Climate Change 2007:

Synthesis Report

Summary for Policymakers

An Assessment of the Intergovernmental Panel on Climate Change

This summary, approved in detail at IPCC Plenary XXVII (Valencia, Spain, 12-17 November 2007), represents the

formally agreed statement of the IPCC concerning key findings and uncertainties contained in the Working Group

contributions to the Fourth Assessment Report.

Based on a draft prepared by:

Lenny Bernstein, Peter Bosch, Osvaldo Canziani, Zhenlin Chen, Renate Christ, Ogunlade Davidson, William Hare, Saleemul

Huq, David Karoly, Vladimir Kattsov, Zbigniew Kundzewicz, Jian Liu, Ulrike Lohmann, Martin Manning, Taroh Matsuno,

Bettina Menne, Bert Metz, Monirul Mirza, Neville Nicholls, Leonard Nurse, Rajendra Pachauri, Jean Palutikof, Martin

Parry, Dahe Qin, Nijavalli Ravindranath, Andy Reisinger, Jiawen Ren, Keywan Riahi, Cynthia Rosenzweig, Matilde

Rusticucci, Stephen Schneider, Youba Sokona, Susan Solomon, Peter Stott, Ronald Stouffer, Taishi Sugiyama, Rob Swart,

Dennis Tirpak, Coleen Vogel, Gary Yohe
Summary for Policymakers

2

Introduction

This Synthesis Report is based on the assessment carried

out by the three Working Groups of the Intergovernmental

Panel on Climate Change (IPCC). It provides an integrated

view of climate change as the final part of the IPCC’s Fourth

Assessment Report (AR4).

A complete elaboration of the Topics covered in this summary can be found in this Synthesis Report and in the underlying reports of the three Working Groups.

1. Observed changes in climate and

their effects

Warming of the climate system is unequivocal, as is

now evident from observations of increases in global

average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level

(Figure SPM.1). {1.1}

Eleven of the last twelve years (1995-2006) rank among

the twelve warmest years in the instrumental record of global

surface temperature (since 1850). The 100-year linear trend

(1906-2005) of 0.74 [0.56 to 0.92]°C1 is larger than the corresponding trend of 0.6 [0.4 to 0.8]°C (1901-2000) given in

the Third Assessment Report (TAR) (Figure SPM.1). The temperature increase is widespread over the globe and is greater

at higher northern latitudes. Land regions have warmed faster

than the oceans (Figures SPM.2, SPM.4). {1.1, 1.2}

Rising sea level is consistent with warming (Figure

SPM.1). Global average sea level has risen since 1961 at an

average rate of 1.8 [1.3 to 2.3] mm/yr and since 1993 at 3.1

[2.4 to 3.8] mm/yr, with contributions from thermal expansion, melting glaciers and ice caps, and the polar ice sheets.

Whether the faster rate for 1993 to 2003 reflects decadal variation or an increase in the longer-term trend is unclear. {1.1}

Observed decreases in snow and ice extent are also consistent with warming (Figure SPM.1). Satellite data since 1978

show that annual average Arctic sea ice extent has shrunk by

2.7 [2.1 to 3.3]% per decade, with larger decreases in summer

of 7.4 [5.0 to 9.8]% per decade. Mountain glaciers and snow

cover on average have declined in both hemispheres. {1.1}

From 1900 to 2005, precipitation increased significantly

in eastern parts of North and South America, northern Europe

and northern and central Asia but declined in the Sahel, the

Mediterranean, southern Africa and parts of southern Asia.

Globally, the area affected by drought has likely2 increased

since the 1970s. {1.1}

It is very likely that over the past 50 years: cold days, cold

nights and frosts have become less frequent over most land

areas, and hot days and hot nights have become more frequent.

It is likely that: heat waves have become more frequent over

most land areas, the frequency of heavy precipitation events

has increased over most areas, and since 1975 the incidence

of extreme high sea level3 has increased worldwide. {1.1}

There is observational evidence of an increase in intense

tropical cyclone activity in the North Atlantic since about 1970,

with limited evidence of increases elsewhere. There is no clear

trend in the annual numbers of tropical cyclones. It is difficult

to ascertain longer-term trends in cyclone activity, particularly

prior to 1970. {1.1}

Average Northern Hemisphere temperatures during the

second half of the 20th century were very likely higher than

during any other 50-year period in the last 500 years and likely

the highest in at least the past 1300 years. {1.1}

Observational evidence4 from all continents and most

oceans shows that many natural systems are being

affected by regional climate changes, particularly temperature increases. {1.2}

Changes in snow, ice and frozen ground have with high confidence increased the number and size of glacial lakes, increased

ground instability in mountain and other permafrost regions and

led to changes in some Arctic and Antarctic ecosystems. {1.2}

There is high confidence that some hydrological systems

have also been affected through increased runoff and earlier

spring peak discharge in many glacier- and snow-fed rivers

and through effects on thermal structure and water quality of

warming rivers and lakes. {1.2}

In terrestrial ecosystems, earlier timing of spring events

and poleward and upward shifts in plant and animal ranges

are with very high confidence linked to recent warming. In

some marine and freshwater systems, shifts in ranges and

changes in algal, plankton and fish abundance are with high

confidence associated with rising water temperatures, as well

as related changes in ice cover, salinity, oxygen levels and

circulation. {1.2}

Of the more than 29,000 observational data series, from

75 studies, that show significant change in many physical and

biological systems, more than 89% are consistent with the

direction of change expected as a response to warming (Fig1 Numbers in square brackets indicate a 90% uncertainty interval around a best estimate, i.e. there is an estimated 5% likelihood that the value

could be above the range given in square brackets and 5% likelihood that the value could be below that range. Uncertainty intervals are not

necessarily symmetric around the corresponding best estimate.

2 Words in italics represent calibrated expressions of uncertainty and confidence. Relevant terms are explained in the Box ‘Treatment of uncertainty’ in the Introduction of this Synthesis Report.

3 Excluding tsunamis, which are not due to climate change. Extreme high sea level depends on average sea level and on regional weather

systems. It is defined here as the highest 1% of hourly values of observed sea level at a station for a given reference period.

4 Based largely on data sets that cover the period since 1970.
3

Summary for Policymakers

(a) Global average surface temperature

(b) Global average sea level

(c) Northern Hemisphere snow cover

Figure SPM.1. Observed changes in (a) global average surface temperature; (b) global average sea level from tide gauge (blue) and satellite

(red) data and (c) Northern Hemisphere snow cover for March-April. All differences are relative to corresponding averages for the period 1961-

1990. Smoothed curves represent decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals

estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c). {Figure 1.1}

Changes in temperature, sea level and Northern Hemisphere snow cover

ure SPM.2). However, there is a notable lack of geographic

balance in data and literature on observed changes, with

marked scarcity in developing countries. {1.2, 1.3}

There is medium confidence that other effects of regional climate change on natural and human environments are emerging, although many are difficult to discern due to adaptation and non-climatic drivers. {1.2}

They include effects of temperature increases on: {1.2}

 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

 some aspects of human health, such as heat-related mortality in Europe, changes in infectious disease vectors in

some areas, and allergenic pollen in Northern Hemisphere

high and mid-latitudes

 some human activities in the Arctic (e.g. hunting and travel

over snow and ice) and in lower-elevation alpine areas

(such as mountain sports).
Summary for Policymakers

4

Changes in physical and biological systems and surface temperature 1970-2004

Figure SPM.2. Locations of significant changes in data series of physical systems (snow, ice and frozen ground; hydrology; and coastal processes) and biological systems (terrestrial, marine and freshwater biological systems), are shown together with surface air temperature changes

over the period 1970-2004. A subset of about 29,000 data series was selected from about 80,000 data series from 577 studies. These met the

following criteria: (1) ending in 1990 or later; (2) spanning a period of at least 20 years; and (3) showing a significant change in either direction,

as assessed in individual studies. These data series are from about 75 studies (of which about 70 are new since the TAR) and contain about

29,000 data series, of which about 28,000 are from European studies. White areas do not contain sufficient observational climate data to

estimate a temperature trend. The 2 × 2 boxes show the total number of data series with significant changes (top row) and the percentage of

those consistent with warming (bottom row) for (i) continental regions: North America (NAM), Latin America (LA), Europe (EUR), Africa (AFR),

Asia (AS), Australia and New Zealand (ANZ), and Polar Regions (PR) and (ii) global-scale: Terrestrial (TER), Marine and Freshwater (MFW), and

Global (GLO). The numbers of studies from the seven regional boxes (NAM, EUR, AFR, AS, ANZ, PR) do not add up to the global (GLO) totals

because numbers from regions except Polar do not include the numbers related to Marine and Freshwater (MFW) systems. Locations of largearea marine changes are not shown on the map. {Figure 1.2}

Physical Biological

Number of

significant

observed

changes

Number of

significant

observed

changes

Observed data series

Physical systems (snow, ice and frozen ground; hydrology; coastal processes)

