The IPCC Third Assessment: Science

The role of the Intergovernmental Panel on Climate Change is to provide the international community with expert guidance regarding scientific and technical aspects of the climate problem. Here, we present the Panel's latest findings on the basic science of global warming.

Since 1990, the Intergovernmental Panel on Climate Change (IPCC) has, at five-yearly intervals, assessed and reported on the current state of knowledge and understanding of the climate issue.

These reports are intended to be used to provide policy makers with an objective assessment and review of the information available on climate change.

The Third Assessment Report, completed this year, consists of three parts.

  • IPCC Working Group I reports on current understanding of the basic science of climate change.
  • Working Group II reports on impacts, adaptation measures and vulnerability to the effects of climate change.
  • Working Group III reports on the scientific, technical, environmental and social aspects together with mitigation options to climate change.

Given the importance of the IPCC reports as a global scientific consensus on our understanding of the climate issue, we consider it to be appropriate and useful that Tiempo presents a summary of each Working Group Report.

The IPCC Working Group I Report was approved in January 2001 as delegates from 99 countries met in Shanghai, PR China. In this issue, we present selected extracts from the Working Group I Summary for Policymakers concerning the basic science of the climate issue. We will be presenting excerpts from the other working group reports in future issues of the bulletin.

The following text is taken verbatim from the Policymakers Summary. All main conclusions are covered, though some explanatory detail has been omitted.

In this summary, the following words have been used by the IPCC where appropriate to indicate judgmental estimates of confidence:

  • virtually certain (greater than 99% chance that a result is true);
  • very likely (90-99% chance);
  • likely (66-90%);
  • medium likelihood (33-66% chance);
  • unlikely (10-33% chance);
  • very unlikely (1-10% chance);
  • exceptionally unlikely (less than 1% chance).

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 usagediffers from that in the 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.


The Third Assessment Report of Working Group I of the Intergovernmental Panel on Climate Change builds upon past assessments and incorporates new results from the past five years of research on climate change. Many hundreds of scientists from many countries participated in its preparation.

This Summary for Policymakers, which was approved by IPCC member governments in Shanghai in January 2001, describes the current state of understanding of the climate system and provides estimates of its projected future evolution and their uncertainties.

An increasing body of observations gives a collective picture of a warming world and other changes in the climate system.

Since the release of the Second Assessment Report (SAR), additional data from new studies of current and palaeoclimates, improved analysis of data sets, more rigorous evaluation of their quality, and comparisons among data from different sources have led to greater understanding of climate change.

The global average surface temperature has increased over the 20th century by about 0.6°C.

  • The global average surface temperature... has increased since 1861. Over the 20th century the increase has been 0.6±0.2°C. This value is about 0.15°C larger than that estimated by the SAR for the period up to 1994, owing to the relatively high temperatures of the additional years (1995 to 2000) and improved methods of processing the data. These numbers take into account various adjustments, including urban heat island effects. The record shows a great deal of variability; for example, most of the warming occurred during the 20th century, during two periods, 1910 to 1945 and 1976 to 2000.
  • Globally, it is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record, since 1861.
  • New analyses of proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year. Because less data are available, less is known about annual averages prior to 1,000 years before present and for conditions prevailing in most of the Southern Hemisphere prior to 1861.

Temperatures have risen during the past four decades in the lowest 8 kilometres of the atmosphere.

  • Since the late 1950s (the period of adequate observations from weather balloons), the overall global temperature increases in the lowest 8 kilometres of the atmosphere and in surface temperature have been similar at 0.1°C per decade.
  • Since the start of the satellite record in 1979, both satellite and weather balloon measurements show that the global average temperature of the lowest 8 kilometres of the atmosphere has changed by +0.05±0.10°C per decade, but the global average surface temperature has increased significantly by +0.15±0.05°C per decade. The difference in the warming rate is statistically significant. This difference occurs primarily over the tropical and sub-tropical regions.
  • The lowest 8 kilometres of the atmosphere and the surface are influenced differently by factors such as stratospheric ozone depletion, atmospheric aerosols, and the El Niño phenomenon. Hence, it is physically plausible to expect that over a short time period (for example, 20 years) there may be differences in temperature trends. In addition, spatial sampling techniques can also explain some of the differences in trends, but these differences are not fully resolved.

