# 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, Ne ville 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 melt ing 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]°C 1 

### is larger than the corresponding trend of 0.6 [0.4 to 0.8]°C (1901-2000) given inthe 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 likely 2 

### 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 level 3 

### 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 20 th 

### 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 evidence **4** 

### from all continents and most **oceans shows that many natural systems are being affected by regional climate changes, particularly tem**  **perature 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 thedirection of change expected as a response to warming (Fig**1** 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 belo w that range . Uncertainty inter vals 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 ‘Treat ment 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 we ather 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 t he 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 re** **gional climate change on natural and human environ ments are emerging, although many are difficult to dis**  **cern 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 tem perature 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. Th ese met the following criteria: (1) ending in 1990 or later; (2) spanning a period of at least 20 years; and (3) showing a significant chan ge 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 co ntain about 29,000 data series, of which about 28,000 are from European studies. White areas do not contain sufficient observational climat e data to estimate a temperature trend. The 2 × 2 boxes show the total number of data series with significant changes (top row) and the p ercentage 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 Freshwa ter (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 (GL O) 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 89% 94% 100% 100% 100% 100% 100% 100% 99% 100% 98% 96% 91% 94% 94% 90% 90% 92% 94% 355 455 53 119



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

5 2 106 8 6 1 85 765 0 120 24 764 5 28,115 28,586 28,671**5 Summary for Policymakers** 



## 2. Causes of change

Changes in atmospheric concentrations of greenhousegases (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 (CO 2 

### ) is the most important anthropogenic GHG. Its annual emissions grew by about 80% between 1970 and 2004. The long-term trend of declining CO 2 

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


### Global atmospheric concentrations of CO **2** 

### , methane **(CH 4**  

### ) and nitrous oxide (N **2** 

### O) 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 CO 2 

### (379ppm) and CH 4 

### (1774ppb) in 2005 exceed by far the natural range over the last 650,000 years. Global increases in CO 2 

### 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 CH 4 

### concentration is predominantly due to agriculture and fossil fuel use. CH 4 

### growthrates 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 N 2 

### 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 tempera**tures since the mid-20 th**  

### century is **very likely due to the** **observed increase in anthropogenic GHG concentra tions.**  **7** 

### It is **likely that there has been significant anthro**  **pogenic 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 bymodels 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 (CO 2 -eq). (c) Share of different sectors in total anthropogenic GHG emissions in 2004 in terms of CO 2 -eq. (Forestry includes deforestation.) {Figure 2.1} F-gases CO 2 from fossil fuel use and other sources

CH 4 from agriculture, waste and energy CO 2 from deforestation, decay and peat

N 2 O from agriculture and othersGtCO2-eq / yr

28.7 35.6 39.4 44.7 49.0

**5** Includes only carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphurhexafluoride (SF 6 ), 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 t o human activities since the pre-industrial era is one of warming (+1.6 [+0.6 to +2.4] W/m 2 ). In comparison, changes in solar irradiance are estimated to have caused a small warming effect (+0.12 [+0.06 to +0.30] W/m 2 ).

**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 da shed 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 model s 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 20 th 

### 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 naturalclimate 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 d o 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 sus**  **tainable 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% (CO 2 

### -eq) between 2000 and 2030 (Figure SPM.5), withfossil 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 21**  **st** 

### century that **would very likely**  **be larger than those observed during the 20**  **th** 

### 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°Cper 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 ofavailable models suggests stronger climate-carbon cycle feedbacks. Warming reduces terrestrial and ocean uptake of atmospheric CO 2 

### , 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 GtCO 2 -eq) in the absence of climate policies: six illustrative SRES marker scenarios (coloured lines) and the 80 th 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 CO 2 , CH 4 , N

2 O and F-gases. **Right Panel:** Solid lines are m ulti-model global av erages of surface warming for scenarios A2, A1B and B1, shown as continuations of the 20 th -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 Circul ation 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 B2Global 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 r anges 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 pro**jected patterns of warming and other regional-scale features, including changes in wind patterns, precipi**  **tation 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 NorthAtlantic Ocean, continuing recent observed trends (Figure SPM.6)

