Working Group II Contribution to the
Intergovernmental Panel on Climate Change
Fourth Assessment Report
Climate Change 2007:
Climate Change Impacts, Adaptation and Vulnerability
Summary for Policymakers
A. Introduction
This Summary sets out the key policy-relevant findings of the Fourth Assessment of Working Group II of the
Intergovernmental Panel on Climate Change (IPCC).
The Assessment is of current scientific understanding of impacts of climate change on natural, managed and
human systems, the capacity of these systems to adapt and their vulnerability1. It builds upon past IPCC
assessments and incorporates new knowledge gained since the Third Assessment.
Statements in this Summary are based on chapters in the Assessment and principal sources are given at the
end of each paragraph2.
B. Current knowledge about observed impacts of climate change on the natural and human environment
From the current Assessment we conclude the following.
Observational evidence from all continents and most oceans shows that many
natural systems are being affected by regional climate changes, particularly
temperature increases.
With regard to changes in snow, ice and frozen ground (including permafrost)4, there is high confidence that
natural systems are affected. Examples are:
• enlargement and increased numbers of glacial lakes [1.3];
• increasing ground instability in permafrost regions, and rock avalanches in mountain regions [1.3];
• changes in some Arctic and Antarctic ecosystems, including those in sea-ice biomes, and also
predators high in the food chain [1.3, 4.4, 15.4].
Based on growing evidence, there is high confidence that the following types of hydrological systems are
being affected around the world:
• increased run-off and earlier spring peak discharge in many glacier- and snow-fed rivers [1.3];
• warming of lakes and rivers in many regions, with effects on thermal structure and water quality
[1.3].
There is very high confidence, based on more evidence from a wider range of species, that recent warming is
strongly affecting terrestrial biological systems, including such changes as:
• earlier timing of spring events, such as leaf-unfolding, bird migration and egg-laying [1.3];
• poleward and upward shifts in ranges in plant and animal species [1.3, 8.2, 14.2].
Based on satellite observations since the early 1980s, there is high confidence that there has been a trend in
many regions towards earlier ‘greening’5 of vegetation in the spring linked to longer thermal growing
seasons due to recent warming. [1.3, 14.2]
There is high confidence, based on substantial new evidence, that observed changes in marine and freshwater
biological systems are associated with rising water temperatures, as well as related changes in ice cover,
salinity, oxygen levels and circulation [1.3]. These include:
• shifts in ranges and changes in algal, plankton and fish abundance in high-latitude oceans [1.3];
• increases in algal and zooplankton abundance in high-latitude and high-altitude lakes [1.3];
• range changes and earlier migrations of fish in rivers [1.3].
The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average
decrease in pH of 0.1 units [IPCC Working Group I Fourth Assessment]. However, the effects of observed
ocean acidification on the marine biosphere are as yet undocumented. [1.3]
A global assessment of data since 1970 has shown it is likely6 that anthropogenic
warming has had a discernible influence on many physical and biological systems.
The consistency between observed and modelled changes in several studies and the spatial
agreement between significant regional warming and consistent impacts at the global scale is sufficient to
conclude with high confidence that anthropogenic warming over the last three decades has had a discernible
influence on many physical and biological systems.
Other effects of regional climate changes on natural and human environments are
emerging, although many are difficult to discern due to adaptation and non-climatic
drivers.
