A report of Working Group I of the 

Intergovernmental Panel on Climate Change

Summary for Policymakers

Drafting Authors:

Richard B. Alley, Terje Berntsen, Nathaniel L. Bindoff, Zhenlin Chen, Amnat Chidthaisong, Pierre Friedlingstein, 

Jonathan M. Gregory, Gabriele C. Hegerl, Martin Heimann, Bruce Hewitson, Brian J. Hoskins, Fortunat Joos, Jean Jouzel, 

Vladimir Kattsov, Ulrike Lohmann, Martin Manning, Taroh Matsuno, Mario Molina, Neville Nicholls, Jonathan Overpeck, 

Dahe Qin, Graciela Raga, Venkatachalam Ramaswamy, Jiawen Ren, Matilde Rusticucci, Susan Solomon, Richard Somerville, 

Thomas F. Stocker, Peter A. Stott, Ronald J. Stouffer, Penny Whetton, Richard A. Wood, David Wratt

Draft Contributing Authors:

J. Arblaster, G. Brasseur, J.H. Christensen, K.L. Denman, D.W. Fahey, P. Forster, E. Jansen, P.D. Jones, R. Knutti, 

H. Le Treut, P. Lemke, G. Meehl, P. Mote, D.A. Randall, D.A. Stone, K.E. Trenberth, J. Willebrand, F. Zwiers 

This Summary for Policymakers should be cited as:

IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working 

Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, 

Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom 

and New York, NY, USA.
2

Summary for Policymakers 

Introduction

The Working Group I contribution to the IPCC Fourth 

Assessment Report describes progress in understanding of 

the human and natural drivers of climate change,1 observed 

climate change, climate processes and attribution, and 

estimates of projected future climate change. It builds 

upon past IPCC assessments and incorporates new fi ndings 

from the past six years of research. Scientifi c progress 

since the Third Assessment Report (TAR) is based upon 

large amounts of new and more comprehensive data, 

more sophisticated analyses of data, improvements in 

understanding of processes and their simulation in models 

and more extensive exploration of uncertainty ranges.

The basis for substantive paragraphs in this Summary 

for Policymakers can be found in the chapter sections 

specifi ed in curly brackets.

Human and Natural Drivers

of Climate Change

Changes in the atmospheric abundance of greenhouse 

gases and aerosols, in solar radiation and in land surface 

properties alter the energy balance of the climate system. 

These changes are expressed in terms of radiative 

forcing,2 which is used to compare how a range of human 

and natural factors drive warming or cooling infl uences 

on global climate. Since the TAR, new observations and 

related modelling of greenhouse gases, solar activity, land 

surface properties and some aspects of aerosols have led 

to improvements in the quantitative estimates of radiative 

forcing.

Global atmospheric concentrations of carbon 

dioxide, methane and nitrous oxide have increased 

markedly as a result of human activities since 1750 

and now far exceed pre-industrial values determined 

from ice cores spanning many thousands of years 

(see Figure SPM.1). The global increases in carbon 

dioxide concentration are due primarily to fossil fuel 

use and land use change, while those of methane 

and nitrous oxide are primarily due to agriculture. 

{2.3, 6.4, 7.3}

• Carbon dioxide is the most important anthropogenic 

greenhouse gas (see Figure SPM.2). The global 

atmospheric concentration of carbon dioxide has 

increased from a pre-industrial value of about 280 ppm 

to 379 ppm3 in 2005. The atmospheric concentration 

of carbon dioxide in 2005 exceeds by far the natural 

range over the last 650,000 years (180 to 300 ppm) as 

determined from ice cores. The annual carbon dioxide 

concentration growth rate was larger during the last 

10 years (1995–2005 average: 1.9 ppm per year), than 

it has been since the beginning of continuous direct 

atmospheric measurements (1960–2005 average: 1.4 

ppm per year) although there is year-to-year variability 

in growth rates. {2.3, 7.3}

• The primary source of the increased atmospheric 

concentration of carbon dioxide since the pre-industrial 

period results from fossil fuel use, with land-use change 

providing another signifi cant but smaller contribution. 

Annual fossil carbon dioxide emissions4 increased 

from an average of 6.4 [6.0 to 6.8]5 GtC (23.5 [22.0 to 

25.0] GtCO2) per year in the 1990s to 7.2 [6.9 to 7.5] 

GtC (26.4 [25.3 to 27.5] GtCO2) per year in 2000–2005 

(2004 and 2005 data are interim estimates). Carbon 

dioxide emissions associated with land-use change 

1 Climate change in IPCC usage refers to any change in climate over time, whether due to natural variability or as a result of human activity. This usage differs from 

that in the United Nations Framework Convention on Climate Change, where climate change refers to a change of climate that is attributed directly or indirectly to 

human activity that alters the composition of the global atmosphere and that is in addition to natural climate variability observed over comparable time periods.

2 Radiative forcing is a measure of the infl uence that 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. Positive forcing tends to warm the surface while negative forcing tends to cool it. In 

this report, radiative forcing values are for 2005 relative to pre-industrial conditions defi ned at 1750 and are expressed in watts per square metre (W m–2). See Glossary and Section 2.2 for further details.

3 ppm (parts per million) or ppb (parts per billion, 1 billion = 1,000 million) is the ratio of the number of greenhouse gas molecules to the total number of molecules of 

dry air. For example, 300 ppm means 300 molecules of a greenhouse gas per million molecules of dry air.

4 Fossil carbon dioxide emissions include those from the production, distribution and consumption of fossil fuels and as a by-product from cement production. An 

emission of 1 GtC corresponds to 3.67 GtCO2.