Biological systems (terrestrial, marine, and freshwater)

,

, ,

Percentage

of significant

changes 

consistent 

with warming

Percentage

of significant

changes 

consistent 

with warming

94% 92% 98% 96% 91% 94% 94% 90% 100% 100% 100% 100% 100% 100% 99% 94% 89% 100% 90%

355 455 53 119

NAM LA EUR AFR AS ANZ PR* TER MFW** GLO

5 5 2 106 8 6 0 120 24 764 1 85 765

28,115 28,586 28,671
5

Summary for Policymakers

2. Causes of change

Changes in atmospheric concentrations of greenhouse

gases (GHGs) and aerosols, land cover and solar radiation alter the energy balance of the climate system. {2.2}

Global GHG emissions due to human activities have

grown since pre-industrial times, with an increase of

70% between 1970 and 2004 (Figure SPM.3).5 {2.1}

Carbon dioxide (CO2) is the most important anthropogenic

GHG. Its annual emissions grew by about 80% between 1970

and 2004. The long-term trend of declining CO2

 emissions

per unit of energy supplied reversed after 2000. {2.1}

Global atmospheric concentrations of CO2, methane

(CH4

) and nitrous oxide (N2O) have increased markedly

as a result of human activities since 1750 and now far

exceed pre-industrial values determined from ice cores

spanning many thousands of years. {2.2}

Atmospheric concentrations of CO2 (379ppm) and CH4

(1774ppb) in 2005 exceed by far the natural range over the

last 650,000 years. Global increases in CO2

 concentrations

are due primarily to fossil fuel use, with land-use change providing another significant but smaller contribution. It is very

likely that the observed increase in CH4

 concentration is predominantly due to agriculture and fossil fuel use. CH4

 growth

rates have declined since the early 1990s, consistent with total emissions (sum of anthropogenic and natural sources) being nearly constant during this period. The increase in N2

O

concentration is primarily due to agriculture. {2.2}

There is very high confidence that the net effect of human

activities since 1750 has been one of warming.6 {2.2}

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the

observed increase in anthropogenic GHG concentrations.7 It is likely that there has been significant anthropogenic warming over the past 50 years averaged over

each continent (except Antarctica) (Figure SPM.4). {2.4}

During the past 50 years, the sum of solar and volcanic

forcings would likely have produced cooling. Observed patterns of warming and their changes are simulated only by

models that include anthropogenic forcings. Difficulties remain in simulating and attributing observed temperature

changes at smaller than continental scales. {2.4}

Global anthropogenic GHG emissions

Figure SPM.3. (a) Global annual emissions of anthropogenic GHGs from 1970 to 2004.5 (b) Share of different anthropogenic GHGs in total

emissions in 2004 in terms of carbon dioxide equivalents (CO2-eq). (c) Share of different sectors in total anthropogenic GHG emissions in 2004

in terms of CO2

-eq. (Forestry includes deforestation.) {Figure 2.1}

F-gases

CO2 from fossil fuel use and other sources

CH4 from agriculture, waste and energy

CO2 from deforestation, decay and peat

N2O from agriculture and others

GtCO2-eq / yr

28.7

35.6

39.4

44.7

49.0

5 Includes only carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and

sulphurhexafluoride (SF6

), whose emissions are covered by the United Nations Framework Convention on Climate Change (UNFCCC). These

GHGs are weighted by their 100-year Global Warming Potentials, using values consistent with reporting under the UNFCCC.

6 Increases in GHGs tend to warm the surface while the net effect of increases in aerosols tends to cool it. The net effect due to human activities

since the pre-industrial era is one of warming (+1.6 [+0.6 to +2.4] W/m2). In comparison, changes in solar irradiance are estimated to have

caused a small warming effect (+0.12 [+0.06 to +0.30] W/m2).

7 Consideration of remaining uncertainty is based on current methodologies.
Summary for Policymakers

6

Figure SPM.4. Comparison of observed continental- and global-scale changes in surface temperature with results simulated by climate models

using either natural or both natural and anthropogenic forcings. Decadal averages of observations are shown for the period 1906-2005 (black

line) plotted against the centre of the decade and relative to the corresponding average for the period 1901-1950. Lines are dashed where spatial

coverage is less than 50%. Blue shaded bands show the 5 to 95% range for 19 simulations from five climate models using only the natural

forcings due to solar activity and volcanoes. Red shaded bands show the 5 to 95% range for 58 simulations from 14 climate models using both

natural and anthropogenic forcings. {Figure 2.5}

Global and continental temperature change

models using only natural forcings

models using both natural and anthropogenic forcings

observations

Advances since the TAR show that discernible human

influences extend beyond average temperature to other

aspects of climate. {2.4}

Human influences have: {2.4}

 very likely contributed to sea level rise during the latter

half of the 20th century

 likely contributed to changes in wind patterns, affecting

extra-tropical storm tracks and temperature patterns

 likely increased temperatures of extreme hot nights, cold

nights and cold days

 more likely than not increased risk of heat waves, area

affected by drought since the 1970s and frequency of heavy

precipitation events.

Anthropogenic warming over the last three decades has likely

had a discernible influence at the global scale on observed

changes in many physical and biological systems. {2.4}

Spatial agreement between regions of significant warming across the globe and locations of significant observed

changes in many systems consistent with warming is very

unlikely to be due solely to natural variability. Several modelling studies have linked some specific responses in physical

and biological systems to anthropogenic warming. {2.4}

More complete attribution of observed natural system responses to anthropogenic warming is currently prevented by

the short time scales of many impact studies, greater natural

climate variability at regional scales, contributions of nonclimate factors and limited spatial coverage of studies. {2.4}
7

Summary for Policymakers

8 For an explanation of SRES emissions scenarios, see Box ‘SRES scenarios’ in Topic 3 of this Synthesis Report. These scenarios do not include

additional climate policies above current ones; more recent studies differ with respect to UNFCCC and Kyoto Protocol inclusion.

9 Emission pathways of mitigation scenarios are discussed in Section 5.

3. Projected climate change

and its impacts

There is high agreement and much evidence that with

current climate change mitigation policies and related sustainable development practices, global GHG emissions

will continue to grow over the next few decades. {3.1}

The IPCC Special Report on Emissions Scenarios (SRES,

2000) projects an increase of global GHG emissions by 25 to

90% (CO2

-eq) between 2000 and 2030 (Figure SPM.5), with

fossil fuels maintaining their dominant position in the global energy mix to 2030 and beyond. More recent scenarios without

additional emissions mitigation are comparable in range.8,9 {3.1}

Continued GHG emissions at or above current rates

would cause further warming and induce many changes

in the global climate system during the 21st century that

would very likely be larger than those observed during

the 20th century (Table SPM.1, Figure SPM.5). {3.2.1}

For the next two decades a warming of about 0.2°C per decade is projected for a range of SRES emissions scenarios. Even

if the concentrations of all GHGs and aerosols had been kept

constant at year 2000 levels, a further warming of about 0.1°C

per decade would be expected. Afterwards, temperature projections increasingly depend on specific emissions scenarios. {3.2}

The range of projections (Table SPM.1) is broadly consistent with the TAR, but uncertainties and upper ranges for

temperature are larger mainly because the broader range of

available models suggests stronger climate-carbon cycle feedbacks. Warming reduces terrestrial and ocean uptake of atmospheric CO2

, increasing the fraction of anthropogenic emissions remaining in the atmosphere. The strength of this feedback effect varies markedly among models. {2.3, 3.2.1}

Because understanding of some important effects driving

sea level rise is too limited, this report does not assess the

likelihood, nor provide a best estimate or an upper bound for

sea level rise. Table SPM.1 shows model-based projections

Scenarios for GHG emissions from 2000 to 2100 (in the absence of additional climate policies)

and projections of surface temperatures

Figure SPM.5. Left Panel: Global GHG emissions (in GtCO2-eq) in the absence of climate policies: six illustrative SRES marker scenarios

(coloured lines) and the 80th percentile range of recent scenarios published since SRES (post-SRES) (gray shaded area). Dashed lines show the

full range of post-SRES scenarios. The emissions include CO2, CH4, N2O and F-gases. Right Panel: Solid lines are multi-model global averages

of surface warming for scenarios A2, A1B and B1, shown as continuations of the 20th -century simulations. These projections also take into

account emissions of short-lived GHGs and aerosols. The pink line is not a scenario, but is for Atmosphere-Ocean General Circulation Model

(AOGCM) simulations where atmospheric concentrations are held constant at year 2000 values. The bars at the right of the figure indicate the

best estimate (solid line within each bar) and the likely range assessed for the six SRES marker scenarios at 2090-2099. All temperatures are

relative to the period 1980-1999. {Figures 3.1 and 3.2}

A1B

B1

A2

B2

Global GHG emissions (GtCO2-eq / yr)

Global surface warming (oC)

Year 2000 constant

concentrations

post-SRES (max)

post-SRES (min)

post-SRES range (80%)

A1FI

A1T

20 century th

2000 2100 1900 2000 2100

Year Year
Summary for Policymakers

8

10 TAR projections were made for 2100, whereas the projections for this report are for 2090-2099. The TAR would have had similar ranges to

those in Table SPM.1 if it had treated uncertainties in the same way.

