Greenhouse Effect - (n.)
The "greenhouse effect" denotes the trapping of heat radiation within the atmosphere by certain trace gases contained in air, mainly water vapor, carbon dioxide and methane. Earth must radiate heat into space to balance the energy received from the Sun. The greenhouse effect makes it impossible to achieve the balance by re-radiation from the surface of Earth, thus forcing a warming of the ground and the lower atmosphere. Balance is then achieved through radiation from a level within the atmosphere, on average at 5000 m elevation. The term "greenhouse effect" also is used in connection with the emission of greenhouse gases associated with human activities, including industrial compounds (chlorofluorocarbons or CFC’s). This is more correctly referred to as the "excess greenhouse effect" or "additional greenhouse effect," unless the meaning is clear from the context.