5 In general, uncertainty ranges for results given in this Summary for Policymakers are 90% uncertainty intervals unless stated otherwise, that is, 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 are 

given where available. Assessed uncertainty intervals are not always symmetric about the corresponding best estimate. Note that a number of uncertainty ranges in 

the Working Group I TAR corresponded to 2 standard deviations (95%), often using expert judgement.
Summary for Policymakers 

3

Figure SPM.1. Atmospheric concentrations of carbon dioxide, 

methane and nitrous oxide over the last 10,000 years (large 

panels) and since 1750 (inset panels). Measurements are shown 

from ice cores (symbols with different colours for different studies) 

and atmospheric samples (red lines). The corresponding radiative 

forcings are shown on the right hand axes of the large panels. 

{Figure 6.4}

are estimated to be 1.6 [0.5 to 2.7] GtC (5.9 [1.8 to 

9.9] GtCO2) per year over the 1990s, although these 

estimates have a large uncertainty. {7.3}

• The global atmospheric concentration of methane has 

increased from a pre-industrial value of about 715 ppb 

to 1732 ppb in the early 1990s, and was 1774 ppb in 

2005. The atmospheric concentration of methane 

in 2005 exceeds by far the natural range of the last 

650,000 years (320 to 790 ppb) as determined from ice 

cores. Growth rates have declined since the early 1990s, 

consistent with total emissions (sum of anthropogenic 

and natural sources) being nearly constant during this 

period. It is very likely6 that the observed increase 

in methane concentration is due to anthropogenic 

activities, predominantly agriculture and fossil fuel 

use, but relative contributions from different source 

types are not well determined. {2.3, 7.4} 

• The global atmospheric nitrous oxide concentration 

increased from a pre-industrial value of about 270 

ppb to 319 ppb in 2005. The growth rate has been 

approximately constant since 1980. More than a third 

of all nitrous oxide emissions are anthropogenic and 

are primarily due to agriculture. {2.3, 7.4}

The understanding of anthropogenic warming and 

cooling infl uences on climate has improved since 

the TAR, leading to very high confi dence7 that the 

global average net effect of human activities since 

1750 has been one of warming, with a radiative 

forcing of +1.6 [+0.6 to +2.4] W m–2 (see Figure 

SPM.2). {2.3., 6.5, 2.9}

• The combined radiative forcing due to increases in 

carbon dioxide, methane, and nitrous oxide is +2.30 

[+2.07 to +2.53] W m–2, and its rate of increase 

during the industrial era is very likely to have been 

unprecedented in more than 10,000 years (see Figures 

CHANGES IN GREENHOUSE GASES FROM ICE CORE

AND MODERN DATA

6 In this Summary for Policymakers, the following terms have been used to 

indicate the assessed likelihood, using expert judgement, of an outcome or 

a result: Virtually certain > 99% probability of occurrence, Extremely likely > 

95%, Very likely > 90%, Likely > 66%, More likely than not > 50%, Unlikely

< 33%, Very unlikely < 10%, Extremely unlikely < 5% (see Box TS.1 for more 

details).

7 In this Summary for Policymakers the following levels of confi dence have 

been used to express expert judgements on the correctness of the underlying science: very high confi dence represents at least a 9 out of 10 chance 

of being correct; high confi dence represents about an 8 out of 10 chance of 

being correct (see Box TS.1) 
4

Summary for Policymakers 

Figure SPM.2. Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2 ), methane 

(CH4 ), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of 

the forcing and the assessed level of scientifi c understanding (LOSU). The net anthropogenic radiative forcing and its range are also 

shown. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. 

Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural 

forcing but are not included in this fi gure due to their episodic nature. The range for linear contrails does not include other possible effects 

of aviation on cloudiness. {2.9, Figure 2.20}

SPM.1 and SPM.2). The carbon dioxide radiative 

forcing increased by 20% from 1995 to 2005, the 

largest change for any decade in at least the last 200 

years. {2.3, 6.4} 

• Anthropogenic contributions to aerosols (primarily 

sulphate, organic carbon, black carbon, nitrate and 

dust) together produce a cooling effect, with a total 

direct radiative forcing of –0.5 [–0.9 to –0.1] W m–2

and an indirect cloud albedo forcing of –0.7 [–1.8 to 

–0.3] W m–2. These forcings are now better understood 

than at the time of the TAR due to improved in situ, 

satellite and ground-based measurements and more 

comprehensive modelling, but remain the dominant 

uncertainty in radiative forcing. Aerosols also infl uence 

cloud lifetime and precipitation. {2.4, 2.9, 7.5}

• Signifi cant anthropogenic contributions to radiative 

forcing come from several other sources. Tropospheric 

ozone changes due to emissions of ozone-forming 

chemicals (nitrogen oxides, carbon monoxide, and 

hydrocarbons) contribute +0.35 [+0.25 to +0.65] 

W m–2. The direct radiative forcing due to changes 

in halocarbons8 is +0.34 [+0.31 to +0.37] W m–2. 

Changes in surface albedo, due to land cover changes 

and deposition of black carbon aerosols on snow, exert 

RADIATIVE FORCING COMPONENTS

8 Halocarbon radiative forcing has been recently assessed in detail in IPCC’s Special Report on Safeguarding the Ozone Layer and the Global Climate System (2005).
Summary for Policymakers 

5

respective forcings of –0.2 [–0.4 to 0.0] and +0.1 [0.0 

to +0.2] W m–2. Additional terms smaller than ±0.1 W 

m–2 are shown in Figure SPM.2. {2.3, 2.5, 7.2}

• Changes in solar irradiance since 1750 are estimated 

to cause a radiative forcing of +0.12 [+0.06 to +0.30] 

W m–2, which is less than half the estimate given in the 

TAR. {2.7}

Direct Observations of Recent

Climate Change

Since the TAR, progress in understanding how climate is 

changing in space and in time has been gained through 

improvements and extensions of numerous datasets and 

data analyses, broader geographical coverage, better 

understanding of uncertainties, and a wider variety of 

measurements. Increasingly comprehensive observations 

are available for glaciers and snow cover since the 1960s, 

and for sea level and ice sheets since about the past 

decade. However, data coverage remains limited in some 

regions. 

