### Core Writing Team 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
**Extended Writing Team** Terry Barker

**Review Editors** Abdelkader Allali, Roxana Bojariu, Sandra Diaz, Ismail Elgizouli, Dave Griggs, David Hawkins, Olav Hohmeyer, Bubu Pateh Jallow, Luc 

### 4 ka Kajfez 

### 4 -Bogataj, Neil Leary, Hoesung Lee, David Wratt 

# Climate Change 2007: **Synthesis Report** 

# Synthesis Report

**An Assessment of the Intergovernmental Panel on Climate Change**
This underlying report, adopted section by section 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:

# Introduction**Introduction**

**26** 

## Introduction

This Synthesis Report is based on the assessment carried out by the three Working Groups (WGs) of the Intergovernmental Panelon Climate Change (IPCC). It provides an integrated view of climate change as the final part of the IPCC’s Fourth Assessment Report (AR4).Topic 1 summarises observed changes in climate and their effects on natural and human systems, regardless of their causes, whileTopic 2 assesses the causes of the observed changes. Topic 3 presents projections of future climate change and related impacts under different scenarios. Topic 4 discusses adaptation and mitigation options over thenext few decades and their interactions with sustainable develop**Figure I.1.** Schematic framework representing anthropogenic drivers, impacts of and responses to climate change, and their linkages. 

### Schematic framework of anthropogenic climate change drivers, impacts and responses 

### ment. Topic 5 assesses the relationship between adaptation andmitigation on a more conceptual basis and takes a longer-term perspective. Topic 6 summarises the major robust findings and remaining key uncertainties in this assessment. A schematic framework representing anthropogenic drivers, impacts of and responses to climate change, and their linkages, is shown in Figure I.1. At the time of the Third Assessment Report (TAR) in 2001, information was mainly available to describe the linkages clockwise, i.e. to derive climatic changes and impacts fromsocio-economic information and emissions. With increased understanding of these linkages, it is now possible to assess the linkagesalso counterclockwise, i.e. to evaluate possible development pathways and global emissions constraints that would reduce the risk of future impacts that society may wish to avoid.**27 Introduction** 

**1** See http://www.ipcc.ch/meetings/ar4-workshops-express-meetings/uncertainty-guidance-note.pdf



### Treatment of uncertainty The IPCC uncertainty guidance note 1 defines a framework for the treatment of uncertainties across all WGs and in this Synthesis Report. This framework is broad because the WGs assess material from different disciplines and cover a diversity of approaches to the t reatment of uncertainty drawn from the literature. The nature of data, indicators and analyses used in the natural sciences is generally di fferent from that used in assessing technology development or the social sciences. WG I focuses on the former, WG III on the latter, and WG II co vers aspects of both. Three different approaches are used to describe uncertainties each with a distinct form of language. Choices among and within t hese three approaches depend on both the nature of the information available and the authors’ expert judgment of the correctness and compl eteness of current scientific understanding. Where uncertainty is assessed qualitatively, it is characterised by providing a relative sense of the amount and quality of evi dence (that is, information from theory, observations or models indicating whether a belief or proposition is true or valid) and the degree of agreement (that is, the level of concurrence in the literature on a particular finding). This approach is used by WG III through a series of self-e xplanatory terms such as: high agreement, much evidence; high agreement, medium evidence; medium agreement, medium evidence; etc

. Where uncertainty is assessed more quantitatively using expert judgement of the correctness of underlying data, models or analy ses, then the following scale of confidence levels is used to express the assessed chance of a finding being correct: v ery high confidence at least 9 out of 10; high confidence about 8 out of 10; medium confidence about 5 out of 10; low confidence about 2 out of 10; and very low confidence less than 1 out of 10. Where uncertainty in specific outcomes is assessed using expert judgment and statistical analysis of a body of evidence (e.g. o bservations or model results), then the following likelihood ranges are used to express the assessed probability of occurrence: virtually certain >99%; extremely likely >95%; very likely >90%; likely >66%; more likely than not > 50%; about as likely as not 33% to 66%; unlikely <33%; very unlikely <10%; extremely unlikely <5%; exceptionally unlikely <1%. WG II has used a combination of confidence and likelihood assessments and WG I has predominantly used likelihood assessments. This Synthesis Report follows the uncertainty assessment of the underlying WGs. Where synthesised findings are based on informa tion from more than one WG, the description of uncertainty used is consistent with that for the components drawn from the respective WG reports. Unless otherwise stated, numerical ranges given in square brackets in this report indicate 90% uncertainty intervals (i.e. ther e 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 best estimate.

# 1

**Observed changes in climate and their effects Topic 1**  **Observed changes in climate and their effects**

**30** 

## 1.1 Observations of climate change

Since the TAR, progress in understanding how climate is changing in space and time has been gained through improvements andextensions of numerous datasets and data analyses, broader geographical coverage, better understanding of uncertainties and a wider variety of measurements. {WGI SPM}



### Definitions of climate change Climate change in IPCC usage refers to a change in the state of the climate that can be identified (e.g. using statistical tests) by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer. It refers to any change in climate over time, whether due to natural variability or as a result of human activity. Thisusage differs from that in the United Nations Framework Convention on Climate Change (UNFCCC), where climate changerefers to a change of climate that is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and that is in addition to natural climate variability observed over comparable time periods.



### 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 1.1). {WGI**  **3.2, 4.8, 5.2, 5.5, SPM}** 

### 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 is larger than the corresponding trend of 0.6 [0.4 to 0.8]°C (1901-2000) given in the TAR (Figure 1.1). The linear warming trend over the 50 years from 1956 to 2005 (0.13 [0.10 to 0.16]°C per decade) is nearly twice that for the 100 years from 1906 to 2005. {WGI 3.2, SPM} 

### The temperature increase is widespread over the globe and isgreater at higher northern latitudes (Figure 1.2). Average Arctic temperatures have increased at almost twice the global average rate in the past 100 years. Land regions have warmed faster than the oceans(Figures 1.2 and 2.5). Observations since 1961 show that the average temperature of the global ocean has increased to depths of at least 3000m and that the ocean has been taking up over 80% of theheat being added to the climate system. New analyses of balloonborne and satellite measurements of lower- and mid-tropospherictemperature show warming rates similar to those observed in surface temperature. {WGI 3.2, 3.4, 5.2, SPM}


### Increases in sea level are consistent with warming (Figure 1.1). Global average sea level rose at an average rate of 1.8 [1.3 to 2.3]mm per year over 1961 to 2003 and at an average rate of about 3.1 [2.4 to 3.8]mm per year from 1993 to 2003. Whether this faster rate for1993 to 2003 reflects decadal variation or an increase in the longerterm trend is unclear. Since 1993 thermal expansion of the oceans has contributed about 57% of the sum of the estimated individual contributions to the sea level rise, with decreases in glaciers and ice caps contributing about 28% and losses from the polar ice sheets contributing the remainder. From 1993 to 2003 the sum of these climate contributions is consistent within uncertainties with the total sea level rise that is directly observed. {WGI 4.6, 4.8, 5.5, SPM, Table SPM.1}


### Observed decreases in snow and ice extent are also consistentwith warming (Figure 1.1). 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 havedeclined in both hemispheres. The maximum areal extent of seasonally frozen ground has decreased by about 7% in the Northern Hemisphere since 1900, with decreases in spring of up to 15%.Temperatures at the top of the permafrost layer have generally increased since the 1980s in the Arctic by up to 3°C. {WGI 3.2, 4.5, 4.6, 4.7, 4.8, 5.5, SPM}

### At continental, regional and ocean basin scales, numerous longterm changes in other aspects of climate have also been observed. Trends from 1900 to 2005 have been observed in precipitationamount in many large regions. Over this period, precipitation increased significantly in eastern parts of North and South America,northern Europe and northern and central Asia whereas precipitation 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. {WGI 3.3, 3.9, SPM} 

### Some extreme weather events have changed in frequency and/ or intensity over the last 50 years:

 

### It is very likely that cold days, cold nights and frosts have become less frequent over most land areas, while hot days and hot nights have become more frequent. {WGI 3.8, SPM}

 

### It is likely that heat waves have become more frequent over most land areas. {WGI 3.8, SPM}

 

### It is likely that the frequency of heavy precipitation events (or proportion of total rainfall from heavy falls) has increased over
most areas. {WGI 3.8, 3.9, SPM}

 

### It is likely that the incidence of extreme high sea level 3 

### has increased at a broad range of sites worldwide since 1975. {WGI 5.5, SPM}


### There is observational evidence of an increase in intense tropicalcyclone activity in the North Atlantic since about 1970, and suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routinesatellite observations in about 1970 complicate the detection of longterm trends in tropical cyclone activity. {WGI 3.8, SPM} 

### 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. {WGI 6.6, SPM}

**2** Likelihood and confidence statements in italics represent calibrated expressions of uncertainty and confidence. See Box ‘Treatm ent of uncertainty’ in the Introduction for an explanation of these terms.

**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.**31 Topic 1**  **Observed changes in climate and their effects**



### (a) Global average surface temperature

(b) Global average sea level

(c) Northern Hemisphere snow cover

**Figure 1.1.** 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). {WGI FAQ 3.1 Figure 1, Figure 4.2, Figure 5.13, Figure SPM.3} 

### Changes in temperature, sea level and Northern Hemisphere snow cover



## 1.2 Observed effects of climate changes

The statements presented here are based largely on data sets that cover the period since 1970. The number of studies of observedtrends in the physical and biological environment and their relationship to regional climate changes has increased greatly since the TAR. The quality of the data sets has also improved. There is anotable lack of geographic balance in data and literature on observed changes, with marked scarcity in developing countries. {WGII SPM}


### These studies have allowed a broader and more confident assessment of the relationship between observed warming and impacts than was made in the TAR. That assessment concluded that “there is high confidence 2 

### that recent regional changes in temperature have had discernible impacts on physical and biological systems”. {WGII SPM} 

### Observational evidence from all continents and most oceans**shows that many natural systems are being affected by re gional climate changes, particularly temperature increases.**  **{WGII SPM}**



### There is high confidence that natural systems related to snow, ice and frozen ground (including permafrost) are affected. Examples are:

 

### enlargement and increased numbers of glacial lakes {WGII 1.3, SPM}

 

### increasing ground instability in permafrost regions and rock avalanches in mountain regions {WGII 1.3, SPM}

 

### changes in some Arctic and Antarctic ecosystems, including those in sea-ice biomes, and predators at high levels of the food web. {WGII 1.3, 4.4, 15.4, SPM}


### Based on growing evidence, there is high confidence that the following effects on hydrological systems are occurring: increasedrunoff and earlier spring peak discharge in many glacier- and snowfed rivers, and warming of lakes and rivers in many regions, with effects on thermal structure and water quality. {WGII 1.3, 15.2, SPM}**Topic 1 Observed changes in climate and their effects** 

**32 Figure 1.2.**  Locations of significant changes in data series of physical systems (snow, ice and frozen ground; hydrology; and coastal proc esses) and biological systems (terrestrial, marine, and freshwater biological systems), are shown together with surface air temperature ch anges 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 x 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) glo bal-scale: Terrestrial (TER), Marine and Freshwater (MFW), and Global (GLO). The numbers of studies from the seven regional boxes (NAM, ..., 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. {WGII Figure SPM.1, Figure 1.8, Figure 1.9; WGI Figure 3.9b} 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



### Changes in physical and biological systems and surface temperature 1970-2004**33 Topic 1**  **Observed changes in climate and their effects**



### There is very high confidence , based on more evidence from a wider range of species, that recent warming is strongly affecting terrestrial biological systems , including such changes as earlier timing of spring events, such as leaf-unfolding, bird migration and egg-laying; and poleward and upward shifts in ranges in plant and animal species. Based on satellite observations since the early 1980s, there is high confidence that there has been a trend in many regions towards earlier ‘greening’ of vegetation in the spring linked to longer thermal growing seasons due to recent warming. {WGII 1.3, 8.2, 14.2, SPM}


### There is high confidence , based on substantial new evidence, that observed changes in marine and freshwater biological systems are associated with rising water temperatures, as well as related changes in ice cover, salinity, oxygen levels and circulation. These include: shifts in ranges and changes in algal, plankton and fishabundance in high-latitude oceans; increases in algal and zooplankton abundance in high-latitude and high-altitude lakes; and rangechanges and earlier fish migrations in rivers. While there is increasing evidence of climate change impacts on coral reefs, separatingthe impacts of climate-related stresses from other stresses (e.g. overfishing and pollution) is difficult. {WGII 1.3, SPM}


### Other effects of regional climate changes on natural and**human environments are emerging, although many are dif ficult to discern due to adaptation and non-climatic drivers.**  **{WGII SPM}** 

### Effects of temperature increases have been documented with medium confidence in the following managed and human systems:

 

### agricultural and forestry management at Northern Hemisphere higher latitudes, such as earlier spring planting of crops, and alterations in disturbances of forests due to fires and pests {WGII 1.3, SPM}

 

### some aspects of human health, such as excess heat-related mortality in Europe, changes in infectious disease vectors in parts of Europe, and earlier onset of and increases in seasonal production of allergenic pollen in Northern Hemisphere high and mid-latitudes {WGII 1.3, 8.2, 8.ES, SPM}

 

### some human activities in the Arctic (e.g. hunting and shorter travel seasons over snow and ice) and in lower-elevation alpine areas (such as limitations in mountain sports). {WGII 1.3, SPM }



### Sea level rise and human development are together contributing to losses of coastal wetlands and mangroves and increasing damage from coastal flooding in many areas. However, based onthe published literature, the impacts have not yet become established trends. {WGII 1.3, 1.ES, SPM}



## 1.3 Consistency of changes in physical and **biological systems with warming**

Changes in the ocean and on land, including observed decreases in snow cover and Northern Hemisphere sea ice extent, thinner seaice, shorter freezing seasons of lake and river ice, glacier melt, decreases in permafrost extent, increases in soil temperatures and borehole temperature profiles, and sea level rise, provide additional evidence that the world is warming. {WGI 3.9}

### 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 (Figure 1.2). {WGII 1.4, SPM}



## 1.4 Some aspects of climate have not been **observed to change**

Some aspects of climate appear not to have changed and, for some, data inadequacies mean that it cannot be determined if theyhave changed. Antarctic sea ice extent shows inter-annual variability and localised changes but no statistically significant average multi-decadal trend, consistent with the lack of rise in near-surface atmospheric temperatures averaged across the continent. There is insufficient evidence to determine whether trends exist in some other variables, for example the meridional overturning circulation (MOC) of the global ocean or small-scale phenomena such as tornadoes, hail, lightning and dust storms. There is no clear trend in the annual numbers of tropical cyclones. {WGI 3.2, 3.8, 4.4, 5.3, SPM}

# 2

**Causes of change Topic 2**  **Causes of change**

**36** 

## Causes of change

This Topic considers both natural and anthropogenic drivers of climate change, including the chain from greenhouse gas (GHG) emissions to atmospheric concentrations to radiative forcing 4 

### to climate responses and effects.



## 2.1 Emissions of long-lived GHGs

The radiative forcing of the climate system is dominated by the long-lived GHGs, and this section considers those whose emissions are covered by the UNFCCC.

