The IPCC was established by the United Nations to bring together the world’s leading scientists to conduct rigorous surveys of the latest technical and scientific literature on climate change.

The main activity of the IPCC is to provide regular Assessment Reports of the state of knowledge on climate change. The latest of these is “Climate Change 2007”, the Fourth IPCC Assessment Report, whose key conclusions are summarised below. This report has been described as a historical landmark in the debate about whether humans are affecting the state of the atmosphere.

1. Warming of the climate systemis unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, wide spread melting of snow and ice, and rising global average sea level.
Eleven of the last twelve years (1995–2006) rank among the twelve warmest years in the instrumental record of global surface temperature (since 1850). The 100-year linear trend (1906–2005) is 0.74°C; temperature increase is widespread over the globe, and is greater at higher northern latitudes. Land regions have warmed faster than the oceans.

2. Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely (over 90% probability) due to the observed increase in anthropogenic (man-made) greenhouse gas concentrations.

3. Rising sea level is consistent with warming. Global average sea level has risen since 1961 at an average rate of 1.8mm/yr and since 1993 at 3.1mm/yr, with contributions from thermal expansion, melting glaciers and ice caps, and the polar ice sheets. Whether the faster rate for 1993 to 2003 reflects decadal variation or an increase in the longer-term trend is unclear.

4. Observed decreases in snow and ice extent are also consistent with warming. Satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.7% per decade, with larger decreases in summer of 7.4% per decade. Mountain glaciers and snow cover on average have declined in both hemispheres.

5. There is observational evidence of an increase in intense tropical cyclone activity in the North Atlantic since about 1970, with limited evidence of increases elsewhere.

Source: Fourth IPCC Assessment Report, “Climate Change 2007”

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).

The IPCC describes warming and cooling effects on the planet in terms of radiative forcing — the rate of change of energy in the system, measured as power per unit area (in SI units, W/m²). Its AR4 report shows in detail the individual warming contributions (positive forcing) of carbon dioxide, methane, nitrous oxide, halocarbons, other human warming factors, and the warming effects of changes in solar activity.

Also shown are the cooling effects (negative forcing) of aerosols, land-use changes, and other human activities. All values are shown as a change from pre-industrial conditions.

— Total radiative forcing from the sum of all human activities is a warming force of about +1.6 watts/m2.

— Radiative forcing from an increase of solar intensity since 1750 is about +0.12 watts/m2.

— Radiative forcing from carbon dioxide, methane, and nitrous oxide combined is very likely (>90%) increasing more quickly during the current era (1750–present) than at any other time in the last 10,000 years.