Warming of the climate system is unequivocal, as is 

now evident from observations of increases in global 

average air and ocean temperatures, widespread 

melting of snow and ice, and rising global average 

sea level (see Figure SPM.3). {3.2, 4.2, 5.5}

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

the 12 warmest years in the instrumental record of 

global surface temperature9 (since 1850). The updated 

100-year linear trend (1906 to 2005) of 0.74°C [0.56°C 

to 0.92°C] is therefore larger than the corresponding 

trend for 1901 to 2000 given in the TAR of 0.6°C 

[0.4°C to 0.8°C]. The linear warming trend over the 

last 50 years (0.13°C [0.10°C to 0.16°C] per decade) 

is nearly twice that for the last 100 years. The total 

temperature increase from 1850–1899 to 2001–2005 is 

0.76°C [0.57°C to 0.95°C]. Urban heat island effects 

are real but local, and have a negligible infl uence (less 

than 0.006°C per decade over land and zero over the 

oceans) on these values. {3.2} 

• New analyses of balloon-borne and satellite 

measurements of lower- and mid-tropospheric 

temperature show warming rates that are similar 

to those of the surface temperature record and are 

consistent within their respective uncertainties, largely 

reconciling a discrepancy noted in the TAR. {3.2, 3.4} 

• The average atmospheric water vapour content has 

increased since at least the 1980s over land and ocean 

as well as in the upper troposphere. The increase is 

broadly consistent with the extra water vapour that 

warmer air can hold. {3.4} 

• Observations since 1961 show that the average 

temperature of the global ocean has increased to depths 

of at least 3000 m and that the ocean has been absorbing 

more than 80% of the heat added to the climate system. 

Such warming causes seawater to expand, contributing 

to sea level rise (see Table SPM.1). {5.2, 5.5} 

• Mountain glaciers and snow cover have declined on 

average in both hemispheres. Widespread decreases 

in glaciers and ice caps have contributed to sea level 

rise (ice caps do not include contributions from the 

Greenland and Antarctic Ice Sheets). (See Table 

SPM.1.) {4.6, 4.7, 4.8, 5.5} 

• New data since the TAR now show that losses from 

the ice sheets of Greenland and Antarctica have very 

likely contributed to sea level rise over 1993 to 2003 

(see Table SPM.1). Flow speed has increased for some 

Greenland and Antarctic outlet glaciers, which drain ice 

from the interior of the ice sheets. The corresponding 

increased ice sheet mass loss has often followed 

thinning, reduction or loss of ice shelves or loss of 

fl oating glacier tongues. Such dynamical ice loss is 

suffi cient to explain most of the Antarctic net mass 

loss and approximately half of the Greenland net mass 

loss. The remainder of the ice loss from Greenland has 

occurred because losses due to melting have exceeded 

accumulation due to snowfall. {4.6, 4.8, 5.5}

• Global average sea level rose at an average rate of 1.8 

[1.3 to 2.3] mm per year over 1961 to 2003. The rate 

was faster over 1993 to 2003: about 3.1 [2.4 to 3.8] 

mm per year. Whether the faster rate for 1993 to 2003 

refl ects decadal variability or an increase in the longerterm trend is unclear. There is high confi dence that 

9 The average of near-surface air temperature over land and sea surface temperature.
6

Summary for Policymakers 

CHANGES IN TEMPERATURE, SEA LEVEL AND NORTHERN HEMISPHERE SNOW COVER

Figure SPM.3. Observed changes in (a) global average surface temperature, (b) global average sea level from tide gauge (blue) and 

satellite (red) data and (c) Northern Hemisphere snow cover for March-April. All changes are relative to corresponding averages for 

the period 1961–1990. Smoothed curves represent decadal average values while circles show yearly values. The shaded areas are the 

uncertainty intervals estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c). {FAQ 3.1, 

Figure 1, Figure 4.2, Figure 5.13}
Summary for Policymakers 

7

the rate of observed sea level rise increased from the 

19th to the 20th century. The total 20th-century rise is 

estimated to be 0.17 [0.12 to 0.22] m. {5.5}

• For 1993 to 2003, the sum of the climate contributions 

is consistent within uncertainties with the total sea level 

rise that is directly observed (see Table SPM.1). These 

estimates are based on improved satellite and in situ

data now available. For the period 1961 to 2003, the 

sum of climate contributions is estimated to be smaller 

than the observed sea level rise. The TAR reported a 

similar discrepancy for 1910 to 1990. {5.5} 

At continental, regional and ocean basin scales, 

numerous long-term changes in climate have 

been observed. These include changes in arctic 

temperatures and ice, widespread changes in 

precipitation amounts, ocean salinity, wind patterns 

and aspects of extreme weather including droughts, 

heavy precipitation, heat waves and the intensity of 

tropical cyclones.10 {3.2, 3.3, 3.4, 3.5, 3.6, 5.2}

• Average arctic temperatures increased at almost twice 

the global average rate in the past 100 years. Arctic 

temperatures have high decadal variability, and a warm 

period was also observed from 1925 to 1945. {3.2}

10 Tropical cyclones include hurricanes and typhoons.

11 The assessed regions are those considered in the regional projections chapter of the TAR and in Chapter 11 of this report.