 

### contraction of snow cover area, increases in thaw depthover most permafrost regions and decrease in sea ice extent; in some projections using SRES scenarios, Arcticlate-summer sea ice disappears almost entirely by the latter part of the 21 st 

### 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 informationfor 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 21 st 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 concentrations b 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 CO 2 -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, A 2 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 1 850- 1899 add 0.5°C.**9 Summary for Policymakers** 



### Geographical pattern of surface warming

**Figure SPM.6.** Projected surface temperature changes for the late 21 stcentury (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-typeecosystems 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-latitudes 13 

### andin 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) canbe 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 CO 2 

### concentrationslead 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 21 st

### century. While the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans isexpected 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 CO 2 where relevant) associated with different amounts of increase in global average surface temperature in the 21 stcentury. 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 assoc iated with the onset of a given impact. Quantitative entries for water scarcity and flooding represent the additional impacts of climate change rela tive 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)

5 °C 0 1234 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

5 °C 0 1234 † 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 agr iculture 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 21 st 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 con tinue 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. Thecorresponding 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 the y 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 confidencestatements, reflecting different sectors (agr iculture, ecosystems, water, coasts, health, industry and settlements). The Working Gro up 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 CO 2 -eq; category IV and V), assessed models project that about 65 to 70% of the estimated global equilibrium temperature increase, assuming a clima te 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 21 st 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 increasedrisk of extinction if increases in global average warming exceed 1.5 to 2.5°C (relative to 1980-1999). As global average Phenomenon a and Likelihood of Examples of major projected impacts by sector direction of trend future trends based on Agr iculture, forestry Water resource s Human health Industry, settlement projections and ecosystems and society for 21 st century using SRES scenarios
Over most land Virtually Increased yields in Effects on water Reduced human Reduced energy demand for areas, warmer and certain b colder environments; resources relying on mortality from heating; increased demand fewer cold days decreased yields in snowmelt; effects on decreased cold f or cooling; declining air quality and nights, warmer war mer environments; some water supplies e xposure in cities; reduced disruption to and more frequent increased insect tr ansport due to snow, ice; hot days and nights outbreaks eff ects on winter tourism Warm spells/heat Very likely Reduced yields in Increased water Increased risk of Reduction in quality of life for waves. Frequency wa rmer 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; inf ectious, respiratory societies due to flooding: areas to waterlogging of contamination of and skin diseases pressures on urban and rural soils w ater supply; water infrastructures; loss of property scarcity may be relieved Area affected by Likely Land degradation; More widespread Increased risk of W ater 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 r isk of malnutrition; potentials; potential for deaths; increased increased risk of population migrationrisk 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, injur ies, 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 migr ations; loss stress disorders of property

Increased incidence Likely d 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 dro wning in floods; relocation; potential for tsunamis) c water systems mig ration-related mov ement 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 hourl y 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 change s 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 21 st 

### 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 21 st 

### 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 CO 2 

### uptake, oceanic oxygen concentrations and terrestrial vegetation. Changes in terrestrial and ocean CO 2 

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



## 4. Adaptation and mitigation options **14**



### A wide array of adaptation options is available, but more**extensive adaptation than is currently occurring is re quired to reduce vulnerability to climate change. There** **are barriers, limits and costs, which are not fully un derstood.**  **{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 adverseimpacts 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 canbe 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 alreadyoccurring 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 somesectors 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 isdynamic 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 sub stantial economic potential for the mitigation of global**  **GHG emissions over the coming decades that could offset the projected growth of global emissions or re**  **duce 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 con siderable 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 GtCO 2 

### -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 discuss ed 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 redu ced). 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 cu rrently 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 eff iciency 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 abo ut 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 stormsurge 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 andintegrated 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 thatintegrate 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 tr ansport 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 sno w-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 trillion 16 

### between 2005 and 2030, will have long-term impacts on GHG emissions, because of the long lifetimes of energy plants and other infrastructurecapital 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 CO 2 

