Research report

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, Neville Nicholls, Leonard Nurse, Rajendra Pachauri, Jean Palutikof, Martin Parry, Dahe Qin, Nijavalli Ravindranath, Andy Reisinger, Jiawen Ren, Keywan Riahi, Cynthia Rosenzweig, Matilde Rusticucci, Stephen Schneider, Youba Sokona, Susan Solomon, Peter Stott, Ronald Stouffer, Taishi Sugiyama, Rob Swart, Dennis Tirpak, Coleen Vogel, Gary Yohe

Extended Writing Team

Terry Barker

Review Editors

Abdelkader Allali, Roxana Bojariu, Sandra Diaz, Ismail Elgizouli, Dave Griggs, David Hawkins, Olav Hohmeyer, Bubu Pateh Jallow, Luc4ka Kajfez4-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 Panel on Climate Change (IPCC). It provides an integrated view of cli- mate change as the final part of the IPCC’s Fourth Assessment Re- port (AR4).

Topic 1 summarises observed changes in climate and their ef- fects on natural and human systems, regardless of their causes, while Topic 2 assesses the causes of the observed changes. Topic 3 pre- sents projections of future climate change and related impacts un- der different scenarios.

Topic 4 discusses adaptation and mitigation options over the next 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 and mitigation on a more conceptual basis and takes a longer-term per- spective. Topic 6 summarises the major robust findings and remain- ing 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 from socio-economic information and emissions. With increased under- standing of these linkages, it is now possible to assess the linkages also counterclockwise, i.e. to evaluate possible development path- ways 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 note1 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 treatment of uncertainty drawn from the literature. The nature of data, indicators and analyses used in the natural sciences is generally different 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 covers aspects of both.

Three different approaches are used to describe uncertainties each with a distinct form of language. Choices among and within these three approaches depend on both the nature of the information available and the authors’ expert judgment of the correctness and completeness of current scientific understanding.

Where uncertainty is assessed qualitatively, it is characterised by providing a relative sense of the amount and quality of evidence (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-explanatory 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 analyses, then the following scale of confidence levels is used to express the assessed chance of a finding being correct: very 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. observations 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 information 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. there is an estimated 5% likelihood that the value could be above the range given in square brackets and 5% likelihood that the value could be below that range). Uncertainty intervals are not necessarily symmetric around the 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 chang- ing in space and time has been gained through improvements and extensions of numerous datasets and data analyses, broader geo- graphical 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. This usage differs from that in the United Nations Framework Con- vention on Climate Change (UNFCCC), where climate change refers to a change of climate that is attributed directly or indi- rectly 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 is greater at higher northern latitudes (Figure 1.2). Average Arctic tem- peratures 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 aver- age temperature of the global ocean has increased to depths of at least 3000m and that the ocean has been taking up over 80% of the heat being added to the climate system. New analyses of balloon- borne and satellite measurements of lower- and mid-tropospheric temperature show warming rates similar to those observed in sur- face 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 for 1993 to 2003 reflects decadal variation or an increase in the longer-

term 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 consistent with warming (Figure 1.1). Satellite data since 1978 show that an- nual average Arctic sea ice extent has shrunk by 2.7 [2.1 to 3.3]% per decade, with larger decreases in summer of 7.4 [5.0 to 9.8]% per decade. Mountain glaciers and snow cover on average have declined in both hemispheres. The maximum areal extent of sea- sonally 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 in- creased 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 long- term changes in other aspects of climate have also been observed. Trends from 1900 to 2005 have been observed in precipitation amount in many large regions. Over this period, precipitation in- creased significantly in eastern parts of North and South America, northern Europe and northern and central Asia whereas precipita- tion declined in the Sahel, the Mediterranean, southern Africa and parts of southern Asia. Globally, the area affected by drought has likely2 increased since the 1970s. {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 be- come 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 level3 has increased at a broad range of sites worldwide since 1975. {WGI 5.5, SPM}

There is observational evidence of an increase in intense tropical cyclone activity in the North Atlantic since about 1970, and sugges- tions of increased intense tropical cyclone activity in some other re- gions where concerns over data quality are greater. Multi-decadal vari- ability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long- term trends in tropical cyclone activity. {WGI 3.8, SPM}

Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years. {WGI 6.6, SPM}

2 Likelihood and confidence statements in italics represent calibrated expressions of uncertainty and confidence. See Box ‘Treatment 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 weather systems. It is defined here as the highest 1% of hourly values of observed sea level at a station for a given reference period.

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 repre- sent 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 observed trends in the physical and biological environment and their rela- tionship to regional climate changes has increased greatly since the TAR. The quality of the data sets has also improved. There is a notable lack of geographic balance in data and literature on ob- served changes, with marked scarcity in developing countries. {WGII SPM}

These studies have allowed a broader and more confident as- sessment of the relationship between observed warming and im- pacts than was made in the TAR. That assessment concluded that “there is high confidence2 that recent regional changes in tempera- ture have had discernible impacts on physical and biological sys- tems”. {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: increased runoff and earlier spring peak discharge in many glacier- and snow- fed 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 processes) and biological systems (terrestrial, marine, and freshwater biological systems), are shown together with surface air temperature changes over the period 1970- 2004. A subset of about 29,000 data series was selected from about 80,000 data series from 577 studies. These met the following criteria: (1) ending in 1990 or later; (2) spanning a period of at least 20 years; and (3) showing a significant change in either direction, as assessed in individual studies. These data series are from about 75 studies (of which about 70 are new since the TAR) and contain about 29,000 data series, of which about 28,000 are from European studies. White areas do not contain sufficient observational climate data to estimate a temperature trend. The 2 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) global-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 large- area 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 7650 120 24 7645

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 tim- ing 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 fish abundance in high-latitude oceans; increases in algal and zooplank- ton abundance in high-latitude and high-altitude lakes; and range changes and earlier fish migrations in rivers. While there is increas- ing evidence of climate change impacts on coral reefs, separating the impacts of climate-related stresses from other stresses (e.g. over- fishing 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 contribut- ing to losses of coastal wetlands and mangroves and increasing damage from coastal flooding in many areas. However, based on the published literature, the impacts have not yet become estab- lished 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 sea ice, shorter freezing seasons of lake and river ice, glacier melt, de- creases 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 stud- ies, 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 they have changed. Antarctic sea ice extent shows inter-annual variabil- ity 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 forcing4 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 system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to pre- industrial conditions defined at 1750 and are expressed in watts per square metre (W/m2). 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) on

the global climate system due to their different radiative prop- erties and lifetimes in the atmosphere. These warming influ- ences 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 long- lived GHG or a mixture of GHGs. The equivalent CO

2 emis-

sion 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 a

standard and useful metric for comparing emissions of dif- ferent 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

CO2 from fossil fuel use and other sources

CH4 from agriculture, waste and energy

CO2 from deforestation, decay and peat

N2O from agriculture and others

G tC

O 2-

eq /

yr

28.7

35.6 39.4

44.7 49.0

The largest growth in GHG emissions between 1970 and 2004

has come from energy supply, transport and industry, while resi- dential 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 appendix 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&T

a n d 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 energy intensity (-33%) during 1970 to 2004 has been smaller than the com- bined effect of global income growth (77%) and global population growth (69%); both drivers of increasing energy-related CO

2 emis-

sions. The long-term trend of declining CO 2 emissions per unit of en-

ergy 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 glo-

bal GHG emissions (Figure 2.2). {WGIII 1.3, SPM}

2.2 Drivers of climate change

Changes in the atmospheric concentrations of GHGs and aero- sols, land cover and solar radiation alter the energy balance of the climate system and are drivers of climate change. They affect the absorption, scattering and emission of radiation within the atmo- sphere and at the Earth’s surface. The resulting positive or negative changes in energy balance due to these factors are expressed as radiative forcing4, 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 a

pre-industrial value of about 280ppm to 379ppm in 2005. The an- nual 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-to- year 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 in- creased 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/m2

(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/m2, 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/m2 and an indirect cloud albedo forcing of -0.7 [-1.8 to -0.3] W/m2. Aerosols also influence precipitation. {WGI 2.4, 2.9, 7.5, SPM}

