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Abstract

This report provides a snapshot of recent scientific literature and new analyses of likely impacts and risks that would be associated with a 4° Celsius warming within this century. It is a rigorous attempt to outline a range of risks, focusing on developing countries and especially the poor. A 4°C world would be one of unprecedented heat waves, severe drought, and major floods in many regions, with serious impacts on ecosystems and associated services. But with action, a 4°C world can be avoided and we can likely hold warming below 2°C.
Why a 4°C Warmer World
Must be Avoided
Turn Down
Heat
the
Why a 4°C Warmer World
Must be Avoided
Turn Down
Heat
the
November 2012
A Report for the World Bank
by the Potsdam Institute for
Climate Impact Research and
Climate Analytics
© 2012 International Bank for Reconstruction and Development / The World Bank
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Washington DC 20433
Telephone: 202-473-1000
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This work is a product of the staff of The World Bank with external contributions.
The findings, interpretations, and conclusions expressed in this work do not
necessarily reflect the views of The World Bank, its Board of Executive Directors,
or the governments they represent.
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iii
Contents
Acknowledgements vii
Foreword ix
Executive Summary xiii
Observed Impacts and Changes to the Climate System xiv
Projected Climate Change Impacts in a 4°C World xv
Rising CO2 Concentration and Ocean Acidication xv
Rising Sea Levels, Coastal Inundation and Loss xv
Risks to Human Support Systems: Food, Water, Ecosystems, and Human Health xvi
Risks of Disruptions and Displacements in a 4°C World xvii
List of Abbreviations xix
1. Introduction 1
2. Observed Climate Changes and Impacts 5
The Rise of CO2 Concentrations and Emissions 5
Rising Global Mean Temperature 6
Increasing Ocean Heat Storage 6
Rising Sea Levels 7
Increasing Loss of Ice from Greenland and Antarctica 8
Ocean Acidication 11
Loss of Arctic Sea Ice 12
Heat Waves and Extreme Temperatures 13
Drought and Aridity Trends 14
Agricultural Impacts 15
Extreme Events in the Period 2000–12 16
Possible Mechanism for Extreme Event Synchronization 16
Welfare Impacts 17
3. 21st Century Projections 21
How Likely is a 4°C World? 23
CO2 Concentration and Ocean Acidication 24
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
iv
Droughts and Precipitation 26
Tropical Cyclones 27
4. Focus: Sea-level Rise Projections 29
Regional Sea-level Rise Risks 31
5. Focus: Changes in Extreme Temperatures 37
A Substantial Increase in Heat Extremes 37
Shifts in Temperature by Region 38
Frequency of Signicantly Warmer Months 39
The Impacts of More Frequent Heat Waves 41
6. Sectoral Impacts 43
Agriculture 43
Water Resources 47
Ecosystems and Biodiversity 49
Human Health 54
7. System Interaction and Non-linearity—The Need for Cross-sector
Risk Assessments 59
Risks of Nonlinear and Cascading Impacts 60
Concluding Remarks 64
Appendix 1. Methods for Modeling Sea-level Rise in a 4°C World 67
Appendix 2 Methods for analyzing extreme heat waves in a 4°C world 71
Bibliography 73
Figures
1. Atmospheric CO2 concentrations at Mauna Loa Observatory 5
2. Global CO2 (a) and total greenhouse gases (b) historic (solid lines) and projected
(dashed lines) emissions 6
3. Temperature data from different sources corrected for short-term temperature variability 7
4. The increase in total ocean heat content from the surface to 2000 m, based
on running ve-year analyses. Reference period is 1955–2006 7
5. Global mean sea level (GMSL) reconstructed from tide-gauge data (blue, red) and
measured from satellite altimetry (black) 8
6. (a) The contributions of land ice thermosteric sea-level rise, and terrestrial,
as well as observations from tide gauges (since 1961) and satellite observations
(since 1993)
(b) the sum of the individual contributions approximates the observed sea-level rise
since the 1970s 9
7. Reconstruction of regional sea-level rise rates for the period 1952–2009, during which
the average sea-level rise rate was 1.8 mm per year (equivalent to 1.8 cm/decade) 9
8. The North Carolina sea-level record reconstructed for the past 2,000 years.
The period after the late 19th century shows the clear effect of human induced
sea-level rise 9
9. Total ice sheet mass balance, dM/dt, between 1992 and 2010 for (a) Greenland,
(b) Antarctica, and c) the sum of Greenland and Antarctica 10
10. Greenland surface melt measurements from three satellites on July 8 and
July 12, 2012 11
CONTENTS
v
11. Observed changes in ocean acidity (pH) compared to concentration of carbon
dioxide dissolved in seawater (p CO2) alongside the atmospheric CO2 record from 1956 11
12. Geographical overview of the record reduction in September’s sea ice extent
compared to the median distribution for the period 1979–2000 12
13. (a) Arctic sea ice extent for 2007–12, with the 1979–2000 average in dark grey;
light grey shading represents two standard deviations.
(b) Changes in multiyear ice from 1983 to 2012 12
14. Russia 2010 and United States 2012 heat wave temperature anomalies as measured
by satellites 13
15. Distribution (top panel) and timeline (bottom) of European summer temperatures
since 1500 13
16. Excess deaths observed during the 2003 heat wave in France. O= observed;
E= expected 14
17. Drought conditions experienced on August 28 in the contiguous United States 14
18. Northern Hemisphere land area covered (left panel) by cold (< -0.43σ), very cold
(< -2σ), extremely cold (< -3σ) and (right panel) by hot (> 0.43σ), very hot (> 2σ)
and extremely hot (> 3σ) summer temperatures 15
19. Observed wintertime precipitation (blue), which contributes most to the annual budget,
and summertime temperature (red), which is most important with respect to evaporative
drying, with their long-term trend for the eastern Mediterranean region 16
20. Probabilistic temperature estimates for old (SRES) and new (RCP) IPCC scenarios 21
21. Probabilistic temperature estimates for new (RCP) IPCC scenarios, based on
the synthesized carbon-cycle and climate system understanding of the IPCC AR4 23
22. Median estimates (lines) from probabilistic temperature projections for
two nonmitigation emission scenarios 24
23. The correlation between regional warming and precipitation changes in the form
of joint distributions of mean regional temperature and precipitation changes
in 2100 is shown for the RCP3-PD and RCP8.5 scenarios 25
24. Simulated historic and 21st century global mean temperature anomalies,
relative to the preindustrial period (1880–1900), for 24 CMIP5 models based on
the RCP8.5 scenario 25
25. Projected impacts on coral reefs as a consequence of a rising atmospheric
CO2 concentration 26
26. Ocean surface pH. Lower pH indicates more severe ocean acidication, which inhibits
the growth of calcifying organisms, including shellsh, calcareous phytoplankton,
and coral reefs 26
27. Sea level (blue, green: scale on the left) and Antarctic air temperature (orange, gray:
scale on the right) over the last 550,000 years, from paleo-records 30
28. As for Figure 22 but for global mean sea-level rise using a semi-empirical approach 32
29. As for Figure 22 but for annual rate of global mean sea-level rise 32
30. Present-day sea-level dynamic topography 32
31. Present-day rates of regional sea-level rise due to land-ice melt only (modeled from
a compilation of land-ice loss observations) 33
32. Sea-level rise in a 4°C warmer world by 2100 along the world’s coastlines, from South
to North 33
33. Multimodel mean of monthly warming over the 21st century (2080–2100 relative to
present day) for the months of JJA and DJF in units of degrees Celsius and in units
of local standard deviation of temperature 38
34. Multimodel mean of the percentage of months during 2080–2100 that are warmer than
3-, 4- and 5-sigma relative to the present-day climatology 39
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
vi
35. Multimodel mean compilation of the most extreme warm monthly temperature
experienced at each location in the period 2080–2100 40
36. Distribution of monthly temperature projected for 2070 (2.9°C warming) across
the terrestrial and freshwater components of WWF’s Global 200 53
A1.1: Regional sea-level projection for the lower ice-sheet scenario and the higher ice
sheet scenario 68
A1.2: Difference in sea-level rise between a 4°C world and a 2°C world for the lower and
higher ice-sheet scenario 68
A2.1: Simulated historic and 21st century global mean temperature anomalies, relative
to the pre-industrial period (1880–1900), for 24 CMIP5 models based
on the RCP8.5 scenario 71
Tables
1. Record Breaking Weather Extremes 2000–12 18
2. Global Mean Sea-Level Projections Between Present-Day (1980–99) and
the 2090–99 Period 31
3. Projected Impacts on Different Crops Without and With Adaptation 45
4. Projected Changes in Median Maize Yields under Different Management Options
and Global Mean Warming Levels 46
5. Number of People Affected by River Flooding in European Regions (1000s) 55
Boxes
1. What are Emission Scenarios? 22
2. Predictability of Future Sea-Level Changes 30
3. Sub-Saharan Africa 62
vii
Acknowledgements
The report Turn Down the Heat: Why a 4°C Warmer World Must be Avoided is a result of contributions
from a wide range of experts from across the globe. We thank everyone who contributed to its richness
and multidisciplinary outlook.
The report has been written by a team from the Potsdam Institute for Climate Impact Research and
Climate Analytics, including Hans Joachim Schellnhuber, William Hare, Olivia Serdeczny, Sophie Adams,
Dim Coumou, Katja Frieler, Maria Martin, Ilona M. Otto, Mahé Perrette, Alexander Robinson, Marcia Rocha,
Michiel Schaeffer, Jacob Schewe, Xiaoxi Wang, and Lila Warszawski.
The report was commissioned by the World Bank’s Global Expert Team for Climate Change Adaptation,
led by Erick C.M. Fernandes and Kanta Kumari Rigaud, who worked closely with the Potsdam Institute
for Climate Impact Research and Climate Analytics. Jane Olga Ebinger coordinated the World Bank team
and valuable insights were provided throughout by Rosina Bierbaum (University of Michigan) and Michael
MacCracken (Climate Institute, Washington DC).
The report received insightful comments from scientific peer reviewers. We would like to thank Ulisses
Confalonieri, Andrew Friend, Dieter Gerten, Saleemul Huq, Pavel Kabat, Thomas Karl, Akio Kitoh, Reto
Knutti, Anthony McMichael, Jonathan Overpeck, Martin Parry, Barrie Pittock, and John Stone.
Valuable guidance and oversight was provided by Rachel Kyte, Mary Barton-Dock, Fionna Douglas and
Marianne Fay.
We are grateful to colleagues from the World Bank for their input: Sameer Akbar, Keiko Ashida, Ferid
Belhaj, Rachid Benmessaoud, Bonizella Biagini, Anthony Bigio, Ademola Braimoh, Haleh Bridi, Penelope
Brook, Ana Bucher, Julia Bucknall, Jacob Burke, Raffaello Cervigni, Laurence Clarke, Francoise Clottes,
Annette Dixon, Philippe Dongier, Milen Dyoulgerov, Luis Garcia, Habiba Gitay, Susan Goldmark, Ellen
Goldstein, Gloria Grandolini, Stephane Hallegatte, Valerie Hickey, Daniel Hoornweg, Stefan Koeberle, Motoo
Konishi, Victoria Kwakwa, Marcus Lee, Marie Francoise Marie-Nelly, Meleesa McNaughton, Robin Mearns,
Nancy Chaarani Meza, Alan Miller, Klaus Rohland, Onno Ruhl, Michal Rutkowski, Klas Sander, Hartwig
Schafer, Patrick Verkooijen Dorte Verner, Deborah Wetzel, Ulrich Zachau and Johannes Zutt.
We would like to thank Robert Bisset and Sonu Jain for outreach efforts to partners, the scientific com-
munity and the media. Perpetual Boateng, Tobias Baedeker and Patricia Braxton provided valuable support
to the team.
We acknowledge with gratitude Connect4Climate that contributed to the production of this report.
ix
Foreword
It is my hope that this report shocks us into action. Even for those of us already committed to fighting
climate change, I hope it causes us to work with much more urgency.
This report spells out what the world would be like if it warmed by 4 degrees Celsius, which is what
scientists are nearly unanimously predicting by the end of the century, without serious policy changes.
The 4°C scenarios are devastating: the inundation of coastal cities; increasing risks for food produc-
tion potentially leading to higher malnutrition rates; many dry regions becoming dryer, wet regions wet-
ter; unprecedented heat waves in many regions, especially in the tropics; substantially exacerbated water
scarcity in many regions; increased frequency of high-intensity tropical cyclones; and irreversible loss of
biodiversity, including coral reef systems.
And most importantly, a 4°C world is so different from the current one that it comes with high uncer-
tainty and new risks that threaten our ability to anticipate and plan for future adaptation needs.
The lack of action on climate change not only risks putting prosperity out of reach of millions of people
in the developing world, it threatens to roll back decades of sustainable development.
It is clear that we already know a great deal about the threat before us. The science is unequivocal
that humans are the cause of global warming, and major changes are already being observed: global mean
warming is 0.8°C above pre industrial levels; oceans have warmed by 0.09°C since the 1950s and are acidi-
fying; sea levels rose by about 20 cm since pre-industrial times and are now rising at 3.2 cm per decade;
an exceptional number of extreme heat waves occurred in the last decade; major food crop growing areas
are increasingly affected by drought.
Despite the global community’s best intentions to keep global warming below a 2°C increase above
pre-industrial climate, higher levels of warming are increasingly likely. Scientists agree that countries’ cur-
rent United Nations Framework Convention on Climate Change emission pledges and commitments would
most likely result in 3.5 to 4°C warming. And the longer those pledges remain unmet, the more likely a
4°C world becomes.
Data and evidence drive the work of the World Bank Group. Science reports, including those produced
by the Intergovernmental Panel on Climate Change, informed our decision to ramp up work on these issues,
leading to, a World Development Report on climate change designed to improve our understanding of the
implications of a warming planet; a Strategic Framework on Development and Climate Change, and a report
on Inclusive Green Growth. The World Bank is a leading advocate for ambitious action on climate change,
not only because it is a moral imperative, but because it makes good economic sense.
But what if we fail to ramp up efforts on mitigation? What are the implications of a 4°C world? We
commissioned this report from the Potsdam Institute for Climate Impact Research and Climate Analytics
to help us understand the state of the science and the potential impact on development in such a world.
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
x
Dr. Jim Yong Kim
President, World Bank Group
It would be so dramatically different from today’s world that it is hard to describe accurately; much relies
on complex projections and interpretations.
We are well aware of the uncertainty that surrounds these scenarios and we know that different scholars
and studies sometimes disagree on the degree of risk. But the fact that such scenarios cannot be discarded
is sufficient to justify strengthening current climate change policies. Finding ways to avoid that scenario is
vital for the health and welfare of communities around the world. While every region of the world will be
affected, the poor and most vulnerable would be hit hardest.
A 4°C world can, and must, be avoided.
The World Bank Group will continue to be a strong advocate for international and regional agreements
and increasing climate financing. We will redouble our efforts to support fast growing national initiatives
to mitigate carbon emissions and build adaptive capacity as well as support inclusive green growth and
climate smart development. Our work on inclusive green growth has shown that—through more efficiency
and smarter use of energy and natural resources—many opportunities exist to drastically reduce the climate
impact of development, without slowing down poverty alleviation and economic growth.
This report is a stark reminder that climate change affects everything. The solutions don’t lie only in
climate finance or climate projects. The solutions lie in effective risk management and ensuring all our
work, all our thinking, is designed with the threat of a 4°C degree world in mind. The World Bank Group
will step up to the challenge.
Executive
Summary
xiii
Executive Summary
-




