoengineering Research”. Geoengineering Our Climate Working Paper, 25 February. Available at: http://
geoengineeringourclimate.com/2014/02/25/environmental-institutions-international-research-programmesand-lessons-for-geoengineering-research-working-paper/.

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VALUE

A

s we noted at the beginning, risk is as the effect of uncertainty on our objectives, or on thethings that we value. The previous three parts of this risk assessment have described someof the effects of climate change. It remains to discuss how we value those effects – how much we
care about them.

In this section, we start with economics, and consider what it can and cannot tell us about the value of therisks of climate change. Then we provide one person’s perspective on the ethical issues at stake. Finally, forthe sake of transparency, we give our own opinion.

22 ECONOMICSThe Stern Review is probably the most famous report on the economics of climate change. It is mostwidely cited for its conclusion that action on climate change would cost far less than inaction. The costs ofaction were estimated at some 1-2% of global GDP per year , and the costs of inaction were estimated tobe equivalent to losing something in the region of 5-20% of global GDP each year.1
What is less widely recognized about the Stern Review is that it framed climate change primarily as aquestion of risk management. The cost-benefit comparison described above was only one of four options it
presented for ways people could compare the values of action and inaction, and take decisions. The others
were:i. An approach focused on reducing the probability of undesired outcomes (such as a certain degreeof temperature increase) to a tolerably low level. The cost of reducing the probability was assessed
quantitatively, but the risks associated with the undesired outcomes were not. (This was the approachtaken in most sections of the Review).
ii. A comparison of the implications of action and inaction for the prospects of long-term economic growthand development.
iii. A comparison of the nature of the world and the quality of life that would arise with action or inaction on
climate change.
The Stern Review’s section on economic modeling acknowledged the difficulties and shortcomings
inherent in its attempt to estimate the aggregate costs of climate change, and urged readers to ‘avoid
an over-literal interpretation of these results’. Only two years after the Review’s publication, its author
Nicholas Stern wrote: “Looking back, I think the Stern Review assumptions led to an underestimationof the costs of inaction whichever of these four approaches are adopted.” 2 In 2013, Stern went further:in a paper entitled The Structure of Economic Modeling of the Potential Impacts of Climate Change:
Grafting Gross Underestimation of Risk onto Already Narrow Science Models,3 he concluded that: “Itis vital that we treat policy analysis as that of a risk-management problem of immense proportions
and discuss risks in a far more realistic way... Many scientists are telling us that our models
are, grossly, underestimating the risks. In these circumstances, it is irresponsible to act as if theeconomic models currently dominating policy analysis represent a sensible central case.”

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Quite apart from the question of underestimation of risk, a deeper problem has been apparent ever since
the earliest attempts to estimate the economic costs of climate change. This is that such estimates, if notpresented transparently, may be misinterpreted as being purely objective, while important subjective value
judgments implicit in them could be obscured.4
Here the economist Cameron Hepburn explains what the limitations of economic cost estimates are, why
economic models tend to underestimate the risks of climate change, and how this relates to decision-making.

Analysis of IAMs suggests that the carbon price can vary quite strongly on the specific response of
ecosystems to temperature rises. As just one example, modelling by Ceronsky et al with FUND, a fairly
standard IAM, suggests that if the thermohaline circulation were to shut down, the corresponding social
cost of carbon (SCC) could increase to as much as US $1,000/t CO2. In short, the applicable social cost ofcarbon is very difficult to pin down because of the wide array of risks that could occur from our meddlingwith the climate system.

2. Value judgments cannot be avoided
Dangerously incomplete climate economic models
Cameron Hepburn, Professor of Environmental Economics, University of Oxford

Introduction
The economic analysis of climate change has had a real influence on the formulation of climate policy.
So it ought to. Climate policy cannot be determined solely by scientists, because there are interests andvalues at stake that go well beyond climate science. Moral philosophers can help us think through someof the relevant issues, including how to account for lives saved, and the trade-offs between the interests ofdifferent people and across different generations. And at some point there must be a reckoning of somekind in order to answer questions for decision makers. Should climate action be more or less ambitious?
What are the various advantages and disadvantages of different policy interventions? Who can, should andwill pay? How ought the inherent risk and ambiguity be evaluated? These are political questions in which
economic and political analysis is unavoidable.
A standard and influential tool used by economists for conducting the reckoning is the Integrated
Assessment Model (IAM). Such models often start with a baseline economic scenario that incorporates anassumed level of emissions. The models then consider the costs and benefits, at the margin, of reducing
emissions to limit the damages that might result from climate change. In other words, the marginal costsof abating a tonne of carbon dioxide emissions are estimated and compared with estimates of the marginal
social damage inflicted by that tonne. The latter is also referred to as the ‘social cost of carbon’ (SCC).
When the process of estimating the SCC arrives at a single number for application to policy, without any
further questions, there is a real risk that decision makers will be misled. It is only natural for a decision
maker to want to know what ‘the number’ is, and some decisions do effectively require such a number.
But there is so much scientific uncertainty in determining the social cost of carbon that any single number
implies a false precision, as discussed in section 1 of this paper. The scientific uncertainty sits alongsidechoices that are ethically contentious, addressed in section 2. And the numbers themselves may be biased,
as noted in section 3.
Nevertheless, such numbers are nevertheless estimated and used. For instance, the United States Officeof Management and Budget and Environmental Protection Agency have conducted a joint analysis of theappropriate social cost of carbon for use in government policy. A high profile and influential group of
economists emerged at the answer that the social cost of carbon was US $36/t CO2. Eighteen months later,
after a period of reflection, the number was updated to US $37/t CO2, where it stands today in June 2015.
The rest of this short paper considers the three key points to bear in mind when interpreting and using
such SCC estimates.

1. Scientific uncertainty about damages remains substantial
While the relationship between carbon dioxide emissions and increases in global mean temperatures
are now fairly well understood, the uncertainty over the specific impacts in specific places at specifictimes remains substantial. Economists have, primarily for convenience, proxied the relationship between
aggregate damages and temperature with a simple ‘damages function’. This expresses the fraction of
GDP lost in a given year due to the relevant increase in temperature. Damages are often assumed, forconvenience, to be a quadratic function of temperature increase. So it is assumed that damages increasesmoothly as temperatures rise, with no abrupt shifts. There are, of course, other possible damages
functions, and the evidence from the physical sciences suggests that functions with thresholds and triggers
are far from ruled out.

Even if we were able to isolate and eliminate all scientific uncertainty in the chain of linkages between
emissions, concentrations, temperatures and economic impacts, it would remain impossible to credibly
specify a single estimate for the social cost of carbon. This is because a range of unavoidable social value
judgments must be made in order to derive any estimate. These value judgments arise in a range of areas,
but the four most contentious and important relate to valuing:• Impacts on future people: The weight placed on impacts in the distant future, compared to impacts
today, is reflected in the discount rate. This was one of the most contested parameters following the
publication of the Stern Review, which used a lower discount rate than previous studies, and in part forthat reason concluded that the social cost of carbon was substantially higher.
• Risk preferences: Value judgements about risk preference are important too, given the risks involved inallowing Earth’s climate to heat. Higher aversion to risk tends to imply a higher social cost of carbon.
• Inequality preferences: It is expected that the impacts of climate change will fall more harshly upon
the poor than the rich. How to value these effects strongly depends upon the assumed aversion toinequality.
• Human lives: Because climate change is expected to lead to a large number of deaths, the monetaryvaluation of a human life, if used, comprises a significant uncertainty in the overall estimate of thesocial cost of carbon.
These various value judgments have been debated at length by the economists and philosophers who workon the integrated assessment modelling of climate change. This is not the place to rehearse those arguments
in detail. However, it is worth noting that the use of market prices and market data – such as using
market interest rates for government bonds as a proxy for the social discount rate – does not avoid thesephilosophical questions. The very decision to use the market is itself a (contested) philosophical choice.

3. Omission bias may lead to misleadingly low estimates
Finally, just as important as the scientific uncertainty and the inevitability of value judgments in SCC
estimates is the concern that estimates emerging from IAMs may be systematically biased. The main
source of concern is that IAMs only model the effects that they are capable of modelling. The implicationis that a wide range of impacts that are uncertain or difficult to quantify are omitted.
It is likely that many of these impacts carry negative consequences. Indeed, some of the omitted impacts
may involve very significant negative consequences, including ecosystem collapse or extreme events suchas the catastrophic risks of irreversible melting of the Greenland ice sheet with the resulting sea level rise.
Other consequences – such as cultural and biodiversity loss – are simply very difficult to quantify andare hence just omitted. While it is also likely that some omitted climate impacts are positive, it is highly
probable that on balance such omitted impacts are strongly negative, leading to SCC estimates that aresystematically too low, and corresponding policy on climate change to be too weak. Indeed, the United
Nations’ IPCC assessment reports themselves accept that their own estimates should be viewed as being
conservative, consistent with the prevailing culture of scientific enquiry.

