The sun's energy drives the Earth's weather and climate and heats its surface. Some of this energy radiates back into space, but some of it is trapped by greenhouse gases (carbon dioxide, water vapor and other gases). A natural "greenhouse effect" keeps the Earth warm enough for life to flourish, but if too much heat is trapped, the Earth's climate could change in disruptive and dangerous ways.

There is a growing scientific consensus that increasing emissions of greenhouse gases (GHG) are affecting the Earth's climate. That consensus is represented by the work of the Intergovernmental Panel on Climate Change (IPCC), a body established by the World Meteorological Organization and the United Nations to assess scientific, technical and socio-economic information relevant for the understanding of climate change, its potential impacts, and options for adaptation and mitigation.

In its Third Assessment Report published in 2001, the IPCC noted that the Earth's surface temperature has increased by about 1 degree Fahrenheit in the past century, with much of that warming occurring the past two decades. The IPCC concluded that "In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations." The IPCC also concluded that these increased concentrations are largely attributable to human activities that result in emissions of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halogenated fluorocarbons (HCFCs) , ozone (O3), perfluorinated carbons (PFCs), hydrofluorocarbons (HFCs). Aerosols, including sulfate particles and black carbon (soot), are also believed to contribute to global warming.

Climate change simulations for the period of 1990 to 2100 based on IPCC scenarios for future GHG emissions yield a globally-averaged surface temperature increase by the end of the century of 1.4 to 5.8°C (2.5 to 10.4°F) relative to 1990, with a mid-range prediction of 3°C (5.4°F). Uncertainty remains in our understanding of how the climate system varies naturally and reacts to emissions of greenhouse gases and aerosols, thus current estimates of the magnitude of future warming will be subject to future adjustments (either upward or downward). The IPCC's Fourth Assessment Report is due in 2007.

For more information on the science of climate change:

• Intergovernmental Panel on Climate Change
• National Academy of Sciences, Testimony before the U.S. Senate Committee on Energy and Natural Resources, July 21, 2005
• National Academy of Sciences, Testimony before the Subcommittee on Global Climate Change and Impacts, U.S. Senate Committee on Commerce, Science, and Transportation, July 20, 2005
• National Research Council, Climate Change Science: An Analysis of Some Key Questions (2001)
• U.S. Department of Energy
• U.S. Energy Information Administration
• U.S. Environmental Protection Agency
• National Oceanic & Atmospheric Administration
• Climate Assessment for the Southwest (CLIMAS)