In comparison, changes in solar irradiance since 1750 are esti- mated to have caused a small radiative forcing of +0.12 [+0.06 to +0.30] W/m2, 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 assess-

ment 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 less than 1.5°C. Values substantially higher than 4.5°C cannot be ex- cluded, 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 forc- ing. Direct emission of water vapour (a greenhouse gas) by human activities makes a negligible contribution to radiative forcing. How- ever, 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 changes represent the largest feedback affecting equilibrium climate sensi- tivity and are now better understood than in the TAR. Cloud feed- backs 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 quantita- tively consistent with the expected response to external forcings (e.g. changes in solar irradiance or anthropogenic GHGs) and in- consistent 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). Measurements are shown from ice cores (symbols with different colours for different stud- ies) 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 understand- ing (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-20th 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 ex- tremely 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 in changes in surface and atmospheric temperatures and in tempera- tures of the upper several hundred metres of the ocean. The ob- served 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 GHG concentrations alone would have caused more warming than ob- served 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 20th cen- tury. {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 using either natural or both natural and anthropogenic forcings. Decadal averages of observations are shown for the period 1906-2005 (black line) plotted against the centre of the decade and relative to the corresponding average for the 1901-1950. Lines are dashed where spatial coverage is less than 50%. Blue shaded bands show the 5 to 95% range for 19 simulations from five climate models using only the natural forcings due to solar activity and volcanoes. Red shaded bands show the 5 to 95% range for 58 simulations from 14 climate models using both natural and anthropogenic forcings. {WGI Figure SPM.4}

Difficulties remain in simulating and attributing observed tem- perature 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 con- tributed 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 20th 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 con- tributed to sea level rise during the latter half of the 20th 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 pat- terns of changes in land precipitation over the 20th century. It is more likely than not that human influence has contributed to a glo- bal 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 re- sponses in physical and biological systems to anthropogenic warm- ing, but only a few such studies have been performed. Taken to- gether 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 attribu- tion of the causes of observed natural system responses to anthro- pogenic warming. The available analyses are limited in the number of systems, length of records and locations considered. Natural tem- perature variability is larger at the regional than the global scale, thus affecting identification of changes to external forcing. At the re- gional scale, other non-climate factors (such as land-use change, pol- lution 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 evidence9 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) have used lower values for some drivers for emissions, notably popula- tion 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 over- all emissions. {WGIII 3.2, TS.3, SPM}

Aerosols have a net cooling effect and the representation of aerosol and aerosol precursor emissions, including sulphur diox- ide, black carbon and organic carbon, has improved in the post- SRES 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 Intro- duction 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 UNFCCC 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 compare GDP between countries. Use of MER is preferable for analyses involving internationally traded products. Use of PPP is preferable for analyses involving comparisons of income between countries at very different stages of development. Most of the monetary units in this report are expressed in MER. This reflects the large majority of emissions mitigation literature that is calibrated in MER. When monetary units are expressed in PPP, this is denoted 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 additional 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 climate 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 introduc- tion of new and more efficient technologies. A1 is divided into three groups that describe alternative directions of technological change: fossil intensive (A1FI), non-fossil energy resources (A1T) and a balance across all sources (A1B). B1 describes a convergent world, with the same global population as A1, but with more rapid changes in economic structures toward a service and information economy. B2 describes a world with intermediate population and economic growth, emphasising local solutions to economic, social, and environ- mental 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 80th percentile range of recent scenarios published since SRES (post-SRES) (gray shaded area). Dashed lines show the full range of post- SRES scenarios. The emissions include 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 policies

G lo

b a l G

H G

e m

is si

o n s

(G t C

O 2 -e

q / y

r)

post-SRES (max)

post-SRES (min)

Purchasing Power Parity, PPP) does not appreciably affect the pro- jected 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 0.15 and 0.3°C per decade from 1990 to 2005. This can now be com- pared with observed values of about 0.2°C per decade, strengthen- ing confidence in near-term projections. {WGI 1.2, 3.2}

3.2.1 21st 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 21st century that would very likely be larger than those observed during the 20th century. {WGI 10.3}

Advances in climate change modelling now enable best esti- mates and likely assessed uncertainty ranges to be given for pro- jected warming for different emissions scenarios. Table 3.1 shows best estimates and likely ranges for global average surface air warm- ing for the six SRES marker emissions scenarios (including cli- mate-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 than in the TAR mainly because the broader range of models now avail- able 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 21st 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 ice sheet flow, because a basis in published literature is lacking. There- fore the upper values of the ranges given are not to be considered upper bounds for sea level rise. The projections include a contribu- tion 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 21st 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 concentrationsb 0.6 0.3 – 0.9 Not available

B1 scenario 1.8 1.1 – 2.9 0.18 – 0.38 A1T scenario 2.4 1.4 – 3.8 0.20 – 0.45 B2 scenario 2.4 1.4 – 3.8 0.20 – 0.43 A1B scenario 2.8 1.7 – 4.4 0.21 – 0.48 A2 scenario 3.4 2.0 – 5.4 0.23 – 0.51 A1FI scenario 4.0 2.4 – 6.4 0.26 – 0.59

Notes: a) These estimates are assessed from a hierarchy of models that encompass a simple climate model, several Earth Models of Intermediate

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

d) Temperature changes are expressed as the difference from the period 1980-1999. To express the change relative to the period 1850-1899 add 0.5°C.

12 TAR projections were made for 2100, whereas the projections for this report are for 2090-2099. The TAR would have had similar ranges to those in Table 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 21st 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 21st century shows scenario-indepen- dent 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 21st century. {WGI 10.3, 10.6, SPM; WGII 15.3.4}

It is very likely that hot extremes, heat waves and heavy pre- cipitation 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 cy- clones (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 simu- lated 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 half- century. {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 most subtropical land regions (by as much as about 20% in the A1B sce- nario 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 21st 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 radia- tive forcing agents constant at B1 or A1B levels in 2100, model experiments show that a further increase in global average tem- perature of about 0.5°C would still be expected by 2200. In addi- tion, 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 20th century simulations. The orange line is for the experiment where concentrations were 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 par t of the figure, as well as results from a hierarchy of independent models and obser vational constraints. Right panels: Projected surface temperature changes for the early and late 21st 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 2090-2099 (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 balance becomes negative (net ice loss) at a global average warming (rela- tive 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. The corresponding future temperatures in Greenland (1.9 to 4.6°C glo- bal) are comparable to those inferred for the last interglacial period 125,000 years ago, when palaeoclimatic information suggests re- ductions 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 in- cluded in current models but suggested by recent observations –

could increase the vulnerability of the ice sheets to warming, in- creasing 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 mass due to increased snowfall. However, net loss of ice mass could oc- cur if dynamical ice discharge dominates the ice sheet mass bal- ance. {WGI 10.7, SPM}

Both past and future anthropogenic CO 2 emissions will con-

tinue 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 averages 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 temperature 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 findings14 regarding the impacts of climate change on systems, sectors and regions, as well as some findings on vulnerability15, for the range of climate changes projected over the 21st century. Unless otherwise stated, the confi- dence 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. land- use change, pollution, fragmentation of natural systems, over- exploitation 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 reverse16, 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 con- fidence). {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 in- crease 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, includ- ing 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 pro- jected 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 af- fected through, for example, increases in malnutrition; increased deaths, diseases and injury due to extreme weather events; in- creased 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 temper- ate areas, such as fewer deaths from cold exposure, and some mixed effects such as changes in range and transmission poten- tial of malaria in Africa. Overall it is expected that benefits will be outweighed by the negative health effects of rising tempera- tures, especially in developing countries. {WGII 8.4, 8.7, 8ES, SPM}

� Critically important will be factors that directly shape the health of populations such as education, health care, public health ini- tiatives, 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 freshwater availability. Widespread mass losses from glaciers and reductions in snow cover over recent decades are projected to accelerate throughout the 21st century, reducing water availability, hydropower potential, and changing seasonality of flows in regions supplied by meltwater from major mountain ranges (e.g. Hindu-Kush, Himalaya, Andes), where more than one-sixth of the world population cur- rently 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 availability. 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 re- sources due to climate change. Drought-affected areas are projected to increase in extent, with the potential for adverse impacts 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 in the frequency and severity of floods and droughts are projected to adversely affect sustainable development. Increased temperatures 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 water 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 agree 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 observed large-scale 20th century runoff is used as a basis for selecting the 12 models from the multi-model ensemble. The global map of annual runoff illustrates 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 decreased dry season runoff. Studies using results from few climate models can be considerably different from the results presented here. {WGII Figure 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 statements, timelines and tempera- tures. The magnitude and timing of impacts that will ultimately be realised will vary with the amount and rate of climate change, emissions scenarios, development pathways and adaptation.