Conclusion
Some scholars have concluded that given these limitations, IAMs are damaging or, at best, useless. It
should certainly be openly and loudly acknowledged that estimates of the social cost of carbon are highly
uncertain, subjective and potentially biased. Estimates should be accompanied with a corresponding

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warning of these weaknesses and advice to take any particular estimate with a grain of salt.
But not having models is not a solution either. Ignoring the intellectual challenges that are intrinsic tothe economics of climate change does not make them vanish. Instead, economists need to do better,with much more transparent models – where value judgments and uncertainties are clear and can beplayed around with by policymakers and the general public – and where wide ranges are employed tocommunicate the sensitivities involved.
Along with transparency, a new generation of IAMs could focus our attention in more useful directions,away from short-term marginal changes and instead towards systemic, transformational change. Rather
than devising policy to balance central estimates of the social cost of carbon and central estimatesof abatement costs, it may be better to seek interventions aimed at two objectives: (i) reducing theprobabilities of very bad outcomes to very low levels, even if this involves relatively high cost; and (ii)increasing the probabilities of a positive transformational ‘surprise’ – for instance a cost breakthrough inclean technology – that could deliver very large social gains.
Determining a central estimate of the SCC does not prevent thinking about transformational change.
However, an exclusive focus on the mean SCC tends to direct policy towards a set of interventions
involving marginal, incremental changes to the existing system. Given the risks, and the potential benefitsof a transition, incremental change is clearly far from enough. Instead, IAMs ought to help decision
makers to consider major disruptive change. Far from being ‘in the tails of the distribution’, disruptivechanges to our natural ecosystems and to our industrial ecosystems are now almost inevitable.

In contrast, we are now deep into the ‘anthropocene’ – the new period in which human activities havea significant global impact on the Earth’s ecosystems. The pressures of a growing human populationand economy, on land and on water, are already high. Humans appropriate around 40% of the world’s
biomass, and that fraction is growing. Extinction rates of plants and animals are rising. On top of this,comes climate change. As the preceding chapters of this report have shown, the risks of climate change
are immense. And unlike those natural disasters with which we are familiar, the risks of climate change are
growing, and will continue to grow over time.
When scientists conduct investigations into facts, they must be as objective as possible. It is on the basisof this objectivity that their authority rests. But when scientists engage in discussion on the economic,
social and ethical aspects of any issue, they speak as citizens and not as experts, and they have noparticular authority. It is important that this distinction is clearly made, so that there is no confusion
between what is science, and what is opinion. So I wish to be clear that with regard to what follows, I am
speaking not as a scientist, but as a concerned citizen.
In my opinion, the present disagreements about climate policy stem less from differences about thescience than from differences in ethics – in particular, in the extent to which we should feel obligations
towards future generations. Those who value the risks of climate change by applying a standard discountrate to estimates of future costs are in effect writing off whatever happens beyond 2050. There is indeed
little risk of catastrophe within that time-horizon, so unsurprisingly such analysis concludes that tacklingclimate change should be given a low priority compared to other public policy aims.

23 ETHICS

But a child born this year could quite possibly live beyond the year 2100. The grandchildren of young
adults alive today could live through several decades of the twenty-second century. Anyone who cares
about those generations, or others further into the future, will deem it worth making an investment now toprotect them from the worst-case scenarios. This is the most compelling argument for acting on climatechange.

Among the ‘wide range of impacts that are uncertain or difficult to quantify’, which Hepburn warns against,
and which are hence likely to be omitted from most economic estimates, are many of the systemic risks
described in the previous section of this report. As we discussed, these risks include state failure, mass
displacement of people, and conflict.

To consider an analogous situation, suppose astronomers had tracked an asteroid, and calculated that it
would hit the Earth in 2080, sixty-five years from now – not with certainty, but with, say, 10% probability.
Would we relax, saying that this is a problem that can be set on one side for fifty years – as people will bythen be richer, and it may turn out that it misses the Earth anyway? I do not think we would. There would
surely be a consensus that we should start straight away and do our damndest to find ways to deflect it, or
mitigate its effects.

It is worth noting that such events rarely have a noticeable impact on global GDP, even in the worst cases.
Both the First World War and the Second World War had a negligible impact on global GDP, despite causing
considerable economic damage in some countries.5 China’s great famine of 1958-1962 reduced its own GDP
by around 5% per annum,6 but since China only contributed around 5% of global GDP at that time,7 theimpact on global output was virtually unnoticeable. Naturally, all of these events are remembered not for theirimpact on GDP, but for the fact that tens of millions of people died.
The point is that the impacts of climate change omitted from economic estimates may not be marginal. They
may be, in human terms, very large. Their valuation is, in large part, a question of ethics. We present herea personal perspective on the ethics of climate change from the host of the final meeting that informed thisreport, Professor Martin Rees.

A personal perspective on the ethics of climate change
Professor Martin Rees, Astronomer Royal, Former President of the Royal Society, Emeritus
Professor of Cosmology and Astrophysics, University of Cambridge
I am an astronomer. I am mindful that our Earth is 45 million centuries old, but that this century is special.
It is the first century when one species – ours – can determine the fate of the biosphere. That is because ofanthropogenic stresses to ecosystems, and the unintended downsides of advanced technologies.
Throughout the centuries, we have been vulnerable to natural threats, such as earthquakes and volcanoes.
But there is one reassuring thing about these threats: their annual probability is not changing much – it
was more or less the same for the Neanderthals (although, of course, their economic consequences aremuch bigger for us now).

A second ethical issue concerns our obligations to people who are remote not in time but in economic
opportunity. It is widely recognized that climate change will hit the hardest those who have contributed theleast to its cause. Heat stress will most hurt those without air conditioning; crop failure will most affectthose who already struggle to afford food; extreme weather events will most endanger those whose homes
are fragile. A decision not to act on climate change is a decision to inflict suffering on a grand scale.
Finally, a third ethical issue concerns the non-human environment. We know that humans are alreadythreatening biodiversity, and that this threat is aggravated by climate change. The level of extinctions
could eventually be comparable to the five mass extinction events in the Earth’s history, of which we havelearned through the fossil record. As some have said, we are destroying the book of life before we haveread it. Clearly, in some cases this has a direct effect on our interests: if fish stocks dwindle to extinction,
we lose a source of food. There are plants in the rain forest whose genes may be useful for medicine. But
for some, this is too anthropocentric a view: biodiversity – life – has a value of its own. To quote the great
ecologist E O Wilson, if our despoliation of nature causes mass extinctions, “it is the action that future
generations will least forgive us for”.
For many people, religious faith is a source of guidance on questions of ethics. As a believer in aconstructive dialogue between science and religion, I have attended meetings to discuss climate changewith Faith leaders at the Vatican. After one such meeting, a group of religious leaders, political leaders,business leaders and scientists released a statement, declaring that the ‘decisive mitigation’ of climatechange was ‘a moral and religious imperative for humanity’.8 People of many different faiths who havethought about climate change have reached this same conclusion.
For me, being there in the Vatican, seeing through the window the dome of St Peter’s Basilica, inspired
some further reflections. Europe’s great cathedrals still overwhelm us today. But think how they seemedat the time they were built – and the vast enterprise their construction entailed. Most of the builders new

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little of the world beyond their own villages. Even the most educated knew of essentially nothing beyond
Europe. They thought that the world was a few thousand years old - and that it might not last anotherthousand. But despite these constricted horizons, in both time and space, despite the deprivation andharshness of their lives, despite their primitive technology, they built these huge and glorious buildings- pushing the boundaries of what was possible. Those who conceived these masterpieces, and those who
built them, knew they would not live to see them finished. Their legacy still elevates our spirits, centuries
later.
What a contrast to so much of our discourse today! Unlike our forebears, we know a great deal about ourworld - and indeed about what lies beyond. New technologies enrich our lives and our understanding.
Many phenomena still make us fearful, but the advance of science spares us from irrational dread. We
know that we are stewards of a precious ‘pale blue dot’ in a vast cosmos - a planet with a future measured
in billions of years - whose fate depends on humanity’s collective actions in the course of this century.
In today’s fast-moving world, we cannot aspire to leave a monument lasting a thousand years, but it issurely shameful to persist in policies that deny future generations a fair inheritance and instead leave themwith a depleted and more hazardous world. To design wise policies, we need all the efforts of scientists,
economists and technologists, and the best knowledge that the 21st century can offer. But to implement
them successfully, we need the full commitment of political leaders and the full support of the voting
public. Our responsibility – to our children, to the poorest, and to our stewardship of the diversity andrichness of life on Earth – surely demands nothing less.

