A number of gases that occur naturally in the atmosphere in small quantities are known as "greenhouse gases". Water vapour, carbon dioxide, ozone, methane, and nitrous oxide trap energy in much the same way as do the glass panes of a closed automobile or a greenhouse. This natural greenhouse effect has kept the earth's atmosphere some 30°C hotter than it would otherwise be, making it possible for humans to exist on earth.

Human activities, however, are now raising the concentrations of these gases in the atmosphere and thus increasing their ability to trap energy. Carbon dioxide levels have risen from 280 parts per million by volume since before the Industrial Revolution to almost 360 today. Man-made carbon dioxide, which is the most important contributor to the "enhanced" greenhouse effect, comes mainly from the use of coal, oil, and natural gas. It is also released by the destruction of forests and other natural "sinks" and "reservoirs" that absorb carbon dioxide from the air.