### emissions to 2005levels 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 = 20×10 12**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 secto r. 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 En ergy Outlook (WEO) 2004 baseline was used; the building sector is based on a baseline in between SRES B2 and A1B; for waste, SRES A1B drivin g forces were used to construct a waste-specific baseline; agriculture and forestry used baselines that mostly used B2 driving fo rces. d) Only global totals for transport are shown because international aviation is included. e) Categories excluded are: non-CO 2 emissions in buildings and transport, part of material efficiency options, heat production and co-generation in energy supply, heavy duty vehicles, shipping and high-oc cupancy passenger transport, most high-cost options for buildin gs, wastewater treatment, emission reduction from coal mines and gas pipelines, and fluorinated gases from energy supply and transport. Th e 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 GtCO 2 -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 CO 2 -eq 

### c) < 0 < 20 < 50 < 100 US$/tCO 2 

### -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$/tCO 2 

### -eq 

### b) Increase in GHG emissions above year 2000 levels 

### Bottom-up Top-down

Gt CO 2 -eq Gt CO 2 -eqEstimated 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 CO2from 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 photovoltaicsMore 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 batteriesEfficient 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 f eedback and control; solar photovoltaics integrated in buildingsMore efficient end-use electrical equipment; heat and power recovery; materialrecycling and substitution; control of non-CO2gas emissions; and a wide array ofprocess-specific technologies; advanced energy efficiency; CCS for cement, ammonia, and iron manufacture; inert electrodes for aluminium manufactureImproved 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 CH4emissions;improved nitrogen fertiliser application techniques to reduce N2O emissions;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 changeLandfill CH4recovery; waste incineration with energy recovery; composting oforganic waste; controlled wastewater treatment; recycling and waste minimisation; biocovers and biofilters to optimise CH 4 oxidation**Policies, measures and instruments shown to be environmentally effective** Reduction 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 facilitiesand 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 wastewatermanagementRenewableenergy incentives or obligationsWaste management regulations**Key constraints or opportunities**(Normal font = constraints; italics = opportunities)Resistance by vested interests may make themdifficult to implement May be appropriate to create markets for lowemissions technologiesPartial coverage of vehicle fleet may limiteffectivenessEffectiveness may drop with higher incomes Particularly appropriate for countries that are building up their transportation systemsPeriodic revision of standards needed Attractive for new buildings.Enforcement can bedifficultNeed 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 industry May encourage synergy with sustainable development and with reducing vulnerability to climate change, thereby overcoming barriers to implementationConstraints include lack of investment capital andland tenure issues. Can help poverty alleviation May stimulate technology diffusionLocal availability of low-cost fuelMost effectively applied at national level withenforcement strategies**Sector Energy supply** **Transport Buildings** **Industry Agriculture** **Forestry/ forests** **Waste Summary for Policymakers** 

**18**

### A wide variety of policies and instruments are avail**able to governments to create the incentives for miti gation 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/tCO 2 

### -eq by 2030 are consistent with stabilisation at around 550ppm CO 2 

### -eq by 2100. For the same stabilisation level, induced technological change may lower these price ranges to US$5-65/tCO 2 

### -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. improvedhealth 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 globalemissions, 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 thatchanges 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 achieve**  **ments 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 cre ation of an international carbon market and new insti**  **tutional mechanisms that may provide the foundation for future mitigation efforts. Progress has also been made** **in addressing adaptation within the UNFCCC and addi tional 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 levelof mitigation, or will improve environmental effectiveness. Efforts can include diverse elements such as emissions targets; sectoral, local, sub-national and regional actions; RD&Dprogrammes; 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. Overthe next half-century, climate change could impede achievement of the Millennium Development Goals. {5.8}



## 5. The long-term perspective

**Determining what constitutes “dangerous anthropo genic 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 vul nerabilities might be labelled ‘key’.**  **{Box ‘Key Vulnerabili ties and Article 2 of the UNFCCC’, Topic 5}**  