Our own opinion
On matters of value, no expert, no leader, and no academic discipline has a monopoly of wisdom. It is for allof us to make up our own minds, having reviewed the evidence.
As the lead authors of this report, we have no more authority to judge the value of the risks of climate change
than anyone else, but we give our own opinion here for the sake of transparency and for the avoidance of
doubt.
We said at the beginning that we assumed our common objectives were human prosperity and security. It isclear to us that the risks posed by climate change to these objectives are very great. We are deeply concerned
about what this means for the future of our families, our countries, and our civilization, all of which we careabout. At present, our exposure to these risks is far higher than we would wish to tolerate.
We do not believe the situation is hopeless. On the contrary, there is much that we can do. The risks ofclimate change cannot be entirely eliminated, but they can certainly be reduced. So in the final section of ourreport, we offer some thoughts on how we can strengthen our efforts towards achieving that goal.

Endnotes
1.

2.
3.

4.

5.

6.
7.

8.

Stern, N. (2007). The Economics of Climate Change – The Stern Review. Available at http://www.cambridge.
org/us/academic/subjects/earth-and-environmental-science/climatology-and-climate-change/economicsclimate-change-stern-review?format=PB
Stern, N. (2009). The Global Deal: Climate Change and The Creation of a New Era of Progress and Prosperity.
PublicAffairs, New York. p.94
Stern, N. (2013). ‘The Structure of Economic Modeling of the Potential Impacts of Climate Change: Grafting
Gross Underestimation of Risk onto Already Narrow Science Models’. Journal of Economic Literature 51(3),838–859. Available at http://personal.lse.ac.uk/sternn/128nhs.pdf .
Funtowicz, S. O. and Ravetz, J. R. (1994). ‘The worth of a songbird: ecological economics as a postnormal science’. Ecological Economics10, 197-207. Available at http://www.nusap.net/downloads/
funtowiczandravetz1994.pdf
Jaeger, C., Schellnhuber, H.J., and Brovkin, V. (2008). ‘Stern’s Review and Adam’s Fallacy’. Climatic Change.
DOI 10.1007/s10584-008-9436-7. Available at http://www.eci.ox.ac.uk/publications/downloads/schellnhuber08stern-and-adam.pdf
Stern N. (2013).
See Figure 12 in Keller, W., Li, B. and Shiue, C.H. (2013). ‘Changhai’s Trade, China’s Growth: Continuity, Recovery,
and Change since the Opium Wars’. IMF Economic Review 61, 336-378. Available at http://www.palgravejournals.com/imfer/journal/v61/n2/fig_tab/imfer201310f12.html
Vatican Radio (2015). ‘Final Declaration on Workshop on Climate Change’. Vatican Radio. Available at http://
en.radiovaticana.va/news/2015/04/28/final_declaration_on_workshop_on_climate_change/1140356

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of risk described by the IPCC as ‘reasons for concern’, made it possible to see that later assessments had
estimated the risks to be larger than earlier assessments had. Defining a set of such indicators in relation
to systemic risks might be more difficult, but should not be impossible. In all three areas, the use of such
indicators could significantly help decision-makers reach their own judgments when confronted by a wide
range of differing expert opinions.

When we began the process of meetings that led to the production of this report, we started out with thesense that there was scope to improve the way that the risks of climate change are assessed, and the way thatsuch assessments are communicated to decision-makers. Our series of meetings reinforced that sense, andprovided a few suggestions for how such an improvement might be brought about.

This risk assessment has attempted to give a rough idea of the big picture, but it has certainly not aimed tobe comprehensive, and it is far from perfect. We hope that these recommendations may be useful when morecomprehensive risk assessments are undertaken in future.
Our first recommendation is that the general principles of risk assessment should be applied to
the greatest extent possible. The principles we have attempted to apply were described in detail in the
introduction. To summarize them briefly here, they were:i. Assess risks in relation to objectives, or interests. Start from an understanding of what it is that wewish to avoid; then assess its likelihood.
ii. Identify the biggest risks. Focus on finding out more about worst-case scenarios in relation to longterm changes, as well as short-term events.
iii. Consider the full range of probabilities, bearing in mind that a very low probability may correspondto a very high risk, if the impact is catastrophic.
iv. Use the best available information, whether this is proven science, or expert judgment. A best
estimate is usually better than no estimate at all.
v. Take a holistic view. Assess systemic risks, as well as direct risks. Assess risks across the fullrange of space and time affected by the relevant decisions.

Our second recommendation is to broaden participation in the risk assessment process. It follows
from our first principle of risk assessment that leaders and decision-makers have a role not only as recipientsof a completed risk assessment, but also at the beginning of the process, in defining the objectives andinterests against which risks should be assessed. This will enable scientists and other experts to ensure theirassessments are as relevant as possible.
Scientists naturally have the lead role in understanding climate change and its direct impacts. At the sametime, experts in politics, technology, economics, and other disciplines can provide information relevant to the
future of global emissions, and the indirect impacts of climate change as it interacts with human systems.
To ensure that the most relevant information or uncertainties are communicated to decision-makers, it canbe helpful to make a distinction between information gathering and risk assessment. Information gathering
activities, such as primary scientific research, may be free to collect whatever is useful or interesting. Risk
assessment has to interrogate that evidence in relation to defined objectives, and according to a specific setof principles. Separating these tasks may allow both to be carried out more effectively. Such a separationof tasks is often made within intelligence agencies. Climate change risk assessments could benefit frominvolving not only scientists, but also those for whom risk assessment is a central part of their professional
expertise. Qualified individuals could be drawn from fields such as defence, intelligence, insurance, and publichealth.
Our third recommendation is that a climate change risk assessment should report to the highest
decision-making authorities. A risk assessment aims to inform those with the power to reduce or manage
the risk. Assessments of specific, local, or sectoral risks of climate change may be directed at those withspecific, local or sectoral responsibilities. Assessments of the risk of climate change as a whole should report
directly to those with responsibility for governance as a whole. At the national level, this is means the headof government, the cabinet, or the national security council. At the global level, it means institutions where
heads of government meet to make decisions.
The aim of improving risk assessment is, of course, to better inform decision-making on risk reduction. Risk
reduction – and the question of what might be a proportionate response to the risks of climate change – is the
subject of the final part of this report.

vi. Be explicit about value judgments. Recognize that they are essentially subjective, and present themtransparently so that they can be subject to public debate.
As we have argued, an assessment of the risks of climate change must apply these principles to at least threeareas: the future pathway of global emissions; the direct risks arising from the climate’s response to thoseemissions; and the risks arising from the interaction of climate change with complex human systems. The last
principle listed above applies particularly to the fourth stage: the valuation of risks.
Risk assessments need to be made on a regular and consistent basis, so that in areas of uncertainty, anychanges or trends in expert judgment are clearly visible over time. In all three areas mentioned above, this
could be facilitated by the identification and use of a consistent set of metrics or indicators. The International
Energy Agency has recently suggested a set of ‘high-level metrics to track energy sector decarbonisation’,including indicators such as public and private investment in low carbon energy research, development anddeployment, and the carbon intensity of total transport fuel demand.1 These and other similar indicators could
help inform future assessments of the relative likelihood of different global emissions pathways. Regarding
the climate’s response to emissions, various such indicators are already in use. Part II of our risk assessment
noted that the use of consistent metrics for estimates of future global sea level rise, and for the categories

Endnote
1.

These and other similar indicators could help inform future assessments of the relative likelihood of different
global emissions pathways. Regarding the climate’s response to emissions, various such indicators are alreadyin use. Part II of our risk assessment noted that the use of consistent metrics for estimates of future global
sea level rise, and for the categories of risk described by the IPCC as ‘reasons for concern’, made it possible tosee that later assessments had estimated the risks to be larger than earlier assessments had. Defining a set ofsuch indicators in relation to systemic risks might be more difficult, but should not be impossible. In all threeareas, the use of such indicators could significantly help decision-makers reach their own judgments whenconfronted by a wide range of differing expert opinions.

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PROPORTIONATE RESPONSE

Arisk assessment is a way to better understand the problems we face; it does not necessarilytell us how to solve them. It may, however, help us think about proportionality – to answer
the question: ‘what would be a proportionate response to the risk?’This was the question we discussed in the last of the series of meetings that informed this report, heldin London in April 2015. Most of the participants in that meeting had expertise in one of three areas:
technology, finance, and politics. Here we present a perspective on the nature of a proportionate response to
the risk of climate change in each of those three areas, before ending with some closing thoughts.