Effects of temperature increases have been documented in the following systems (medium confidence):
• effects on agricultural and forestry management at Northern Hemisphere higher latitudes, such as
earlier spring planting of crops, and alterations in disturbance regimes of forests due to fires and
pests [1.3];
• some aspects of human health, such as heat-related mortality in Europe, infectious disease vectors in
some areas, and allergenic pollen in Northern Hemisphere high and mid-latitudes [1.3, 8.2, 8.ES];
• some human activities in the Arctic (e.g., hunting and travel over snow and ice) and in lowerelevation
alpine areas (such as mountain sports). [1.3]
Recent climate changes and climate variations are beginning to have effects on many other natural and
human systems. However, based on the published literature, the impacts have not yet become established
trends. Examples include:
• Settlements in mountain regions are at enhanced risk to glacier lake outburst floods caused by
melting glaciers. Governmental institutions in some places have begun to respond by building dams
and drainage works. [1.3]
• In the Sahelian region of Africa, warmer and drier conditions have led to a reduced length of
growing season with detrimental effects on crops. In southern Africa, longer dry seasons and more
uncertain rainfall are prompting adaptation measures. [1.3]
• Sea-level rise and human development are together contributing to losses of coastal wetlands and
mangroves and increasing damage from coastal flooding in many areas. [1.3]
C. Current knowledge about future impacts
The following is a selection of the key findings regarding projected impacts, as well as some findings on
vulnerability and adaptation, in each system, sector and region for the range of (unmitigated) climate changes
projected by the IPCC over this century8 judged to be relevant for people and the environment9. The impacts
frequently reflect projected changes in precipitation and other climate variables in addition to temperature,
sea level and concentrations of atmospheric carbon dioxide. The magnitude and timing of impacts will vary
with the amount and timing of climate change and, in some cases, the capacity to adapt. These issues are
discussed further in later sections of the Summary.
Fresh water resources and their management
By mid-century, annual average river runoff and water availability are projected to increase by 10-40% at
high latitudes and in some wet tropical areas, and decrease by 10-30% over some dry regions at mid-latitudes
and in the dry tropics, some of which are presently water stressed areas. In some places and in particular
seasons, changes differ from these annual figures. ** D10 [3.4]
Drought-affected areas will likely increase in extent. Heavy precipitation events, which are very likely to
increase in frequency, will augment flood risk. ** N [Working Group I Fourth Assessment, 3.4]
Adaptation procedures and risk management practices for the water sector are being developed in some
countries and regions that have recognised projected hydrological changes with related uncertainties. *** N
[3.6]
In the course of the century, water supplies stored in glaciers and snow cover are projected to decline,
reducing water availability in regions supplied by meltwater from major mountain ranges, where more than
one-sixth of the world population currently lives.
Ecosystems
The resilience of many ecosystems is likely to be exceeded this century by an unprecedented combination of
climate change, associated disturbances (e.g., flooding, drought, wildfire, insects, ocean acidification), and
other global change drivers (e.g., land use change, pollution, over-exploitation of resources). ** N [4.1 to
4.6]
Over the course of this century net carbon uptake by terrestrial ecosystems is likely to peak before midcentury
and then weaken or even reverse11, thus amplifying climate change. ** [4.ES]
Approximately 20-30% of plant and animal species assessed so far are likely to be at increased risk of
extinction if increases in global average temperature exceed 1.5-2.5oC. * N [4.4, T4.1]
For increases in global average temperature exceeding 1.5-2.5°C and in concomitant atmospheric carbon
dioxide concentrations, there are projected to be major changes in ecosystem structure and function, species’
ecological interactions, and species’ geographic ranges, with predominantly negative consequences for
biodiversity, and ecosystem goods and services e.g., water and food supply. ** N [4.4]
The progressive acidification of oceans due to increasing atmospheric carbon dioxide is expected to have
negative impacts on marine shell forming organisms (e.g., corals) and their dependent species. * N [B4.4,
6.4]
Food, fibre and forest products
Crop productivity is projected to increase slightly at mid to high latitudes for local mean temperature
increases of up to 1-3°C depending on the crop, and then decrease beyond that in some regions. * D [5.4]
At lower latitudes, especially seasonally dry and tropical regions, crop productivity is projected to decrease
for even small local temperature increases (1-2°C), which would increase risk of hunger. * D [5.4]
Globally, the potential for food production is projected to increase with increases in local average
temperature over a range of 1-3°C, but above this it is projected to decrease. * D [5.4, 5.ES]
Adaptations such as altered cultivars and planting times allow low and mid- to high latitude cereal yields to
be maintained at or above baseline yields for modest warming. * N [5.5]
Increases in the frequency of droughts and floods are projected to affect local production negatively,
especially in subsistence sectors at low latitudes. ** D [5.4, 5.ES]
Globally, commercial timber productivity rises modestly with climate change in the short- to medium-term,
with large regional variability around the global trend. * D [5.4]
Regional changes in the distribution and production of particular fish species are expected due to continued
warming, with adverse effects projected for aquaculture and fisheries.