**Global GHG emissions due to human activities have grown since pre-industrial times, with an increase of 70% between**  **1970 and 2004 (Figure 2.1). 5**  **{WGIII 1.3, SPM}**


### Carbon dioxide (CO 2 

### ) is the most important anthropogenic GHG. Its annual emissions have grown between 1970 and 2004 by about 80%, from 21 to 38 gigatonnes (Gt), and represented 77% of total anthropogenic GHG emissions in 2004 (Figure 2.1). The rate of growth of CO 2 

### -eq emissions was much higher during the recent 10-year period of 1995-2004 (0.92 GtCO 2 

### -eq per year) than during the previous period of 1970-1994 (0.43 GtCO 2 

### -eq per year). {WGIII 1.3, TS.1, SPM}

**4** Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere sys tem and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values ar e for changes relative to preindustrial conditions defined at 1750 and are expressed in watts per square metre (W/m 2 ).

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

**6** This report uses 100-year GWPs and numerical values consistent with reporting under the UNFCCC.

**7** Such values may consider only GHGs, or a combination of GHGs and aerosols. 

### Carbon dioxide-equivalent (CO **2** 

### -eq) emissions and **concentrations** GHGs differ in their warming influence (radiative forcing) onthe global climate system due to their different radiative properties and lifetimes in the atmosphere. These warming influences may be expressed through a common metric based on the radiative forcing of CO 2 .

• **CO 2**  **-equivalent emission** is the amount of CO 2 emission that would cause the same time-integrated radiative forcing,over a given time horizon, as an emitted amount of a longlived GHG or a mixture of GHGs. The equivalent CO 2 emission is obtained by multiplying the emission of a GHG by its Global Warming Potential (GWP) for the given time horizon. 6 For a mix of GHGs it is obtained by summing the equivalent CO 2 emissions of each gas. Equivalent CO 2 emission is astandard and useful metric for comparing emissions of different GHGs but does not imply the same climate change responses (see WGI 2.10).

• **CO 2**  **-equivalent concentration** is the concentration of CO 2 that would cause the same amount of radiative forcing as a given mixture of CO 2 and other forcing components. 7

**Figure 2.1.** (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 CO 2 -eq. (c) Share of different sectors in total anthropogenic GHG emissions in 2004 in terms of CO 2 -eq. (Forestry includes deforestation.) {WGIII Figures TS.1a, TS.1b, TS.2b} 

### Global anthropogenic GHG emissions

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 

### The largest growth in GHG emissions between 1970 and 2004has come from energy supply, transport and industry, while residential and commercial buildings, forestry (including deforestation) and agriculture sectors have been growing at a lower rate. The**37 Topic 2**  **Causes of change**

**Figure 2.2.** (a) Distribution of regional per capita GHG emissions according to the population of different country groupings in 2004 (see a ppendix for definitions of country groupings). (b) Distribution of regional GHG emissions per US$ of GDP PPP

over the GDP of different country groupings in 2004. The percentages in the bars in both panels indicate a region’s share in global GHG emissions. {WGIII Figures SPM.3a, SPM.3b} 

### Regional distribution of GHG emissions by population and by GDP PPP

and M&Tand M&T 

### sectoral sources of GHGs in 2004 are considered in Figure 2.1c. {WGIII 1.3, SPM} 

### The effect on global emissions of the decrease in global energyintensity (-33%) during 1970 to 2004 has been smaller than the combined effect of global income growth (77%) and global population growth (69%); both drivers of increasing energy-related CO 2 

### emissions. The long-term trend of declining CO 2 

### emissions per unit of energy supplied reversed after 2000. {WGIII 1.3, Figure SPM.2, SPM} 

### Differences in per capita income, per capita emissions and energy intensity among countries remain significant. In 2004, UNFCCC Annex I countries held a 20% share in world population, produced 57% of the world’s Gross Domestic Product based on Purchasing Power Parity (GDP PPP 

### ) and accounted for 46% of global GHG emissions (Figure 2.2). {WGIII 1.3, SPM}



## 2.2 Drivers of climate change

Changes in the atmospheric concentrations of GHGs and aerosols, land cover and solar radiation alter the energy balance of the climate system and are drivers of climate change. They affect theabsorption, scattering and emission of radiation within the atmosphere and at the Earth’s surface. The resulting positive or negative changes in energy balance due to these factors are expressed as radiative forcing 4 

### , which is used to compare warming or cooling influences on global climate. {WGI TS.2} Human activities result in emissions of four long-lived GHGs: CO 2 

### , methane (CH 4 

### ), nitrous oxide (N 2 

### O) and halocarbons (a group of gases containing fluorine, chlorine or bromine). Atmospheric concentrations of GHGs increase when emissions are larger than removal processes.

**Global atmospheric concentrations of CO 2**  

### , CH **4** 

### and N **2** 

### O **have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values deter**  **mined from ice cores spanning many thousands of years (Figure 2.3). The atmospheric concentrations of CO**  **2** 

### and CH **4** 

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

### concentrations are due pri**marily 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 predomi**nantly due to agriculture and fossil fuel use. The increase in N**  **2** 

### O concentration is primarily due to agriculture. **{WGI 2.3, 7.3, SPM}**  

### The global atmospheric concentration of CO 2 

### increased from apre-industrial value of about 280ppm to 379ppm in 2005. The annual CO 2 

### concentration growth rate was larger during the last 10 years (1995-2005 average: 1.9ppm per year) than it has been since the beginning of continuous direct atmospheric measurements(1960-2005 average: 1.4ppm per year), although there is year-toyear variability in growth rates. {WGI 2.3, 7.3, SPM; WGIII 1.3} 

### The global atmospheric concentration of CH 4 

### has increased from a pre-industrial value of about 715ppb to 1732ppb in the early 1990s, and was 1774ppb in 2005. Growth rates have declined since the early 1990s, consistent with total emissions (sum of anthropogenic and natural sources) being nearly constant during this period. {WGI 2.3, 7.4, SPM} 

### The global atmospheric N 2 

### O concentration increased from a pre-industrial value of about 270ppb to 319ppb in 2005. {WGI 2.3, 7.4, SPM}

### Many halocarbons (including hydrofluorocarbons) have increased from a near-zero pre-industrial background concentration, primarily due to human activities. {WGI 2.3, SPM; SROC SPM}



### There is **very high confidence that the global average net** **effect of human activities since 1750 has been one of warm ing, with a radiative forcing of +1.6 [+0.6 to +2.4] W/m**  **2** 

### (Figure 2.4). **{WGI 2.3, 6.5, 2.9, SPM}** 

### The combined radiative forcing due to increases in CO 2 

### , CH 4 

### and N 2 

### O is +2.3 [+2.1 to +2.5] W/m 2 

### , and its rate of increase during**Topic 2 Causes of change** 

**38** 

### the industrial era is very likely to have been unprecedented in more than 10,000 years (Figures 2.3 and 2.4). The CO 2 

### radiative forcing increased by 20% from 1995 to 2005, the largest change for any decade in at least the last 200 years. {WGI 2.3, 6.4, SPM} 

### Anthropogenic contributions to aerosols (primarily sulphate, organic carbon, black carbon, nitrate and dust) together produce a cooling effect, with a total direct radiative forcing of -0.5 [-0.9 to -0.1] W/m 2 

### and an indirect cloud albedo forcing of -0.7 [-1.8 to -0.3] W/m 2 

### . Aerosols also influence precipitation. {WGI 2.4, 2.9, 7.5, SPM}

### In comparison, changes in solar irradiance since 1750 are estimated to have caused a small radiative forcing of +0.12 [+0.06 to +0.30] W/m 2 

### , which is less than half the estimate given in the TAR. {WGI 2.7, SPM}



## 2.3 Climate sensitivity and feedbacks

The equilibrium climate sensitivity is a measure of the climate system response to sustained radiative forcing. It is defined as the equilibrium global average surface warming following a doubling of CO 2 

### concentration. Progress since the TAR enables an assessment that climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be lessthan 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values. {WGI 8.6, 9.6, Box 10.2, SPM}

### Feedbacks can amplify or dampen the response to a given forcing. Direct emission of water vapour (a greenhouse gas) by humanactivities makes a negligible contribution to radiative forcing. However, as global average temperature increases, tropospheric water vapour concentrations increase and this represents a key positive feedback but not a forcing of climate change. Water vapour changesrepresent the largest feedback affecting equilibrium climate sensitivity and are now better understood than in the TAR. Cloud feedbacks remain the largest source of uncertainty. Spatial patterns of climate response are largely controlled by climate processes and feedbacks. For example, sea-ice albedo feedbacks tend to enhance the high latitude response. {WGI 2.8, 8.6, 9.2, TS.2.1.3, TS.2.5, SPM} 

### Warming reduces terrestrial and ocean uptake of atmospheric CO 2 

### , increasing the fraction of anthropogenic emissions remaining in the atmosphere. This positive carbon cycle feedback leads to larger atmospheric CO 2 

### increases and greater climate change for a given emissions scenario, but the strength of this feedback effect varies markedly among models. {WGI 7.3, TS.5.4, SPM; WGII 4.4}



## 2.4 Attribution of climate change

Attribution evaluates whether observed changes are quantitatively consistent with the expected response to external forcings(e.g. changes in solar irradiance or anthropogenic GHGs) and inconsistent with alternative physically plausible explanations. {WGI TS.4, SPM} **Figure 2.3.** Atmospheric concentrations of CO 2 , CH 4 and N 2 O over the last 10,000 years (large panels) and since 1750 (inset panels). Measurementsare shown from ice cores (symbols with different colours for different studies) and atmospheric samples (red lines). The corresponding radiative forcings relative to 1750 are shown on the right hand axes of the large panels. {WGI Figure SPM.1} 

### Changes in GHGs from ice core and modern data**39 Topic 2**  **Causes of change**



### Radiative forcing components

**Figure 2.4.** Global average radiative forcing (RF) in 2005 (best estimates and 5 to 95% uncertainty ranges) with respect to 1750 for CO 2 , CH 4 , N

2 O and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). Aerosols from explosive volcanic eruptions contribute an additional episodic cooling term for a few years following an eruption. The range for linear contrails does not include other possible effects of aviation on cloudiness. {WGI Figure SPM.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 concentrations. 8**  

### This is an advance since the TAR’s conclusion that “most **of the observed warming over the last 50 years is likely**  **to have been due to the increase in GHG concentrations” (Fig**  **ure 2.5). {WGI 9.4, SPM}**  

### The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years can be explained without external forcing and very likely that it is not due to known natural causes alone. During this period, the sum of solar and volcanic forcings would likely have produced cooling, not warming. Warming of the climate system has been detected inchanges in surface and atmospheric temperatures and in temperatures of the upper several hundred metres of the ocean. The observed pattern of tropospheric warming and stratospheric cooling is very likely due to the combined influences of GHG increases and stratospheric ozone depletion. It is likely that increases in GHGconcentrations alone would have caused more warming than observed because volcanic and anthropogenic aerosols have offset some warming that would otherwise have taken place. {WGI 2.9, 3.2, 3.4, 4.8, 5.2, 7.5, 9.4, 9.5, 9.7, TS.4.1, SPM}



### It is **likely that there has been significant anthropogenic** **warming over the past 50 years averaged over each conti nent (except Antarctica) (Figure 2.5).**  **{WGI 3.2, 9.4, SPM}** 

### The observed patterns of warming, including greater warming over land than over the ocean, and their changes over time, are simulated only by models that include anthropogenic forcing. No coupled global climate model that has used natural forcing only has reproduced the continental mean warming trends in individual continents (except Antarctica) over the second half of the 20 th 

### century. {WGI 3.2, 9.4, TS.4.2, SPM}

**8** Consideration of remaining uncertainty is based on current methodologies.**Topic 2 Causes of change** 

**40** 

### Global and continental temperature change

**Figure 2.5.** Comparison of observed continental- and global-scale changes in surface temperature with results simulated by climate models us ing 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 1901-1950. Lines are dashed where spatial coverage is le ss 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. {WGI Fig ure SPM.4}



### Difficulties remain in simulating and attributing observed temperature changes at smaller scales. On these scales, natural climate variability is relatively larger, making it harder to distinguish changes expected due to external forcings. Uncertainties in local forcings, such as those due to aerosols and land-use change, and feedbacks also make it difficult to estimate the contribution of GHG increases to observed small-scale temperature changes. {WGI 8.3, 9.4, SPM}



### Advances since the TAR show that discernible human in**fluences extend beyond average temperature to other as pects of climate, including temperature extremes and wind**  **patterns. {WGI 9.4, 9.5, SPM}**  

### Temperatures of the most extreme hot nights, cold nights and cold days are likely to have increased due to anthropogenic forcing. It is more likely than not that anthropogenic forcing has increased the risk of heat waves. Anthropogenic forcing is likely to have contributed to changes in wind patterns, affecting extra-tropical storm tracks and temperature patterns in both hemispheres. However, the observed changes in the Northern Hemisphere circulation are larger than simulated by models in response to 20 th 

### century forcing change. {WGI 3.5, 3.6, 9.4, 9.5, 10.3, SPM}


### It is very likely that the response to anthropogenic forcing contributed to sea level rise during the latter half of the 20 th 

### century. There is some evidence of the impact of human climatic influence models using only natural forcings

models using both natural and anthropogenic forcings observations**41 Topic 2**  **Causes of change**



### on the hydrological cycle, including the observed large-scale patterns of changes in land precipitation over the 20 th 

### century. It is more likely than notthat human influence has contributed to a global trend towards increases in area affected by drought since the 1970s and the frequency of heavy precipitation events. {WGI 3.3, 5.5, 9.5, TS.4.1, TS.4.3}


### Anthropogenic warming over the last three decades has **likely had a discernible influence at the global scale on ob**  **served changes in many physical and biological systems. {WGII 1.4}**  