24 TECHNOLOGY
Sir David King,i Professor Jim Skea,ii Professor Tim Green.iii
Energy produced without greenhouse gas emissions needs to become less costly than production based oncoal, gas or oil. Clean energy must be able to compete directly on cost before a world-wide roll-out can be
expected, particularly in the emerging and developing economies. The challenge needs to be a major focus
for scientists and engineers. Progress in technology is happening at an impressive rate, but it is not yet fastenough to meet the 2°C target with reasonable probability.

The technologies we need to progress
There is no single magic bullet. We need progress in six major areas. First, there are the three main typesof clean energy supply: renewables (especially solar and wind), nuclear power, and coal and gas subject tocarbon capture and storage (CCS). All have important roles to play, depending on the country and region inquestion. In sunny areas like India, Africa and south-east Asia solar can play a central role. In more northernareas, like Japan and Northern Europe, wind and nuclear have an important role, as does CCS in areas rich incoal and natural gas.
There are also three elements that are common to all sources of energy. First is our ability to store the energy
cheaply for when it is needed. Then there is our ability to transmit it cheaply to where it is needed. And finally
there is our ability to rein in our overall demand for energy through energy efficiency. Storage, for example
through batteries, capacitors, and large-scale mechanical methods, is needed at all size scales up to gridscale, and for a variety of durations from minutes to seasons.

Recent progress
In recent years, the price of solar and wind power installation has fallen dramatically. This has largely been
driven by the introduction of feed-in tariffs across the European Union, starting in Germany in 1989. The
prices of photovoltaic (PV) panels have been falling by 17% for every doubling of capacity, and the prices ofi.

Sir David King, UK Foreign Secretary’s Special Representative for Climate Change, former Government Chief Scientific Adviser;ii.

Professor Jim Skea, RCUK Energy Strategy Fellow and Professor of Sustainable Energy Centre for Environmental Policy, Imperial
College London

iii. Professor Tim Green, Director, The Energy Future Lab, Imperial College London
ST PAUL’S CATHEDRAL, LONDON. BUILT 1675 – 1708. FINANCED BY A TAX ON COAL
Image by Thomas Wesenberg.

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PV installation and on-shore wind installation are now comparable with new fossil fuel power installation in
many parts of the world. With more research, both plant and installation costs can fall even faster. There isalso scope for further progress in developing concentrated solar power, for deployment in desert sites.
However, feed-in tariffs have done nothing for the development of competitively priced energy storage andsmart grids. While there has been some significant innovation, particularly in batteries, many of the otherstorage technologies are immature with uncertain costs, performances and system implications. With smart
grids, at-scale demonstration is needed to extract lessons for deployment and to understand behavioural andsocial implications. This particularly is where a major thrust of publicly-funded research, development anddeployment (RD&D) activity is urgently required.1

Accelerating progress
We have four key messages relevant to accelerating energy innovation. These are: the scale of investment
required; the need for a mission focus that will send signals to the private sector and underpin specificactions; the need to create the right conditions for market entry for novel technologies; and the need for acoordinated international approach.
1. Scale. We need to put a much higher priority on the discovery of new, cheaper ways to produce, storeand distribute clean energy. In the face of this urgent need it is remarkable to note that there is a
declining trend in the fraction of national income devoted to public investment in a cleaner energy future.
The public sector has an urgent role to play in addressing this imbalance. The International Energy
Agency concludes that our RD&D investment in clean energy technology needs to increase by three tosix times in order to hold the global temperature within 2°C above preindustrial levels.2 The country withthe highest proportion of national income devoted to energy RD&D investment is Finland, at 0.13%; theglobal median level in developed countries (2012) is only 0.035%. In even the best performing segmentsof the energy sector – such as solar and wind – the ratio of R&D to sales is under 2%. This compares
unfavourably with over 5% in consumer electronics, and 15% in pharmaceuticals.3
2. A mission focus. Most of the energy sector’s own RD&D investment focuses on extending the fossil
fuel resource base and enabling it to be extracted at lower cost. Public sector RD&D has a unique roleto play since it can be deliberately directed at overcoming barriers to the development and adoption oftransformational low carbon technologies. The challenge - which brings focus to the mission - is to bringthe costs of low carbon technologies down to a level where they can successfully compete with incumbent
technologies. If research, development and deployment programmes are directed at achieving thisspecific goal for each of the key technologies that are essential to a low carbon transition, then resources
will be allocated more efficiently, and the desired results will be achieved more quickly.
A clear RD&D mission focused on low-carbon transformation sends the right signals to the private sector,
which will have a key role in bringing these technologies to the market. A parallel can be drawn withthe highly successful arrangement for the semiconductor industry, where Moore’s law (the number oftransistors on a computer chip doubles every two years) has been maintained through a mission-oriented
Roadmap driven by a publicly- and industry-funded pre-competitive RD&D programme over the past 30
years (the International Technology Roadmap for Engineers, ITRS).
3. Creating the right conditions for market entry of clean energy technologies. Market-pull policies
that turn low carbon technologies into attractive commercial propositions are needed to complement
RD&D investments. Subsidies for clean energy technologies, and carbon pricing, can both help tocreate the right market conditions. By increasing demand, and hence the volume of production and theeconomies of scale, they bring forward the point at which clean energy prices become competitive withmature fossil-fuel based systems.
This approach has already been seen to work, with simple measures leading to high volume deploymentof small-scale technologies – with examples including feed-in tariffs for photovoltaics, efficiency
regulations for vehicles, and product standards and labelling for efficient domestic appliances). Thisapproach needs to be replicated further. Support for large-scale technologies – such as offshore wind andnuclear energy, bioenergy, and carbon capture and storage – tailored to technological maturity, is needed
so that risks are shared. In particular, market design needs to be adjusted so that it rewards the systembenefits of facilitating technologies, such as storage and smart grids.

Support is also needed to assist new technologies through the ‘valley of death’ – the period in which atechnology may already be ready for market, but not yet able to overcome the advantages of incumbency
that benefit existing technologies. Public support can help start-up technology companies survive throughthis period, when they might otherwise fail.
4. A coordinated international approach. The scale of RD&D investment involved calls for a coordinatedinternational approach. Benefits would arise in terms of economies of scale, shared research objectives,
the pooling of research findings, the agreement of common RD&D targets and milestones, and the
avoidance of duplicated effort. Each country would continue to organise and manage its own programme,
but could coordinate its efforts with partners through an overarching technological committee. Such acommittee would set out roadmaps for RD&D (as in the semiconductor industry, as mentioned above)
which would lead to cost-competitive low carbon technologies. An international approach of this kind isvitally needed to enhance the productivity of RD&D investments, accelerate the diffusion of low carbon
energy technologies, encourage the private sector to enter a potentially enormous market, and thereby
increase the chance of avoiding dangerous climate change.

25 FINANCE
Rowan Douglas, Steve Waygood, James Cameron.iv
For generations, governments have sought to align the interests of the financial markets and society. Nowhereis this tension more keenly and persistently felt than in the area of climate risks, which is arguably the mostsignificant contemporary market failure. To correct this failure, and respond effectively to the risk of climatechange, we need to do better in three areas: disclosing risk; pricing carbon; and incentivising long-term
investment.

Disclosing risk
There is a collective call across a wide range of sectors and policy interests to avoid further unreported andunmanaged accumulations of risk. If accumulated risk – whether in the form of short-term bubbles or longterm environmental externalities – is not rationally accounted for and managed at an early stage, then it tends
to manifest itself through market shocks, disorder, and system-wide losses at the point when it can no longer
be ignored, often with devastating consequences.
Over the last two decades, there has been a revolution in Environmental, Social and Governance reportingand disclosure initiatives. Some of these have improved disclosure of risks to individual enterprises or tosociety, and this represents valuable progress. However, the proliferation of initiatives can be confusing toinvestors, and the assessment of many risks – including those associated with climate change – is not yet
integrated into the financial system sufficiently for those risks to be effectively managed.
The experience of the insurance industry is instructive. In the late 1980s and early 1990s, traditional risk
assessment approaches based on loss experience data were found inadequate to cope with a range of
unprecedented, extreme and emerging risks. Since then, the industry has transformed its approach, using
science, systems modelling and scenarios to better understand current risk distributions and extremes. The
full spectrum of risks is now assessed in estimating three key metrics: i) the annual average loss; ii) the 1 in
20 year annual loss; and c) the 1 in 200 year annual loss. Risk management, based on these metrics, ensuresthat insurers remain solvent and can fulfil their commitments even in the worst years. This is similar to thepractice of ‘stress tests’ in other sectors. What is distinctive is how the stress test is employed in mainstream
financial regulation, credit ratings, equity analysis and annual reporting across the whole insurance andreinsurance sector.
iv.