Without further commitments and action to reduce greenhouse
gas emissions, the world is likely to warm by more than 3°C
above the preindustrial climate. Even with the current mitigation
commitments and pledges fully implemented, there is roughly a
20 percent likelihood of exceeding 4°C by 2100. If they are not
met, a warming of 4°C could occur as early as the 2060s. Such a
warming level and associated sea-level rise of 0.5 to 1 meter, or
more, by 2100 would not be the end point: a further warming to
levels over 6°C, with several meters of sea-level rise, would likely
occur over the following centuries.
Thus, while the global community has committed itself to
holding warming below 2°C to prevent “dangerous” climate
change, and Small Island Developing states (SIDS) and Least
Developed Countries (LDCs) have identified global warming of
1.5°C as warming above which there would be serious threats to
their own development and, in some cases, survival, the sum total
of current policies—in place and pledged—will very likely lead to
warming far in excess of these levels. Indeed, present emission
trends put the world plausibly on a path toward 4°C warming
within the century.
This report is not a comprehensive scientific assessment, as
will be forthcoming from the Intergovernmental Panel on Climate
Change (IPCC) in 2013–14 in its Fifth Assessment Report. It is
focused on developing countries, while recognizing that developed
countries are also vulnerable and at serious risk of major damages
from climate change. A series of recent extreme events worldwide
continue to highlight the vulnerability of not only the developing
world but even wealthy industrialized countries.
Uncertainties remain in projecting the extent of both climate
change and its impacts. We take a risk-based approach in which
risk is defined as impact multiplied by probability: an event with
low probability can still pose a high risk if it implies serious
consequences.
No nation will be immune to the impacts of climate change.
However, the distribution of impacts is likely to be inherently
unequal and tilted against many of the world’s poorest regions,
which have the least economic, institutional, scientific, and tech-
nical capacity to cope and adapt. For example:
• Even though absolute warming will be largest in high latitudes,
the warming that will occur in the tropics is larger when com-
pared to the historical range of temperature and extremes to
which human and natural ecosystems have adapted and coped.
The projected emergence of unprecedented high-temperature
extremes in the tropics will consequently lead to significantly
larger impacts on agriculture and ecosystems.
• Sea-level rise is likely to be 15 to 20 percent larger in the trop-
ics than the global mean.
• Increases in tropical cyclone intensity are likely to be felt
disproportionately in low-latitude regions.
• Increasing aridity and drought are likely to increase substan-
tially in many developing country regions located in tropical
and subtropical areas.
A world in which warming reaches 4°C above preindustrial
levels (hereafter referred to as a 4°C world), would be one of
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
xiv
unprecedented heat waves, severe drought, and major floods in
many regions, with serious impacts on human systems, ecosystems,
and associated services.
Warming of 4°C can still be avoided: numerous studies show
that there are technically and economically feasible emissions
pathways to hold warming likely below 2°C. Thus the level of
impacts that developing countries and the rest of the world expe-
rience will be a result of government, private sector, and civil
society decisions and choices, including, unfortunately, inaction.
Observed Impacts and Changes to the
Climate System
The unequivocal effects of greenhouse gas emission–induced
change on the climate system, reported by IPCC’s Fourth Assess-
ment Report (AR4) in 2007, have continued to intensify, more or
less unabated:
• The concentration of the main greenhouse gas, carbon diox-
ide (CO2), has continued to increase from its preindustrial
concentration of approximately 278 parts per million (ppm)
to over 391 ppm in September 2012, with the rate of rise now
at 1.8 ppm per year.
• The present CO2 concentration is higher than paleoclimatic
and geologic evidence indicates has occurred at any time in
the last 15 million years.
• Emissions of CO2 are, at present, about 35,000 million metric
tons per year (including land-use change) and, absent further
policies, are projected to rise to 41,000 million metric tons of
CO2 per year in 2020.
• Global mean temperature has continued to increase and is
now about 0.8°C above preindustrial levels.
A global warming of 0.8°C may not seem large, but many
climate change impacts have already started to emerge, and the
shift from 0.8°C to 2°C warming or beyond will pose even greater
challenges. It is also useful to recall that a global mean temperature
increase of 4°C approaches the difference between temperatures
today and those of the last ice age, when much of central Europe
and the northern United States were covered with kilometers of ice
and global mean temperatures were about 4.5°C to 7°C lower. And
this magnitude of climate change—human induced—is occurring
over a century, not millennia.
The global oceans have continued to warm, with about 90
percent of the excess heat energy trapped by the increased green-
house gas concentrations since 1955 stored in the oceans as heat.
The average increase in sea levels around the world over the 20th
century has been about 15 to 20 centimeters. Over the last decade
the average rate of sea-level rise has increased to about 3.2 cm per
decade. Should this rate remain unchanged, this would mean over
30 cm of additional sea-level rise in the 21st century.
The warming of the atmosphere and oceans is leading to an
accelerating loss of ice from the Greenland and Antarctic ice sheets,
and this melting could add substantially to sea-level rise in the
future. Overall, the rate of loss of ice has more than tripled since
the 1993–2003 period as reported in the IPCC AR4, reaching 1.3
cm per decade over 2004–08; the 2009 loss rate is equivalent to
about 1.7 cm per decade. If ice sheet loss continues at these rates,
without acceleration, the increase in global average sea level due to
this source would be about 15 cm by the end of the 21st century.
A clear illustration of the Greenland ice sheet’s increasing vulner-
ability to warming is the rapid growth in melt area observed since
the 1970s. As for Arctic sea ice, it reached a record minimum in
September 2012, halving the area of ice covering the Arctic Ocean
in summers over the last 30 years.
The effects of global warming are also leading to observed
changes in many other climate and environmental aspects of the
Earth system. The last decade has seen an exceptional number of
extreme heat waves around the world with consequential severe
impacts. Human-induced climate change since the 1960s has
increased the frequency and intensity of heat waves and thus also
likely exacerbated their societal impacts. In some climatic regions,
extreme precipitation and drought have increased in intensity and/
or frequency with a likely human influence. An example of a recent
extreme heat wave is the Russian heat wave of 2010, which had
very significant adverse consequences. Preliminary estimates for
the 2010 heat wave in Russia put the death toll at 55,000, annual
crop failure at about 25 percent, burned areas at more than 1
million hectares, and economic losses at about US$15 billion (1
percent gross domestic product (GDP)).
In the absence of climate change, extreme heat waves in Europe,
Russia, and the United States, for example, would be expected to
occur only once every several hundred years. Observations indicate
a tenfold increase in the surface area of the planet experiencing
extreme heat since the 1950s.
The area of the Earth’s land surface affected by drought has
also likely increased substantially over the last 50 years, somewhat
faster than projected by climate models. The 2012 drought in the
United States impacted about 80 percent of agricultural land,
making it the most severe drought since the 1950s.
Negative effects of higher temperatures have been observed on
agricultural production, with recent studies indicating that since
the 1980s global maize and wheat production may have been
reduced significantly compared to a case without climate change.
Effects of higher temperatures on the economic growth of poor
countries have also been observed over recent decades, suggesting
a significant risk of further reductions in the economic growth
in poor countries in the future due to global warming. An MIT
study1 used historical fluctuations in temperature within countries
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to identify its effects on aggregate economic outcomes. It reported
that higher temperatures substantially reduce economic growth in
poor countries and have wide-ranging effects, reducing agricultural
output, industrial output, and political stability. These findings
inform debates over the climate’s role in economic development
and suggest the possibility of substantial negative impacts of
higher temperatures on poor countries.
Projected Climate Change Impacts in a
4°C World
The effects of 4°C warming will not be evenly distributed around
the world, nor would the consequences be simply an extension of
those felt at 2°C warming. The largest warming will occur over
land and range from 4°C to 10°C. Increases of 6°C or more in
average monthly summer temperatures would be expected in large
regions of the world, including the Mediterranean, North Africa,
the Middle East, and the contiguous United States
Projections for a 4°C world show a dramatic increase in the
intensity and frequency of high-temperature extremes. Recent
extreme heat waves such as in Russia in 2010 are likely to become
the new normal summer in a 4°C world. Tropical South America,
central Africa, and all tropical islands in the Pacific are likely to
regularly experience heat waves of unprecedented magnitude and
duration. In this new high-temperature climate regime, the coolest
months are likely to be substantially warmer than the warmest
months at the end of the 20th century. In regions such as the
Mediterranean, North Africa, the Middle East, and the Tibetan
plateau, almost all summer months are likely to be warmer than
the most extreme heat waves presently experienced. For example,
the warmest July in the Mediterranean region could be 9°C warmer
than today’s warmest July.
Extreme heat waves in recent years have had severe impacts,
causing heat-related deaths, forest fires, and harvest losses. The
impacts of the extreme heat waves projected for a 4°C world have
not been evaluated, but they could be expected to vastly exceed
the consequences experienced to date and potentially exceed the
adaptive capacities of many societies and natural systems.
Rising CO2 Concentration and Ocean
Acidication
Apart from a warming of the climate system, one of the most
serious consequences of rising carbon dioxide concentration in
the atmosphere occurs when it dissolves in the ocean and results
in acidification. A substantial increase in ocean acidity has been
observed since preindustrial times. A warming of 4°C or more
by 2100 would correspond to a CO2 concentration above 800 ppm
and an increase of about 150 percent in acidity of the ocean. The
observed and projected rates of change in ocean acidity over the
next century appear to be unparalleled in Earth’s history. Evidence
is already emerging of the adverse consequences of acidification
for marine organisms and ecosystems, combined with the effects
of warming, overfishing, and habitat destruction.
Coral reefs in particular are acutely sensitive to changes in
water temperatures, ocean pH, and intensity and frequency of
tropical cyclones. Reefs provide protection against coastal floods,
storm surges, and wave damage as well as nursery grounds and
habitat for many fish species. Coral reef growth may stop as CO2
concentration approaches 450 ppm over the coming decades (cor-
responding to a warming of about 1.4°C in the 2030s). By the
time the concentration reaches around 550 ppm (corresponding
to a warming of about 2.4°C in the 2060s), it is likely that coral
reefs in many areas would start to dissolve. The combination
of thermally induced bleaching events, ocean acidification, and
sea-level rise threatens large fractions of coral reefs even at 1.5°C
global warming. The regional extinction of entire coral reef eco-
systems, which could occur well before 4°C is reached, would
have profound consequences for their dependent species and for
the people who depend on them for food, income, tourism, and
shoreline protection.
Rising Sea Levels, Coastal Inundation
and Loss
Warming of 4°C will likely lead to a sea-level rise of 0.5 to 1
meter, and possibly more, by 2100, with several meters more to be
realized in the coming centuries. Limiting warming to 2°C would
likely reduce sea-level rise by about 20 cm by 2100 compared to
a 4°C world. However, even if global warming is limited to 2°C,
global mean sea level could continue to rise, with some estimates
ranging between 1.5 and 4 meters above present-day levels by the
year 2300. Sea-level rise would likely be limited to below 2 meters
only if warming were kept to well below 1.5°C.
Sea-level rise will vary regionally: for a number of geophysically
determined reasons, it is projected to be up to 20 percent higher
in the tropics and below average at higher latitudes. In particular,
the melting of the ice sheets will reduce the gravitational pull on
the ocean toward the ice sheets and, as a consequence, ocean
water will tend to gravitate toward the Equator. Changes in wind
and ocean currents due to global warming and other factors will
also affect regional sea-level rise, as will patterns of ocean heat
uptake and warming.
1 Dell, Melissa, Benjamin F. Jones, and Benjamin A. Olken. 2012. “Temperature
Shocks and Economic Growth: Evidence from the Last Half Century.” American
Economic Journal: Macroeconomics, 4(3): 66–95.
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Sea-level rise impacts are projected to be asymmetrical even
within regions and countries. Of the impacts projected for 31
developing countries, only 10 cities account for two-thirds of the
total exposure to extreme floods. Highly vulnerable cities are to
be found in Mozambique, Madagascar, Mexico, Venezuela, India,
Bangladesh, Indonesia, the Philippines, and Vietnam.
For small island states and river delta regions, rising sea levels
are likely to have far ranging adverse consequences, especially
when combined with the projected increased intensity of tropical
cyclones in many tropical regions, other extreme weather events,
and climate change–induced effects on oceanic ecosystems (for
example, loss of protective reefs due to temperature increases and
ocean acidification).
Risks to Human Support Systems: Food,
Water, Ecosystems, and Human Health
Although impact projections for a 4°C world are still preliminary
and it is often difficult to make comparisons across individual
assessments, this report identifies a number of extremely severe
risks for vital human support systems. With extremes of tempera-
ture, heat waves, rainfall, and drought are projected to increase
with warming; risks will be much higher in a 4°C world compared
to a 2°C world.
In a world rapidly warming toward 4°C, the most adverse
impacts on water availability are likely to occur in association
with growing water demand as the world population increases.
Some estimates indicate that a 4°C warming would significantly
exacerbate existing water scarcity in many regions, particularly
northern and eastern Africa, the Middle East, and South Asia,
while additional countries in Africa would be newly confronted
with water scarcity on a national scale due to population growth.
• Drier conditions are projected for southern Europe, Africa (except
some areas in the northeast), large parts of North America
and South America, and southern Australia, among others.
• Wetter conditions are projected in particular for the northern
high latitudes—that is, northern North America, northern
Europe, and Siberia—and in some monsoon regions. Some
regions may experience reduced water stress compared to a
case without climate change.
• Subseasonal and subregional changes to the hydrological
cycle are associated with severe risks, such as flooding and
drought, which may increase significantly even if annual
averages change little.
With extremes of rainfall and drought projected to increase
with warming, these risks are expected to be much higher in a
4°C world as compared to the 2°C world. In a 2°C world:
• River basins dominated by a monsoon regime, such as the
Ganges and Nile, are particularly vulnerable to changes in
the seasonality of runoff, which may have large and adverse
effects on water availability.
• Mean annual runoff is projected to decrease by 20 to 40 percent
in the Danube, Mississippi, Amazon, and Murray Darling river
basins, but increase by roughly 20 percent in both the Nile
and the Ganges basins.
All these changes approximately double in magnitude in a
4°C world.
The risk for disruptions to ecosystems as a result of ecosystem
shifts, wildfires, ecosystem transformation, and forest dieback
would be significantly higher for 4°C warming as compared to
reduced amounts. Increasing vulnerability to heat and drought
stress will likely lead to increased mortality and species extinction.
Ecosystems will be affected by more frequent extreme weather
events, such as forest loss due to droughts and wildfire exacerbated
by land use and agricultural expansion. In Amazonia, forest fires
could as much as double by 2050 with warming of approximately
1.5°C to 2°C above preindustrial levels. Changes would be expected
to be even more severe in a 4°C world.
In fact, in a 4°C world climate change seems likely to become
the dominant driver of ecosystem shifts, surpassing habitat
destruction as the greatest threat to biodiversity. Recent research
suggests that large-scale loss of biodiversity is likely to occur in a
4°C world, with climate change and high CO2 concentration driv-
ing a transition of the Earth´s ecosystems into a state unknown
in human experience. Ecosystem damage would be expected to
dramatically reduce the provision of ecosystem services on which
society depends (for example, fisheries and protection of coast-
line—afforded by coral reefs and mangroves).
Maintaining adequate food and agricultural output in the
face of increasing population and rising levels of income will be
a challenge irrespective of human-induced climate change. The
IPCC AR4 projected that global food production would increase
for local average temperature rise in the range of 1°C to 3°C, but
may decrease beyond these temperatures.
New results published since 2007, however, are much less opti-
mistic. These results suggest instead a rapidly rising risk of crop
yield reductions as the world warms. Large negative effects have
been observed at high and extreme temperatures in several regions
including India, Africa, the United States, and Australia. For example,
significant nonlinear effects have been observed in the United
States for local daily temperatures increasing to 29°C for corn and
30°C for soybeans. These new results and observations indicate a
significant risk of high-temperature thresholds being crossed that
could substantially undermine food security globally in a 4°C world.
Compounding these risks is the adverse effect of projected sea-
level rise on agriculture in important low-lying delta areas, such
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xvii
as in Bangladesh, Egypt, Vietnam, and parts of the African coast.
Sea-level rise would likely impact many mid-latitude coastal areas
and increase seawater penetration into coastal aquifers used for
irrigation of coastal plains. Further risks are posed by the likeli-
hood of increased drought in mid-latitude regions and increased
flooding at higher latitudes.
The projected increase in intensity of extreme events in the
future would likely have adverse implications for efforts to reduce
poverty, particularly in developing countries. Recent projections
suggest that the poor are especially sensitive to increases in
drought intensity in a 4°C world, especially across Africa, South
Asia, and other regions.
Large-scale extreme events, such as major floods that interfere
with food production, could also induce nutritional deficits and
the increased incidence of epidemic diseases. Flooding can intro-
duce contaminants and diseases into healthy water supplies and
increase the incidence of diarrheal and respiratory illnesses. The
effects of climate change on agricultural production may exacerbate
under-nutrition and malnutrition in many regions—already major
contributors to child mortality in developing countries. Whilst eco-
nomic growth is projected to significantly reduce childhood stunt-
ing, climate change is projected to reverse these gains in a number
of regions: substantial increases in stunting due to malnutrition
are projected to occur with warming of 2°C to 2.5°C, especially
in Sub-Saharan Africa and South Asia, and this is likely to get
worse at 4°C. Despite significant efforts to improve health services
(for example, improved medical care, vaccination development,
surveillance programs), significant additional impacts on poverty
levels and human health are expected. Changes in temperature,
precipitation rates, and humidity influence vector-borne diseases
(for example, malaria and dengue fever) as well as hantaviruses,
leishmaniasis, Lyme disease, and schistosomiasis.
Further health impacts of climate change could include injuries
and deaths due to extreme weather events. Heat-amplified levels of
smog could exacerbate respiratory disorders and heart and blood
vessel diseases, while in some regions climate change–induced
increases in concentrations of aeroallergens (pollens, spores) could
amplify rates of allergic respiratory disorders.
Risks of Disruptions and Displacements
in a 4°C World
Climate change will not occur in a vacuum. Economic growth
and population increases over the 21st century will likely add
to human welfare and increase adaptive capacity in many, if
not most, regions. At the same time, however, there will also
be increasing stresses and demands on a planetary ecosystem
already approaching critical limits and boundaries. The resil-
ience of many natural and managed ecosystems is likely to be
undermined by these pressures and the projected consequences
of climate change.
The projected impacts on water availability, ecosystems, agri-
culture, and human health could lead to large-scale displacement
of populations and have adverse consequences for human security
and economic and trade systems. The full scope of damages in a
4°C world has not been assessed to date.
Large-scale and disruptive changes in the Earth system are
generally not included in modeling exercises, and rarely in impact
assessments. As global warming approaches and exceeds 2°C, the
risk of crossing thresholds of nonlinear tipping elements in the
Earth system, with abrupt climate change impacts and unprec-
edented high-temperature climate regimes, increases. Examples
include the disintegration of the West Antarctic ice sheet leading
to more rapid sea-level rise than projected in this analysis or
large-scale Amazon dieback drastically affecting ecosystems, riv-
ers, agriculture, energy production, and livelihoods in an almost
continental scale region and potentially adding substantially to
21st-century global warming.
There might also be nonlinear responses within particular
economic sectors to high levels of global warming. For example,
nonlinear temperature effects on crops are likely to be extremely
relevant as the world warms to 2°C and above. However, most of
our current crop models do not yet fully account for this effect,
or for the potential increased ranges of variability (for example,
extreme temperatures, new invading pests and diseases, abrupt
shifts in critical climate factors that have large impacts on yields
and/or quality of grains).
Projections of damage costs for climate change impacts typically
assess the costs of local damages, including infrastructure, and do not
provide an adequate consideration of cascade effects (for example,
value-added chains and supply networks) at national and regional
scales. However, in an increasingly globalized world that experi-
ences further specialization in production systems, and thus higher
dependency on infrastructure to deliver produced goods, damages
to infrastructure systems can lead to substantial indirect impacts.
Seaports are an example of an initial point where a breakdown
or substantial disruption in infrastructure facilities could trigger
impacts that reach far beyond the particular location of the loss.
The cumulative and interacting effects of such wide-ranging
impacts, many of which are likely to be felt well before 4°C warm-
ing, are not well understood. For instance, there has not been a
study published in the scientific literature on the full ecological,
human, and economic consequences of a collapse of coral reef
ecosystems, much less when combined with the likely concomitant
loss of marine production due to rising ocean temperatures and
increasing acidification, and the large-scale impacts on human
settlements and infrastructure in low-lying fringe coastal zones
that would result from sea-level rise of a meter or more this cen-
tury and beyond.
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xviii
As the scale and number of impacts grow with increasing global
mean temperature, interactions between them might increasingly
occur, compounding overall impact. For example, a large shock to
agricultural production due to extreme temperatures across many
regions, along with substantial pressure on water resources and
changes in the hydrological cycle, would likely impact both human
health and livelihoods. This could, in turn, cascade into effects on
economic development by reducing a population´s work capacity,
which would then hinder growth in GDP.
With pressures increasing as warming progresses toward
4°C and combining with nonclimate–related social, economic,
and population stresses, the risk of crossing critical social system
thresholds will grow. At such thresholds existing institutions that
would have supported adaptation actions would likely become
much less effective or even collapse. One example is a risk
that sea-level rise in atoll countries exceeds the capabilities of
controlled, adaptive migration, resulting in the need for complete
abandonment of an island or region. Similarly, stresses on human
health, such as heat waves, malnutrition, and decreasing quality
of drinking water due to seawater intrusion, have the potential
to overburden health-care systems to a point where adaptation is
no longer possible, and dislocation is forced.
Thus, given that uncertainty remains about the full nature
and scale of impacts, there is also no certainty that adaptation to
a 4°C world is possible. A 4°C world is likely to be one in which
communities, cities and countries would experience severe disrup-
tions, damage, and dislocation, with many of these risks spread
unequally. It is likely that the poor will suffer most and the global
community could become more fractured, and unequal than
today. The projected 4°C warming simply must not be allowed
to occur—the heat must be turned down. Only early, cooperative,
international actions can make that happen.
xix
Abbreviations
°C degrees Celsius
AIS Antarctic Ice Sheet
AOGCM Atmosphere-Ocean General Circulation Model
AOSIS Alliance of Small Island States
AR4 Fourth Assessment Report of the Intergovernmental Panel on Climate Change
AR5 Fifth Assessment Report of the Intergovernmental Panel on Climate Change
BAU Business as Usual
CaCO
3
Calcium Carbonate
cm Centimeter
CMIP5 Coupled Model Intercomparison Project Phase 5
CO
2
Carbon Dioxide
CO
2
e Carbon Dioxide Equivalent
DIVA Dynamic Interactive Vulnerability Assessment
DJF December January February
GCM General Circulation Model
GDP Gross Domestic Product
GIS Greenland Ice Sheet
GtCO
2
e Gigatonnes—billion metric tons—of Carbon Dioxide Equivalent
IAM Integrated Assessment Model
IBAU “IMAGE (Model) Business As Usual” Scenario (Hinkel et al. 2011)
ISI-MIP Inter-Sectoral Model Inter-comparison Project
IPCC Intergovernmental Panel on Climate Change
JJA June July August
LDC Least Developed Country
MGIC Mountain Glaciers and Ice Caps
NH Northern Hemisphere
NOAA National Oceanic and Atmospheric Administration (United States)
OECD Organisation for Economic Cooperation and Development
PG Population Growth
PGD Population Growth Distribution
ppm Parts per Million
RBAU “Rahmstorf Business As Usual” Scenario (Hinkel et al. 2011)
RCP Representative Concentration Pathway
SH Southern Hemisphere
SLR Sea-Level Rise
SRES IPCC Special Report on Emissions Scenarios
SREX IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation
SSA Sub-Saharan Africa
UNFCCC United National Framework Convention on Climate Change
WBG World Bank Group
WBGT Wet-Bulb Global Temperature
WDR World Development Report
WHO World Health Organization
Chapter
1
1
Introduction
           