Snow cover and ice extent have decreased.

  • Satellite data show that there are very likely to have been decreases of about 10% in the extent of snow cover since the late 1960s, and ground-based observations show that there is very likely to have been a reduction of about two weeks in the annual duration of lake and river ice cover in the mid- and high latitudes of the Northern Hemisphere, over the 20th century.
  • There has been a widespread retreat of mountain glaciers in non-polar regions during the 20th century.
  • Northern Hemisphere spring and summer sea-ice extent has decreased by about 10 to 15% since the 1950s...

Global average sea level has risen and ocean heat content has increased.

  • Tide gauge data show that global average sea level rose between 0.1 and 0.2 metres during the 20th century.
  • Global ocean heat content has increased since the late 1950s, the period for which adequate observations of sub-surface ocean temperatures have been available.

Changes have also occurred in other important aspects of climate.

  • It is very likely that precipitation has increased by 0.5 to 1% per decade in the 20th century over most mid- and high latitudes of the Northern Hemisphere continents, and it is likely that rainfall has increased by 0.2 to 0.3% per decade over the tropical (10°N to 10°S) land areas. Increases in the tropics are not evident over the past few decades. It is also likely that rainfall has decreased over much of the Northern Hemisphere sub-tropical (10°N to 30°N) land areas during the 20th century by about 0.3% per decade. In contrast to the Northern Hemisphere, no comparable systematic changes have been detected in broad latitudinal averages over the Southern Hemisphere...
  • Warm episodes of the El-Niño-Southern Oscillation (ENSO) phenomenon... have been more frequent, persistent and intense since the mid-1970s, compared with the previous 100 years.
  • In some regions, such as parts of Asia and Africa, the frequency and intensity of droughts have been observed to increase in recent decades.

Some important aspects of climate appear not to have changed.

  • A few areas of the globe have not warmed in recent decades, mainly over some parts of the Southern Hemisphere oceans and parts of Antarctica.
  • Changes globally in tropical and extra-tropical storm intensity and frequency are dominated by inter-decadal to multi-decadal variations, with no significant trends evident over the 20th century. Conflicting analyses make it difficult to draw definitive conclusions about changes in storm activity, especially in the extra-tropics.

Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate.

Changes in climate occur as a result of both internal variability within the climate system and external factors (both natural and anthropogenic). The influence of external factors on climate can be broadly compared using the concept of radiative forcing. A positive radiative forcing, such as that produced by increasing concentrations of greenhouse gases, tends to warm the surface. A negative radiative forcing, which can arise from an increase in some types of aerosols (microscopic airborne particles) tends to cool the surface. Natural factors, such as changes in solar output or explosive volcanic activity, can also cause radiative forcing.

Concentrations of atmospheric greenhouse gases and their radiative forcing have continued to increase as a result of human activities.