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 Fig- ure 3.6 could be associated with ‘key vulnerabilities’, based on a num- ber of criteria in the literature (magnitude, timing, persistence/ reversibility, the potential for adaptation, distributional aspects, likeli- hood and ‘importance’ of the impacts) (see Topic 5.2). {WGII SPM}

3.3.2 Impacts on regions17

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 agriculture could be reduced by up to 50%. Agricultural production, in- cluding access to food, in many African countries is projected to be severely compromised. This would further adversely af- fect food security and exacerbate malnutrition. {WGII 9.4, SPM}

� Towards the end of the 21st century, projected sea level rise will affect low-lying coastal areas with large populations. The cost of adaptation could amount to at least 5 to 10% of 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, East and South-East Asia, particularly in large river basins, is pro- jected 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 natu- ral resources and the environment associated with rapid urbanisation, industrialisation and economic development. {WGII 10.4, SPM}

� Endemic morbidity and mortality due to diarrhoeal disease pri- marily 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 drought and fire. However, in New Zealand, initial benefits are pro- jected 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 to exacerbate risks from sea level rise and increases in the sever- ity 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 con- ditions (high temperatures and drought) in a region already vul- nerable 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 de- creases in soil water are projected to lead to gradual replace- ment of tropical forest by savanna in eastern Amazonia. Semi- arid vegetation will tend to be replaced by arid-land vegeta- tion. {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 consequences for food security. In temperate zones, soybean yields are pro- jected to increase. Overall, the number of people at risk of hun- ger is projected to increase (medium confidence). {WGII 13.4, Box TS.6}

� Changes in precipitation patterns and the disappearance of gla- ciers 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 21st century. The black lines link impacts; broken-line arrows indicate impacts continuing with increasing temperature. Entries are placed so that the left-hand side of text indicates the approximate level of warming that is associated with the onset of a given impact. Quantitative entries for water scarcity and flooding represent the additional impacts of climate change relative to the conditions projected across the range of SRES scenarios A1FI, A2, B1 and B2. Adaptation to climate change is not included in these estimations. Confidence levels for all statements are high. 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 °C0 1 2 3 4

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-respirator y and infectious diseases

Increased morbidity and mor tality from heat waves, floods and droughts

Substantial burden on health ser vices

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 °C0 1 2 3 4

† 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 agriculture by 5 to 20%, but with important variability among regions. Ma- jor 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 be further challenged by an increased number, intensity and dura- tion of heat waves during the course of the century, with poten- tial 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 thick- ness and extent of glaciers, ice sheets and sea ice, and changes in natural ecosystems with detrimental effects on many organ- isms 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 pro- jected 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 infra- structure, settlements and facilities that support the livelihood of island communities. {WGII 16.4, SPM}

� Deterioration in coastal conditions, for example through ero- sion of beaches and coral bleaching, is expected to affect local resources. {WGII 16.4, SPM}

� By mid-century, climate change is expected to reduce water resources in many small islands, e.g. in the Caribbean and Pa- cific, to the point where they become insufficient to meet de- mand during low-rainfall periods. {WGII 16.4, SPM}

� With higher temperatures, increased invasion by non-native species is expected to occur, particularly on mid- and high-lati- tude 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 be- cause of sensitivity to warming; mediterranean-type ecosys- tems 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-latitudes19 and in

the dry tropics, due to changes in rainfall and evapotranspira- tion, 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 particu- larly 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 further

acidification. Projections based on SRES scenarios give a reduc- tion in average global surface ocean pH of between 0.14 and 0.35 units over the 21st century. While the effects of observed ocean acidi- fication on the marine biosphere are as yet undocumented, the pro- gressive acidification of oceans is expected to have negative im- pacts on marine shell-forming organisms (e.g. corals) and their de- pendent 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 the mid- to late 21st century. These do not take into account any changes or developments in adaptive capacity. The likelihood estimates in column two relate to the phenomena listed in column one. {WGII Table SPM.1}

Phenomenona and Likelihood of Examples of major projected impacts by sector direction of trend future trends

based on Agriculture, forestry Water resources Human health Industry, settlement projections and ecosystems {WGII 3.4} {WGII 8.2, 8.4} and society for 21st 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 warmer environments; some water supplies exposure in cities; reduced disruption to and more frequent increased insect transport due to snow, ice; hot days and nights outbreaks effects on winter tourism

Warm spells/heat Very likely Reduced yields in Increased water Increased risk of Reduction in quality of life for waves. Frequency warmer regions demand; water heat-related people in warm areas without increases over most due to heat stress; quality problems, mortality, especially appropriate housing; impacts land areas increased danger of e.g. algal blooms for the elderly, on the elderly, very young and

wildfire chronically sick, poor very young and socially isolated

Heavy precipitation Very likely Damage to crops; Adverse effects on Increased risk of Disruption of settlements, events. Frequency soil erosion, inability quality of surface deaths, injuries and commerce, transport and increases over most to cultivate land due and groundwater; infectious, respiratory societies due to flooding: areas to waterlogging of contamination of and skin diseases pressures on urban and rural

soils water supply; water infrastructures; loss of property scarcity may be relieved

Area affected by Likely Land degradation; More widespread Increased risk of Water shortage for settlements, drought increases lower yields/crop water stress food and water industry and societies;

damage and failure; shortage; increased reduced hydropower generation increased livestock risk of malnutrition; potentials; potential for deaths; increased increased risk of population migration risk of wildfire water- and food-

borne diseases

Intense tropical Likely Damage to crops; Power outages Increased risk of Disruption by flood and high cyclone activity windthrow (uprooting) causing disruption deaths, injuries, winds; withdrawal of risk increases of trees; damage to of public water supply water- and food- coverage in vulnerable areas

coral reefs borne diseases; by private insurers; potential post-traumatic for population migrations; loss stress disorders of property

Increased incidence 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 drowning in floods; relocation; potential for tsunamis)c water systems migration-related movement of populations and

health effects infrastructure; also see tropical cyclones above

Notes: a) See 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 hourly values of observed

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

systems on sea level extremes has not been assessed. {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 play a role. If a large-scale abrupt climate change were to occur, its im- pact 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 ex- pansion 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 spe- cies 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 21st 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 21stcentury. Longer-term changes in the MOC cannot be assessed with confi- dence. {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, fisher- ies, ocean CO

2 uptake, oceanic oxygen concentrations and terres-

trial 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 comple- mentary roles on a more conceptual basis over a longer timeframe.

The capacity to adapt and mitigate is dependent on socio-eco- nomic and environmental circumstances and the availability of in- formation and technology20 . 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 eco- nomic 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 and reducing their vulnerability to the impacts of weather- and climate- related events such as floods, droughts and storms. Nevertheless, additional adaptation measures will be required at regional and lo- cal 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 mag- nitude. {WGII 17.2, SPM; WGIII 1.2}

A wide array of adaptation options is available, but more ex- tensive 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 artefacts (hardware, equipment) and (social) information (‘software’, know-how for production and use of artefacts).

adaptation options by sector. Many adaptation actions have mul- tiple drivers, such as economic development and poverty allevia- tion, and are embedded within broader development, sectoral, re- gional and local planning initiatives such as water resources plan- ning, coastal defence and disaster risk reduction strategies. Ex- amples of this approach are the Bangladesh National Water Man- agement 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 adapta- tion 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 con- fidence that there are viable adaptation options that can be imple- mented in some of these sectors at low cost and/or with high ben- efit-cost ratios. Empirical research also suggests that higher ben- efit-cost ratios can be achieved by implementing some adaptation measures at an early stage compared to retrofitting long-lived in- frastructure 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 af- fected by multiple climate and non-climate stresses, as well as de- velopment 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 cli- mate change, variability and extremes. For example, a heat wave in 2003 caused high levels of mortality in European cities (especially among the elderly), and Hurricane Katrina in 2005 caused large hu- man 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