### A synthesis of studies strongly demonstrates that the spatial agreement between regions of significant warming across the globe and the locations of significant observed changes in many natural systems consistent with warming is very unlikely to be due solely to natural variability of temperatures or natural variability of the systems. Several modelling studies have linked some specific responses in physical and biological systems to anthropogenic warming, but only a few such studies have been performed. Taken together with evidence of significant anthropogenic warming over the past 50 years averaged over each continent (except Antarctica), it is likely that anthropogenic warming over the last three decades has had a discernible influence on many natural systems. {WGI 3.2, 9.4, SPM; WGII 1.4, SPM}

### Limitations and gaps currently prevent more complete attribution of the causes of observed natural system responses to anthropogenic warming. The available analyses are limited in the numberof systems, length of records and locations considered. Natural temperature variability is larger at the regional than the global scale,thus affecting identification of changes to external forcing. At the regional scale, other non-climate factors (such as land-use change, pollution and invasive species) are influential. {WGII 1.2, 1.3, 1.4, SPM}

# 3

**Climate change and its impacts in the near and long term under different scenarios** **Topic 3 Climate change and its**  **impacts in the near and long term under different scenarios**

**44** 

## 3.1 Emissions scenarios

**There is high agreement**  **and much evidence**  **9** 

### that with cur**rent climate change mitigation policies and related sustain able development practices, global GHG emissions will con** **tinue to grow over the next few decades. Baseline emis sions scenarios published since the IPCC Special Report**  **on Emissions Scenarios (SRES, 2000) are comparable in range to those presented in SRES (see Box on SRES sce**  **narios and Figure 3.1). 10**  **{WGIII 1.3, 3.2, SPM}** 

### The SRES scenarios project an increase of baseline global GHG emissions by a range of 9.7 to 36.7 GtCO 2 

### -eq (25 to 90%) between 2000 and 2030. In these scenarios, fossil fuels are projected to maintain their dominant position in the global energy mix to 2030 and beyond. Hence CO 2 

### emissions from energy use between 2000 and 2030 are projected to grow 40 to 110% over that period. {WGIII 1.3, SPM} 

### Studies published since SRES (i.e. post-SRES scenarios) haveused lower values for some drivers for emissions, notably population projections. However, for those studies incorporating these new population projections, changes in other drivers, such as economic growth, result in little change in overall emission levels. Economic growth projections for Africa, Latin America and the Middle East to 2030 in post-SRES baseline scenarios are lower than in SRES,but this has only minor effects on global economic growth and overall emissions. {WGIII 3.2, TS.3, SPM} 

### Aerosols have a net cooling effect and the representation ofaerosol and aerosol precursor emissions, including sulphur dioxide, black carbon and organic carbon, has improved in the postSRES scenarios. Generally, these emissions are projected to be lower than reported in SRES. {WGIII 3.2, TS.3, SPM} 

### Available studies indicate that the choice of exchange rate for Gross Domestic Product (GDP) (Market Exchange Rate, MER or

**9** Agreement/evidence statements in italics represent calibrated expressions of uncertainty and confidence. See Box ‘Treatment of uncertainty’ in the Introduction for an explanation of these terms.

**10** Baseline scenarios do not include additional climate policies above current ones; more recent studies differ with respect to UN FCCC and Kyoto Protocol inclusion. Emission pathways of mitigation scenarios are discussed in Topic 5.

**11** Since the TAR, there has been a debate on the use of different exchange rates in emissions scenarios. Two metrics are used to c ompare GDP between countries. Use of MER is preferable for analyses involving internationally traded products. Use of PPP is preferable for analys es involving comparisons of income between countries at very different stages of development. Most of the monetary units in this report are expressed in ME R. This reflects the large majority of emissions mitigation literature that is calibrated in MER. When monetary units are expressed in PPP, this is denote d by GDP PPP . {WGIII SPM}



### SRES scenarios SRES refers to the scenarios described in the IPCC Special Report on Emissions Scenarios (SRES, 2000). The SRES scenarios are grouped into four scenario families (A1, A2, B1 and B2) that explore alternative development pathways, covering a wide range of demographic, economic and technological driving forces and resulting GHG emissions. The SRES scenarios do not include additiona l climate policies above current ones. The emissions projections are widely used in the assessments of future climate change, and their underlying assumptions with respect to socio-economic, demographic and technological change serve as inputs to many recent clim ate change vulnerability and impact assessments. {WGI 10.1; WGII 2.4; WGIII TS.1, SPM} The A1 storyline assumes a world of very rapid economic growth, a global population that peaks in mid-century and rapid introdu ction of new and more efficient technologies. A1 is divided into three groups that describe alternative directions of technologi cal change: fossil intensive (A1FI), non-fossil energy resources (A1T) and a balance across all sources (A1B). B1 describes a convergent wo rld, with the same global population as A1, but with more rapid changes in economic structures toward a service and information econ omy. B2 describes a world with intermediate population and economic growth, emphasising local solutions to economic, social, and env ironmental sustainability. A2 describes a very heterogeneous world with high population growth, slow economic development and slow technological change. No likelihood has been attached to any of the SRES scenarios. {WGIII TS.1, SPM} **Figure 3.1.** Global GHG emissions (in GtCO 2 -eq per year) in the absence of additional climate policies: six illustrative SRES marker scenarios (coloured lines) and 80 th percentile range of recent scenarios published since SRES(post-SRES) (gray shaded area). Dashed lines show the full range of postSRES scenarios. The emissions include CO 2 , CH 4 , N

2 O and F-gases. {WGIII 1.3, 3.2, Figure SPM.4} 

### Scenarios for GHG emissions from 2000 to 2100 in the absence of additional climate policiesGlobal GHG emissions (Gt CO2-eq / yr) post-SRES (max)

post-SRES (min)



### Purchasing Power Parity, PPP) does not appreciably affect the projected emissions, when used consistently. 11 

### The differences, if any, are small compared to the uncertainties caused by assumptions on other parameters in the scenarios, e.g. technological change. {WGIII 3.2, TS.3, SPM}**45 Topic 3**  **Climate change and its impacts in the near and long term under different scenarios** 



## 3.2 Projections of future changes in climate

**For the next two decades a warming of about 0.2°C per de cade 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, tem perature projections increasingly depend on specific emis**  **sions scenarios (Figure 3.2). {WGI 10.3, 10.7; WGIII 3.2}**  

### Since the IPCC’s first report in 1990, assessed projections have suggested global averaged temperature increases between about 0.15and 0.3°C per decade from 1990 to 2005. This can now be compared with observed values of about 0.2°C per decade, strengthening confidence in near-term projections. {WGI 1.2, 3.2}



### 3.2.1 21 **st** 

### century global changes

**Continued GHG emissions at or above current rates would cause further warming and induce many changes in the glo**  **bal climate system during the 21 st**  

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

### century. **{WGI 10.3}**

### Advances in climate change modelling now enable best estimates and likely assessed uncertainty ranges to be given for projected warming for different emissions scenarios. Table 3.1 shows best estimates and likely ranges for global average surface air warming for the six SRES marker emissions scenarios (including climate-carbon cycle feedbacks). {WGI 10.5} 

### Although these projections are broadly consistent with the span quoted in the TAR (1.4 to 5.8°C), they are not directly comparable. Assessed upper ranges for temperature projections are larger thanin the TAR mainly because the broader range of models now available suggests stronger climate-carbon cycle feedbacks. For the A2 scenario, for example, the climate-carbon cycle feedback increases the corresponding global average warming at 2100 by more than 1°C. Carbon feedbacks are discussed in Topic 2.3. {WGI 7.3, 10.5, SPM} 

### 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. Model-based projections of global average sea level rise at the end of the 21 st 

### century (2090-2099) are shown in Table 3.1. For each scenario, the mid-point of the range in Table 3.1 is within 10% of the TAR model average for 2090-2099. The ranges are narrower than in the TAR mainly because of improved information about some uncertainties in the projected contributions. 12 

### The sea level projections do not include uncertainties in climate-carbon cycle feedbacks nor do they include the full effects of changes in icesheet flow, because a basis in published literature is lacking. Therefore the upper values of the ranges given are not to be consideredupper bounds for sea level rise. The projections include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but these flow rates could increase or decrease in the future. If this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios shown in Table 3.1 would increase by 0.1 to 0.2m. 13 {WGI 10.6, SPM}

**Table 3.1.** Projected global average surface warming and sea level rise at the end of the 21 st century. {WGI 10.5, 10.6, Table 10.7, Table SPM.3}

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) These estimates are assessed from a hierarchy of models that encompass a simple climate model, several Earth Models of Interm ediate Complexity, and a large number of Atmosphere-Ocean General Circulation Models (AOGCMs) as well as observational constraints. b) Year 2000 constant composition is derived from 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 WGI TAR) for the SRES B1, AIT, B2, A1B, A2 and A1FI illustrative mar ker 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.

**12** 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 3.1 if it had treated uncertainties in the same way.

**13** For discussion of the longer term see Sections 3.2.3 and 5.2.**Topic 3 Climate change and its**  **impacts in the near and long term under different scenarios**

**46** 

### 3.2.2 21 **st** 

### century regional changes
**There is now higher confidence than in the TAR in projected patterns of warming and other regional-scale features, in**  **cluding changes in wind patterns, precipitation and some aspects of extremes and sea ice.**  **{WGI 8.2, 8.3, 8.4, 8.5, 9.4, 9.5, 10.3, 11.1}**  

### Projected warming in the 21 st

### century shows scenario-independent geographical patterns similar to those observed over the past several decades. Warming is expected to be greatest over land and at most high northern latitudes, and least over the Southern Ocean (near Antarctica) and northern North Atlantic, continuing recent observed trends (Figure 3.2 right panels). {WGI 10.3, SPM} 

### Snow cover area is projected to contract. Widespread increases in thaw depth are projected over most permafrost regions. Sea ice is projected to shrink in both the Arctic and Antarctic under all SRES scenarios. In some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21 st 

### century. {WGI 10.3, 10.6, SPM; WGII 15.3.4}


### It is very likely that hot extremes, heat waves and heavy precipitation events will become more frequent. {SYR Table 3.2; WGI 10.3, SPM} 

### Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical sea-surface temperatures. There is less confidence in projections of a global decrease in numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period. {WGI 3.8, 9.5, 10.3, SPM} 

### Extra-tropical storm tracks are projected to move poleward, with consequent changes in wind, precipitation and temperature patterns,continuing the broad pattern of observed trends over the last halfcentury. {WGI 3.6, 10.3, SPM} 

### Since the TAR there is an improving understanding of projected patterns of precipitation. Increases in the amount of precipitation

are very likely in high-latitudes, while decreases are likely in mostsubtropical land regions (by as much as about 20% in the A1B scenario in 2100, Figure 3.3), continuing observed patterns in recent trends. {WGI 3.3, 8.3, 9.5, 10.3, 11.2-11.9, SPM}


### 3.2.3 Changes beyond the 21 **st** 

### century
**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.**  **{WGI 10.4, 10.5, 10.7, SPM}**

### If radiative forcing were to be stabilised, keeping all the radiative forcing agents constant at B1 or A1B levels in 2100, modelexperiments show that a further increase in global average temperature of about 0.5°C would still be expected by 2200. In addition, thermal expansion alone would lead to 0.3 to 0.8m of sea level rise by 2300 (relative to 1980-1999). Thermal expansion would continue for many centuries, due to the time required to transport heat into the deep ocean. {WGI 10.7, SPM}



### Atmosphere-Ocean General Circulation Model projections of surface warming

**Figure 3.2. Left panel:**  Solid lines are multi-model global averages of surface warming (relative to 1980-1999) for the SRES scenarios A2, A1B and B1, shown as continuations of the 20 th century simulations. The orange line is for the experiment where concentrations w ere held constant at year 2000 values. The bars in the middle 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 relative to 1980-1999. The assessment of the best estimate and likely ranges in the bars includes the Atmosphere-Ocean General Circulation Models (AOGCMs) in the left part of the figure, as well as results from a hierarchy of independent models and observational con straints. **Right panels:** Projected surface temperature changes for the early and late 21 st century relative to the period 1980-1999. The panels show the multi-AOGCM average projections for the A2 (top), A1B (middle) and B1 (bottom) SRES scenarios averaged over decades 2020-2029 (left) and 20 90-2099 (right). {WGI 10.4, 10.8, Figures 10.28, 10.29, SPM} A2 A1B B1 Year 2000 constant concentrations 20 century**47 Topic 3**  **Climate change and its impacts in the near and long term under different scenarios** 



### Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100. Current models suggest ice mass losses increase with temperature more rapidly than gains due to increased precipitation and that the surface mass balancebecomes negative (net ice loss) at a global average warming (relative to pre-industrial values) in excess of 1.9 to 4.6°C. If such a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7m. Thecorresponding future temperatures in Greenland (1.9 to 4.6°C global) are comparable to those inferred for the last interglacial period125,000 years ago, when palaeoclimatic information suggests reductions of polar land ice extent and 4 to 6m of sea level rise. {WGI 6.4, 10.7, SPM}

### Dynamical processes related to ice flow – which are not included in current models but suggested by recent observations – could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude. {WGI 4.6, 10.7, SPM} 

### Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and gain massdue to increased snowfall. However, net loss of ice mass could occur if dynamical ice discharge dominates the ice sheet mass balance. {WGI 10.7, SPM} 

### Both past and future anthropogenic CO 2 

### emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the time scales required for the removal of this gas from the atmosphere. {WGI 7.3, 10.3, Figure 7.12, Figure 10.35, SPM} 

### Estimated long-term (multi-century) warming corresponding to the six AR4 WG III stabilisation categories is shown in Figure 3.4. Multi-model projected patterns of precipitation changes

**Figure 3.3.** Relative changes in precipitation (in percent) for the period 2090-2099, relative to 1980-1999. Values are multi-model average s based on the SRES A1B scenario for December to February (left) and June to August (right). White areas are where less than 66% of the models agree in the sign of the change and stippled areas are where more than 90% of the models agree in the sign of the change. {WGI Figure 10.9, SPM} 

## %
-20 -10 -5 5 10 20

**Figure 3.4.** Estimated long-term (multi-century) warming corresponding to the six AR4 WG III stabilisation categories (Table 5.1). The temp erature scale has been shifted by -0.5°C compared to Table 5.1 to account approximately for the warming between pre-industrial 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 850 ppm CO 2 -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 5.1), the equilibrium temperature may be reached earlier. {WGI 10.7.2} 

### 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)**Topic 3 Climate change and its**  **impacts in the near and long term under different scenarios**

**48** 

## 3.3 Impacts of future climate changes

**More specific information is now available across a wide range of systems and sectors concerning the nature of fu**  **ture impacts, including some fields not covered in previous assessments.**  **{WGII TS.4, SPM}** 

