The IPCC Third Assessment Report (TAR), Climate Change 2001, is an assessment of available scientific and socio-economic information on climate change by the IPCC. Statements of the IPCC or information from the TAR were often used as a reference showing a scientific consensus on the subject of global warming. The Third Assessment Report (TAR) was completed in 2001 and consists of four reports, three of them from its Working Groups: Working Group I: The Scientific Basis;TAR Working Group 1 , IPCC. Working Group II: Impacts, Adaptation and Vulnerability;TAR Working Group 2 , IPCC. Working Group III: Mitigation;TAR Working Group 3 , IPCC. Synthesis Report.TAR Synthesis Report , IPCC. A number of the TAR's conclusions are given quantitative estimates of how probable it is that they are correct, e.g., greater than 66% probability of being correct., in IPCC TAR SYR 2001 Q2 p 44 These are "Bayesian" probabilities, which are based on an expert assessment of all the available evidence., in IPCC TAR WG2 2001 Ch 2 p 129, pp. 19–20, 27–28. Report website.
"Robust findings" of the Synthesis Report include: "Observations show Earth's surface is warming. Globally, 1990s very likely warmest decade in instrumental record". Atmospheric concentrations of anthropogenic (i.e., human-emitted) greenhouse gases have increased substantially. Since the mid-20th century, most of the observed warming is "likely" (greater than 66% probability, based on expert judgement) due to human activities. Projections based on the Special Report on Emissions Scenarios suggest warming over the 21st century at a more rapid rate than that experienced for at least the last 10,000 years. "Projected climate change will have beneficial and adverse effects on both environmental and socioeconomic systems, but the larger the changes and the rate of change in climate, the more the adverse effects predominate." "Ecosystems and species are vulnerable to climate change and other stresses (as illustrated by observed impacts of recent regional temperature changes) and some will be irreversibly damaged or lost." "Greenhouse gas emission reduction (mitigation) actions would lessen the pressures on natural and human systems from climate change." "Adaptation [to the effects of climate change] has the potential to reduce adverse effects of climate change and can often produce immediate ancillary benefits, but will not prevent all damages." An example of adaptation to climate change is building levees in response to sea level rise., in IPCC AR4 WG2 2007 Ch 6-1 p 343}}
Working groups
The IPCC was established in 1988 by the United Nations Environment Programme (UNEP) and the UN's World Meteorological Organization (WMO) "... to assess scientific, technical and socio-economic information relevant for the understanding of climate change, its potential impacts and options for adaptation and mitigation."IPCC website & The IPCC is organized as three working groups (WG) and a task force :
WGI: Scientific aspects of climate (see ). WGII: Vulnerability, consequences, and options (see ). WGIII: Limitation and mitigation options (see ). Task Force: National Greenhouse Gas Inventories Programme
WG I covers the same areas as the Second Assessment Report (SAR) of 1995, but WG II & III cover slightly different areas in the TAR.
Conclusions Working Group I
The key conclusions of Working Group I The Scientific Basis, Summary for Policymakers , in were:
An increasing body of observations gives a collective picture of a warming world and other changes in the climate system (The global average surface temperature has increased over the 20th century by about 0.6 °C; Temperatures have risen during the past four decades in the lowest 8 kilometres of the atmosphere; Snow cover and ice extent have decreased) Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate (Anthropogenic aerosols (i.e., human emitted aerosols) are short-lived and mostly produce negative radiative forcing; Natural factors have made small contributions to radiative forcing over the past century) 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 (e.g., 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 their demonstrated performance on a range of space and time-scales .) There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities Human influences will continue to change atmospheric composition throughout the 21st century Global average temperature and sea level are projected to rise under all IPCC SRES scenarios.
The TAR estimate for the climate sensitivity is 1.5 to 4.5 °C; and the average surface temperature is projected to increase by 1.4 to 5.8 Celsius degrees over the period 1990 to 2100, and the sea level is projected to rise by 0.1 to 0.9 metres over the same period. The wide range in projections is based upon several different scenarios that assume different levels of future CO2 emissions (see the section below on Projections in the TAR).
Synthesis Report
The TAR Synthesis Report includes a summary of the TAR's main findings and uncertainties., in "Robust findings" of the TAR include: Observed warming of the Earth's surface, attribution of observed warming to human activities, projected increases in future global mean temperature, rising sea levels, and increased frequency of heat waves. Future warming will have both beneficial and adverse effects, but for higher levels of warming, adverse effects will predominate. Developing countries and poor persons are most vulnerable to climate change. "Key uncertainties" in the TAR include: Estimated climate forcings of natural climatic factors and anthropogenic aerosols (e.g., sulfate, which is produced when sulfur-rich coal is burnt), future changes in greenhouse gas (GHG) emissions, and the role of climate feedbacks, which may amplify or reduce the magnitude of future climate change; Assigning probabilities to projections of changes in sea level and temperature, as well as uncertainties related to regional projections of climate change.
List of greenhouse gases The following tables are adapted from Chapter 6 of the Working Group 1 report.