-


While the global community has committed itself to holding
warming below 2°C to prevent “dangerous” climate change, the
sum total of current policies—in place and pledged—will very
likely lead to warming far in excess of this level. Indeed, present
emission trends put the world plausibly on a path toward 4°C
warming within this century.
Levels greater than 4°C warming could be possible within
this century should climate sensitivity be higher, or the carbon
cycle and other climate system feedbacks more positive, than
anticipated. Current scientific evidence suggests that even with
the current commitments and pledges fully implemented, there
is roughly a 20 percent likelihood of exceeding 4°C by 2100, and
a 10 percent chance of 4°C being exceeded as early as the 2070s.
Warming would not stop there. Because of the slow response
of the climate system, the greenhouse gas emissions and con-
centrations that would lead to warming of 4°C by 2100 would
actually commit the world to much higher warming, exceeding
6°C or more, in the long term, with several meters of sea-level
rise ultimately associated with this warming (Rogelj et al. 2012;
IEA 2012; Schaeffer & van Vuuren 2012).
Improvements in knowledge have reinforced the findings of
the Fourth Assessment Report (AR4) of the Intergovernmental
Panel on Climate Change (IPCC), especially with respect to an
increasing risk of rapid, abrupt, and irreversible change with
high levels of warming. These risks include, but are not limited,
to the following:
• Meter-scale sea-level rise by 2100 caused by the rapid loss of
ice from Greenland and the West Antarctic Ice Sheet
• Increasing aridity, drought, and extreme temperatures in many
regions, including Africa, southern Europe and the Middle East,
most of the Americas, Australia, and Southeast Asia
• Rapid ocean acidification with wide-ranging, adverse implica-
tions for marine species and entire ecosystems
• Increasing threat to large-scale ecosystems, such as coral reefs
and a large part of the Amazon rain forest
Various climatic extremes can be expected to change in intensity
or frequency, including heat waves, intense rainfall events and
related floods, and tropical cyclone intensity.
There is an increasing risk of substantial impacts with
consequences on a global scale, for example, concerning food
production. A new generation of studies is indicating adverse
impacts of observed warming on crop production regionally and
globally (for example, Lobell et al. 2011). When factored into
analyses of expected food availability under global warming
scenarios, these results indicate a greater sensitivity to warm-
ing than previously estimated, pointing to larger risks for global
and regional food production than in earlier assessments. Such
potential factors have yet to be fully accounted for in global risk
assessments, and if realized in practice, would have substantial
consequences for many sectors and systems, including human
health, human security, and development prospects in already
vulnerable regions. There is also a growing literature on the
potential for cascades of impacts or hotspots of impacts, where
impacts projected for different sectors converge spatially. The
increasing fragility of natural and managed ecosystems and their
services is in turn expected to diminish the resilience of global
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2
socioeconomic systems, leaving them more vulnerable to noncli-
matic stressors and shocks, such as emerging pandemics, trade
disruptions, or financial market shocks (for example, Barnosky
et al. 2012; Rockström et al. 2009).
This context has generated a discussion in the scientific com-
munity over the implications of 4°C, or greater, global warming
for human societies and natural ecosystems (New et al. 2011).
The IPCC AR4 in 2007 provided an overview of the impacts and
vulnerabilities projected up to, and including, this level of global
mean warming. The results of this analysis confirm that global
mean warming of 4°C would result in far-reaching and profound
changes to the climate system, including oceans, atmosphere,
and cryosphere, as well as natural ecosystems—and pose major
challenges to human systems. The impacts of these changes are
likely to be severe and to undermine sustainable development
prospects in many regions. Nevertheless, it is also clear that the
assessments to date of the likely consequences of 4°C global mean
warming are limited, may not capture some of the major risks and
may not accurately account for society’s capacity to adapt. There
have been few systematic attempts to understand and quantify the
differences of climate change impacts for various levels of global
warming across sectors.
This report provides a snapshot of recent scientific literature
and new analyses of likely impacts and risks that would be
associated with a 4°C warming within this century. It is a rigor-
ous attempt to outline a range of risks, focusing on developing
countries, especially the poor.
This report is not a comprehensive scientific assessment, as
will be forthcoming from the Intergovernmental Panel on Climate
Change (IPCC) in 2013/14 in its Fifth Assessment Report (AR5). It
is focused on developing countries while recognizing that devel-
oped countries are also vulnerable and at serious risk of major
damages from climate change.
Chapter 2 summarizes some of the observed changes to the
Earth’s climate system and their impacts on human society that
are already being observed. Chapter 3 provides some background
on the climate scenarios referred to in this report and discusses
the likelihood of a 4°C warming. It also examines projections for
the coming century on the process of ocean acidification, changes
in precipitation that may lead to droughts or floods, and changes
in the incidence of extreme tropical cyclones. Chapters 4 and 5
provide an analysis of projected sea-level rise and increases in
heat extremes, respectively. Chapter 6 discusses the implications
of projected climate changes and other factors for society, specifi-
cally in the sectors of agriculture, water resources, ecosystems,
and human health. Chapter 7 provides an outlook on the potential
risks of nonlinear impacts and identifies where scientists’ under-
standing of a 4°C world is still very limited.
Uncertainties remain in both climate change and impact
projections. This report takes a risk-based approach where risk
is defined as impact times probability: an event with low prob-
ability can still pose a high risk if it implies serious consequences.
While not explicitly addressing the issue of adaptation, the
report provides a basis for further investigation into the potential
and limits of adaptive capacity in the developing world. Developed
countries are also vulnerable and at serious risk of major dam-
ages from climate change. However, as this report reflects, the
distribution of impacts is likely to be inherently unequal and tilted
against many of the world’s poorest regions, which have the least
economic, institutional, scientific, and technical capacity to cope
and adapt proactively. The low adaptive capacity of these regions
in conjunction with the disproportionate burden of impacts places
them among the most vulnerable parts of the world.
The World Development Report 2010 (World Bank Group
2010a) reinforced the findings of the IPCC AR4: the impacts of
climate change will undermine development efforts, which calls
into question whether the Millennium Development Goals can
be achieved in a warming world. This report is, thus, intended
to provide development practitioners with a brief sketch of the
challenges a warming of 4°C above preindustrial levels (hereafter,
referred to as a 4°C world) would pose, as a prelude to further
and deeper examination. It should be noted that this does not
imply a scenario in which global mean temperature is stabilized
by the end of the century.
Given the uncertainty of adaptive capacity in the face of
unprecedented climate change impacts, the report simultaneously
serves as a call for further mitigation action as the best insurance
against an uncertain future.
Chapter
2
5
Observed Climate Changes and Impacts

-
State of the Climate 2011


The Rise of CO2 Concentrations and
Emissions
In order to investigate the hypothesis that atmospheric CO2 con-
centration influences the Earth’s climate, as proposed by John
Tyndall (Tyndall 1861), Charles D. Keeling made systematic mea-
surements of atmospheric CO2 emissions in 1958 at the Mauna Loa
Observatory, Hawaii (Keeling et al. 1976; Pales & Keeling 1965).
Located on the slope of a volcano 3,400 m above sea level and
remote from external sources and sinks of carbon dioxide, the site
was identified as suitable for long-term measurements (Pales and
Keeling 1965), which continue to the present day. Results show
an increase from 316 ppm (parts per million) in March 1958 to
391 ppm in September 2012. Figure 1 shows the measured carbon
dioxide data (red curve) and the annual average CO2 concentrations
in the period 1958–2012. The seasonal oscillation shown on the red
curve reflects the growth of plants in the Northern Hemisphere,
which store more CO2 during the boreal spring and summer than
is respired, effectively taking up carbon from the atmosphere
(Pales and Keeling 1965). Based on ice-core measurements,2 pre-
industrial CO2 concentrations have been shown to have been in
the range of 260 to 280 ppm (Indermühle 1999). Geological and
paleo-climatic evidence makes clear that the present atmospheric
CO2 concentrations are higher than at any time in the last 15 mil-
lion years (Tripati, Roberts, and Eagle 2009).
Since 1959, approximately 350 billion metric tons of carbon
(or GtC)3 have been emitted through human activity, of which 55
2 The report adopts 1750 for defining CO2 concentrations. For global mean tem-
perature pre-industrial is defined as from mid-19th century.
3 Different conventions are used are used in the science and policy communities.
When discussing CO2 emissions it is very common to refer to CO2 emissions by the
weight of carbon—3.67 metric tons of CO2 contains 1 metric ton of carbon, whereas
when CO2 equivalent emissions are discussed, the CO2 (not carbon) equivalent is
almost universally used. In this case 350 billion metric tons of carbon is equivalent
to 1285 billion metric tons of CO2.
Figure 1: Atmospheric CO2 concentrations at Mauna Loa
Observatory.
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
6
percent has been taken up by the oceans and land, with the rest
remaining in the atmosphere (Ballantyne et al. 2012). Figure 2a
shows that CO2 emissions are rising. Absent further policy, global
CO2 emissions (including emissions related to deforestation) will
reach 41 billion metric tons of CO2 per year in 2020. Total green-
house gases will rise to 56 GtCO2e4 in 2020, if no further climate
action is taken between now and 2020 (in a “business-as-usual”
scenario). If current pledges are fully implemented, global total
greenhouse gases emissions in 2020 are likely to be between 53
and 55 billion metric tons CO2e per year (Figure 2b).
Rising Global Mean Temperature
The Fourth Assessment Report (AR4) of the Intergovernmental
Panel on Climate Change (IPCC) found that the rise in global mean
temperature and warming of the climate system were “unequivo-
cal.” Furthermore, “most of the observed increase in global average
temperature since the mid-20th century is very likely due to the
observed increase in anthropogenic greenhouse gas concentra-
tions” (Solomon, Miller et al. 2007). Recent work reinforces this
conclusion. Global mean warming is now approximately 0.8°C
above preindustrial levels.5
The emergence of a robust warming signal over the last three
decades is very clear, as has been shown in a number of studies.
For example, Foster and Rahmstorf (2011) show the clear signal that
emerges after removal of known factors that affect short-term tempera-
ture variations. These factors include solar variability and volcanic
aerosol effects, along with the El Niño/Southern oscillation events
(Figure 3). A suite of studies, as reported by the IPCC, confirms that
the observed warming cannot be explained by natural factors alone
and thus can largely be attributed to anthropogenic influence (for
example, Santer et al 1995; Stott et al. 2000). In fact, the IPCC (2007)
states that during the last 50 years “the sum of solar and volcanic
forcings would likely have produced cooling, not warming”, a result
which is confirmed by more recent work (Wigley and Santer 2012).
Increasing Ocean Heat Storage
While the warming of the surface temperature of the Earth is perhaps
one of the most noticeable changes, approximately 93 percent of
the additional heat absorbed by the Earth system resulting from
an increase in greenhouse gas concentration since 1955 is stored
4 Total greenhouse gas emissions (CO2e) are calculated by multiplying emissions
of each greenhouse gas by its Global Warming Potential (GWPs), a measure that
compares the integrated warming effect of greenhouses to a common base (carbon
dioxide) on a specified time horizon. This report applies 100-year GWPs from IPCC’s
Second Assessment Report, to be consistent with countries reporting national com-
munications to the UNFCCC.
5 See HadCRUT3v: http://www.cru.uea.ac.uk/cru/data/temperature/ and (Jones
et al. 2012).
Figure 2: Global CO2 (a) and total greenhouse gases (b) historic (solid lines) and projected (dashed lines) emissions. CO2 data source:
PRIMAP4BISa baseline and greenhouse gases data source: Climate Action Trackerb. Global pathways include emissions from international transport.
Pledges ranges in (b) consist of the current best estimates of pledges put forward by countries and range from minimum ambition, unconditional
pledges, and lenient rules to maximum ambition, conditional pledges, and more strict rules.
A. B.
a https://sites.google.com/a/primap.org/www/the-primap-model/documentation/baselines
b http://climateactiontracker.org/