### The following is a selection of key findings 14 

### regarding the impacts of climate change on systems, sectors and regions, as well as some findings on vulnerability 15 

### , for the range of climate changes projected over the 21 st

### century. Unless otherwise stated, the confidence level in the projections is high . Global average temperature increases are given relative to 1980-1999. Additional information on impacts can be found in the WG II report. {WGII SPM}


### 3.3.1 Impacts on systems and sectors
Ecosystems

 

### The resilience of many ecosystems is likely to be exceeded this century by an unprecedented combination of climate change, associated disturbances (e.g. flooding, drought, wildfire, insects,ocean acidification) and other global change drivers (e.g. landuse change, pollution, fragmentation of natural systems, overexploitation of resources). {WGII 4.1-4.6, SPM}

 

### Over the course of this century, net carbon uptake by terrestrial ecosystems is likely to peak before mid-century and then weaken

or even reverse 16 

### , thus amplifying climate change. {WGII 4.ES, Figure 4.2, SPM}

 

### 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 ( medium confidence ). {WGII 4.ES, Figure 4.2, SPM}

 

### For increases in global average temperature exceeding 1.5 to 2.5°C and in concomitant atmospheric CO 2 

### concentrations, there are projected to be major changes in ecosystem structure and function, species’ ecological interactions and shifts in species’ geographical ranges, with predominantly negative consequences for biodiversity and ecosystem goods and services, e.g. water and food supply. {WGII 4.4, Box TS.6, SPM}



### Food

 

### Crop productivity is projected to increase slightly at mid- to high latitudes for local mean temperature increases of up to 1 to 3°C depending on the crop, and then decrease beyond that in some regions ( medium confidence ). {WGII 5.4, SPM}

 

### At lower latitudes, especially in seasonally dry and tropical regions, crop productivity is projected to decrease for even small local temperature increases (1 to 2°C), which would increase the risk of hunger ( medium confidence ). {WGII 5.4, SPM}

 

### Globally, the potential for food production is projected to increase with increases in local average temperature over a range of 1 to 3°C, but above this it is projected to decrease ( medium confidence ). {WGII 5.4, 5.5, SPM}



### Coasts

 

### Coasts are projected to be exposed to increasing risks, including coastal erosion, due to climate change and sea level rise. The effect will be exacerbated by increasing human-induced pressures on coastal areas ( very high confidence ). {WGII 6.3, 6.4, SPM}

 

### By the 2080s, many millions more people than today are projected to experience floods every year due to sea level rise. The numbers affected will be largest in the densely populated and low-lying megadeltas of Asia and Africa while small islands are especially vulnerable ( very high confidence ). {WGII 6.4, 6.5, Table 6.11, SPM}



### Industry, settlements and society

 

### The most vulnerable industries, settlements and societies are generally those in coastal and river flood plains, those whose economies are closely linked with climate-sensitive resources and those in areas prone to extreme weather events, especially where rapid urbanisation is occurring. {WGII 7.1, 7.3, 7.4, 7.5, SPM}

 

### Poor communities can be especially vulnerable, in particular those concentrated in high-risk areas. {WGII 7.2, 7.4, 5.4, SPM}



### Health

 

### The health status of millions of people is projected to be affected through, for example, increases in malnutrition; increaseddeaths, diseases and injury due to extreme weather events; increased burden of diarrhoeal diseases; increased frequency of cardio-respiratory diseases due to higher concentrations of ground-level ozone in urban areas related to climate change; and the altered spatial distribution of some infectious diseases. {WGI 7.4, Box 7.4; WGII 8.ES, 8.2, 8.4, SPM}

 

### Climate change is projected to bring some benefits in temperate areas, such as fewer deaths from cold exposure, and somemixed effects such as changes in range and transmission potential of malaria in Africa. Overall it is expected that benefits willbe outweighed by the negative health effects of rising temperatures, especially in developing countries. {WGII 8.4, 8.7, 8ES, SPM}

 

### Critically important will be factors that directly shape the healthof populations such as education, health care, public health initiatives, and infrastructure and economic development. {WGII 8.3, SPM}



### Water

 

### Water impacts are key for all sectors and regions. These are discussed below in the Box ‘Climate change and water’.

**14** Criteria of choice: magnitude and timing of impact, confidence in the assessment, representative coverage of the system, sector and region.

**15** Vulnerability to climate change is the degree to which systems are susceptible to, and unable to cope with, adverse impacts.

**16** Assuming continued GHG emissions at or above current rates and other global changes including land-use changes.**49 Topic 3**  **Climate change and its impacts in the near and long term under different scenarios** 



### Climate change and water Climate change is expected to exacerbate current stresses on water resources from population growth and economic and land-use change, including urbanisation. On a regional scale, mountain snow pack, glaciers and small ice caps play a crucial role in fre shwater availability. Widespread mass losses from glaciers and reductions in snow cover over recent decades are projected to accelerate throughout the 21 st century, reducing water availability, hydropower potential, and changing seasonality of flows in regions supplied bymeltwater from major mountain ranges (e.g. Hindu-Kush, Himalaya, Andes), where more than one-sixth of the world population currently lives. {WGI 4.1, 4.5; WGII 3.3, 3.4, 3.5} Changes in precipitation (Figure 3.3) and temperature (Figure 3.2) lead to changes in runoff (Figure 3.5) and water availabilit y. Runoff is projected with high confidence to increase by 10 to 40% by mid-century at higher latitudes and in some wet tropical areas, including populous areas in East and South-East Asia, and decrease by 10 to 30% over some dry regions at mid-latitudes and dry tropics, due to decreases in rainfall and higher rates of evapotranspiration. There is also high confidence that many semi-arid areas (e.g. the Mediterranean Basin, western United States, southern Africa and north-eastern Brazil) will suffer a decrease in water resources due to climate change. Drought-affected areas are projected to increase in extent, with the potential for adverse impac ts on multiple sectors, e.g. agriculture, water supply, energy production and health. Regionally, large increases in irrigation water demand as a result of climate changes are projected. {WGI 10.3, 11.2-11.9; WGII 3.4, 3.5, Figure 3.5, TS.4.1, Box TS.5, SPM} The negative impacts of climate change on freshwater systems outweigh its benefits ( high confidence ). Areas in which runoff is projected to decline face a reduction in the value of the services provided by water resources ( very high confidence ). The beneficial impacts of increased annual runoff in some areas are likely to be tempered by negative effects of increased precipitation variability and seasonal runoff shifts on water supply, water quality and flood risk. {WGII 3.4, 3.5, TS.4.1} Available research suggests a significant future increase in heavy rainfall events in many regions, including some in which the mean rainfall is projected to decrease. The resulting increased flood risk poses challenges to society, physical infrastructure and water quality. It is likely that up to 20% of the world population will live in areas where river flood potential could increase by the 2080s. Increases i n the frequency and severity of floods and droughts are projected to adversely affect sustainable development. Increased temperat ures will further affect the physical, chemical and biological properties of freshwater lakes and rivers, with predominantly adverse impacts on many individual freshwater species, community composition and water quality. In coastal areas, sea level rise will exacerbate w ater resource constraints due to increased salinisation of groundwater supplies. {WGI 11.2-11.9; WGII 3.2, 3.3, 3.4, 4.4}



### Projections and model consistency of relative changes in runoff by the end of the 21st century

**Figure 3.5.** Large-scale relative changes in annual runoff (water availability, in percent) for the period 2090-2099, relative to 1980-1999 . Values represent the median of 12 climate models using the SRES A1B scenario. White areas are where less than 66% of the 12 models agr ee on the sign of change and hatched areas are where more than 90% of models agree on the sign of change. The quality of the simulation of the ob served large-scale 20

th century runoff is used as a basis for selecting the 12 models from the multi-model ensemble. The global map of annual runoff i llustrates a large scale and is not intended to refer to smaller temporal and spatial scales. In areas where rainfall and runoff is very low (e.g. desert areas), small changes in runoff can lead to large percentage changes. In some regions, the sign of projected changes in runoff differs from recently observed trends. In some areas with projected increases in runoff, different seasonal effects are expected, such as increased wet season runoff and decr eased dry season runoff. Studies using results from few climate models can be considerably different from the results presented here. {WGII Figu re 3.4, adjusted to match the assumptions of Figure SYR 3.3; WGII 3.3.1, 3.4.1, 3.5.1}**Topic 3 Climate change and its**  **impacts in the near and long term under different scenarios**

**50 17**  Unless stated explicitly, all entries are from WG 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 WG II SPM refers to the source of the statement s, timelines and temperatures. The magnitude and timing of impacts that will ultimately be realised will vary with the amount and rate of climate chang e, emissions scenarios, development pathways and adaptation.

### Studies since the TAR have enabled more systematic un**derstanding of the timing and magnitude of impacts related to differing amounts and rates of climate change.**  **{WGII SPM}** 

### Examples of this new information for systems and sectors are presented in Figure 3.6. The upper panel shows impacts increasing with increasing temperature change. Their estimated magnitude and timing is also affected by development pathways (lower panel). {WGII SPM}

### Depending on circumstances, some of the impacts shown in Figure 3.6 could be associated with ‘key vulnerabilities’, based on a number of criteria in the literature (magnitude, timing, persistence/reversibility, the potential for adaptation, distributional aspects, likelihood and ‘importance’ of the impacts) (see Topic 5.2). {WGII SPM}


### 3.3.2 Impacts on regions **17**



### Africa

 

### By 2020, between 75 and 250 million of people are projected to be exposed to increased water stress due to climate change. {WGII 9.4, SPM}

 

### By 2020, in some countries, yields from rain-fed agriculturecould be reduced by up to 50%. Agricultural production, including access to food, in many African countries is projectedto be severely compromised. This would further adversely affect food security and exacerbate malnutrition. {WGII 9.4, SPM}

 

### 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 GDP. {WGII 9.4, SPM}

 

### By 2080, an increase of 5 to 8% of arid and semi-arid land in Africa is projected under a range of climate scenarios ( high confidence ). {WGII Box TS.6, 9.4.4}



### Asia

 

### By the 2050s, freshwater availability in Central, South, Eastand South-East Asia, particularly in large river basins, is projected to decrease. {WGII 10.4, SPM}

 

### 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. {WGII 10.4, SPM}

 

### Climate change is projected to compound the pressures on natural resources and the environment associated with rapid urbanisation, industrialisation and economic development. {WGII 10.4, SPM}

 

### 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. {WGII 10.4, SPM}



### Australia and New Zealand

 

### By 2020, significant loss of biodiversity is projected to occur in some ecologically rich sites, including the Great Barrier Reef and Queensland Wet Tropics. {WGII 11.4, SPM}  

### By 2030, water security problems are projected to intensify in southern and eastern Australia and, in New Zealand, in Northland and some eastern regions. {WGII 11.4, SPM}

 

### 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 droughtand fire. However, in New Zealand, initial benefits are projected in some other regions. {WGII 11.4, SPM}

 

### By 2050, ongoing coastal development and population growth in some areas of Australia and New Zealand are projected toexacerbate risks from sea level rise and increases in the severity and frequency of storms and coastal flooding. {WGII 11.4, SPM}



### 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). {WGII 12.4, SPM}

 

### 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). {WGII 12.4, SPM}

 

### 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. {WGII 12.4, SPM}

 

### Climate change is also projected to increase the health risks due to heat waves and the frequency of wildfires. {WGII 12.4, SPM}



### 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. Semiarid vegetation will tend to be replaced by arid-land vegetation. {WGII 13.4, SPM}

 

### There is a risk of significant biodiversity loss through species extinction in many areas of tropical Latin America. {WGII 13.4, SPM}

 

### Productivity of some important crops is projected to decrease and livestock productivity to decline, with adverse consequencesfor food security. In temperate zones, soybean yields are projected to increase. Overall, the number of people at risk of hunger is projected to increase ( medium confidence ). {WGII 13.4, Box TS.6}

 

### Changes in precipitation patterns and the disappearance of glaciers are projected to significantly affect water availability for human consumption, agriculture and energy generation. {WGII 13.4, SPM}**51 Topic 3**  **Climate change and its impacts in the near and long term under different scenarios** 



### 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 3.6.** Examples of impacts associated with global average temperature change. **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 st 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. Quantita tive entries for water scarcity and flooding represent the additional impacts of climate change relative to the conditions projected across the range of SRES scena rios A1FI, A2, B1 and B2. Adaptation to climate change is not included in these estimations. Confidence levels for all statements are high . The upper right panel gives the WG II references for the statements made in the upper left panel.* **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. {WGI Figure SPM.5, 10.7; WGII Figure SPM.2; WGIII Table TS.2, Table 3.10} *Where ES = Executive Summary, T = Table, B = Box and F = Figure. Thus B4.5 indicates Box 4.5 in Chapter 4 and 3.5.1 indicates Section 3.5.1 in Chapter 3. 