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ro te

ct io

n o

f e xi

st in

g n

a tu

ra l

b a

rr ie

rs

H e

a t-

h e

a lth

a ct

io n

p la

n s;

e

m e

rg e

n cy

m e

d ic

a l

se rv

ic e

s; i

m p

ro ve

d c

lim a

te -s

e n

si tiv

e d

is e

a se

s u

rv e

ill a

n ce

a n

d c

o n

tr o

l; sa

fe w

a te

r a

n d

i m

p ro

ve d

s a

n ita

tio n

D iv

e rs

ifi ca

tio n

o f

to u

ri sm

a tt

ra ct

io n

s a

n d

re ve

n u

e s;

s h

ift in

g s

ki s

lo p

e s

to h

ig h

e r

a lti

tu d

e s

a n

d g

la ci

e rs

; a

rt ifi

ci a

l sn

o w

-m a

ki n

g

R e

a lig

n m

e n

t/ re

lo ca

tio n

; d

e si

g n

s ta

n d

a rd

s a

n d

p la

n n

in g

f o

r ro

a d

s, r

a il

a n

d o

th e

r in

fr a

st ru

ct u

re to

c o

p e

w ith

w a

rm in

g a

n d

d ra

in a

g e

S tr

e n

g th

e n

in g

o f

o ve

rh e

a d

t ra

n sm

is si

o n

a n

d d

is tr

ib u

tio n

i n

fr a

st ru

ct u

re ;

u n

d e

rg ro

u n

d c

a b lin

g fo

r u

til iti

e s;

e n

e rg

y e

ff ic

ie n

cy ;

u se

o f

re n

e w

a b

le so

u rc

e s;

r e

d u

ce d

d e

p e

n d

e n

ce o

n s

in g

le so

u rc

e s

o f

e n

e rg

y

U n

d e

rl y

in g

p o

li c

y f

ra m

e w

o rk

N a

tio n

a l

w a

te r

p o

lic ie

s a

n d

i n

te g ra

te d

w a

te r

re so

u rc

e s

m a

n a

g e

m e

n t;

w a

te r-

re la

te d

h a

za rd

s m

a n

a g

e m

e n

t

R &

D p

o lic

ie s;

i n

st itu

tio n

a l

re fo

rm ;

la n

d t

e n

u re

a n

d l

a n

d r

e fo

rm ;

tr a

in in

g ;

ca p

a ci

ty b

u ild

in g

; cr

o p

i n

su ra

n ce

; fin

a n

ci a

l in

ce n

tiv e s,

e .g

. su

b si

d ie

s a

n d

t a

x cr

e d

its

S ta

n d

a rd

s a

n d

r e

g u

la tio

n s

th a

t in

te g ra

te cl

im a

te c

h a

n g

e c

o n

si d

e ra

tio n

s in

to d

e si

g n

; la

n d

-u se

p o

lic ie

s; b

u ild

in g

c o

d e

s; i

n su

ra n

ce

P u b lic

h e

a lth

p o

lic ie

s th

a t

re co

g n

is e

c lim

a te

ri sk

; st

re n

g th

e n

h e

a lth

s e

rv ic

e s;

r e

g io

n a

l a

n d

in te

rn a

tio n

a l

co o

p e

ra tio

n

In te

g ra

te d

p la

n n

in g

( e .g

. ca

rr yi

n g

c a

p a

ci ty

; lin

ka g

e s

w ith

o th

e r

se ct

o rs

); f

in a

n ci

a l

in ce

n -

tiv e s,

e .g

. su

b si

d ie

s a

n d

t a

x cr

e d

its

In te

g ra

tin g

c lim

a te

c h

a n

g e

c o

n si

d e

ra tio

n s

in to

n a

tio n

a l

tr a

n sp

o rt

p o

lic y;

i n ve

st m

e n

t in

R &

D f

o r

sp e

ci a

l si

tu a

tio n

s, e

.g .

p e

rm a

fr o

st a

re a

s

N a

tio n

a l

e n

e rg

y p

o lic

ie s,

r e

g u

la tio

n s,

a n

d f

is ca

l a

n d

f in

a n

ci a

l in

ce n

tiv e

s to

e n

co u

ra g

e u

se o

f a

lte rn

a tiv

e s

o u

rc e

s; i

n co

rp o

ra tin

g c

lim a

te ch

a n

g e

i n

d e

si g

n s

ta n

d a

rd s

K e y

c o

n s

tr a

in ts

a n

d o

p p

o rt

u n

it ie

s t

o im

p le

m e

n ta

ti o

n (

N o

rm a

l fo

n t

= c

o n

st ra

in ts

; ita

lic s

= o

p p

o rt

u n

iti e

s)

F in

a n

ci a

l, h

u m

a n

r e

so u

rc e

s a

n d

p h ys

ic a

l b

a rr

ie rs

; in

te g

ra te

d w

a te

r re

so u

rc e

s m

a n

a g

e m

e n

t; s

yn e

rg ie

s w

ith o

th e

r se

ct o

rs

Te ch

n o

lo g

ic a

l a

n d

f in

a n

ci a

l co

n st

ra in

ts ;

a cc

e ss

t o

n e w

v a ri

e tie

s; m

a rk

e ts

; lo

n g

e r

g ro

w in

g s

e a

so n

i n

h ig

h e

r la

tit u

d e

s; r

ev e n

u e s

fr o

m ‘n

e w

’ p ro

d u

ct s

F in

a n

ci a

l a

n d

t e

ch n

o lo

g ic

a l

b a

rr ie

rs ;

a va

ila b

ili ty

o f

re lo

ca tio

n s

p a

ce ;

in te

g ra

te d

p o

lic ie

s a

n d

m a

n a

g e

m e

n t;

s yn

e rg

ie s

w ith

su st

a in

a b le

d e ve

lo p

m e

n t

g o

a ls

L im

its t

o h

u m

a n

t o

le ra

n ce

( vu

ln e

ra b le

g ro

u p

s) ;

kn o

w le

d g

e l

im ita

tio n

s; f

in a

n ci

a l

ca p

a ci

ty ;

u p g ra

d e

d h

e a

lth s

e rv

ic e

s; im

p ro

ve d

q u

a lit

y o

f lif

e

A p

p e

a l/m

a rk

e tin

g o

f n

e w

a tt

ra ct

io n

s; fin

a n

ci a

l a

n d

l o

g is

tic a

l ch

a lle

n g

e s;

p o

te n

tia l

a d ve

rs e

i m

p a

ct o

n o

th e

r se

ct o

rs (

e .g

. a

rt ifi

ci a

l sn

o w

-m a

ki n

g m

a y

in cr

e a

se e

n e

rg y

u se

); r

ev e n

u e

s fr

o m

‘ n

e w

’ a tt

ra ct

io n

s; in

vo lv

e m

e n

t o

f w

id e

r g

ro u

p o

f st

a ke

h o

ld e

rs

F in

a n

ci a

l a

n d

t e

ch n

o lo

g ic

a l

b a

rr ie

rs ;

a va

ila b

ili ty

o f

le ss

v u

ln e

ra b le

r o

u te

s; im

p ro

ve d

t e

ch n

o lo

g ie

s a

n d

i n

te g ra

tio n

w ith

ke y

se ct

o rs

( e .g

. e

n e

rg y)

A cc

e ss

t o

v ia

b le

a lte

rn a

tiv e

s; f

in a

n ci

a l

a n

d te

ch n

o lo

g ic

a l

b a

rr ie

rs ;

a cc

e p

ta n

ce o

f n

e w

te ch

n o

lo g

ie s;

s tim

u la

tio n

o f

n e w

t e

ch n

o lo

- g

ie s;

u se

o f

lo ca

l re

so u

rc e

s

N o

te :

O th

e r

e xa

m p

le s

fr o

m m

a n

y se

ct o

rs w

o u

ld i

n cl

u d

e e

a rl

y w

a rn

in g

s ys

te m

s.

S e

c to

r

W a

te r

{W G

II ,

5 .5

, 1

6 .4

; Ta

b le

s 3

.5 ,

1 1

.6 ,1

7 .1

}

A g

ri c

u lt

u re

{W G

II 1

0 .5

, 1

3 .5

; Ta

b le

1 0

.8 }

In fr

a s

tr u

c tu

re /

s e

tt le

m e

n t

(i n

c lu

d in

g c

o a

s ta

l zo

n e

s )

{W G

II 3

.6 ,

1 1

.4 ;

Ta b

le s

6 .1

1 ,

1 7

.1 }

H u

m a

n h

e a

lt h

{W G

II 1

4 .5

, Ta

b le

1 0

.8 }

T o

u ri

s m

{W G

II 1

2 .5

, 1

5 .5

, 1

7 .5

; Ta

b le

1 7

.1 }

T ra

n s

p o

rt {W

G II

7 .6

, 1

7 .2

}

E n

e rg

y {W

G II

7 .4

, 1

6 .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 studies21 indicate that there is high agreement and much evidence of substantial eco- nomic potential21 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 costs22 have the potential to reduce emissions by about 6 GtCO

2 -eq/yr in 2030. Realising these requires dealing with imple-

mentation 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 mar- ginal 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 mitigation potential in any sector. Table 4.2 lists selected examples of key tech- nologies, policies, constraints and opportunities by sector. {WGIII SPM}

Future energy infrastructure investment decisions, expected to total over US$20 trillion23 between 2005 and 2030, will have long- term impacts on GHG emissions, because of the long lifetimes of energy plants and other infrastructure capital stock. The widespread diffusion of low-carbon technologies may take many decades, even if early investments in these technologies are made attractive. Ini- tial estimates show that returning global energy-related CO

2 emis-

sions to 2005 levels by 2030 would require a large shift in the pat- tern 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). Mitigation potential is further differen- tiated in terms of ‘market mitigation potential’ and ‘economic mitigation potential’.

