Introduction 2 (26 November 2006) 3 4 In accordance with a ...download.repubblica.it/pdf/2007/syr.pdf · 41 large regions: significantly increased precipitation in eastern parts of

Preliminary Draft for informal review – 27 Nov 2006 to 7 Jan 2007 IPCC Fourth Assessment Report - Synthesis Report

DRAFT – Do Not Cite, Quote or Distribute Page 1 of 1 Introduction

Introduction 1

(26 November 2006) 2

3

In accordance with a decision taken at the 22nd

Session of the IPCC (New Delhi, 9 to 11 4

November 2004), the IPCC decided to prepare a Synthesis Report of the Fourth Assessment 5

Report (AR4). It was agreed to produce the Synthesis Report up to 30 pages including maps 6

and figures, and a Summary for Policymakers up to 5 pages of text. The topics to be covered 7

in the Synthesis Report, as approved by the Panel, would include: 8

9

1 Observed changes in climate change and its effects 10

2 Causes of change 11

3 Climate change and its impacts in the near and long term under different scenarios 12

4 Adaptation and mitigation options and responses, the inter-relationship with sustainable 13

development, at global and regional levels 14

5 The long-term perspective: scientific and socio-economic aspects relevant to adaptation 15

and mitigation, consistent with the objectives and provisions of the Convention, and in 16

the context of sustainable development 17

6 Robust findings, key uncertainties 18

19

This Synthesis Report has been produced in accordance with the decision of the Panel, based 20

on the material presented in the three Working Group Reports. 21


DRAFT – Do Not Cite, Quote or Distribute Page 1 of 7 Topic 1

Topic 1 – Observed changes in climate and its effects 1


3

4

1.1 Warming of the climate system is unequivocal, as is now evident from increases in 5

global average air and ocean temperatures, melting of snow and ice, and rising sea level 6

[WGI SPM p4-17]. 7

8

Global mean surface temperature has increased with a linear trend of 0.74±0.18°C over the 9

last 100 years (1906-2005) [WGI TS.3.1.1], a larger trend than the corresponding trend at the 10

time of the TAR. The rate of warming over the last 50 years is almost twice that for the last 11

100 years. The warming is widespread over the globe, and is a maximum at higher northern 12

latitudes [WGI TS.3.1.2]. Warming in sea surface temperatures is strongly evident at all 13

latitudes in each of the oceans [WGI 3.ES]. The heat content of the ocean has increased over 14

the past 50 years [WGI TS.3.3.1]. Land regions have warmed at a faster rate than the oceans. 15

Land temperatures have warmed at about 0.25ºC per decade since 1979, with the greatest 16

warming in winter and spring in the Northern Hemisphere [WGI 3.ES]. New analyses of 17

balloon-borne and satellite measurements of lower- and mid-tropospheric temperature show 18

warming rates similar to the surface temperature, largely reconciling a discrepancy noted in 19

the TAR. 20

21

Increases in sea level [WGI TS.3.3.3], and decreases in snow and ice [WGI TS.3.2], are 22

consistent with warming (Figure 1.1). Global average sea level has risen since 1961 at an 23

average rate of 1.8 mm yr-1

. Cryospheric changes include the strong retreat of the Arctic sea 24

ice (2.7±0.6% per decade) [WGI 4.4], the continued shrinking of mountain glaciers, the 25

decrease of the extent of snow cover and seasonally frozen ground, and the increase in 26

temperature of permafrost. Average Arctic temperatures have increased at almost twice the 27

global average rate in the past 100 years [WGI 3.2]. The maximum area covered by seasonally 28

frozen ground has decreased by about 7% in the Northern Hemisphere since 1900 [WGI 4.7]. 29

30

It is likely that the second half of the 20th

century was the warmest 50-year period for the 31

Northern Hemisphere in the past millennium [WGI TS.3.5]. In earlier times, paleoclimate 32

evidence indicates that very large sea level rises have been associated with warming about 125 33

thousand years ago causing large-scale retreat of the Greenland Ice Sheet and other Arctic ice 34

fields, contributing to sea level rises of 4-6m above current conditions [WGI TS.3.5]. 35

36

There is evidence of other large-scale changes in climate since the mid-20th

century, including 37

atmospheric circulation changes with a poleward shift and strengthening of the westerly winds 38

[WGI TS.3.1.2], precipitation in some large areas, and in some climate and weather extremes. 39

Long-term trends from 1900 to 2005 have been observed in precipitation amount in many 40

large regions: significantly increased precipitation in eastern parts of North and South 41

America, northern Europe and northern and central Asia, with drying observed in the Sahel, 42

the Mediterranean, southern Africa and parts of southern Asia [WGI TS.3.1.3]. More intense 43

and longer droughts have been observed over wider areas since the 1970s, particularly in the 44

tropics and subtropics. Increased drying due to higher temperatures has contributed to this 45

change. As well, changes in sea surface temperatures, atmospheric circulation patterns, and 46

decreased snowpack and snow cover have been linked to droughts [WGI 3.3]. Cold days, cold 47

nights and frost have become rarer, while hot days, hot nights, and heat waves have become 48

more frequent [WGI Table SPM-1]. Observations suggest increases in extreme high water at a 49



broad range of sites worldwide since 1975 [WGI TS.3.3.3, Table SPM-1]. There is increasing 1

evidence for a strengthening of mid-latitude winds, increased mid-latitude storm activity, and 2

increases in wave heights [WGI TS.3.1.2, 3.ES]. The frequency of heavy precipitation events 3

has increased [WGI 3.8, 3.9]. There is evidence that tropical cyclones have become more 4

intense, although there are concerns about the quality of the historical data [WGI TS.3.1.3]. 5

6

7

8

9 Figure 1.1. Changes in global mean temperature, sea level, and snow cover area. Panel (a) 10

shows global mean temperatures as annual values (open circles) and a smoothed curve (black 11

line) with uncertainty in the smoothed curve shown by the yellow shaded area. Panel (b) 12

shows global mean sea level from tide gauge data (circles) and recent satellite measurements 13

(red line). Panel (c) shows April Northern Hemisphere snow cover area each year (circles) 14

with smoothed values (black line). [WGI Figure SPM-3] 15

16



1.2 Physical and biological systems on all continents and in some oceans are already 1

being affected by recent climate changes, particularly regional temperature increases 2

[WGII SPM]. Effects on human systems, although more difficult to discern due to 3

adaptation and non-climatic drivers, are emerging [WGII SPM]. 4

5

1.2.1 Physical systems 6

7

For physical systems there is more and stronger evidence since the TAR that climate changes 8

since 1970 are affecting natural and managed systems in the cryosphere and there is now 9

emerging evidence of effects on hydrology and water resources, and coastal processes and 10

zones [WGII SPM.B.1, 1.3.1]. 11

12

1.2.1.1 Cryosphere 13

14

The cryospheric changes have led to changes in slope instability in mountain and permafrost 15

regions and increase of glacial lakes and destabilization of moraines damming glacier lakes 16

[WGII 1.3.1.2]. Decreases in glaciers and large and small ice-caps have contributed to the 17

observed sea level rise [WGI 4.6, 4.7, 4.8, 5.5]. 18

19

1.2.1.2 Water resources 20

21

The spring peak discharge is occurring earlier in rivers affected by snow melt [WGII 1.3.2]. 22

Runoff has increased in large basins in Northern Hemisphere higher latitudes [WGII 1.3.2.1]. 23

In some other regions snowmelt changes have led to water supply restrictions [WGII TS.C.1]. 24

Lakes and rivers are warming, with effects on thermal structure and chemistry [WGII 1.3.2.3]. 25

Water availability is being increasingly restricted, associated with more droughts in drier 26

regions [WGII 1.3.2]. 27

28

1.2.1.3 Coastal zones 29

30

The effects of sea level rise, enhanced wave heights, and intensification of storms are found in 31

some coastal regions not modified by humans, e.g. polar areas and barrier beaches, mainly 32

through coastal erosion [WGII 1.ES, 1.3.3.1]. Sea level rise is contributing to losses of coastal 33

wetlands and mangroves, and increased damage from coastal flooding in many areas, although 34

human modification of coasts such as increased construction in vulnerable zones plays an 35

important role also [WGII 1.ES, 1.3.3.2]. 36

37

1.2.2 Biological systems 38

39

There is more evidence from a wider range of species and communities in terrestrial 40

ecosystems than was reported in the TAR that recent warming is already strongly affecting 41

natural biological systems, while there is substantial new evidence in marine and freshwater 42

systems relating changes to warming. [WGII 1.ES, 1.3.4, 1.3.5] 43

44

1.2.2.1 Marine ecosystems 45

46

There have been many observed changes in marine species phenology and distribution, e.g. 47

poleward movement of plankton and fish by 10º latitude over the last four decades in the 48

North Atlantic [WGII 1.ES, 1.3.4.2, 1.3.4.3]. As well, climate change and variability, in 49



combination with direct human impacts, have already caused substantial damage to coral reefs 1

[WGII 1.ES, 1.3.4.1]. 2

3

1.2.2.2 Freshwater ecosystems 4

5

Warming lakes and rivers are affecting abundance and productivity, community composition, 6

phenology, distribution and migration in freshwater systems [WGII 1.ES, 1.3.4.4]. For 7

example, in high-latitude or high-altitude lakes where warmer temperature and reduced ice 8

cover has led to longer growing seasons, many lakes are showing increased algal abundance 9

and productivity over the past century. There have also been similar increases in the 10

abundance of zooplankton. In contrast, some lakes, particularly in the tropics, are 11

experiencing reduced algal abundance and declines in productivity because warming has led 12

to stronger stratification and reduced upwelling of nutrient-rich deep water [WGII 1.3.4.4]. 13

14

1.2.2.3 Terrestrial ecosystems 15

16

The overwhelming majority of studies of regional climate effects on terrestrial species reveal 17

consistent responses to warming trends, including poleward and elevational range expansions 18

of flora and fauna [WGII 1.ES, 1.3.5.4], phenological changes across the Northern 19

Hemisphere especially earlier onset of spring events, migration, and lengthening of growing 20

seasons [WGII 1.ES, 1.3.5.2], and changes in abundance of certain species over the last few 21

decades including limited evidence of a few local disappearances and changes in community 22

composition [WGII 1.ES, 1.3.5.4]. Changes in Arctic and Antarctic Peninsula flora and fauna 23

are well-documented [WGII 1.3.1.2]. 24

25

1.2.3 Human systems 26

27

Effects of climate change on human systems, although often difficult to discern due to 28

adaptation and non-climatic drivers, are emerging [WGII 1.ES]. 29

30

1.2.3.1 Agriculture and forestry 31

32

In comparison with other factors, recent warming has been of limited consequence in the 33

agriculture and forestry sectors [WGII 1.3.6]. A significant advance in phenology, however, 34

has been observed for agriculture and forestry in large parts of the Northern Hemisphere, with 35

limited responses in crop management [WGII 1.3.6.1, 1.3.6.2]. The lengthening of the 36

growing season has contributed to an observed increase in forest productivity in many regions, 37

while warmer and drier conditions are partly responsible for reduced forest productivity and 38

increased forest fires in North America and the Mediterranean basin [WGII 1.3.6.2]. Both 39

agriculture and forestry have shown vulnerability to recent trends in heat waves, droughts and 40

floods [WGII 1.3.6] (medium confidence). 41

42

1.2.3.2 Health 43

44

There is emerging evidence of temperature changes having some effects on human health. 45

There have been changes in the distribution of some human disease vectors and of changes in 46

the seasonal production of pollens that cause allergenic diseases [WGII 1.ES, 1.3.7.2, 1.3.7.5]. 47

High temperature extremes were associated with excess mortality in the 2003 heat wave in 48



Europe, prompting adaptation measures, although changes in health effects related to 1

increasing heat extremes elsewhere have not been demonstrated [WGII TS.B.3]. 2

3

1.2.3.3 Livelihoods and infrastructure 4

5

There have been changes in indigenous livelihoods in the Arctic and limitations on mountain 6

sports in lower-elevation alpine areas [WGII 1.3.1.2]. Increases in storminess and tropical 7

cyclone intensity can lead to increased infrastructure damage, especially when combined with 8

sea level rise [WGII 1.2.1.3]. 9

10

1.3 Some aspects of climate have not changed, or the data are insufficient to determine 11

whether they have changed, and some responses to climate changes are difficult to detect 12

or isolate. 13

14

Some important aspects of climate appear not to have changed, and for some others we cannot 15

determine if they are changing because of data inadequacies. For instance, total Antarctic sea 16

ice exhibits inter-annual variability and localized changes but no statistically significant 17

average trends, consistent with the lack of warming in atmospheric temperatures averaged 18

across the continent [WGI SPM, TS.3.2]. There is insufficient evidence to determine whether 19

trends exist in some other variables, for example the meridional overturning circulation of the 20

global ocean or tornadoes, hail, lightning and dust-storms [WGI SPM]. There is no clear trend 21

in the annual number of tropical cyclones. The average difference between day and night time 22

temperatures, after apparently declining for some decades [TAR], has not changed since 1979, 23

both having risen at about the same rate. [WGI 3.2]. 24

25

Responses to climate changes in human systems are difficult to detect because of multiple 26

non-climate driving forces and the presence of adaptations. Thus the major factors leading to 27

increased vulnerability of tropical coasts is related to increased population, and development 28

of vulnerable infrastructure [WGII 1.3.3.1]. Changes in river systems related to water supply 29

make it more difficult to determine how streamflow may be changing. Non-climatic factors 30

important for changes in hydrological variables are human interventions in water catchments, 31

such as land-use and land-cover changes, and changes in rates of water consumption for 32

agricultural, industrial, commercial, and domestic uses. These in turn affect evaporation and 33

transpiration, soil moisture storage, infiltration and percolation into soil and groundwater, as 34

well as runoff quantity and timing, and water quality. In some regions, such as the North 35

China Plains, the combination of intensive pumping for irrigation and several years of below-36

normal precipitation and rising temperature since the 1980s accelerated the downward trend in 37

water levels and has caused groundwater depression, land subsidence, disappearance of 38

wetlands, and intrusion of seawater in coastal zone [WGII 1.3.2.1]. 39

40

Since the TAR, there has been further research on the role of observed climate change on the 41

geographical distribution of malaria and its transmission intensity in African highland areas 42

but the evidence remains unclear. Malaria incidence has increased since the 1970s at some 43

sites in East Africa, but it has yet to be proven whether this is due solely to warming of the 44

environment. A range of studies have demonstrated the importance of temperature variability 45

in malaria transmission in these highland sites. While a few studies have shown the effect of 46

long-term upward trends in temperature on malaria in some highland sites, other studies 47

indicate that increase in resistance of the malaria parasite to drugs, decrease in vector-control 48

activities, and ecological changes may have been the most likely driving forces behind the 49



malaria resurgence in recent years. Thus, while climate is a major limiting factor in the spatial 1

and temporal distribution of malaria, many non-climatic factors (drug resistance and HIV 2

prevalence, and secondarily, cross-border movement of people, agricultural activities, 3

emergence of insecticide resistance and the use of DDT for indoor residual spraying) may 4

alter or override the effects of climate [WGII 1.3.7.2]. 5

6

1.4 Direct (non-climate) effects of carbon dioxide 7

8

Ocean acidification is occurring although the impacts, for example on corals and the marine 9

biosphere, are as yet uncertain [WGII 1.3.4]. There is evidence that the average pH of surface 10

seawater has fallen by 0.1 units in the last 200 years, i.e. a 30% increase in the concentration 11

of hydrogen ions in the surface oceans [WGII 1.3.4]. The effects of this ocean acidification on 12

the marine biosphere are as yet undocumented [WGII 1.3.4]. 13

14

Plant response to elevated CO2 alone − without climate change − is positive. Effects depend 15

on photosynthetic pathway, species, growth stage, and management. Plant physiologists and 16

modelers now recognize that effects of elevated CO2 measured in experimental settings and 17

implemented in models may overestimate actual field and farm-level responses, due to many 18

limiting factors such as pests, weeds, competition for resources, soil, water and air quality, 19

etc., which are neither well understood at large scales, nor well implemented in leading 20

models [WGII 5.4.1.1]. 21

22

1.5 Observed changes in physical and biological systems are consistent with a warming 23

world 24

25

Changes in the cryosphere, ocean and land, with observed decreases in snow cover and sea 26

ice, thinner sea ice, shorter freezing seasons of lake and river ice, glacier melt, decreases in 27

permafrost extent, increases in soil temperatures and borehole temperature profiles, and sea 28

level rise, all strongly support the view that the world is warming [WGI 3.9]. 29