### 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. WGII 3.4.1, 3.4.3
3.ES, 3.4.1, 3.4.3
3.5.1, T3.3, 20.6.2, TS.B5

4.ES, 4.4.11

4.ES, T4.1, F4.2, F4.4

19.3.5 4.2.2, 4.4.1, 4.4.4, 4.4.5, 4.4.6, 4.4.10, B4.5

5.ES, 5.4.7

5.ES, 5.4.2, F5.2

5.ES, 5.4.2, F5.2

6.ES, 6.3.2, 6.4.1, 6.4.2

6.4.1

T6.6, F6.8, TS.B5

8.ES, 8.4.1, 8.7, T8.2, T8.4 8.ES, 8.2.2, 8.2.3, 8.4.1, 8.4.2, 8.7, T8.3, F8.3 8.ES, 8.2.8, 8.7, B8.4 T4.1, F4.4, B4.4, 6.4.1, 6.6.5, B6.1

8.6.1**Topic 3 Climate change and its**  **impacts in the near and long term under different scenarios**

**52** 

### 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. {WGII 14.4, SPM}

 

### In the early decades of the century, moderate climate change is projected to increase aggregate yields of rain-fed agricultureby 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. {WGII 14.4, SPM}

 

### Cities that currently experience heat waves are expected to befurther challenged by an increased number, intensity and duration of heat waves during the course of the century, with potential for adverse health impacts. {WGII 14.4, SPM}

 

### Coastal communities and habitats will be increasingly stressed by climate change impacts interacting with development and pollution. {WGII 14.4, SPM}



### Polar Regions

 

### The main projected biophysical effects are reductions in thickness and extent of glaciers, ice sheets and sea ice, and changesin natural ecosystems with detrimental effects on many organisms including migratory birds, mammals and higher predators. {WGII 15.4, SPM}

 

### For human communities in the Arctic, impacts, particularly those resulting from changing snow and ice conditions, are projected to be mixed. {WGII 15.4, SPM}

 

### Detrimental impacts would include those on infrastructure and traditional indigenous ways of life. {WGII 15.4, SPM}

 

### In both polar regions, specific ecosystems and habitats are projected to be vulnerable, as climatic barriers to species invasions are lowered. {WGII 15.4, SPM}



### 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. {WGII 16.4, SPM}

 

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

resources. {WGII 16.4, SPM}

 

### By mid-century, climate change is expected to reduce waterresources 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. {WGII 16.4, SPM}

 

### With higher temperatures, increased invasion by non-nativespecies is expected to occur, particularly on mid- and high-latitude islands. {WGII 16.4, SPM} 

### 3.3.3 Especially affected systems, sectors and regions
**Some systems, sectors and regions are likely**  **to be espe cially affected by climate change.**  18 **{WGII TS.4.5}** 

### Systems and sectors: {WGII TS.4.5}

 

### 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-latitudes 19 

### and inthe 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: {WGII TS.4.5}

 

### 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. {WGII 7.1, 7.2, 7.4, 8.2, 8.4, TS.4.5}



### 3.3.4 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 

### concentrations lead to furtheracidification. 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 is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species. {WGI SPM; WGII SPM}



### 3.3.5 Extreme events

**Altered frequencies and intensities of extreme weather, to gether with sea level rise, are expected to have mostly adverse**  **effects on natural and human systems (Table 3.2). {WGII SPM}**  

### Examples for selected extremes and sectors are shown in Table 3.2.

**18** 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.

**19** Including arid and semi-arid regions.**53 Topic 3**  **Climate change and its impacts in the near and long term under different scenarios** 

**Table 3.2.** Examples of possible impacts of climate change due to changes in extreme weather and climate events, based on projections to t he mid- to late 21 st century. These do not take into account any changes or developments in adaptive capacity. The likelihood estimates in column tw o relate to the phenomena listed in column one. {WGII Table SPM.1}

Phenomenon a and Likelihood of Examples of major projected impacts by sector direction of trend future trends based on Agriculture, forestry W ater resources Human health Industry, settlement projections and ecosystems {WGII 3.4} {WGII 8.2, 8.4} and society for 21 st century {WGII 4.4, 5.4} {WGII 7.4} 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 for 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 transport 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 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; inf ectious, respiratory societies due to flooding: areas to w aterlogging 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 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 mal nutrition; 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 disr uption deaths, injuries, winds; withdrawal of risk increases of trees; damage to of pub lic 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 WGI 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. {WGI 10.6}



## 3.4 Risk of abrupt or irreversible changes

**Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and**  **magnitude of the climate change. {WGII 12.6, 19.3, 19.4, SPM}**  

### Abrupt climate change on decadal time scales is normally thought of as involving ocean circulation changes. In addition on longer time scales, ice sheet and ecosystem changes may also playa role. If a large-scale abrupt climate change were to occur, its impact could be quite high (see Topic 5.2). {WGI 8.7, 10.3, 10.7; WGII 4.4, 19.3}

### Partial loss of ice sheets on polar land and/or the thermal expansion of seawater over very long time scales 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**Topic 3 Climate change and its**  **impacts in the near and long term under different scenarios**

**54** 

### islands. Current models project that such changes would occur over very long time scales (millennial) if a global temperature increase of 1.9 to 4.6°C (relative to pre-industrial) were to be sustained. Rapid sea level rise on century time scales cannot be excluded. {SYR 3.2.3; WGI 6.4, 10.7; WGII 19.3, SPM} 

### 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 temperature increase exceeds about 3.5°C, model projections suggest significant extinctions (40 to 70% of species assessed) around the globe. {WGII 4.4, Figure SPM.2} 

### Based on current model simulations, it is very likely that the meridional overturning circulation (MOC) of the Atlantic Ocean will slow down during the 21 st 

### century; nevertheless temperatures in the region are projected to increase. It is very unlikely that the MOC will undergo a large abrupt transition during the 21 st 

### century.Longer-term changes in the MOC cannot be assessed with confidence. {WGI 10.3, 10.7; WGII Figure, Table TS.5, SPM.2} 

### 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. {WGII 12.6, 19.3, Figure SPM.2}

# 4

**Adaptation and mitigation options and responses, and the inter-relationship with sustainable**  **development, at global and regional levels Topic 4 Adaptation and mitigation options and responses, and the inter-relationship with sustainable development, at global and**  **regional levels**

**56** 

## 4.1 Responding to climate change

Societies can respond to climate change by adapting to its impacts and by reducing GHG emissions (mitigation), thereby reducing the rate and magnitude of change. This Topic focuses on adaptation and mitigation options that can be implemented over the next two to three decades, and their inter-relationship with sustainable development.These responses can be complementary. Topic 5 addresses their complementary roles on a more conceptual basis over a longer timeframe.The capacity to adapt and mitigate is dependent on socio-economic and environmental circumstances and the availability of information and technology 20 

### . However, much less information is available about the costs and effectiveness of adaptation measures than about mitigation measures. {WGII 17.1, 17.3; WGIII 1.2}



## 4.2 Adaptation options

**Adaptation can reduce vulnerability, both in the short and the long term.**  **{WGII 17.2, 18.1, 18.5, 20.3, 20.8}** 

### Vulnerability to climate change can be exacerbated by other stresses. These arise from, for example, current climate hazards,poverty, unequal access to resources, food insecurity, trends in economic globalisation, conflict and incidence of diseases such as HIV/ AIDS. {WGII 7.2, 7.4, 8.3, 17.3, 20.3, 20.4, 20.7, SPM} 

### Societies across the world have a long record of adapting andreducing their vulnerability to the impacts of weather- and climaterelated events such as floods, droughts and storms. Nevertheless,additional adaptation measures will be required at regional and local levels 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. However, adaptation alone is not expected to cope with all the projected effects of climate change,especially not over the long term as most impacts increase in magnitude. {WGII 17.2, SPM; WGIII 1.2}

### 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. Some planned adaptation is already occurring on a limited basis. Table 4.1 provides examples of planned

**20** Technology is defined as the practical application of knowledge to achieve particular tasks that employs both technical artefac ts (hardware, equipment) and (social) information (‘software’, know-how for production and use of artefacts).

### adaptation options by sector. Many adaptation actions have multiple drivers, such as economic development and poverty alleviation, and are embedded within broader development, sectoral, regional and local planning initiatives such as water resources planning, coastal defence and disaster risk reduction strategies. Examples of this approach are the Bangladesh National Water Management Plan and the coastal defence plans of The Netherlands and Norway, which incorporate specific climate change scenarios. {WGII 1.3, 5.5.2, 11.6, 17.2}

### Comprehensive estimates of the costs and benefits of adaptation at the global level are limited in number. However, the number of adaptation cost and benefit estimates at the regional and project levels for impacts on specific sectors, such as agriculture, energy demand for heating and cooling, water resources management and infrastructure, is growing. Based on these studies there is high confidence that there are viable adaptation options that can be implemented in some of these sectors at low cost and/or with high benefit-cost ratios. Empirical research also suggests that higher benefit-cost ratios can be achieved by implementing some adaptationmeasures at an early stage compared to retrofitting long-lived infrastructure at a later date. {WGII 17.2}



### Adaptive capacity is intimately connected to social and eco**nomic development, but it is not evenly distributed across and within societies.**  **{WGII 7.1, 7.2, 7.4, 17.3}** 

### 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. It is also affected by multiple climate and non-climate stresses, as well as development policy. {WGII 17.3} 

### Recent studies reaffirm the TAR finding that adaptation will be vital and beneficial. However, financial, technological, cognitive, behavioural, political, social, institutional and cultural constraints limit both the implementation and effectiveness of adaptation measures.Even societies with high adaptive capacity remain vulnerable to climate change, variability and extremes. For example, a heat wave in 2003 caused high levels of mortality in European cities (especiallyamong the elderly), and Hurricane Katrina in 2005 caused large human and financial costs in the United States. {WGII 7.4, 8.2, 17.4}**57 Topic 4 Adaptation and mitigation options and responses, and the inter-relationship with sustainable development, at global and**  **regional levels Table 4.1.**  Selected examples of planned adaptation by sector.**Adaptation option/strategy**Expanded rainwater harvesting; waterstorage and conservation techniques; waterreuse; desalination; water-useand irrigationefficiencyAdjustment of planting dates and crop variety;crop relocation; improved land management,e.g. erosion control and soil protection throughtree plantingRelocation; seawalls and storm surge barriers;dune reinforcement; land acquisition andcreation of marshlands/wetlands as bufferagainst sea level rise and flooding; protectionof existing natural barriersHeat-health action plans; emergencymedical services; improved climate-sensitivedisease surveillance and control; safe waterand improved sanitationDiversification of tourism attractions andrevenues; shifting ski slopes to higher altitudesand glaciers; artificial snow-makingRealignment/relocation; design standards andplanning for roads, rail and other infrastructureto cope with warming and drainageStrengthening of overhead transmission anddistribution infrastructure; underground cablingfor utilities; energy efficiency; use of renewablesources; reduced dependence on singlesources of energy**Underlying policy framework**National water policies and integrated waterresources management; water-related hazardsmanagementR&D policies; institutional reform; land tenureand land reform; training; capacity building;crop insurance; financial incentives, e.g.subsidies and tax creditsStandards and regulations that integrateclimate change considerations into design;land-use policies; building codes; insurancePublic health policies that recognise climaterisk; strengthen health services; regional andinternational cooperationIntegrated planning (e.g. carrying capacity;linkages with other sectors); financial incen-tives, e.g. subsidies and tax creditsIntegrating climate change considerations intonational transport policy; investment in R&D forspecial situations, e.g. permafrost areasNational energy policies, regulations, and fiscaland financial incentives to encourage use ofalternative sources; incorporating climatechange in design standards**Key constraints and opportunities to implementation** (Normal font = constraints; italics = opportunities)Financial, human resources and physicalbarriers; integrated water resources management; synergies with other sectorsTechnological and financial constraints;access to new varieties; markets; longer growing season in higher latitudes; revenues from ‘new’ productsFinancial and technological barriers;availability of relocation space; integrated policies and management; synergies with sustainable development goalsLimits to human tolerance (vulnerablegroups); knowledge limitations; financialcapacity; upgraded health services; improved quality of lifeAppeal/marketing of new attractions;financial and logistical challenges; potentialadverse impact on other sectors (e.g.artificial snow-making may increase energyuse); revenues from ‘new’ attractions; involvement of wider group of stakeholdersFinancial and technological barriers;availability of less vulnerable routes; improved technologies and integration with key sectors (e.g. energy)Access to viable alternatives; financial andtechnological barriers; acceptance of newtechnologies; stimulation of new technolo- gies; use of local resourcesNote:Other examples from many sectors would include early warning systems.**Sector Water** {WGII, 5.5, 16.4;Tables 3.5, 11.6,17.1}**Agriculture**{WGII 10.5, 13.5;Table 10.8}**Infrastructure/ settlement (including** **coastal zones)**{WGII 3.6, 11.4; Tables6.11, 17.1}**Human health**{WGII 14.5, Table 10.8}**Tourism**{WGII 12.5, 15.5, 17.5;Table 17.1}**Transport**{WGII 7.6, 17.2}**Energy**{WGII 7.4, 16.2}**Topic 4 Adaptation and mitigation options and responses, and the inter-relationship with sustainable development, at global and regional levels** 

**58** 

## 4.3 Mitigation options

**Both bottom-up and top-down studies 21**  

### indicate that there **is high agreement**  **and much evidence** **of substantial eco nomic potential**  **21** 

### for the mitigation of global GHG emissions **over the coming decades that could offset the projected growth of global emissions or reduce emissions below cur**  **rent levels. {WGIII 11.3, SPM}**  

### Figure 4.1 compares global economic mitigation potential in 2030 with the projected emissions increase from 2000 to 2030. Bottom-up studies suggest that mitigation opportunities with net negative costs 22 

### have the potential to reduce emissions by about 6 GtCO 2 

### -eq/yr in 2030. Realising these requires dealing with implementation barriers. 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. 21 {WGIII 11.3, SPM} 

### Sectoral estimates of economic mitigation potential and marginal costs derived from bottom-up studies corrected for double counting of mitigation potential are shown in Figure 4.2. While top-down and bottom-up studies are in line at the global level, there are considerable differences at the sectoral level. {WGIII 11.3, SPM} 

### No single technology can provide all of the mitigationpotential in any sector. Table 4.2 lists selected examples of key technologies, policies, constraints and opportunities by sector. {WGIII SPM} 

### Future energy infrastructure investment decisions, expected to total over US$20 trillion 23 

### between 2005 and 2030, will have longterm 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, evenif early investments in these technologies are made attractive. Initial estimates show that returning global energy-related CO 2 

### emissions to 2005 levels by 2030 would require a large shift in the pattern of investment, although the net additional investment required ranges from negligible to 5 to 10%. {WGIII 4.1, 4.4, 11.6, SPM}

**21** 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). Mitigatio n 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 t hat 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 low er 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 mitigati on options and capture macro-economic and market feedbacks.

**22** Net negative costs (no regrets opportunities) are defined as those options whose benefits such as reduced energy costs and redu ced emissions of local/ regional pollutants equal or exceed their costs to society, excluding the benefits of avoided climate change.

**23** 20 trillion = 20,000 billion = 20×10 12 

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

**Figure 4.1.** Global economic mitigation potential in 2030 estimated from bottom-up (Panel a) and top-down (Panel b) studies, compared with t he 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 so ils, to ensure consistency with the SRES emissions results. {WGIII Figures SPM.4, SPM.5a, SPM.5b} 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**59 Topic 4 Adaptation and mitigation options and responses, and the inter-relationship with sustainable development, at global and**  **regional levels**



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

**Figure 4.2.** Estimated economic mitigation potential by sector and region using technologies and practices expected to be available in 2030 . The potentials do not include non-technical options such as lifestyle changes. {WGIII Figure SPM.6}
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 Ener gy 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 we re 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-CO 2 emissions in buildings and transport, part of material efficiency options, heat production and cogeneration in energy supply, heavy duty vehicles, shipping and high-occupancy passenger tr ansport, 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%. 