Rowan Douglas is CEO, Capital, Science & Policy Practice and Chairman, Willis Research Network, Willis Group. Steve Waygood is
Chief Responsible Investment Officer, Aviva Investors. James Cameron is Chairman of the Overseas Development Institute, and former
chairman and founder of Climate Change Capital. With thanks to Bryony Worthington, Mark Campanale, Nick Robins, Paul Dickinson
and Ingrid Holmes for additional ideas and input.

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As this approach has revealed true levels of risk exposure, it has transformed risk governance withininsurance companies and the wider market, making organisations across public, private and mutual4 sectors
far more resilient, transparent and comparable than before. Markets have developed much greater confidence
in the insurance sector’s ability to underwrite its risks sustainably; uncertainty has been classified andreduced, and capital has remained abundantly available for appropriately risk-adjusted allocation. Valuations
and decision-making have been better able to take risks into account and resilience has been rewarded.

overall cost on the economy. However, we have also learned that carbon pricing at relatively low levels does
not bring about the technological transformation that is required to effectively manage climate risk. Given
the inability of economic models to predict such transformations, we have no way of knowing what level ofpricing will be effective. A logical approach is therefore to set carbon pricing on a path of rising stringency
that keeps pace with revealed information about abatement potential, until we see the desired changes taking
place. If this direction of travel is clear to all, then the desired changes will come earlier and at lower cost.

Based on the insurance methodology, it would be possible to design a common risk reporting framework that
could be applied to all entities (public, private and mutual) including companies, cities and even countries.
This would assess risks to each entity’s interests under a common template of risk classes, probability
thresholds, and periods of time, and translate this into annualised costs, which would be proportionately
reduced with greater resilience or reductions in risk. At the company level, this would provide a riskorientated lens to interpret and formalise much of the work already undertaken in Environmental, Socialand Governance reporting. At the national level, it would build on the practice of countries such as the
UK in publishing national assessments of civil emergency and security risks. At all levels, it would provide
information that would better enable planning and accounting for the future, increasing resilience, andmanagement of emerging risks.

Who should pay the price? To date, governments have mostly placed the burden of change on the
consumption end of the carbon life cycle, through carbon pricing and product standards. Carbon prices
are paid by fossil fuel consumers. An alternative, potentially additional approach would be for fossil-fuelproducers to pay the price. This would have two significant advantages: it is administratively simpler to levy
a tax at the point of extraction (or import) than at any other point; and taxing at source will ensure thatthe price is integrated into the whole economy, not just part of it. A third advantage could be to stimulate
investment in carbon capture and storage. This could be achieved by exempting fossil fuel producers fromany carbon price to the extent they are able to demonstrate that the carbon they produce remains unburnedor equivalent emissions are safely returned to the ground. This would be identical in principle to successful
regulations in Europe that have made producers of packaging, automobiles and appliances responsible for
dealing with the waste created when those products come to the end of their useful lives.

As an example of how this could contribute to the management of climate change risk, consider an energy
firm focussed on coal production. The firm may identify a carbon price of US$100 per tonne, or theannouncement of an EU target to cut coal consumption by 50%, as one of its extreme loss scenarios. The
probability of this may be estimated as ‘1 in 100’ or 1% for the following year, with the contingent liability
evaluated, but could rise to high values in future years, in responses to changes in the policy landscape. As
the insurance sector has experienced, within a short time, managers and regulators would become familiar
with this approach, and leading risk scenarios would become objects for discussion and refinement among
practitioners, markets and stakeholders. Markets, benefiting from the increased transparency, robust
framework, and tractable numbers produced by this approach, would be better able to discount the value of
assets proportionately and ensure that capital was allocated in a way that took into account not only current
risks, but also the way in which risks may to change over time.
Of course, some firms may have an incentive to lobby governments not to manage climate risks. While thismay be good for the firm’s bottom line, it may be antithetical to the broader interests of their shareholders.
Shareholders – and society at large – should be able to make up their own minds on this question, but atpresent it is very difficult for them to see: (i) which trade associations companies are members of; (ii) whatthese trade bodies are saying about climate change; or (iii) whether this work is in their interests. So in
addition to the need to improve disclosure of risks, it should become standard practice that all corporate
advocacy relating to climate change is a matter of public record.

Pricing carbon
Governments should recognise that valuing carbon in the real economy is essential. It will be almost
impossible to get to grips with greenhouse gas emissions in a complex, dynamic and diverse economy without
using carbon pricing in some form. The great virtue of putting a clear price on carbon is that it can engage
many, with an interest they can valorise, in the enterprise of achieving a public policy goal in a multitudeof ways that no policymaker, however wise, could possibly have determined in advance. At the same time,carbon pricing is not a panacea, and should always been seen as one of a number of price-based and nonprice-based policy interventions that can easily co-exist and which will all be necessary to deliver an adequate
response.
The vast majority of governments and investors are aligned in wanting a price on carbon in order to rationallyand systematically allocate capital away from climate risk, which affects the value of their whole portfolios
in ways which are difficult to calculate, and towards better risk-adjusted returns from other, low carbon or
resource efficient investments.
There are many ways to implement carbon pricing, and the most appropriate form will vary according topolitical and legal cultures. However, in any country, a carbon pricing system needs to be simple, enduring andtransparent enough for the market to understand and act on. And it needs to cover enough of the economy tomake a difference (achieving this is likely to require a portfolio of policies rather than a single one).
But what should the price be? We have learned from our experience so far that even low carbon prices canachieve significant reductions in emissions, while promoting resource efficiency, and without imposing any

Who should pocket the proceeds? Any carbon pricing system that is significant enough to be effective will
face the opposition of vested interests. Governments may find it easier to overcome this opposition if they
have strong public support. This may be achieved by making the system revenue-neutral, giving the proceeds
back to the public. An added advantage is that such a system can incorporate a positive feedback: if a price islevied as a fixed tax per tonne of carbon extracted, and if a fixed proportion of total tax revenues are returned
to market participants, then action taken to reduce the tax burden by one participant increases the incentive
for others to act similarly (since a decrease in total revenues results in an increase in net tax paid). This
contrasts with the negative feedback experienced with fixed quantity based carbon pricing policies such as
cap-and-trade, where any reduction in carbon consumption by one participant reduces overall demand, andso reduces the incentive for others to follow.
Finally, even more important than any of the specifics discussed above, is the question of credibility. Carbon
pricing is effective when investors are confident that the system will endure over time. This requires visible
political will and strong institutions. Internationally, further confidence may be generated if major economies
agree to implement robust and rising carbon prices together.

Incentivising long-term investment
The International Energy Agency estimates that to limit global warming to 2°C, we will need to invest around$1trn a year in clean energy between now and the middle of the century. Further investment will be needed toreduce emissions in other sectors. This figure may change as our understanding of the severity of the climate
risk improves; however, it is already clear that significant sums of money will be required.
Fortunately, with over $50 trillion invested in the global stock markets, and a further $100 trillion held insovereign and intergovernmental debt, on the face of it, there should be no shortage of capital available. The
speed and scale of the growth in sovereign debt that was issued to underpin the global financial system during
the financial crisis demonstrates that it is possible to secure financing at the speed and scale implied here.
The key is the existence of political will.
Raising and spending this money will need considerable planning, effort and international coordination. If weare to achieve this efficiently, effectively and sustainably, we will need the international community to develop
a long-term capital raising plan. The overall direction for such a plan could be set by government leaders ina group such as the G20. The plan would need to consider all forms of large-scale finance. Sovereign debt is
one of the asset classes best able to finance investment for the long-term, but its issuance during the financial
crisis has already brought a significant number of countries to debt levels that have tested their credit ratings.
So the plan must include a new more nuanced approach to evaluating debt-based risk, and also make use ofmultilateral development banks, infrastructure investment, project finance, corporate debt, foreign direct
investment, and equity investment. For all these assets classes, the plan will need to consider which asset
owners have the capital available, and, importantly, what incentive structures will allow them to profit frominvesting sustainably for the long-term. Policies that create ‘bankable’ projects will be key to this.

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A critical weakness of the financial system at present is that almost all its actors are strongly incentivised
to focus on the short term – the so-called ‘tragedy of the horizons’. Fund managers are assessed on theirquarterly performance; investment consultants benefit from switching fund managers; brokers are paid
according to trading volumes and so are incentivised to encourage market activity, and so are stock
exchanges, whose revenues are linked to the number of transactions that take place. Almost nobody looks
more than a year or two ahead. The overwhelming focus on the short term is a problem for the economy as awhole, since it results in misallocation of capital. Company directors are deterred from investing in long-term
opportunities, at the same time as markets fail to manage long-term risks, and companies focused on shortterm gain can profit at the expense of those seeking to do the right thing over the longer term.

incompatible with the changes needed to reduce the risk of climate change. This creates political tension,
where efforts to regulate existing activities are met with stiff opposition, resulting in blocking or watering
down of proposed interventions. The global nature of the problem, coupled with fears about economic
competitiveness, can be a powerful argument against unilateral action. In this context, politicians need tothink carefully about helping to manage transition in key sectors of the economy. This may mean providing
compensations and targeting incentives towards technologies that can be deployed in the most carbon
intensive sectors.