7
in the ocean. Recent work by Levitus and colleagues (Levitus et al.
2012) extends the finding of the IPCC AR4. The observed warming
of the world’s oceans “can only be explained by the increase in
atmospheric greenhouse gases.” The strong trend of increasing
ocean heat content continues (Figure 4). Between 1995 and 2010
the world’s oceans as a whole have warmed on average by 0.09°C.
In concert with changes in marine chemistry, warming waters
are expected to adversely affect fisheries, particularly in tropical
regions as stocks migrate away from tropical countries towards
cooler waters (Sumaila 2010). Furthermore, warming surface
waters can enhance stratification, potentially limiting nutrient
availability to primary producers. Another particularly severe
consequence of increasing ocean warming could be the expan-
sion of ocean hypoxic zones,6 ultimately interfering with global
ocean production and damaging marine ecosystems. Reductions
in the oxygenation zones of the ocean are already occurring, and
in some ocean basins have been observed to reduce the habitat
for tropical pelagic fishes, such as tuna (Stramma et al. 2011).
Rising Sea Levels
Sea levels are rising as a result of anthropogenic climate warm-
ing. This rise in sea levels is caused by thermal expansion of the
oceans and by the addition of water to the oceans as a result
of the melting and discharge of ice from mountain glaciers and
ice caps and from the much larger Greenland and Antarctic ice
sheets. A significant fraction of the world population is settled
along coastlines, often in large cities with extensive infrastructure,
making sea-level rise potentially one of the most severe long-term
6 The ocean hypoxic zone is a layer in the ocean with very low oxygen concentra-
tion (also called OMZ – Oxygen Minimum Zone), due to stratification of vertical
layers (limited vertical mixing) and high activity of microbes, which consume oxygen
in processing organic material deposited from oxygen-rich shallower ocean layers
with high biological activity. An hypoxic zone that expands upwards to shallower
ocean layers, as observed, poses problems for zooplankton that hides in this zone
for predators during daytime, while also compressing the oxygen-rich surface zone
above, thereby stressing bottom-dwelling organisms, as well as pelagic (open-sea)
species. Recent observations and modeling suggest the hypoxic zones globally
expand upward (Stramma et al 2008; Rabalais 2010) with increased ocean-surface
temperatures, precipitation and/or river runoff, which enhances stratification, as
well as changes in ocean circulation that limit transport from colder, oxygen-rich
waters into tropical areas and finally the direct outgassing of oxygen, as warmer
waters contain less dissolved oxygen. “Hypoxic events” are created by wind changes
that drive surface waters off shore, which are replaced by deeper waters from the
hypoxic zones entering the continental shelves, or by the rich nutrient content of
such waters stimulating local plankton blooms that consume oxygen when abruptly
dying and decomposing. The hypoxic zones have also expanded near the continents
due to increased fertilizer deposition by precipitation and direct influx of fertilizers
transported by continental runoff, increasing the microbe activity creating the hypoxic
zones. Whereas climate change might enhance precipitation and runoff, other human
activities might enhance, or suppress fertilizer use, as well as runoff.
Figure 3:
Temperature data from different sources (GISS: NASA
Goddard Institute for Space Studies GISS; NCDC: NOAA National
Climate Data Center; CRU: Hadley Center/ Climate Research Unit UK;
RSS: data from Remote Sensing Systems; UAH: University of Alabama
at Huntsville) corrected for short-term temperature variability. When the
data are adjusted to remove the estimated impact of known factors on
short-term temperature variations (El Nino/Southern Oscillation, volcanic
aerosols and solar variability), the global warming signal becomes evident.
Source:
Figure 4:
The increase in total ocean heat content from the surface
to 2000 m, based on running ve-year analyses. Reference period is
1955–2006. The black line shows the increasing heat content at depth
(700 to 2000 m), illustrating a signicant and rising trend, while most of
the heat remains in the top 700 m of the ocean. Vertical bars and shaded
area represent +/–2 standard deviations about the ve-year estimate for
respective depths.
Source: 
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
8
impacts of climate change, depending upon the rate and ultimate
magnitude of the rise.
Substantial progress has been made since the IPCC AR4 in the
quantitative understanding of sea-level rise, especially closure of
the sea-level rise budget. Updated estimates and reconstructions
of sea-level rise, based on tidal gauges and more recently, satel-
lite observations, confirm the findings of the AR4 (Figure 5) and
indicate a sea-level rise of more than 20 cm since preindustrial
times7 to 2009 (Church and White 2011). The rate of sea-level rise
was close to 1.7 mm/year (equivalent to 1.7 cm/decade) during
the 20th century, accelerating to about 3.2 mm/year (equivalent
to 3.2 cm/decade) on average since the beginning of the 1990s
(Meyssignac and Cazenave 2012).
In the IPCC AR4, there were still large uncertainties regarding
the share of the various contributing factors to sea-level rise, with
the sum of individually estimated components accounting for less
than the total observed sea-level rise. Agreement on the quantita-
tive contribution has improved and extended to the 1972–2008
period using updated observational estimates (Church et al.
2011) (Figure 6): over that period, the largest contributions have
come from thermal expansion (0.8 mm/year or 0.8 cm/decade),
mountain glaciers, and ice caps (0.7 mm/year or 0.7 cm/decade),
followed by the ice sheets (0.4 mm/year or 0.4 cm/decade). The
study by Church et al. (2011) concludes that the human influence
on the hydrological cycle through dam building (negative con-
tribution as water is retained on land) and groundwater mining
(positive contribution because of a transfer from land to ocean)
contributed negatively (–0.1 mm/year or –0.1 cm/decade), to
sea-level change over this period. The acceleration of sea-level
rise over the last two decades is mostly explained by an increas-
ing land-ice contribution from 1.1 cm/decade over 1972–2008
period to 1.7 cm/decade over 1993–2008 (Church et al. 2011), in
particular because of the melting of the Greenland and Antarctic
ice sheets, as discussed in the next section. The rate of land ice
contribution to sea level rise has increased by about a factor of
three since the 1972–1992 period.
There are significant regional differences in the rates of observed
sea-level rise because of a range of factors, including differential
heating of the ocean, ocean dynamics (winds and currents),
and the sources and geographical location of ice melt, as well as
subsidence or uplifting of continental margins. Figure 7 shows
reconstructed sea level, indicating that many tropical ocean regions
have experienced faster than global average increases in sea-level
rise. The regional patterns of sea-level rise will vary according
to the different causes contributing to it. This is an issue that is
explored in the regional projections of sea-level rise later in this
report (see Chapter 4).
Longer-term sea-level rise reconstructions help to locate the
contemporary rapid rise within the context of the last few thousand
years. The record used by Kemp et al. (2011), for example, shows
a clear break in the historical record for North Carolina, starting
in the late 19th century (Figure 8). This picture is replicated in
other locations globally.
Increasing Loss of Ice from Greenland
and Antarctica
Both the Greenland and Antarctic ice sheets have been losing mass
since at least the early 1990s. The IPCC AR4 (Chapter 5.5.6 in work-
ing group 1) reported 0.41 ±0.4 mm/year as the rate of sea-level
rise from the ice sheets for the period 1993–2003, while a more
recent estimate by Church et al. in 2011 gives 1.3 ±0.4 mm/year for
the period 2004–08. The rate of mass loss from the ice sheets has
thus risen over the last two decades as estimated from a combina-
tion of satellite gravity measurements, satellite sensors, and mass
balance methods (Velicogna 2009; Rignot et al. 2011). At present,
the losses of ice are shared roughly equally between Greenland
and Antarctica. In their recent review of observations (Figure 9),
Figure 5: Global mean sea level (GMSL) reconstructed from tide-
gauge data (blue, red) and measured from satellite altimetry (black).
The blue and red dashed envelopes indicate the uncertainty, which
grows as one goes back in time, because of the decreasing number of
tide gauges. Blue is the current reconstruction to be compared with one
from 2006. Source: Church and White 2011. Note the scale is in mm of
sea-level-rise—divide by 10 to convert to cm.
Source:
7 While the reference period used for climate projections in this report is the pre-
industrial period (circa 1850s), we reference sea-level rise changes with respect to
contemporary base years (for example, 1980–1999 or 2000), because the attribution
of past sea-level rise to different potential causal factors is difficult.

9
Figure 6: Left panel (a): The contributions of land ice (mountain glaciers and ice caps and Greenland and Antarctic ice sheets), thermosteric sea-
level rise, and terrestrial storage (the net effects of groundwater extraction and dam building), as well as observations from tide gauges (since 1961)
and satellite observations (since 1993). Right panel (b): the sum of the individual contributions approximates the observed sea-level rise since the
1970s. The gaps in the earlier period could be caused by errors in observations.
Source: 
continues, but without further acceleration, there would be a 13
cm contribution by 2100 from these ice sheets. Note that these
numbers are simple extrapolations in time of currently observed
trends and, therefore, cannot provide limiting estimates for projec-
tions about what could happen by 2100.
Observations from the pre-satellite era, complemented by
regional climate modeling, indicate that the Greenland ice sheet
moderately contributed to sea-level rise in the 1960s until early
Figure 8: The North Carolina sea-level record reconstructed for the
past 2,000 years. The period after the late 19th century shows the clear
effect of human induced sea-level rise.
Temperature (C)
A
EIV Global (Land + Ocean) Reconstruction
(Mann et al., 2008)
HADCrutv3
Instrumental Record
-0.4
-0.2
0.0
0.2
0500 1000 1500 2000
853-1076
1274 -1476
0mm/yr +0.6mm/yr -0.1mm/yr +2.1
1865-1892
Sand Point
Tump Point
Change Point
GIA Adjusted Sea Level (m)
Summary of North Carolina sea-level
reconstruction (1 and 2σ error bands)
C
Year (AD)
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
B
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
1900 1940 1980
-0.4
-0.2
0.0
Sand Point
Tump Point
Tide-gauge records
North Carolina
Charleston, SC
Relative Sea Level (m MSL)
(inset)
Reconstructions
Year (AD)
RSL (m MSL)
1860
Source:
Figure 7: Reconstruction of regional sea-level rise rates for the
period 1952–2009, during which the average sea-level rise rate was 1.8
mm per year (equivalent to 1.8 cm/decade). Black stars denote the 91
tide gauges used in the global sea-level reconstruction.
Source:
Rignot and colleagues (Rignot et al. 2011) point out that if the pres-
ent acceleration continues, the ice sheets alone could contribute
up to 56 cm to sea-level rise by 2100. If the present-day loss rate
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
10
1970s, but was in balance until the early 1990s, when it started los-
ing mass again, more vigorously (Rignot, Box, Burgess, and Hanna
2008). Earlier observations from aerial photography in southeast
Greenland indicate widespread glacier retreat in the 1930s, when
air temperatures increased at a rate similar to present (Bjørk et
al. 2012). At that time, many land-terminating glaciers retreated
more rapidly than in the 2000s, whereas marine terminating
glaciers, which drain more of the inland ice, experienced a more
rapid retreat in the recent period in southeast Greenland. Bjørk
and colleagues note that this observation may have implications
for estimating the future sea-level rise contribution of Greenland.
Recent observations indicate that mass loss from the Greenland
ice sheet is presently equally shared between increased surface
melting and increased dynamic ice discharge into the ocean (Van
den Broeke et al. 2009). While it is clear that surface melting will
continue to increase under global warming, there has been more
debate regarding the fate of dynamic ice discharge, for which
physical understanding is still limited. Many marine-terminating
glaciers have accelerated (near doubling of the flow speed) and
retreated since the late 1990s (Moon, Joughin, Smith, and Howat
2012; Rignot and Kanagaratnam 2006). A consensus has emerged
that these retreats are triggered at the terminus of the glaciers, for
example when a floating ice tongue breaks up (Nick, Vieli, Howat,
and Joughin 2009). Observations of intrusion of relatively warm
ocean water into Greenland fjords (Murray et al. 2010; Straneo et
al. 2010) support this view. Another potential explanation of the
recent speed-up, namely basal melt-water lubrication,8 seems not
to be a central mechanism, in light of recent observations (Sundal
et al. 2011) and theory (Schoof 2010).
Increased surface melting mainly occurs at the margin of the
ice sheet, where low elevation permits relatively warm air tem-
peratures. While the melt area on Greenland has been increasing
since the 1970s (Mernild, Mote, and Liston 2011), recent work also
shows a period of enhanced melting occurred from the early 1920s
to the early 1960s. The present melt area is similar in magnitude
as in this earlier period. There are indications that the greatest
melt extent in the past 225 years has occurred in the last decade
(Frauenfeld, Knappenberger, and Michaels 2011). The extreme
surface melt in early July 2012, when an estimated 97 percent of
the ice sheet surface had thawed by July 12 (Figure 10), rather
than the typical pattern of thawing around the ice sheet’s margin,
represents an uncommon but not unprecedented event. Ice cores
from the central part of the ice sheet show that similar thawing
has occurred historically, with the last event being dated to 1889
and previous ones several centuries earlier (Nghiem et al. 2012).
Figure 9: Total ice sheet mass balance, dM/dt, between 1992 and
2010 for (a) Greenland, (b) Antarctica, and c) the sum of Greenland
and Antarctica, in Gt/year from the Mass Budget Method (MBM) (solid
black circle) and GRACE time-variable gravity (solid red triangle), with
associated error bars.
Source: 8 When temperatures rise above zero for sustained periods, melt water from surface
melt ponds intermittently flows down to the base of the ice sheet through crevasses
and can lubricate the contact between ice and bedrock, leading to enhanced sliding
and dynamic discharge.

11
The Greenland ice sheet’s increasing vulnerability to warming is
apparent in the trends and events reported here—the rapid growth
in melt area observed since the 1970s and the record surface melt
in early July 2012.
Ocean Acidication
The oceans play a major role as one of the Earth´s large CO2 sinks.
As atmospheric CO2 rises, the oceans absorb additional CO2 in an
attempt to restore the balance between uptake and release at the
oceans’ surface. They have taken up approximately 25 percent of
anthropogenic CO2 emissions in the period 2000–06 (Canadell et al.
2007). This directly impacts ocean biogeochemistry as CO2 reacts
with water to eventually form a weak acid, resulting in what has
been termed “ocean acidification.” Indeed, such changes have been
observed in waters across the globe. For the period 1750–1994, a
decrease in surface pH9 of 0.1 pH has been calculated (Figure 11),
which corresponds to a 30 percent increase in the concentration
of the hydrogen ion (H+) in seawater (Raven 2005). Observed
increases in ocean acidity are more pronounced at higher latitudes
than in the tropics or subtropics (Bindoff et al. 2007).
Acidification of the world’s oceans because of increasing
atmospheric CO
2
concentration is, thus, one of the most tangible
consequences of CO
2
emissions and rising CO
2
concentration.
Ocean acidification is occurring and will continue to occur, in
the context of warming and a decrease in dissolved oxygen in the
world’s oceans. In the geological past, such observed changes
in pH have often been associated with large-scale extinction
events (Honisch et al. 2012). These changes in pH are projected
to increase in the future. The rate of changes in overall ocean
biogeochemistry currently observed and projected appears to
be unparalleled in Earth history (Caldeira and Wickett 2003;
Honisch et al. 2012).
Critically, the reaction of CO2 with seawater reduces the
availability of carbonate ions that are used by various marine
biota for skeleton and shell formation in the form of calcium
carbonate (CaCO3). Surface waters are typically supersaturated
with aragonite (a mineral form of CaCO3), favoring the forma-
tion of shells and skeletons. If saturation levels are below a value
of 1.0, the water is corrosive to pure aragonite and unprotected
aragonite shells (Feely, Sabine, Hernandez-Ayon, Ianson, and
Hales 2008). Because of anthropogenic CO2 emissions, the levels
at which waters become undersaturated with respect to aragonite
have become shallower when compared to preindustrial levels.
Aragonite saturation depths have been calculated to be 100 to 200
m shallower in the Arabian Sea and Bay of Bengal, while in the
Pacific they are between 30 and 80 m shallower south of 38°S
and between 30 and 100 m north of 3°N (Feely et al. 2004). In
upwelling areas, which are often biologically highly productive,
undersaturation levels have been observed to be shallow enough
for corrosive waters to be upwelled intermittently to the surface.
9 Measure of acidity. Decreasing pH indicates increasing acidity and is on a loga-
rithmic scale; hence a small change in pH represents quite a large physical change.
Figure 10: Greenland surface melt measurements from three
satellites on July 8 (left panel) and July 12 (right panel), 2012.
Source:
Figure 11: Observed changes in ocean acidity (pH) compared to
concentration of carbon dioxide dissolved in seawater (p CO2) alongside
the atmospheric CO2 record from 1956. A decrease in pH indicates an
increase in acidity.
Source: 
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12
Without the higher atmospheric CO2 concentration caused by
human activities, this would very likely not be the case (Fabry,
Seibel, Feely, and Orr 2008).
Loss of Arctic Sea Ice
Arctic sea ice reached a record minimum in September 2012
(Figure 12). This represents a record since at least the beginning
of reliable satellite measurements in 1973, while other assessments
estimate that it is a minimum for about at least the last 1,500
years (Kinnard et al. 2011). The linear trend of September sea ice
extent since the beginning of the satellite record indicates a loss
of 13 percent per decade, the 2012 record being equivalent to an
approximate halving of the ice covered area of the Arctic Ocean
within the last three decades.
Apart from the ice covered area, ice thickness is a relevant
indicator for the loss of Arctic sea ice. The area of thicker ice
(that is, older than two years) is decreasing, making the entire ice
cover more vulnerable to such weather events as the 2012 August
storm, which broke the large area into smaller pieces that melted
relatively rapidly (Figure 13).
Recent scientific studies consistently confirm that the
observed degree of extreme Arctic sea ice loss can only be
explained by anthropogenic climate change. While a variety
of factors have influenced Arctic sea ice during Earth’s history
(for example, changes in summer insolation because of varia-
tions in the Earth’s orbital parameters or natural variability of
wind patterns), these factors can be excluded as causes for the
Figure 12: Geographical overview of the record reduction in
September’s sea ice extent compared to the median distribution for the
period 1979–2000.
Source:
Figure 13: Left panel: Arctic sea ice extent for 2007–12, with the 1979–2000 average in dark grey; light grey shading represents two standard
deviations. Right panel: Changes in multiyear ice from 1983 to 2012.
Source:

13
recently observed trend (Min, Zhang, Zwiers, and Agnew 2008;
Notz and Marotzke 2012).
Apart from severe consequences for the Arctic ecosystem
and human populations associated with them, among the
potential impacts of the loss of Arctic sea ice are changes in
the dominating air pressure systems. Since the heat exchange
between ocean and atmosphere increases as the ice disappears,
large-scale wind patterns can change and extreme winters in
Europe may become more frequent (Francis and Vavrus 2012;
Jaiser, Dethloff, Handorf, Rinke, and Cohen 2012; Petoukhov
and Semenov 2010).
Heat Waves and Extreme Temperatures
The past decade has seen an exceptional number of extreme heat
waves around the world that each caused severe societal impacts
(Coumou and Rahmstorf 2012). Examples of such events include
the European heat wave of 2003 (Stott et al. 2004), the Greek heat
wave of 2007 (Founda and Giannaopoulos 2009), the Australian
heat wave of 2009 (Karoly 2009), the Russian heat wave of 2010
(Barriopedro et al. 2011), the Texas heat wave of 2011 (NOAA 2011;
Rupp et al. 2012), and the U.S. heat wave of 2012 (NOAA 2012,
2012b) (Figure 14).
These heat waves often caused many heat-related deaths, for-
est fires, and harvest losses (for example, Coumou and Rahmstorf
2012). These events were highly unusual with monthly and seasonal
temperatures typically more than 3 standard deviations (sigma)
warmer than the local mean temperature—so-called 3-sigma events.
Without climate change, such 3-sigma events would be expected to
occur only once in several hundreds of years (Hansen et al. 2012).
The five hottest summers in Europe since 1500 all occurred after
2002, with 2003 and 2010 being exceptional outliers (Figure 15)
(Barriopedro et al. 2011). The death toll of the 2003 heat wave is
estimated at 70,000 (Field et al. 2012), with daily excess mortality
reaching up to 2,200 in France (Fouillet et al. 2006) (Figure 16).
The heatwave in Russia in 2010 resulted in an estimated death toll
of 55,000, of which 11,000 deaths were in Moscow alone, and more
than 1 million hectares of burned land (Barriopedro et al. 2011).
In 2012, the United States, experienced a devastating heat wave
Figure 14: Russia 2010 and United States 2012 heat wave temperature anomalies as measured by satellites.
Source:
Figure 15: Distribution (top panel) and timeline (bottom) of
European summer temperatures since 1500.
Source:
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
14
and drought period (NOAA 2012, 2012b). On August 28, about 63
percent of the contiguous United States was affected by drought
conditions (Figure 17) and the January to August period was the
warmest ever recorded. That same period also saw numerous
wildfires, setting a new record for total burned area—exceeding
7.72 million acres (NOAA 2012b).
Recent studies have shown that extreme summer temperatures
can now largely be attributed to climatic warming since the 1960s
(Duffy and Tebaldi 2012; Jones, Lister, and Li 2008; Hansen et al.
2012; Stott et al. 2011). In the 1960s, summertime extremes of
more than three standard deviations warmer than the mean of the
climate were practically absent, affecting less than 1 percent of
the Earth’s surface. The area increased to 4–5 percent by 2006–08,
and by 2009–11 occurred on 6–13 percent of the land surface. Now
such extremely hot outliers typically cover about 10 percent of the
land area (Figure 18) (Hansen et al. 2012).
The above analysis implies that extremely hot summer months
and seasons would almost certainly not have occurred in the absence
of global warming (Coumou, Robinson, and Rahmstorf, in review;
Hansen et al. 2012). Other studies have explicitly attributed indi-
vidual heat waves, notably those in Europe in 2003 (Stott, Stone,
and Allen 2004), Russia in 2010 (Otto et al. 2012), and Texas in
2011 (Rupp et al. 2012) to the human influence on the climate.
Drought and Aridity Trends
On a global scale, warming of the lower atmosphere strengthens
the hydrologic cycle, mainly because warmer air can hold more
water vapor (Coumou and Rahmstorf 2012; Trenberth 2010). This
strengthening causes dry regions to become drier and wet regions
to become wetter, something which is also predicted by climate
models (Trenberth 2010). Increased atmospheric water vapor
loading can also amplify extreme precipitation, which has been
detected and attributed to anthropogenic forcing over Northern
Hemisphere land areas (Min, Zhang, Zwiers, and Hegerl 2011).
Observations covering the last 50 years show that the intensi-
fication of the water cycle indeed affected precipitation patterns
over oceans, roughly at twice the rate predicted by the models
(Durack et al. 2012). Over land, however, patterns of change are
generally more complex because of aerosol forcing (Sun, Roder-
ick, and Farquhar 2012) and regional phenomenon including soil,
moisture feedbacks (C.Taylor, deJeu, Guichard, Harris and Dorigo,
2012). Anthropogenic aerosol forcing likely played a key role in
observed precipitation changes over the period 1940–2009 (Sun
et al. 2012). One example is the likelihood that aerosol forcing
has been linked to Sahel droughts (Booth, Dunstone, Halloran,
Andrews, and Bellouin 2012), as well as a downward precipita-
tion trend in Mediterranean winters (Hoerling et al. 2012). Finally,
changes in large-scale atmospheric circulation, such as a poleward
migration of the mid-latitudinal storm tracks, can also strongly
affect precipitation patterns.
Warming leads to more evaporation and evapotranspiration,
which enhances surface drying and, thereby, the intensity and
duration of droughts (Trenberth 2010). Aridity (that is, the degree
to which a region lacks effective, life-promoting moisture) has
increased since the 1970s by about 1.74 percent per decade,
but natural cycles have played a role as well (Dai 2010, 2011).
Figure 16: Excess deaths observed during the 2003 heat wave in
France. O= observed; E= expected.
Source:
Figure 17: Drought conditions experienced on August 28 in the
contiguous United States.
Source:

15
Dai (2012) reports that warming induced drying has increased
the areas under drought by about 8 percent since the 1970s. This
study, however, includes some caveats relating to the use of the
drought severity index and the particular evapotranspiration
parameterization that was used, and thus should be considered
as preliminary.
One affected region is the Mediterranean, which experienced
10 of the 12 driest winters since 1902 in just the last 20 years
(Hoerling et al. 2012). Anthropogenic greenhouse gas and aero-
sol forcing are key causal factors with respect to the downward
winter precipitation trend in the Mediterranean (Hoerling et al.
2012). In addition, other subtropical regions, where climate models
project winter drying when the climate warms, have seen severe
droughts in recent years (MacDonald 2010; Ummenhofer et al.
2009), but specific attribution studies are still lacking. East Africa
has experienced a trend towards increased drought frequencies
since the 1970s, linked to warmer sea surface temperatures in the
Indian-Pacific warm pool (Funk 2012), which are at least partly
attributable to greenhouse gas forcing (Gleckler et al. 2012). Fur-
thermore, a preliminary study of the Texas drought event in 2011
concluded that the event was roughly 20 times more likely now
than in the 1960s (Rupp, Mote, Massey, Rye, and Allen 2012).
Despite these advances, attribution of drought extremes remains
highly challenging because of limited observational data and
the limited ability of models to capture meso-scale precipitation
dynamics (Sun et al. 2012), as well as the influence of aerosols.
Agricultural Impacts
Since the 1960s, sown areas for all major crops have increasingly
experienced drought, with drought affected areas for maize more
than doubling from 8.5 percent to 18.6 percent (Li, Ye, Wang, and
Yan 2009). Lobell et al. 2011 find that since the 1980s, global crop
production has been negatively affected by climate trends, with
maize and wheat production declining by 3.8 percent and 5.5
percent, respectively, compared to a model simulation without
climate trends. The drought conditions associated with the Russian
heat wave in 2010 caused grain harvest losses of 25 percent, lead-
ing the Russian government to ban wheat exports, and about $15
billion (about 1 percent gross domestic product) of total economic
loss (Barriopedro et al. 2011).
The high sensitivity of crops to extreme temperatures can
cause severe losses to agricultural yields, as has been observed
in the following regions and countries:
• Africa: Based on a large number of maize trials (covering
varieties that are already used or intended to be used by
African farmers) and associated daily weather data in Africa,
Lobell et al. (2011) have found a particularly high sensitivity
of yields to temperatures exceeding 30°C within the grow-
ing season. Overall, they found that each “growing degree
day” spent at a temperature above 30°C decreases yields by
1 percent under optimal (drought-free) rainfed conditions.
A test experiment where daily temperatures were artificially
increased by 1°C showed that—based on the statistical model
the researchers fitted to the data—65 percent of the currently
maize growing areas in Africa would be affected by yield
losses under optimal rainfed conditions. The trial conditions
the researchers analyzed were usually not as nutrient limited
as many agricultural areas in Africa. Therefore, the situation
is not directly comparable to “real world” conditions, but the
study underlines the nonlinear relationship between warm-
ing and yields.
Figure 18: Northern Hemisphere land area covered (left panel) by cold (< –0.43σ), very cold (< –2σ), extremely cold (< –3σ) and (right panel) by
hot (> 0.43σ), very hot (> 2σ) and extremely hot (> 3σ) summer temperatures.
Source:
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16
• United States: In the United State, significant nonlinear effects
are observed above local temperatures of 29°C for maize, 30°C
for soybeans, and 32°C for cotton (Schlenker and Roberts 2009).
• Australia: Large negative effects of a “surprising” dimension
have been found in Australia for regional warming variations
of +2°C, which Asseng, Foster, and Turner argue have general
applicability and could indicate a risk that “could substantially
undermine future global food security” (Asseng, Foster, and
Turner 2011).
• India: Lobell et al. 2012 analyzed satellite measurements
of wheat growth in northern India to estimate the effect of
extreme heat above 34°C. Comparison with commonly used
process-based crop models led them to conclude that crop
models probably underestimate yield losses for warming of
2°C or more by as much as 50 percent for some sowing dates,
where warming of 2°C more refers to an artificial increase of
daily temperatures of 2°C. This effect might be significantly
stronger under higher temperature increases.
High impact regions are expected to be those where trends in
temperature and precipitation go in opposite directions. One such
“hotspot” region is the eastern Mediterranean where wintertime
precipitation, which contributes most to the annual budget, has
been declining (Figure 19), largely because of increasing anthro-
pogenic greenhouse gas and aerosol forcing (Hoerling et al. 2012).
At the same time, summertime temperatures have been increas-
ing steadily since the 1970s (Figure 19), further drying the soils
because of more evaporation.
These climatic trends accumulated to produce four consecutive
dry years following 2006 in Syria, with the 2007–08 drought being
particularly devastating (De Schutter 2011; Trigo et al. 2010). As the
vast majority of crops in this country are nonirrigated (Trigo et al.
2010), the region is highly vulnerable to meteorological drought. In
combination with water mismanagement, the 2008 drought rapidly
led to water stress with more than 40 percent of the cultivated land
affected, strongly reducing wheat and barley production (Trigo et
al. 2010). The repeated droughts resulted in significant losses for
the population, affecting in total 1.3 million people (800,000 of
whom were severely affected), and contributing to the migration
of tens of thousands of families (De Schutter 2011). Clearly, these
impacts are also strongly influenced by nonclimatic factors, such
as governance and demography, which can alter the exposure
and level of vulnerability of societies. Accurate knowledge of the
vulnerability of societies to meteorological events is often poorly
quantified, which hampers quantitative attribution of impacts
(Bouwer 2012). Nevertheless, qualitatively one can state that the
largely human-induced shift toward a climate with more frequent
droughts in the eastern Mediterranean (Hoerling et al. 2012) is
already causing societal impacts in this climatic “hotspot.”
Extreme Events in the Period 2000–12
Recent work has begun to link global warming to recent record-
breaking extreme events with some degree of confidence. Heat
waves, droughts, and floods have posed challenges to affected
societies in the past. Table 1 below shows a number of unusual
weather events for which there is now substantial scientific evidence
linking them to global warming with medium to high levels of con-
fidence. Note that while floods are not included in this table, they
have had devastating effects on human systems and are expected
to increase in frequency and size with rising global temperatures.
Possible Mechanism for Extreme Event
Synchronization
The Russian heat wave and Pakistan flood in 2010 can serve as an
example of synchronicity between extreme events. During these
events, the Northern Hemisphere jet stream exhibited a strongly
meandering pattern, which remained blocked for several weeks.
Such events cause persistent and, therefore, potentially extreme
weather conditions to prevail over unusually longtime spans. These
patterns are more likely to form when the latitudinal temperature
gradient is small, resulting in a weak circumpolar vortex. This is just
what occurred in 2003 as a result of anomalously high near-Arctic
sea-surface temperatures (Coumou and Rahmstorf 2012). Ongoing
melting of Arctic sea ice over recent decades has been linked to
Figure 19: Observed wintertime precipitation (blue), which
contributes most to the annual budget, and summertime temperature
(red), which is most important with respect to evaporative drying, with
their long-term trend for the eastern Mediterranean region.

17
observed changes in the mid-latitudinal jet stream with possible
implications for the occurrence of extreme events, such as heat waves,
floods, and droughts, in different regions (Francis and Vavrus 2012).
Recent analysis of planetary-scale waves indicates that with
increasing global warming, extreme events could occur in a glob-
ally synchronized way more often (Petoukhov, Rahmstorf, Petri,
and Schellnhuber, in review). This could significantly exacerbate
associated risks globally, as extreme events occurring simultaneously
in different regions of the world are likely to put unprecedented
stresses on human systems. For instance, with three large areas
of the world adversely affected by drought at the same time, there
is a growing risk that agricultural production globally may not be
able to compensate as it has in the past for regional droughts (Dai
2012). While more research is needed here, it appears that extreme
events occurring in different sectors would at some point exert
pressure on finite resources for relief and damage compensation.
Welfare Impacts
A recent analysis (Dell and Jones 2009) of historical data for the
period 1950 to 2003 shows that climate change has adversely
affected economic growth in poor countries in recent decades.
Large negative effects of higher temperatures on the economic
growth of poor countries have been shown, with a 1°C rise in
regional temperature in a given year reducing economic growth
in that year by about 1.3 percent. The effects on economic
growth are not limited to reductions in output of individual sec-
tors affected by high temperatures but are felt throughout the
economies of poor countries. The effects were found to persist
over 15-year time horizons. While not conclusive, this study is
arguably suggestive of a risk of reduced economic growth rates in
poor countries in the future, with a likelihood of effects persisting
over the medium term.
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18
Table 1: Selection of record-breaking meteorological events since 2000, their societal impacts and qualitative condence level that the
meteorological event can be attributed to climate change. Adapted from Ref.1
Region (Year) Meteorological Record-breaking Event
Condence in
attribution to
climate change Impact, costs
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  



  
 

 



  
-

  
on
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
  
on



  


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

 




 



 and severe
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 

 
 
 
 
 
 
 

 
 
 
 
 
 
 
 
 
 
 
 


19
Table 1: Selection of record-breaking meteorological events since 2000, their societal impacts and qualitative condence level that the
meteorological event can be attributed to climate change. Adapted from Ref.1
 
 
 
 
 

 
 
 
 
 
 
 
 
 
 
 

continued
Chapter
3
21
21st Century Projections





The nonmitigation IPCC Special Report on Emissions Scenarios
(SRES) (Nakicenovic and Swart 2000), assessed in the IPCC AR4,
gave a warming range for 2100 of 1.6–6.9°C above preindustrial
temperatures. In these projections, about half the uncertainty range
is due to the uncertainties in the climate system response to green-
house gas emissions. Assuming a “best guess” climate response,
the warming response was projected at 2.3–4.5°C by 2100, the
remaining uncertainty being due to different assumptions about
how the world population, economy, and technology will develop
during the 21st century. No central, or most likely, estimate was
provided of future emissions for the SRES scenarios, as it was not
possible to choose one emissions pathway over another as more
likely (Nakicenovic and Swart 2000). The range from the SRES
scenarios, nevertheless, indicates that there are many nonmitiga-
tion scenarios that could lead to warming in excess of 4°C. The
evolution of policies and emissions since the SRES was completed
points to a warming of above 3°C being much more likely than
those levels below, even after including mitigation pledges and
targets adopted since 2009.
While the SRES generation of scenarios did not include mitiga-
tion of greenhouse gas emissions to limit global warming, a range of
new scenarios was developed for the IPCC AR5, three of which are
derived from mitigation scenarios. These so-called Representative
Concentration Pathways (RCPs) (Moss et al. 2010) are compared
with the SRES scenarios in Figure 20. Three of the RCPs are derived
from mitigation scenarios produced by Integrated Assessment
Models (IAMs) that are constructed to simulate the international
energy-economic system and allow for a wide variety of energy
Figure 20: Probabilistic temperature estimates for old (SRES) and
new (RCP) IPCC scenarios. Depending on which global emissions path
is followed, the 4°C temperature threshold could be exceeded before
the end of the century.
Source:
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
22
Box 1: What are Emissions Scenarios?
-












SRES Scenarios


 

 



 



 



Representative Concentration Pathways
-




-


 
 
 
 