Gases relevant to radiative forcing only GasAlternate nameFormula1998 levelIncrease since 1750Radiative forcing (Wm−2)Specific heat at STP (J kg−1)Carbon dioxideCarbon Dioxide(CO2)365 μmol/mol87 μmol/mol1.460.819MethaneMarsh gas(CH4)1,745 nmol/mol1,045 nmol/mol0.482.191Nitrous oxideLaughing gas(N2O)314 nmol/mol44 nmol/mol0.150.88TetrafluoromethaneCarbon tetrafluoride(CF4)80 pmol/mol40 pmol/mol0.0031.33HexafluoroethanePerfluoroethane(C2F6)3 pmol/mol3 pmol/mol0.0010.067Sulfur hexafluorideSulfur fluoride(SF6)4.2 pmol/mol4.2 pmol/mol0.0020.074HFC-23Trifluoromethane(CHF3)14 pmol/mol14 pmol/mol0.0020.064HFC-134a1,1,1,2-TetrafluoroethaneC2H2F47.5 pmol/mol7.5 pmol/mol0.0010.007HFC-152a1,1-Difluoroethane(C2H4F2)0.5 pmol/mol0.5 pmol/mol0.0000.04
Gases relevant to radiative forcing and ozone depletion GasAlternate nameFormula1998 levelIncrease since 1750Radiative forcing (Wm−2)CFC-11§Trichlorofluoromethane(CFCl3)268 pmol/mol268 pmol/mol0.07CFC-12§Dichlorodifluoromethane(CF2Cl2)533 pmol/mol533 pmol/mol0.17CFC-13§Chlorotrifluoromethane(CClF3)4 pmol/mol4 pmol/mol0.001CFC-1131,1,1-Trichlorotrifluoroethane(C2F3Cl3)84 pmol/mol84 pmol/mol0.03CFC-1141,2-Dichlorotetrafluoroethane(C2F4Cl2)15 pmol/mol15 pmol/mol0.005CFC-115Chloropentafluoroethane(C2F5Cl)7 pmol/mol7 pmol/mol0.001Carbon tetrachlorideTetrachloromethane(CCl4)102 pmol/mol102 pmol/mol0.011,1,1-TrichloroethaneMethyl chloroform(CH3CCl3)69 pmol/mol69 pmol/mol0.004HCFC-141b1,1-Dichloro-1-fluoroethane(C2H3FCl2)10 pmol/mol10 pmol/mol0.001HCFC-142b1-Chloro-1,1-difluoroethane(C2H3F2Cl)11 pmol/mol11 pmol/mol0.002Halon-1211Bromochlorodifluoromethane(CClF2Br)3.8 pmol/mol3.8 pmol/mol0.001Halon-1301Bromotrifluoromethane(CF3Br)2.5 pmol/mol2.5 pmol/mol0.001
Projections Projections are used in the TAR as a guide to the possible future effects of climate change, e.g., changes in global mean temperature and sea level., in In the TAR, the word "projection" is favoured over "prediction"., in This is because many future changes related to climate are highly uncertain., in For example, climate change projections are affected by highly uncertain changes in future GHG emissions., in
The TAR projects impacts according to possible future changes in global mean temperature., in Other projections are based on scenarios that the IPCC has developed. In 2000,, p.143 in the IPCC published 40 different scenarios, pp.143-144 in (the "SRES" scenarios) which contain estimates of future changes in anthropogenic emissions of greenhouse gases and aerosols. The SRES scenarios project a wide range of possible changes in future social and economic development, and projected climate change impacts vary according to the scenario considered., in The IPCC has not assigned probabilities to the 40 SRES scenarios. Some authors, in have argued that some SRES scenarios are more likely to occur than others.
Scientific opinion
The IPCC is backed by the scientific community., in . This document is also available in PDF format For example, a joint statement of support was issued in May 2001 by the science academies of Australia, Belgium, Brazil, Canada, the Caribbean, China, France, Germany, India, Indonesia, Ireland, Italy, Malaysia, New Zealand, Sweden and the UK. It states: "We recognise the IPCC as the world's most reliable source of information on climate change and its causes, and we endorse its method of achieving consensus."
In 2001, the executive branch of the US federal government asked the US National Research Council (US NRC, 2001), in to produce an assessment of climate change science. Part of the assessment by US NRC (2001), in looks at the report produced by Working Group I (WG I) in the TAR. Working Group I's contribution to the TAR assesses the physical scientific aspects of the climate system and climate change. US NRC (2001) generally agrees with findings of the WG I report, for example, US NRC (2001), in state that "[the] IPCC's conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue."
US NRC (2001), in emphasise the need for governments to have a good understanding of uncertainties in climate change science. The example cited by US NRC (2001) is the uncertainty over future changes in GHG emissions, which may be less or more than that projected by the TAR. US NRC (2001) also state: The most valuable contribution U.S. scientists can make is to continually question basic assumptions and conclusions, promote clear and careful appraisal and presentation of the uncertainties about climate change as well as those areas in which science is leading to robust conclusions, and work toward a significant improvement in the ability to project the future.
Reception Endorsements In 2001, 16 national science academies issued a joint statement on climate change.