The IPCC Third Assessment Report was issued in 2001 and is a product of the Intergovernmental Panel on Climate Change, which is based in Geneva Switzerland, and was established in 1988 by two United Nations organisations, the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). IPCC is an international body comprised of many political, governmental and scientific organizations, and has "...been established by WMO and UNEP 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 membership is open to members of the UN and WMO. 
The IPCC has made a series of reports related to climate change. The Third Assessment Report (TAR) was issued in 2001, and it has released other specialised reports since then. The IPCC Fourth Assessment Report (AR4) is due in 2007. Statements of the IPCC or information from the TAR are often used as a reference showing a scientific consensus on the subject of global warming.
The IPCC's organisation is currently structured as three working groups (WG) and a task force :
- WGI: Scientific aspects of climate 
- WGII: Vulnerability, consequences, and options 
- WGIII: Limitation and mitigation options 
- 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.
The "headlines" from the summary for policymakers  in The Scientific Basis 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 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 predictions is based upon several different scenarios that assume different levels of future CO2 emissions. Each scenario then has a range of possible outcomes associated with it. The most optimistic outcome assumes an aggressive campaign to reduce CO2 emissions, while the most pessimistic is a "business as usual" scenario. The more realistic scenarios fall in between.
IPCC predictions are based on the same models used to establish the importance of the different factors in global warming. These models need data about anthropogenic emissions of greenhouse gases and aerosols. These data are predicted from economic models based on 35 different scenarios. Scenarios go from pessimistic to optimistic, and predictions of global warming depend on the kind of scenario considered.
IPCC uses the best available predictions and their reports are under strong scientific scrutiny. The IPCC concedes that there is a need for better models and better scientific understanding of some climate phenomena, as well as the uncertainties involved. Critics assert that the available data is not sufficient to determine the real importance of greenhouse gases in climate change. Sensitivity of climate to greenhouse gases may be overestimated or underestimated because of some flaws in the models and because the importance of some external factors may be misestimated. The predictions are based on scenarios, and the IPCC did not assign any probability to the 35 scenarios used.
See also Edit
- climate change
- global climate model
- effects of global warming
- attribution of recent climate change
- global warming controversy
Adapted from the Wikipedia article "IPCC Third Assessment Report" http://en.wikipedia.org/wiki/IPCC_Third_Assessment_Report