Coastal systems and low-lying areas
Coasts are projected to be exposed to increasing risks, including coastal erosion, due to climate change and
sea-level rise and the effect will be exacerbated by increasing human-induced pressures on coastal areas. ***
D [6.3, 6.4]
Corals are vulnerable to thermal stress and have low adaptive capacity. Increases in sea surface temperature
of about 1 to 3°C are projected to result in more frequent coral bleaching events and widespread mortality,
unless there is thermal adaptation or acclimatisation by corals. *** D [B6.1, 6.4]
Coastal wetlands including salt marshes and mangroves are projected to be negatively affected by sea-level
rise especially where they are constrained on their landward side, or starved of sediment. *** D [6.4]
Many millions more people are projected to be flooded every year due to sea-level rise by the 2080s. Those
densely-populated and low-lying areas where adaptive capacity is relatively low, and which already face
other challenges such as tropical storms or local coastal subsidence, are especially at risk. The numbers
affected will be largest in the mega-deltas of Asia and Africa while small islands are especially vulnerable.
*** D [6.4]
Adaptation for coastal regions will be more challenging in developing countries than developed countries
due to constraints on adaptive capacity. ** D [6.4, 6.5, T6.11]
Industry, Settlement and Society
Costs and benefits of climate change for industry, settlement, and society will vary widely by location and
scale. In the aggregate, however, net effects will tend to be more negative the larger the change in climate. **
N [7.4, 7.6]
The most vulnerable industries, settlements and societies are generally those in coastal and river flood plains,
those whose economies are closely linked with climate-sensitive resources, and those in areas prone to
extreme weather events, especially where rapid urbanisation is occurring. ** D [7.1, 7.3, 7.4, 7.5]
Poor communities can be especially vulnerable, in particular those concentrated in high-risk areas. They
tend to have more limited adaptive capacities, and are more dependent on climate-sensitive resources such as
local water and food supplies. ** N [7.2, 7.4, 5.4]
Where extreme weather events become more intense and/or more frequent, the economic and social costs of
those events will increase, and these increases will be substantial in the areas most directly affected. Climate
change impacts spread from directly impacted areas and sectors to other areas and sectors through extensive
and complex linkages.
Health
Projected climate change-related exposures are likely to affect the health status of millions of people,
particularly those with low adaptive capacity, through:
• increases in malnutrition and consequent disorders, with implications for child growth and
development;
• increased deaths, disease and injury due to heat waves, floods, storms, fires and droughts;
• the increased burden of diarrhoeal disease;
• the increased frequency of cardio-respiratory diseases due to higher concentrations of ground level
ozone related to climate change; and,
• the altered spatial distribution of some infectious disease vectors. ** D [8.4, 8.ES, 8.2]
Climate change is expected to have some mixed effects, such as the decrease or increase of the range and transmission potential of malaria in Africa. ** D [8.4]
Studies in temperate areas12 have shown that climate change is projected to bring some benefits, such as
fewer deaths from cold exposure. Overall it is expected that these benefits will be outweighed by the
negative health effects of rising temperatures world-wide, especially in developing countries. ** D [8.4]
The balance of positive and negative health impacts will vary from one location to another, and will alter
over time as temperatures continue to rise. Critically important will be factors that directly shape the health
of populations such as education, health care, public health prevention and infrastructure and economic
development.
