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World Bank

Executive Summary

This report provides a snapshot of recent scientific  literature and new analyses of likely impacts
and risks that would be asso­ ciated 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.

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 oc 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 trap·
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
unprecedented heat waves,  severe drought, and major  floods in many regions, with serious impacts
on human systems, ecosystems, and associated  services.
Warming of 4 oc 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 warld  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  (CO<), 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  CO<  concentration is higher  than  paleoclimatic and geologic evidence  indicates
has occurred  at any time  in the last 1S million years.
• Emissions of C0  are, at present,  about 3S,OOO million metric tons per year (including land-use
change) and, absent further policies, are projected to rise to 41,000 million metric tons of C0
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 oc warming or beyond will pose even greater
challenges. It is also useful to recall that a global mean temperature increase  of 4 oc 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.S°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 19SS stored in the oceans as heat.
The average increase in sea levels around  the world over the 20th century  has been about 1S to 20
centimeters. Over the last decade the average rate of sea-level rise has increased to about 3.2 em
per decade. Should this rate remain unchanged, this would mean over
30 em 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 em per decade  over 2004-08;  the  2009 loss rate is
equivalent to about 1.7 em 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 1S em 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 SS,OOO,
annual crop  failure  at about  2S percent, burned areas  at  more than  1 million hectares, and
economic losses  at about  US$1S 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 oc= only once every several hundred years. Observations indicate a
tenfold increase  in the surface  area of the planet  experiencing extreme  heat since the 19S0s.
The area of the  Earth’s land surface  affected by drought has also likely increased substantially
over the last SO 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 19SOs.
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 study! used historical
fluctuations in temperature within countries

EXECUTIVE SUMMARY
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 zoe 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 oc 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 oc 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 oc warld 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.

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 C0
2
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 em by 2100 compared  to
a 4oc world. However, even if global warming is limited to 2 oc,
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 rising C02
Concentration and Ocean
the  ocean  toward  the  ice sheets  and,  as a consequence, ocean  acification

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 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. a nd  Benjamin A Olken. 2012. “Temperature
Shocks  and  Economic Growth:  Evidence  from  the  L3st Half  Cent ury.” American

by 2100 would correspond to a C02 concentration above 800 ppm
Economic Journal: Macroeconomics. 4 (3): 66-95.
TURN DOWN THE HEAT  WHY A  4 °C WARMER WORLD MUST BE  AVOIDED

vel 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 4oc 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 z o e world.
In a world  rapidly  warming toward  4°C,  the  most  adverse
• 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 ZO to 40 percent in the Danube, Mississippi,
Amazon, and Murray Darling river basins,  but  increase  by roughly  ZO  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 Z050 with warming  of approximately
1.5 oc to Z o C above preindustrial levels. Changes would be expected
to be even more severe in a 4°C world.
In fact, in a 4o 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

impacts  on  water  availability  are likely to occur  in association

4o c world, with climate change and high C0

concentration driv­

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 z oe world.  In a z oe world:

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 1o cto 3° C, but may decrease beyond  these temperatures.
New results published since Z007, 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 Z9°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

EXECUTIVE SUMMARY

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 z oe 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 z oe 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 msts 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.

TURN DOWN THE  HEAT WHY  A 4 °C WARMER WORLD MUST BE  AVOIDED

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.