In its February 2007 report, the Intergovernmental Panel on Climate Change, established by the World Meteorological Organization and the United Nations Environment Programme, expressed more than 90 percent confidence that global warming is caused by human activity -- namely, an increase in the amount of carbon dioxide, methane, and other heat-trapping "greenhouse" gases in the atmosphere.

 

Like glass in a greenhouse, these gases allow the sun's heat through the atmosphere, but then trap much of it near the Earth's surface. For billions of years they have played an important role in maintaining the proper temperatures for life to thrive. But since the Industrial Revolution, human activity has led to a dramatic increase in the amount of these heat-trapping gases. Carbon dioxide levels have risen from pre-Industrial levels of 280 parts per million to 379 parts per million. The amount of methane, another greenhouse gas, has more than doubled. The result has been a measurable warming trend called global climate change or, more specifically, global warming.