11 For discussion of the longer term, see material below.

of global average sea level rise for 2090-2099.10 The projections do not include uncertainties in climate-carbon cycle feedbacks nor the full effects of changes in ice sheet flow, therefore the upper values of the ranges are not to be considered

upper bounds for sea level rise. They include a contribution

from increased Greenland and Antarctic ice flow at the rates

observed for 1993-2003, but this could increase or decrease

in the future.11 {3.2.1}

There is now higher confidence than in the TAR in projected patterns of warming and other regional-scale

features, including changes in wind patterns, precipitation and some aspects of extremes and sea ice. {3.2.2}

Regional-scale changes include: {3.2.2}

 warming greatest over land and at most high northern latitudes and least over Southern Ocean and parts of the North

Atlantic Ocean, continuing recent observed trends (Figure SPM.6)

 contraction of snow cover area, increases in thaw depth

over most permafrost regions and decrease in sea ice extent; in some projections using SRES scenarios, Arctic

late-summer sea ice disappears almost entirely by the latter part of the 21st century

 very likely increase in frequency of hot extremes, heat

waves and heavy precipitation

 likely increase in tropical cyclone intensity; less confidence

in global decrease of tropical cyclone numbers

 poleward shift of extra-tropical storm tracks with consequent changes in wind, precipitation and temperature patterns

 very likely precipitation increases in high latitudes and

likely decreases in most subtropical land regions, continuing observed recent trends.

There is high confidence that by mid-century, annual river

runoff and water availability are projected to increase at high

latitudes (and in some tropical wet areas) and decrease in some

dry regions in the mid-latitudes and tropics. There is also high

confidence that many semi-arid areas (e.g. Mediterranean

Basin, western United States, southern Africa and

north-eastern Brazil) will suffer a decrease in water resources

due to climate change. {3.3.1, Figure 3.5}

Studies since the TAR have enabled more systematic

understanding of the timing and magnitude of impacts

related to differing amounts and rates of climate

change. {3.3.1, 3.3.2}

Figure SPM.7 presents examples of this new information

for systems and sectors. The top panel shows impacts increasing with increasing temperature change. Their estimated magnitude and timing is also affected by development pathway

(lower panel). {3.3.1}

Examples of some projected impacts for different regions

are given in Table SPM.2.

Table SPM.1. Projected global average surface warming and sea level rise at the end of the 21st century. {Table 3.1}

Temperature change Sea level rise

(°C at 2090-2099 relative to 1980-1999) a, d (m at 2090-2099 relative to 1980-1999)

Case Best estimate Likely range Model-based range

excluding future rapid dynamical changes in ice flow

Constant year 2000

concentrationsb 0.6 0.3 – 0.9 Not available

B1 scenario 1.8 1.1 – 2.9 0.18 – 0.38

A1T scenario 2.4 1.4 – 3.8 0.20 – 0.45

B2 scenario 2.4 1.4 – 3.8 0.20 – 0.43

A1B scenario 2.8 1.7 – 4.4 0.21 – 0.48

A2 scenario 3.4 2.0 – 5.4 0.23 – 0.51

A1FI scenario 4.0 2.4 – 6.4 0.26 – 0.59

Notes:

a) Temperatures are assessed best estimates and likely uncertainty ranges from a hierarchy of models of varying complexity as well as

observational constraints.

b) Year 2000 constant composition is derived from Atmosphere-Ocean General Circulation Models (AOGCMs) only.

c) All scenarios above are six SRES marker scenarios. Approximate CO2

-eq concentrations corresponding to the computed radiative

forcing due to anthropogenic GHGs and aerosols in 2100 (see p. 823 of the Working Group I TAR) for the SRES B1, AIT, B2, A1B, A2

and A1FI illustrative marker scenarios are about 600, 700, 800, 850, 1250 and 1550ppm, respectively.

d) Temperature changes are expressed as the difference from the period 1980-1999. To express the change relative to the period 1850-

1899 add 0.5°C.
9

Summary for Policymakers

Geographical pattern of surface warming

Figure SPM.6. Projected surface temperature changes for the late 21st century (2090-2099). The map shows the multi-AOGCM average projection for the A1B SRES scenario. Temperatures are relative to the period 1980-1999. {Figure 3.2}

Some systems, sectors and regions are likely to be especially affected by climate change.12 {3.3.3}

Systems and sectors: {3.3.3}

 particular ecosystems:

- terrestrial: tundra, boreal forest and mountain regions

because of sensitivity to warming; mediterranean-type

ecosystems because of reduction in rainfall; and tropical rainforests where precipitation declines

- coastal: mangroves and salt marshes, due to multiple

stresses

- marine: coral reefs due to multiple stresses; the sea ice

biome because of sensitivity to warming

 water resources in some dry regions at mid-latitudes13 and

in the dry tropics, due to changes in rainfall and evapotranspiration, and in areas dependent on snow and ice melt

 agriculture in low latitudes, due to reduced water availability

 low-lying coastal systems, due to threat of sea level rise

and increased risk from extreme weather events

 human health in populations with low adaptive capacity.

Regions: {3.3.3}

 the Arctic, because of the impacts of high rates of projected

warming on natural systems and human communities

 Africa, because of low adaptive capacity and projected

climate change impacts

 small islands, where there is high exposure of population

and infrastructure to projected climate change impacts

 Asian and African megadeltas, due to large populations

and high exposure to sea level rise, storm surges and river

flooding.

Within other areas, even those with high incomes, some

people (such as the poor, young children and the elderly) can

be particularly at risk, and also some areas and some activities. {3.3.3}

Ocean acidification

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. Increasing atmospheric CO2

 concentrations

lead to further acidification. Projections based on SRES scenarios give a reduction in average global surface ocean pH of

between 0.14 and 0.35 units over the 21st century. While the effects of observed ocean acidification on the marine biosphere are

as yet undocumented, the progressive acidification of oceans is

expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species. {3.3.4}

12 Identified on the basis of expert judgement of the assessed literature and considering the magnitude, timing and projected rate of climate

change, sensitivity and adaptive capacity.

13 Including arid and semi-arid regions.
Summary for Policymakers

10

Examples of impacts associated with global average temperature change

(Impacts will vary by extent of adaptation, rate of temperature change and socio-economic pathway)

Figure SPM.7. Examples of impacts associated with projected global average surface warming. Upper panel: Illustrative examples of global

impacts projected for climate changes (and sea level and atmospheric CO2 where relevant) associated with different amounts of increase in

global average surface temperature in the 21st century. The black lines link impacts; broken-line arrows indicate impacts continuing with increasing temperature. Entries are placed so that the left-hand side of text indicates the approximate level of warming that is associated with the onset

of a given impact. Quantitative entries for water scarcity and flooding represent the additional impacts of climate change relative to the conditions

projected across the range of SRES scenarios A1FI, A2, B1 and B2. Adaptation to climate change is not included in these estimations. Confidence levels for all statements are high. Lower panel: Dots and bars indicate the best estimate and likely ranges of warming assessed for the

six SRES marker scenarios for 2090-2099 relative to 1980-1999. {Figure 3.6}

Warming by 2090-2099 relative to 1980-1999 for non-mitigation scenarios

6.4°C

5.4°C

0 1 2 3 4 5 °C

Global average annual temperature change relative to 1980-1999 (°C)

0 1234 5 °C

About 30% of 

global coastal 

wetlands lost‡

Increased water availability in moist tropics and high latitudes

Decreasing water availability and increasing drought in mid-latitudes and semi-arid low latitudes

Hundreds of millions of people exposed to increased water stress

Up to 30% of species at 

increasing risk of extinction

Increased coral bleaching Most corals bleached Widespread coral mortality

Increasing species range shifts and wildfire risk

Terrestrial biosphere tends toward a net carbon source as:

~15% ~40% of ecosystems affected 

Tendencies for cereal productivity

to decrease in low latitudes Productivity of all cereals 

decreases in low latitudes

Cereal productivity to

decrease in some regions

Complex, localised negative impacts on small holders, subsistence farmers and fishers

Tendencies for some cereal productivity 

to increase at mid- to high latitudes

 Significant† extinctions 

around the globe

Changed distribution of some disease vectors

Increasing burden from malnutrition, diarrhoeal, cardio-respiratory and infectious diseases

Increased morbidity and mortality from heat waves, floods and droughts

Substantial burden on health services

Ecosystem changes due to weakening of the meridional 

overturning circulation

Millions more people could experience 

coastal flooding each year

Increased damage from floods and storms

WATER

ECOSYSTEMS

FOOD

COASTS

HEALTH

0 1234 5 °C

† Significant is defined here as more than 40%. ‡ Based on average rate of sea level rise of 4.2mm/year from 2000 to 2080.
11

Summary for Policymakers

Table SPM.2. Examples of some projected regional impacts. {3.3.2}

Africa  By 2020, between 75 and 250 million of people are projected to be exposed to increased water stress due to

climate change.