30

A statistical comparison shows that the agreement between observed warming and the many 31

observed changes in physical, biological and human systems consistent with warming (Figure 32

1.2, Table 1.1) is very unlikely to be due to natural variability in the systems [WGII 1.4.2.3]. 33

That is, the vast majority (>85%) of changes in these systems have been in the direction that 34

would be expected to occur as a response to warming [WGII TS.B]. 35



1 Figure 1.2. Observed regional trends in surface air temperature over the period 1970-2004 2

and locations of observed changes in cryosphere, hydrology, coastal zones, marine, freshwater 3

and terrestrial biological systems with at least 20 years of data in the period. Temperature 4

trends are averaged over 5o x 5

o gridboxes. Size of circles indicates numbers of systems 5

observed at each location. [WGII Figure SPM-1] 6 7 8 9 10

Table 1.1. Percentage of 5o x 5

o cells with observed changes in physical and biological 11

systems and regional temperature change (significant warming, warming, cooling, significant 12

cooling), with characterization of observed changes as ‘consistent with warming,’ or ‘not 13

consistent with warming.’ Expected values are for the null hypothesis: There is no 14

relationship between significant changes in systems and significant warming. [WGII 1.4] 15 16

Temperature cells

Cells with significant observed change consistent with warming

Cells with significant observed change not consistent with warming

Significant warming

35%

(expected 2.5%)

3%

(expected 2.5%)

Warming 51%

(expected 22.5%)

4%

(expected 22.5%)

Cooling 5%

(expected 22.5%)

1%

(expected 22.5%)

Significant cooling

1%

(expected 2.5%)

0%

(expected 2.5%)

17



Topic 2 – Causes of change 1


3

4

2.1 Emissions 5

6

2.1.1 Greenhouse gas emission trends (1970-2004) 7

8

Anthropogenic emissions of global greenhouse gases covered under the Kyoto Protocol have 9

grown since 1970, but with several important changes in the rates of growth over time. 10

Greenhouse gas emissions have increased by about 75% during the period 1970-2004 (Figure 11

2.1). Among the greenhouse gases, CO2 is the largest source and has grown by 87%. CO2 12

accounted for 75% of the total anthropogenic emissions in 2004. Methane and nitrous oxide 13

emissions have grown by 40% and 50% since 1970, respectively, although methane emissions 14

have stayed fairly constant since the early 1990s. CO2 from fossil fuel combustion followed 15

by deforestation dominates the greenhouse gas emissions. [WGIII TS.1.2, 1.3.1] 16

17

18

19

20

21

Global anthropogenic greenhouse gas emissions 1970-2004

0

10

20

30

40

50

60

1970 1975 1980 1985 1990 1995 2000

Pg CO2-eq.

ODP gases 6)

HFCs, PFCs, SF6

N2O other

N2O agriculture

CH4 other

CH4 waste

CH4 agriculture

CH4 energy 3)

CO2 other 2)

CO2 decay, non-CO2, peat 5)

CO2 deforestation 1) 4)

CO2 fossil fuel use

22 23

24

25

26

27

Figure 2.1. Global greenhouse gas emission trends 1970-2004. [WGIII Figure SPM-1] 28

ODP gases (6)

CO2 fossil fuel use

CO2 decay, non-CO2, peat (5)

CO2 other (2)

CH4 energy (3)

CH4 agriculture

CH4 waste

CH4 other

N2O agriculture

HFCs, PFCs, SF6

CO2 deforestation (1) 4)

Note: 100 year GWPs from IPCC 1996 (SAR) were used to convert emissions to CO2-eq. (cf. UNFCCC reporting guidelines). 1) Including biofuel combustion at 10% (assuming 90% sustainable production); excluding 10% carbon in forest fire emissions of non-CO2 gases. 2) Cement production and natural gas venting / flaring. 3) Including biofuels. 4) For large-scale biomass burning averaged activity data for 1997-2002 were used from GFED. based on satellite data. 5) CO2 emissions from decay (decomposition) of aboveground biomass that remains after logging and deforestation, 10% carbon in forest fire emissions of non-CO2 gases (CO, NMVOC, CH4) and CO2 from peat fires and decay of drained peat soils. Excluding fossil fuel fires. 6) CO2-eq. emissions of CFCs, halons, MCF, CTC and CH3Br (direct effect only). The indirect effects are much more uncertain and as a whole estimated to be significantly smaller than the direct effects.

N2O other



2.1.2 Sources of emissions and sectoral contribution 1

2

The largest growth in CO2 emissions has come from power generation, which has increased 3

threefold, followed by emissions from road transport, which have nearly doubled. Methane 4

emissions from the agriculture sector have roughly stabilized due to compensating reductions 5

and increases in livestock and rice production, respectively. N2O emissions, mainly from 6

fertilizer use, increased during the period 1970-2004; however, emissions from industrial 7

processes declined. [WGIII TS.1.2] 8

9

2.1.3 Linkage between GDP, population, energy intensity and greenhouse gas emissions 10

11

Global primary energy use nearly doubled from 229 EJ in 1971 to 449 EJ in 2003, with an 12

average annual growth of 2.1% per year over this period. A decomposition analysis shows that 13

GDP per capita growth and population growth were the main drivers of growth in global CO2 14

emissions during the last three decades (Figure 2.2). As can be seen from Figure 2.2, the 15

increases in population and GDP per capita (and therefore energy use per capita) have 16

outweighed the decrease in energy use per unit of GDP, while the carbon intensity of energy 17

supply was more or less constant in the period 1983-2003. The role of carbon intensity in 18

offsetting the emissions growth has been declining over the last two decades. At the global 19

scale, declining carbon and energy intensities have not offset the growth in population and 20

GDP, leading to growth in CO2 emissions. [WGIII TS.1.2, 1.3.1.2] 21

22

23

24 25

Figure 2.2. Decomposition of CO2 emission growth at global scale, shown for three decades. 26

[WGIII Figure TS-3] 27

28

2.1.4 Regional contribution to greenhouse gas emissions 29

30

Developed (Annex-I) countries account for 20% of the global population and contributed 31

46.4% of GHG emissions during 2003 (Figure 2.3). In contrast, developing countries (non 32

Annex-I), which account for 80% of the global population, contributed 53.6% of the CO2 33

emissions during 2003. Per capita emissions were highest in the USA and Canada at 34

approximately 20 t CO2 per capita compared to approximately 4.22 t CO2 per capita for non-35

Annex-I countries. [WGIII SPM, TS.1.2] 36



1 Figure 2.3. Distribution of regional per capita CO2 emissions over different country groupings 2

in 2003. [WGIII Figure SPM-4] 3

4

5

2.2 Drivers and feedbacks 6

7

2.2.1 Drivers 8

9

Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation 10

and in land surface properties affect the absorption, scattering and emission of radiation 11

within the atmosphere and at the Earth’s surface. The resulting positive or negative changes 12

in energy balance due to these factors are expressed as radiative forcing1, which is used to 13

compare warming or cooling influences on global climate. 14

15

Current atmospheric concentrations of carbon dioxide and methane far exceed pre-industrial 16

values determined from ice cores spanning the last 650,000 years. The increases in these 17

greenhouse gases since 1750 (Figure 2.4) are due primarily to emissions from fossil fuel use, 18

land-use changes and agriculture. [WGI SPM] 19

20

• Carbon dioxide is the most important anthropogenic greenhouse gas. Its atmospheric 21

concentration increased from a pre-industrial value of about 280 ppm to 379 ppm in 22

2005. [WGI SPM] 23

1 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 m–2).



• The atmospheric methane concentration increased from a pre-industrial value of 1

about 715 ppb to 1774 ppb in 2005. Growth rates have declined since 1993, 2

consistent with total emissions being nearly constant during this period. [WGI SPM] 3

• The atmospheric nitrous oxide concentration increased from a pre-industrial value of 4

about 270 ppb to 319 ppb in 2005. [WGI SPM] 5

6

7 8

Figure 2.4. Atmospheric concentrations of carbon dioxide, methane and nitrous oxide over 9

the last 10,000 years (large panels) and since 1750 (inset panels). Measurements are shown 10

from ice cores (symbols with different colours for different studies) and atmospheric samples 11

(lines). The corresponding radiative forcings are shown on the right hand axes of the large 12

panels. [WGI Figure SPM-1] 13



The globally averaged net effect of human activities since 1750 has very likely been one of 1

warming, with a radiative forcing of +1.6 (+0.6 to +2.4)2 W m

-2. This is likely to have been at 2

least five times greater than that due to solar output changes (Figure 2.5). [WGI SPM] 3

4

• The combined radiative forcing due to increases in carbon dioxide, methane, and 5

nitrous oxide is +2.3 (+2.1 to +2.5) W m-2

, and its rate of increase during the 6

industrial era is very likely to have been unprecedented in more than 10,000 years 7

(Figures 2.4 and 2.5). The CO2 radiative forcing increased by 20% during the last 10 8

years (1995-2005), the largest change observed or inferred for any decade in at least 9

the last 200 years. [WGI SPM] 10

• Anthropogenic aerosols (primarily sulphate, organic carbon, black carbon, nitrate and 11

dust) together produce a total direct radiative forcing of -0.5 (-0.9 to -0.1) W m–2

and 12

an indirect cloud albedo forcing of -0.7 (-1.8 to -0.3) W m

-2. [WGI SPM] 13

• The net radiative forcing due to ozone changes is dominated by anthropogenic 14

tropospheric ozone changes of +0.35 (+0.25 to +0.65) W m–2

. [WGI SPM] 15

• Changes in surface albedo, due to land-cover changes and deposition of black carbon 16

aerosols on snow, exert respective forcings of -0.2 (-0.4 to 0.0) and +0.1 (0.0 to +0.2) 17

W m-2

. [WGI SPM] 18

• Changes in solar output since 1750 are estimated to have caused a radiative forcing of 19

+0.12 (+0.06 to +0.30) W m–2

, which is less than half the estimate given in the TAR. 20

[WGI SPM] 21

• The spatial patterns of radiative forcings for ozone, aerosol direct effects, aerosol-22

cloud interactions and land-use have considerable uncertainties. This is in contrast to 23

the relatively high confidence in the spatial pattern of radiative forcing for the long-24

lived greenhouse gases. The net positive radiative forcing in the Southern Hemisphere 25

very likely exceeds that in the Northern Hemisphere because of smaller aerosol 26

concentrations in the Southern Hemisphere. [WGI TS.2.5] 27

28

29

2 Assessed uncertainty ranges given in the SYR are 90% confidence 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. Best

estimates given where available. Assessed confidence intervals are not always symmetric about the corresponding best

estimate.



1 2

Figure 2.5. Global-average radiative forcing (RF) estimates and ranges in 2005 for carbon 3

dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important agents and 4

mechanisms, together with the typical geographical extent (spatial scale) of the forcing and 5

the assessed level of scientific understanding (LOSU). Volcanic aerosols contribute an 6

additional natural forcing but are not included in this figure due to their episodic nature. [WGI 7

Figure SPM-2] 8

9

10

11

12

2.2.2 Feedbacks 13

14

Warming tends to reduce land and ocean uptake of atmospheric carbon dioxide, increasing the 15

fraction of anthropogenic emissions that remains in the atmosphere. The carbon cycle 16

feedback is positive but its magnitude is uncertain. [WGI SPM] 17

18

Analysis of models together with constraints from observations suggest that the equilibrium 19

global average warming expected for a doubling of pre-industrial CO2 concentrations (550 20

ppm) is likely to be in the range 2 to 4.5°C above pre-industrial values, with a best estimate of 21

about 3°C. This warming is very unlikely to be less than 1.5°C. Values substantially higher 22

than 4.5°C cannot be excluded, but agreement of models with observations is not as good for 23

those values. Water vapour changes dominate the feedbacks affecting climate sensitivity and 24

are now better understood than in the TAR, while cloud feedbacks remain the largest source 25

of uncertainty. [WGI SPM, 8.6, 9.6, Box 10.2] 26

27

28

29



2.3 Attribution 1

2

2.3.1 Attribution of temperature changes 3

4

The observed widespread warming of the atmosphere and ocean, together with ice mass loss, 5

support the conclusion that it is extremely unlikely (<5%) that global climate change of the 6

past fifty years was caused by unforced variability alone [WGI SPM, 4.8, 5.2, 9.4, 9.5, 9.7]. 7

8

It is very likely that anthropogenic greenhouse gas increases caused most of the observed 9

increase in globally averaged temperatures since the mid-20th

century (Figure 2.6) [WGI SPM, 10

9.4]. During this time, solar and volcanic forcings would be likely to have produced cooling, 11

not warming [WGI TS.4.1]. Warming of the climate system has been detected and attributed 12

to anthropogenic forcing in surface and free atmosphere temperatures, in temperatures of the 13

upper several hundred metres of the ocean and in contributions to sea level rise [WGI SPM]. 14

The observed pattern of tropospheric warming and stratospheric cooling can be largely 15

attributed to the combined influences of greenhouse gas increases and stratospheric ozone 16

depletion. [WGI SPM, 3.2, 3.4, 9.4, 9.5] 17

18

19

20 Figure 2.6. Changes in continental- and global-scale decadal surface air temperature for 1906–21

2005, relative to the corresponding average for the 1901–1950 period, compared with model 22

simulations. Black lines indicate observed changes and are dashed where spatial coverage is 23

less than 50%. Blue bands show the 5–95% range for 19 simulations from 5 climate models 24

using only natural forcings, and red bands show the 5–95% range for 58 model simulations 25

from 14 climate models using both natural and anthropogenic forcings. The changes shown are 26

unadjusted model output in regions where observations are available. [WGI Figure SPM-4] 27



It is likely that greenhouse gases alone would have caused more warming than observed 1

because volcanic and anthropogenic aerosols have offset some warming that would otherwise 2

have taken place [WGI SPM, 2.9, 7.5, 9.4]. A key factor in identifying the aerosol fingerprint, 3

and therefore the amount of cooling counteracting greenhouse warming, is the temperature 4

change through time, as well as the hemispheric warming contrast [WGI TS.4.1]. 5

6

It is likely that there has been significant anthropogenic warming over the past 50 years 7

averaged over each continent except Antarctica (Figure 2.6) [WGI SPM, 3.2, 9.4]. The ability 8

of coupled climate models to simulate the temperature evolution on each of six continents 9

provides stronger evidence of human influence on the global climate than was available in the 10

TAR [WGI TS.4.2]. The observed patterns of warming, including greater warming over land 11

than over the ocean, and their changes over time, are simulated by models that include anthro-12

pogenic forcing [WGI SPM, 3.2, 9.4]. No coupled global climate model that has used natural 13

forcing only has reproduced the continental mean warming trends in individual continents 14

(except Antarctica) over the second half of the 20th

century [WGI TS.4.2, 9.4]. 15

16

Difficulties remain in reliably simulating and attributing observed temperature changes at 17

smaller scales. Unforced variability becomes more important for sub-continental or smaller 18

scales. This together with uncertainties in local forcings and feedbacks makes it difficult to 19

estimate the contribution of greenhouse gas increases to observed small-scale temperature 20

changes [WGI SPM, 8.3, 9.4]. 21

22

2.3.2 Attribution of changes in other aspects of the climate system 23

24

Discernible human influences now extend to other aspects of climate, including atmospheric 25

circulation patterns, and some types of extremes [WGI SPM, 9.5]. Human influences are 26

likely to have contributed to changes in atmospheric circulation3, affecting storm tracks, 27

winds, and temperature patterns in both hemispheres. However, the observed changes in the 28