• Satellite data since 1978 show that annual average 

arctic sea ice extent has shrunk by 2.7 [2.1 to 3.3]% 

per decade, with larger decreases in summer of 7.4 [5.0 

to 9.8]% per decade. These values are consistent with 

those reported in the TAR. {4.4}

• Temperatures at the top of the permafrost layer have 

generally increased since the 1980s in the Arctic (by 

up to 3°C). The maximum area covered by seasonally 

frozen ground has decreased by about 7% in the 

Northern Hemisphere since 1900, with a decrease in 

spring of up to 15%. {4.7}

• Long-term trends from 1900 to 2005 have been observed 

in precipitation amount over many large regions.11

Signifi cantly increased precipitation has been observed 

in eastern parts of North and South America, northern 

Europe and northern and central Asia. Drying has been 

observed in the Sahel, the Mediterranean, southern 

Africa and parts of southern Asia. Precipitation is 

highly variable spatially and temporally, and data are 

limited in some regions. Long-term trends have not 

been observed for the other large regions assessed.11

{3.3, 3.9}

• Changes in precipitation and evaporation over the 

oceans are suggested by freshening of mid- and highlatitude waters together with increased salinity in lowlatitude waters. {5.2}

Table SPM.1. Observed rate of sea level rise and estimated contributions from different sources. {5.5, Table 5.3} 

 Rate of sea level rise (mm per year)

Source of sea level rise 1961–2003 1993–2003

Thermal expansion 0.42 ± 0.12 1.6 ± 0.5

Glaciers and ice caps 0.50 ± 0.18 0.77 ± 0.22

Greenland Ice Sheet 0.05 ± 0.12 0.21 ± 0.07

Antarctic Ice Sheet 0.14 ± 0.41 0.21 ± 0.35

Sum of individual climate 1.1 ± 0.5 2.8 ± 0.7

contributions to sea level rise

Observed total sea level rise 1.8 ± 0.5a 3.1 ± 0.7a

Difference

(Observed minus sum of 0.7 ± 0.7 0.3 ± 1.0

estimated climate contributions)

Table note:

a Data prior to 1993 are from tide gauges and after 1993 are from satellite altimetry.
8

Summary for Policymakers 

Table notes: 

a See Table 3.7 for further details regarding defi nitions.

b See Table TS.4, Box TS.5 and Table 9.4.

c Decreased frequency of cold days and nights (coldest 10%).

d Warming of the most extreme days and nights each year.

e Increased frequency of hot days and nights (hottest 10%).

f

 Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgement rather than formal attribution 

studies. 

g Extreme high sea level depends on average sea level and on regional weather systems. It is defi ned here as the highest 1% of hourly values of observed sea level at a station for a given reference period. 

h Changes in observed extreme high sea level closely follow the changes in average sea level. {5.5} It is very likely that anthropogenic activity contributed 

to a rise in average sea level. {9.5} 

i

 In all scenarios, the projected global average sea level at 2100 is higher than in the reference period. {10.6} The effect of changes in regional weather 

systems on sea level extremes has not been assessed.

• Mid-latitude westerly winds have strengthened in both 

hemispheres since the 1960s. {3.5}

• More intense and longer droughts have been observed 

over wider areas since the 1970s, particularly in the 

tropics and subtropics. Increased drying linked with 

higher temperatures and decreased precipitation has 

contributed to changes in drought. Changes in sea 

surface temperatures, wind patterns and decreased 

snowpack and snow cover have also been linked to 

droughts. {3.3}

• The frequency of heavy precipitation events has 

increased over most land areas, consistent with warming 

and observed increases of atmospheric water vapour. 

{3.8, 3.9}

• Widespread changes in extreme temperatures have been 

observed over the last 50 years. Cold days, cold nights 

and frost have become less frequent, while hot days, 

hot nights and heat waves have become more frequent 

(see Table SPM.2). {3.8}

Table SPM.2. Recent trends, assessment of human infl uence on the trend and projections for extreme weather events for which there 

is an observed late-20th century trend. {Tables 3.7, 3.8, 9.4; Sections 3.8, 5.5, 9.7, 11.2–11.9}

 Likelihood that trend Likelihood of a Likelihood of future trends

 Phenomenona and occurred in late 20th human contribution based on projections for

 direction of trend century (typically to observed trendb 21st century using

 post 1960) SRES scenarios

Warmer and fewer cold

 days and nights over Very likelyc Likelyd Virtually certaind

most land areas

Warmer and more frequent

hot days and nights over Very likelye Likely (nights)d Virtually certaind

most land areas

Warm spells/heat waves.

Frequency increases over Likely More likely than notf Very likely

most land areas

Heavy precipitation events.

Frequency (or proportion of Likely More likely than notf

 Very likely total rainfall from heavy falls)

increases over most areas 

Area affected by Likely in many More likely than not Likely droughts increases regions since 1970s 

Intense tropical cyclone Likely in some

More likely than notf

 Likely activity increases regions since 1970

Increased incidence of

extreme high sea level Likely More likely than notf,h Likelyi

(excludes tsunamis)g
Summary for Policymakers 

9

• There is observational evidence for an increase in 

intense tropical cyclone activity in the North Atlantic 

since about 1970, correlated with increases of tropical 

sea surface temperatures. There are also suggestions 

of increased intense tropical cyclone activity in some 

other regions where concerns over data quality are 

greater. Multi-decadal variability and the quality of 

the tropical cyclone records prior to routine satellite 

observations in about 1970 complicate the detection 

of long-term trends in tropical cyclone activity. There 

is no clear trend in the annual numbers of tropical 

cyclones. {3.8} 

Some aspects of climate have not been observed to 

change. {3.2, 3.8, 4.4, 5.3}

• A decrease in diurnal temperature range (DTR) was 

reported in the TAR, but the data available then extended 

only from 1950 to 1993. Updated observations reveal 

that DTR has not changed from 1979 to 2004 as both 

day- and night-time temperature have risen at about 

the same rate. The trends are highly variable from one 

region to another. {3.2}

• Antarctic sea ice extent continues to show interannual 

variability and localised changes but no statistically 

signifi cant average trends, consistent with the lack 

of warming refl ected in atmospheric temperatures 

averaged across the region. {3.2, 4.4}

• There is insuffi cient evidence to determine whether 

trends exist in the meridional overturning circulation 

(MOC) of the global ocean or in small-scale phenomena 

such as tornadoes, hail, lightning and dust-storms. 

{3.8, 5.3}

A Palaeoclimatic Perspective

Palaeoclimatic studies use changes in climatically sensitive 

indicators to infer past changes in global climate on time 

scales ranging from decades to millions of years. Such proxy 

data (e.g., tree ring width) may be infl uenced by both local 

temperature and other factors such as precipitation, and 

are often representative of particular seasons rather than 

full years. Studies since the TAR draw increased confi dence 

from additional data showing coherent behaviour across 

multiple indicators in different parts of the world. However, 

uncertainties generally increase with time into the past due 

to increasingly limited spatial coverage. 