### 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



### While studies use different methodologies, there is **high agreement**  **and much evidence**  **that in all analysed world regions near-term health co-benefits from reduced air pol**  **lution, as a result of actions to reduce GHG emissions, can be substantial and may offset a substantial fraction of miti**  **gation costs. {WGIII 11.8, SPM}** 

### Energy efficiency and utilisation of renewable energy offer synergies with sustainable development. In least developed countries, energy substitution can lower mortality and morbidity by reducing indoor air pollution, reduce the workload for women and childrenand decrease the unsustainable use of fuelwood and related deforestation. {WGIII 11.8, 11.9, 12.4}



### Literature since the TAR confirms with **high agreement and**  **medium evidence that there may be effects from Annex I** **countries’ action on the global economy and global emis sions, although the scale of carbon leakage remains uncer**  **tain. {WGIII 11.7, SPM}**  

### Fossil fuel exporting nations (in both Annex I and non-Annex I countries) may expect, as indicated in the TAR, lower demand andprices 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. {WGIII 11.7, SPM} 

### Critical uncertainties remain in the assessment of carbon leakage. Most equilibrium modelling supports the conclusion in the TAR of economy-wide leakage from Kyoto action in the order of 5to 20%, which would be less if competitive low-emissions technologies were effectively diffused. {WGIII 11.7, SPM}



### There is also **high agreement and**  **medium evidence that**  **changes in lifestyle and behaviour patterns can contribute to climate change mitigation across all sectors. Manage**  **ment practices can also have a positive role. {WGIII SPM}**  

### Examples that can have positive impacts on mitigation include changes in consumption patterns, education and training, changes in building occupant behaviour, transport demand management and management tools in industry. {WGIII 4.1, 5.1, 6.7, 7.3, SPM}



### Policies that provide a real or implicit price of carbon could**create incentives for producers and consumers to signifi cantly invest in low-GHG products, technologies and pro** 

**cesses. {WGIII SPM}** 

### 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**Topic 4 Adaptation and mitigation options and responses, and the inter-relationship with sustainable development, at global and regional levels** 

**60 Table 4.2**  Selected examples of key sectoral mitigation technologies, policies and measures, constraints and opportunities. {WGIII Tables SPM.3, SPM.7}**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 vehiclepurchase, 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 ofinternationalcompetitivenessPredictable 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**{WGIII 4.3, 4.4}**Transport**{WGIII 5.4}**Buildings**{WGIII 6.5}**Industry**{WGIII 7.5}**Agriculture**{WGIII 8.4}**Forestry/forests**{WGIII 9.4}**Waste**{WGIII 10.4}**61 Topic 4 Adaptation and mitigation options and responses, and the inter-relationship with sustainable development, at global and**  **regional levels**



### stabilisation level, studies since the TAR that take into account induced technological change may lower these price ranges to US$5- 65/tCO 2 

### -eq in 2030. 24 {WGIII 3.3, 11.4, 11.5, SPM}



### There is **high agreement and**  **much evidence that a wide**  **variety of national policies and instruments are available to governments to create the incentives for mitigation action.**  **Their applicability depends on national circumstances and an understanding of their interactions, but experience from**  **implementation in various countries and sectors shows there are advantages and disadvantages for any given in**  **strument. {WGIII 13.2, SPM}**  

### Four main criteria are used to evaluate policies and instruments: environmental effectiveness, cost effectiveness, distributional effects including equity, and institutional feasibility. {WGIII 13.2, SPM} 

### General findings about the performance of policies are: {WGIII 13.2, SPM}

 

### Integrating climate policies in broader development policies makes implementation and overcoming barriers easier.

 

### Regulations and standards generally provide some certaintyabout emission levels. They may be preferable to other instruments when information or other barriers prevent producers and consumers from responding to price signals. However, they may not induce innovations and more advanced technologies.

 

### Taxes and charges can set a price for carbon, but cannot guarantee a particular level of emissions. Literature identifies taxes as an efficient way of internalising costs of GHG emissions.

 

### Tradable permits will establish a carbon price. The volume of allowed emissions determines their environmental effectiveness, while the allocation of permits has distributional consequences. Fluctuation in the price of carbon makes it difficult to estimate the total cost of complying with emission permits.

 

### Financial incentives (subsidies and tax credits) are frequentlyused by governments to stimulate the development and diffusion of new technologies. While economic costs are generallyhigher than for the instruments listed above, they are often critical to overcome barriers.

 

### Voluntary agreements between industry and governments are politically attractive, raise awareness among stakeholders and have played a role in the evolution of many national policies.The majority of agreements have not achieved significant emissions reductions beyond business as usual. However, some recent agreements, in a few countries, have accelerated the application of best available technology and led to measurable emission reductions.

 

### Information instruments (e.g. awareness campaigns) may positively affect environmental quality by promoting informedchoices and possibly contributing to behavioural change, however, their impact on emissions has not been measured yet.  

### Research, development and demonstration (RD&D) can stimulate technological advances, reduce costs and enable progress toward stabilisation.

Some corporations, local and regional authorities, NGOs and civil groups are adopting a wide variety of voluntary actions. These voluntary actions may limit GHG emissions, stimulate innovative policies and encourage the deployment of new technologies. Ontheir own, they generally have limited impact on national- or regional-level emissions. {WGIII 13.4, SPM}



## 4.4 Relationship between adaptation and **mitigation options and relationship with sustainable development** 

**There is growing understanding of the possibilities to choose and implement climate response options in several**  **sectors to realise synergies and avoid conflicts with other dimensions of sustainable development.**  **{WGIII SPM}**

### Climate change policies related to energy efficiency and renewable energy are often economically beneficial, improve energy security and reduce local pollutant emissions. Reducing both loss of natural habitat and deforestation can have significant biodiversity, soil and water conservation benefits, and can be implemented in a socially and economically sustainable manner. Forestation andbioenergy plantations can restore degraded land, manage water runoff, retain soil carbon and benefit rural economies, but could compete with food production and may be negative for biodiversity, if not properly designed. {WGII 20.3, 20.8; WGIII 4.5, 9.7, 12.3, SPM} 

### There is growing evidence that decisions about macro-economic policy, agricultural policy, multilateral development bank lending, insurance practices, electricity market reform, energy security and forest conservation, for example, which are often treated as being apart from climate policy, can significantly reduce emissions (Table 4.3). Similarly, non-climate policies can affect adaptive capacity and vulnerability. {WGII 20.3; WGIII SPM, 12.3}


### Both synergies and trade-offs exist between adaptation and **mitigation options. {WGII 18.4.3; WGIII 11.9)** 

### Examples of synergies include properly designed biomass production, formation of protected areas, land management, energy use in buildings, and forestry, but synergies are rather limited in other sectors. Potential trade-offs include increased GHG emissionsdue to increased consumption of energy related to adaptive responses. {WGII 18.4.3, 18.5, 18.7, TS.5.2; WGIII 4.5, 6.9, 8.5, 9.5, SPM}

**24** 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 thu s 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 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 stabil isation level are reduced; the reductions are greater at lower stabilisation level.**Topic 4 Adaptation and mitigation options and responses, and the inter-relationship with sustainable development, at global and regional levels** 

**62** 

## 4.5 International and regional cooperation

**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 the climate change problem, stimulation of an array of national policies, the** **creation of an international carbon market and the estab lishment of 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 initiatives have been suggested. {WGII 18.7;**  **WGIII 13.3, SPM}** 

### The impact of the Protocol’s first commitment period relative to global emissions is projected to be limited. Its economic impacts on participating Annex-B countries are projected to be smaller than presented in the TAR, which showed 0.2 to 2% lower GDP in 2012without emissions trading and 0.1 to 1.1% lower GDP with emissions trading among Annex-B countries. To be more environmentally effective, future mitigation efforts would need to achieve deeper reductions covering a higher share of global emissions (see Topic 5). {WGIII 1.4, 11.4, 13.3, SPM} 

### The literature provides **high agreement and**  **much evidence of many options for achieving reductions of global GHG**  **emissions at the international level through cooperation. It also suggests that successful agreements are environmen** **tally effective, cost-effective, incorporate distributional con siderations and equity, and are institutionally feasible.**  **{WGIII 13.3, SPM}** 

### Greater cooperative efforts to reduce emissions will help to reduce global costs for achieving a given level of mitigation, or will improve environmental effectiveness. Improving and expanding thescope of market mechanisms (such as emission trading, Joint Implementation and Clean Development Mechanism) could reduce overall mitigation costs. {WGIII 13.3, SPM} 

### Efforts to address climate change can include diverse elements such as emissions targets; sectoral, local, sub-national and regionalactions; RD&D programmes; adopting common policies; implementing development-oriented actions; or expanding financing instruments. These elements can be implemented in an integrated fashion, but comparing the efforts made by different countries quantitatively would be complex and resource intensive. {WGIII 13.3, SPM} 

### Actions that could be taken by participating countries can be differentiated both in terms of when such action is undertaken, who participates and what the action will be. Actions can be binding or non-binding, include fixed or dynamic targets, and participation can be static or vary over time. {WGIII 13.3, SPM} **Table 4.3.** Integrating climate change considerations into development policies – selected examples in the area of mitigation. {WGIII 12.2 .4.6}

Selected sectors Non-climate change policy instruments and actions Potenti ally affects:

Macro-economy Implement non-climate taxes/subsidies and/or other fiscal and T otal global GHG emissions regulatory policies that promote sustainable development
Forestry Adoption of forest conservation and sustainable management practices GHG emissions from deforestation
Electricity Ado ption of cost-effective renewables, demand-side management Electricity sector CO 2 emissions programmes, and transmission and distribution loss reduction
Petroleum imports Diversifying imported and domestic fuel mix and reducing Emissions from crude oil and product economy’s energy intensity to impro ve energy secur ity imports
Insurance for building, Differentiated premiums, liability insurance e xclusions, Transport and building sector GHG transport sectors improved terms for green products em issions
International finance Country and sector strategies and project lending that reduces emissions Emissions from developing countrie s

# 5

**The long-term perspective: scientific and socio-economic aspects relevant to adaptation**  **and mitigation, consistent with the objectives and provisions of the Convention, and in the context of**  **sustainable development Topic 5**  **The long-term perspective**

**64** 

## 5.1 Risk management perspective

**Responding to climate change involves an iterative risk management process that includes both mitigation and ad**  **aptation, taking into account actual and avoided climate change damages, co-benefits, sustainability, equity and at**  **titudes to risk. {WGII 20. 9, SPM; WGIII SPM}**  

### Risk management techniques can explicitly accommodate sectoral,regional and temporal diversity, but their application requires information about not only impacts resulting from the most likely climate scenarios, but also impacts arising from lower-probability but higher-consequence events and the consequences of proposed policies and measures. Risk is generally understood to be the product of the likelihood of an event and its consequences. Climate change impacts depend on the characteristics of natural and human systems, their development pathways and their specific locations. {SYR 3.3, Figure 3.6; WGII 20.2, 20.9, SPM; WGIII 3.5, 3.6, SPM}



## 5.2 Key vulnerabilities, impacts and risks – **long-term perspectives**

**The five ‘reasons for concern’ identified in the TAR are now assessed to be stronger with many risks identified with**  **higher confidence. Some are projected to be larger or to occur at lower increases in temperature. This is due to (1)**  **better understanding of the magnitude of impacts and risks associated with increases in global average temperature and**  **GHG concentrations, including vulnerability to present-day climate variability, (2) more precise identification of the cir**  **cumstances that make systems, sectors, groups and regions especially vulnerable and (3) growing evidence that the risk**  **of very large impacts on multiple century time scales would continue to increase as long as GHG concentrations and**  **temperature continue to increase. Understanding about the relationship between impacts (the basis for ‘reasons for con**  **cern’ in the TAR) and vulnerability (that includes the ability to adapt to impacts) has improved.**  **{WGII 4.4, 5.4, 19.ES, 19.3.7, TS.4.6; WGIII 3.5, SPM}** 

### The TAR concluded that vulnerability to climate change is a function of exposure, sensitivity and adaptive capacity. Adaptation can reduce sensitivity to climate change while mitigation can reduce theexposure to climate change, including its rate and extent. Both conclusions are confirmed in this assessment. {WGII 20.2, 20.7.3} 

### No single metric can adequately describe the diversity of keyvulnerabilities or support their ranking. A sample of relevant impacts is provided in Figure 3.6. The estimation of key vulnerabilities in any system, and damage implied, will depend on exposure (the rate and magnitude of climate change), sensitivity, which is determined in part and where relevant by development status, andadaptive capacity. Some key vulnerabilities may be linked to thresholds; in some cases these may cause a system to shift from one stateto another, whereas others have thresholds that are defined subjectively and thus depend on societal values. {WGII 19.ES, 19.1}


### The five ‘reasons for concern’ that were identified in the TAR were intended to synthesise information on climate risks and keyvulnerabilities and to “aid readers in making their own determination” about risk. These remain a viable framework to consider key vulnerabilities, and they have been updated in the AR4. {TAR WGII Chapter 19; WGII SPM}

 

### Risks to unique and threatened systems. There is new and stronger evidence of observed impacts of climate change onunique and vulnerable systems (such as polar and high mountain communities and ecosystems), with increasing levels ofadverse 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. Confidencehas increased that a 1 to 2°C increase in global mean temperature above 1990 levels (about 1.5 to 2.5°C above pre-indus**25** 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.



