Science of climate change

The Intergovenrmental Panel on Climate Change (IPCC) 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.
Vital Climate Graphics The basics of climate change presented through a series of charts and diagrams. UNEP

BBC World Service Clickable, simple background information on climate change with the focus on Europe

Tiempo This site on climate change run by the University of East Anglia in the UK provides information on climate science - and much more
Fact sheets on climate change from the Atmosphere, Climate and Environment Unit at the Manchester Metropolitan University
US Environmental Protection Agency site has a clear well-organised section on Science Q & A
US Global Change Programmeregularly posts news on scientific research in climate change
Environmental Defense Fund site sorts myth from fact on the science of climate change