Market mitigation potential is the mitigation potential based on private costs and private discount rates (reflecting the perspective of private consumers and companies ), which might be expected to occur under forecast market conditions, including policies and measures currently in place, noting that barriers limit actual uptake. Economic mitigation potential is the mitigation potential that takes into account social costs and benefits and social discount rates (reflecting the perspective of society; social discount rates are lower than those used by private investors ), assuming that market efficiency is improved by policies and measures and barriers are removed. Mitigation potential is estimated using different types of approaches. Bottom-up studies are based on assessment of mitigation options, emphasising specific technologies and regulations. They are typically sectoral studies taking the macro-economy as unchanged. Top-down studies assess the economy-wide potential of mitigation options. They use globally consistent frameworks and aggregated information about mitigation options and capture macro-economic and market feedbacks.

22 Net negative costs (no regrets opportunities) are defined as those options whose benefits such as reduced energy costs and reduced 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 = 20×1012

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 the projected emissions increases from SRES scenarios relative to year 2000 GHG emissions of 40.8 GtCO

2 -eq (Panel c). Note: GHG emissions in 2000 are exclusive of

emissions of decay of above-ground biomass that remains after logging and deforestation and from peat fires and drained peat soils, to ensure consistency with the SRES emissions results. {WGIII Figures SPM.4, SPM.5a, SPM.5b}

0

5

10

15

20

25

30

35

A1FI A2 A1B A1T B1B2

Gt CO2-eq

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

low end of range high end of range 0

5

10

15

20

25

30

35

a)

low end of range high end of range 0

5

10

15

20

25

30

35

< 20 < 50 < 100 US$/tCO2-eq b) Increase in GHG emissions

above year 2000 levels Bottom-up Top-down

Gt CO2-eq Gt CO2-eq

E st

im a

te d

m iti

g a

tio n

p o

te n

tia l i

n 2

0 3

0

E st

im a

te d

m iti

g a

tio n

p o

te n

tia l i

n 2

0 3

0

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 sector. b) The estimated potentials have been constrained by the availability of studies particularly at high carbon price levels. c) Sectors used different baselines. For industry the SRES B2 baseline was taken, for energy supply and transport the World Energy Outlook (WEO) 2004

baseline was used; the building sector is based on a baseline in between SRES B2 and A1B; for waste, SRES A1B driving forces were used to construct a waste-specific baseline; agriculture and forestry used baselines that mostly used B2 driving forces.

d) Only global totals for transport are shown because international aviation is included. e) Categories excluded are non-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 transport, most high-cost options for buildings, wastewater treatment, emission reduction from coal mines and gas pipelines, and fluorinated gases from energy supply and transport. The underestimation of the total economic potential from these emissions is of the order of 10 to 15%.

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 syn- ergies 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 children and decrease the unsustainable use of fuelwood and related defor- estation. {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 and prices and lower GDP growth due to mitigation policies. The ex- tent 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 leak- age. Most equilibrium modelling supports the conclusion in the TAR of economy-wide leakage from Kyoto action in the order of 5 to 20%, which would be less if competitive low-emissions tech- nologies 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 miti- gation 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

T a b

le 4

.2

S e

le ct

e d

e xa

m p

le s

o f

ke y

se ct

o ra

l m

iti g

a tio

n t

e ch

n o

lo g

ie s,

p o

lic ie

s a

n d

m e

a su

re s,

c o

n st

ra in

ts a

n d

o p

p o

rt u

n iti

e s.

{ W

G II

I Ta

b le

s S

P M

.3 ,

S P

M .7

}

K e y

m it

ig a

ti o

n t

e c h

n o

lo g

ie s

a n

d p

ra c

ti c

e s

c u

rr e

n tl

y c

o m

m e

rc ia

ll y

a va

il a

b le

. K

e y

m it

ig a

ti o

n t

e c h

n o

lo g

ie s

a n

d p

ra c

ti c

e s

p ro

je c

te d

t o

b e

c o

m m

e rc

ia li

s e

d b

e fo

re 2

0 3

0 s

h o

w n

i n

i ta

li c

s .

Im p

ro ve

d s

u p

p ly

a n

d d

is tr

ib u

tio n

e ff

ic ie

n cy

; fu

e l

sw itc

h in

g f

ro m

c o

a l

to g

a s;

n u

cl e

a r

p o w

e r;

r e

n e w

a b le

h e

a t

a n

d p

o w

e r

(h yd

ro p

o w

e r,

s o

la r,

w in

d ,

g e

o th

e rm

a l

a n

d b

io e

n e

rg y)

; co

m b

in e

d h

e a

t a

n d

p o

w e r;

e a

rl y

a p

p lic

a tio

n s

o f

ca rb

o n

d io

xi d

e c

a p

tu re

a n

d s

to ra

g e

( C

C S

) (e

.g . st

o ra

g e

o f

re m

o ve

d C

O 2 f

ro m

n a

tu ra

l g

a s)

; C

C S

f o

r g

a s,

b io

m a

ss a

n d

c o

a l-

fir e

d e

le ct

ri ci

ty g

e n

e ra

tin g

f a

ci lit

ie s;

a d

va n

ce d

n u

cl e

a r

p o w

e r;

a d va

n ce

d r

e n

e w

a b

le e

n e

rg y,

i n

cl u

d in

g t

id a

l a

n d

w a ve

e n

e rg

y, c

o n

ce n

tr a

tin g

s o

la r,

a n

d s

o la

r p

h o

to vo

lta ic

s

M o

re f

u e

l- e

ff ic

ie n

t ve

h ic

le s;

h yb

ri d

v e

h ic

le s;

c le

a n

e r

d ie

se l

ve h

ic le

s; b

io fu

e ls

; m

o d

a l

sh ift

s fr

o m

r o

a d

t ra

n sp

o rt

t o

r a

il a

n d

p u

b lic

t ra

n sp

o rt

s ys

te m

s; n

o n

-m o

to ri

se d

tr a

n sp

o rt

( cy

cl in

g ,

w a

lk in

g );

l a

n d

-u se

a n

d t

ra n

sp o

rt p

la n

n in

g ;

se co

n d

g e

n e

ra tio

n b

io fu

e ls

; h

ig h

e r

e ff

ic ie

n cy

a ir

cr a

ft ;

a d va

n ce

d e

le ct

ri c

a n

d h

yb ri

d v

e h

ic le

s w

ith m

o re

p o w

e rf

u l

a n

d r

e lia

b le

b a

tt e

ri e

s

E ff

ic ie

n t

lig h

tin g

a n

d d

a yl

ig h

tin g

; m

o re

e ff

ic ie

n t

e le

ct ri

ca l

a p

p lia

n ce

s a

n d

h e

a tin

g a

n d

c o

o lin

g d

e vi

ce s;

i m

p ro

ve d

c o

o k

st o ve

s, i

m p

ro ve

d i

n su

la tio

n ;

p a

ss iv

e a

n d

a ct

iv e

so la

r d

e si

g n

f o

r h

e a

tin g

a n

d c

o o

lin g

; a

lte rn

a tiv

e r

e fr

ig e

ra tio

n f

lu id

s, r

e co

ve ry

a n

d re

cy cl

in g

o f

flu o

ri n

a te

d g

a se

s; i

n te

g ra

te d

d e

si g

n o

f co

m m

e rc

ia l

b u

ild in

g s

in cl

u d

in g

te ch

n o

lo g

ie s,

s u

ch a

s in

te lli

g e

n t

m e

te rs

t h

a t

p ro

vi d

e f

e e

d b

a ck

a n

d c

o n

tr o

l; so

la r

p h

o to

vo lta

ic s

in te

g ra

te d

i n

b u

ild in

g s

M o

re e

ff ic

ie n

t e

n d

-u se

e le

ct ri

ca l

e q

u ip

m e

n t; h

e a

t a

n d

p o w

e r

re co

ve ry

; m

a te

ri a

l re

cy cl

in g

a n

d s

u b

st itu

tio n

; co

n tr

o l o

f n

o n

-C O

2 g

a s

e m

is si

o n

s; a

n d

a w

id e

a rr

a y

o f

p ro

ce ss

-s p

e ci

fic t

e ch

n o

lo g

ie s;

a d va

n ce

d e

n e

rg y

e ff

ic ie

n cy

; C

C S

f o

r ce

m e

n t,

a m

m o

n ia

, a

n d

i ro

n m

a n u fa

ct u

re ;