 By 2020, in some countries, yields from rain-fed agriculture could be reduced by up to 50%. Agricultural

production, including access to food, in many African countries is projected to be severely compromised. This

would further adversely affect food security and exacerbate malnutrition.

 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 to 10% of Gross Domestic Product (GDP).

 By 2080, an increase of 5 to 8% of arid and semi-arid land in Africa is projected under a range of climate

scenarios (TS).

Asia  By the 2050s, freshwater availability in Central, South, East and South-East Asia, particularly in large river

basins, is projected to decrease.

 Coastal areas, especially heavily populated megadelta regions in South, East and South-East Asia, will be at

greatest risk due to increased flooding from the sea and, in some megadeltas, flooding from the rivers.

 Climate change is projected to compound the pressures on natural resources and the environment

associated with rapid urbanisation, industrialisation and economic development.

 Endemic morbidity and mortality due to diarrhoeal disease primarily associated with floods and droughts

are expected to rise in East, South and South-East Asia due to projected changes in the hydrological cycle.

Australia and  By 2020, significant loss of biodiversity is projected to occur in some ecologically rich sites, including the

New Zealand Great Barrier Reef and Queensland Wet Tropics.

 By 2030, water security problems are projected to intensify in southern and eastern Australia and, in

New Zealand, in Northland and some eastern regions.

 By 2030, production from agriculture and forestry 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 are projected in some other regions.

 By 2050, ongoing coastal development and population growth in some areas of Australia and New Zealand

are projected to exacerbate risks from sea level rise and increases in the severity and frequency of storms

and coastal flooding.

Europe  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 sea level rise).

 Mountainous areas will face glacier retreat, reduced snow cover and winter tourism, and extensive species

losses (in some areas up to 60% under high emissions 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.

 Climate change is also projected to increase the health risks due to heat waves and the frequency of wildfires.

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.

 Productivity of some important crops is projected to decrease and livestock productivity to decline, with

adverse consequences for food security. In temperate zones, soybean yields are projected to increase.

Overall, the number of people at risk of hunger is projected to increase (TS; medium confidence).

 Changes in precipitation patterns and the disappearance of glaciers are projected to significantly affect

water availability for human consumption, agriculture and energy generation.

North America  Warming in western mountains is projected to cause decreased snowpack, more winter flooding and

reduced summer flows, exacerbating competition for over-allocated water resources.

 In the early decades of the century, moderate climate change is projected to increase aggregate yields of

rain-fed agriculture by 5 to 20%, but with important variability among regions. Major challenges are

projected for crops that are near the warm end of their suitable range or which depend on highly utilised

water resources.

 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.

 Coastal communities and habitats will be increasingly stressed by climate change impacts interacting

with development and pollution.

continued...
Summary for Policymakers

12

Altered frequencies and intensities of extreme weather,

together with sea level rise, are expected to have mostly

adverse effects on natural and human systems. {3.3.5}

Examples for selected extremes and sectors are shown in

Table SPM.3.

Anthropogenic warming and sea level rise would continue for centuries due to the time scales associated

with climate processes and feedbacks, even if GHG

concentrations were to be stabilised. {3.2.3}

Estimated long-term (multi-century) warming corresponding to the six AR4 Working Group III stabilisation categories

is shown in Figure SPM.8.

Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100. Current models

suggest virtually complete elimination of the Greenland ice

sheet and a resulting contribution to sea level rise of about 7m

if global average warming were sustained for millennia in

excess of 1.9 to 4.6°C relative to pre-industrial values. The

corresponding future temperatures in Greenland are comparable to those inferred for the last interglacial period 125,000

years ago, when palaeoclimatic information suggests reductions

of polar land ice extent and 4 to 6m of sea level rise. {3.2.3}

Current global model studies project that the Antarctic ice

sheet will remain too cold for widespread surface melting and

gain mass due to increased snowfall. However, net loss of ice

mass could occur if dynamical ice discharge dominates the

ice sheet mass balance. {3.2.3}

Table SPM.2. continued...

Polar Regions  The main projected biophysical effects are reductions in thickness and extent of glaciers, ice sheets

and sea ice, and changes in natural ecosystems with detrimental effects on many organisms including

migratory birds, mammals and higher predators.

 For human communities in the Arctic, impacts, particularly those 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.

 In both polar regions, specific ecosystems and habitats are projected to be vulnerable, as climatic barriers to

species invasions are lowered.

 Small Islands  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.

 Deterioration in coastal conditions, for example through erosion of beaches and coral bleaching, is expected

to affect local resources.

 By mid-century, climate change is expected 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.

 With higher temperatures, increased invasion by non-native species is expected to occur, particularly on

mid- and high-latitude islands.

Note:

Unless stated explicitly, all entries are from Working Group II SPM text, and are either very high confidence or high confidence statements, reflecting different sectors (agriculture, ecosystems, water, coasts, health, industry and settlements). The Working Group II SPM

refers to the source of the statements, timelines and temperatures. The magnitude and timing of impacts that will ultimately be realised

will vary with the amount and rate of climate change, emissions scenarios, development pathways and adaptation.

Figure SPM.8. Estimated long-term (multi-century) warming corresponding to the six AR4 Working Group III stabilisation categories (Table

SPM.6). The temperature scale has been shifted by -0.5°C compared to Table SPM.6 to account approximately for the warming between preindustrial and 1980-1999. For most stabilisation levels global average temperature is approaching the equilibrium level over a few centuries. For

GHG emissions scenarios that lead to stabilisation at levels comparable to SRES B1 and A1B by 2100 (600 and 850ppm CO2-eq; category IV

and V), assessed models project that about 65 to 70% of the estimated global equilibrium temperature increase, assuming a climate sensitivity

of 3°C, would be realised at the time of stabilisation. For the much lower stabilisation scenarios (category I and II, Figure SPM.11), the equilibrium temperature may be reached earlier. {Figure 3.4}

Estimated multi-century warming relative to 1980-1999 for AR4 stabilisation categories

 0 1 2 3 4 5 6 °C

Global average temperature change relative to 1980-1999 (°C)
13

Summary for Policymakers

Table SPM.3. Examples of possible impacts of climate change due to changes in extreme weather and climate events, based on

projections to the mid- to late 21st century. These do not take into account any changes or developments in adaptive capacity. The

likelihood estimates in column two relate to the phenomena listed in column one. {Table 3.2}

Anthropogenic warming could lead to some impacts

that are abrupt or irreversible, depending upon the rate

and magnitude of the climate change. {3.4}

Partial loss of ice sheets on polar land could imply metres

of sea level rise, major changes in coastlines and inundation

of low-lying areas, with greatest effects in river deltas and

low-lying islands. Such changes are projected to occur over

millennial time scales, but more rapid sea level rise on century time scales cannot be excluded. {3.4}

Climate change is likely to lead to some irreversible impacts. There is medium confidence that approximately 20 to

30% of species assessed so far are likely to be at increased

risk of extinction if increases in global average warming exceed 1.5 to 2.5°C (relative to 1980-1999). As global average

Phenomenona and Likelihood of Examples of major projected impacts by sector

direction of trend future trends

based on Agriculture, forestry Water resources Human health Industry, settlement

projections and ecosystems and society

for 21st century

using SRES

scenarios

Over most land Virtually Increased yields in Effects on water Reduced human Reduced energy demand for

areas, warmer and certainb colder environments; resources relying on mortality from heating; increased demand

fewer cold days decreased yields in snowmelt; effects on decreased cold for cooling; declining air quality

and nights, warmer warmer environments; some water supplies exposure in cities; reduced disruption to

and more frequent increased insect transport due to snow, ice;

hot days and nights outbreaks effects on winter tourism

Warm spells/heat Very likely Reduced yields in Increased water Increased risk of Reduction in quality of life for

waves. Frequency warmer regions demand; water heat-related people in warm areas without

increases over most due to heat stress; quality problems, mortality, especially appropriate housing; impacts

land areas increased danger of e.g. algal blooms for the elderly, on the elderly, very young and

wildfire chronically sick, poor

very young and

socially isolated

Heavy precipitation Very likely Damage to crops; Adverse effects on Increased risk of Disruption of settlements,

events. Frequency soil erosion, inability quality of surface deaths, injuries and commerce, transport and

increases over most to cultivate land due and groundwater; infectious, respiratory societies due to flooding:

areas to waterlogging of contamination of and skin diseases pressures on urban and rural

soils water supply; water infrastructures; loss of property

scarcity may be

relieved

Area affected by Likely Land degradation; More widespread Increased risk of Water shortage for settlements,

drought increases lower yields/crop water stress food and water industry and societies;

damage and failure; shortage; increased reduced hydropower generation

increased livestock risk of malnutrition; potentials; potential for

deaths; increased increased risk of population migration

risk of wildfire water- and foodborne diseases

Intense tropical Likely Damage to crops; Power outages Increased risk of Disruption by flood and high

cyclone activity windthrow (uprooting) causing disruption deaths, injuries, winds; withdrawal of risk

increases of trees; damage to of public water supply water- and food- coverage in vulnerable areas

coral reefs borne diseases; by private insurers; potential

post-traumatic for population migrations; loss

stress disorders of property

Increased incidence Likelyd Salinisation of Decreased fresh- Increased risk of Costs of coastal protection

of extreme high irrigation water, water availability due deaths and injuries versus costs of land-use

sea level (excludes estuaries and fresh- to saltwater intrusion by drowning in floods; relocation; potential for

tsunamis)c water systems migration-related movement of populations and

health effects infrastructure; also see tropical

cyclones above

Notes:

a) See Working Group I Table 3.7 for further details regarding definitions.