Northern Hemisphere circulation are larger than simulated. Temperatures of the most extreme 29

hot nights, cold nights and cold days are likely to have increased due to anthropogenic forcing. 30

Anthropogenic forcing may have increased the risk of heat waves. [WGI SPM, 3.5, 3.6, 9.4, 31

9.5, 10.3] 32

33

Anthropogenic forcing has likely contributed to recent decreases in Arctic sea ice extent [WGI 34

TS.4.1]. Changes in Arctic sea ice are expected given the observed enhanced Arctic warming. 35

36

It is very likely that the response to anthropogenic forcing contributed to sea level rise during 37

the latter half of the 20th

century. Modelled estimates of the contribution to sea level rise from 38

thermal expansion are in good agreement with estimates based on observations during 1961-39

2003, although the budget for sea level rise over that interval is not closed. The observed 40

increase in the rate of loss of mass from glaciers and ice caps is proportional to the global 41

average temperature rise expected from physical considerations. [WGI TS.4.1] 42

43

There is some evidence of the impact of human influence on the hydrological cycle, including 44

the observed large-scale patterns of changes in land precipitation over the 20th

century and a 45

global trend towards increases in drought in the second half of the 20th

century [WGI TS.4.3]. 46

3 In particular, the Southern and Northern Annular Modes and related changes in the North Atlantic Oscillation [3.6, 9.5,

Box TS.3.1]



Comparisons between observations and models suggest that changes in monsoons, storm 1

intensities and Sahelian rainfall are related at least in part to changes in observed sea surface 2

temperatures (SSTs). Changes in global SSTs are expected to be affected by anthropogenic 3

forcing, but an association of regional SST changes with forcing has not been established. 4

[WGI TS.4.3] 5

6

2.3.3 Increased confidence in understanding of the climate system response to radiative 7

forcing 8

9

There is now increased confidence in the understanding of the climate system response to 10

radiative forcing [WGI SPM, 6.6, 8.6, 9.6, Box 10.2]. A significant fraction of the 11

reconstructed Northern Hemisphere interdecadal temperature variability over the seven 12

centuries prior to 1950 is very likely attributable to volcanic eruptions and changes in solar 13

output, and it is likely that anthropogenic forcing contributed to the early 20th

century warming 14

evident in these records [WGI SPM, 2.7, 2.8, 6.6, 9.3]. 15

16

2.3.4 Effects of human-induced climate change on physical and biological systems 17

18

Global-scale assessment of the consistency of observed significant changes in physical and 19

biological systems and observed warming shows that it is likely that anthropogenic warming 20

over the last three decades has had a discernible influence on physical and biological systems 21

[WGII SPM, 1.4]. 22

23

Topic 1 showed that significant trends in physical and biological systems over at least the past 24

30 years are consistent with concurrent warming trends in many regions [Table 1, Topic 1, 25

WGII SPM, 1.3]. Further, a small number of analyses have specifically linked responses in 26

some physical and biological systems directly to anthropogenic climate change using climate, 27

process and statistical models [WGII SPM, 1.4]. The evidence presented in Topic 2 also 28

shows that it is likely that there has been significant anthropogenic warming over the past 50 29

years averaged over each continent except Antarctica [WGI SPM, 3.2, 9.4]. A global-scale 30

assessment of the consistency of observed significant changes in physical and biological 31

systems and observed warming shows that it is likely that the observed changes in systems 32

cannot be explained entirely due to natural variability or other confounding non-climate 33

factors [WGII SPM]. 34

35

Limitations remain in the evidence chain that would permit causal linkage of the observed 36

system responses to anthropogenic warming. The few end-to-end analyses which have been 37

performed are limited in the systems, time scales, and locations considered. Although 38

significant anthropogenic continental-mean warming has been detected, attribution becomes 39

more difficult at the regional scales which are relevant for the study of system responses. 40

Natural temperature variability becomes larger at regional scales relative to the changes due to 41

external forcing, making the latter more difficult to identify at these scales. Many impacts 42

studies are limited to shorter timescales than the 50-year timescale over which the attribution 43

of global-mean and continental mean surface temperature changes has been assessed. There 44

are other non-climate factors, such as land use change, invasive species or pollution, which 45

may have contributed to the observed system changes in some regions, although in many cases 46

these have been explicitly ruled out in observed impact studies. However a global-scale 47

assessment shows that it is likely that anthropogenic warming over the last three decades has 48

had a discernible influence on physical and biological systems. 49



Topic 3 – Climate change and its impacts in the near and long term under different 1

scenarios 2


4

5

3.1 Emissions scenarios 6

7

A large number of new baseline (non-intervention) and stabilization/mitigation scenarios have 8

been developed since the publication of SRES and TAR. The scenarios explore future 9

emissions pathways, their main underlying driving forces and how these might be affected by 10

policy interventions. The focus of this assessment is two-fold. First, the new baseline scenario 11

literature is reviewed as to how representative the SRES ranges of driving forces and 12

emissions are of the newer scenarios in the literature. In addition, this assessment reviews 13

main findings and methodological advances of the recent stabilization scenarios compared to 14

TAR. [WGIII 3.3] 15

16

The main finding from the comparison of SRES and new baseline scenarios in the literature is 17

that overall greenhouse gas emissions ranges have not changed appreciably since SRES 18

(Figure 3.1a) [WGIII 3.2]. The SRES scenarios thus remain an important part of the literature 19

as they provide internally consistent and comprehensive information with respect to possible 20

future development pathways and corresponding emissions rages in absence of climate 21

policies. Their resulting emissions projections are widely used in recent assessments of future 22

climate change (Section 3.2), and their underlying assumptions with respect to socio-23

economic, demographic and technological change serve as inputs to many recent climate 24

change vulnerability and impact assessments (Sections 3.3). Other findings of the assessment 25

of recent baseline scenarios and comparisons to SRES include [WGIII 3.2]: 26

27

• Some drivers for emissions, notably population projections, have been revised 28

downwards. Studies incorporating these new population projections, assume 29

compensatory effects for other emissions drivers (such as economic growth), and thus 30

report little change in overall emission level. [WGIII 3.2.1] 31

• Short-term economic growth projections for Africa, Latin America and the Middle East (to 32

2030) in recent scenarios are lower than in the SRES. These regional changes have only a 33

minor effect on global economic growth and overall emissions. [WGIII 3.2.1, 3.2.2] 34

• Aerosol and aerosol precursor emissions, which have a net cooling effect, are projected 35

to be lower than reported in the SRES. [WGIII 3.2.2] 36

• Most long-range economic scenarios reported in the literature continue to use market 37

exchange rates. For emission projections, it is not likely that the choice of market 38

exchange rate or purchasing power parity will have a substantial effect. [WGIII 3.2.1.4] 39

40

The main methodological advance of the recent stabilization scenarios compared to TAR is 41

the shift away from the traditional focus on CO2 to multigas stabilization strategies. Recent 42

scenarios thus consider the full basket of greenhouse gases and a wider range of mitigation 43

options, including non-CO2 sources in the energy, industry, forestry and agricultural sector. A 44

robust finding (consistent with a wide range of stabilization targets and baselines) is that even 45

when non-CO2 mitigation options are considered, the bulk of the emissions reductions (60-46

80% of total mitigation) have to come from reduction of CO2 in the energy and industry 47

sectors. Including non-CO2 options, however, provides greater flexibility and thus significant 48

cost reductions for achieving stabilization. [WGIII TS.3.3, 3.3, 3.5] 49



1

The CO2 emissions pathways of the recent stabilization scenarios (Figure 3.1b) show that 2

global emissions must peak and decline thereafter to meet any of the analyzed stabilization 3

targets. Recent studies show also that stabilization is possible at lower targets than reported in 4

TAR. The lower the target, the less cumulative emissions are allowed to be vented to the 5

atmosphere over the course of the century (Figure 3.1c). In the lowest stabilization scenarios 6

(2.5-3 W/m2 corresponding to stabilization at 440 ppm CO2-eq. to 490 ppm CO2-eq.), 7

emissions peak already as early as 2010-2020 and are reduced thereafter to almost zero or 8

below in the long term (by 2100). [WGIII 3.3, 3.5] 9

10

-5

0

5

10

15

20

25

30

35

40

45

50

2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

Glo

ba

l C

O2

Em

iss

ion

s (

GtC

)

Post-SRES range (80%)

A1B (SRES)

B1 (SRES)

A2 (SRES)

B2 (SRES)

Post-SRES (max)

Post-SRES (min)

a)

-5

0

5

10

15

20

25

30

35

40

45

50

2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

Glo

ba

l C

O2

Em

iss

ion

s (

GtC

)E: 6 -7 W/m2

D: 5 -6 W/m2

C: 4 -5 W/m2

B: 3 -4 W/m2

A2: 3 -3.5 W/m2

A1: 2.5 -3 W/m2

Stabilization target:

b)

0

400

800

1200

1600

2000

A1 A2 B C D E

Stabilization scenarios (categories)

Cu

mu

lati

ve

CO

2 e

mis

sio

ns

, 2

00

0-2

10

0 (

GtC

)

80th percentile range

median

c)

11 Figure 3.1. Global development of CO2 emissions in baseline and stabilization scenarios and 12

the cumulative CO2 emissions (2000-2100) in the stabilization scenarios. Panel (a) shows the 13

range of CO2 emissions in recent Post-SRES baseline scenarios compared to the SRES marker 14

scenarios. Panel (b) gives CO2 emissions ranges of recent Post-TAR stabilization scenarios 15

(grouped according to different stabilization targets). Panel (c) denotes the range of 16

cumulative CO2 emissions (2000-2100) for each group of Post-TAR stabilization scenarios. 17

[WGIII Figure-3.10, Figure-3.21, Figure-3.22, 3.2, 3.3] Notes: 18

• All emissions ranges give the 80th

percentile of the full scenario distribution 19

• Panel (b) to be modified to give also the corresponding CO2-eq. stabilization levels (in 20

addition to radiative forcing as presented now) 21

• Preliminary Figure. 22

23

3.2 Projections of future changes in climate 24

25

A major advance of this assessment of climate change projections compared with the TAR is 26

the large number of simulations available, which together with new approaches to constraints 27

from observations provide a quantitative basis for estimating likelihoods of expected 28

warmings. During the past few years, there has been an unprecedented effort in the climate 29

modelling community to provide climate model results and analysis based on a set of common 30

integrations, including both past and future climate forcings. This activity is a major step 31

forward relative to the earlier IPCC assessments. [WGI 8.3, 9.4, 10.3] 32

33

Since the IPCC’s first report 1990, assessed projections have suggested global averaged 34

temperature increases between about 0.15 and 0.3°C per decade from 1990 to 2005. This can 35

now be compared with observed values of about 0.2°C per decade, strengthening confidence 36

in near-term projections. For the next two decades a warming of about 0.2°C per decade is 37

projected for a range of SRES emission scenarios (Figure 3.2). [WGI, 1.2, 3.2, 9.4, 10.3] 38



3.2.1 21st century global changes 1

2

Continued greenhouse gas emissions at or above current rates would cause further warming 3

and induce many changes in the global climate system during the 21st century that would very 4

likely be larger than those observed during the 20th

century. [WGI 10.3] 5

6

• Projected globally-averaged surface warming for the end of the 21st century (2090–7

2099) is scenario-dependent so that the actual warming will be significantly affected by 8

the actual emissions that occur. Warmings relative to 1980–1999 for six SRES 9

scenarios, given as best estimates and corresponding likely ranges in °C, are: B1: 1.7 10

(1.0 to 2.7)°C; A1T: 2.4 (1.4 to 3.8)°C; B2: 2.4 (1.4 to 3.8)°C; A1B: 2.7 (1.6 to 4.3)°C; 11

A2: 3.2 (1.9 to 5.1)°C; A1FI: 4.0 (2.4 to 6.3)°C. [WGI 10.5] 12

13

• Warming tends to reduce land and ocean uptake of atmospheric carbon dioxide, 14

increasing the fraction of anthropogenic emissions that remains in the atmosphere. For 15

the A2 scenario, the carbon dioxide feedback increases the corresponding global average 16

warming at 2100 by more than 1°C. Assessed uncertainty ranges for temperature 17

projections are larger than in the TAR because a broader range of models and climate 18

carbon-cycle feedbacks have been considered. [WGI 7.3, 10.5] 19

20

• Projections of sea level rise are smaller than given in the TAR, mainly due to improved 21

estimates of ocean heat uptake. Smaller assessed uncertainties in glacier and ice cap7

22

changes also contribute to a reduced upper bound. However, these ranges do not include 23

uncertainties in carbon-cycle feedbacks or ice flow processes because a basis in 24

published literature is lacking. If recently observed increases in ice discharge rates from 25

the Greenland and Antarctic ice sheets were to grow linearly with global average 26

temperature change, that would add 10 to 25% of the central estimate to each scenario, 27

but understanding of these effects is too limited to assess their likelihood. [WGI 10.6] 28

29

• Increasing atmospheric carbon dioxide concentrations lead to increasing acidification of 30

the ocean. Projections based on SRES scenarios give reductions in pH1 of between 0.14 31

and 0.35 units in the 21st century, extending the decrease of 0.1 units since pre-industrial 32

times. [WGI Box 7.3, 10.4] 33

34

3.2.2 21st century regional changes 35

36

There is now higher confidence in projected patterns of warming and other regional-scale 37

features, including changes in circulation patterns, precipitation, and some aspects of extremes 38

and of ice. [WGI 8.2, 8.3, 8.4, 8.5, 9.4, 9.5, 10.3, 11.1] 39

40

• Projected warming in the 21st century shows scenario-independent geographical patterns 41

similar to those observed over the past 50 years. Warming is expected to be greatest 42

over land and at high northern latitudes, and least over the Southern Ocean and North 43

Atlantic (Figure 3.2). [WGI 10.3] 44

45

46

1 Decreases in pH correspond to increases in acidity of a solution. See Glossary for further details.



AOGCM projections of surface temperatures 1

2

3 4

Figure 3.2. Projected global average temperature changes for the early and late 21st century 5

relative to the period 1980–1999. The central and right panels show the AOGCM multi-model 6

average projections for the B1 (top), A1B (middle) and A2 (bottom) SRES scenarios averaged 7

over decades 2020–2029 (center) and 2090–2099 (right). The left panel shows corresponding 8

uncertainties as the relative probabilities of estimated global average warming from several 9

different studies for the same periods. [WGI Figures 10.8 and 10.28] 10

11

12

• Snow cover is projected to contract. Widespread increases in thaw depth are projected 13

over most permafrost regions. [WGI 10.3, 10.6] 14

15

• Sea ice is projected to shrink in both the Arctic and Antarctic under all SRES scenarios. 16

Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st 17

century in some projections. [WGI 10.3] 18

19

• It is very likely that hot extremes, heat waves, and heavy precipitation events will 20

continue to become more frequent and the number of cool days and nights and frost will 21

decrease. [WGI 10.3] 22

23

• The number of tropical cyclones (typhoons and hurricanes) per year is projected to 24

decrease but their intensity is expected to increase, with larger peak wind speeds and 25

more intense precipitation. The apparent increase in the proportion of very intense 26

storms since 1970 is much larger than simulated by current models for that period. 27

[WGI 9.5, 10.3, 3.8] 28

29



• Storm tracks are projected to move poleward, with consequent changes in wind, 1

precipitation, and temperature patterns outside the tropics, continuing the broad pattern 2

of observed trends over the last half-century. [WGI 3.6, 10.3] 3

4

• Since the TAR there is an improving understanding of projected patterns of 5

precipitation. Increases in the amount of precipitation are very likely in high-latitudes, 6

while decreases are likely in most subtropical land regions (by as much as about 20% in 7

the A1B scenario in 2100, Figure 3.3), continuing observed patterns in recent trends. 8