in e

rt e

le ct

ro d

e s

fo r

a lu

m in

iu m

m a

n u fa

ct u

re

Im p

ro ve

d c

ro p

a n

d g

ra zi

n g

l a

n d

m a

n a

g e

m e

n t

to i n

cr e

a se

s o

il ca

rb o

n s

to ra

g e

; re

st o

ra tio

n o

f cu

lti va

te d

p e

a ty

s o

ils a

n d

d e

g ra

d e

d l

a n

d s;

i m

p ro

ve d

r ic

e c

u lti

va tio

n te

ch n

iq u

e s

a n

d l

iv e

st o

ck a

n d

m a

n u

re m

a n

a g

e m

e n

t to

r e

d u

ce C

H 4 e

m is

si o

n s;

im p

ro ve

d n

itr o

g e

n f

e rt

ili se

r a

p p

lic a

tio n

t e

ch n

iq u

e s

to r

e d

u ce

N 2 O

e m

is si

o n

s; d

e d

ic a

te d

e n

e rg

y cr

o p

s to

r e

p la

ce f

o ss

il fu

e l u

se ;

im p

ro ve

d e

n e

rg y

e ff

ic ie

n cy

; im

p ro

ve m

e n

ts o

f cr

o p

y ie

ld s

A ff o

re st

a tio

n ;

re fo

re st

a tio

n ;

fo re

st m

a n

a g

e m

e n

t; r

e d

u ce

d d

e fo

re st

a tio

n ;

h a

rv e

st e

d w

o o

d p

ro d

u ct

m a

n a

g e

m e

n t; u

se o

f fo

re st

ry p

ro d

u ct

s fo

r b

io e

n e

rg y

to r

e p

la ce

f o

ss il

fu e

l u

se ;

tr e

e s

p e

ci e

s im

p ro

ve m

e n

t to

i n

cr e

a se

b io

m a

ss p

ro d

u ct

iv ity

a n

d c

a rb

o n

se q

u e

st ra

tio n

; im

p ro

ve d

r e

m o

te s

e n

si n

g t

e ch

n o

lo g

ie s

fo r

a n

a ly

si s

o f

ve g

e ta

tio n

/s o

il ca

rb o

n s

e q

u e

st ra

tio n

p o

te n

tia l

a n

d m

a p

p in

g l

a n

d -u

se c

h a

n g

e

L a

n d

fil l C

H 4 r

e co

ve ry

; w

a st

e i

n ci

n e

ra tio

n w

ith e

n e

rg y

re co

ve ry

; co

m p

o st

in g

o f

o rg

a n

ic w

a st

e ;

co n

tr o

lle d

w a

st e w

a te

r tr

e a

tm e

n t; r

e cy

cl in

g a

n d

w a

st e

m in

im is

a tio

n ;

b io

co ve

rs a

n d

b io

fil te

rs t

o o

p tim

is e

C H

4 o

xi d

a tio

n

P o

li c

ie s

, m

e a

s u

re s

a n

d i

n s

tr u

m e

n ts

s h

o w

n t

o b

e e n

v ir

o n

m e

n ta

ll y

e ff

e c

ti v

e

R e

d u

ct io

n o

f fo

ss il

fu e

l su

b si

d ie

s; t

a xe

s o

r ca

rb o

n c

h a

rg e

s o

n f

o ss

il fu

e ls

F e

e d

-i n

t a

ri ff

s fo

r re

n e w

a b le

e n

e rg

y te

ch n

o lo

g ie

s; re

n e w

a b le

e n

e rg

y o

b lig

a tio

n s;

p ro

d u

ce r

su b

si d

ie s

M a

n d

a to

ry f

u e

l e

co n

o m

y; b

io fu

e l

b le

n d

in g

a n

d C

O 2

st a

n d

a rd

s fo

r ro

a d

t ra

n sp

o rt

Ta xe

s o

n v

e h

ic le

p u

rc h

a se

, re

g is

tr a

tio n

, u

se a

n d

m o

to r

fu e

ls ;

ro a

d a

n d

p a

rk in

g p

ri ci

n g

In flu

e n

ce m

o b

ili ty

n e

e d

s th

ro u

g h

l a

n d

-u se

r e

g u

la tio

n s

a n

d in

fr a

st ru

ct u

re p

la n

n in

g ;

in ve

st m

e n

t in

a tt

ra ct

iv e

p u

b lic

tr a

n sp

o rt

f a

ci lit

ie s

a n

d n

o n

-m o

to ri

se d

f o rm

s o

f tr

a n

sp o

rt

A p

p lia

n ce

s ta

n d

a rd

s a

n d

l a

b e

lli n

g

B u

ild in

g c

o d

e s

a n

d c

e rt

ifi ca

tio n

D e

m a

n d

-s id

e m

a n

a g

e m

e n

t p

ro g

ra m

m e

s

P u b lic

s e

ct o

r le

a d

e rs

h ip

p ro

g ra

m m

e s,

i n

cl u

d in

g p

ro cu

re m

e n

t

In ce

n tiv

e s

fo r

e n

e rg

y se

rv ic

e c

o m

p a

n ie

s (E

S C

O s)

P ro

vi si

o n

o f

b e

n ch

m a

rk i

n fo

rm a

tio n

; p

e rf

o rm

a n

ce st

a n

d a

rd s;

s u

b si

d ie

s; t

a x

cr e

d its

Tr a

d a

b le

p e

rm its

V o

lu n

ta ry

a g re

e m

e n

ts

F in

a n

ci a

l in

ce n

tiv e

s a

n d

r e

g u

la tio

n s

fo r

im p

ro ve

d l

a n

d m

a n

a g

e m

e n

t; m

a in

ta in

in g

s o

il ca

rb o

n c

o n

te n

t; e

ff ic

ie n

t u

se o

f fe

rt ili

se rs

a n

d i

rr ig

a tio

n

F in

a n

ci a

l in

ce n

tiv e

s (n

a tio

n a

l a

n d

i n

te rn

a tio

n a

l) t

o in

cr e

a se

f o

re st

a re

a ,

to r

e d

u ce

d e

fo re

st a

tio n

a n

d t

o m

a in

ta in

a n

d m

a n

a g

e f

o re

st s;

l a

n d

-u se

r e

g u

la tio

n a

n d

e n

fo rc

e m

e n

t

F in

a n

ci a

l in

ce n

tiv e

s fo

r im

p ro

ve d w

a st

e a

n d

w a

st e w

a te

r m

a n

a g

e m

e n

t

R e

n e

w a b le

e n

e rg

y in

ce n

tiv e

s o

r o

b lig

a tio

n s

W a

st e

m a

n a

g e

m e

n t

re g

u la

tio n

s

K e y

c o

n s

tr a

in ts

o r

o p

p o

rt u

n it

ie s

(N o

rm a

l fo

n t

= c

o n

st ra

in ts

; ita

lic s

= o

p p

o rt

u n

iti e

s)

R e

si st

a n

ce b

y ve

st e

d i

n te

re st

s m

a y

m a

ke t

h e

m d

iff ic

u lt

to i

m p

le m

e n

t

M a y

b e

a p

p ro

p ri

a te

t o

c re

a te

m a

rk e

ts f

o r

lo w

- e

m is

si o

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te ch

n o

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ie s

P a rt

ia l co

ve ra

g e

o f

ve h

ic le

f le

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m a y

lim it

e ff e

ct iv

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E ff e

ct iv

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e ss

m ay

d ro

p w

ith h

ig h

e r

in co

m e

s

P a rt

ic u

la rl

y a

p p

ro p

ri a

te f

o r

co u

n tr

ie s

th a

t a

re b u

ild in

g u

p t

h e

ir t

ra n

sp o

rt a

tio n

s ys

te m

s

P e ri

o d

ic r

e vi

si o

n o

f st

a n

d a

rd s

n e

e d

e d

A tt

ra ct

iv e

f o

r n

e w

b u

ild in

g s.

E n

fo rc

e m

e n

t ca

n b

e d

if fi cu

lt

N e e d f o r

re g u la

tio n s

so t

h a t

u til

iti e s

m ay

p ro

fit

G o ve

rn m

e n

t p

u rc

h a

si n

g c

a n

e xp

a n

d d

e m

a n

d f

o r

e n

e rg

y- e

ff ic

ie n

t p

ro d

u ct

s

S u

cc e

ss f

a ct

o r:

A cc

e ss

t o

t h

ir d

p a

rt y

fin a

n ci

n g

M a y

b e

a p

p ro

p ri

a te

t o

s tim

u la

te t

e ch

n o

lo g

y u

p ta

ke .