b) Warming of the most extreme days and nights each year.

c) Extreme high sea level depends on average sea level and on regional weather systems. It is defined as the highest 1% of hourly values

of observed sea level at a station for a given reference period.

d) In all scenarios, the projected global average sea level at 2100 is higher than in the reference period. The effect of changes in regional

weather systems on sea level extremes has not been assessed.
Summary for Policymakers

14

temperature increase exceeds about 3.5°C, model projections

suggest significant extinctions (40 to 70% of species assessed)

around the globe. {3.4}

Based on current model simulations, the meridional overturning circulation (MOC) of the Atlantic Ocean will very likely

slow down during the 21st century; nevertheless temperatures

over the Atlantic and Europe are projected to increase. The

MOC is very unlikely to undergo a large abrupt transition during the 21st century. Longer-term MOC changes cannot be assessed with confidence. Impacts of large-scale and persistent

changes in the MOC are likely to include changes in marine

ecosystem productivity, fisheries, ocean CO2

 uptake, oceanic

oxygen concentrations and terrestrial vegetation. Changes in

terrestrial and ocean CO2

 uptake may feed back on the climate system. {3.4}

4. Adaptation and mitigation options14

A wide array of adaptation options is available, but more

extensive adaptation than is currently occurring is required to reduce vulnerability to climate change. There

are barriers, limits and costs, which are not fully understood. {4.2}

Societies have a long record of managing the impacts of

weather- and climate-related events. Nevertheless, additional

adaptation measures will be required to reduce the adverse

impacts of projected climate change and variability, regardless of the scale of mitigation undertaken over the next two to

three decades. Moreover, vulnerability to climate change can

be exacerbated by other stresses. These arise from, for example, current climate hazards, poverty and unequal access to

resources, food insecurity, trends in economic globalisation,

conflict and incidence of diseases such as HIV/AIDS. {4.2}

Some planned adaptation to climate change is already

occurring on a limited basis. Adaptation can reduce vulnerability, especially when it is embedded within broader sectoral

initiatives (Table SPM.4). There is high confidence that there

are viable adaptation options that can be implemented in some

sectors at low cost, and/or with high benefit-cost ratios. However, comprehensive estimates of global costs and benefits of

adaptation are limited. {4.2, Table 4.1}

Adaptive capacity is intimately connected to social and

economic development but is unevenly distributed

across and within societies. {4.2}

A range of barriers limits both the implementation and

effectiveness of adaptation measures. The capacity to adapt is

dynamic and is influenced by a society’s productive base, including natural and man-made capital assets, social networks

and entitlements, human capital and institutions, governance,

national income, health and technology. Even societies with

high adaptive capacity remain vulnerable to climate change,

variability and extremes. {4.2}

Both bottom-up and top-down studies indicate that

there is high agreement and much evidence of substantial economic potential for the mitigation of global

GHG emissions over the coming decades that could

offset the projected growth of global emissions or reduce emissions below current levels (Figures SPM.9,

SPM.10).15 While top-down and bottom-up studies are

in line at the global level (Figure SPM.9) there are considerable differences at the sectoral level. {4.3}

No single technology can provide all of the mitigation

potential in any sector. The economic mitigation potential,

which is generally greater than the market mitigation potential, can only be achieved when adequate policies are in place

and barriers removed (Table SPM.5). {4.3}

Bottom-up studies suggest that mitigation opportunities

with net negative costs have the potential to reduce emissions

by around 6 GtCO2

-eq/yr in 2030, realising which requires

dealing with implementation barriers. {4.3}

14 While this Section deals with adaptation and mitigation separately, these responses can be complementary. This theme is discussed in

Section 5.

15 The concept of ‘mitigation potential’ has been developed to assess the scale of GHG reductions that could be made, relative to emission

baselines, for a given level of carbon price (expressed in cost per unit of carbon dioxide equivalent emissions avoided or reduced). Mitigation

potential is further differentiated in terms of ‘market mitigation potential’ and ‘economic mitigation potential’.

Market mitigation potential is the mitigation potential based on private costs and private discount rates (reflecting the perspective of private

consumers and companies), which might be expected to occur under forecast market conditions, including policies and measures currently in

place, noting that barriers limit actual uptake.

Economic mitigation potential is the mitigation potential that takes into account social costs and benefits and social discount rates (reflecting the perspective of society; social discount rates are lower than those used by private investors), assuming that market efficiency is

improved by policies and measures and barriers are removed.

Mitigation potential is estimated using different types of approaches. Bottom-up studies are based on assessment of mitigation options,

emphasising specific technologies and regulations. They are typically sectoral studies taking the macro-economy as unchanged. Top-down

studies assess the economy-wide potential of mitigation options. They use globally consistent frameworks and aggregated information about

mitigation options and capture macro-economic and market feedbacks.
15

Summary for Policymakers

Table SPM.4. Selected examples of planned adaptation by sector. {Table 4.1}

Adaptation option/strategy

Expanded rainwater harvesting;

water storage and conservation

techniques; water re-use;

desalination; water-use and

irrigation efficiency

Adjustment of planting dates and

crop variety; crop relocation;

improved land management, e.g.

erosion control and soil protection

through tree planting

Relocation; seawalls and storm

surge barriers; dune reinforcement; land acquisition and

creation of marshlands/wetlands

as buffer against sea level rise

and flooding; protection of existing

natural barriers

Heat-health action plans;

emergency medical services;

improved climate-sensitive

disease surveillance and control;

safe water and improved

sanitation

Diversification of tourism

attractions and revenues; shifting

ski slopes to higher altitudes and

glaciers; artificial snow-making

Ralignment/relocation; design

standards and planning for roads,

rail and other infrastructure to

cope with warming and drainage

Strengthening of overhead

transmission and distribution

infrastructure; underground

cabling for utilities; energy

efficiency; use of renewable

sources; reduced dependence on

single sources of energy

Underlying policy framework

National water policies and

integrated water resources management; water-related hazards

management

R&D policies; institutional reform;

land tenure and land reform; training;

capacity building; crop insurance;

financial incentives, e.g. subsidies

and tax credits

Standards and regulations that

integrate climate change considerations into design; land-use policies;

building codes; insurance

Public health policies that recognise

climate risk; strengthened health

services; regional and international

cooperation

Integrated planning (e.g. carrying

capacity; linkages with other

sectors); financial incentives, e.g.

subsidies and tax credits

Integrating climate change considerations into national transport policy;

investment in R&D for special

situations, e.g. permafrost areas

National energy policies, regulations,

and fiscal and financial incentives to

encourage use of alternative

sources; incorporating climate

change in design standards

Key constraints and opportunities

to implementation (Normal font =

constraints; italics = opportunities)

Financial, human resources and

physical barriers; integrated water

resources management; synergies with

other sectors

Technological and financial

constraints; access to new varieties;

markets; longer growing season in

higher latitudes; revenues from ‘new’

products

Financial and technological barriers;

availability of relocation space;

integrated policies and management;

synergies with sustainable development

goals

Limits to human tolerance (vulnerable

groups); knowledge limitations; financial

capacity; upgraded health services;

improved quality of life

Appeal/marketing of new attractions;

financial and logistical challenges;

potential adverse impact on other

sectors (e.g. artificial snow-making may

increase energy use); revenues from

‘new’ attractions; involvement of wider

group of stakeholders

Financial and technological barriers;

availability of less vulnerable routes;

improved technologies and integration

with key sectors (e.g. energy)

Access to viable alternatives; financial

and technological barriers; acceptance

of new technologies; stimulation of new

technologies; use of local resources

Note:

Other examples from many sectors would include early warning systems.