[WGI 3.3, 8.3, 9.5, 10.3, 11.2 to 11.9] 9

10

Projected patterns of precipitation changes 11

12 Figure 3.3. Relative changes in precipitation (in percent) for the period 2090–2099, relative to 13

1980–1999. Values are multi-model averages based on the SRES A1B scenario for December 14

to February (left) and June to August (right). White areas are where less than 66% of the 15

models agree in the sign of the change and stippled areas are where more than 90% of the 16

models agree in the sign of the change. [WGI Figure 10.9] 17

18

19

3.2.3 Beyond 21st century changes 20

21

Climate processes, feedbacks, and their timescales imply that anthropogenic warming and sea 22

level rise would continue for centuries even if greenhouse gas concentrations were to be 23

stabilized. [WGI 10.4, 10.5, 10.7] 24

25

• Uncertainty in the magnitude of the positive feedback between climate change and the 26

carbon cycle leads to uncertainty in the trajectory of carbon dioxide emissions required 27

for a particular stabilization level. A number of models suggest that this feedback effect 28

would require reductions in cumulative emissions in the 21st century, compared with 29

simulations that do not include carbon cycle feedback, by 105 to 300 GtC and by 165 to 30

510 GtC for stabilization at 450 and 1000 ppm respectively. [WGI 7.3, 10.4] 31

32

• Stabilization of radiative forcing in 2100 at B1 or A1B levels2

would be expected to lead 33

to further warming of about 0.5°C, mostly in the following century. [WGI 10.7] 34

2 Approximate CO2 equivalent concentrations corresponding to the computed radiative forcing due to anthropogenic

greenhouse gases and aerosols in 2100 (see p.823 of the TAR) for the SRES B1, A1T, B2, A1B, A2 and A1FI illustrative

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



• If radiative forcing were to be stabilized in 2100 at A1B levels2, thermal expansion 1

alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980–1999) and 2

would continue at decreasing rates for many centuries, due to the time required to mix 3

heat into the deep ocean. [WGI 10.7] 4

5

• Contraction of the Greenland ice sheet is projected to continue to contribute to sea level 6

rise after 2100. Current models suggest that a global average warming (relative to pre-7

industrial values) of 1.9 to 4.6°C would lead to virtually complete elimination of the 8

Greenland ice sheet and a resulting sea level rise of about 7 m, if sustained for 9

millennia. These temperatures are comparable to those inferred for the last interglacial 10

period 125,000 years ago, when paleoclimatic information suggests reductions of polar 11

ice extent and 4 to 6 m of sea level rise. [WGI 6.4, 10.7] 12

13

• Dynamical processes not included in current models but suggested by recent 14

observations could increase the vulnerability of the ice sheets to warming, increasing 15

future sea level rise. Understanding of these processes is limited and there is no 16

consensus on their magnitude. [WGI 4.6, 10.7] 17

18

• Current global model studies project that the Antarctic ice sheet will remain too cold for 19

widespread surface melting and is expected to gain in mass due to increased snowfall. 20

However, net loss of ice mass could occur if dynamical ice discharge dominates the ice 21

sheet mass balance. [WGI 10.7] 22

23

• 21st century anthropogenic carbon dioxide emissions will contribute to warming and sea 24

level rise for more than a millennium, due to the timescales required for removal of this 25

gas. [WGI 7.3, 10.3] 26

27

3.3 Impacts of future climate changes 28

29

3.3.1 Framing issues: Hazards, risks and opportunities; timescales, inertia and lags 30

31

Climate change will lead to changes in geophysical, biological and socio-economic systems. 32

An impact describes a specific change in a system caused by its exposure to climate change. 33

Impacts may be judged to be harmful or beneficial. Vulnerability to climate change is the 34

degree to which these systems are susceptible to, and unable to cope with, the adverse 35

impacts. The concept of risk, which combines the magnitude of the impact with the 36

probability of its occurrence, captures uncertainty in the underlying processes of climate 37

change, exposure, impacts and adaptation. [WGII 19.1.1] 38

39

3.3.2 Most vulnerable sectors and systems 40

41

Key vulnerabilities are associated with many climate-sensitive systems, including water 42

resources, ecosystems, coastal areas, food supply, industries and settlements, and health. 43

[WGII SPM.C, 19.1.2.1] 44

45

Water resources are very likely to be reduced in much of mid-latitudes and dry tropics, which 46

are under water stress already. Runoff and water availability are very likely to increase at 47

higher latitudes and in some wet tropics. Risk of extreme events, floods and droughts, is likely 48



to increase. Water volumes stored in glaciers and snow cover are very likely to decline, 1

reducing summer and autumn flows in populous regions. [WGII 3.4, 13.4.3] 2

3

Ecosystems are virtually certain to experience a loss of species, reductions of biodiversity and 4

changes in range. Polar, mountain, Mediterranean-climate, and savanna ecosystems are at 5

particular risk. In the oceans, coral reef ecosystems are under immediate threat from 6

temperature rise and under longer-term threat from rising ocean acidification. The resilience 7

of many ecosystems is likely to be exceeded this century by an unprecedented combination of 8

change in climate and other global change drivers. [WGII 4.2, 4.4] 9

10

Low-lying coastal areas are virtually certain to be threatened by sea level rise, leading to 11

coastal land loss and increased risk of flooding. This will be exacerbated by increasing 12

human-induced pressures. Hundreds of millions of people are vulnerable, especially in dense 13

and low-lying settlements with low adaptive capacity, already facing other pressures. Many 14

risk areas are in Asia (particularly the megadeltas) and Africa, while small islands face the 15

highest relative increase in risks. [WGII 6.3, 6.4] 16

17

Agricultural productivity is likely to increase at higher latitudes, up to around 3ºC global 18

temperature rise. At lower latitudes, especially in the seasonally dry tropics, crop yield 19

potential is likely to decrease for even small temperature increases, which would increase risk 20

of hunger in areas where food security is already relatively low. Beyond around 3ºC global 21

temperature increase, productivity is likely to decrease in most regions. Increased frequency of 22

droughts and floods would affect local production negatively, especially in subsistence sectors 23

at low latitudes. [WGII 5.3, 5.4, 5.6] 24

25

Industries, settlements, and societies in coastal and riverine areas are most vulnerable, being 26

closely linked with climate-sensitive resources, and in areas prone to extreme weather events, 27

especially where rapid urbanization is occurring. Poor communities can be especially 28

vulnerable, because they tend to be concentrated in relatively high-risk areas, to have more 29

limited coping capacities, and are more dependent on climate-sensitive resources such as local 30

water and food supplies. Economic costs of extreme weather events are likely to increase. 31

[WGII 7.4, 7.5, 7.6] 32

33

Health of millions of people is likely to be affected by climate change. Among important 34

health impacts are malnutrition and consequent disorders, which would affect child growth 35

and development; disease and injury due to heat waves, floods, storms, fires, and droughts; 36

water-related diseases; cardio-respiratory diseases due to higher concentrations of ground-37

level ozone. The geographic distribution of some infectious diseases is very likely to change. 38

Climate change is likely to have some mixed effects, e.g. expansion and contraction of the 39

range of malaria, and in some places may bring benefits to health such as fewer deaths from 40

cold exposure. [WGII 8.2, 8.4, 8.7] 41

42

Figure 3.4 shows a sample of sectoral impacts of climate change, which depend on capacity to 43

adapt. It is assumed that adaptive capacity develops over time in line with recent experience, 44

and is not enhanced by climate policy. However, enhancing adaptive capacity leads to increase 45

of the tolerance to climate change, particularly for economic and social systems; and efforts to 46

mitigate climate change by emissions reduction may delay and reduce extent of climate change 47

and its impacts [WGII 17.ES, 20.7]. Some impacts are likely to result from relatively small 48

climate changes, while other impacts are likely to occur for large climate changes. Some 49



impacts have been observed already, for a global average temperature increase of 0.74ºC over 1

the last hundred years (1906-2005) [WGII 1.3]. 2

3

4 5

Figure 3.4. [FIGURE TO COME] 6 7 8

3.3.3 Most vulnerable regions 9

10

Two sources of risk can be identified: exposure to climate change, and vulnerability due to 11

development circumstance (intrinsic vulnerability). The regions most at risk are: the Arctic, 12

Sub-Saharan Africa, small islands and Asian mega-deltas. [WGII SPM.C, 9.4.4, 10.4, 15.5, 13

16.4] 14

15

In the Arctic, rates of warming are projected to be higher than the global average. The positive 16

impacts include opening of sea routes, increases in biological productivity and agricultural 17

potential and less severe climate conditions, with positive implications for human health, 18

savings on heating energy and opportunities for tourism. At risk are ecosystems and 19

indigenous human communities following traditional lifestyles. In Sub-Saharan Africa, 20

intrinsic vulnerability is high because of relatively low adaptive capacity and heavy economic 21

dependence on rain-fed agriculture. In small islands, large impacts can be expected due to the 22

combination of high exposure (e.g., to sea level rise and storm surge) and high vulnerability 23

(e.g., concentration of population and infrastructure along coasts). In Asian megadeltas, large 24

impacts can be expected due to high exposure (especially to sea level rise and increased risk 25

of storm surge) together with large populations. [WGII 9.4, 9.6, 10.4.3, 15.4, 15.5, 15.6.3, 26

16.4] 27

28

However, all regions have vulnerable areas, communities and sectors. There are vulnerable 29

people, places, and activities even in high-income economies, as demonstrated by the impacts 30

of recent extreme events, not necessarily related to human-induced climate change, such as the 31

extreme heat of 2003 in much of Europe and Hurricane Katrina in the southern U.S. in 2005 32

[WGII 6.3.1, 7.4, 8.ES, 17.2, 17.3]. A sample of regional impacts, for all regions, is presented 33

in Figure 3.5. 34

35

36 37

Figure 3.5. [FIGURE TO COME] 38 39 40

3.3.4 Key vulnerabilities 41 42

Assessment of potential key vulnerabilities is intended to provide guidance to decision-makers 43

for identifying levels and rates of climate change that, in the terminology of the UNFCCC 44

- Figure placeholder -




Article 2, could result from ‘dangerous anthropogenic interference’ (DAI) with the climate 1

system. Ultimately, the definition of DAI cannot be based on scientific arguments alone, but 2

involves other judgements informed by the state of scientific knowledge. No single metric can 3

adequately describe the diversity of key vulnerabilities, nor determine their ranking but a 4

summary of this knowledge is illustrated in Figure 3.4 (for systems and sectors, in global 5

coverage) and Figure 3.5 (regional). [WGII 19.ES, 19.1.2.3, 19.1.2.5] 6

7

Working Group II identifies seven criteria from the literature that are often used to identify 8

key vulnerabilities [WGII 19.2]: 9

• magnitude of impacts 10

• timing of impacts 11

• persistence and reversibility of impacts 12

• potential for adaptation 13

• distributional aspects of impacts and vulnerabilities 14

• likelihood (estimates of uncertainty) of impacts and vulnerabilities and confidence in 15

those estimates 16

• importance of the system(s) at risk 17

18

3.4 Risk of abrupt or irreversible changes 19

20

Abrupt climate change on decadal time scales is normally thought of as involving ocean 21

circulation changes. On longer time scales, ice sheet changes may also play a role. It is very 22

likely that the Atlantic meridional overturning circulation (MOC) will slow down during the 23

21st century, with an average model-estimated reduction by 2100 of 25% (range from zero to 24

more than 50%). Temperatures in the Atlantic region are projected to increase despite such 25

changes due to the much larger warming associated with projected increases of greenhouse 26

gases. It is very unlikely that the MOC will undergo a large abrupt transition during the 21st 27

century. Longer-term changes in the MOC cannot be assessed with confidence. However, 28

meltwater from ice sheets and continued warming may further weaken the ocean overturning 29

circulations. In addition, there may be thresholds where the ice sheets continue melting even if 30

the climate returns to present day conditions. [WGI 10.3, 10.7; WGII Figure SPM-3, Table 31

TS-5] 32



Topic 4 – Adaptation and mitigation options and responses, the 1

inter-relationship with sustainable development, at global and regional levels 2


4

5

4.1 Framing issues 6

7

This topic synthesises the body of knowledge on adaptation and mitigation options and 8

responses, and their inter-relationship with sustainable development at global and regional 9

levels. Development pathways influence climate change vulnerability, adaptation and 10

mitigation, and climate change itself. Adaptation and mitigation policies can have a 11

significant impact on development paths. 12

13

Adaptation can be both reactive to experienced climate change and proactive; while mitigation 14

can only be proactive in relation to benefits from avoided climate change occurring over 15

centuries. Adaptation options have been practiced for many centuries to cope with the adverse 16

impacts of climate. In recent years, mitigation efforts at public and private levels are 17

receiving importance. 18

19

There are synergies between adaptation and mitigation; which can be attractive to 20

stakeholders provided that they are cost-effective. The notion of a global ‘optimal mix’ of 21

adaptation and mitigation is a theoretical construct; it cannot be implemented in practice. 22

Both adaptation and mitigation can enhance sustainability of development. This can be 23

achieved through increasing adaptive and mitigative capacities of society. Technological 24

development, transfer and diffusion can enhance the goals of sustainable development 25

together with international and regional cooperation. 26

27

4.2 Adaptation options 28

29

Adaptation can reduce vulnerability to climate variability and change (high confidence). 30

Societies across the world have a long record of adapting to the impacts of weather and 31

climate through a diverse range of practices. However, (assuming no adaptation 32

interventions), vulnerability to climate variability and change is likely1 to increase as a result 33

of the interaction of many factors such as type and rate of economic development, population 34

growth, rapid urbanization, expansion of human settlements into high risk areas, and a loss of 35

traditional coping skills. Equally, vulnerability to climate change will be further exacerbated 36

by multiple stresses from other sources such as poverty, unequal access to resources, food 37

insecurity and environmental degradation. Given the current concentrations of greenhouse 38

gases, some climate change is inevitable in the few decades. Irrespective of the scale of 39

mitigation undertaken in the next twenty years, additional adaptation measures will be 40

required at regional and local levels to reduce the impacts of climate change. While 41

adaptation can offset some potential impacts of climate change, there may be no feasible or 42

cost-effective options to deal with other effects. Adaptation will be necessary even if drastic 43

mitigation is implemented. [WGIII TS.1.4; WGII 17.2, WGII SPM] 44

45

Some adaptations to climate change are being implemented now on a limited basis (high 46

confidence). In both developed and developing countries, adaptation measures that consider 47

1 Likely is defined as having 66-90% probability of being true.



climate change are being implemented, on a limited basis by public and private actors. 1

Examples are consideration of future sea level rise in the design of communication 2

infrastructure such as the Confederation Bridge (Canada) and a coastal highway (Micronesia). 3

However, adaptation measures may have social and environmental externalities. For example, 4

air-conditioning can overcome heat stress or extreme heat related discomfort, but it increases 5

energy demand. If this increased demand is supplied from fossil fuels, greenhouse gas 6

emissions will be increased. In addition to climate, many adaptations have multiple drivers, 7

such as economic development and poverty alleviation. Many planned adaptation initiatives 8

are embedded within broader development and sectoral planning initiatives, which include 9

water resources planning, coastal defence and disaster planning. Examples include an 10

initiative to take into account climate change in the National Water Management Plan 11

(NWMP) of Bangladesh, and the design of flood protection and cyclone-resistant 12

infrastructure in Tonga. These examples show that some technologies, institutions and 13

resources for adaptation are already available, and can provide a basis for the implementation 14

of appropriate measures to reduce climate change risks. [WGII 1.3, 17.2; WGIII TS.6.4] 15

16

Many adaptation measures can be implemented at low cost, but comprehensive estimates of 17

adaptation are currently lacking (high confidence). Comprehensive estimates of the costs of 18

adaptation at the global level are limited and largely speculative. These approximations are 19

largely inferred from global damage estimates of climate change, which tend to conflate 20

estimated costs of adaptation and those of residual impacts. Even less is known about the 21

global benefits of adaptation, in terms of damages avoided. However, adaptation cost and 22

benefit estimates at the regional and project levels for specific impacts; such as sea level rise, 23

agriculture, energy demand for heating and cooling, water resources management and 24

infrastructure, are growing. These studies show that there are viable adaptation options that 25

can be implemented in these sectors at low cost, and/or with high benefit-cost ratios. 26

