"Many human activities release 'greenhouse gases' into the atmosphere. The levels of these gases are increasing at a faster rate than at any time in hundreds of thousands of years. […] This fact sheet addresses the most frequently asked questions about the science of global warming and climate change. The Intergovernmental Panel on Climate Change's (IPCC) Fourth Assessment Report (2007) serves as the key reference for this brochure. The IPCC was formed jointly in 1988 by the United Nations Environment Programme and the United Nations World Meteorological Organization. The IPCC brings together the world's top scientists, economists and other experts, synthesizes peer-reviewed scientific literature on climate change studies, and produces authoritative assessments of the current state of knowledge of climate change."
Environmental Protection Agency: http://www.epa.gov/