Future energy infrastructure investment decisions, expected to exceed US$20 trillion16 between 2005 and 2030,

will have long-term impacts on GHG emissions, because of

the long lifetimes of energy plants and other infrastructure

capital stock. The widespread diffusion of low-carbon technologies may take many decades, even if early investments in

these technologies are made attractive. Initial estimates show

that returning global energy-related CO2

 emissions to 2005

levels by 2030 would require a large shift in investment patterns, although the net additional investment required ranges

from negligible to 5 to 10%. {4.3}

Sector

Water

Agriculture

Infrastructure/

settlement

(including

coastal zones)

Human health

Tourism

Transport

Energy

16 20 trillion = 20,000 billion = 20×1012
Summary for Policymakers

16

Figure SPM.10. Estimated economic mitigation potential by sector in 2030 from bottom-up studies, compared to the respective baselines

assumed in the sector assessments. The potentials do not include non-technical options such as lifestyle changes. {Figure 4.2}

Notes:

a) The ranges for global economic potentials as assessed in each sector are shown by vertical lines. The ranges are based on end-use allocations of

emissions, meaning that emissions of electricity use are counted towards the end-use sectors and not to the energy supply sector.

b) The estimated potentials have been constrained by the availability of studies particularly at high carbon price levels.

c) Sectors used different baselines. For industry, the SRES B2 baseline was taken, for energy supply and transport, the World Energy Outlook

(WEO) 2004 baseline was used; the building sector is based on a baseline in between SRES B2 and A1B; for waste, SRES A1B driving

forces were used to construct a waste-specific baseline; agriculture and forestry used baselines that mostly used B2 driving forces.

d) Only global totals for transport are shown because international aviation is included.

e) Categories excluded are: non-CO2 emissions in buildings and transport, part of material efficiency options, heat production and co-generation in energy supply, heavy duty vehicles, shipping and high-occupancy passenger transport, most high-cost options for buildings, wastewater treatment, emission reduction from coal mines and gas pipelines, and fluorinated gases from energy supply and transport. The underestimation of the total economic potential from these emissions is of the order of 10 to 15%.

Economic mitigation potentials by sector in 2030 estimated from bottom-up studies

2.4-4.7 1.6-2.5 5.3-6.7 2.5-5.5 2.3-6.4 1.3-4.2 0.4-1.0

total sectoral potential at <US$100/tCO -eq in GtCO -eq/yr: 2 2

Energy supply Transport Buildings Industry Agriculture Forestry Waste

World total

Figure SPM.9. Global economic mitigation potential in 2030 estimated from bottom-up (Panel a) and top-down (Panel b) studies, compared with

the projected emissions increases from SRES scenarios relative to year 2000 GHG emissions of 40.8 GtCO2-eq (Panel c). Note: GHG emissions

in 2000 are exclusive of emissions of decay of above ground biomass that remains after logging and deforestation and from peat fires and

drained peat soils, to ensure consistency with the SRES emission results. {Figure 4.1}

Comparison between global economic mitigation potential and projected emissions increase in 2030

0

5

10

15

20

25

30

35

A1FI A2 A1B A1T B1 B2

Gt CO2-eq

c)

< 0 < 20 < 50 < 100 US$/tCO2-eq

low end of range high end of range

0

5

10

15

20

25

30

35

a)

low end of range high end of range

0

5

10

15

20

25

30

35

< 20 < 50 < 100 US$/tCO2-eq

b) Increase in GHG emissions 

above year 2000 levels

Bottom-up Top-down

Gt CO2-eq Gt CO2-eq 

Estimated mitigation potential in 2030Estimated mitigation potential in 2030
17

Summary for Policymakers

Table SPM.5 Selected examples of key sectoral mitigation technologies, policies and measures, constraints and opportunities. {Table 4.2}

Key mitigation technologies and practices currently commercially available.Key mitigation technologies and practices projected to be commercialised

before 2030 shown in italics.

Improved supply and distribution efficiency; fuel switching from coal to gas; nuclearpower; renewable heat and power (hydropower, solar, wind, geothermal andbioenergy); combined heat and power; early applications of carbon dioxide captureand storage (CCS) (e.g. storage of removed CO2 from natural gas); CCS for gas,

biomass and coal-fired electricity generating facilities; advanced nuclear power;

advanced renewable energy, including tidal and wave energy, concentrating solar,

and solar photovoltaics

More fuel-efficient vehicles; hybrid vehicles; cleaner diesel vehicles; biofuels; modalshifts from road transport to rail and public transport systems; non-motorisedtransport (cycling, walking); land-use and transport planning; second generation

biofuels; higher efficiency aircraft; advanced electric and hybrid vehicles with more

powerful and reliable batteries

Efficient lighting and daylighting; more efficient electrical appliances and heatingand cooling devices; improved cook stoves, improved insulation; passive and activesolar design for heating and cooling; alternative refrigeration fluids, recovery andrecycling of fluorinated gases; integrated design of commercial buildings including

technologies, such as intelligent meters that provide feedback and control; solar

photovoltaics integrated in buildings

More efficient end-use electrical equipment; heat and power recovery; materialrecycling and substitution; control of non-CO2 gas emissions; and a wide array of

process-specific technologies; advanced energy efficiency; CCS for cement,

ammonia, and iron manufacture; inert electrodes for aluminium manufacture

Improved crop and grazing land management to increase soil carbon storage;restoration of cultivated peaty soils and degraded lands; improved rice cultivationtechniques and livestock and manure management to reduce CH4 emissions;

improved nitrogen fertiliser application techniques to reduce N O emissions; 2

dedicated energy crops to replace fossil fuel use; improved energy efficiency;

improvements of crop yieldsAfforestation; reforestation; forest management; reduced deforestation; harvestedwood product management; use of forestry products for bioenergy to replace fossilfuel use; tree species improvement to increase biomass productivity and carbon

sequestration; improved remote sensing technologies for analysis of vegetation/soil

carbon sequestration potential and mapping land-use change

Landfill CH4 recovery; waste incineration with energy recovery; composting of

organic waste; controlled wastewater treatment; recycling and waste minimisation;

biocovers and biofilters to optimise CH4 oxidation

Policies, measures and instruments shown to beenvironmentally effectiveReduction of fossil fuel subsidies; taxes or carbon chargeson fossil fuelsFeed-in tariffs for renewable energy technologies;renewable energy obligations; producer subsidiesMandatory fuel economy; biofuel blending and CO2standards for road transportTaxes on vehicle purchase, registration, use and motorfuels; road and parking pricingInfluence mobility needs through land-use regulations andinfrastructure planning; investment in attractive publictransport facilities and non-motorised forms of transportAppliance standards and labellingBuilding codes and certificationDemand-side management programmesPublic sector leadership programmes, includingprocurementIncentives for energy service companies (ESCOs)Provision of benchmark information; performancestandards; subsidies; tax creditsTradable permitsVoluntary agreementsFinancial incentives and regulations for improved landmanagement; maintaining soil carbon content; efficient useof fertilisers and irrigationFinancial incentives (national and international) toincrease forest area, to reduce deforestation and tomaintain and manage forests; land-use regulation andenforcementFinancial incentives for improved waste and wastewatermanagementRenewable energy incentives or obligationsWaste management regulations

Key constraints or opportunities(Normal font = constraints;italics = opportunities)Resistance by vested interests may make themdifficult to implementMay be appropriate to create markets for lowemissions technologiesPartial coverage of vehicle fleet may limiteffectivenessEffectiveness may drop with higher incomesParticularly appropriate for countries that are

building up their transportation systems

Periodic revision of standards neededAttractive for new buildings. Enforcement can be

difficultNeed for regulations so that utilities may profit

Government purchasing can expand demand for

energy-efficient products

Success factor: Access to third party financing

May be appropriate to stimulate technology uptake.

Stability of national policy important in view ofinternational competitivenessPredictable allocation mechanisms and stableprice signals important for investmentsSuccess factors include: clear targets, a baselinescenario, third-party involvement in design andreview and formal provisions of monitoring, closecooperation between government and industryMay encourage synergy with sustainable

development and with reducing vulnerability toclimate change, thereby overcoming barriers to

implementation

Constraints include lack of investment capital andland tenure issues. Can help poverty alleviation

May stimulate technology diffusion

Local availability of low-cost fuelMost effectively applied at national level withenforcement strategies

SectorEnergy supplyTransportBuildingsIndustryAgricultureForestry/forestsWaste
Summary for Policymakers

18

A wide variety of policies and instruments are available to governments to create the incentives for mitigation action. Their applicability depends on national

circumstances and sectoral context (Table SPM.5). {4.3}

They include integrating climate policies in wider development policies, regulations and standards, taxes and charges,

tradable permits, financial incentives, voluntary agreements,

information instruments, and research, development and demonstration (RD&D). {4.3}

An effective carbon-price signal could realise significant

mitigation potential in all sectors. Modelling studies show that

global carbon prices rising to US$20-80/tCO2

-eq by 2030 are

consistent with stabilisation at around 550ppm CO2

-eq by 2100.