Palaeoclimatic information supports the interpretation that the warmth of the last half century 

is unusual in at least the previous 1,300 years. 

The last time the polar regions were signifi cantly 

warmer than present for an extended period (about 

125,000 years ago), reductions in polar ice volume 

led to 4 to 6 m of sea level rise. {6.4, 6.6}

• Average Northern Hemisphere temperatures during the 

second half of the 20th century were very likely higher 

than during any other 50-year period in the last 500 

years and likely the highest in at least the past 1,300 

years. Some recent studies indicate greater variability 

in Northern Hemisphere temperatures than suggested 

in the TAR, particularly fi nding that cooler periods 

existed in the 12th to 14th, 17th and 19th centuries. 

Warmer periods prior to the 20th century are within the 

uncertainty range given in the TAR. {6.6}

• Global average sea level in the last interglacial period 

(about 125,000 years ago) was likely 4 to 6 m higher 

than during the 20th century, mainly due to the retreat 

of polar ice. Ice core data indicate that average polar 

temperatures at that time were 3°C to 5°C higher than 

present, because of differences in the Earth’s orbit. The 

Greenland Ice Sheet and other arctic ice fi elds likely

contributed no more than 4 m of the observed sea level 

rise. There may also have been a contribution from 

Antarctica. {6.4} 
10

Summary for Policymakers 

Understanding and Attributing

Climate Change

This assessment considers longer and improved records, 

an expanded range of observations and improvements in 

the simulation of many aspects of climate and its variability 

based on studies since the TAR. It also considers the results 

of new attribution studies that have evaluated whether 

observed changes are quantitatively consistent with the 

expected response to external forcings and inconsistent 

with alternative physically plausible explanations.

Most of the observed increase in global average 

temperatures since the mid-20th century is very 

likely due to the observed increase in anthropogenic 

greenhouse gas concentrations.12 This is an 

advance since the TAR’s conclusion that “most of 

the observed warming over the last 50 years is likely

to have been due to the increase in greenhouse gas 

concentrations”. Discernible human infl uences 

now extend to other aspects of climate, including 

ocean warming, continental-average temperatures, 

temperature extremes and wind patterns (see 

Figure SPM.4 and Table SPM.2). {9.4, 9.5}

• It is likely that increases in greenhouse gas 

 concentrations alone would have caused more 

 warming than observed because volcanic and 

anthropogenic aerosols have offset some warming that 

would otherwise have taken place. {2.9, 7.5, 9.4}

• The observed widespread warming of the atmosphere 

and ocean, together with ice mass loss, support the 

conclusion that it is extremely unlikely that global 

climate change of the past 50 years can be explained 

without external forcing, and very likely that it is not 

due to known natural causes alone. {4.8, 5.2, 9.4, 9.5, 

9.7} 

• Warming of the climate system has been detected in 

changes of surface and atmospheric temperatures in 

the upper several hundred metres of the ocean, and 

in contributions to sea level rise. Attribution studies 

have established anthropogenic contributions to all of 

these changes. The observed pattern of tropospheric 

warming and stratospheric cooling is very likely due to 

the combined infl uences of greenhouse gas increases 

and stratospheric ozone depletion. {3.2, 3.4, 9.4, 9.5} 

• It is likely that there has been signifi cant anthropogenic 

warming over the past 50 years averaged over each 

continent except Antarctica (see Figure SPM.4). 

The observed patterns of warming, including greater 

warming over land than over the ocean, and their 

changes over time, are only simulated by models that 

include anthropogenic forcing. The ability of coupled 

climate models to simulate the observed temperature 

evolution on each of six continents provides stronger 

evidence of human infl uence on climate than was 

available in the TAR. {3.2, 9.4}

• Diffi culties remain in reliably simulating and attributing 

observed temperature changes at smaller scales. On 

these scales, natural climate variability is relatively 

larger, making it harder to distinguish changes expected 

due to external forcings. Uncertainties in local forcings 

and feedbacks also make it diffi cult to estimate the 

contribution of greenhouse gas increases to observed 

small-scale temperature changes. {8.3, 9.4} 

• Anthropogenic forcing is likely to have contributed 

to changes in wind patterns,13 affecting extratropical storm tracks and temperature patterns in 

both hemispheres. However, the observed changes in 

the Northern Hemisphere circulation are larger than 

simulated in response to 20th-century forcing change. 

{3.5, 3.6, 9.5, 10.3} 

• Temperatures of the most extreme hot nights, cold 

nights and cold days are likely to have increased due 

to anthropogenic forcing. It is more likely than not that 

anthropogenic forcing has increased the risk of heat 

waves (see Table SPM.2). {9.4} 

12 Consideration of remaining uncertainty is based on current methodologies. 

13 In particular, the Southern and Northern Annular Modes and related changes in the North Atlantic Oscillation. {3.6, 9.5, Box TS.2}
Summary for Policymakers 

11

GLOBAL AND CONTINENTAL TEMPERATURE CHANGE

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

models using natural and anthropogenic forcings. Decadal averages of observations are shown for the period 1906 to 2005 (black line) 

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

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

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

both natural and anthropogenic forcings. {FAQ 9.2, Figure 1}
12

Summary for Policymakers 

Projections of Future

Changes in Climate

A major advance of this assessment of climate change 

projections compared with the TAR is the large number of 

simulations available from a broader range of models. Taken 

together with additional information from observations, 

these provide a quantitative basis for estimating likelihoods 

for many aspects of future climate change. Model 

simulations cover a range of possible futures including 

idealised emission or concentration assumptions. These 

include SRES14 illustrative marker scenarios for the 2000 

to 2100 period and model experiments with greenhouse 

gases and aerosol concentrations held constant after year 

2000 or 2100. 