Africa
By 2020, between 75 and 250 million people are projected to be exposed to an increase of water stress due to
climate change. If coupled with increased demand, this will adversely affect livelihoods and exacerbate
water-related problems. ** D [9.4, 3.4, 8.2, 8.4]
Agricultural production, including access to food, in many African countries and regions is projected to be
severely compromised by climate variability and change. The area suitable for agriculture, the length of
growing seasons and yield potential, particularly along the margins of semi-arid and arid areas, are expected
to decrease. This would further adversely affect food security and exacerbate malnutrition in the continent. In
some countries, yields from rain-fed agriculture could be reduced by up to 50% by 2020. ** D [9.2, 9.4,
F9.4, 9.6, 8.4]
Local food supplies are projected to be negatively affected by decreasing fisheries resources in large lakes
due to rising water temperatures, which may be exacerbated by continued over-fishing. ** N [9.4, 5.4, 8.4]
Towards the end of the 21st century, projected sea-level rise will affect low-lying coastal areas with large
populations. The cost of adaptation could amount to at least 5-10% of GDP. Mangroves and coral reefs are
projected to be further degraded, with additional consequences for fisheries and tourism. ** D [9.4]
New studies confirm that Africa is one of the most vulnerable continents to climate variability and change
because of multiple stresses and low adaptive capacity. Some adaptation to current climate variability is
taking place, however, this may be insufficient for future changes in climate. ** N [9.5]
Asia
Glacier melt in the Himalayas is projected to increase flooding, rock avalanches from destabilised slopes,
and affect water resources within the next two to three decades. This will be followed by decreased river
flows as the glaciers recede. * N [10.2, 10.4]
Freshwater availability in Central, South, East and Southeast Asia particularly in large river basins is
projected to decrease due to climate change which, along with population growth and increasing demand
arising from higher standards of living, could adversely affect more than a billion people by the 2050s.
Coastal areas, especially heavily-populated mega-delta regions in South, East and Southeast Asia, will be at
greatest risk due to increased flooding from the sea and in some mega-deltas flooding from the rivers. ** D
[10.4]
Climate change is projected to impinge on sustainable development of most developing countries of Asia as
it compounds the pressures on natural resources and the environment associated with rapid urbanisation,
industrialisation, and economic development. ** D [10.5]
It is projected that crop yields could increase up to 20% in East and Southeast Asia while it could decrease
up to 30% in Central and South Asia by the mid-21st century. Taken together and considering the influence
of rapid population growth and urbanization, the risk of hunger is projected to remain very high in several
developing countries. * N [10.4.1]
Endemic morbidity and mortality due to diarrhoeal disease primarily associated with floods and droughts are
expected to rise in East, South and Southeast Asia due to projected changes in hydrological cycle associated
with global warming. Increases in coastal water temperature would exacerbate the abundance and/or toxicity
of cholera in South Asia. **N [10.4.5]
Australia and New Zealand
As a result of reduced precipitation and increased evaporation, water security problems are projected to
intensify by 2030 in southern and eastern Australia and, in New Zealand, in Northland and some eastern
regions. ** D [11.4]
Significant loss of biodiversity is projected to occur by 2020 in some ecologically-rich sites including the
Great Barrier Reef and Queensland Wet Tropics. Other sites at risk include Kakadu wetlands, south-west
Australia, sub-Antarctic islands and the alpine areas of both countries. *** D [11.4]
Ongoing coastal development and population growth in areas such as Cairns and Southeast Queensland
(Australia) and Northland to Bay of Plenty (New Zealand), are projected to exacerbate risks from sea-level
rise and increases in the severity and frequency of storms and coastal flooding by 2050. *** D [11.4, 11.6]
Production from agriculture and forestry by 2030 is projected to decline over much of southern and eastern
Australia, and over parts of eastern New Zealand, due to increased drought and fire. However, in New
Zealand, initial benefits to agriculture and forestry are projected in western and southern areas and close to
major rivers due to a longer growing season, less frost and increased rainfall. ** N [11.4]
The region has substantial adaptive capacity due to well-developed economies and scientific and technical
capabilities, but there are considerable constraints to implementation and major challenges from changes in
extreme events. Natural systems have limited adaptive capacity. ** N [11.2, 11.5]
Europe
For the first time, wide ranging impacts of changes in current climate have been documented: retreating
glaciers, longer growing seasons, shift of species ranges, and health impacts due to a heat wave of
unprecedented magnitude. The observed changes described above are consistent with those projected for
future climate change. *** N [12.2, 12.4, 12.6]
Nearly all European regions are anticipated to be negatively affected by some future impacts of climate
change and these will pose challenges to many economic sectors. Climate change is expected to magnify
regional differences in Europe’s natural resources and assets. Negative impacts will include increased risk of
inland flash floods, and more frequent coastal flooding and increased erosion (due to storminess and sealevel
rise). The great majority of organisms and ecosystems will have difficulties adapting to climate change.