For the same stabilisation level, induced technological change

may lower these price ranges to US$5-65/tCO2

-eq in 2030.17 {4.3}

There is high agreement and much evidence that mitigation actions can result in near-term co-benefits (e.g. improved

health due to reduced air pollution) that may offset a substantial fraction of mitigation costs. {4.3}

There is high agreement and medium evidence that Annex

I countries’ actions may affect the global economy and global

emissions, although the scale of carbon leakage remains uncertain.18 {4.3}

Fossil fuel exporting nations (in both Annex I and non-Annex I countries) may expect, as indicated in the TAR, lower demand and prices and lower GDP growth due to mitigation policies. The extent of this spillover depends strongly on assumptions related to policy decisions and oil market conditions. {4.3}

There is also high agreement and medium evidence that

changes in lifestyle, behaviour patterns and management practices can contribute to climate change mitigation across all sectors. {4.3}

Many options for reducing global GHG emissions

through international cooperation exist. There is high

agreement and much evidence that notable achievements of the UNFCCC and its Kyoto Protocol are the

establishment of a global response to climate change,

stimulation of an array of national policies, and the creation of an international carbon market and new institutional mechanisms that may provide the foundation

for future mitigation efforts. Progress has also been made

in addressing adaptation within the UNFCCC and additional international initiatives have been suggested. {4.5}

Greater cooperative efforts and expansion of market mechanisms will help to reduce global costs for achieving a given level

of mitigation, or will improve environmental effectiveness. Efforts can include diverse elements such as emissions targets;

sectoral, local, sub-national and regional actions; RD&D

programmes; adopting common policies; implementing development-oriented actions; or expanding financing instruments. {4.5}

In several sectors, climate response options can be

implemented to realise synergies and avoid conflicts

with other dimensions of sustainable development.

Decisions about macroeconomic and other non-climate

policies can significantly affect emissions, adaptive

capacity and vulnerability. {4.4, 5.8}

Making development more sustainable can enhance mitigative and adaptive capacities, reduce emissions and reduce

vulnerability, but there may be barriers to implementation. On

the other hand, it is very likely that climate change can slow

the pace of progress towards sustainable development. Over

the next half-century, climate change could impede achievement of the Millennium Development Goals. {5.8}

5. The long-term perspective

Determining what constitutes “dangerous anthropogenic interference with the climate system” in relation

to Article 2 of the UNFCCC involves value judgements.

Science can support informed decisions on this issue,

including by providing criteria for judging which vulnerabilities might be labelled ‘key’. {Box ‘Key Vulnerabilities and Article 2 of the UNFCCC’, Topic 5}

Key vulnerabilities19 may be associated with many climate-sensitive systems, including food supply, infrastructure,

health, water resources, coastal systems, ecosystems, global

biogeochemical cycles, ice sheets and modes of oceanic and

atmospheric circulation. {Box ‘Key Vulnerabilities and Article 2 of

the UNFCCC’, Topic 5}

17 Studies on mitigation portfolios and macro-economic costs assessed in this report are based on top-down modelling. Most models use a

global least-cost approach to mitigation portfolios, with universal emissions trading, assuming transparent markets, no transaction cost, and

thus perfect implementation of mitigation measures throughout the 21st century. Costs are given for a specific point in time. Global modelled

costs will increase if some regions, sectors (e.g. land use), options or gases are excluded. Global modelled costs will decrease with lower

baselines, use of revenues from carbon taxes and auctioned permits, and if induced technological learning is included. These models do not consider

climate benefits and generally also co-benefits of mitigation measures, or equity issues. Significant progress has been achieved in applying approaches based on induced technological change to stabilisation studies; however, conceptual issues remain. In the models that consider induced

technological change, projected costs for a given stabilisation level are reduced; the reductions are greater at lower stabilisation level.

18 Further details may be found in Topic 4 of this Synthesis Report.

19 Key vulnerabilities can be identified based on a number of criteria in the literature, including magnitude, timing, persistence/reversibility, the

potential for adaptation, distributional aspects, likelihood and ‘importance’ of the impacts.
19

Summary for Policymakers

The five ‘reasons for concern’ identified in the TAR remain a viable framework to consider key vulnerabilities. These ‘reasons’ are assessed here to be stronger

than in the TAR. Many risks are identified with higher confidence. Some risks are projected to be larger or to occur

at lower increases in temperature. Understanding about

the relationship between impacts (the basis for ‘reasons

for concern’ in the TAR) and vulnerability (that includes

the ability to adapt to impacts) has improved. {5.2}

This is due to more precise identification of the circumstances that make systems, sectors and regions especially vulnerable and growing evidence of the risks of very large impacts on multiple-century time scales. {5.2}

 Risks to unique and threatened systems. There is new

and stronger evidence of observed impacts of climate

change on unique and vulnerable systems (such as polar

and high mountain communities and ecosystems), with

increasing levels of adverse impacts as temperatures increase further. An increasing risk of species extinction and

coral reef damage is projected with higher confidence than

in the TAR as warming proceeds. There is medium confidence that approximately 20 to 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 to 2.5°C over 1980-1999 levels. Confidence has

increased that a 1 to 2°C increase in global mean temperature above 1990 levels (about 1.5 to 2.5°C above preindustrial) poses significant risks to many unique and

threatened systems including many biodiversity hotspots.

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. Increasing

vulnerability of indigenous communities in the Arctic and

small island communities to warming is projected. {5.2}

 Risks of extreme weather events. Responses to some recent extreme events reveal higher levels of vulnerability

than the TAR. There is now higher confidence in the projected increases in droughts, heat waves and floods, as

well as their adverse impacts. {5.2}

 Distribution of impacts and vulnerabilities. There are

sharp differences across regions and those in the weakest

economic position are often the most vulnerable to climate change. There is increasing evidence of greater vulnerability of specific groups such as the poor and elderly

not only in developing but also in developed countries.

Moreover, there is increased evidence that low-latitude

and less developed areas generally face greater risk, for

example in dry areas and megadeltas. {5.2}

 Aggregate impacts. Compared to the TAR, initial net market-based benefits from climate change are projected to

peak at a lower magnitude of warming, while damages

would be higher for larger magnitudes of warming. The

net costs of impacts of increased warming are projected

to increase over time. {5.2}

 Risks of large-scale singularities. There is high confidence that global warming over many centuries would lead

to a sea level rise contribution from thermal expansion

alone that is projected to be much larger than observed

over the 20th century, with loss of coastal area and associated impacts. There is better understanding than in the TAR

that the risk of additional contributions to sea level rise

from both the Greenland and possibly Antarctic ice sheets

may be larger than projected by ice sheet models and could

occur on century time scales. This is because ice dynamical processes seen in recent observations but not fully included in ice sheet models assessed in the AR4 could increase the rate of ice loss. {5.2}

There is high confidence that neither adaptation nor

mitigation alone can avoid all climate change impacts;

however, they can complement each other and together

can significantly reduce the risks of climate change. {5.3}

Adaptation is necessary in the short and longer term to address impacts resulting from the warming that would occur even

for the lowest stabilisation scenarios assessed. There are barriers,

limits and costs, but these are not fully understood. Unmitigated

climate change would, in the long term, be likely to exceed the

capacity of natural, managed and human systems to adapt. The

time at which such limits could be reached will vary between

sectors and regions. Early mitigation actions would avoid further

locking in carbon intensive infrastructure and reduce climate

change and associated adaptation needs. {5.2, 5.3}

Many impacts can be reduced, delayed or avoided by

mitigation. Mitigation efforts and investments over the

next two to three decades will have a large impact on

opportunities to achieve lower stabilisation levels. Delayed emission reductions significantly constrain the

opportunities to achieve lower stabilisation levels and

increase the risk of more severe climate change impacts. {5.3, 5.4, 5.7}

In order to stabilise the concentration of GHGs in the atmosphere, emissions would need to peak and decline thereafter. The lower the stabilisation level, the more quickly this

peak and decline would need to occur.20 {5.4}

Table SPM.6 and Figure SPM.11 summarise the required

emission levels for different groups of stabilisation concentrations and the resulting equilibrium global warming and long20 For the lowest mitigation scenario category assessed, emissions would need to peak by 2015, and for the highest, by 2090 (see Table SPM.6).