For the next two decades, a warming of about 

0.2°C per decade is projected for a range of SRES 

emission scenarios. Even if the concentrations of 

all greenhouse gases and aerosols had been kept 

constant at year 2000 levels, a further warming of 

about 0.1°C per decade would be expected. {10.3, 

10.7}

• Since IPCC’s fi rst report in 1990, assessed projections 

have suggested global average temperature increases 

between about 0.15°C and 0.3°C per decade for 1990 to 

2005. This can now be compared with observed values 

of about 0.2°C per decade, strengthening confi dence in 

near-term projections. {1.2, 3.2} 

• Model experiments show that even if all radiative 

forcing agents were held constant at year 2000 levels, 

a further warming trend would occur in the next two 

decades at a rate of about 0.1°C per decade, due mainly 

to the slow response of the oceans. About twice as 

much warming (0.2°C per decade) would be expected 

if emissions are within the range of the SRES scenarios. 

Best-estimate projections from models indicate 

that decadal average warming over each inhabited 

continent by 2030 is insensitive to the choice among 

SRES scenarios and is very likely to be at least twice 

as large as the corresponding model-estimated natural 

variability during the 20th century. {9.4, 10.3, 10.5, 

11.2–11.7, Figure TS-29} 

Analysis of climate models together with 

constraints from observations enables an assessed 

likely range to be given for climate sensitivity for 

the fi rst time and provides increased confi dence in 

the understanding of the climate system response 

to radiative forcing. {6.6, 8.6, 9.6, Box 10.2}

• The equilibrium climate sensitivity is a measure of the 

climate system response to sustained radiative forcing. 

It is not a projection but is defi ned as the global average 

surface warming following a doubling of carbon 

dioxide concentrations. It is likely to be in the range 

2°C to 4.5°C with a best estimate of about 3°C, and is 

very unlikely to be less than 1.5°C. Values substantially 

higher than 4.5°C cannot be excluded, but agreement 

of models with observations is not as good for those 

values. Water vapour changes represent the largest 

feedback affecting climate sensitivity and are now 

better understood than in the TAR. Cloud feedbacks 

remain the largest source of uncertainty. {8.6, 9.6, Box 

10.2} 

• It is very unlikely that climate changes of at least the 

seven centuries prior to 1950 were due to variability 

generated within the climate system alone. A signifi cant 

fraction of the reconstructed Northern Hemisphere 

inter-decadal temperature variability over those 

centuries is very likely attributable to volcanic eruptions 

and changes in solar irradiance, and it is likely that 

anthropogenic forcing contributed to the early 20thcentury warming evident in these records. {2.7, 2.8, 

6.6, 9.3}

14 SRES refers to the IPCC Special Report on Emission Scenarios (2000). The SRES scenario families and illustrative cases, which did not include additional climate 

initiatives, are summarised in a box at the end of this Summary for Policymakers. Approximate carbon dioxide 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 1,550 ppm respectively. Scenarios B1, A1B and A2 have been the focus of model intercomparison 

studies and many of those results are assessed in this report.
Summary for Policymakers 

13

Continued greenhouse gas emissions at or above 

current rates would cause further warming and 

induce many changes in the global climate system 

during the 21st century that would very likely be 

larger than those observed during the 20th century. 

{10.3}

• Advances in climate change modelling now enable 

best estimates and likely assessed uncertainty ranges to 

be given for projected warming for different emission 

scenarios. Results for different emission scenarios are 

provided explicitly in this report to avoid loss of this 

policy-relevant information. Projected global average 

surface warmings for the end of the 21st century 

(2090–2099) relative to 1980–1999 are shown in Table 

SPM.3. These illustrate the differences between lower 

and higher SRES emission scenarios, and the projected 

warming uncertainty associated with these scenarios. 

{10.5}

• Best estimates and likely ranges for global average 

surface air warming for six SRES emissions marker 

scenarios are given in this assessment and are shown 

in Table SPM.3. For example, the best estimate for 

the low scenario (B1) is 1.8°C (likely range is 1.1°C 

to 2.9°C), and the best estimate for the high scenario 

(A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C). 

Although these projections are broadly consistent with 

the span quoted in the TAR (1.4°C to 5.8°C), they are 

not directly comparable (see Figure SPM.5). The Fourth 

Assessment Report is more advanced as it provides best 

estimates and an assessed likelihood range for each of 

the marker scenarios. The new assessment of the likely

ranges now relies on a larger number of climate models 

of increasing complexity and realism, as well as new 

information regarding the nature of feedbacks from the 

carbon cycle and constraints on climate response from 

observations. {10.5}

• Warming tends to reduce land and ocean uptake of 

atmospheric carbon dioxide, increasing the fraction of 

anthropogenic emissions that remains in the atmosphere. 

For the A2 scenario, for example, the climate-carbon 

cycle feedback increases the corresponding global 

average warming at 2100 by more than 1°C. Assessed 

upper ranges for temperature projections are larger 

than in the TAR (see Table SPM.3) mainly because 

the broader range of models now available suggests 

stronger climate-carbon cycle feedbacks. {7.3, 10.5} 

• Model-based projections of global average sea level 

rise at the end of the 21st century (2090–2099) are 

shown in Table SPM.3. For each scenario, the midpoint 

of the range in Table SPM.3 is within 10% of the 

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

Table notes:

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

Complexity and a large number of Atmosphere-Ocean General Circulation Models (AOGCMs).

b Year 2000 constant composition is derived from AOGCMs only.