Mountainous areas will face glacier retreat, reduced snow cover and winter tourism, and extensive species
losses (in some areas up to 60% under high emission scenarios by 2080).
In Southern Europe, climate change is projected to worsen conditions (high temperatures and drought) in a
region already vulnerable to climate variability, and to reduce water availability, hydropower potential,
summer tourism, and in general, crop productivity. It is also projected to increase health risks due to heat
waves and the frequency of wildfires. ** D [12.2, 12.4, 12.7]
In Central and Eastern Europe, summer precipitation is projected to decrease, causing higher water stress.
Health risks due to heat waves are projected to increase. Forest productivity is expected to decline and the
frequency of peatland fires to increase. ** D [12.4]
In Northern Europe, climate change is initially projected to bring mixed effects, including some benefits such
as reduced demand for heating, increased crop yields and increased forest growth. However, as climate
change continues, its negative impacts (including more frequent winter floods, endangered ecosystems and
increasing ground instability) are likely to outweigh its benefits. ** D [12.4]
Adaptation to climate change is likely to benefit from experience gained in reaction to extreme climate
events, by specifically implementing proactive climate change risk management adaptation plans. *** N
[12.5]
Latin America
By mid-century, increases in temperature and associated decreases in soil water are projected to lead to
gradual replacement of tropical forest by savanna in eastern Amazonia. Semi-arid vegetation will tend to be
replaced by arid-land vegetation. There is a risk of significant biodiversity loss through species extinction in
many areas of tropical Latin America. ** D [13.4]
In drier areas, climate change is expected to lead to salinisation and desertification of agricultural land.
Productivity of some important crops are projected to decrease and livestock productivity to decline, with
adverse consequences for food security. In temperate zones soybean yields are projected to increase. ** N
[13.4, 13.7]
Sea-level rise is projected to cause increased risk of flooding in low-lying areas. ** N [13.4, 13.7] Increases
in sea surface temperature due to climate change are projected to have adverse effects on Mesoamerican
coral reefs, and cause shifts in the location of south-east Pacific fish stocks. ** N [13.4]
Changes in precipitation patterns and the disappearance of glaciers are projected to significantly affect water
availability for human consumption, agriculture and energy generation. ** D [13.4]
Some countries have made efforts to adapt, particularly through conservation of key ecosystems, early
warning systems, risk management in agriculture, strategies for flood drought and coastal management, and
disease surveillance systems. However, the effectiveness of these efforts is outweighed by: lack of basic
information, observation and monitoring systems; lack of capacity building and appropriate political,
institutional and technological frameworks; low income; and settlements in vulnerable areas, among others.
** D [13.2]
North America
Moderate climate change in the early decades of the century is projected to increase aggregate yields of rainfed
agriculture by 5-20%, but with important variability among regions. Major challenges are projected for
crops that are near the warm end of their suitable range or depend on highly utilised water resources. ** D
[14.4]
Warming in western mountains is projected to cause decreased snowpack, more winter flooding, and reduced
summer flows, exacerbating competition for over-allocated water resources.
