The greenhouse effect is the physical mechanism by which the atmosphere helps to maintain Earth's surface temperature within a range comfortable for organisms and ecological processes. The greenhouse effect is largely a natural phenomenon, but its intensity may be changing because of increasing concentrations of carbon dioxide and some other gases in the atmosphere. These increased concentrations are occurring as a result of human activities, especially the burning of fossil fuels and the clearing of forests. A probable consequence of an intensification of Earth's greenhouse effect will be a significant warming of the atmosphere. This could likely result in important secondary changes, such as a rise in sea level, variations in the patterns of precipitation, and large and difficult ecological and socio-economic adjustments.

Earth's greenhouse effect is a well-understood physical phenomenon. Scientists believe that in the absence of the greenhouse...