Temperature Change Sea Level Rise 

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

 Best Likely Model-based range excluding future

Case estimate range rapid dynamical changes in ice fl ow

Constant Year 2000 

concentrationsb 0.6 0.3 – 0.9 NA

B1 scenario 1.8 1.1 – 2.9 0.18 – 0.38

A1T scenario 2.4 1.4 – 3.8 0.20 – 0.45

B2 scenario 2.4 1.4 – 3.8 0.20 – 0.43

A1B scenario 2.8 1.7 – 4.4 0.21 – 0.48

A2 scenario 3.4 2.0 – 5.4 0.23 – 0.51

A1FI scenario 4.0 2.4 – 6.4 0.26 – 0.59
14

Summary for Policymakers 

TAR model average for 2090–2099. The ranges are 

narrower than in the TAR mainly because of improved 

information about some uncertainties in the projected 

contributions.15 {10.6}

• Models used to date do not include uncertainties in 

climate-carbon cycle feedback nor do they include 

the full effects of changes in ice sheet fl ow, because a 

basis in published literature is lacking. The projections 

include a contribution due to increased ice fl ow from 

Greenland and Antarctica at the rates observed for 1993 

to 2003, but these fl ow rates could increase or decrease 

in the future. For example, if this contribution were to 

grow linearly with global average temperature change, 

the upper ranges of sea level rise for SRES scenarios 

shown in Table SPM.3 would increase by 0.1 to 0.2 m. 

Larger values cannot be excluded, but understanding of 

these effects is too limited to assess their likelihood or 

provide a best estimate or an upper bound for sea level 

rise. {10.6}

• Increasing atmospheric carbon dioxide concentrations 

lead to increasing acidifi cation of the ocean. Projections 

based on SRES scenarios give reductions in average 

global surface ocean pH16 of between 0.14 and 0.35 

units over the 21st century, adding to the present 

decrease of 0.1 units since pre-industrial times. {5.4, 

Box 7.3, 10.4}

Figure SPM.5. Solid lines are multi-model global averages of surface warming (relative to 1980–1999) for the scenarios A2, A1B and B1, 

shown as continuations of the 20th century simulations. Shading denotes the ±1 standard deviation range of individual model annual 

averages. The orange line is for the experiment where concentrations were held constant at year 2000 values. The grey bars at right 

indicate the best estimate (solid line within each bar) and the likely range assessed for the six SRES marker scenarios. The assessment of 

the best estimate and likely ranges in the grey bars includes the AOGCMs in the left part of the fi gure, as well as results from a hierarchy 

of independent models and observational constraints. {Figures 10.4 and 10.29}

MULTI-MODEL AVERAGES AND ASSESSED RANGES FOR SURFACE WARMING

15 TAR projections were made for 2100, whereas projections in this report are for 2090–2099. The TAR would have had similar ranges to those in Table SPM.3 if it had 

treated the uncertainties in the same way.

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

15

PROJECTIONS OF SURFACE TEMPERATURES

• Sea ice is projected to shrink in both the Arctic and 

Antarctic under all SRES scenarios. In some projections, 

arctic late-summer sea ice disappears almost entirely 

by the latter part of the 21st century. {10.3} 

• It is very likely that hot extremes, heat waves and heavy 

precipitation events will continue to become more 

frequent. {10.3}

• Based on a range of models, it is likely that future 

tropical cyclones (typhoons and hurricanes) will 

become more intense, with larger peak wind speeds 

and more heavy precipitation associated with ongoing 

increases of tropical sea surface temperatures. There is 

less confi dence in projections of a global decrease in 

numbers of tropical cyclones. The apparent increase 

in the proportion of very intense storms since 1970 in 

some regions is much larger than simulated by current 

models for that period. {9.5, 10.3, 3.8} 

There is now higher confi dence in projected patterns 

of warming and other regional-scale features, 

including changes in wind patterns, precipitation 

and some aspects of extremes and of ice. {8.2, 8.3, 

8.4, 8.5, 9.4, 9.5, 10.3, 11.1}

• Projected warming in the 21st century shows scenarioindependent geographical patterns similar to those 

observed over the past several decades. Warming is 

expected to be greatest over land and at most high 

northern latitudes, and least over the Southern Ocean 

and parts of the North Atlantic Ocean (see Figure 

SPM.6). {10.3} 

• Snow cover is projected to contract. Widespread 

increases in thaw depth are projected over most 

permafrost regions. {10.3, 10.6} 

Figure SPM.6. Projected surface temperature changes for the early and late 21st century relative to the period 1980–1999. The central 

and right panels show the AOGCM multi-model average projections for the B1 (top), A1B (middle) and A2 (bottom) SRES scenarios 

averaged over the decades 2020–2029 (centre) and 2090–2099 (right). The left panels show corresponding uncertainties as the relative 

probabilities of estimated global average warming from several different AOGCM and Earth System Model of Intermediate Complexity 

studies for the same periods. Some studies present results only for a subset of the SRES scenarios, or for various model versions. 

Therefore the difference in the number of curves shown in the left-hand panels is due only to differences in the availability of results. 

{Figures 10.8 and 10.28}
16

Summary for Policymakers 

PROJECTED PATTERNS OF PRECIPITATION CHANGES

Figure SPM.7. Relative changes in precipitation (in percent) for the period 2090–2099, relative to 1980–1999. Values are multi-model 

averages based on the SRES A1B scenario for December to February (left) and June to August (right). White areas are where less than 

66% of the models agree in the sign of the change and stippled areas are where more than 90% of the models agree in the sign of the 

change. {Figure 10.9}

• Extratropical storm tracks are projected to move 

poleward, with consequent changes in wind, 

precipitation and temperature patterns, continuing the 

broad pattern of observed trends over the last halfcentury. {3.6, 10.3} 

• Since the TAR, there is an improving understanding 

of projected patterns of precipitation. Increases in the 

amount of precipitation are very likely in high latitudes, 

while decreases are likely in most subtropical land 

regions (by as much as about 20% in the A1B scenario 

in 2100, see Figure SPM.7), continuing observed 

patterns in recent trends. {3.3, 8.3, 9.5, 10.3, 11.2 to 

11.9} 

• Based on current model simulations, it is very likely that 

the meridional overturning circulation (MOC) of the 

Atlantic Ocean will slow down during the 21st century. 