  • The atmospheric concentration of carbon dioxide (CO2) has increased by 31% since 1750. The present CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years.
  • About three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation.
  • Currently the ocean and the land together are taking up about half of the anthropogenic CO2 emissions. On land, the uptake of anthropogenic CO2 very likely exceeded the release of CO2 by deforestation during the 1990s.
  • The atmospheric concentration of methane (CH4) has increased by 1060 ppb (151%) since 1750 and continues to increase. The present CH4 concentration has not been exceeded during the past 420,000 years... Slightly more than half of current CH4 emissions are anthropogenic (for example, use of fossil fuels, cattle, rice agriculture and landfills). In addition, carbon monoxide (CO) emissions have recently been identified as a cause of increasing CH4 concentration.
  • The atmospheric concentration of nitrous oxide (N2O) has increased by 46 ppb (17%) since 1750 and continues to increase. The present N2O concentration has not been exceeded during at least the past thousand years. About a third of current N2O emissions are anthropogenic (for example, agricultural soils, cattle feed lots and chemical industry).
  • Since 1995, the atmospheric concentrations of many of those halocarbon gases that are both ozone-depleting and greenhouse gases (for example, CFCl3 and CF2Cl2), are either increasing more slowly or decreasing, both in response to reduced emissions under the regulations of the Montreal Protocol and its Amendments...
  • The observed depletion of the stratospheric ozone (O3) layer from 1979 to 2000 is estimated to have caused a negative radiative forcing (-0.15 Wm-2). Assuming full compliance with current halocarbon regulations, the positive forcing of the halocarbons will be reduced as will the magnitude of the negative forcing from stratospheric ozone depletion as the ozone layer recovers over the 21st century.
  • The total amount of O3 in the troposphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic emissions of several O3-forming gases...

Anthropogenic aerosols are short-lived and mostly produce negative radiative forcing.

  • The major sources of anthropogenic aerosols are fossil fuel and biomass burning. These sources are also linked to degradation of air quality and acid deposition.
  • Since the SAR, significant progress has been achieved in better characterising the direct radiative roles of different types of aerosols. Direct radiative forcing is estimated to be -0.4 Wm-2 for sulphate, -0.2 Wm-2 for biomass burning aerosols, -0.1 Wm-2 for fossil fuel organic carbon and +0.2 Wm-2 for fossil fuel black carbon aerosols. There is much less confidence in the ability to quantify the total aerosol direct effect, and its evolution over time, than that for the gases listed above. Aerosols also vary considerably by region and respond quickly to changes in emissions.
  • In addition to their direct radiative forcing, aerosols have an indirect radiative forcing through their effects on clouds. There is now more evidence for this indirect effect, which is negative, although of very uncertain magnitude.

Natural factors have made small contributions to radiative forcing over the past century.

  • The radiative forcing due to changes in solar irradiance for the period since 1750 is estimated to be about +0.3 Wm-2, most of which occurred during the first half of the 20th century. Since the late 1970s, satellite instruments have observed small oscillations due to the 11-year solar cycle. Mechanisms for the amplification of solar effects on climate have been proposed, but currently lack a rigorous theoretical or observational basis.
  • Stratospheric aerosols from explosive volcanic eruptions lead to negative forcing, which lasts a few years. Several major eruptions occurred in the periods 1880 to 1920 and 1960 to 1991.
  • The combined change in radiative forcing of the two major natural factors (solar variations and volcanic aerosols) is estimated to be negative for the past two, and possibly the past four, decades.

Confidence in the ability of models to project future climate has increased.

Complex physically-based climate models are required to provide detailed estimates of feedbacks and of regional features. Such models cannot yet simulate all aspects of climate (for example, they still cannot account fully for the observed trend in the surface-troposphere temperature difference since 1979) and there are particular uncertainties associated with clouds and their interaction with radiation and aerosols. Nevertheless, confidence in the ability of these models to provide useful projections of future climate has improved due to the demonstrated performance on a range of space and time-scales.

  • Simulations that include estimates of natural and anthropogenic forcing reproduce the observed large-scale changes in surface temperature over the 20th century. However, contributions from some additional processes and forcings may not have been included in the models. Nevertheless, the large-scale consistency between models and observations can be used to provide an independent check on projected warming rates over the next few decades under a given emissions scenario.
  • Some aspects of model simulations of ENSO, monsoons and the North Atlantic Oscillation, as well as selected periods of past climate, have improved.

There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.

The SAR concluded: “The balance of evidence suggests a discernible human influence on global climate”. That report also noted that the anthropogenic signal was still emerging from the background of natural climate variability. Since the SAR, progress has been made in reducing uncertainty, particularly with respect to distinguishing and quantifying the magnitude of responses to different external influences. Although many of the sources of uncertainty identified in the SAR still remain to some degree, new evidence and improved understanding support an updated conclusion.