### Key Vulnerabilities and Article 2 of the UNFCCC
Article 2 of the UNFCCC states: “The ultimate objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve, in accordance with the relevant provisions of the Convention, stabilisation of greenhouse gas concentrations in the at mosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achie ved within a time frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner.” Determining what constitutes “dangerous anthropogenic interference with the climate system” in relation to Article 2 of the UNF CCC involves value judgements. Science can support informed decisions on this issue, including by providing criteria for judging wh ich vulnerabilities might be labelled ‘key’. {SYR 3.3, WGII 19.ES} Key vulnerabilities 25 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 circulati on. {WGII 19.ES} More specific information is now available across the regions of the world concerning the nature of future impacts, including f or some places not covered in previous assessments. {WGII SPM}**65 Topic 5**  **The long-term perspective**



### trial) 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 acclimatisationby corals. Increasing vulnerability of Arctic indigenous communities and small island communities to warming is projected. {SYR 3.3, 3.4, Figure 3.6, Table 3.2; WGII 4.ES, 4.4, 6.4, 14.4.6, 15.ES, 15.4, 15.6, 16.ES, 16.2.1, 16.4, Table 19.1, 19.3.7, TS.5.3, Figure TS.12, Figure TS.14}

 

### Risks of extreme weather events. Responses to some recent extreme climate events reveal higher levels of vulnerability in both developing and developed countries than was assessed inthe TAR. There is now higher confidence in the projected increases in droughts, heat waves and floods, as well as their adverse impacts. As summarised in Table 3.2, increases in drought, heat waves and floods are projected in many regions and would have mostly adverse impacts, including increased water stress and wild fire frequency, adverse effects on food production, adverse health effects, increased flood risk and extreme high sea level, and damage to infrastructure. {SYR 3.2, 3.3, Table 3.2; WGI 10.3, Table SPM.2; WGII 1.3, 5.4, 7.1, 7.5, 8.2, 12.6, 19.3, Table 19.1, Table SPM.1}

 

### 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 and are frequently the most susceptible to climate-related damages, especially when they face multiple stresses. 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. There is greater confidence in the projected regionalpatterns of climate change (see Topic 3.2) and in the projections of regional impacts, enabling better identification of particularly vulnerable systems, sectors and regions (see Topic 3.3).Moreover, there is increased evidence that low-latitude and lessdeveloped areas generally face greater risk, for example in dry areas and megadeltas. New studies confirm that Africa is oneof the most vulnerable continents because of the range of projected impacts, multiple stresses and low adaptive capacity. Substantial risks due to sea level rise are projected particularly for Asian megadeltas and for small island communities. {SYR 3.2, 3.3, 5.4; WGI 11.2-11.7, SPM; WGII 3.4.3, 5.3, 5.4, Boxes 7.1 and 7.4, 8.1.1, 8.4.2, 8.6.1.3, 8.7, 9.ES, Table 10.9, 10.6, 16.3, 19.ES, 19.3, Table 19.1, 20.ES, TS.4.5, TS.5.4, Tables TS.1, TS.3, TS.4, SPM}

 

### Aggregate impacts. Compared to the TAR, initial net marketbased benefits from climate change are projected to peak at a lower magnitude and therefore sooner than was assessed in the TAR. It is likely that there will be higher damages for larger magnitudes of global temperature increase than estimated in the TAR, and the net costs of impacts of increased warming are projected to increase over time. Aggregate impacts have also been quantified in other metrics (see Topic 3.3): for example, climate change over the next century is likely to adversely affect hundreds of millions of people through increased coastal flooding, reductions in water supplies, increased malnutrition and increased health impacts. {SYR 3.3, Figure 3.6; WGII 19.3.7, 20.7.3, TS.5.3}

 

### Risks of large-scale singularities. 26 

### As discussed in Topic 3.4, during the current century, a large-scale abrupt change in the meridional overturning circulation is very unlikely . 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. Thereis 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. Complete deglaciation of the Greenland ice sheet would raise sea level by 7m and could be irreversible. {SYR 3.4; WGI 10.3, Box 10.1; WGII 19.3.7, SPM}



## 5.3 Adaptation and mitigation

**There is high confidence** **that neither adaptation nor mitiga tion alone can avoid all climate change impacts. Adaptation** **is necessary both in the short term and longer term to ad dress impacts resulting from the warming that would occur**  **even for the lowest stabilisation scenarios assessed. There are barriers, limits and costs that are not fully understood.**  **Adaptation and mitigation can complement each other and together can significantly reduce the risks of climate change** 

**. {WGII 4.ES, TS 5.1, 18.4, 18.6, 20.7, SPM; WGIII 1.2, 2.5, 3.5, 3.6}**  

### Adaptation will be ineffective for some cases such as naturalecosystems (e.g. loss of Arctic sea ice and marine ecosystem viability), the disappearance of mountain glaciers that play vital rolesin water storage and supply, or adaptation to sea level rise of several metres 27 

### . It will be less feasible or very costly in many cases for the projected climate change beyond the next several decades (such as deltaic regions and estuaries). There is high confidence that the ability of many ecosystems to adapt naturally will be exceeded this century. In addition, multiple barriers and constraints to effective adaptation exist in human systems (see Topic 4.2). {SYR 4.2; WGII 17.4.2, 19.2, 19.4.1} 

### Unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems toadapt. Reliance on adaptation alone could eventually lead to a magnitude of climate change to which effective adaptation is not possible, or will only be available at very high social, environmental and economic costs. {WGII 18.1, SPM}

**26** See glossary

**27** While it is technically possible to adapt to several metres of sea le vel rise, the resources required are so unevenly distribut ed that in reality this risk is outside the scope of adaptation. {WGII 17.4.2, 19.4.1}**Topic 5 The long-term perspective** 

**66 28**  Peaking means that the emissions need to reach a maximum before they decline later.

**29** For the lowest mitigation scenario category assessed, emissions would need to peak by 2015 and for the highest by 2090 (see Tab le 5.1). Scenarios that use alternative emission pathways show substantial differences on the rate of global climate change. {WGII 19.4} 

### Efforts to mitigate GHG emissions to reduce the rate and **magnitude of climate change need to account for inertia in the climate and socio-economic systems.**  **{SYR 3.2; WGI 10.3, 10.4, 10.7, SPM; WGIII 2.3.4}** 


### After GHG concentrations are stabilised, the rate at which the global average temperature increases is expected to slow within a few decades. Small increases in global average temperature could still be expected for several centuries. Sea level rise from thermalexpansion would continue for many centuries at a rate that eventually decreases from that reached before stabilisation, due to ongoing heat uptake by oceans. {SYR 3.2, WGI 10.3, 10.4, 10.7, SPM}

### Delayed emission reductions significantly constrain the opportunities to achieve lower stabilisation levels and increase the risk of more severe climate change impacts. Even though benefits of mitigation measures in terms of avoided climate change would take several decades to materialise, mitigation actions begun in the shortterm would avoid locking in both long-lived carbon intensive infrastructure and development pathways, reduce the rate of climatechange and reduce the adaptation needs associated with higher levels of warming. {WGII 18.4, 20.6, 20.7, SPM; WGIII 2.3.4, 3.4, 3.5, 3.6, SPM} 

## 5.4 Emission trajectories for stabilisation

**In order to stabilise the concentration of GHGs in the atmo sphere, emissions would need to peak and decline thereaf**  **ter. 28**  

### The lower the stabilisation level, the more quickly this **peak and decline would need to occur (Figure 5.1). 29**  **{WGIII 3.3, 3.5, SPM}**  

### Advances in modelling since the TAR permit the assessment of multi-gas mitigation strategies for exploring the attainability and costs for achieving stabilisation of GHG concentrations. These scenarios explore a wider range of future scenarios, including lower levels of stabilisation, than reported in the TAR. {WGIII 3.3, 3.5, SPM}


### Mitigation efforts over the next two to three decades will **have a large impact on opportunities to achieve lower stabilisation levels (Table 5.1 and Figure 5.1).**  **{WGIII 3.5, SPM}**  

### Table 5.1 summarises the required emission levels for different groups of stabilisation concentrations and the resulting equilibrium

CO 2 

### emissions and equilibrium temperature increases for a range of stabilisation levels

**Figure 5.1.** Global CO 2 emissions for 1940 to 2000 and emissions ranges for categories of stabilisation scenarios from 2000 to 2100 (left-hand panel); and the corresponding relationship between the stabilisation target and the likelyequilibrium global average temperature increase above pre-industrial (righthand 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). Emi ssions 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 deforestati on, and from peat fires and drained peat soils. {WGIII Figures SPM.7 and SPM.8}Equilibrium global average temperatureincrease above pre-industrial (°C)



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

### YearWorld CO emissions (GtCO /yr)2 2**67 Topic 5**  **The long-term perspective**

**Table 5.1.** 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 {WGI 10.7; WGIII Table TS.2, Table 3.10, Table SPM.5}

Category Change in global CO 2 emissions in 2050 (percent of 2000 emissions) a,c Global average temperature increase above pre-industrial at equilibrium, using ‘best estimate’ climate sensitivity d,e Global average sea level rise above pre-industrial at equilibrium from thermal expansion only f

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 un derestimated due to missing carbon cycle feedbacks (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 2.1). 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 concentra tions occurs between 2100 and 2150 (see also Footnote 30). 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 th e 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 Atmos phere-Ocean General Circulation Model and EMICs to Earth System Models of Intermediate Complexity.)



### global average temperature increases, using the ‘best estimate’ of climate sensitivity (see Figure 5.1 for the likely range of uncertainty). Stabilisation at lower concentration and related equilibrium temperature levels advances the date when emissions need to peak and requires greater emissions reductions by 2050. 30 

### Climate sensitivity is a key uncertainty for mitigation scenarios that aim to meet specific temperature levels. 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. {WGIII 3.3, 3.4, 3.5, 3.6, SPM} 

### 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 (Table 5.1). If GHG and aerosol concentrations had been stabilised at year 2000 levels, thermal expansion alone would beexpected to lead to further sea level rise of 0.3 to 0.8m. 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 manycenturies. These long-term consequences would have major implications for world coastlines. The long time scale of thermal expansion and ice sheet response to warming imply that mitigation strategies that seek to stabilise GHG concentrations (or radiative forcing) at or above present levels do not stabilise sea level for many centuries. {WG1 10.7} 

### Feedbacks between the carbon cycle and climate change affect the required mitigation and adaptation response to climate change. Climate-carbon cycle coupling is expected to increase the fraction of anthropogenic emissions that remains in the atmosphere as the climate system warms (see Topics 2.3 and 3.2.1), but mitigation studies have not yet incorporated the full range of these feedbacks. As a consequence, the emission reductions to meet a particular stabilisation level reported in the mitigation studies assessed in Table 5.1 might be underestimated. Based on current understanding of climate-carbon cycle feedbacks, model studies suggest that stabilising CO 2 

### concentrations at, for example, 450ppm 31 

### could require cumulative emissions over the 21 st 

### century to be less than 1800 [1370 to 2200] GtCO 2 

### , which is about 27% less than the 2460 [2310 to 2600] GtCO 2 

### determined without consideration of carbon cycle feedbacks. {SYR 2.3, 3.2.1; WGI 7.3, 10.4, SPM}

**30** 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 ov er a few centuries. For the much lower stabilisation s cenarios (category I and II, Figure 5.1), the equilibrium temperature may be reached earlier.

**31** To stabilise at 1000ppm CO 2 , this feedback could require that cumulative emissions be reduced from a model average of approximately 5190 [4910 to 5460] GtCO 2 to approximately 4030 [3590 to 4580] GtCO 2 . {WGI 7.3, 10.4, SPM} CO 2 concentration at stabilisation (2005 = 379 ppm) b CO 2 -equivalent concentration at stabilisation including GHGs and aerosols (2005=375 ppm) b Peaking year for CO 2 emissions a,c Number of assessed scenarios**Topic 5 The long-term perspective** 

**68** 

### Illustrative mitigation portfolios for achieving stabilisation of GHG concentrations

**Figure 5.2** Cumulative emissions reductions for alternative mitigation measures for 2000-2030 (left-hand panel) and for 2000-2100 (right-ha nd panel). The figure shows illustrative scenarios from four models (AIM, IMA GE, IPAC and MESSAGE) aiming at the stabilisation at low (490 to 540ppm CO 2 -eq) and intermediate levels (650ppm CO 2 -eq) respectively. Dark bars denote reductions for a target of 650ppm CO 2-eq and light bars denote the additional reductions to achieve 490 to 540ppm CO 2 -eq. Note that some models do not consider mitigation through forest sink enhancement (AIM and IPAC) or CCS (AIM) and that the share of low-carbon energy options in total energy supply is also determined by inclusion of these options in the baseline. CCS includes CO 2 capture and storage from biomass. Forest sinks include reducing emissions from deforestation. The figure shows emissions reduct ions from baseline scenarios with cumulative emissions between 6000 to 7000 GtCO 2 -eq (2000-2100). {WGIII Figure SPM.9} 

## 5.5 Technology flows and development

**There is high agreement**  **and much evidence**  **that all stabilisation levels assessed can be achieved by deploy**  **ment of a portfolio of technologies that are either currently available or expected to be commercialised in coming de**  **cades, assuming appropriate and effective incentives are in place for development, acquisition, deployment and dif**  **fusion of technologies and addressing related barriers. {WGIII**  **SPM}** 

### Worldwide deployment of low-GHG emission technologies as well as technology improvements through public and private RD&D would be required for achieving stabilisation targets as well as cost reduction. 32 

### Figure 5.2 gives illustrative examples of the contribution of the portfolio of mitigation options. The contribution of different technologies varies over time and region and depends on the baseline development path, available technologies and relative costs, and the analysed stabilisation levels. Stabilisation at the lower of the assessed levels (490 to 540ppm CO 2 

### -eq) requires early investments and substantially more rapid diffusion and commercialisation of advanced low-emissions technologies over the next decades (2000-2030) and higher contributions across abatement options inthe long term (2000-2100). This requires that barriers to development, acquisition, deployment and diffusion of technologies are effectively addressed with appropriate incentives. {WGIII 2.7, 3.3, 3.4, 3.6, 4.3, 4.4, 4.6, SPM} 

### Without sustained investment flows and effective technologytransfer, it may be difficult to achieve emission reduction at a significant scale. Mobilising financing of incremental costs of lowcarbon technologies is important. {WGIII 13.3, SPM}

### There are large uncertainties concerning the future contribution of different technologies. However, all assessed stabilisation scenarios concur that 60 to 80% of the reductions over the courseof the century would come from energy supply and use and industrial processes. Including non-CO 2 

### and CO 2 

### land-use and forestrymitigation options provides greater flexibility and cost-effectiveness. Energy efficiency plays a key role across many scenarios formost regions and time scales. For lower stabilisation levels, scenarios put more emphasis on the use of low-carbon energy sources, such as renewable energy, nuclear power and the use of CO 2 

### capture and storage (CCS). In these scenarios, improvements of carbon intensity of energy supply and the whole economy needs to be much faster than in the past (Figure 5.2). {WGIII 3.3, 3.4, TS.3, SPM}

**32** By comparison, government funding in real absolute terms for most energy research programmes has been flat or declining for nea rly two decades (even after the UNFCCC came into force) and is now about half of the 1980 level. {WGIII 2.7, 3.4, 4.5, 11.5, 13.2}**69 Topic 5**  **The long-term perspective**



## 5.6 Costs of mitigation and long-term **stabilisation targets**

**The macro-economic costs of mitigation generally rise with the stringency of the stabilisation target and are relatively**  **higher when derived from baseline scenarios characterised by high emission levels.**  **{WGIII SPM}**



### There is high agreement and medium evidence that in 2050 global average macro-economic costs for multi-gas mitigation towards stabilisation between 710 and 445ppm CO 2 