Unfortunately, the transitional and distributional costs of adaptation are generally not 27

included in such estimates. Empirical research also suggests that high benefit/cost can be 28

achieved by implementing many adaptation measures now compared with the costs of 29

retrofitting long-lived infrastructure at a later date. [WGII 17.2; WGIII TS.3.5] 30 31

Adaptive capacity is uneven across and within societies and is intimately connected to 32

social and economic development (high confidence). There are individuals and groups 33

within all societies throughout the world with insufficient capacity to adapt to climate change. 34

These include the poor, elderly, women (especially in developing countries), and other 35

disadvantaged groups. The capacity to adapt is a dynamic process and is influenced by 36

societies’ productive base including: natural and man made capital assets, social networks and 37

entitlements, human capital and institutions, good governance, income, technology, and 38

exposure to climate risk. The effectiveness of specific mitigation and adaptation policies is 39

influenced by many of the same drivers that influence socio-economic development 40

(investments, consumption, technology, population, governance, and environmental 41

priorities). Multiple stresses related to HIV/AIDS, land degradation, economic globalization 42

and market change, and violent conflict affect both the exposure to climate risks and the 43

capacity to adapt. [WGII 17.3; WGIII TS.2] 44

45

There are substantial limits and constraints to adaptation (high confidence). Studies in 46

various parts of the world reaffirm the TAR finding that while adaptation will be vital and 47

beneficial in a number of cases, there are limits to both the effectiveness and implementation 48

of adaptation measures. Many societies have high adaptive capacity, but this does not 49



necessarily translate into real action on adaptation to climate change, variability and extremes. 1

For example, despite a high capacity to adapt to heat stress through relatively inexpensive 2

adaptations, residents (especially elderly) of European cities continue to experience high 3

levels of mortality. Hurricane Katrina that hit New Orleans in USA in 2005 is another 4

example. [WGII 17.4; WGIII TS.3] 5

6

There are many significant barriers, including financial, technological, cognitive, behavioural, 7

political, social, and cultural constraints to implementing adaptation. Decisions relating to 8

adaptation are often undertaken at a hierarchy of levels, and actions at one level may enhance 9

or constrain options at another. There are also impediments related to knowledge and 10

information flows that are vital to making adaptation decisions. However, it has been shown 11

that these may be overcome by participatory practices and the application of local and 12

traditional knowledge. [WGII 17.4, 20.8; WGIII TS.3] 13

14

4.3 Mitigation options 15

16

Since 1970 global emissions of GHGs covered by the Kyoto Protocol have increased by more 17

that 50%, with CO2, the largest source, growing by about 60%. Assuming current policies 18

remain unchanged, CO2 emissions are projected to increase 50-100% by 2030 relative to 19

2000, with two-thirds of the increase originating in developing countries, though the per 20

capita emissions in developed countries will remain substantially higher (high confidence). 21

[WGIII SPM.1] 22

23

There is a large low-cost mitigation potential between now and 2030. The overall economic 24

reduction potential2 by 2030 at costs <US$ 20/tCO2-eq (2005$) is estimated at 8-12 GtCO2-25

eq. At costs <US$ 100/tCO2-eq, it is estimated at 18-25 GtCO2-eq, which is consistent with 26

emissions profiles for stabilization between 450 and 550 ppm CO2-eq. These estimates of 27

mitigation potential and marginal costs are derived from bottom-up studies covering all 28

sectors corrected for double counting, and are consistent with intermediate results from long-29

term, top-down modelling studies (Table 4.1 / Figure 4.1). 30

31

[Note: EITHER Table 4.1 OR Figure 4.1 will be used. If Figure 4.1 is used, the information 32

in the right-most column of Table 4.1 would be presented as text.] 33

34

Approximately 30% of the total mitigation potential is in OECD countries, 10% in countries 35

with economies-in-transition, and 60% in developing nations (medium confidence). These 36

potentials can only be achieved when adequate government policies are in place (high 37

confidence). [WGIII SPM.9] 38

39

Carbon pricing, either through taxes or cap-and-trade systems, is an essential incentive for 40

implementing mitigation options (high confidence). Both sectoral bottom-up and top-down 41

assessments suggest that carbon prices of US$ 20-25/tCO2-eq can begin to drive large scale 42

shifts to zero carbon energy supply and make many end-use mitigation options attractive. 43

Additional incentives, e.g. direct government funding and regulation, are also important, 44

particularly in increasing innovation, where market signals can be insufficient. [WGIII SPM.26] 45

2 Economic potential is defined as cost-effective GHG mitigation when non-market costs and (non-climate)

benefits are included with market costs and benefits in assessing the options for particular levels of carbon prices

and when using social discount rates (< 5%) instead of private ones (typically > 15%).



Table 4.1. Estimated global mitigation potential in 2030 compared to SRES B2 or World 1

Energy Outlook (2004) Baselines. [WGIII Table TS-19; for additional detail see WGIII 11.3] 2

3

Sector Economic Potential, MtCO2-eq Key Technologies

No-regrets <$0/tCO2-eq

Low Cost <$20/tCO2-eq

High Cost <$100/tCO2-eq

Energy Supply 850 1,850 2,200-5,100 improved supply efficiency, renewable energy (particularly biomass), fuel switching from coal to gas, advanced nuclear power, CO2 capture and storage (CCS) for gas or coal-fired generating facilities.

Transport 3,000 efficient hybrid vehicles, cleaner diesel, and use of bio-fuels, with use of hydrogen powered fuel cell vehicles in the longer term.

Buildings 3,200 3,650 3,700-4,100 efficient lighting, more effective building envelopes, passive solar design for heating, cooling and ventilation, and more efficient electrical appliances and heating and cooling devices.

Industry 1,300 2,800-5,600 more efficient end-use electrical equipment, heat and power recovery, material recycling and substitution, control of non-CO2 gas emissions, and a wide array of process-specific technologies.

Agriculture 2,100 3,300 improved crop and grazing land management to increase soil carbon storage; improved rice cultivation techniques, and livestock and manure management, to reduce CH4 emissions; improved nitrogen fertilizer application techniques to reduce N2O emissions, dedicated bio-energy crops to replace fossil fuel use.

Forestry 150 1,250 2,700 afforestation and reduced deforestation, use of forestry products for bio-energy to replace fossil fuel use.

Waste Management

700 550-1,300 landfill methane recovery, waste-to-energy applications.

Other geo-engineering options, e.g. ocean fertilization to remove CO2 from the atmosphere, or blocking sunlight by injecting reflective materials into the upper atmosphere; remain largely speculative, uncosted, and with the potential for unknown side effects.

Total 4,200 10,700 18,200-25,000



1 2

Figure 4.1. Estimated mitigation potential in OECD, EIT, and non-OECD countries in 2030 3

compared to SRES B2 or World Energy Outlook (2004) Baselines. [Draft figure; based on 4

WGIII Table TS-19; for additional detail see WGIII 11.3] 5



Four main criteria are used in evaluating policies: environmental effectiveness, economic 1

efficiency, equity, and political feasibility. Using these criteria, general conclusions about the 2

performance of policies are (high agreement, much evidence) [WGIII SPM.27]: 3

4

• Integrating climate policies into broader development policies makes it easier to 5

implement them and overcome barriers. 6

• Regulatory measures and standards generally provide environmental certainty, and may 7

be preferred when barriers prevent a response to price signals. 8

• Taxes and charges are economically efficient, but cannot guarantee a particular level of 9

emissions and make be politically difficult to implement. 10

• Tradable permits are effective at establishing a carbon price, and can be environmentally 11

effective depending on the volume of emissions allowed. 12

• Voluntary agreements between industry and governments are politically attractive, serve 13

to change attitudes, increase awareness, lower barriers to innovation and technology 14

adoption, and facilitate cooperation with stakeholders. Well-designed agreements can 15

provide more than business-as-usual energy savings or emission reductions. 16

• Some recent voluntary actions, taken unilaterally by industry, have achieved substantial 17

emission reductions. 18

• Information campaigns and government leadership can be effective in addressing barriers 19

to mitigation. 20

• Financial incentives can stimulate the diffusion of new technologies. While their 21

economic cost are generally higher, they are often critical to overcoming barriers. 22

23

No one policy can be expect to achieve the available mitigation potential, a suite of policies 24

will be needed. [WGIII SPM.27, TS.13.2, 7.ES] 25

26

For 20-30 $/tCO2 prices, modelling studies suggest that for an emission trajectory 27

consistent with stabilization at 650 ppmv CO2-eq, gross world product would be at worst 28

about 0.5% below baseline by 2030, depending on policy mix and incentives for innovation 29

and deployment of low-carbon technologies. Effects for the more stringent long-run targets 30

and prices up to $100/ tCO2 are more uncertain, with most studies suggesting costs less than 31

1.0% global output by 2030, with the estimates heavily dependent on approaches and 32

assumptions (high agreement, much evidence). [WGIII SPM.10] 33

34

This report confirms the conclusions in the TAR on carbon leakage3 as the result of Kyoto 35

policies (in the order of 5-20%, which could be reduced by the diffusion of low-emission 36

technology). It also confirms the spill-over effects of emissions reductions in Annex I 37

countries on non-Annex I countries (oil exporting countries can expect lower oil prices and 38

GDP loss, but the extent to which this will occur depends on both Annex I policies and oil 39

exporting country responses). Recent studies indicate that widespread relocation of energy-40

intensive industries is unlikely and due to higher oil prices, revenues from oil exports are now 41

projected to be much higher than in earlier studies (confidence statement to come). [WGIII 42

SPM.11] 43

44

While studies use different methodologies, there is general agreement for all analyzed 45

world regions that near-term health benefits from reduced air pollution, as a result of GHG 46

3 Carbon leakage is defined as the increase in emissions in non-Annex I countries due to the implementation of

emissions reductions in Annex I countries, expressed as a percentage of Annex I emissions.



mitigation policies that reduce fossil fuel use, can be substantial and may offset a 1

substantial fraction of mitigation costs. Other co-benefits include reduced energy 2

dependence and improved agricultural productivity. Co-benefits of reduced deforestation 3

include improved biodiversity, agricultural production, and soil and water conservation. , and 4

would decrease net costs. An integrated approach in designing air pollution and climate 5

change mitigation policies offers potentially large cost reductions (high agreement, much 6

evidence). [WGIII SPM.12] 7

8

The portfolio of technologies that are either on the market or are expected to be on the 9

market in coming decades can reduce GHG emissions to the levels needed to achieve the 10

assessed range of stabilization pathways, provided that the necessary incentives are in place 11

for their implementation and further development. However, implementation implies that 12

large numbers of new low-emission installations would be needed in a relatively short period. 13

The “lock-in” effects of infrastructure, technology and product design choices made by 14

industrialized countries in the period of low energy prices are responsible for the major 15

increase of world GHG emissions. However, a major part of the infrastructure needed for 16

development is still to be built in developing countries, and the spectrum of future options is 17

considerably wider than in industrialized countries. Due to long life-times of energy and other 18

infrastructure capital stock, widespread diffusion of low-carbon technologies may take many 19

decades. Therefore the use of the projected investment in the expansion and renewal of 20

energy supply till 2030 of at least US$ 20 trillion is critical for the penetration of low carbon 21

technologies (high agreement, much evidence). [WGIII SPM.6] 22

23

4.4 Relationship between adaptation and mitigation options and relationship with 24

sustainable development 25

26

Adaptation and mitigation generally complement one another as response measures to climate 27

change (very high confidence). Mitigation is needed to achieve the deep emissions reductions 28

that will be required if atmospheric greenhouse gas concentrations are to be stabilized. Reliance 29

on adaptation alone could allow the climate to change so significantly that effective reduction in 30

climate risks would be possible only at very high social, environmental and economic costs. 31

Impacts can be reduced or delayed by emissions control, but even the most stringent mitigation 32

efforts cannot avoid further climate change in the next few decades. It follows that adaptation is 33

unavoidable. There is no single optimal mix. Climate policy is not about making a choice 34

between adapting to and mitigating climate change, and neither adaptation nor mitigation alone is 35

likely to be able effectively to avoid dangerous levels of climate change. Taken together, though, 36

progress can be made [WGII 18.4, 18.6; WGIII SPM.31, TS.1.4, TS.3.5]. 37

38

Synergetic options between adaptation and mitigation can be identified in some sectors, but 39

trade-offs are also possible (very high confidence). Synergies can be identified in 40

agriculture, forestry, buildings and urban infrastructure. However, some adaptation options 41

(e.g. cooling, irrigation, coastal protection infrastructure) can lead to increased greenhouse gas 42

emissions when fossil fuels are used to deliver the required energy. In these cases, achieving 43

the same mitigation targets can therefore take more effort. [WGII TS.D.2; WGIII TS.6.4, 44

TS.11.6]. 45

46

Capacities to adapt and to mitigate are driven by similar sets of underlying factors that 47

match well with the precursors and goals of sustainable development (very high 48

confidence). Development pathways influence emissions of greenhouse gases and climate 49



change vulnerability, and vice versa. Climate change and response policies could have 1

significant impacts on development. Enhancing society’s response capacity through the 2

pursuit of sustainable development pathways is therefore one way of promoting both 3

adaptation and mitigation [WGII TS.D.2; WGIII SPM.31]. 4

5

Climate change can make sustainable development more difficult. Response options, per 6

se, can have positive or negative effects on sustainable development (very high confidence). 7

The impacts of climate change are expected to be greatest where they occur in the context of 8

multiple stresses from other sources, such as poverty, unequal access to resources, food 9

security, and environmental degradation. They are likely to impede the achievement of the 10

Millennium Development Goals, particularly over the longer term. Adaptive and mitigative 11

response options can have positive impacts on sustainable development by reducing climate 12

risks and thereby making development more sustainable. In other cases, responses can 13

compete with meeting other vital development objectives. There is a growing understanding 14

of how to implement response options that minimize conflict with other dimensions of 15

sustainable development. Where trade-offs are unavoidable, the same understanding can 16

support rational decision making [WGII SPM.E, TS.D.2, TS.F; WGIII SPM.31, SPM.32, TS. 17

12]. 18

19

Mitigation and adaptation opportunities in development decisions can be substantial, but 20

they vary widely (very high confidence). Decisions about fiscal policy, multilateral 21

development bank lending, insurance practices, industrial policies, electricity market 22

liberalization, energy security, forest conservation, for example, can have profound impacts on 23

GHG emissions, the extent of mitigation possible, and the associated ability of mitigation to 24

reduce climate risks. Conversely, climate policies that implicitly address social, 25

environmental, economic and security issues may turn out to be important levers for creating a 26

sustainable world. Decisions in the same areas can have similarly profound impacts on 27

vulnerability to climate change and adaptation [WGII TS.F; WGIII SPM.31, TS.1] 28

29

4.5 Technology 30

31

Technology is the broad set of processes covering know-how, experience and equipment, 32

used by humans to produce services and transform resources [WG III TS.2.7], and is 33

required to implement both adaptation and mitigation options. Examples of adaptation 34

technology include: alternative crops, improved drinking water systems, and tropical cyclone 35

resistant housing [WG II, Table 17.1]. Examples of mitigation technology include: use of 36

biomass and other renewables to replace fossil fuels, energy efficient electric motor systems 37

and appliances, and control techniques for non-CO2 GHGs [WGIII SPM.13-SPM.24]. 38

39

A portfolio of technologies will be needed to achieve both short and long-term mitigation 40

objectives. Figure 4.2 shows the role that various technologies are expected to play in 41

achieving the 18-25 GtCO2 mitigation potential in 2030 at <$100/tCO2 shown in Table 4.1 42