  • There is a longer and more closely scrutinized temperature record and new model estimates of variability. The warming over the past 100 years is very unlikely to be due to internal variability alone, as estimated by current models. Reconstructions of climate data for the past 1,000 years also indicate that this warming was unusual and is unlikely to be entirely natural in its origin.
  • There are new estimates of the climate response to natural and anthropogenic forcing, and new detection techniques have been applied. Detection and attribution studies consistently find evidence for an anthropogenic signal in the climate record of the last 35 to 50 years.
  • In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.
  • Furthermore, it is very likely that the 20th century warming has contributed significantly to the observed sea level rise, through thermal expansion of sea water and widespread loss of land ice. Within present uncertainties, observations and models are both consistent with a lack of significant acceleration of sea level rise during the 20th century.

Human influences will continue to change atmospheric composition throughout the 21st century.

Models have been used to make projections of atmospheric concentrations of greenhouse gases and aerosols, and hence of future climate, based upon emissions scenarios from the IPCC Special Report on Emissions Scenarios (SRES).

  • Emissions of CO2 due to fossil fuel burning are virtually certain to be the dominant influence on the trends in atmospheric CO2 concentration during the 21st century.
  • By 2100, carbon cycle models project atmospheric CO2 concentrations of 540 to 970 ppm for the illustrative SRES scenarios (90 to 250% above the concentration of 280 ppm in the year 1750). These projections include the land and ocean climate feedbacks. Uncertainties, especially about the magnitude of the climate feedback from the terrestrial biosphere, cause a variation of about -10 to +30% around each scenario. The total range is 490 to 1260 ppm (75 to 350% above the 1750 concentration).
  • Reductions in greenhouse gas emissions and the gases that control their concentration would be necessary to stabilize radiative forcing. For example, for the most important anthropogenic greenhouse gas, carbon cycle models indicate that stabilization of atmospheric CO2 concentrations at 450, 650 or 1,000 ppm would require global anthropogenic CO2 emissions to drop below 1990 levels, within a few decades, about a century, or about two centuries, respectively, and continue to decrease steadily thereafter. Eventually CO2 emissions would need to decline to a very small fraction of current emissions.
  • The SRES scenarios include the possibility of either increases or decreases in anthropogenic aerosols (for example, sulphate aerosols, biomass aerosols, black and organic carbon aerosols) depending on the extent of fossil fuel use and policies to abate polluting emissions. In addition, natural aerosols (for example, sea salt, dust and emissions leading to the production of sulphate and carbon aerosols) are projected to increase as a result of changes in climate.
  • For the SRES illustrative scenarios, relative to the year 2000, the global mean radiative forcing due to greenhouse gases continues to increase through the 21st century, with the fraction due to CO2 projected to increase from slightly more than half to about three quarters. The change in the direct plus indirect aerosol radiative forcing is projected to be smaller in magnitude than that of CO2.

Global average temperature and sea level are projected to rise under all IPCC SRES scenarios.

In order to make projections of future climate, models incorporate past, as well as future emissions of greenhouse gases and aerosols. Hence, they include estimates of warming to date and the commitment to future warming from past emissions.

Figure 1: The range of projected global-mean surface air temperature responses taking account of key uncertainties in scientific understanding and in future emissions. The results are derived from a simple climate model tuned to a number of more complex models with a range of climate sensitivities over the full range of 35 SRES greenhouse gas emissions scenarios. Graphic amended from the Working Group I Summary for Policymakers, courtesy of Sarah Raper and Mike Salmon.

