of climate change, its potential impacts and options for adaptation and mitigation." 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