23
technologies to satisfy demand (Masui et al. 2011; Thomson et al.
2011; Vuuren et al. 2011; Rao and Riahi 2006).
The purpose of the RCP exercise was to derive a wide range
of plausible pathways through 2100 (and beyond) to be used to
drive the climate and climate impact models, the results of which
would be summarized in the IPCC.
The highest RCP scenario, RCP8.5 (Riahi, Rao, et al. 2011), is
the only nonmitigation pathway within this AR5 scenario group
and is comparable to the highest AR4 SRES scenario (SRES A1FI).
It projects warming by 2100 of close to 5°C. However, RCP6,
one of the RCP mitigation scenarios that assumes only a limited
degree of climate policy intervention, already projects warming
exceeding 4°C by 2100 with a probability of more than 15 percent.
As illustrated in Figure 20, the range of changes in temperature
for the RCP scenarios is wider than for the AR4 SRES scenarios.
The main reason for this is that the RCPs span a greater range of
plausible emissions scenarios, including both scenarios assuming
no mitigation efforts (RCP8.5) and scenarios that assume relatively
ambitious mitigation efforts (RCP3PD). This wide variety of the
RCP pathway range is further illustrated in Figure 21. The median
estimate of warming in 2100 under the nonmitigation RCP8.5
pathway is close to 5°C and still steeply rising, while under the
much lower RCP3PD pathway temperatures have already peak
and slowly transition to a downward trajectory before the end
of this century.
How Likely is a 4°C World?
The emission pledges made at the climate conventions in Copen-
hagen and Cancun, if fully met, place the world on a trajectory for
a global mean warming of well over 3°C. Even if these pledges
are fully implemented there is still about a 20 percent chance of
exceeding 4°C in 2100.
10
If these pledges are not met then there
is a much higher likelihood—more than 40 percent—of warm-
ing exceeding 4°C by 2100, and a 10 percent possibility of this
occurring already by the 2070s, assuming emissions follow the
medium business-as-usual reference pathway. On a higher fos-
sil fuel intensive business-as-usual pathway, such as the IPCC
SRESA1FI, warming exceeds 4°C earlier in the 21st century. It is
important to note, however, that such a level of warming can
still be avoided. There are technically and economically feasible
emission pathways that could still limit warming to 2°C or below
in the 21st century.
To illustrate a possible pathway to warming of 4°C or more,
Figure 22 uses the highest SRES scenario, SRESA1FI, and compares
it to other, lower scenarios. SRESA1FI is a fossil-fuel intensive, high
economic growth scenario that would very likely cause mean the
global temperature to exceed a 4°C increase above preindustrial
temperatures.
Most striking in Figure 22 is the large gap between the pro-
jections by 2100 of current emissions reduction pledges and the
(lower) emissions scenarios needed to limit warming to 1.5–2°C
above pre-industrial levels. This large range in the climate change
implications of the emission scenarios by 2100 is important in its
Figure 21:
Probabilistic temperature estimates for new (RCP) IPCC
scenarios, based on the synthesized carbon-cycle and climate system
understanding of the IPCC AR4. Grey ranges show 66 percent ranges,
yellow lines are the medians. Under a scenario without climate policy
intervention (RCP8.5), median warming could exceed 4°C before the
last decade of this century. In addition, RCP6 (limited climate policy)
shows a more than 15 percent chance to exceed 4°C by 2100.
Source
10 Probabilities of warming projections are based on the approach of (Meinshausen
et al. 2011), which involves running a climate model ensemble of 600 realizations
for each emissions scenario. In the simulations each ensemble member is driven by
a different set of climate-model parameters that define the climate-system response,
including parameters determining climate sensitivity, carbon cycle characteristics, and
many others. Randomly drawn parameter sets that do not allow the climate model to
reproduce a set of observed climate variables over the past centuries (within certain
tolerable “accuracy” levels) are filtered out and not used for the projections, leaving
the 600 realizations that are assumed to have adequate predictive skill.
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
24
own right, but it also sets the stage for an even wider divergence
in the changes that would follow over the subsequent centuries,
given the long response times of the climate system, including
the carbon cycle and climate system components that contribute
to sea-level rise.
The scenarios presented in Figure 22 indicate the likely onset
time for warming of 4°C or more. It can be seen that most of the
scenarios remain fairly close together for the next few decades
of the 21st century. By the 2050s, however, there are substantial
differences among the changes in temperature projected for the
different scenarios. In the highest scenario shown here (SRES A1FI),
the median estimate (50 percent chance) of warming reaches 4°C
by the 2080s, with a smaller probability of 10 percent of exceeding
this level by the 2060s. Others have reached similar conclusions
(Betts et al. 2011). Thus, even if the policy pledges from climate
convention in Copenhagen and Cancun are fully implemented,
there is still a chance of exceeding 4°C in 2100. If the pledges are
not met and present carbon intensity trends continue, then the
higher emissions scenarios shown in Figure 22 become more likely,
raising the probability of reaching 4°C global mean warming by
the last quarter of this century.
Figure 23 shows a probabilistic picture of the regional patterns
of change in temperature and precipitation for the lowest and
highest RCP scenarios for the AR4 generation of AOGCMS. Patterns
are broadly consistent between high and low scenarios. The high
latitudes tend to warm substantially more than the global mean.
RCP8.5, the highest of the new IPCC AR5 RCP scenarios, can
be used to explore the regional implications of a 4°C or warmer
world. For this report, results for RCP8.5 (Moss et al. 2010) from
the new IPCC AR5 CMIP5 (Coupled Model Intercomparison Proj-
ect; Taylor, Stouffer, & Meehl 2012) climate projections have been
analyzed. Figure 24 shows the full range of increase of global mean
temperature over the 21st century, relative to the 1980–2000 period
from 24 models driven by the RCP8.5 scenario, with those eight
models highlighted that produce a mean warming of 4–5°C above
preindustrial temperatures averaged over the period 2080–2100.
In terms of regional changes, the models agree that the most
pronounced warming (between 4°C and 10°C) is likely to occur
over land. During the boreal winter, a strong “arctic amplifica-
tion” effect is projected, resulting in temperature anomalies of
over 10°C in the Arctic region. The subtropical region consisting
of the Mediterranean, northern Africa and the Middle East and
the contiguous United States is likely to see a monthly summer
temperature rise of more than 6°C.
CO2 Concentration and Ocean
Acidication
The high emission scenarios would also result in very high carbon
dioxide concentrations and ocean acidification, as can be seen in
Figure 25 and Figure 26. The increase of carbon dioxide concen-
tration to the present-day value of 390 ppm has caused the pH
to drop by 0.1 since preindustrial conditions. This has increased
ocean acidity, which because of the logarithmic scale of pH is
equivalent to a 30 percent increase in ocean acidity (concentration
of hydrogen ions). The scenarios of 4°C warming or more by 2100
correspond to a carbon dioxide concentration of above 800 ppm
and lead to a further decrease of pH by another 0.3, equivalent to
a 150 percent acidity increase since preindustrial levels.
Ongoing ocean acidification is likely to have very severe
consequences for coral reefs, various species of marine calcifying
organisms, and ocean ecosystems generally (for example, Vézina
& Hoegh-Guldberg 2008; Hofmann and Schellnhuber 2009).
A recent review shows that the degree and timescale of ocean
acidification resulting from anthropogenic CO2 emissions appears
to be greater than during any of the ocean acidification events
identified so far over the geological past, dating back millions of
years and including several mass extinction events (Zeebe 2012).
If atmospheric CO2 reaches 450 ppm, coral reef growth around
the world is expected to slow down considerably and at 550 ppm
reefs are expected to start to dissolve (Cao and Caldeira 2008;
Silverman et al. 2009). Reduced growth, coral skeleton weakening,
Figure 22: Median estimates (lines) from probabilistic temperature
projections for two nonmitigation emission scenarios (SRES A1FI and a
reference scenario close to SRESA1B), both of which come close to, or
exceed by a substantial margin, 4°C warming by 2100. The results for
these emissions are compared to scenarios in which current pledges
are met and to mitigation scenarios holding warming below 2°C with
a 50 percent chance or more (Hare, Cramer, Schaeffer, Battaglini,
and Jaeger 2011; Rogelj et al. 2010; Schaeffer, Hare, Rahmstorf, and
Vermeer 2012). The 2 standard deviation uncertainty range is provided
for one scenario only to enhance readability. A hypothetical scenario
is also plotted for which global emissions stop are ended in 2016, as
an illustrative comparison against pathways that are technically and
economically feasible. The spike in warming after emissions are cut to
zero is due to the removal of the shading effect of sulfate aerosols.
Illustrative low-emission scenario with negative CO2 emissions
from upper half of literature range
in 2nd half of 21st Century
Historical observations
2100
1900 1950 2000 2050
0
1
2°C
1.5°C
3
4
5
Global average surface temperature increase
above pre-industrial levels (°C)
Current Pledges
virtually certain to exceed 2°C; 50% chance above 3°C
Reference (close to SRES A1B)
likely to exceed 3°C
Effect of current
pledges
RCP3PD
likely below 2°C; medium chance to exceed 1.5°C
Global sudden stop to emissions in 2016
likely below 1.5°C
Geophysical
intertia
IPCC SRES A1FI
very likely to exceed 4°C
Stabilization at 50% chance to exceed 2°C

25
and increased temperature dependence would start to affect coral
reefs already below 450 ppm. Thus, a CO2 level of below 350 ppm
appears to be required for the long-term survival of coral reefs,
if multiple stressors, such as high ocean surface-water tempera-
ture events, sea-level rise, and deterioration in water quality, are
included (Veron et al. 2009).
Based on an estimate of the relationship between atmo-
spheric carbon dioxide concentration and surface ocean acidity
(Bernie, Lowe, Tyrrell, and Legge 2010), only very low emission
scenarios are able to halt and ultimately reverse ocean acidifica-
tion (Figure 26). An important caveat on these results is that the
approach used here is likely to be valid only for relatively short
timescales. If mitigation measures are not implemented soon to
reduce carbon dioxide emissions, then ocean acidification can be
expected to extend into the deep ocean. The calculations shown
refer only to the response of the ocean surface layers, and once
ocean acidification has spread more thoroughly, slowing and
reversing this will be much more difficult. This would further
add significant stress to marine ecosystems already under pres-
sure from human influences, such as overfishing and pollution.
Figure 24: Simulated historic and 21st century global mean
temperature anomalies, relative to the preindustrial period (1880–1900),
for 24 CMIP5 models based on the RCP8.5 scenario. The colored
(and labeled) curves show those simulations reaching a global mean
warming of 4°C–5°C warmer than preindustrial for 2080–2100, which
are used for further analysis.
Figure 23: The correlation between regional warming and precipitation changes in the form of joint distributions of mean regional temperature
and precipitation changes in 2100 is shown for the RCP3-PD (blue) and RCP8.5 (orange) scenarios. The latter exceeds 4°C warming globally by
2100. The distributions show the uncertainty in the relationship between warming and precipitation for 20 of the AOGCMs used in the IPCC AR4, and
take into account the signicant effects of aerosols on regional patterns. The boxes indicate the inner 80 percent of the marginal distributions and the
labeling of the axes is the same in all subpanels and given in the legend. The region denitions are based on Giorgi and Bi (2005) and are often used
to describe large-scale climate changes over land areas. Here, they are amended by those for the West and East Antarctic Ice Sheets separated by
the Transantarctic Mountains.
Source
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
26
Droughts and Precipitation
As explained earlier, modeling, observations and theoretical
considerations suggest that greenhouse-gas forcing leads to an
intensification of the water cycle (Trenberth 2010). This implies
that on the planetary scale, in a warmer world generally dry areas
will become drier and wet areas wetter, in the absence of additional
forcing by aerosols (Chen et al. 2011), which are projected to play
a much smaller role relative to greenhouse gases compared to the
20th century. The most robust large-scale feature of climate model
projections seems to be an increase in precipitation in the tropics
and a decrease in the subtropics, as well as an increase in mid
to high latitudes (Trenberth 2010; Allen 2012). On the regional
scale, observational evidence suggests soil-moisture feedbacks
might lead to increased vertical air transport (convection) trig-
gering afternoon rains over drier soils, hence providing a negative
feedback that dampens an increased dryness trend, although it
is as yet unclear if and how the small-scale feedbacks involved
translate to longer time scales and larger subcontinental spatial
scales (Taylor de Jenet 2012).
Using the results from the latest generations of 13 climate
models (CMIP5) that will form major input for IPCC AR5, Sill-
mann et al. (2012) showed that total precipitation on wet days is
generally projected to increase by roughly 10 percent. They also
found that extreme precipitation events, expressed as total annual
precipitation during the five wettest days in the year, is projected to
increase by 20 percent in RCP8.5 (4+°C), indicating an additional
risk of flooding. Large increases in mean total precipitation are
projected for large parts of the Northern Hemisphere, East Africa,
and South and Southeast Asia, as well as Antarctica, while changes
are amplified in high northern and southern latitudes for scenarios
in which global mean warming exceeds 4°C.
Significant increases in extreme precipitation are projected
to be more widespread. The strongest increases of 20–30 percent
precipitation during the annually wettest days were found for
South Asia, Southeast Asia, western Africa, eastern Africa, Alaska,
Greenland, northern Europe, Tibet, and North Asia. The projected
increases in extreme precipitation seem to be concentrated in
the Northern Hemisphere winter season (December, January,
and February) over the Amazon Basin, southern South America,
western North America, central North America, northern Europe,
and Central Asia.
Overall drier conditions and droughts are caused by net
decreases in precipitation and evaporation, the latter enhanced
by higher surface temperatures (Trenberth 2010), as explained in
Chapter 2 on observations. Since the net change determines soil
moisture content, and since increased precipitation might occur in
more intense events, an increase in overall precipitation might be
consistent with overall drier conditions for some regions. Trenberth
(2010) and more recently Dai (2012), who used the CMIP5 model
results mentioned above, showed that particularly significant
soil moisture decreases are projected to occur over much of the
Americas, as well as the Mediterranean, southern Africa, and
Australia. He also found that soil moisture content is projected
to decrease in parts of the high northern latitudes.
Figure 25: Projected impacts on coral reefs as a consequence
of a rising atmospheric CO2 concentration. Coral reef limits from
Silverman et al. (2009) indicate the approximate levels of atmospheric
CO2 concentration at which the reaction of CO2 with seawater reduces
the availability of calcium carbonate to the point that coral reefs stop
growing (450 ppm), or even start to resolve (550 ppm). Based on further
considerations of coral bleaching resulting from associated warming at
high CO2 while also considering other human inuences, Veron et al.
(2009) estimated that the CO2 concentration might have to be reduced
to below 350 ppm to ensure the long-term survival of coral reefs. See
caption of Figure 22 for legend.
1900 2000 2100
300
400
500
600
700
800
900
1000
Year
CO2 concentration (ppm)
1950 2050
Coral reefs start dissolving
Coral reefs stop growing
Long-term limit for reefs
Illustrative low-emission scenario with
strong negative CO2 emissions
Current Pledges
Reference (close to SRES A1B)
RCP3PD
Global sudden stop to emissions in 2016
IPCC SRES A1FI
50% chance to exceed 2°C
Sources: 
Figure 26: Ocean surface pH. Lower pH indicates more severe
ocean acidication, which inhibits the growth of calcifying organisms,
including shellsh, calcareous phytoplankton, and coral reefs. The
SRES A1FI scenarios show increasing ocean acidication likely to be
associated with 4°C warming. Method for estimating pH from Bernie et
al. (2010). Median estimates from probabilistic projections. See Hare et
al. 2011; Rogelj et al. 2010; Schaeffer et al. 2012. See caption of Figure
22 for more details.
1900 1950 2000 2050 2100
7.7
7.8
7.9
8
8.1
Year
Ocean Acidity (pH)
Illustrative low-emission scenario with
strong negative CO2 emissions
Current Pledges
Reference (close to SRES A1B)
RCP3PD
Global sudden stop to emissions in 2016
IPCC SRES A1FI
50% chance to exceed 2°C

27
A different indicator of drought is the Palmer Drought Index,
which measures the cumulative balance of precipitation and
evaporation relative to local conditions, therefore indicating what
is normal for a geographical location. The most extreme droughts
compared to local conditions are projected over the Amazon,
western United States, the Mediterranean, southern Africa, and
southern Australia (Dai 2012). Further discussion of droughts and
their implications for agriculture appears in section 6.