Scenarios that use alternative emission pathways show substantial differences in the rate of global climate change.
Summary for Policymakers

20

Table SPM.6. Characteristics of post-TAR stabilisation scenarios and resulting long-term equilibrium global average temperature and

the sea level rise component from thermal expansion only.a {Table 5.1} Category CO2 concentration at stabilisation (2005 = 379 ppm) b CO2-equivalent concentration at stabilisation including GHGs and aerosols (2005 = 375 ppm) b Peaking year for CO2 emissions a,c Change in global CO2 emissions in 2050

(percent of 2000emissions) a,c

Global averagetemperature increaseabove pre-industrial atequilibrium, using ‘bestestimate’ climatesensitivity d, eGlobal average sea levelrise above pre-industrialat equilibrium fromthermal expansiononlyfNumber of assessedscenarios

ppm ppm year percent °C metres

I 350 – 400 445 – 490 2000 – 2015 -85 to -50 2.0 – 2.4 0.4 – 1.4 6

II 400 – 440 490 – 535 2000 – 2020 -60 to -30 2.4 – 2.8 0.5 – 1.7 18

III 440 – 485 535 – 590 2010 – 2030 -30 to +5 2.8 – 3.2 0.6 – 1.9 21

IV 485 – 570 590 – 710 2020 – 2060 +10 to +60 3.2 – 4.0 0.6 – 2.4 118

V 570 – 660 710 – 855 2050 – 2080 +25 to +85 4.0 – 4.9 0.8 – 2.9 9

VI 660 – 790 855 – 1130 2060 – 2090 +90 to +140 4.9 – 6.1 1.0 – 3.7 5

Notes:

a) The emission reductions to meet a particular stabilisation level reported in the mitigation studies assessed here might be underestimated due to missing carbon cycle feedbacks (see also Topic 2.3).

b) Atmospheric CO2 concentrations were 379ppm in 2005. The best estimate of total CO2-eq concentration in 2005 for all long-lived

GHGs is about 455ppm, while the corresponding value including the net effect of all anthropogenic forcing agents is 375ppm CO2

-eq.

c) Ranges correspond to the 15th to 85th percentile of the post-TAR scenario distribution. CO2 emissions are shown so multi-gas scenarios

can be compared with CO2

-only scenarios (see Figure SPM.3).

d) The best estimate of climate sensitivity is 3°C.

e) Note that global average temperature at equilibrium is different from expected global average temperature at the time of stabilisation of

GHG concentrations due to the inertia of the climate system. For the majority of scenarios assessed, stabilisation of GHG concentrations occurs between 2100 and 2150 (see also Footnote 21).

f) Equilibrium sea level rise is for the contribution from ocean thermal expansion only and does not reach equilibrium for at least many

centuries. These values have been estimated using relatively simple climate models (one low-resolution AOGCM and several EMICs

based on the best estimate of 3°C climate sensitivity) and do not include contributions from melting ice sheets, glaciers and ice caps.

Long-term thermal expansion is projected to result in 0.2 to 0.6m per degree Celsius of global average warming above pre-industrial.

(AOGCM refers to Atmosphere-Ocean General Circulation Model and EMICs to Earth System Models of Intermediate Complexity.)

21 Estimates for the evolution of temperature over the course of this century are not available in the AR4 for the stabilisation scenarios. For most

stabilisation levels, global average temperature is approaching the equilibrium level over a few centuries. For the much lower stabilisation

scenarios (category I and II, Figure SPM.11), the equilibrium temperature may be reached earlier.

term sea level rise due to thermal expansion only.21 The timing and level of mitigation to reach a given temperature

stabilisation level is earlier and more stringent if climate sensitivity is high than if it is low. {5.4, 5.7}

Sea level rise under warming is inevitable. Thermal expansion would continue for many centuries after GHG concentrations have stabilised, for any of the stabilisation levels

assessed, causing an eventual sea level rise much larger than

projected for the 21st century. The eventual contributions from

Greenland ice sheet loss could be several metres, and larger

than from thermal expansion, should warming in excess of

1.9 to 4.6°C above pre-industrial be sustained over many centuries. The long time scales of thermal expansion and ice sheet

response to warming imply that stabilisation of GHG concentrations at or above present levels would not stabilise sea level

for many centuries. {5.3, 5.4}

There is high agreement and much evidence that

all stabilisation levels assessed can be achieved by

deployment of a portfolio of technologies that are either currently available or expected to be commercialised

in coming decades, assuming appropriate and effective incentives are in place for their development,

acquisition, deployment and diffusion and addressing

related barriers. {5.5}

All assessed stabilisation scenarios indicate that 60 to 80%

of the reductions would come from energy supply and use

and industrial processes, with energy efficiency playing a key

role in many scenarios. Including non-CO2

 and CO2 land-use

and forestry mitigation options provides greater flexibility and

cost-effectiveness. Low stabilisation levels require early investments and substantially more rapid diffusion and

commercialisation of advanced low-emissions technologies. {5.5}

Without substantial investment flows and effective technology transfer, it may be difficult to achieve emission reduction at a significant scale. Mobilising financing of incremental costs of low-carbon technologies is important. {5.5}
21

Summary for Policymakers

Figure SPM.11. Global CO2 emissions for 1940 to 2000 and emissions ranges for categories of stabilisation scenarios from 2000 to 2100 (lefthand panel); and the corresponding relationship between the stabilisation target and the likely equilibrium global average temperature increase

above pre-industrial (right-hand panel). Approaching equilibrium can take several centuries, especially for scenarios with higher levels of stabilisation.

Coloured shadings show stabilisation scenarios grouped according to different targets (stabilisation category I to VI). The right-hand panel

shows ranges of global average temperature change above pre-industrial, using (i) ‘best estimate’ climate sensitivity of 3°C (black line in middle

of shaded area), (ii) upper bound of likely range of climate sensitivity of 4.5°C (red line at top of shaded area) (iii) lower bound of likely range of

climate sensitivity of 2°C (blue line at bottom of shaded area). Black dashed lines in the left panel give the emissions range of recent baseline

scenarios published since the SRES (2000). Emissions ranges of the stabilisation scenarios comprise CO2-only and multigas scenarios and

correspond to the 10th to 90th percentile of the full scenario distribution. Note: CO2 emissions in most models do not include emissions from decay

of above ground biomass that remains after logging and deforestation, and from peat fires and drained peat soils. {Figure 5.1}

CO2 emissions and equilibrium temperature increases for a range of stabilisation levels Equilibrium global average temperature increase above pre-industrial (°C)

GHG concentration stabilisation level (ppm CO -eq) Year 2

World CO emissions (GtCO /yr) 2 2

22 See Footnote 17 for more detail on cost estimates and model assumptions.

The macro-economic costs of mitigation generally rise

with the stringency of the stabilisation target (Table

SPM.7). For specific countries and sectors, costs vary

considerably from the global average.22 {5.6}

In 2050, global average macro-economic costs for mitigation towards stabilisation between 710 and 445ppm CO2

-eq are

between a 1% gain and 5.5% decrease of global GDP (Table

SPM.7). This corresponds to slowing average annual global GDP

growth by less than 0.12 percentage points. {5.6}

Table SPM.7. Estimated global macro-economic costs in 2030 and 2050. Costs are relative to the baseline for least-cost trajectories

towards different long-term stabilisation levels. {Table 5.2}

Stabilisation levels Median GDP reductiona (%) Range of GDP reductionb (%) Reduction of average annual GDP

(ppm CO2

-eq) growth rates (percentage points) c,e

2030 2050 2030 2050 2030 2050

445 – 535d Not available < 3 < 5.5 < 0.12 < 0.12

535 – 590 0.6 1.3 0.2 to 2.5 slightly negative to 4 < 0.1 < 0.1

590 – 710 0.2 0.5 -0.6 to 1.2 -1 to 2 < 0.06 < 0.05

Notes:

Values given in this table correspond to the full literature across all baselines and mitigation scenarios that provide GDP numbers.

a) Global GDP based on market exchange rates.

b) The 10th and 90th percentile range of the analysed data are given where applicable. Negative values indicate GDP gain. The first row

(445-535ppm CO2-eq) gives the upper bound estimate of the literature only.

c) The calculation of the reduction of the annual growth rate is based on the average reduction during the assessed period that would

result in the indicated GDP decrease by 2030 and 2050 respectively.

d) The number of studies is relatively small and they generally use low baselines. High emissions baselines generally lead to higher costs.

e) The values correspond to the highest estimate for GDP reduction shown in column three.
Summary for Policymakers

22

23 Net economic costs of damages from climate change aggregated across the globe and discounted to the specified year.

Responding to climate change involves an iterative risk

management process that includes both adaptation and

mitigation and takes into account climate change damages, co-benefits, sustainability, equity and attitudes

to risk. {5.1}

Impacts of climate change are very likely to impose net

annual costs, which will increase over time as global temperatures increase. Peer-reviewed estimates of the social cost

of carbon23 in 2005 average US$12 per tonne of CO2

, but the

range from 100 estimates is large (-$3 to $95/tCO2

). This is

due in 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. Aggregate

estimates of costs mask significant differences in impacts

across sectors, regions and populations and very likely underestimate damage costs because they cannot include many nonquantifiable impacts. {5.7}

Limited and early analytical results from integrated analyses of the costs and benefits of mitigation indicate that they

are broadly comparable in magnitude, but do not as yet permit

an unambiguous determination of an emissions pathway or

stabilisation level where benefits exceed costs. {5.7}

Climate sensitivity is a key uncertainty for mitigation scenarios for specific temperature levels. {5.4}

Choices about the scale and timing of GHG mitigation

involve balancing the economic costs of more rapid emission

reductions now against the corresponding medium-term and

long-term climate risks of delay. {5.7}