  • The globally averaged surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 to 2100 (Figure 1). These results are for the full range of 35 SRES scenarios, based on a number of climate models.
  • The projected rate of warming is much larger than the observed changes during the 20th century and is very likely to be without precedent during at least the last 10,000 years, based on palaeoclimate data.
  • Based on recent global model simulations, it is very likely that nearly all land areas will warm more rapidly than the global average, particularly those at northern high latitudes in the cold season. Most notable of these is the warming in the northern regions of North America, and northern and central Asia, which exceeds global mean warming in each model by more than 40%. In contrast, the warming is less than the global mean change in south and southeast Asia in summer and in southern South America in winter.
  • Recent trends for surface temperature to become more El Niño-like in the tropical Pacific, with the eastern tropical Pacific warming more than the western tropical Pacific, with a corresponding eastward shift of precipitation, are projected to continue in many models.
  • Based on global model simulations and for a wide range of scenarios, global average water vapour concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter. At low latitudes there are both regional increases and decreases over land areas. Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected.

Confidence in observed changes (latter half of 20th century)

Changes in phenomenon

Confidence in projected changes (during the 21st century)


Higher maximum temperatures and more hot days over nearly all land areas

Very likely

Very likely

Higher minimum temperatures, fewer cold days and frost days over nearly all land areas

Very likely

Very likely

Reduced diurnal temperature range over most land areas

Very likely

Likely, over many areas

Increase of heat index over land areas

Very likely, over most areas

Likely, over many Northern Hemisphere mid- to high latitude land areas

More intense precipitation events*

Very likely, over most areas

Likely, in a few areas

Increased summer continental drying and associated risk of drought

Likely, over most mid-latitude continental interiors. (Lack of consistent projections in other areas.)

Not observed in the few analyses available

Increase in tropical cyclone peak wind intensities**

Likely, over some areas

Insufficient data for assessment

Increase in tropical cyclone mean and peak precipitation intensities

Likely, over some areas

Table 1: Estimates of confidence in observed and projected changes in extreme weather and climate events. See introduction for an explanation of terms ‘likely’ and ‘very likely.’

* For other areas, there are either insufficient data or conflicting analyses
** Past and future changes in tropical cyclone location and frequency are uncertain

  • Table 1 depicts an assessment of confidence in observed changes in extremes of weather and climate during the latter half of the 20th century (left column) and in projected changes during the 21st century (right column). This assessment relies on observational and modelling studies, as well as the physical plausibility of future projections across all commonly-used scenarios and is based on expert judgement.
  • For some other extreme phenomena, many of which may have important impacts on the environment and society, there is currently insufficient information to assess recent trends, and climate models currently lack the spatial detail required to make confident projections. For example, very small-scale phenomena, such as thunderstorms, tornadoes, hail and lightning, are not simulated in climate models.
  • Confidence in projections of changes in future frequency, amplitude, and spatial pattern of El Niño events in the tropical Pacific is tempered by some shortcomings in how well El Niño is simulated in complex models. Current projections show little change or a small increase in amplitude for El Niño events over the next 100 years.
  • Even with little or no change in El Niño amplitude, global warming is likely to lead to greater extremes of drying and heavy rainfall and increase the risk of droughts and floods that occur with El Niño events in many different regions.
  • It is likely that warming associated with increasing greenhouse gas concentrations will cause an increase of Asian summer monsoon precipitation variability. Changes in monsoon mean duration and strength depend on the details of the emission scenario. The confidence in such projections is also limited by how well the climate models simulate the detailed seasonal evolution of the monsoons.
  • Most models show weakening of the ocean thermohaline circulation which leads to a reduction of the heat transport into high latitudes of the Northern Hemisphere. However, even in models where the thermohaline circulation weakens, there is still a warming over Europe due to increased greenhouse gases. The current projections using climate models do not exhibit a complete shut-down of the thermohaline circulation by 2100. Beyond 2100, the thermohaline circulation could completely, and possibly irreversibly, shut-down in either hemisphere if the change in radiative forcing is large enough and applied long enough.
  • Northern Hemisphere snow cover and sea-ice extent are projected to decrease further.
  • Glaciers and ice caps are projected to continue their widespread retreat during the 21st century.
  • The Antarctic ice sheet is likely to gain mass because of greater precipitation, while the Greenland ice sheet is likely to lose mass because the increase in runoff will exceed the precipitation increase.
  • Global mean sea level is projected to rise by 0.09 to 0.88 metres between 1990 and 2100, for the full range of SRES scenarios. This is due primarily to thermal expansion and loss of mass from glaciers and ice caps. The range of sea level rise presented in the SAR was 0.13 to 0.94 metres based on the IS92 scenarios. Despite the higher temperature change projections in this assessment, the sea level projections are slightly lower, primarily due to the use of improved models, which give a smaller contribution from glaciers and ice sheets.