This is not a situation that we are obliged to accept. Governments, stock exchanges, institutional investors
and others set the rules of the game by the laws they pass, the regulations they enforce, and the contracts
they write. Any of these players can change the rules of the game, and so change behavioural incentives.
For example, laws could be amended to explicitly require good stewardship and the assessment of long-term
environmental, social and governance (ESG) risk as part of the fiduciary duty of asset managers. In linewith this principle, it has been proposed5 that clauses on responsible investment should be written into fundmanagement contracts, that fund managers’ performance should be evaluated on the basis of their long-term
ability to beat benchmarks as well as their performance as stewards, and that clients should be able to sue
fund managers and investment consultants for negligence if they fail to properly consider the potential for
ESG risks and unsustainable development to harm the value of long-term investment portfolios.
In the same spirit, we recommend that governments take a close look at the national financial regulators andtheir global coordinating bodies, to assess whether their mandates need to be updated in order to ensure thatthey are attending to the profound, long-term financial risks associated with climate change. If measures aretaken to incorporate the assessment of long-term risk into the financial system, we will make much fasterprogress towards managing risk effectively.

26 POLITICS
Baroness Bryony Worthington, Member of the House of Lords, UK Parliament
In 2008 the UK passed a landmark piece of legislation to address climate risk: The Climate Change Act. It
introduced a legal framework committing the UK to an 80% cut in greenhouse gases by 2050 compared to1990, through a series of consecutive five yearly carbon budgets. The UK was the first country in the world
to translate the scientific evidence of climate risk into a long-term legal framework. This was possible due avery strong political consensus that UK could and should act to reduce its contribution to the global climate
problem. As was argued at the time, demonstrating leadership in this way subsequently catalysed othercountries to follow suit, and now more than 60 countries have passed laws that seek to address their impact
on the climate.
The UK’s approach to managing the risk of climate change reflects our political situation. The actors andactions contributing to climate change are, for the most part, within the private sector. Political leaders
therefore used the rule of law to create a new carbon emissions management system, guiding future policymaking, which includes additional laws and regulations, at an EU and UK level, and influencing company
decision-making.
The political context in other countries differs and different responses to climate risk can be expected. For
example, in China where there is a higher degree of state involvement in planning the economy, the response
to climate change is embodied in the five-year plan. This blueprint sets out a vision with specific targets toachieve it, which in turn provides a framework for the financing of programmes and projects. In the US,
with the political deadlock at a Federal level, progress on climate change is being pursued at a State and citylevel. Some states, such as California, have passed comprehensive legislative packages applying emissions
standards and introducing carbon pricing. Elsewhere, in the absence of new laws, the existing Clean Air Act isbeing used to trigger action, though it is still subject to legal challenge.
There are, however, irrespective of jurisdiction, some common challenges in the politics of tacklinggreenhouse gas emissions. There are many powerful interest groups whose current business models are

Moves to incentivise new, cleaner, potentially disruptive technologies have proven in many cases politicallyeasier to introduce. However, if they are perceived as too costly, political support can be withdrawn,
sometimes abruptly. Increasing the number of voices representing cleaner ways of doing things can create amore balanced political climate, as can well resourced and targeted civil society representations.
The next international climate change agreement will not be a top-down legislative framework based onpolitically determined carbon budgets. It will instead be a more inclusive, less deterministic documentthat reflects a shared will to act. The degree of action, however, could easily fall well short of what a risk
based assessment of the science would imply is necessary. So we must use the time that remains to build
confidence, strengthen political will, and ensure that this agreement represents the best that is possible within
this current framework. At the same time, we must think hard about how to increase the effectiveness of ouraction in the years ahead.
A priority must be to find ways in which international action can increase the momentum that is alreadyapparent in the deployment of cleaner technologies and prevent re-investment in high carbon infrastructure.
The Montreal Protocol to phase out ozone-destroying chemicals showed that an international agreement canbe remarkably effective when it is focussed on technological standards in a specific economic sector. The
same approach can be applied to many of the sources of carbon emissions. For example, the EU, China andthe US all have different standards regulating the emissions from power stations and vehicles. If these few
large economies agreed to have the same standards, sector participants across the world would be strongly
incentivised to meet them; and as technological progress is likely to outpace expectations, governments could
agree together to a timetable of progressively tighter standards. In any sector, a critical mass of countries
taking a lead will help to incentivise others to follow.
The fight to secure a proportionate response to climate risk will continue, and a deeper, shared understandingof that risk can only help to serve as a spur to action.
Ridding the world of slavery was a hundred year battle. The battle to rid the world of excessive greenhousegases is already a quarter of a century old and much progress has been made. The human mind is geared
for problem solving, and more and more people are applying themselves to the task. Over the next quarter
century, the pace of positive change looks set to increase. With sufficient political will, I remain confident that
we can and will avert the worst-case climate change scenarios that threaten our very existence.

Endnotes
1.
2.
3.

4.
5.

King, D.A., Browne, J., Layard, R., O’Donnell, G, Rees, M., Stern, N. and Turner,A. (2015). A Global Apollo
Programme to Combat Climate Change. London School of Economics, in print.
International Energy Agency (2013). Tracking Clean Energy Progress 2013: IEA input to the Clean Energy
Ministerial. Available at http://www.iea.org/publications/tcep_web.pdf
Laleman, R. and Albrecht, J. (2012) Comparing push and pull measures for PV and Wind in Europe, Renewable
Energy, 61: 33-37, p.3. See also Grubb, M., with Hourcade, J.-C. and Neuhoff, K. (2014) Planetary Economics,:
Energy, Climate Change and the Three Domains of Sustainable Development. Oxford, :Earthscan, Routledge,
p.321
The ‘mutual’ sector refers to building societies and credit unions run for the benefit of member investors rather
than shareholders or directors
By the United Nations Environment Programme Finance Initiative’s Asset Management Working Group

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CLIMATE CHANGE: A RISK ASSESSMENTRISK REDUCTION - ELEMENTS OF A PROPORTIONATE RESPONSERISK REDUCTION - ELEMENTS OF A PROPORTIONATE RESPONSE

27 CLOSING THOUGHTS

those resources, and take the lead in bringing them to the global market. The largest producing andconsuming countries must work together to make sure the global market benefits from their economies ofscale, bringing down the costs of clean energy for all, at the same time as improving access to energy for thepoorest. And while all countries have a role to play in reducing the subsidies given to polluting energy, thosethat host the hubs of global finance should take the lead in revising the rules of the game.

As the last of our contributing authors wrote, we have a battle on our hands: a battle topreserve a safe climate for the future. Powerful forces are engaged in this battle, whether wenotice them or not. The power of vested interests to resist change, the inertia of infrastructure
systems, and the unyielding laws of thermodynamics all seem to be arrayed against us.

Just as the risks of climate change are both immediate and long-term, we must act both immediately and witha long-term view. A risk that grows over time will not be managed successfully if our horizons are short-term.
Ultimately, the risks of climate change will only be under control when we have reduced global emissions tonear zero. So while we must do all in our power to reduce emissions now, we must also follow a path that
increases our power to do more in the future.

To win this battle, we must deploy equally powerful forces in favour of change: the power of human ingenuity,
the power of technology, and the power of leadership. We must match the laws of physics with a will and adetermination that is equally unyielding.

The risks of climate change may be greater than is commonly realized, but so is our capacity to confront
them. An honest assessment of risk is no reason for fatalism. If we counter inertia with ingenuity, match
feedback with feedback, and find and cross the thresholds of non-linear change, then the goal of preserving asafe climate for the future need not be beyond our reach.