S ta

b ili

ty o

f n

a tio

n a

l p

o lic

y im

p o

rt a

n t

in v

ie w

o f

in te

rn a

tio n

a l

co m

p e

tit iv

e n

e ss

P re

d ic

ta b le

a llo

ca tio

n m

e ch

a n

is m

s a

n d

s ta

b le

p ri

ce s

ig n

a ls

i m

p o

rt a

n t

fo r

in ve

st m

e n

ts

S u

cc e

ss f

a ct

o rs

i n

cl u

d e

: cl

e a

r ta

rg e

ts ,

a b

a se

lin e

sc e

n a

ri o ,

th ir

d -p

a rt

y in

vo lv

e m

e n

t in

d e

si g

n a

n d

re vi

e w

a n

d f

o rm

a l

p ro

vi si

o n

s o

f m

o n

ito ri

n g

, cl

o se

co o

p e

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n b

e tw

e e

n g

o ve

rn m

e n

t a

n d

i n

d u

st ry

M a y

e n

co u

ra g

e s

yn e

rg y

w ith

s u

st a

in a

b le

d e ve

lo p

m e

n t

a n

d w

ith r

e d

u ci

n g

v u

ln e

ra b

ili ty

t o

cl im

a te

c h

a n

g e ,

th e

re b y

o ve

rc o

m in

g b

a rr

ie rs

t o

im p

le m

e n

ta tio

n

C o

n st

ra in

ts i

n cl

u d

e l

a ck

o f

in ve

st m

e n

t ca

p ita

l a

n d

la n

d t

e n u

re i

ss u

e s.

C a

n h

e lp

p o ve

rt y

a lle

vi a

tio n

.

M a y

st im

u la

te t

e ch

n o

lo g

y d

iff u

si o

n

L o

ca l

a va

ila b

ili ty

o f

lo w

-c o

st f

u e

l

M o

st e

ff e

ct iv

e ly

a p

p lie

d a

t n

a tio

n a

l le

ve l

w ith

e n

fo rc

e m

e n

t st

ra te

g ie

s

S e

c to

r

E n

e rg

y S

u p

p ly

{W G

II I

4 .3

, 4

.4 }

T ra

n s

p o

rt {W

G II

I 5

.4 }

B u

il d

in g

s {W

G II

I 6

.5 }

In d

u s

tr y

{W G

II I

7 .5

}

A g

ri c

u lt

u re

{W G

II I

8 .4

}

F o

re s

tr y

/f o

re s

ts {W

G II

I 9

.4 }

W a

s te

{ W

G II

I 1

0 .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 in- duced 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 certainty about emission levels. They may be preferable to other instru- ments 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 guar- antee 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 frequently used by governments to stimulate the development and diffu- sion of new technologies. While economic costs are generally higher than for the instruments listed above, they are often criti- cal 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 emis- sions reductions beyond business as usual. However, some re- cent agreements, in a few countries, have accelerated the appli- cation of best available technology and led to measurable emis- sion reductions.

� Information instruments (e.g. awareness campaigns) may posi- tively affect environmental quality by promoting informed choices and possibly contributing to behavioural change, how- ever, their impact on emissions has not been measured yet.

� Research, development and demonstration (RD&D) can stimu- late 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. On their own, they generally have limited impact on national- or re- gional-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 renew- able energy are often economically beneficial, improve energy se- curity 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 and bioenergy plantations can restore degraded land, manage water run- off, retain soil carbon and benefit rural economies, but could com- pete 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 pro- duction, 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 emissions due to increased consumption of energy related to adaptive re- sponses. {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 thus perfect implementation of mitigation measures throughout the 21st century. Costs are given for a specific point in time. Global modelled costs will increase if some regions, sectors (e.g. land use), options or gases are excluded. Global modelled costs will decrease with lower baselines, use of revenues from carbon taxes and auctioned permits, and if induced technological learning is included. These models do not consider climate benefits and generally also co-benefits of mitigation measures, or equity issues. Significant progress has been achieved in applying approaches based on induced technological change to stabilisation studies; however, conceptual issues remain. In the models that consider induced technological change, projected costs for a given stabilisation level are reduced; the reductions are greater at lower stabilisation level.

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 2012 without emissions trading and 0.1 to 1.1% lower GDP with emis- sions trading among Annex-B countries. To be more environmen- tally 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 re- duce global costs for achieving a given level of mitigation, or will improve environmental effectiveness. Improving and expanding the scope of market mechanisms (such as emission trading, Joint Imple- mentation 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 regional actions; RD&D programmes; adopting common policies; imple- menting development-oriented actions; or expanding financing in- struments. 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 Potentially affects:

Macro-economy Implement non-climate taxes/subsidies and/or other fiscal and Total global GHG emissions regulatory policies that promote sustainable development

Forestry Adoption of forest conservation and sustainable management practices GHG emissions from deforestation

Electricity Adoption 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 improve energy security imports

Insurance for building, Differentiated premiums, liability insurance exclusions, Transport and building sector GHG transport sectors improved terms for green products emissions

International finance Country and sector strategies and project lending that reduces emissions Emissions from developing countries

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 informa- tion about not only impacts resulting from the most likely climate sce- narios, but also impacts arising from lower-probability but higher-con- sequence events and the consequences of proposed policies and mea- sures. 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 func- tion of exposure, sensitivity and adaptive capacity. Adaptation can re- duce sensitivity to climate change while mitigation can reduce the exposure to climate change, including its rate and extent. Both conclu- sions are confirmed in this assessment. {WGII 20.2, 20.7.3}

No single metric can adequately describe the diversity of key vulnerabilities or support their ranking. A sample of relevant im- pacts is provided in Figure 3.6. The estimation of key vulnerabili- ties 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, and adaptive capacity. Some key vulnerabilities may be linked to thresh- olds; in some cases these may cause a system to shift from one state to another, whereas others have thresholds that are defined subjec- tively 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 key vulnerabilities and to “aid readers in making their own determina- tion” 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 on unique and vulnerable systems (such as polar and high moun- tain communities and ecosystems), with increasing levels of adverse impacts as temperatures increase further. An increas- ing 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 in- creased risk of extinction if increases in global average tem- perature exceed 1.5 to 2.5°C over 1980-1999 levels. Confidence has increased that a 1 to 2°C increase in global mean tempera- ture 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 atmo- sphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved 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 UNFCCC involves value judgements. Science can support informed decisions on this issue, including by providing criteria for judging which vulnerabilities might be labelled ‘key’. {SYR 3.3, WGII 19.ES}

Key vulnerabilities25 may be associated with many climate-sensitive systems, including food supply, infrastructure, health, water resources, coastal systems, ecosystems, global biogeochemical cycles, ice sheets and modes of oceanic and atmospheric circulation. {WGII 19.ES}

More specific information is now available across the regions of the world concerning the nature of future impacts, including for 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 sys- tems including many biodiversity hotspots. Corals are vulner- able to thermal stress and have low adaptive capacity. Increases in sea surface temperature of about 1 to 3°C are projected to result in more frequent coral bleaching events and widespread mortality, unless there is thermal adaptation or acclimatisation by corals. Increasing vulnerability of Arctic indigenous com- munities 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 in the TAR. There is now higher confidence in the projected in- creases in droughts, heat waves and floods, as well as their ad- verse 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 regional patterns of climate change (see Topic 3.2) and in the projec- tions of regional impacts, enabling better identification of par- ticularly vulnerable systems, sectors and regions (see Topic 3.3). Moreover, there is increased evidence that low-latitude and less- developed areas generally face greater risk, for example in dry areas and megadeltas. New studies confirm that Africa is one of the most vulnerable continents because of the range of pro- jected 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 market- based 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 af- fect 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 20th

century, with loss of coastal area and associated impacts. There is better understanding than in the TAR that the risk of addi- tional 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 obser- vations 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 natural ecosystems (e.g. loss of Arctic sea ice and marine ecosystem vi- ability), the disappearance of mountain glaciers that play vital roles in water storage and supply, or adaptation to sea level rise of sev- eral metres27. 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 to adapt. Reliance on adaptation alone could eventually lead to a mag- nitude of climate change to which effective adaptation is not pos- sible, 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 level rise, the resources required are so unevenly distributed 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 Table 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 thermal expansion would continue for many centuries at a rate that eventu- ally decreases from that reached before stabilisation, due to ongo- ing heat uptake by oceans. {SYR 3.2, WGI 10.3, 10.4, 10.7, SPM}