### -eq are between a 1%gain to a 5.5% decrease of global GDP (Table 5.2). This corresponds to slowing average annual global GDP growth by less than 0.12 percentage points. Estimated GDP losses by 2030 are on average lower and show a smaller spread compared to 2050 (Table 5.2). For specific countries and sectors, costs vary considerably from the global average. 33 {WGIII 3.3, 13.3, SPM}



## 5.7 Costs, benefits and avoided climate

**impacts at global and regional levels**

**Impacts of climate change will vary regionally. Aggregated and discounted to the present, they are**  **very likely to im** **pose net annual costs, which will increase over time as glo bal temperatures increase.**  **{WGII SPM}** 

### For increases in global average temperature of less than 1 to 3°C above 1980-1999 levels, some impacts are projected to produce market benefits in some places and sectors while, at the same time, imposing costs in other places and sectors. Global mean losses could be 1 to 5% of GDP for 4°C of warming, but regional losses could be substantially higher. {WGII 9.ES, 10.6, 15.ES, 20.6, SPM}

### Peer-reviewed estimates of the social cost of carbon (net economic costs of damages from climate change aggregated across the globe and discounted to the present) for 2005 have an average value of US$12 per tonne of CO 2 

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

### ). The range of published evidence indicates that the net damage costs of climate change are projected to be significant and to increase over time. {WGII 20.6, SPM}


### It is very likely that globally aggregated figures underestimatethe damage costs because they cannot include many non-quantifiable impacts. It is virtually certain that aggregate estimates of costsmask significant differences in impacts across sectors, regions, countries and populations. In some locations and amongst some groups of people with high exposure, high sensitivity and/or low adaptivecapacity, net costs will be significantly larger than the global average. {WGII 7.4, 20.ES, 20.6, 20.ES, SPM}


### Limited and early analytical results from integrated analy**ses of the global costs and benefits of mitigation indicate that these are broadly comparable in magnitude, but do not** **as yet permit an unambiguous determination of an emis sions pathway or stabilisation level where benefits exceed**  **costs. {WGIII SPM}**  

### Comparing the costs of mitigation with avoided damages would require the reconciliation of welfare impacts on people living indifferent places and at different points in time into a global aggregate measure of well-being. {WGII 18.ES} 

### 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. {WGIII SPM}



### Many impacts can be avoided, reduced or delayed by miti**gation. {WGII SPM}** 

### Although the small number of impact assessments that evaluate stabilisation scenarios do not take full account of uncertainties in projected climate under stabilisation, they nevertheless provide indications of damages avoided and risks reduced for different

**Table 5.2.** 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. {WGIII 3.3, 13.3, Tables SPM.4 and SPM.6}

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 applic able. 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 gener ally 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.

**33** See Footnote 24 for further details on cost estimates and model assumptions.**Topic 5 The long-term perspective** 

**70** 

### amounts of emissions reduction. The rate and magnitude of futurehuman-induced climate change and its associated impacts are determined by human choices defining alternative socio-economic futures and mitigation actions that influence emission pathways. Figure 3.2 demonstrates that alternative SRES emission pathways could lead to substantial differences in climate change throughout the 21 st 

### century. Some of the impacts at the high temperature end ofFigure 3.6 could be avoided by socio-economic development pathways that limit emissions and associated climate change towards the lower end of the ranges illustrated in Figure 3.6. {SYR 3.2, 3.3; WGIII 3.5, 3.6, SPM} 

### Figure 3.6 illustrates how reduced warming could reduce the risk of, for example, affecting a significant number of ecosystems, the risk of extinctions, and the likelihood that cereal productivity in some regions would tend to fall. {SYR 3.3, Figure 3.6; WGII 4.4, 5.4, Table 20.6}



## 5.8 Broader environmental and **sustainability issues**

**Sustainable development can reduce vulnerability to climate change, and climate change could impede nations’ abilities**  **to achieve sustainable development pathways. {WGII SPM}** 



### It is very likely that climate change can slow the pace of progress toward sustainable development either directly through increased exposure to adverse impacts or indirectly through erosion of the capacity to adapt. Over the next half-century, climate change could impede achievement of the Millennium Development Goals. {WGII SPM} 

### Climate change will interact at all scales with other trends in global environmental and natural resource concerns, includingwater, soil and air pollution, health hazards, disaster risk, and deforestation. Their combined impacts may be compounded in future in the absence of integrated mitigation and adaptation measures. {WGII 20.3, 20.7, 20.8, SPM}


### Making development more sustainable can enhance miti**gative and adaptive capacities, reduce emissions, and re duce vulnerability, but there may be barriers to implementa**  **tion. {WGII 20.8; WGIII 12.2, SPM}**  

### Both adaptive and mitigative capacities can be enhanced through sustainable development. Sustainable development can, thereby, reduce vulnerability to climate change by reducing sensitivities (through adaptation) and/or exposure (through mitigation). Atpresent, however, few plans for promoting sustainability have explicitly included either adapting to climate change impacts, or promoting adaptive capacity. Similarly, changing development pathscan make a major contribution to mitigation but may require resources to overcome multiple barriers. {WGII 20.3, 20.5, SPM; WGIII 2.1, 2.5, 12.1, SPM}

# 6

**Robust findings, key uncertainties Topic 6**  **Robust findings, key uncertainties**

**72** 

## Robust findings, key uncertainties

As in the TAR, a robust finding for climate change is defined as one that holds under a variety of approaches, methods, models and assumptions, and is expected to be relatively unaffected by uncertainties. Key uncertainties are those that, if reduced, could lead to new robust findings. {TAR SYR Q.9} 

### Robust findings do not encompass all key findings of the AR4. Some key findings may be policy-relevant even though they are associated with large uncertainties. {WGII 20.9} 

### The robust findings and key uncertainties listed below do not represent an exhaustive list.



## 6.1 Observed changes in climate and their **effects, and their causes**

**Robust findings**

Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and oceantemperatures, widespread melting of snow and ice and rising global average sea level. {WGI 3.9, SPM} 

### Many natural systems, on all continents and in some oceans, are being affected by regional climate changes. Observed changesin many physical and biological systems are consistent with warming. As a result of the uptake of anthropogenic CO 2 

### since 1750, the acidity of the surface ocean has increased. {WGI 5.4 , WGII 1.3} 

### Global total annual anthropogenic GHG emissions, weighted by their 100-year GWPs, have grown by 70% between 1970 and2004. As a result of anthropogenic emissions, atmospheric concentrations of N 2 

### O now far exceed pre-industrial values spanning many thousands of years, and those of CH 4 

### and CO 2 

### now far exceed the natural range over the last 650,000 years. {WGI SPM; WGIII 1.3} 

### Most of the global average warming over the past 50 years is very likely due to anthropogenic GHG increases and it is likely that there is a discernible human-induced warming averaged over each continent (except Antarctica). {WGI 9.4, SPM} 

### 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. {WGII 1.4, SPM}


### Key uncertainties

Climate data coverage remains limited in some regions and there is a notable lack of geographic balance in data and literature on observed changes in natural and managed systems, with marked scarcity in developing countries. {WGI SPM; WGII 1.3, SPM}

### Analysing and monitoring changes in extreme events, including drought, tropical cyclones, extreme temperatures and the frequency and intensity of precipitation, is more difficult than for climatic averages as longer data time-series of higher spatial and temporal resolutions are required. {WGI 3.8, SPM} 

### Effects of climate changes on human and some natural systems are difficult to detect due to adaptation and non-climatic drivers. {WGII 1.3} 

### Difficulties remain in reliably simulating and attributing observed temperature changes to natural or human causes at smallerthan continental scales. At these smaller scales, factors such as landuse change and pollution also complicate the detection of anthropogenic warming influence on physical and biological systems. {WGI 8.3, 9.4, SPM; WGII 1.4, SPM} 

### The magnitude of CO 2 

### emissions from land-use change and CH 4 

### emissions from individual sources remain as key uncertainties. {WGI 2.3, 7.3, 7.4; WGIII 1.3, TS.14}



## 6.2 Drivers and projections of future climate **changes and their impacts**

**Robust findings**

With current climate change mitigation policies and related sustainable development practices, global GHG emissions will continue to grow over the next few decades. {WGIII 3.2, SPM} 

### For the next two decades a warming of about 0.2°C per decade is projected for a range of SRES emissions scenarios. {WGI 10.3, 10.7, SPM}


### 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. {WGI 10.3, 11.1, SPM} 

### The pattern of future warming where land warms more than the adjacent oceans and more in northern high latitudes is seen in all scenarios. {WGI 10.3, 11.1, SPM}

### Warming tends to reduce terrestrial ecosystem and ocean uptake of atmospheric CO 2 

### , increasing the fraction of anthropogenic emissions that remains in the atmosphere. {WGI 7.3, 10.4, 10.5, SPM} 

### Anthropogenic warming and sea level rise would continue for centuries even if GHG emissions were to be reduced sufficientlyfor GHG concentrations to stabilise, due to the time scales associated with climate processes and feedbacks. {WGI 10.7, SPM} 

### Equilibrium climate sensitivity is very unlikely to be less than 1.5°C. {WGI 8.6, 9.6, Box 10.2, SPM} 

### Some systems, sectors and regions are likely to be especiallyaffected by climate change. The systems and sectors are some ecosystems (tundra, boreal forest, mountain, mediterranean-type, mangroves, salt marshes, coral reefs and the sea-ice biome), low-lying coasts, water resources in some dry regions at mid-latitudes and inthe dry topics and in areas dependent on snow and ice melt, agriculture in low-latitude regions, and human health in areas with low adaptive capacity. The regions are the Arctic, Africa, small islands and Asian and African megadeltas. Within other regions, even thosewith high incomes, some people, areas and activities can be particularly at risk. {WGII TS.4.5} 

### Impacts are very likely to increase due to increased frequencies and intensities of some extreme weather events. Recent events havedemonstrated the vulnerability of some sectors and regions, including in developed countries, to heat waves, tropical cyclones, floods and drought, providing stronger reasons for concern as compared to the findings of the TAR. {WGII Table SPM.2, 19.3}**73 Topic 6**  **Robust findings, key uncertainties**



### Key uncertainties

Uncertainty in the equilibrium climate sensitivity creates uncertainty in the expected warming for a given CO 2 

### -eq stabilisationscenario. Uncertainty in the carbon cycle feedback creates uncertainty in the emissions trajectory required to achieve a particular stabilisation level. {WGI 7.3, 10.4, 10.5, SPM} 

### Models differ considerably in their estimates of the strength ofdifferent feedbacks in the climate system, particularly cloud feedbacks, oceanic heat uptake and carbon cycle feedbacks, althoughprogress has been made in these areas. Also, the confidence in projections is higher for some variables (e.g. temperature) than for others (e.g. precipitation), and it is higher for larger spatial scales and longer time averaging periods. {WGI 7.3, 8.1-8.7, 9.6, 10.2, 10.7, SPM; WGII 4.4} 

### Aerosol impacts on the magnitude of the temperature response, on clouds and on precipitation remain uncertain. {WGI 2.9, 7.5, 9.2, 9.4, 9.5} 

### Future changes in the Greenland and Antarctic ice sheet mass,particularly due to changes in ice flow, are a major source of uncertainty that could increase sea level rise projections. The uncertainty in the penetration of the heat into the oceans also contributes to the future sea level rise uncertainty. {WGI 4.6, 6.4, 10.3, 10.7, SPM} 

### Large-scale ocean circulation changes beyond the 21 st 

### centurycannot be reliably assessed because of uncertainties in the meltwater supply from the Greenland ice sheet and model response to the warming. {WGI 6.4, 8.7, 10.3 } 

### Projections of climate change and its impacts beyond about 2050 are strongly scenario- and model-dependent, and improved projections would require improved understanding of sources of uncertainty and enhancements in systematic observation networks. {WGII TS.6}

### Impacts research is hampered by uncertainties surrounding regional projections of climate change, particularly precipitation. {WGII TS.6} 

### Understanding of low-probability/high-impact events and thecumulative impacts of sequences of smaller events, which is required for risk-based approaches to decision-making, is generally limited. {WGII 19.4, 20.2, 20.4, 20.9, TS.6}



## 6.3 Responses to climate change

**Robust findings**

Some planned adaptation (of human activities) is occurring now;more extensive adaptation is required to reduce vulnerability to climate change. {WGII 17.ES, 20.5, Table 20.6, SPM} 

### Unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems to adapt. {WGII 20.7, SPM} 

### A wide range of mitigation options is currently available or projected to be available by 2030 in all sectors. The economic mitigation potential, at costs that range from net negative up to US$100/ tCO 2 

### -equivalent, is sufficient to offset the projected growth of global emissions or to reduce emissions to below current levels in 2030. {WGIII 11.3, SPM}

### 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 lowerstabilisation levels. Delayed emissions reductions significantly constrain the opportunities to achieve lower stabilisation levels and increase the risk of more severe climate change impacts. {WGII SPM, WGIII SPM} 

### The range of stabilisation levels for GHG concentrations that have been assessed can be achieved by deployment of a portfolioof technologies that are currently available and those that are expected to be commercialised in coming decades, provided that appropriate and effective incentives are in place and barriers are removed. In addition, further RD&D would be required to improvethe technical performance, reduce the costs and achieve social acceptability of new technologies. The lower the stabilisation levels, the greater the need for investment in new technologies during the next few decades. {WGIII 3.3, 3.4}

### Making development more sustainable by changing development paths can make a major contribution to climate change mitigation and adaptation and to reducing vulnerability. {WGII 18.7, 20.3, SPM; WGIII 13.2, SPM} 

### Decisions about macro-economic and other policies that seem unrelated to climate change can significantly affect emissions. {WGIII 12.2}


### Key uncertainties

Understanding of how development planners incorporate information about climate variability and change into their decisions is limited. This limits the integrated assessment of vulnerability. {WGII 18.8, 20.9}

### The evolution and utilisation of adaptive and mitigative capacity depend on underlying socio-economic development pathways. {WGII 17.3, 17.4, 18.6, 19.4, 20.9} 

### Barriers, limits and costs of adaptation are not fully understood, partly because effective adaptation measures are highly dependenton specific geographical and climate risk factors as well as institutional, political and financial constraints. {WGII SPM}

### Estimates of mitigation costs and potentials depend on assumptions about future socio-economic growth, technological change and consumption patterns. Uncertainty arises in particular from assumptions regarding the drivers of technology diffusion and thepotential of long-term technology performance and cost improvements. Also little is known about the effects of changes in behaviour and lifestyles. {WGIII 3.3, 3.4, 11.3} 

### The effects of non-climate policies on emissions are poorly quantified. {WGIII 12.2}