Increasing intensity of extreme dry events appears likely to have
adverse implications for poverty, particularly in developing coun-
tries in the future. According to models that bring together the
biophysical impacts of climate change and economic indicators,
food prices can be expected to rise sharply, regardless of the
exact amount of warming (Nelson et al. 2010). A recent projec-
tion of the change in poverty and changes in extreme dry event
intensity in the 2071 to 2100 period under the SRES A2 scenario
(with warming of about 4.1°C above preindustrial temperatures)
indicates a significant risk of increased climate-induced poverty
(Ahmed, Diffenbaugh, and Hertel 2009). The largest increase in
poverty because of climate change is likely to occur in Africa,
with Bangladesh and Mexico also projected to see substantial
climate-induced poverty increases.
Tropical Cyclones
For some regions, the projected increased intensity of tropical
cyclones poses substantial risks. The IPCC´s Special Report on
Managing the Risks of Extreme Events and Disasters to Advance
Climate Change Adaptation (SREX) reports that the average maxi-
mum cyclone intensity (defined by maximum speed) is likely to
increase in the future (Field et al. 2012). This is to be expected
from both theory and high-resolution modeling (Bender et al.
2010; Knutson et al. 2010), although uncertainty remains as to
whether the global frequency of tropical cyclones will decrease
or remain essentially the same. Increasing exposure through
economic growth and development is likely to lead to higher
economic losses in the future, with floodwaters in many locations
increasing in the absence of additional protection measures. In
the East Asia and Pacific and South Asian regions as a whole,
gross domestic product (GDP) has outpaced increased losses
because of tropical cyclone damage, but in all other regions the
risk of economic losses from tropical cyclones appears to be
growing faster than GDP per capita; in other words, the risk of
loss of wealth because of tropical cyclone disasters appears to be
increasing faster than wealth (UNISDR 2011). Recent work has
demonstrated that the mortality risk from tropical cyclones depends
on such factors as tropical cyclone intensity, exposure, levels of
poverty, and governance structures (Peduzzi et al. 2012). In the
short term, over the next 20 years or so, increases in population
and development pressure combined with projected increases
in tropical cyclone intensity appear likely to greatly increase
the number of people exposed to risk and exacerbate disasters
(Peduzzi et al. 2012). Mendelsohn, Emanuel, Chonabayashi, and
Bakkensen (2012) project that warming reaching roughly 4°C by
2100 is likely to double the present economic damage resulting
from the increased projected frequency of high-intensity tropical
cyclones accompanying global warming, with most damages
concentrated in North America, East Asia, and the Caribbean
and Central American region.
Chapter
4
29
Focus: Sea-level Rise Projections

       
-




It is now understood that, in addition to global rise in sea levels,
a number of factors, such as the respective contribution of the ice
sheets or ocean dynamics, will affect what could happen in any
particular location. Making estimates of regional sea-level rise,
therefore, requires having to make estimates of the loss of ice on
Greenland and Antarctica and from mountain glaciers and ice caps.
Furthermore, there is at present an unquantifiable risk of
nonlinear responses from the West Antarctic Ice Sheet and pos-
sibly from other components of Greenland and Antarctica. In the
1970s, Mercer hypothesized that global warming could trigger
the collapse of the West Antarctic Ice Sheet, which is separated
from the East Antarctic Ice Sheet by a mountain range. The West
Antarctic Ice Sheet is grounded mainly below sea level, with the
deepest points far inland, and has the potential to raise eustatic
global sea level by about 3.3 m (Bamber, Riva, Vermeersen, and
LeBrocq 2009). This estimate takes into account that the reverse
bedslope could trigger instability of the ice sheet, leading to an
unhalted retreat. Since the first discussion of a possible collapse of
the West Antarctic Ice Sheet because of this so-called Marine Ice
Sheet Instability (Weertman 1974) induced by global anthropogenic
greenhouse gas concentrations (Hughes 1973; Mercer 1968, 1978),
the question of if and how this might happen has been debated.
In their review of the issue in 2011, Joughin and Alley conclude
that the possibility of a collapse of the West Antarctic Ice Sheet
cannot be discarded, although it remains unclear how likely such
a collapse is and at what rate it would contribute to sea-level rise.
A range of approaches have been used to estimate the regional
consequences of projected sea-level rise with both a small and
a substantial ice sheet contribution over the 21st century (see
Appendix 1 and Table 2 for a summary).
Using a semi-empirical model indicates that scenarios that
approach 4°C warming by 2100 (2090–2099) lead to median esti-
mates of sea-level rise of nearly 1 m above 1980–1999 levels on this
time frame (Table 2). Several meters of further future sea-level rise
would very likely be committed to under these scenarios (Schaef-
fer et al. 2012). In this scenario, as described in Appendix 1, the
Antarctic and Greenland Ice Sheets (AIS and GIS) contributions
to the total rise are assumed to be around 26 cm each over this
time period. Applying the lower ice-sheet scenario assumption,
the total rise is approximately 50 cm, the AIS and GIS contribu-
tions to the total rise 0 and around 3 cm, respectively (Table 2).
Process-based modeling considerations at the very high end of
physically plausible ice-sheet melt, not used in this report, suggest
that sea-level rise of as much as 2 m by 2100 might be possible at
maximum (Pfeffer et al. 2008).
For a 2°C warming by 2100 (2090–99), the median estimate
of sea-level rise from the semi-empirical model is about 79 cm
above 1980–99 levels. In this case, the AIS and GIS contributions
to the total rise are assumed to be around 23 cm each. Applying
the lower ice-sheet scenario assumption, the median estimate of
the total rise is about 34 cm, with the AIS and GIS contributing
0 and around 2 cm respectively (Table 2).
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
30
Box 2: Predictability of Future Sea-level Changes



-


-
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






Figure 27. Sea level (blue, green: scale on the left) and Antarctic air temperature (orange, gray: scale on the right) over the last 550,000
years, from paleo-records (from right to left: present-day on the left). Sea level varied between about 110 m below and 10 m above present,
while air temperature in Antarctica varied between about 10°C below and 4°C above present, with a very good correlation between both
quantities. Variations in Antarctic air temperature are about two-fold those of global mean air temperature. Low sea-level stands correspond to
glacial periods and high stands to interglacials (see main text).
-120
-80
-40
0
40
RSL (m)
-50
0
50
Residuals around
RSL* (m)
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000 550000
-120
-80
-40
0
40
RSL (m)
-50 0 50
residua ls (m)
0
100
200
300
N
-120
-80
-40
0
40
RSL (m)
-12
-8
-4
0
4
!
T
AA
(°C )
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000 550000
Age (y BP, EDC3)
a.
b.
c. d.
e.
Source:
continued on next page

31
The benefit of choosing a 2°C pathway rather than a 4°C
pathway can be to limit up to about 20 cm of total global sea-level
rise by the end of the century.
Schaeffer et al. (2012) report, with a semi-empirical model,
significant potential to reduce the rate of sea-level rise by 2100 with
deep mitigation scenarios, such as RCP3PD, and even more so with a
scenario consistent with limiting warming to 1.5°C by 2100 (Figure 28).
For example, under deep mitigation scenarios the rate of sea-level
rise could be either stabilized (albeit at three times the present level
under RCP3PD) or reduced from peak levels reached at mid-century
(under a 1.5°C consistent scenario). Under emissions scenarios that
reach or exceed 4°C warming by 2100 the rate of sea-level rise would
continue to increase throughout the 21st century (Figure 29).
Regional Sea-level Rise Risks
Sea level is not “flat” nor uniformly distributed over the Earth.
The presence of mountains, deep-ocean ridges, and even ice sheets
perturb the gravity field of the Earth and give the ocean surface
mountains and valleys. Wind and ocean currents further shape
the sea surface (Yin, Griffies, and Stouffer 2010), with strong cur-
rents featuring a cross-current surface slope (because of Earth
rotation). This effect results in a so-called “dynamic” sea-level
pattern (Figure 30), which describes local deviations from the
gravity-shaped surface (also called geoid), which the ocean would
have if it were at rest. This dynamic topography also adjusts to the
temperature and salinity structure, and thereby the local density
distribution of the underlying water. Apart from those changes in
the sea level itself (or in the absolute sea level, as measured from
the center of the Earth), the vertical motion of the Earth’s crust also
influences the perceived sea level at the coast (also called relative
sea level, as measured from the coast). The elevation of the land
surface responds to current and past changes in ice loading, in
particular the glacial isostatic adjustment since the last deglacia-
tion (Peltier and Andrews 1976). Local land subsidence can also
occur in response to mining (Poland and Davis 1969), leading to
a perceived sea-level rise. In what follows, this publication refers
to sea-level changes regardless of whether they are absolute or
relative changes.






continued
Table 2: Global Mean Sea-Level Projections between Present-Day (1980–99) and the 2090–99 Period

th

th










Scenario Thermal expansion (cm) MGIC (cm)
Thermal
+MGIC (cm) GIS (cm) AIS (cm) Total (cm)
   

  
   
       
   


  
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
32
Climate change perturbs both the geoid and the dynamic topog-
raphy. The redistribution of mass because of melting of continental
ice (mountain glaciers, ice caps, and ice sheets) changes the gravity
field (and therefore the geoid). This leads to above-average rates
of rise in the far field of the melting areas and to below-average
rise—sea-level drop in extreme cases—in the regions surround-
ing shrinking ice sheets and large mountain glaciers (Farrell and
Clark 1976) (Figure 31). That effect is accentuated by local land
uplift around the melting areas. These adjustments are mostly
instantaneous.
Changes in the wind field and in the ocean currents can
also—because of the dynamic effect mentioned above—lead to
strong local sea-level changes (Landerer, Jungclaus, and Marotzke
2007; Levermann, Griesel, Hofmann, Montoya, and Rahmstorf
2005). In certain cases, however, these large deviations from the
global mean rate of rise are caused by natural variability (such as
the El Niño phenomenon) and will not be sustained in the future.
The very high rates of rise observed in the western tropical Pacific
since the 1960s (Becker et al. 2012) likely belong to this category
(B. Meyssignac, Salas y Melia, Becker, Llovel, and Cazenave 2012).
In the following, the authors apply two scenarios (lower
ice-sheet and higher ice-sheet) in a 4°C world to make regional
sea-level rise projections. For methods, please see Appendix 1 and
Table 2 for global-mean projections.
A clear feature of the regional projections for both the lower
and higher ice-sheet scenarios is the relatively high sea-level rise
at low latitudes (in the tropics) and below-average sea-level rise
at higher latitudes (Figure 32). This is primarily because of the
polar location of ice masses whose reduced gravitational pull
accentuates the rise in their far-field, the tropics, similarly to
present-day ice-induced pattern of rise (Figure 31). Close to the
main ice-melt sources (Greenland, Arctic Canada, Alaska, Pata-
gonia, and Antarctica), crustal uplift and reduced self-attraction
cause a below-average rise, and even a sea-level fall in the very
Figure 30: Present-day sea-level dynamic topography. This gure
shows the sea-level deviations from the geoid (that is, the ocean surface
determined by the gravity eld, if the oceans were at rest). Above-
average sea-level is shown in orange/red while below-average sea level
is indicated in blue/purple. The contour lines indicate 10 cm intervals.
This “dynamic topography” reects the equilibrium between the surface
slope and the ocean current systems. Noteworthy is the below-average
sea level along the northeastern coast of the United States, associated
with the Gulf Stream. Climate change is projected to provoke a slow-
down of the Gulf Stream during the 21st century and a corresponding
attening of the ocean surface. This effect alone would, in turn, cause
sea level to rise in that area. Note however that there is no systematic
link between present-day dynamic topography (shown in this gure) and
the future sea-level rise under climate warming.
Source: 
Figure 28: As for Figure 22 but for global mean sea-level rise
using a semi-empirical approach. The indicative/xed present-day rate
of 3.3 mm.yr-1 is the satellite-based mean rate 1993–2007 (Cazenave
and Llovel 2010). Median estimates from probabilistic projections. See
Schaeffer et al. (2012) and caption of Figure 22 for more details.
1900 1950 2000 2050 2100
-25
0
25
50
75
100
125
Year
Sea level (cm above 2000)
Fixed present-day rate
Illustrative low-emission scenario with
strong negative CO2 emissions
Current Pledges
Reference (close to SRES A1B)
RCP3PD
Global sudden stop to emissions in 2016
IPCC SRES A1FI
50% chance to exceed 2°C
Figure 29: As for Figure 22 but for annual rate of global mean
sea-level rise. The indicative/xed present-day rate of 3.3 mm.yr-1 is
the satellite based mean rate 1993–2007 (Cazenave and Llovel 2010).
Median estimates from probabilistic projections. See Schaeffer et al.
(2012 and caption of Figure 22 for more details.
1900 1950 2000 2050 2100
0
5
10
15
20
Year
Rate of Sea Level Rise (mm/year)
Illustrative low-emission scenario with
strong negative CO2 emissions
Current Pledges
Reference (close to SRES A1B)
RCP3PD
Global sudden stop to emissions in 2016
IPCC SRES A1FI
50% chance to exceed 2°C
Fixed present-day rate

33
near-field of a mass source. Further away, the eastern Asian coast
and the Indian Ocean experience above-average contribution
from land-ice melt.
While this is clearly the dominant effect in the higher ice-sheet
case, where the median land-ice contribution makes up around
70 percent of the total, it explains only part of the pattern in the
lower ice-sheet case, where land ice accounts for only 40 percent
of the total median. Ocean dynamics also shape the pattern of
projected sea-level. In particular, above-average contribution
from ocean dynamics is projected along the northeastern North
American and eastern Asian coasts, as well as in the Indian Ocean
(Figure A1.3). In the northeastern North American coast, gravi-
tational forces counteract dynamic effects because of the nearby
location of Greenland. Along the eastern Asian coast and in the
Indian Ocean, however, which are far from melting glaciers, both
gravitational forces and ocean dynamics act to enhance sea-level
rise, which can be up to 20 percent higher than the global mean.
In summary, projected sea-level rise by 2100 presents regional
variations, which are generally contained within ±20 percent of
the global mean rise, although higher values are also possible
(Figure 32). Sea-level rise tends to be larger than the global mean at
low latitudes, such as in vulnerable locations in the Indian Ocean
or in the western Pacific, and less than the global mean at high
latitudes, for example along the Dutch coast, because of the polar
location of the ice sheets and their reduced gravitational pull after
melting. On top of ice-induced patterns, changes in ocean currents
can also lead to significant deviations from the global mean rise.
The northeastern North American coast has indeed been identified
as a “hotspot” where the sea level is rising faster than the global
mean (Sallenger et al. 2012), and might continue to do so (Yin et
al. 2009), if the gravitational depression from the nearby melting
Greenland and Canadian glaciers is moderate.
The biggest uncertainties in regional projections of sea-level
rise are caused by insufficient knowledge of the contributions
from the large ice sheets, especially from dynamic changes in the
Antarctic ice sheet. So far, semi-empirical models or approaches
using kinematic constraints11 have been used to bridge the gap
11 A kinematic constraint is, for example, estimating the maximum ice flux that can
in total pass through the narrow fjords around the Greenland ice sheet assuming an
upper limit of a physically reasonable speed of the glaciers.
Figure 32: Sea-level rise in a 4°C warmer world by 2100 along the
world’s coastlines, from South to North. Each color line indicates an
average over a particular coast as shown in the inlet map in the upper
panel. The scale on the right-hand side represents the ratio of regional
sea-level compared to global-mean sea level (units of percent), and
the vertical bars represent uncertainty thereof, showing 50 percent, 68
percent, and 80 percent ranges.
−100
−90
−80
−70
−60
−50
−40
−30
−20
−10
0
10
20
30
40
50
60
%
−100
−90
−80
−70
−60
−50
−40
−30
−20
−10
0
10
20
30
40
50
60
%
−70 −60 −50 −40 −30 −20 −10 010 20 30 40 50 60 70
0
50
100
150
Melbourne
Cape Town
Mauritius Tuvalu
Mombasa
Maldives
Bay of Bengal
Hong Kong
Lisbon
New York
Vancouver
Dutch Coast
Latitude
Sea−level change (cm)
b. High ice−sheet scenario
0
10
20
30
40
50
60
70
Sea−level change (cm)
a. Low ice−sheet scenario
Figure 31: Present-day rates of regional sea-level rise due to land-
ice melt only (modeled from a compilation of land-ice loss observations).
This features areas of sea-level drop in the regions close to ice sheets
and mountain glaciers (in blue) and areas of sea-level rise further
away (red), as a consequence of a modied gravity eld (reduced
self-attraction from the ice masses) or land uplift. The thick green
contour indicates the global sea-level rise (1.4 mm/yr): locations inside
the contour experience above-average rise, while locations outside
the contour experience below-average sea-level rise or even drop.
Compare Figure A1.3 for projected sea-level contribution from land ice
in a 4°C world
Source:
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
34
between the few available projections of ice-sheet contribution and
the need to provide estimates of future sea-level rise. It should be
noted that warming of 4°C above preindustrial temperatures by
2100 implies a commitment to further sea-level rise beyond this
point, even if temperatures were stabilized.