The multi-model average reduction by 2100 is 25% 

(range from zero to about 50%) for SRES emission 

scenario A1B. Temperatures in the Atlantic region 

are projected to increase despite such changes due to 

the much larger warming associated with projected 

increases in greenhouse gases. It is very unlikely that 

the MOC will undergo a large abrupt transition during 

the 21st century. Longer-term changes in the MOC 

cannot be assessed with confi dence. {10.3, 10.7} 

Anthropogenic warming and sea level rise would 

continue for centuries due to the time scales 

associated with climate processes and feedbacks, 

even if greenhouse gas concentrations were to be 

stabilised. {10.4, 10.5, 10.7}

• Climate-carbon cycle coupling is expected to add 

carbon dioxide to the atmosphere as the climate system 

warms, but the magnitude of this feedback is uncertain. 

This increases the uncertainty in the trajectory of 

carbon dioxide emissions required to achieve a 

particular stabilisation level of atmospheric carbon 

dioxide concentration. Based on current understanding 

of climate-carbon cycle feedback, model studies 

suggest that to stabilise at 450 ppm carbon dioxide 

could require that cumulative emissions over the 21st 

century be reduced from an average of approximately 

670 [630 to 710] GtC (2460 [2310 to 2600] GtCO2) to 

approximately 490 [375 to 600] GtC (1800 [1370 to 

2200] GtCO2). Similarly, to stabilise at 1000 ppm, this 

feedback could require that cumulative emissions be 

reduced from a model average of approximately 1415 

[1340 to 1490] GtC (5190 [4910 to 5460] GtCO2) to 

approximately 1100 [980 to 1250] GtC (4030 [3590 to 

4580] GtCO2). {7.3, 10.4}
Summary for Policymakers 

17

• If radiative forcing were to be stabilised in 2100 at B1 

or A1B levels14 a further increase in global average 

temperature of about 0.5°C would still be expected, 

mostly by 2200. {10.7}

• If radiative forcing were to be stabilised in 2100 at A1B 

levels14, thermal expansion alone would lead to 0.3 to 

0.8 m of sea level rise by 2300 (relative to 1980–1999). 

Thermal expansion would continue for many centuries, 

due to the time required to transport heat into the deep 

ocean. {10.7}

• Contraction of the Greenland Ice Sheet is projected 

to continue to contribute to sea level rise after 2100. 

Current models suggest that ice mass losses increase 

with temperature more rapidly than gains due to 

precipitation and that the surface mass balance 

becomes negative at a global average warming 

(relative to pre-industrial values) in excess of 1.9°C 

to 4.6°C. If a negative surface mass balance were 

sustained for millennia, that would lead to virtually 

complete elimination of the Greenland Ice Sheet and 

a resulting contribution to sea level rise of about 7 m. 

The corresponding future temperatures in Greenland 

are comparable to those inferred for the last interglacial 

period 125,000 years ago, when palaeoclimatic 

information suggests reductions of polar land ice extent 

and 4 to 6 m of sea level rise. {6.4, 10.7} 

• Dynamical processes related to ice fl ow not included 

in current models but suggested by recent observations 

could increase the vulnerability of the ice sheets to 

warming, increasing future sea level rise. Understanding 

of these processes is limited and there is no consensus 

on their magnitude. {4.6, 10.7}

• Current global model studies project that the Antarctic 

Ice Sheet will remain too cold for widespread surface 

melting and is expected to gain in mass due to increased 

snowfall. However, net loss of ice mass could occur if 

dynamical ice discharge dominates the ice sheet mass 

balance. {10.7}

• Both past and future anthropogenic carbon dioxide 

emissions will continue to contribute to warming and 

sea level rise for more than a millennium, due to the 

time scales required for removal of this gas from the 

atmosphere. {7.3, 10.3}
18

Summary for Policymakers 

17 Emission scenarios are not assessed in this Working Group I Report of the IPCC. This box summarising the SRES scenarios is taken from the TAR and has been 

subject to prior line-by-line approval by the Panel.

THE EMISSION SCENARIOS OF THE IPCC SPECIAL REPORT ON EMISSION SCENARIOS (SRES)17

A1. The A1 storyline and scenario family describes a future world of very rapid economic growth, global 

population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more effi cient 

technologies. Major underlying themes are convergence among regions, capacity building and increased cultural 

and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario 

family develops into three groups that describe alternative directions of technological change in the energy system. 

The three A1 groups are distinguished by their technological emphasis: fossil-intensive (A1FI), non-fossil energy 

sources (A1T) or a balance across all sources (A1B) (where balanced is defi ned as not relying too heavily on one 

particular energy source, on the assumption that similar improvement rates apply to all energy supply and end 

use technologies).

A2. The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme is selfreliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results 

in continuously increasing population. Economic development is primarily regionally oriented and per capita 

economic growth and technological change more fragmented and slower than other storylines.

B1. The B1 storyline and scenario family describes a convergent world with the same global population, that 

peaks in mid-century and declines thereafter, as in the A1 storyline, but with rapid change in economic structures 

toward a service and information economy, with reductions in material intensity and the introduction of clean 

and resource-effi cient technologies. The emphasis is on global solutions to economic, social and environmental 

sustainability, including improved equity, but without additional climate initiatives.

B2. The B2 storyline and scenario family describes a world in which the emphasis is on local solutions to 

economic, social and environmental sustainability. It is a world with continuously increasing global population, at 

a rate lower than A2, intermediate levels of economic development, and less rapid and more diverse technological 

change than in the B1 and A1 storylines. While the scenario is also oriented towards environmental protection and 

social equity, it focuses on local and regional levels.

An illustrative scenario was chosen for each of the six scenario groups A1B, A1FI, A1T, A2, B1 and B2. All 

should be considered equally sound.

The SRES scenarios do not include additional climate initiatives, which means that no scenarios are included 

that explicitly assume implementation of the United Nations Framework Convention on Climate Change or the 

emissions targets of the Kyoto Protocol.