Disturbances from pests, diseases, and fire are projected to have increasing impacts on forests, with an
extended period of high fire risk and large increases in area burned. *** N [14.4, B14.1]
Cities that currently experience heat waves are expected to be further challenged by an increased number,
intensity and duration of heat waves during the course of the century, with potential for adverse health
impacts. The growing number of the elderly population is most at risk. *** D [14.4]
Coastal communities and habitats will be increasingly stressed by climate change impacts interacting with
development and pollution. Population growth and the rising value of infrastructure in coastal areas increase
vulnerability to climate variability and future climate change, with losses projected to increase if the intensity
of tropical storms increases. Current adaptation is uneven and readiness for increased exposure is low. *** N
[14.4]
Polar Regions
In the Polar Regions, the main projected biophysical effects are reductions in thickness and extent of glaciers
and ice sheets, and changes in natural ecosystems with detrimental effects on many organisms including
migratory birds, mammals and higher predators. In the Arctic, additional impacts include reductions in the
extent of sea ice and permafrost, increased coastal erosion, and an increase in the depth of permafrost
seasonal thawing. ** D [15.3, 15.4, 15.2]
For Arctic human communities, impacts, particularly resulting from changing snow and ice conditions, are
projected to be mixed. Detrimental impacts would include those on infrastructure and traditional indigenous
ways of life. ** D [15.4]
Beneficial impacts would include reduced heating costs and more navigable northern sea routes. * D [15.4]
In both polar regions, specific ecosystems and habitats are projected to be vulnerable, as climatic barriers to
species’ invasions are lowered. ** D [15.6, 15.4]
Already Arctic human communities are adapting to climate change, but both external and internal stressors
challenge their adaptive capacities. Despite the resilience shown historically by Arctic indigenous
communities, some traditional ways of life are being threatened and substantial investments are needed to
adapt or re-locate physical structures and communities. ** D [15.ES]
Small Islands
Small islands, whether located in the Tropics or higher latitudes, have characteristics which make them
especially vulnerable to the effects of climate change, sea level rise and extreme events. *** [16.1, 16.5]
Deterioration in coastal conditions, for example through erosion of beaches and coral bleaching, is expected
to affect local resources, e.g., fisheries, and reduce the value of these destinations for tourism. ** D [16.4]
Sea-level rise is expected to exacerbate inundation, storm surge, erosion and other coastal hazards, thus
threatening vital infrastructure, settlements and facilities that support the livelihood of island communities.
*** D [16.4]
Climate change is projected by the mid-century to reduce water resources in many small islands, e.g., in the
Caribbean and Pacific, to the point where they become insufficient to meet demand during low rainfall
periods. *** D [16.4]
With higher temperatures, increased invasion by non-native species is expected to occur, particularly on
middle and high-latitude islands.
Impacts due to altered frequencies and intensities of extreme weather, climate, and
sea level events are very likely to change.
Since the IPCC Third Assessment, confidence has increased that some weather events and extremes will
become more frequent, more widespread and/or more intense during the 21st century; and more is known
about the potential effects of such changes.
Some large-scale climate events have the potential to cause very large impacts,
especially after the 21st century.
Very large sea-level rises that would result from widespread deglaciation of Greenland and West Antarctic
ice sheets imply major changes in coastlines and ecosystems, and inundation of low-lying areas, with
greatest effects in river deltas. Relocating populations, economic activity, and infrastructure would be costly
and challenging. There is medium confidence that at least partial deglaciation of the Greenland ice sheet, and
possibly the West Antarctic ice sheet, would occur over a period of time ranging from centuries to millennia
for a global average temperature increase of 1- 4°C (relative to 1990-2000), causing a contribution to sea
level rise of 4-6 m or more. The complete melting of the Greenland ice sheet and the West Antarctic ice
sheet would lead to a contribution to sea-level rise of up to 7 m and about 5 m, respectively. [Working Group
I Fourth Assessment 6.4, 10.7; Working Group II Fourth Assessment 19.3]
Based on climate model results, it is very unlikely that the Meridional Overturning Circulation (MOC) in the
North Atlantic will undergo a large abrupt transition during the 21st century. Slowing of the MOC this
century is very likely, but temperatures over the Atlantic and Europe are projected to increase nevertheless,
due to global warming. Impacts of large-scale and persistent changes in the MOC are likely to include
changes to marine ecosystem productivity, fisheries, ocean carbon dioxide uptake, oceanic oxygen
concentrations and terrestrial vegetation.
D. Current knowledge about responding to climate change
Some adaptation is occurring now, to observed and projected future climate change,
but on a limited basis.
There is growing evidence since the IPCC Third Assessment of human activity to adapt to observed and
anticipated climate change. For example, climate change is considered in the design of infrastructure projects
such as coastal defence in the Maldives and The Netherlands, and the Confederation Bridge in Canada.
Other examples include prevention of glacial lake outburst flooding in Nepal, and policies and strategies
such as water management in Australia and government responses to heat waves in, for example, some
European countries.
Adaptation will be necessary to address impacts resulting from the warming which
is already unavoidable due to past emissions.
Past emissions are estimated to involve some unavoidable warming (about a further 0.6°C by the end of the
century) even if atmospheric greenhouse gas concentrations remain at 2000 levels (see Working Group I
Fourth Assessment). There are some impacts for which adaptation is the only available and appropriate
response. An indication of these impacts can be seen in Table SPM-1.