Anthropogenic climate change will persist for many centuries.

  • Emissions of long-lived greenhouse gases (that is, CO2, N2O, PFCs, SF6) have a lasting effect on atmospheric composition, radiative forcing and climate. For example, several centuries after CO2 emissions occur, about a quarter of the increase in CO2 concentration caused by these emissions is still present in the atmosphere.
  • After greenhouse gas concentrations have stabilized, global average surface temperatures would rise at a rate of only a few tenths of a degree per century rather than several degrees per century as projected for the 21st century without stabilization. The lower the level at which concentrations are stabilized, the smaller the total temperature change.
  • Global mean surface temperature increases and rising sea level from thermal expansion of the ocean are projected to continue for hundreds of years after stabilization of greenhouse gas concentrations (even at present levels), owing to the long timescales on which the deep ocean adjusts to climate change.
  • Ice sheets will continue to react to climate warming and contribute to sea level rise for thousands of years after climate has been stabilized. Climate models indicate that the local warming over Greenland is likely to be one to three times the global average. Ice sheet models project that a local warming of larger than 3°C, if sustained for millennia, would lead to virtually a complete melting of the Greenland ice sheet with a resulting sea level rise of about 7 metres. A local warming of 5.5°C, is sustained for 1000 years, would be likely to result in a contribution from Greenland of about 3 metres to sea level rise.
  • Current ice dynamic models suggest that the West Antarctic ice sheet could contribute up to 3 metres to sea level rise over the next 1000 years, but such results are strongly dependent on model assumptions regarding climate change scenarios, ice dynamics and other factors.

Further action is required to address the remaining gaps in information and understanding.

Further research is required to improve the ability to detect, attribute and understand climate change, to reduce uncertainties and to project future climate changes. In particular, there is a need for additional systematic and sustained observations, modelling and process studies. A serious concern is the decline of observational networks. The following are high priority areas for action.

Systematic observations and reconstructions:

  • Reverse the decline of observational networks in many parts of the world.
  • Sustain and expand the observational foundation for climate studies by providing accurate, long-term, consistent data including implementation of a strategy for integrated global observations.
  • Enhance the development of reconstructions of past climate periods.
  • Improve the observations of the spatial distribution of greenhouse gases and aerosols.

Modelling and process studies:

  • Improve understanding of the mechanisms and factors leading to changes in radiative forcing.
  • Understand and characterize the important unresolved processes and feedbacks, both physical and biogeochemical, in the climate system.
  • Improve methods to quantify uncertainties of climate projections and scenarios, including long-term ensemble simulations using complex models.
  • Improve the integrated hierarchy of global and regional climate models with a focus on the simulation of climate variability, regional climate changes and extreme events.
  • Link more effectively models of the physical climate and the biogeochemical system, and in turn improve coupling with descriptions of human activities.

Cutting across these foci are crucial needs associated with strengthening international cooperation and coordination in order to better utilize scientific, computational and observational resources. This should also promote the free exchange of data among scientists. A special need is to increase the observational and research capacities in many regions, particularly in developing countries. Finally, as is the goal of this assessment, there is a continuing imperative to communicate research advances in terms that are relevant to decision making.

Further information
The Working Group I “Summary for Policymakers” is available on the IPCC web site or can be obtained from the IPCC Secretariat, c/o World Meteorological Organization, PO Box 2300, CH-1211 Geneva 2, Switzerland. The full Working Group 1 report, “Climate Change 2001: The Scientific Basis,” is published by Cambridge University Press.