The greatest risks of climate change arise when thresholds are crossed: what had been gradual becomessudden; what had been inconvenient becomes intolerable. The greatest reductions in risk will be won in thesame way. Gradual, incremental measures will not be enough: we must seek out non-linear, discontinuous,transformational change.
Political leadership can and should be a source of non-linear change. It can move a government from inaction
to action, and a society from apathy to engagement. With existing technology, there is already the opportunity
for political leadership to dramatically change the trajectory of any country’s emissions in the short term.
Technological innovation is a natural source of non-linear change. New technologies can emerge slowly, butthen displace old ones rapidly and suddenly when some invisible threshold is crossed. We need to accelerate
this pace of change, and bring forward those thresholds, in all the technologies that are needed to win thebattle. Above all, we must use both technological progress, and policy measures such as carbon pricing, tocross as soon as possible the threshold at which clean energy becomes cheaper than fossil energy.
In finance, small changes in the rules of the game can produce large changes in results. The right adjustments
to regulations and incentives will dramatically alter the flow of money, sending more of it in a direction thatserves our long-term economic interests.
The power of non-linear change is not reserved to political leaders, technologists and markets. Social change
can also be discontinuous, unpredictable, and dramatic. The battle against slavery (or colonialism) may havetaken a century, but when change finally came, it came quickly.
The risks of climate change are amplified by feedbacks: rising temperatures melt ice; sea stripped of ice takes
in more heat; and the temperatures rise faster. To win this battle, we must set up our own cycles of positive
feedback. Political interventions must change market sentiment, so that the market sends more investment
into clean energy technologies, so that this accelerates technological progress, so that new politicalinterventions become possible.
In this battle, every country must contribute according to what it can do best. Those most responsible for the
cumulative emissions in the atmosphere must do the most. Countries that are rich in energy from sunlight,
wind, rivers, tides, and geothermal heat must push the boundaries of technological innovation in exploiting

Many of our parents and grandparents fought for the future of their countries, making greater sacrifices thanmost of us will ever have to. Never mind what we owe to future generations; it is the least we owe to the pastthat we should make a decent fight of this battle now.

London, Beijing, Delhi, Cambridge (MA)14 June 2015

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RECOMMENDATIONS FOR CONTINUING RISK ASSESSMENT

ANNEXESANNEXES

Geographic areas
The three geographic areas in this study were in North India, South-East China and South-East USA. The
numbers of grid cells included were 281, 953 and 602 respectively.
N India: Includes Rajastan, Punjab, Haryana, Chandighar, Delhi, Bihar and Uttar Pradesh

ANNEX: RISK OF HEAT STRESS TO HUMANS – 			METHODS AND ADDITIONAL RESULTSSE USA: Louisiana, Mississipi, Alabama, Georgia, Florida, S Carolina, N Carolina, Virginia, W Virginia,
Kentucky, Tennessee, Arkansas
SE China: The area below the two straight lines in the map.

Methods
Relating global temperature rise to local climatic conditions relevant to heat stress
We used the HadGEM2-ES climate model values1 for temperature and humidity (dew point) to calculate
the heat stress index Wet Bulb Globe Temperature (WBGT) for 67,000 global grid cells over land (0.5 x
0.5 degrees), for different time periods (1993, 2025, 2050, 2075 and 2200; each estimated as 25-30-year
averages), on the RCP8.5 scenario pathway. This provided approximate data on grid-cell level heat stresschange during two centuries.
The heat stress levels were calculated as 30-year averages for each calendar month, so the June values arean average of 900 June days. We calculated the intra-month daily variability of the heat levels and standard
deviations (assuming Gaussian distributions), which can be used to identify lower limits for the hottest 10%of calendar month days (‘hottest 3 days’).
Relating climatic conditions to core body temperatures and heat stress
The WBGT index combines temperature and humidity with estimates of likely air movement over the skinand additional heat from heat radiation to produce a physiologically relevant heat exposure level, which canbe used in climate change health impact assessments.2 The heat stress level also depends on metabolic rate(physical activity level) and clothing.
Details of the WBGT method and its calculation and interpretation can be found in the ClimateCHIP Technical
Report 2014:4 at www.ClimateCHIP.org.3 The mapping of heat stress in relation to work and other physicalactivities has been described in detail for parts of Asia.4
The relationship between WBGT levels and physiological limits was estimated using the de Dear (University
of NSW, Australia) formula, which is available as the WWW Thermal Comfort Index Calculator (http://web.
arch.usyd.edu.au/~rdedear/) to estimate effects on core body temperature. The WBGT calculations containthe variables that are inserted into the de Dear formulas that then calculate the time trend up to four hours of
the core body temperature (70kg person, light clothing).
How uncertainty in the modelled future climate change is addressed.
There is uncertainty as to how much climate will change due to increased emissions of greenhouse gases andconsequently how WBGT will change in the future. To address some of this uncertainty, the WBGT for theend of the 21st century was calculated at different levels of global mean temperature change using a patternscaling technique to derive the local temperature change5 and assuming the HadGEM2-ES spatial patternof warming. The different levels of temperature change chosen were based on the distribution of modelled
end of the 21st century temperature rise for RCP8.5 from the World Climate Research Programme’s (WCRP)
Couple Model Inter-comparison Project (phase 5; CMIP5)6 and presented in the International Panel on
Climate Change (IPCC) Fifth Assessment Report.7 The calculated WBGT values in this work therefore reflect
some of the uncertainty in the temperature response to increased climate forcing but do not include theuncertainty in the local response to climate change since only one spatial pattern of warming is used (i.e one
model, HadGEM2-ES) or the uncertainty in the future changes to humidity.

Population
For each grid cell we also had estimates of local population numbers in four age groups, and we calculated
population-weighted heat exposure probabilities for the adult and elderly population (ages >= 15 years). The
population database until 2099 was based on UN Population office estimates (calculations by Briggs, 2014)
and after 2099 we used total estimated population changes:
Since our ‘probability’ estimates for the highest global temperature changes (in 2200) were based on
proportion of population, changes in population number between 2085 and 2200 did not need to be taken
into account. For simplicity, we assumed that there was no change over time in the relative spatial distributionof population during the second century.
Hours of work lost (see additional results, on the following page)The risk functions for lost work capacity due to heat were presented in the ClimateCHIP Technical Report
2014:4 at www.ClimateCHIP.org.8

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CLIMATE CHANGE: A RISK ASSESSMENT

ANNEXESANNEXESADDITIONAL RESULTS

5. Collins, M., R. Knutti, J. Arblaster, J.-L. Dufresne, T. Fichefet, P. Friedlingstein, X. Gao, W.J. Gutowski, T. Johns, G.
Krinner, M. Shongwe, C. Tebaldi, A.J. Weaver and M. Wehner (2013). ‘Long-term Climate Change: Projections,
Commitments and Irreversibility’. Climate Change 2013: The Physical Science Basis. Contribution of Working
Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F.,
D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp, doi:10.1017/
CBO9781107415324.
6. Taylor, K.E., R.J. Stouffer, G.A. Meehl (2012). ‘An Overview of CMIP5 and the experiment design.’ Bull. Amer.
Meteor. Soc., 93, 485-498, doi:10.1175/BAMS-D-11-00094.1, 2012.
7. IPCC (2013). ‘Summary for Policymakers’. Climate Change 2013: The Physical Science Basis. Contributionof Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[Stocker,T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
8. Kjellstrom et al., 2014

This figure displays the results in an alternative way, showing estimates of the proportion of work hours
lost in the three study areas due to heat, at different levels of physical activity, in relation to global averagetemperature increase (0 to +8 degrees C).i
Lost annual daylight work hours (%) for people working
hourly at 300W

Lost annual daylight work hours (%) for people working
hourly at 200W

60

Percentage of hours lost

Percentage of hours lost

45
40
35
30
25
20
15
10
5

50
40
30
20
10

ANNEX: RISK OF WATER STRESS – METHODS

0

0
0

2

4

6

8

0

10

2

4

6

8

N India

N India

SE China

SE China

SE USASE USA

Lost annual daylight work hours (%) for people working
hourly at 400W
70
60
50
40
30

For a given climate pathway (or emissions scenario), socio-economic scenario and impact indicator, theprincipal sources of uncertainty which determine the risk that climate change has a specified impact can
be classified into three; (i) the overall magnitude of climate change (as indicated by change in global mean
temperature, for example), (ii) the spatial and seasonal distribution of changes in relevant climatic variables
associated with a given overall magnitude of climate change, and (iii) the translation of these local andregional climate changes into ‘impact’. The effects of all these uncertainties on risk and how it changes overtime could be assessed by undertaking multiple simulations under all possible plausible conditions, but this is
not feasible in practice. The range in potential impacts of climate change through the 21st century is therefore
estimated here by combining estimates of the probability distributions of changes in global mean surface
temperature – under a given climate pathway - for each year with damage functions relating change in global
mean surface temperature to impact.
Changes in temperature through the 21st century were estimated using a probabilistic version1 of the MAGICC
simple climate model,2 which is run with multiple parameter combinations to produce distributions oftemperature change by year.

20
10
0
0

2

4

6

8

Global average temperature increase, degr. C
N India
SE China

10i.

Global average temperature increase is shown here as
relative to a 'present day' baseline, as defined by the
30-year average centred on 1995 (i.e. from 1980 to 2009).
This is about 0.6 degrees C higher than the 'pre-industrial'baseline used in the temperature increase chapter, andreferred to generally throughout this report.