OR Figure 4.1. 43

44



1 2

Figure 4.2. (This figure is not yet available. Its development was agreed at the WG III LA 3

meeting on 9-13 October, but it will not be finalized until 15 December.) 4

5

While substantial adaptation and mitigation can be achieved with existing technology, new 6

and lower cost technology will be needed to increase adaptive capacity and reduce the cost 7

of achieving stabilization (high agreement, much evidence). The cost and pace of any 8

mitigation response to climate change concerns will depend critically on the cost, 9

performance, and availability of technologies that can lower future emissions. Numerous 10

studies show significant economic value to the improvement of emissions mitigating 11

technologies that are currently in use, and the development and deployment of advanced 12

emissions mitigation technologies [WGII 17.4.1; WGIII TS.2.7]. 13

14

Three important drivers of new technology deployment are: R&D, learning by doing, and 15

spillovers (transfer of knowledge or economic benefits of innovation from one entity to 16

another) (high agreement, much evidence). The evidence strongly suggests that all three of 17

these sources play important roles in technological advance and there is no compelling reason 18

to believe that one is broadly more important than the others. The evidence also suggests that 19

these sources are not simply substitutes, but may have highly complementary interactions 20

[WGIII TS.2.7]. 21

22

Better understanding of the mechanisms of technology development and transfer highlights 23

the need for government support of private sector technology innovation through financial 24

contributions, taxation measures, standard setting and market creation (high agreement, 25

much evidence). Public benefits of R&D investments are much bigger than private benefits, 26

justifying government R&D funding. Global estimates of R&D funding for adaptation 27

technology are not available. Funding for many energy research programs, such as renewables, 28

has been flat or declining for nearly two decades [WGIII SPM.28]. 29

30

4.6 International and regional cooperation 31

32

The Kyoto Protocol has set a significant precedent as a means to solve a long-term 33

international environmental problem (high agreement, much evidence). Its most notable 34

achievements are the stimulation of an array of national policies, the creation of an 35

international carbon market and the establishment of new institutional mechanisms. Its 36

economic impacts on the participating countries are yet to be demonstrated. The CDM, in 37

particular, has created a large project pipeline and mobilized substantial financial resources, 38

but it has faced methodological challenges regarding the determination of baselines and 39

additionality. The Protocol has also stimulated the development of emissions trading systems, 40

which are an increasingly applied implementation mechanism for addressing climate change 41

in nations around the world, but a fully global system has yet to be implemented. However, 42

the Kyoto Protocol has some limitations. For example, its effect on atmospheric 43

concentrations will be limited unless its first commitment period is followed-up by measures 44

to achieve deeper reductions and the implementation of policy instruments by all major 45

emitters. [WGIII 13.3] 46




Numerous options are identified in the literature for achieving emission reductions both 1

under and outside of the Convention and its Kyoto Protocol (high agreement, much 2

evidence), for example, by expanding the scope of market mechanisms through sectoral and 3

sub-national crediting agreements and by enhanced international R&D technology 4

programmes. Sectorally-focused market mechanisms are attractive because they can 5

contribute to sustainable development and attract additional investments and participants and 6

may be more cost effective than project based mechanisms; although they are generally less 7

efficient than broad based market policies. As in the case of project based mechanisms like 8

CDM, there may be methodological challenges in setting baselines and determining 9

additionality. International R&D programmes can induce cost savings, build national capacity 10

and create goodwill. However, they may benefit only a few sectors and may target the wrong 11

technologies. Other cooperative agreements, related to research and development, but not 12

specifically focused on near-term GHG mitigation, are less extensively analyzed in the climate 13

literature. These include technology agreements, such as the implementing agreements of the 14

International Energy Agency. There is no evidence that investments in R&D activities will 15

achieve the same level of emission reductions as quantitative emission objectives, such as 16

those of the Kyoto Protocol, unless supplemented with policies to promote technology 17

adoption. Integrating elements such as technology development and cap and trade 18

programmes in an agreement is possible, but comparing the efforts made by different 19

countries would be very complex and resource consuming. [WGIII 12.2., 13.2, 13.4] 20

21

A great deal of new literature is available on potential structures for and substance of 22

future international agreements (high agreement, much evidence). As has been noted in 23

previous IPCC reports, because climate change is a global commons problem, any approach 24

that does not include a large portion of the world, and at a minimum the world’s major 25

emitters, will be more costly and less environmentally effective – in other words, a second 26

best approach. There is a broad consensus in the literature that a successful agreement will 27

have to be fair/equitable, flexible (accommodate changes while providing adequate 28

investment certainty), scientifically sound, economically efficient and lead ultimately toward 29

universal participation and a more sustainable development path. Most proposals for future 30

agreements in the literature include a discussion of goals, specific actions, timetables, 31

participation, institutional arrangements, reporting and compliance provisions. Other elements 32

address incentives, non-participation and non-compliance penalties. [WGIII 13.3] 33

34

Numerous authors note the need for goals as an important element of any climate 35

agreement (high agreement, much evidence). They determine the extent of participation, the 36

stringency of measures and the timing of actions. There is considerable literature assessing 37

different goals and the pathways to reach them. There is a broad consensus that to limit global 38

temperature to a goal of 2°C above the pre-industrial level, developed countries would need to 39

reduce emissions in 2020 by between 10% to 30% below 1990 levels and in 2050 by 40

approximately 40% to 95%. Emissions in developing countries would need to deviate from 41

their current path by 2020, and emissions in all countries would need to deviate substantially 42

from their current path by 2050. Reaching lower temperature levels requires earlier 43

reductions and greater participation compared to higher levels of greenhouse gases. [WGIII 44

TS.3.3, TS.3.5, TS.13.4] 45

46

While the preponderance of the literature reviews nationally based governmental regimes; 47

regional entities, corporations, sub-national governments, NGOs and civil groups play a 48

key role, and are adopting a wide variety of actions to reduce emissions of greenhouse 49



gases (high agreement, much evidence). Actions by regional, state, provincial and local 1

governments have limited geographical scope, but include for example, renewable portfolio 2

standards, energy efficiency programs, emission registries and sectoral cap and trade 3

mechanisms. Corporate actions range from voluntary initiatives to specific emissions or 4

intensity targets and, in a few cases, internal trading systems. The literature suggests a number 5

of reasons that lead corporations to act, including a desire to influence or pre-empt 6

government action, to create financial value and to differentiate a company and its products. 7

Regional and sub-national entities take actions for other reasons, including a desire to 8

influence national policies, address stakeholder concerns, create incentives for new industries 9

or to create environmental co-benefits. Many of the above actions may limit GHG emissions, 10

stimulate innovative policies, and encourage the deployment of new technologies, but they are 11

by their nature limited in scope (and often in duration) and are thus less than optimal in terms 12

of economic efficiency and environmental effectiveness. There is no evidence indicating that 13

these actions by themselves, will lead to significant national emission reductions, unless 14

supplemented by national government policies. [WGIII 13.4] 15



Topic 5 – The long-term perspective: scientific and socio-economic aspects relevant to 1

adaptation and mitigation, consistent with the objectives and provisions of the 2

Convention, and in the context of sustainable development 3


5

6

5.1 Costs, benefits and avoided damages and risks, timing of mitigation and equity 7

implications, relationship between adaptation and mitigation 8

9

In this section we synthesize information from Working Groups I, II and III in relation to the 10

costs, benefits and avoided damage and risks at global and regional levels from climate 11

change under different scenarios. These factors are compared across a range of 1ºC 12

temperature bands from present to up to 6ºC warming as estimated in Working Group II and 13

placed in the context of the mitigation assessment of WGIII. 14

15

A summary of results from the WGII assessment of impacts and vulnerabilities are shown in 16

Table 5.1. This summarizes the relationships seen in the scientific literature between levels of 17

climate change and risks to biological, social, and regional systems. The key vulnerabilities 18

identified in this table show that the risks tend to increase in nearly all cases with increasing 19

global mean temperature. 20

21

[INSERT TABLE 5.1] 22 23

To place these risks in the context of the mitigation task, Table 5.2 shows the CO2 equivalent 24

concentrations and radiative forcing corresponding to different warming levels at equilibrium 25

above the pre-industrial (1860) climate. The second column gives CO2 equivalent 26

concentrations and radiative forcing that would lead to the warming levels in the first column 27

if the climate sensitivity were equal to the best estimate (3ºC). For temperature thresholds 28

such as 2ºC and 4ºC above preindustrial, the CO2 equivalent concentrations that would lead to 29

those equilibrium warming levels as best estimate are about 441 ppmv CO2 equivalent and 30

701 ppmv CO2 equivalent respectively. The last column shows, based on the WGI assessment 31

of climate sensitivity, the CO2 equivalent concentration that may limit warming at or below 32

the warming levels in the first column with an estimated probability of at least 83%.1 33

34

1 This probability level is used for consistency with the WGI assessment of climate sensitivity. [WGI SPM]



Table 5.2. Global mean temperature increase, greenhouse gas concentration and radiative 1

forcing. 2

3

CO2 equivalent concentration and radiative forcing corresponding to best estimate of climate sensitivity for warming level in column 1 (1) (2)

Temperature increase in ºC above 1990 (above 1860)

CO2 equivalent (ppmv)

Radiative forcing (W/m

2)

CO2 equivalent concentration that may limit warming below warming level in column 1 with an estimated probability of at least 83% (3)

0.0 (0.6) 319 0.7 305 1.0 (1.6) 402 2.0 356 1.4 (2.0) 441 2.5 378 2.0 (2.6) 507 3.2 415 2.4 (3.0) 556 3.7 441 3.0 (3.6) 639 4.5 484 3.4 (4.0) 701 4.9 515 4.0 (4.6) 805 5.7 565 4.4 (5.0) 883 6.2 601 5.0 (5.6) 1014 6.9 659 5.4 (6.0) 1112 7.4 701 6.0 (6.6) 1277 8.2 768

4 Notes: 5 6 1. WGI finds that the climate sensitivity is likely to be in the range 2 to 4.5°C, with a best estimate of about 7

3°C, very unlikely to be less than 1.5°C and values substantially higher than 4.5°C “cannot be excluded. 8 [WGI SPM] 9

2. The simple relationships Teq = T2×CO2 × ln([CO2]/278)/ln(2) and ∆Q = 5.35 × ln([CO2]/278) are used. 10 3. This probability as used as the standard likelihood classifications used in WGI cannot be applied to the 11

conclusions on climate sensitivity. The CO2 concentration equivalent in this column is calculated on the 12 basis that there is at least an 83% chance of the climate sensitivity being at or below 4.5ºC, consistent with 13 WGI findings. 14

15

16

17

Working Group III has analyzed mitigation scenarios according to their achieved radiative 18

forcing levels and Table 5.3 summarizes some of the key results from this assessment. Global 19

scenario studies indicate that the cost of mitigation is rising with increasing stringency of the 20

stabilization target (Figure 5.1). Medium-term costs by 2030 are generally below about 2% 21

loss of Gross Domestic Product (GDP) for a stabilization level of about 4.5 W/m2 (650 ppmv 22

CO2-eq.) and increase to less than 3% for a lower stabilization level of about 3.5 W/m2 (530 23

ppmv CO2-eq.). Mitigation costs are subject to uncertainty and depend critically on the 24

baseline development path, related emissions and the assumed rate of technological change. 25

Uncertainties with respect to the drivers of costs are increasing over time, leading to relatively 26

wider ranges of mitigation costs in the long term (right panel of Figure 5.1). The vast majority 27

of studies report GDP losses below 7% by 2100 with a few studies indicating potential 28

negative losses. Recent literature on multigas abatement strategies indicates also that 29

comprehensive climate mitigation policies, encompassing all sectors and the full basket of 30

GHG emissions (including carbon sinks), would lead to significantly lower costs than 31

abatement policies focusing on CO2 only. [WGIII 3.3] 32 33



-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Stabilization level (W/m2)

GD

P l

os

ses

in 2

03

0 (

pe

rce

nt

pe

r ye

ar)

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Stabilization level (W/m2)

GD

P lo

sse

s in

21

00

(p

erc

en

t p

er

yea

r)

AIM - IMCP

ENTICE - IMCP

MIND - IMCP

DEMETER - IMCP

RICE FAST - IMCP

ISGM - CCSP

MERGE - CCSP

MiniCAM - CCSP

MESSAGE - B1

MESSAGE - B2

MESSAGE - A2

AIM - A1 - PS

ASF - A2 PS

MARIA - B2 - PS

MiniCam - A1 - PS

WorldScan - A1 - PS

WorldScan - B2 - PS

1 2

Figure 5.1. Relationship between cost of mitigation (loss of GDP in percent) and the long-3

term stabilization target (radiative forcing compared to pre-industrial level, W/m2). Left-hand 4

panel gives GDP losses for 2030 and right-hand panel for 2100. Individual lines denote 5

studies with multiple stabilization targets. Stabilization scenarios sharing similar baseline 6

assumptions are shown in the same colour. Blue range represents the 80th

percentile of the 7

literature (TAR and post-TAR scenarios). [WGIII Figure SPM-6] 8 9 10 11 12

Table 5.3 also shows estimates of the probability of scenarios staying below certain warming 13

levels above pre industrial at equilibrium should the stabilized radiative forcing levels be 14

maintained. The lowest scenarios (A1), for example, stabilize CO2 equivalent forcing in the 15

range 444 - 487 ppmv CO2-eq. and at equilibrium with a best estimate climate sensitivity 16

would result in a warming of 2.0 - 2.4ºC with a likely range of 1.4 - 3.6ºC warming above 17

preindustrial. Scenario range B stabilizes CO2 equivalent forcing in the range 535 - 587 ppmv 18

CO2-eq. and at equilibrium with a best estimate climate sensitivity would result in a warming 19

of 2.4 - 3.2ºC with a likely range of 1.9 - 4.9ºC warming above preindustrial. 20 21

A strict comparison between the equilibrium warming associated with each of the scenario 22

range in Table 5.3 and the key vulnerabilities at different temperature ranges in Table 5.1 is 23

not possible. The temperature scale in the latter in most cases reflects realized temperature 24

increases during the 21st century, and time dependent transient warming ranges for the 25

mitigation scenarios which would permit a more direct comparison are not available. 26

27

One of the risks identified in Working Group I that has implications for the mitigation results 28

is that of an increasing carbon cycle feedback. The strength of feedbacks between terrestrial 29

vegetation and climate change varies markedly among models and additional CO2 and CH4 30

releases are possible from permafrost, peat lands, wetlands, and large stores of marine 31

hydrates. The range of these feedbacks is not however presently included in atmospheric CO2 32



concentration or radiative forcing stabilization scenarios. A positive climate-carbon cycle 1

feedback would reduce the land and ocean uptake of CO2, implying a reduction of the 2

compatible emissions required to achieve a given atmospheric CO2 stabilization. [WGI 10.ES] 3

4

Table 5.3. Mitigation and climate change. 5

6

Class Anthropogenic addition to radiative forcing at

stabilisation (W/m

2)

Multigas concentration level (ppmv

CO2-eq)

Stabilisation level for CO2

only, consistent with multi-gas level

(ppmv CO2)

Number of

scenario studies

Global mean temperature ºC increase above pre-industrial at equilibrium, using best

guess climate

sensitivity

Likely range of global

mean temperature ºC increase above pre-industrial at equilibrium

(1)

Peaking year for CO2 emissions

Change in global

emissions in 2050 (% of

2000 emissions)

A1 2.5 - 3.0 444 - 487 350 - 398 6 2.0 - 2.4 1.4 - 3.6 2000 - 2015 -86 to -48

A2 3.0 - 3.5 487 - 535 398 - 442 18 2.4 - 2.8 1.6 - 4.2 2000 - 2020 -62 to -31

B 3.5 - 4.0 535 - 587 442 - 484 21 2.4 - 3.2 1.9 - 4.9 2010 - 2030 -29 to +5

C 4.0 - 5.0 587 - 708 484 - 571 118 3.2 - 4.0 2.2 - 6.1 2020 - 2060 +9 to +58

D 5.0 - 6.0 708 - 853 571 - 657 9 4.0 - 4.9 2.7 - 7.3 2050 - 2080 +27 to +84

E 6.0 - 7.5 853 - 1129 657 - 789 5 4.9 - 6.1 3.2 - 8.5 2060 - 2090 +91 to +142

7 Notes: 8 1. Warming estimated using the likely (66%) range of climate sensitivity of 2-4.5ºC and taking the range 9

from the lower and uppers boundaries of the radiative forcing scenario class. 10

11

12

(Note: The next draft of Topic 5 may include further material that complements that in Topic 13