While a review of the regional impacts of sea level rise has not
been undertaken here, it is useful to indicate some particular risks.
Because of high population densities and often inadequate
urban planning, coastal cities in developing regions are particu-
larly vulnerable to sea-level rise in concert with other impacts of
climate change. Coastal and urban migration, with often associated
unplanned urban sprawl, still exacerbates risks in the future. Sea-
level rise impacts are projected to be asymmetrical even within
regions and countries. Of the impacts projected for 31 developing
countries, only ten cities account for two-thirds of the total expo-
sure to extreme floods. Highly vulnerable cities are to be found in
Mozambique, Madagascar, Mexico, Venezuela, India, Bangladesh,
Indonesia, the Philippines, and Vietnam (Brecht et al. 2012)
Because of the small population of small islands and poten-
tial problems with adaptation implementation, Nicholls et al.
(2011) conclude that forced abandonment seems a possible outcome
even for small changes in sea level. Similarly, Barnett and Adger
(2003) point out that physical impacts might breach a threshold that
pushes social systems into complete abandonment, as institutions
that could facilitate adaptation collapse. Projecting such collapses,
however, can potentially lead to self-fulfilling prophecies, if foreign
aid decreases. Barnett and Adger cite Tuvalu as a case in which
negotiations over migration rights to New Zealand might have
undermined foreign aid investor confidence and thereby indirectly
undermined the potential for adaptive capacity.
A recent detailed review (Simpson et al. 2010) of the conse-
quences for 1 m sea-level rise in the Caribbean illustrates the scale
of the damage that could be caused to small island developing
states by the 2080s. Total cumulative capital GDP loss was estimated
at US$68.2 billion equivalent to about 8.3 percent of projected
GDP in 2080, including present value of permanently lost land,
as well as relocation and reconstruction costs. Annual GDP costs
were estimated by the 2080s at $13.5 billion (1.6 percent of GDP),
mainly in the tourism and agricultural sectors. These estimates
do not include other potential factors, such as water supply costs,
increased health care costs, nonmarket damages, and increased
tropical cyclone damages. The tourism industry, a major source of
economic growth in these regions, was found to be very sensitive
to sea-level rise. Large areas of important wetlands would be lost,
affecting fisheries and water supply for many communities: losses
of 22 percent in Jamaica, 17 percent in Belize, and 15 percent in
the Bahamas are predicted.
Nicholls and Cazenave (2010) stress that geological processes
also drive sea-level rise and, therefore, its impacts. In additional,
human activities, such as drainage and groundwater fluid with-
drawal, exacerbate subsidence in regions of high population density
and economic activity. River deltas are particularly susceptible to
such additional stresses. These observations highlight the potential
for coastal management to alleviate some of the projected impacts.
At the same time, they hint at the double challenge of adapting to
climate change induced sea-level rise and impacts of increasing
coastal urbanization, particularly in developing regions. It thus
appears paramount to include sea-level rise projections in coastal
planning and decisions on long-term infrastructure developments.
Chapter
5
37
Focus: Changes in Extreme Temperatures





Meehl and Tebaldi (2004) found significant increases in intensity,
duration, and frequency of three-day heat events under a business-
as-usual scenario. The intensity of such events increases by up to
3°C in the Mediterranean and the western and southern United
States. Based on the SRES A2 transient greenhouse-gas scenario,
Schär et al. (2004) predict that toward the end of the century about
every second European summer could be as warm as or warmer
than the summer of 2003. Likewise, Stott et al. (2004) show that
under unmitigated emission scenarios, the European summer of
2003 would be classed as an anomalously cold summer relative
to the new climate by the end of the century. Barnett et al. (2006)
show that days exceeding the present-day 99th percentile occur more
than 20 times as frequently in a doubled CO2 climate. In addition,
extremely warm seasons are robustly predicted to become much
more common in response to doubled CO2 (Barnett et al. 2006).
Based on the same ensemble of simulations, Clark, Brown, and
Murphy (2006) conclude that the intensity, duration, and frequency
of summer heat waves are expected to be substantially greater
over all continents, with the largest increases over Europe, North
and South America, and East Asia.
These studies, which analyze extreme weather events in
simulations with a doubling of CO2 and those following a business-
as-usual emissions path, can provide useful insights. Without
exception, such studies show that heat extremes, whether on
daily or seasonal time scales, greatly increase in climates more
than 3°C warmer than today.
To the authors’ knowledge, no single study has specifically
analyzed the number of extremes in a world beyond 4°C warmer
than preindustrial conditions. The authors address this gap in the
science and provide statistical analysis of heat extremes in CMIP5
(Coupled Model Intercomparison Project) climate projections that
reach a 4°C world by the end of the 21st century (Taylor et al.
2012). Methods are described in Appendix 2.
A Substantial Increase in Heat Extremes
The authors’ statistical analysis indicates that monthly heat extremes
will increase dramatically in a world with global mean temperature
more than 4
°
C warmer than preindustrial temperatures. Temperature
anomalies that are associated with highly unusual heat extremes
today (namely, 3-sigma events occurring only once in several hun-
dreds of years in a stationary climate)
12
will have become the norm
over most (greater than 50 percent) continental areas by the end
of the 21st century. Five-sigma events, which are now essentially
12 In general, the standard deviation (sigma) shows how far a variable tends to devi-
ate from its mean value. In the authors’ study it represents the possible year-to-year
changes in local monthly temperature because of natural variability. For a normal
distribution, events warmer than 3 sigma away from the mean have a return time
of 740 years and events warmer than 5 sigma have a return time of several million
years. Monthly temperature data do not necessarily follow a normal distribution (for
example, the distribution can have “long” tails making warm events more likely)
and the return times can be different. Nevertheless, 3-sigma events are extremely
unlikely and 4-sigma events almost certainly have not occurred over the lifetime
of key infrastructure. A warming of 5 sigma means that the average change in the
climate is 5 times larger than the normal year-to-year variation experienced today.
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
38
absent, will become common, especially in the tropics and in the
Northern Hemisphere (NH) mid-latitudes during summertime.
According to the authors’ analysis, the most pronounced
warming will occur over land (see Figure 33, top row). Monthly
mean temperatures over oceans will increase between 0°C and
4°C and over continents between 4°C and 10°C. Warming over
continental regions in the tropics and in the Southern Hemisphere
(SH) is distributed rather evenly without strong spatial and
seasonal variations. The only exception is Argentina, which is
expected to see less wintertime (JJA) warming. In the NH, much
stronger spatial and seasonal variations in continental warming
patterns are observed. During the boreal winter, strong warming
in the near Arctic region is observed due to the so-called “arctic
amplification” effect, resulting in temperature anomalies of over
10°C. Two NH regions can be identified that are expected to see
more warming in summertime than in wintertime: The subtropical
region consisting of the Mediterranean, northern Africa, and the
Middle East, as well as the contiguous United States, are likely to
see monthly summer temperatures rise by more than 6°C.
All land areas show a mean warming of at least 1-sigma above
the present-day mean and most land areas (greater than 80 per-
cent) show warming of at least 2-sigma. Roughly half of the land
area will likely experience a mean warming of more than 3-sigma
during the boreal winter and more than 4-sigma during the boreal
summer. This seasonal difference is due to enhanced warming
over NH mid-latitudinal land areas during the boreal summer.
Shifts in Temperature by Region
In the authors’ analysis, a 4°C warmer world will consistently
cause temperatures in the tropics to shift by more than 6 standard
deviations for all months of the year (Figure 33 bottom panels).
Particularly, countries in tropical South America, Central Africa,
and all tropical islands in the Pacific will see unprecedented
extreme temperatures become the new norm in all months of
the year. In fact, a temperature shift of 6 standard deviations or
more implies a new climatic regime with the coolest months in
2080–2100 being substantially warmer than the warmest months
in the end of the 20th century. In the SH mid-latitudes, monthly
temperatures over the continents by the end of the 21st century
lie in the range of 2- to 4-sigma above the present-day mean in
both seasons. Over large regions of the NH mid-latitudes, the con-
tinental warming (in units of sigma) is much stronger in summer,
Figure 33: Multimodel mean of monthly warming over the 21st century (2080–2100 relative to present day) for the months of JJA (left) and DJF
(right) in units of degrees Celsius (top) and in units of local standard deviation of temperature (bottom). The intensity of the color scale has been
reduced over the oceans for distinction.

39
reaching 4- to 5-sigma, than in winter. This includes large regions
of North America, southern Europe, and central Asia, including
the Tibetan plateau.
From this analysis, the tropics can be identified as high
impact regions, as highlighted in previous studies (Diffenbaugh
and Scherer 2011). Here, continental warming of more than 4
°
C
shifts the local climate to a fundamentally new regime. This
implies that anomalously cold months at the end of the 21st
century will be substantially warmer than record warm months
experienced today.
Outside the tropics, the NH subtropics and mid-latitudes are
expected to experience much more intense heat extremes during
the boreal summer. In the Mediterranean, North Africa, the Middle
East, the Tibetan plateau, and the contiguous United States, almost
all (80 percent to 100 percent) summer months will be warmer
than 3-sigma and approximately half (about 50 percent) will be
warmer than 5-sigma. This implies that temperatures of the warm-
est July within the period 2080–2100 in the Mediterranean region,
for example, are expected to approach 35°C, which is about 9°C
warmer than the warmest July estimated for the present day. This
strong increase in the intensity of summertime extremes over NH
continental regions is likely because of soil moisture feedbacks
(Schär and et al. 2004; Zwiers and Kharin 1998). Once the soil
has completely dried out due to strong evaporation during heat
waves, no more heat can be converted into latent heat, thus further
increasing temperatures. This effect is much more important dur-
ing summers (Schär and et al. 2004) and has been a characteristic
of major heat and drought events in Europe and North America.
Frequency of Signicantly Warmer
Months
Figure 34 shows the frequency of months warmer than 3-, 4-,
and 5-sigma occurring during 2080–2100 for JJA and DJF. This
figure clearly shows that the tropics would move to a new
Figure 34:
Multimodel mean of the percentage of months during 2080–2100 that are warmer than 3- (top), 4- (middle) and 5-sigma (bottom) relative to
the present-day climatology, for the months of JJA (left) and DJF (right). The intensity of the color scale has been reduced over the oceans for distinction.
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
40
climatic regime. In the authors’ analysis, even months warmer
than 5-sigma are very common over tropical regions, reaching 100
percent frequencies in central Africa and parts of tropical South
America. In addition, the tropical ocean maintains anomalies
above 3-sigma 100 percent of the time for all months. Over SH
extra-tropical land areas, the patterns are again broadly similar
between the warm and cold season. Australia and Argentina are
expected to see summer months (DJF) warmer than 3-sigma about
50 percent of the time, but 5-sigma events will still be rare. In the
NH mid-latitudes, especially summertime extremes (3-, 4- and
5-sigma events) will increase dramatically. In the Mediterranean,
North Africa, and Middle East almost all (80 percent to 100 per-
cent) summer months will be warmer than 3-sigma and about
half (about 50 percent) will be warmer than 5-sigma. The same
approximate values hold for summer extremes over the contiguous
United States and the Tibetan plateau. For the Mediterranean,
North Africa, and Middle East, the strong increase in summer-
time extremes is directly related to the enhanced summertime
warming trends in these areas (Figure 33). In contrast, the high
number of summertime extremes over the Tibetan plateau is due
to much smaller standard deviations here in JJA in combination
with a moderate warming. Over the continental both effects play
a role. Warm extremes during the boreal winter hardly increase
over some areas of NH continents, including the eastern United
States and central Europe.
Figure 35 plots the multi-model mean of the warmest July and
January temperatures encountered during the period 2080–2100.
The warmest July month in the Sahara and the Middle East will see
temperatures as high as 45°C, or 6–7°C above the warmest July
simulated for the present day. In the Mediterranean and central
United States, the warmest July in the period 2080–2100 will see
temperatures close to 35°C, or up to 9°C above the warmest July
for the present day. Finally, in the Southern Hemisphere, record
monthly summer extremes (namely, January) will be as warm as
40°C in Australia, or about 5°C warmer than the most extreme
present-day January. Note that temperatures presented here are
monthly averages, which include night-time temperatures. Day-
time temperatures can be expected to significantly exceed the
monthly average.
Monthly heat extremes exceeding 3 standard deviations or more
that occur during summer months are associated with the most
prolonged, and therefore high-impact, heat waves. The authors’
results show that the number of such prolonged heat waves will
increase dramatically in a 4°C warmer world over essentially all
continental regions, with the tropics and the NH subtropics and
mid-latitudes most severely impacted. This is consistent with
Figure 35: Multimodel mean compilation of the most extreme warm monthly temperature experienced at each location in the period 2080–2100
for the months of July (left) and January (right) in absolute temperatures (top) and anomalies compared to the most extreme monthly temperature
simulated during present day (bottom). The intensity of the color scale has been reduced over the oceans for distinction.

41
modeling studies on the increase of heat wave intensity over
the 21st century based on business-as-usual emission scenarios
(Meehl and Tebaldi 2004; Schär and et al. 2004; Stott et al. 2004) or
doubled CO2 simulations (Barnett et al. 2005; Clark et al. 2006;
Zwiers and Kharin 1998). These results also corroborate recent
modeling studies indicating that the tropics are especially vulnerable
to unprecedented heat extremes in the next century (Beaumont
et al. 2011; Diffenbaugh and Scherer 2011).
The Impacts of More Frequent Heat
Waves
Given the humanitarian impacts of recent extreme heat waves,
the strong increase in the number of extreme heat waves in a
4°C world as reported here would pose enormous adaptation
challenges for societies. Prolonged heat waves are generally the
most destructive as mortality and morbidity rates are strongly
linked to heat wave duration, with excess deaths increasing each
additional hot day (Kalkstein and Smoyer 1993; Smoyer 1998; Tan
et al. 2006; Fouillet et al. 2006). Temperature conditions experi-
enced during these recent events would become the new norm in
a 4°C warmer world and a completely new class of heat waves,
with magnitudes never experienced before in the 20th century,
would occur regularly. Societies and ecosystems can be expected
to be especially vulnerable to the latter as they are not adapted to
extremes never experienced before. In particular, the agricultural
sector would be strongly impacted as extreme heat can cause severe
yield losses (Lobell et al. 2012) (see Section 6). Ecosystems in
tropical and sub-tropical regions would be particularly vulnerable
to climate change. The authors’ analysis show that the increase in
absolute temperatures relative to the past variability is largest in
these regions and thus the impacts on ecosystems would become
extreme here (see Section 6).
Chapter
6
43
Sectoral Impacts
-




In light of the knowledge gaps with respect to future effects of
climate change, there are two international research projects that
were recently initiated to quantify impacts within a sector and
across sectors at different levels of global warming, including
high-end scenarios. First, the Agriculture Model Intercomparison
and Improvement Project AgMIP (launched in October 2010) is
bringing together a large number of biophysical and agro-economic
modelling groups explicitly covering regional to global scales to
compare their results and improve their models with regard to
observations (Rötter, Carter, Olesen, and Porter 2011). Second,
the first Inter-Sectoral Model Intercomparison Project (ISI-MIP)
was launched in December 2011 with a fast-track phase designed
to provide a synthesis of cross-sectoral global impact projections
at different levels of global warming (Schiermeier 2012). Both
projects will profit from the new RCPs where the highest reaches
about 5°C of global warming.
Agriculture
The overall conclusions of IPCC AR4 concerning food production
and agriculture included the following:
• Crop productivity is projected to increase slightly at mid- to
high latitudes for local mean temperature increases of up to
1 to 3°C depending on the crop, and then decrease beyond
that in some regions (medium confidence) {WGII 5.4, SPM}.
• At lower latitudes, especially in seasonally dry and tropical
regions, crop productivity is projected to decrease for even small
local temperature increases (1 to 2°C) which would increase
the risk of hunger (medium confidence) {WGII 5.4, SPM}.
• Globally, the potential for food production is projected to
increase with increases in local average temperature over a
range of 1 to 3°C, but above this it is projected to decrease
(medium confidence) {WGII 5.4, 5.5, SPM}.
These findings clearly indicate a growing risk for low-latitude
regions at quite low levels of temperature increase and a grow-
ing risk for systemic global problems above a warming of a few
degrees Celsius. While a comprehensive review of literature is
forthcoming in the IPCC AR5, the snapshot overview of recent
scientific literature provided here illustrates that the concerns
identified in the AR4 are confirmed by recent literature and in
important cases extended. In particular, impacts of extreme heat
waves deserve mention here for observed agricultural impacts
(see also Chapter 2).
This chapter will focus on the latest findings regarding possible
limits and risks to large-scale agriculture production because of
climate change, summarizing recent studies relevant to this risk
assessment, including at high levels of global warming approach-
ing 4°C. In particular, it will deliberately highlight important
13 (http://www.cru.uea.ac.uk/cru/data/temperature/ – October 17, 2012.
Turn Down The heaT: why a 4°C warmer worlD musT Be avoiDeD
44
findings that point to the risks of assuming a forward projection
of historical trends.
Projections for food and agriculture over the 21st century indi-
cate substantial challenges irrespective of climate change. As early
as 2050, the world’s population is expected to reach about 9 billion
people (Lutz and Samir 2010) and demand for food is expected to
increase accordingly. Based on the observed relationship between
per capita GDP and per capita demand for crop calories (human
consumption, feed crops, fish production and losses during food
production), Tilman et al. (2011) project a global increase in the
demand for crops by about 100 percent from 2005 to 2050. Other
estimates for the same period project a 70 percent increase of
demand (Alexandratos 2009). Several projections suggest that
global cereal and livestock production may need to increase by
between 60 and 100 percent to 2050, depending on the warming
scenario (Thornton et al. 2011).
The historical context can on the one hand provide reassurance
that despite growing population, food production has been able
to increase to keep pace with demand and that despite occasional
fluctuations, food prices generally stabilize or decrease in real
terms (Godfray, Crute, et al. 2010). Increases in food production
have mainly been driven by more efficient use of land, rather than
by the extension of arable land, with the former more widespread
in rich countries and the latter tending to be practiced in poor
countries (Tilman et al. 2011). While grain production has more
than doubled, the area of land used for arable agriculture has
only increased by approximately 9 percent (Godfray, Beddington,
et al. 2010).
However, although the expansion of agricultural produc-
tion has proved possible through technological innovation and
improved water-use efficiency, observation and analysis point to
a significant level of vulnerability of food production and prices
to the consequences of climate change, extreme weather, and
underlying social and economic development trends. There are
some indications that climate change may reduce arable land in
low-latitude regions, with reductions most pronounced in Africa,
Latin America, and India (Zhang and Cai 2011). For example,
flooding of agricultural land is also expected to severely impact
crop yields in the future: 10.7 percent of South Asia´s agricultural
land is projected to be exposed to inundation, accompanied by a
10 percent intensification of storm surges, with 1 m sea-level rise
(Lange et al. 2010). Given the competition for land that may be
used for other human activities (for example, urbanization and
biofuel production), which can be expected to increase as climate
change places pressure on scarce resources, it is likely that the main
increase in production will have to be managed by an intensification
of agriculture on the same—or possibly even reduced—amount of
land (Godfray, Beddington et al. 2010; Smith et al. 2010). Declines
in nutrient availability (for example, phosphorus), as well as the
spread in pests and weeds, could further limit the increase of
agricultural productivity. Geographical shifts in production pat-
terns resulting from the effects of global warming could further
escalate distributional issues in the future. While this will not be
taken into consideration here, it illustrates the plethora of factors
to take into account when thinking of challenges to promoting
food security in a warming world.
New results published since 2007 point to a more rapidly
escalating risk of crop yield reductions associated with warming
than previously predicted (Schlenker and Lobell 201