### Key vulnerabilities 19 

### 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 transac tion cost, and thus perfect implementation of mitigation measures throughout the 21 st 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 decreas e with lower baselines, use of revenues from carbon taxes and auctioned permits, and if induced technological learning is included. These mo dels do not consider climate benefits and generally also co-benefits of mitigation measures, or equity issues. Significant progress has been achieve d 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 stabilis ation 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 re**main a viable framework to consider key vulnerabili ties. These ‘reasons’ are assessed here to be stronger** **than in the TAR. Many risks are identified with higher con fidence. 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), withincreasing 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 ofextinction if increases in global average temperature exceed 1.5 to 2.5°C over 1980-1999 levels. Confidence hasincreased 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 vulnerabilitythan 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 weakesteconomic 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 20 th 

### 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 couldoccur 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. De layed emission reductions significantly constrain the**  **opportunities to achieve lower stabilisation levels and increase the risk of more severe climate change im**  **pacts. {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 requiredemission levels for different groups of stabilisation concentrations and the resulting equilibrium global warming and long**20** For the lowest mitigation scenario category assessed, emissions would need to peak by 2015, and for the highest, by 2090 (see T able 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}CategoryCO2concentrationat stabilisation(2005 = 379 ppm)bCO2-equivalentconcentration atstabilisation includingGHGs and aerosols(2005 = 375 ppm)bPeaking year for CO2emissionsa,cChange in global CO2emissions in 2050(percent of 2000emissions)a,cGlobal averagetemperature increaseabove pre-industrial atequilibrium, using ‘bestestimate’ climatesensitivityd, 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 lev el reported in the mitigation studies assessed here might be un derestimated due to missing carbon cycle feedbac ks (see also Topic 2.3). b) Atmospheric CO 2 concentrations were 379ppm in 2005. The best estimate of total CO 2 -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 CO 2 -eq. c) Ranges correspond to the 15 th to 85 th percentile of the post-TAR scenario distribution. CO 2 emissions are shown so multi-gas scenarios can be compared with CO 2 -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 stab ilisation of GHG concentrations due to the inertia of the climate system. For the majority of scenarios assessed, stabilisation of GHG conce ntrations 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 le ast 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 i ce caps. Long-term thermal expansion is projected to result in 0.2 to 0.6m per degree Celsius of global average warming above pre-indust rial. (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 s cenarios. 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 temperaturestabilisation 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 21 st 

### century. The eventual contributions from Greenland ice sheet loss could be several metres, and larger than from thermal expansion, should warming in excess of1.9 to 4.6°C above pre-industrial be sustained over many centuries. The long time scales of thermal expansion and ice sheetresponse 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 ei**  **ther currently available or expected to be commercialised in coming decades, assuming appropriate and effec**  **tive 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-CO 2 

### and CO 2 

### land-use and forestry mitigation options provides greater flexibility andcost-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 CO 2emissions 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 high er levels of stabilisation. Coloured shadings show stabilisation scenarios grouped according to different targets (stabilisation category I to VI). The rig ht-hand panel shows ranges of global average temperature change above pre-industrial, using (i) ‘best estimate’ climate sensitivity of 3°C (b lack 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 CO 2 -only and multigas scenarios and correspond to the 10 th to 90 th percentile of the full scenario distribution. Note: CO 2 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} 

### CO 2 

### emissions and equilibrium temperature increases for a range of stabilisation levelsEquilibrium global average temperatureincrease above pre-industrial (°C)

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

### -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 reduction a (%) Range of GDP reduction b (%) Reduction of average annual GDP (ppm CO 2 -eq) growth rates (percentage points) c,e

2030 2050 2030 2050 2030 2050

445 – 535 d 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 num bers. a) Global GDP based on market exchange rates. b) The 10 th and 90 th percentile range of the analysed data are given where applicable. Negative values indicate GDP gain. The first row (445-535ppm CO 2 -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 hi gher 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 dam**  **ages, co-benefits, sustainability, equity and attitudes to risk.**  **{5.1}** 

### Impacts of climate change are very likely to impose netannual costs, which will increase over time as global temperatures increase. Peer-reviewed estimates of the social cost of carbon 23 

### in 2005 average US$12 per tonne of CO 2 

### , but the range from 100 estimates is large (-$3 to $95/tCO 2 

### ). This isdue 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}