3 on geophysical issues such as climatic extremes, the meridional overturning circulation, 14

and the Greenland and Antarctic ice sheets, etc. drawing on appropriate material in the 15

WG1 report [e.g., WG1 SPM, TS, chapter 6, 10, and 11].) 16

17

18

19

Figure 5.2 offers a glimpse into the global distribution of an index of national vulnerability to 20

climate impacts in 2100 with and without mitigation (to a 550 ppmv limit) and with and 21

without enhanced adaptive capacity. The upper left panel presents a portrait of the 22

geographical distribution of vulnerability in 2100 along the A2 emissions scenario with a 23

climate sensitivity of 5.5ºC under the limiting assumption that adaptive capacities are fixed at 24

current levels. It is a benchmark of maximum vulnerability against which other options can 25

be assessed. Notice that nearly every nation displays extreme vulnerability. The upper right 26

panel presents comparable geographic distributions under the assumption that adaptive 27

capacity improves everywhere with special emphasis on developing countries whose 28

capacities are assumed to advance to the current global mean by 2100. Some improvement is 29

seen, but adaptation alone still cannot reduce extreme vulnerability worldwide. The lower 30



panels present the effect of limiting equivalent atmospheric concentrations of greenhouse 1

gases to 550 ppmv. In the lower left panels, adaptive capacity is again held constant at current 2

levels, and extreme vulnerability persists in developing countries and threatens developed 3

countries. Mitigation, alone, cannot overcome climate risk. The lower right panels show the 4

complementary effects of investments in enhanced adaptive capacity and mitigation. 5

Developing countries are still most vulnerable. Developed countries are also vulnerable, but 6

they see noticeable benefits from the complementary effects of the policy portfolio. While 7

other results suggest that global mitigation efforts through 2050 would benefit developing 8

countries more than developed countries when combined with enhanced adaptation, climate 9

change through 2100 would produce significant vulnerabilities ubiquitously even if a 10

relatively restrictive concentration cap were implemented in combination with a program 11

designed to enhance adaptive capacity significantly. [WGII 20.ES, 20.7] 12

13

14

15

16

17 Panel A Panel B 18 19 20

21 Panel C Panel D 22

23

Figure 5.2. Geographical distribution of vulnerability in 2100 with and without mitigation 24

along an A2 emissions scenario with a climate sensitivity of 5.5ºC. Panel A portrays vulne-25

rability with a static representation of current adaptive capacity. Panel B shows vulnerability 26

with enhanced adaptive capacity worldwide. Panel C displays the geographical implications 27

of mitigation designed to cap effective atmospheric concentrations of greenhouse gases at 550 28

ppmv. Panel D offers a portrait of the combined complementary effects of mitigation to the 29

same 550 ppmv concentration limit and enhanced adaptive capacity. [WGII 20.7] 30

31



5.2 Technology flows and development 1

2

A large body of literature has illustrated the importance of technological change and its 3

uncertainty for future GHG emission levels and hence the magnitude of possible climate 4

change. The rate of technological change and the future availability of advanced technologies 5

are major determinants of the cost and attainability of achieving stabilization of GHG 6

concentrations. The pace of this change depends critically on the intensity of "demand-pull" 7

and "supply-push" policies to promote technology diffusion, including investments into R&D, 8

learning by doing and technology transfer (or spillovers). Given the inertia of the energy 9

system and the long life-time of its infrastructure, however, the widespread diffusion of low-10

carbon technologies may require many decades. This is also illustrated by the emissions 11

profiles of the stabilization scenarios (Table 5.3), which are characterized by initially 12

increasing emissions over the short-term, reaching a maximum, followed eventually by deep 13

emissions reductions as carbon-saving technologies acquire larger market shares to achieve 14

stabilization. [WGIII 2.9, 3.3, 3.4, 3.6, 4.3, 4.4, 4.6] 15

16

Scenario studies suggest that it is technically feasible to stabilise GHG concentrations in the 17

atmosphere at low levels of about 450-490 ppmv CO2-eq. Such stabilization levels require 18

investments in low-carbon carbon emissions technologies, technology improvements through 19

public and private R&D, as well as rapid diffusion of advanced technologies to reduce CO2 20

emissions to less than half of today's level by 2050 (Table 5.3). [WGIII 3.3, 3.4, 3.6] 21

22

The large variation of national and local circumstances and the uncertainty about future 23

performance of individual options, calls for the implementation of a wide portfolio of 24

mitigation and adaptation technologies. The contribution of individual technologies varies 25

strongly over time and region and depends on the baseline scenario and the analyzed 26

stabilization level. Stabilization studies conclude that the prime target of emissions reductions 27

will be the energy and industry sector, while additional utilization of land-use and forestry 28

mitigation options (both non- CO2 and CO2) provides greater flexibility and thus cost 29

effectiveness. With increasing stringency of the stabilization level more emphasis is put in 30

scenarios on energy efficiency and the use of low carbon energy sources, including renewable 31

energy, nuclear power, and CO2 capture and storage (CCS - in combination with fossil fuels 32

and bioenergy). Figure 5.3 illustrates the contribution of individual mitigation options and the 33

order of magnitude of abatement that is needed when moving from a stabilization level of 34

4.5 W/m2 (650 ppmv CO2-eq.) down to lower stabilization levels such as 3 W/m

2 (500 ppmv 35

CO2-eq.). [WGIII 3.3, 3.4, 4.3, 11.2] 36

37



0 50 100 150 200 250 300 350

Energy conservation & efficiency

Nuclear

Biofuels (incl. CCS)

Other renewables

Fossil CCS

Fossil fuel switch

Forest sinks

Non-CO2

Cumulative emissions reductions (2000-2100), GtC

4.5 W/m2

3 W/m2

IMAGE

MESSAGE

1 Figure 5.3. Figure 5.3: Cumulative emissions reductions for alternative mitigation measures 2

(2000-2100). Scenarios from illustrative models (IMAGE and MESSAGE) aiming at the 3

stabilization of radiative forcing for 3 and 4.5 W/m2 respectively. Black bars denote 4

reductions for a target of 4.5 W/m2 and grey bars the additional reductions to achieve 3 W/m

2. 5

Both models assume intermediate baseline emissions of about 1600 GtC (2000-2100). 6

[preliminary figure - will be revised following WGIII decisions; more models to be added] 7

8

9

5.3 Broader environmental and integration issues 10

11

It is very likely that climate change will result in net costs into the future. Aggregated across 12

the globe and discounted to today, it is also very likely that these costs will grow over time. 13

[WGII SPM.D] 14

15

It is very likely that vulnerability to climate change will impede nations’ abilities to achieve 16

sustainable development pathways, as measured for example as long-term progress towards 17

the Millennium Development Goals. Over the next half-century, for example, it is very likely 18

that climate change will make it more difficult for nations to achieve their Millennium 19

Development Goals. It is, though, very likely that climate change attributed with high 20

confidence to anthropogenic sources, per se, will not be a significant extra impediment to 21

nations’ reaching their 2015 Millennium Development Targets; many other obstacles with 22

more immediate impacts stand in the way. [WGII SPM.E] 23

24

Vulnerability to the impacts of climate change is expected to be most significant when and 25

where they are felt together with these other obstacles – other stresses on human systems such 26

as poverty, unequal access to resources, food insecurity, environmental degradation, and risks 27

from natural hazards. Climate change can, itself, be a source of multiple stress when some of 28

its various manifestations occur in the same location. In these cases, total vulnerability is 29

larger than the sum of vulnerabilities computed in isolation for each impact. [WGII SPM.E] 30



The challenge of coping with multiple stresses can, though, illuminate opportunities for 1

complementary activity. Efforts to cope with the impacts of climate change, either through 2

mitigation or adaptation, and attempts to promote sustainable development share common 3

goals and determinants including, access to resources (including information and technology), 4

equity in the distribution of resources, stocks of human and social capital, access to risk 5

sharing mechanisms, abilities of decision-support mechanisms to cope with uncertainty. 6

Exploiting this coincidence of underlying determinants is the key to exploiting these 7

opportunities. Reducing vulnerability to the hazards associated with current and future 8

climate variability and extremes through specific policies and programs, individual initiatives, 9

participatory planning processes, and other community approaches can, for example, reduce 10

vulnerability to climate change and support sustainable development. [WGII SPM.E; WGIII 11

2.ES] 12

13

It is therefore very likely that significant synergies can be found by bringing climate change to 14

the development community and critical development issues to the climate change 15

community. While synergies between adaptation and mitigation measures are likely to be 16

effective through the middle of this century, however, it is more likely than not that even a 17

combination of aggressive mitigation and significant investment in adaptive capacity will be 18

overwhelmed by the end of the century along likely development scenarios. It is, indeed, very 19

unlikely that the efforts to reduce vulnerability either directly or indirectly through improving 20

underlying determinants of adaptive and mitigative capacities will be sufficient to eliminate 21

all damages associated with climate change. It must be emphasized, as well, that some 22

development activities directed at non-climate goals and objectives can actually exacerbate 23

climate-related vulnerabilities. [WGII SPM.E] 24

25



Table 5.1. Key vulnerabilities. 1

2

Temperature

increase ºC

above 1990

(1860)

Biological

systems

Social systems Regional systems

0 (0.6) Terrestrial: Many ecosystems already affected [WGII 1.3.4, 1.3.5] Marine: Increased coral reef bleaching. [WGII 4.4.9] Freshwater ecosystems: Some lakes already showing decreased fisheries output; pole-ward migration of aquatic species. [ WGII 3.4.3, 19.3.2.2]

Health: Some evidence of health impacts observed. [WGII 19.3.1] Water resources: Some evidence of increased flooding and drought severity. [WGII 3.4.3]

Small Island States: very substantial risks from sea level rise, increased intensity of extreme events, loss of coral reefs, shifts in tourism, and other factors. [WGII 16.4] High-Mountain Communities: Glacial melt is causing flooding in some areas, shifts in ecosystems and water security problems due to decreased storage. Indigenous, poor or isolated communities: Climate change and sea level rise adds significantly to other stresses. Polar Regions [WGII 19.3.2] Climate change is already having substantial impacts on societal and ecological systems. [WGII 15.4.1, 15.4.2, 15.4.6, 15.4.7]

1 (1.6) Terrestrial: Widespread disturbance, sensitive to rate of climate change and land use [WGII 4.4.1] (1-3ºC) Marine: Potential regional extinction of coral reefs. [WGII 4.4.9] Freshwater ecosystems: Poleward migration of aquatic species and reduced lake production.

Food: Reduced low-latitude production. Increased high latitude production (1-3ºC) [WGII 5.6.4] Potential for increased global production. [WGII 5.6.1] Health: Increased morbidity and mortality from some infectious diseases and heat waves. [WGII 19.3.1] Water resources: Saline intrusion of coastal freshwater resources. [WGII 3.4.3] Risks: Increased flooding and drought severity Infrastructure: Likely exponentially increasing risk of infrastructure damage. [WGII Ch 3.5, 6.5.3, 7.5]

Latin America: Tens of millions of people at risk of water shortages [WGII 13.4.3]; many endemic species at risk from land use and climate change. [WGII 13.2.5.1] Africa: Decreased agricultural production and increased risk of hunger, reduced water supplies, and increased risks to human health. [WGII 9.4.1, 9.4.3, 9.4.4] Asia: About a billion people face risks from reduced agricultural production, reduced water supplies, and increases in extreme events. [WGII 10.4.2, 10.6.2]

1.4 (2.0)

2 (2.6) Marine: Widespread degradation of coral reefs, sensitive to pH, runoff, overfishing. [WGII 4.49] Marine: Severe ecological implications from ocean acidification.

Food: Global production peaks and begins to decline. [WGII 5.6.1] Water resources: Severity of floods, droughts, erosion, water quality deterioration will increase with increasing climate change (medium confidence). Hundreds of millions people could face reduced water supplies [WGII 13.4.3] (low-medium confidence). Risks: Severity of droughts, floods and deterioration of water quality increasing.

Africa: All seasons extremely warm by the end of the 21st century. East and West Africa. Frequency of extremely dry seasons generally decreasing and an increase in the number of extremely wet seasons. South Africa the frequency of extremely dry winter and springs increases. Severe drying in southwest Africa likely. [WGI Ch 11] Polar regions: Continued warming will lead to further loss of ice cover and permafrost. Arctic ecosystems and many species will be threatened, although net ecosystem productivity could increase. While some economic opportunities will likely open up (e.g., shipping), traditional ways of life will most likely be disrupted. [WGII 15.4]

2.4 (3.0)



Temperature

increase ºC

above 1990

(1860)

Biological

systems

Social systems Regional systems

3 (3.6) Terrestrial: Large-scale transformation of ecotypes and ecosystem services Loss of 1/3 of species (3ºC). [WGII 4.4.11] Terrestrial: Becomes net source of C above 3ºC. [WGII 4.4.1]

3.4 (4.0)

4 (4.6) Freshwater systems: Extinction of many aquatic species. Increased salinity of inland lakes. [WGII 3.4.3, 19.3.2.2]

Food: Further declines global food production [WGII 5.6.1]

High-Mountain Communities: Widespread impacts on most communities. Many areas will lose their mountain glaciers.

4.4 (5.0)

5 (5.6)

5.4 (6.0)

6 (6.6)

1



Topic 6 – Robust findings, key uncertainties 1


3

4

6.1 Definition and overview 5

6

As in the TAR, a robust finding for climate change is defined as one that holds under a variety 7

of approaches, methods, models, and assumptions and one that is expected to be relatively 8

unaffected by uncertainties. Key uncertainties in this context are those that, if reduced, may 9

lead to new and robust findings in relation to the issues discussed in this report. 10

11

Robust findings, as defined above, are only a part of the key findings of the AR4. Some key 12

findings may be policy-relevant even though, or in some cases because, they are associated 13

with large uncertainties or depend on assumptions and possible futures. Robust findings 14

provide important cornerstones for climate change decision-making, but they do not 15

summarise all scientific knowledge that may be relevant for prudent risk management. 16

17

Compared with earlier assessments, the AR4 includes a greater number, with a greater level of 18

detail, of robust findings on climate change science, impacts, adaptation and vulnerability, and 19

mitigation. Nonetheless, several key uncertainties identified in the TAR continue to exist. In 20

some areas of research, there is now greater recognition of the sources and magnitude of 21

uncertainties that limit our current ability to produce more robust quantitative findings. 22

23

6.2 Robust findings and key uncertainties 24

25

Table 6.1 provides examples of robust findings and key uncertainties; the table entries do not 26

represent an exhaustive list. 27

28

Table 6.1. Robust findings and key uncertainties. 29

30

Area of knowledge Robust findings Key uncertainties

Observed climate

change and its

effects, and

attribution of

observed changes

and effects to natural

and human causes

Global mean surface temperatures continue to

rise. Rates of surface warming increased in the

mid-1970s and the global land surface has been

warming at about double the rate of ocean surface

warming since then. Changes in surface

temperature extremes are consistent with warming

of the climate. [WGI TS.6.2.1]

Physical and biological systems on all continents

and some oceans are already being affected by

recent regional temperature increases. Responses

of terrestrial species to warming across the

northern hemisphere are well documented by

phenological changes, especially the earlier onset

of spring events, migration, and lengthening of the

growing season. Warming of lakes and rivers is

affecting abundance and productivity, community

composition, phenology, distribution and

migration of some freshwater species. [WGII

TS.B]

Radiosonde records are much less complete

spatially than surface records and it is likely that

all records of tropospheric temperature trends still

contain residual errors. [WGI TS.6.2.1]

Analyses of atmospheric circulation are best only

after 1979, making discrimination between change

and variability difficult. [WGI TS.6.2.1]

Records of soil moisture and streamflow are often

very short, and are available for only a few

regions, which impedes analyses of changes in

droughts. [WGI TS.6.2.1]

Information on hurricane frequency and intensity

is limited prior to the satellite era. There are

questions about the interpretation of the satellite

hurricane record. There is insufficient evidence to

determine whether trends exist in tornadoes, hail,

lightning and dust-storms. [WGI TS.6.2.1]

There is no global compilation of snow data prior



There has been widespread retreat of mountain

glaciers since the end of the 19th century. The

extent of Northern Hemisphere snow cover has

declined. Since 1978, annual mean Arctic sea ice

extent has been declining and summer minimum

Arctic ice extent has decreased. [WGI TS.6.2.2]

Effects due to changes in the cryosphere include

slope instability in mountain and permafrost

regions, shorter travel season for vehicles over

frozen roads in the Arctic, increase of glacial

lakes in mountain regions and destabilisation of

moraines damming these lakes, changes in Arctic

and Antarctic Peninsula flora and fauna; limitation

on mountain sports in lower –elevation alpine

areas, and changes in indigenous livelihoods in

the Arctic [WGII TS.B]

Global average sea level rose during the 20th

century and the rate of sea level rise increased

between the mid-19th and mid-20th centuries.