SE USA

Endnotes
1.

10

Global average temperature increase, degr. C

Global average temperature increase, degr. C

Percentage of hours lost152

Downloaded from the ISI-MIP database at Potsdam Institute (Hempel, S., Frieler, K., Warszawski, L., Schewe,J., and Piontek, F. (2013). ‘A trend-preserving bias correction – the ISI-MIP approach’. Earth Syst. Dynam., 4,219-236, doi:10.5194/esd-4-219-2013, 2013.) The climate model data were from the RCP8.5 simulation whichassumes a high forcing by the end of the century close to 8.5Wm-2.
2. Kjellstrom T, Holmer I, Lemke B. (2009). ‘Workplace heat stress, health and productivity: An increasing
challenge for low and middle income countries during climate change’. Global Health Action; 2. doi:10.3402/
gha.v2i0.2047.
3. Kjellstrom T, Lemke B, Otto M, Dear K, Hyatt O. (2014) ‘Occupational heat stress. Contribution to WHO projecton global assessment of the health impacts of climate change, which started in 2009’. Technical Report 2014:4
at www.ClimateCHIP.org.
4. Kjellstrom T, Lemke B, Otto M (2013). ‘Mapping occupational heat exposure and effects in South-East Asia:
Ongoing time trends 1980-2009 and future estimates to 2050’. Indust Health 51: 56-67.

Damage functions for each impact indicator were constructed by scaling the patterns of climate change
as simulated by different climate models to defined changes in global mean temperature and estimating
impacts corresponding to each temperature change by combining a spatial impacts model with definedsocio-economic scenario.3 In this application, 21 climate model patterns were used,4 so there are therefore
21 different damage functions (representing the 21 different patterns of change in climate) for each time
period (because the socio-economic characteristics vary over time). Each of the 21 climate model patterns
is assumed to be equally plausible. Impacts were assessed under two climate pathways – represented by the
RCP2.6 and RCP8.5 scenarios – and under three socio-economic scenarios defining change in population.
The socio-economic scenarios represent medium, low and high population growth assumptions, and are takenfrom the Shared Socio-economic Pathways (SSPs) projections.5
Impacts on exposure to water stress and river flooding are based on river flows as simulated using the
MacPDM global hydrological model.6 which operates at a spatial resolution of 0.5x0.5o. The period
1961-1990 is used to define the reference baseline climate. Water resources per capita is calculated forapproximately 1300 river basins and islands, and regional and global totals of people in different water stressclasses are calculated by summing the numbers of people in watersheds in those classes.7
The average annual number of people in major floodplains affected by floods greater than the current 30-year
return period flood is estimated by calculating the change in the probability of experiencing the magnitudeof the current 30-year flood in the future in each grid cell, and multiplying by the grid cell floodplain
population.8 Basin-scale changes in the annual probability of experiencing a flood exceeding the current
30-year return period flood were estimated by calculating the weighted average of changes in each gridcell within the basin, weighting by floodplain population (so unpopulated grid cells do not contribute to theaverage).

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ANNEXESANNEXESImpacts on drought frequency are based on changes in the 12-month standardised precipitation index(SPI) calculated from monthly precipitation. A ‘drought’ is defined as a month with an index of less than
-2 as calculated over the climate baseline period (1961-1990). This occurs just under 2% of the time in thebaseline period, and can be regarded as an extreme drought. Change in the average annual area of cropland9
experiencing such a drought is estimated by calculating the change in frequency of this threshold. The UNDP
2009 Global Assessment Report on Disaster Risks10 uses the SPI (calculated over 6 months) alongside ameasure of precipitation variability to characterise drought risks; their assessment uses an SPI threshold of-1, which occurs approximately 15% of the time.
There are, of course, a number of caveats with this assessment. The estimated distribution of potentialchanges in temperature in a year is based on one simple climate model – MAGICC – with one set of plausible
parameters. The spatial patterns of change in climate are estimated from 21 climate models, all of which
are assumed equally plausible: a different ensemble of climate models could give a different indication of
the range of potential changes in a region. The damage functions are based on scaling patterns of change
from these climate models, which basically assumes that climate in a region varies linearly with global mean
temperature:11 this may not be the case, and there may be substantial ‘step changes’ or non-linearities in
specific regions. Only one impact model is used to estimate hydrological changes, and different models cangive different responses to the same change in climate.12 The range in potential impacts is therefore probably
underestimated, although the probability of exceeding a threshold is likely to be more robust (there are no
indications that the MacPDM model used here is systematically different from other global hydrologicalmodels). Finally, there are different potential indicators for change in water stress, flood risk and drought
risk, and these may give different indications of the global and regional effect of climate change.
The results should therefore be interpreted as indicative only: the differences between climate pathways andbetween places are more robust than the actual magnitudes of impact and risk.

Endnotes
1.

Lowe, J. A., C. Huntingford, S. C. B. Raper, C. D. Jones, S. K. Liddicoat, and L. K. Gohar (2009). ‘How difficult
is it to recover from dangerous levels of global warming?’ Environmental Research Letters, 4(1),014012
10.1088/1748-9326/4/1/014012.
2. Meinshausen, M., S. C. B. Raper, and T. M. L. Wigley (2011). ‘Emulating coupled atmosphere-ocean andcarbon cycle models with a simpler model, MAGICC6-Part 1: Model description and calibration’. Atmospheric
Chemistry and Physics, 11(4), 1417-1456, 10.5194/acp-11-1417-2011.
3. Arnell, N.W., Brown, S., Gosling, S.N., Hinkel, J., Huntingford, C., Lloyd-Hughes, B., Lowe, J.A., Osborn, T.,
Nicholls, R.J. Zelazowski, P. (2014). ‘Global-scale climate impact functions: the relationship between climate
forcing and impact’. Climatic Change doi 10.1007/s10584-013-1034-.
4. Meehl, G. A., C. Covey, T. Delworth, M. Latif, B. McAvaney, J. F. B. Mitchell, R. J. Stouffer, and K. E. Taylor (2007).
‘The WCRP CMIP3 multimodel dataset - A new era in climate change research.’ Bulletin of the American
Meteorological Society, 88(9), 13835. O’Neill, BC, Kriegler, E., Riahi, K., Ebi KL, Hallegatte, S, Carter, TR, Mathur, R, van Vuuren DP (2014). ‘A new
scenario framework for climate change research: the concept of shared socio-economic pathways.’ Climatic
Change 122 387-400. The scenarios are available from https://secure.iiasa.ac.at/web-apps/ene/SspDb/.
6. Gosling, S.N. & Arnell, N.W. (2010). ‘Simulating current global river runoff with a global hydrological model:
model revisions, validation and sensitivity analysis.’ Hydrological Processes 25 (7), 1129-1145. doi:10.1002/
hyp.7727.
7. The methodology is the same as in Gosling, S.N. & Arnell, N.W. (2013). ‘A global assessment of the impact ofclimate change on water scarcity.’ Climatic Change doi:10.1007/s10584-013-0853-x, but using different socioeconomic scenarios.
8. The simulation of flood flows is described in Arnell, N.W. & Gosling, S.N. (2014). ‘The impacts of climate change
on river flood risk at the global scale.’ Climatic Change doi:10.1007/s10584-014-1084-5, although the indicatorhere is different.
9. Ramankutty N, Evan AT, Monfreda C, Foley JA (2008). ‘Farming the planet: 1. Geographic distribution of global
agricultural lands in the year 2000.’ Global Biogeochemical Cycles 22. doi: Gb1003 10.1029/2007gb002952.
10. UNDP (2009) Risk and poverty in a changing climate: 2009 Global Assessment Report on Disaster Risk
Reduction. UNDP: New York.
11. Arnell, N.W., Brown, S., Gosling, S.N., Hinkel, J., Huntingford, C., Lloyd-Hughes, B., Lowe, J.A., Osborn, T.,
Nicholls, R.J. Zelazowski, P. (2014). ‘Global-scale climate impact functions: the relationship between climate
forcing and impact.’ Climatic Change doi 10.1007/s10584-013-1034-.
12. Schewe, J., Heinke, J., Gerten, D., Haddeland, I., Arnell, N.W., Clark, D.B., Dankers, R., Eisner, S., Fekete, B., ColonGonzalez, F.J., Gosling, S.N., Kim, H., Liu, X., Masaki, Y., Portmann, F.T., Satoh, Y., Stacke, T., Tang, Q., Wada,
Y., Wisser, D., Albrecht, T., Frieler, K., Piontek, F., Warszawski, L. & Kabat, P. (2014). ‘Multi-model assessmentof water scarcity under climate change.’ Proceedings of the National Academy of Sciences 111, 3245-3250
doi.10.1073/pnas.1222460110.

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