Delayed emission reductions significantly constrain the oppor- tunities 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 short term would avoid locking in both long-lived carbon intensive in- frastructure and development pathways, reduce the rate of climate change and reduce the adaptation needs associated with higher lev- els 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 likely equilibrium global average temperature increase above pre-industrial (right- hand panel). Approaching equilibrium can take several centuries, especially for scenarios with higher levels of stabilisation. Coloured shadings show stabilisation scenarios grouped according to different targets (stabilisation category I to VI). The right-hand panel shows ranges of global average tempera- ture change above pre-industrial, using (i) ‘best estimate’ climate sensitivity of 3°C (black line in middle of shaded area), (ii) upper bound of likely range of climate sensitivity of 4.5°C (red line at top of shaded area) (iii) lower bound of likely range of climate sensitivity of 2°C (blue line at bottom of shaded area). Black dashed lines in the left panel give the emissions range of recent baseline scenarios published since the SRES (2000). Emissions ranges of the stabilisation scenarios comprise CO

2 -only and multigas scenarios and correspond to the 10th to 90th percentile of the full scenario distribution. Note: CO

2

emissions in most models do not include emissions from decay of above ground biomass that remains after logging and deforestation, and from peat fires and drained peat soils. {WGIII Figures SPM.7 and SPM.8}

E q

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a se

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

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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 underestimated 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 15th to 85th 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 stabilisation of GHG

concentrations due to the inertia of the climate system. For the majority of scenarios assessed, stabilisation of GHG concentrations occurs between 2100 and 2150 (see also Footnote 30).

f) Equilibrium sea level rise is for the contribution from ocean thermal expansion only and does not reach equilibrium for at least many centuries. These values have been estimated using relatively simple climate models (one low-resolution AOGCM and several EMICs based on the best estimate of 3°C climate sensitivity) and do not include contributions from melting ice sheets, glaciers and ice caps. Long-term thermal expansion is projected to result in 0.2 to 0.6m per degree Celsius of global average warming above pre-industrial. (AOGCM refers to Atmosphere-Ocean General Circulation Model and EMICs to Earth System Models of Intermediate Complexity.)

global average temperature increases, using the ‘best estimate’ of climate sensitivity (see Figure 5.1 for the likely range of uncer- tainty). 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 sen- sitivity 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 21st cen- tury (Table 5.1). If GHG and aerosol concentrations had been stabilised at year 2000 levels, thermal expansion alone would be expected to lead to further sea level rise of 0.3 to 0.8m. The even- tual contributions from Greenland ice sheet loss could be several metres, and larger than from thermal expansion, should warming in excess of 1.9 to 4.6°C above pre-industrial be sustained over many centuries. These long-term consequences would have major impli-

cations for world coastlines. The long time scale of thermal expan- sion and ice sheet response to warming imply that mitigation strat- egies that seek to stabilise GHG concentrations (or radiative forc- ing) 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, 450ppm31 could re-

quire cumulative emissions over the 21st 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 scenarios. For most stabilisation levels global average temperature is approaching the equilibrium level over a few centuries. For the much lower stabilisation scenarios (category I and II, Figure 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-hand panel). The figure shows illustrative scenarios from four models (AIM, IMAGE, 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 reduc-

tions 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 reductions 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 contribu- tion of the portfolio of mitigation options. The contribution of dif- ferent 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 invest-

ments and substantially more rapid diffusion and commercialisation of advanced low-emissions technologies over the next decades

(2000-2030) and higher contributions across abatement options in the long term (2000-2100). This requires that barriers to develop- ment, 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 technology transfer, it may be difficult to achieve emission reduction at a sig- nificant scale. Mobilising financing of incremental costs of low- carbon technologies is important. {WGIII 13.3, SPM}

There are large uncertainties concerning the future contribu- tion of different technologies. However, all assessed stabilisation scenarios concur that 60 to 80% of the reductions over the course of the century would come from energy supply and use and indus- trial processes. Including non-CO

2 and CO

2 land-use and forestry

mitigation options provides greater flexibility and cost-effective- ness. Energy efficiency plays a key role across many scenarios for most regions and time scales. For lower stabilisation levels, sce- narios put more emphasis on the use of low-carbon energy sources, such as renewable energy, nuclear power and the use of CO

2 cap-

ture and storage (CCS). In these scenarios, improvements of car- bon 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 nearly 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 glo- bal 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 corre- sponds to slowing average annual global GDP growth by less than 0.12 percentage points. Estimated GDP losses by 2030 are on aver- age 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 eco- nomic 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 underestimate the damage costs because they cannot include many non-quantifi- able impacts. It is virtually certain that aggregate estimates of costs mask significant differences in impacts across sectors, regions, coun- tries and populations. In some locations and amongst some groups of people with high exposure, high sensitivity and/or low adaptive capacity, net costs will be significantly larger than the global aver- age. {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 in different places and at different points in time into a global aggre- gate 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 evalu- ate 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 numbers. a) Global GDP based on market exchange rates. b) The 10th and 90th percentile range of the analysed data are given where applicable. Negative values indicate GDP gain. The first row (445-535ppm

CO 2 -eq) gives the upper bound estimate of the literature only.

c) The calculation of the reduction of the annual growth rate is based on the average reduction during the assessed period that would result in the indicated GDP decrease by 2030 and 2050 respectively.

d) The number of studies is relatively small and they generally use low baselines. High emissions baselines generally lead to higher 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 future human-induced climate change and its associated impacts are de- termined 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 21st century. Some of the impacts at the high temperature end of Figure 3.6 could be avoided by socio-economic development path- ways 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, including water, soil and air pollution, health hazards, disaster risk, and de- forestation. 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). At present, however, few plans for promoting sustainability have ex- plicitly included either adapting to climate change impacts, or pro- moting adaptive capacity. Similarly, changing development paths can make a major contribution to mitigation but may require re- sources 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 evi- dent from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising glo- bal 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 changes in many physical and biological systems are consistent with warm- ing. 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 and 2004. As a result of anthropogenic emissions, atmospheric concen- trations 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, includ- ing drought, tropical cyclones, extreme temperatures and the fre- quency and intensity of precipitation, is more difficult than for cli- matic averages as longer data time-series of higher spatial and tem- poral 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 ob- served temperature changes to natural or human causes at smaller than continental scales. At these smaller scales, factors such as land- use change and pollution also complicate the detection of anthro- pogenic 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 uncertain-

ties. {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 sus- tainable development practices, global GHG emissions will con- tinue 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 21st century that would very likely be larger than those observed during the 20th 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 up- take 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 sufficiently for GHG concentrations to stabilise, due to the time scales associ- ated 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 especially affected by climate change. The systems and sectors are some eco- systems (tundra, boreal forest, mountain, mediterranean-type, man- groves, salt marshes, coral reefs and the sea-ice biome), low-lying coasts, water resources in some dry regions at mid-latitudes and in the dry topics and in areas dependent on snow and ice melt, agri- culture 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 those with high incomes, some people, areas and activities can be par- ticularly 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 have demonstrated the vulnerability of some sectors and regions, includ- ing 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}

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Topic 6 Robust findings, key uncertainties

Key uncertainties

Uncertainty in the equilibrium climate sensitivity creates un- certainty in the expected warming for a given CO

2 -eq stabilisation

scenario. Uncertainty in the carbon cycle feedback creates uncer- tainty 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 of different feedbacks in the climate system, particularly cloud feed- backs, oceanic heat uptake and carbon cycle feedbacks, although progress has been made in these areas. Also, the confidence in pro- jections 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 uncer- tainty 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 21st century cannot be reliably assessed because of uncertainties in the meltwa- ter 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 re- gional projections of climate change, particularly precipitation. {WGII TS.6}

Understanding of low-probability/high-impact events and the cumulative impacts of sequences of smaller events, which is re- quired 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 cli- mate 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 pro- jected to be available by 2030 in all sectors. The economic mitiga- tion potential, at costs that range from net negative up to US$100/ tCO

2 -equivalent, is sufficient to offset the projected growth of glo-

bal emissions or to reduce emissions to below current levels in 2030. {WGIII 11.3, SPM}

Many impacts can be reduced, delayed or avoided by mitiga- tion. Mitigation efforts and investments over the next two to three decades will have a large impact on opportunities to achieve lower stabilisation levels. Delayed emissions reductions significantly con- strain 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 portfolio of technologies that are currently available and those that are ex- pected to be commercialised in coming decades, provided that ap- propriate and effective incentives are in place and barriers are re- moved. In addition, further RD&D would be required to improve the technical performance, reduce the costs and achieve social ac- ceptability 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 develop- ment paths can make a major contribution to climate change miti- gation 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 in- formation 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 capac- ity 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 dependent on specific geographical and climate risk factors as well as institu- tional, political and financial constraints. {WGII SPM}

Estimates of mitigation costs and potentials depend on assump- tions about future socio-economic growth, technological change and consumption patterns. Uncertainty arises in particular from assumptions regarding the drivers of technology diffusion and the potential of long-term technology performance and cost improve- ments. 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}