During 1993–2003 sea-level rose more rapidly

than during 1961–2003. [WGI TS.6.2.3]

It is very likely that average Northern Hemisphere

temperatures during the second half of the 20th

century were warmer than in any other 50-year

period in the last 500 years. [WGI TS.6.2.4]

Extremely unlikely (< 5%) that the global pattern

of warming during the past half century can be

explained without external forcing, and very

unlikely that it is due to known natural external

causes alone which would likely have produced

cooling. [WGI TS.6.3]

Greenhouse gas forcing has very likely caused

most of the observed global warming over the last

50 years and would likely have resulted in greater

than the observed warming if there had not been

an offsetting cooling effect from aerosol and other

forcings. [WGI TS.6.3]

Anthropogenic forcing has likely contributed to

the general warming observed in the upper several

hundred metres of the ocean during the latter half

of the twentieth century and has very likely

contributed to sea level rise during the latter half

of the 20th

century through thermal expansion

from ocean warming and glacier mass loss. [WGI

TS.6.3]

to 1960. Mass balance estimates for ice shelves

and ice sheets, especially for Antarctica, are

limited. [TS.WGI 6.2.2]

Decadal variability in global heat content, salinity,

and sea-level changes can only be evaluated with

moderate confidence. There is low confidence in

observations of trends in the meridional

overturning circulation. Global average sea level

rise from 1961–2003 appears to be larger than can

be explained by thermal expansion and land ice

melting. [WGI TS.6.2.3]

Effects of recent ocean acidification on the marine

biosphere are as yet undocumented. [WGII TS.B]

While there is increasing evidence for climate

change impacts on coral reefs, discerning the

impacts of climate-related stresses from other

stresses (e.g., overfishing and pollution) is

difficult. [WGII TS.B]

Impacts of human activities on coasts, such as

urban development, are in general still greater

than impacts that can be attributed to sea-level

rise. [WGII TS.B]

There is little evidence about the effects of

observed climate change on health for two

reasons: the lack of long epidemiological or

health-related data series, and the importance of

non-climate drivers in determining the distribution

and intensity of human disease. [WGII TS.B]

Confidence in attributing some climate change

phenomena to anthropogenic influence is

currently limited by uncertainties in radiative

forcings, feedbacks and observations. [WGII

TS.B]

Attribution at scales smaller than continental and

over timescales less than 50 years is limited by

larger climate variability on smaller scales, by

uncertainties in the small-scale details of the

climate’s response to external forcing and by

uncertainties in simulation of internal variability

on small scales. [WGI TS.6.3]

Less confidence in understanding of forced

changes in precipitation and surface pressure than

there is of temperature. [WGI TS.6.3]

Formal detection and attribution studies are absent

or limited in number for some phenomena

including some types of extreme events. [WGI

TS.6.3]

Apparent discrepancies between estimates of

ocean heat content variability from models and

observations. [WGI TS.6.3]

The sea level budget for 1961-2003 is not closed,

contributing to the uncertainty in attribution of sea

level rise. [WGI TS.6.2.3, TS.6.3]



1


Drivers and

projections of future

changes in climate and

their impacts

In the absence of additional policies, global CO2

emissions from energy use are projected to increase

with 50-100% by 2030 relative to 2000, mainly due

to the continued dominance of fossil fuel use.

[WGIII 1.3]

GHG emissions ranges derived from long-term

baseline scenarios (i.e. without additional climate

policies) have not changed appreciably compared

with SRES. [WGIII 3.2]

For stabilisation to occur at any level, global

emissions would ultimately have to decline to levels

much below current emissions; the lower the

stabilisation level, the earlier global emissions would

have to peak. [WGIII 3.3, 3.5]

Current atmospheric concentrations of carbon

dioxide and methane, and their associated positive

radiative forcing, far exceed those determined from

ice core measurements spanning the last 650,000

years. [WGI TS.6.1]

Fossil fuel use, agriculture, and land use have been

the dominant cause of increases in greenhouse gases

over the last 250 years. [WGI TS.6.1]

Solar contributions to global average radiative

forcing are likely at least five times smaller than the

contribution of increases in greenhouse gases over

the industrial period. [WGI TS.6.1]

Climate models are based on well-established

physical principles and have been demonstrated to

reproduce observed features of recent climate and

past climate changes. There is considerable

confidence that AOGCMs provide credible

quantitative estimates of future climate change,

particularly at continental scales and above.

Confidence in these estimates is higher for some

climate variables (e.g., temperature) than for others

(e.g., precipitation). [WGI FAQ 8.1]

Equilibrium climate sensitivity is likely to be in the

range 2–4.5°C with a most likely value of about 3°C,

based upon multiple observational and modelling

constraints. It is very unlikely to be less than 1.5°C.

There is a good understanding of the origin of

differences in equilibrium climate sensitivity found

in different models. [WGI 8.6, 9.6, Box 10.2]

The large heat capacity of the oceans leads to very

long response time scales in the climate system. Even

if concentrations of radiative forcing agents were to

be stabilized today, further warming and related

Gaps in remain in emissions statistics and

projections for some sectors and applications,

particularly for non-CO2 greenhouse gases.

[WGIII TS.14]

Emission scenarios can support policymaking

by clarifying the results of coherent patterns of

societal choices and trends, but they cannot

predict the future. [WGIII 3.2, 3.3, TS.14]

The full range of processes leading to

modification of cloud properties by aerosols is

not well understood. [WGI TS.6.1]

The causes of recent changes in the growth rate

of atmospheric methane are not well

understood. [WGI TS.6.1]

The roles of different factors increasing

tropospheric ozone concentrations since pre-

industrial times are not well characterized.

[WGI TS.6.1]

The radiative forcings due to land-surface

properties, land-atmosphere interactions,

aerosols, stratospheric water vapour changes

and past solar changes are not well quantified.

[WGI TS.6.1]

A proven set of model metrics comparing

simulations with observations, that might be

used to narrow the range of plausible climate

projections, has yet to be developed. Climate

models remain limited by the spatial resolution

that can be achieved with present computer

resources, by the need for more extensive

ensemble runs, and by the need to include

some additional processes. This hinders

reliable projections of important climate

factors such as tropical storms and other

changes in extremes, ENSO and other major

modes of variability, and oceanic heat uptake.

In many regions where fine spatial scales in

climate are generated by topography, there is

insufficient information on how climate change

will be expressed at these scales. [WGI 8.1-8.5,

10.3, 11.1-11.9]

Models differ considerably in their estimates of

the strength of different feedbacks in the

climate system. Cloud feedbacks are the

primary source of inter-model differences in

equilibrium climate sensitivity, with low cloud

being the largest contributor. [WGI 8.6]



climate changes would be expected to occur well into

the future. Also, the near term warming projections

are little affected by different scenario assumptions

or different model sensitivities, and are consistent

with that observed for the past few decades. Sea level

rise due to thermal expansion and loss of mass from

ice sheets would continue for centuries or millennia

even if radiative forcing were to be stabilized. [WGI

10.3, 10.7]

The projections of warming where the high northern

latitudes and land areas warm more than the

Southern Ocean and the North Atlantic lead to

changes in other variables. For example,

precipitation tends to increase in high latitudes and in

the tropical maxima with decreases in the subtropics.

Snow and land ice reduce in aerial extent and

volume. Sea level increases due to thermal expansion

and land ice melting. Heat waves become more

frequent and longer lasting in a future warmer

climate. Decreases in frost days are shown to occur

almost everywhere in the mid and high latitudes, with

an increase in growing season length. There is a

tendency for summer drying of the mid-continental

areas during summer, indicating a greater risk of

droughts in those regions. [WGI 10.3, FAQ 10.1]

Based on current simulations, it is very likely that the

Atlantic Ocean meridional overturning circulation

(MOC) will slow down by 2100. However, it is very

unlikely that the MOC will undergo a large abrupt

transition during the course of the 21st century.

[WGI 10.3]

Future warming would tend to reduce the capacity of

the Earth system (land and ocean) to absorb

anthropogenic carbon dioxide. As a result, an

increasingly large fraction of anthropogenic CO2

would stay airborne in the atmosphere under a

warmer climate. This feedback requires reductions in

the cumulative emissions consistent with stabilization

at a given atmospheric CO2 level compared to the

hypothetical case of no such feedback. The higher

the stabilization scenario, the larger the amount of

climate change and the larger the required

reductions. [WGI 7.3, 10.4]

[WGII statements on projected impacts to come in

next draft]

The likelihood of a large abrupt change of the

MOC beyond the end of the 21st century

cannot yet be assessed reliably. For low and

medium emission scenarios with atmospheric

greenhouse gas concentrations stabilized

beyond 2100, the MOC recovers from initial

weakening within one to several centuries. A

permanent reduction of the MOC cannot be

excluded if the forcing is strong and long

enough. [WGI 10.7]

The magnitude of future carbon cycle

feedbacks is still poorly determined. [WGI 7.3,

10.4]

The sensitivity of ice sheet surface mass

balance (melting and precipitation) to global

climate change is not well constrained by

observations and has a large spread in models.

There is consequently a large uncertainty in the

magnitude of global warming which, if

sustained, would lead to the elimination of the

Greenland ice sheet. Models do not yet exist

that address key processes that could contribute

to large rapid dynamical changes in the

Antarctic and Greenland ice sheets that could

increase the discharge of ice into the ocean.

[WGI 10.6, 10.7]

[WGII statements related to projected impacts

to come in next draft]

1


Options to respond to

changes and manage

risks by adaptation and

mitigation

Some limited adaptation is occurring now, and more

is projected as response to future climate change.

More extensive adaptation is required to reduce

vulnerability to higher levels and rates of warming,

but there are barriers, limits and costs. Moreover,

future vulnerability depends strongly on development

Comparison of impacts across different scales,

groups and among different metrics (e.g.

market system changes, species lost,

distributional effects, the possibility of

catastrophic losses, choices of equity weights

and discount rates, etc) creates very large



pathway. [WGII 2.4.6, 17.ES, 19.4.1]

Vulnerability to the impacts of climate change will

be greatest when and where these impacts are

experienced together with stresses from other

sources. Because climate change manifests itself in

many ways (changes in temperature, changes in the

frequency of temperature extremes, extreme events,

changes in precipitation, sea level rise, etc), climate

change can itself be the source of multiple stresses.

Vulnerability across multiple manifestations of

climate change can therefore be greater than the sum

of the vulnerabilities to specific impacts taken in

isolation [WGII 19.3.4, 20.3, 20.7]

Climate change will result in net costs into the future,

aggregated across the globe and discounted to today;

these costs will grow over time. Impacts are very

likely to be reduced (but not eliminated) and/or

delayed by a portfolio of adaptation and mitigation

measures. Aggregate estimates also hide an

enormous range of impacts characterized in multiple

numeraires and distributed across regions, nations,

and individuals. [WGII 18.4, 19.4.1, 19.4.2, 20.6]

Given the uncertainties in factors such as climate

sensitivity, regional climate change, vulnerability to

climate change, adaptive capacity and the likelihood

of bringing such capacity to bear, a risk management

framework emerges as an appropriate framework to

address key vulnerabilities. However, the

assignment of probabilities to specific key impacts is

often very difficult due to the large uncertainties

involved. [WGII 2.2, 19.2, 20.9]

A wide range of mitigation options currently exist in

all sectors at reasonable cost. [WGIII 11.3]

Mitigation potentials can only be reached when

adequate government policies are in place. A wide

variety of national policies and instruments are

available to governments to create incentives.

[WGIII 13.2]

The Kyoto protocol has stimulated an array of

national policies, the creation of a global carbon

market and the establishment of new institutional

mechanisms that may provide the foundation for

future mitigation efforts. [WGIII 13.3]

Stabilisation cannot be reached by applying a single

technology: a portfolio of measures would be

needed. [WGIII 3.3]

The lower the assumed stabilisation level the higher

the cost and the uncertainty about the cost, but the

greater the avoided climate change damages. [WGIII

3.3, 3.5]

uncertainties in the estimation of the social

costs of carbon. [WGII 19.4.2, 18.ES, 20.6]

Estimation of adaptive capacity becomes more

uncertain the greater the level of warming and

the faster that level is reached. Rates of

climate change thus add a considerable element

of uncertainty in estimating the benefits of

various stabilization pathways in reducing key

vulnerabilities, especially for groups or systems

with inherently lower adaptive potential.

[WGII 18.ES, 19.4.2]

Applying risk management techniques require

information about impacts that are located

outside prevailing tendencies, but their

possibilities and relative likelihoods are

currently poorly quantified. [WGII Box 19.3,

20.9]

A wide range of estimates of mitigation

potential exists as a result of the following

uncertainties:

• Emission baselines, which are functions of

development trends, policy choices, and

consumer (both individual and industrial)

preferences. [WGIII TS.14, 3.2]

• Rates of technological development and

implementation, which are functions of

technology cost and policy drivers, both

climate and non-climate. [WGIII 3.4, 11.5]

• In the case of carbon sinks, the persistence

of mitigation measures. [WGIII 8.4, 9.4]

• Ability to identify links and capture

synergies between mitigation and

sustainable development, including

methods, modalities and roles of various

actors in the integration of mitigation in

non-climate policies. [WGIII TS.14]

There are fewer studies on mitigation costs,

potentials, and instruments for countries

belonging to economies in transition and most

developing regions than for developed

countries. [WGIII TS.14, 11.3]

The mechanism and impacts of “technology

learning”, diffusion and transfer are

insufficiently known. [WGIII TS.14, 2.8, 3.4,

11.5]



The range of stabilisation levels assessed can be

achieved by application of a combination of

technologies that are on the market and those that are

projected to be commercialised in coming decades,

provided the necessary incentives are in place for

investments, cost reduction and further development

and deployment of technologies. [WGIII 3.3, 3.4,

3.6, 11.3]

Making development more sustainable by changing

development paths can make a major contribution to

climate change mitigation and there are many

possibilities to choose and implement mitigation

options to realise synergies and avoid conflicts with

other dimensions of sustainable development.

[WGIII 2.2, 3.3, 12.1, 12.2]

If the prices of fossil fuels continue to rise, more

low-carbon alternatives will become competitive but

on the other hand, oil sands, oil shales, heavy oils,

and synthetic fuels from coal and gas will also

become more competitive, leading to increasing

GHG emissions, unless power plants are equipped

with CCS. [WGIII 4.2-4.5, 5.3]

For all analyzed world regions, near-term health

benefits from reduced air pollution following GHG

reductions can be substantial and may offset a

substantial fraction of mitigation costs. [WGIII 11.8]

1

Documents

Introduction 2 (26 November 2006) 3 4 In accordance with a ...download.repubblica.it/pdf/2007/syr.pdf · 41 large regions: significantly increased precipitation in eastern parts of