A wide array of adaptation options is available, but more extensive adaptation than
is currently occurring is required to reduce vulnerability to future climate change.
There are barriers, limits and costs, but these are not fully understood.
Impacts are expected to increase with increases in global average temperature, as indicated in Table SPM-1.
Although many early impacts of climate change can be effectively addressed through adaptation, the options
for successful adaptation diminish and the associated costs increase with increasing climate change. At
present we do not have a clear picture of the limits to adaptation, or the cost, partly because effective
adaptation measures are highly dependent on specific, geographical and climate risk factors as well as
institutional, political and financial constraints.
Vulnerability to climate change can be exacerbated by the presence of other
stresses.
Non-climate stresses can increase vulnerability to climate change by reducing resilience and can also reduce
adaptive capacity because of resource deployment to competing needs. For example, current stresses on
some coral reefs include marine pollution and chemical runoff from agriculture as well as increases in water
temperature and ocean acidification. Vulnerable regions face multiple stresses that affect their exposure and
sensitivity as well as their capacity to adapt. These stresses arise from, for example, current climate hazards,
poverty and unequal access to resources, food insecurity, trends in economic globalisation, conflict, and
incidence of disease such as HIV/AIDS. [7.4, 8.3, 17.3, 20.3] Adaptation measures are seldom undertaken in
response to climate change alone but can be integrated within, for example, water resource management,
coastal defence, and disaster planning.
Many impacts can be avoided, reduced or delayed by mitigation.
A small number of impact assessments have now been completed for scenarios in which future atmospheric
concentrations of greenhouse gases are stabilised. Although these studies do not take full account of
uncertainties in projected climate under stabilisation, they nevertheless provide indications of damages
avoided or vulnerabilities and risks reduced for different amounts of emissions reduction. [2.4, T20.6]
A portfolio of adaptation and mitigation measures can diminish the risks associated
with climate change.
Even the most stringent mitigation efforts cannot avoid further impacts of climate change in the next few
decades, which makes adaptation essential, particularly in addressing near-term impacts. Unmitigated
climate change would, in the long term, be likely to exceed the capacity of natural, managed and human
systems to adapt.
Impacts of climate change will vary regionally but, aggregated and discounted to
the present, they are very likely to impose net annual costs which will increase over
time as global temperatures increase.
This Assessment makes it clear that the impacts of future climate change will be mixed across regions. For
increases in global mean temperature of less than 1 to 3oC above 1990 levels, some impacts are projected to
produce benefits in some places and some sectors, and produce costs in other places and other sectors . It is,
however, projected that some low latitude and polar regions will experience net costs even for small
increases in temperature. It is very likely that all regions will experience either declines in net benefits or
increases in net costs for increases in temperature greater than about 2 to 3°C [9.ES, 9.5, 10.6, T109, 15.3,
15.ES]. These observations re-confirm evidence reported in the Third Assessment that, while developing
countries are expected to experience larger percentage losses, global mean losses could be 1-5% Gross
Domestic Product (GDP) for 4oC of warming. [F20.3]
Many estimates of aggregate net economic costs of damages from climate change across the globe (i.e., the
social cost of carbon (SCC), expressed in terms of future net benefits and costs that are discounted to the
present) are now available. Peer-reviewed estimates of the social cost of carbon for 2005 have an average
value of US$43 per tonne of carbon (tC) (i.e., US$12 per tonne of carbon dioxide) but the range around this
mean is large. For example, in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon
(US$-3 per tonne of carbon dioxide) up to US$350/tC (US$130 per tonne of carbon dioxide)
The large ranges of SCC are due in the large part to differences in assumptions regarding climate sensitivity,
response lags, the treatment of risk and equity, economic and non-economic impacts, the inclusion of
potentially catastrophic losses and discount rates. It is very likely that globally aggregated figures
underestimate the damage costs because they cannot include many non-quantifiable impacts. Taken as a
whole, the range of published evidence indicates that the net damage costs of climate change are likely to be
significant and to increase over time. |
ipcc_summary_policimakers_q2007.pdf