A gas that absorbs radiation at specific wavelengths within the spectrum of radiation (infrared radiation) emitted by the EarthÕs surface and by clouds. The gas in turn emits infrared radiation from a level where the temperature is colder than the surface. The net effect is a local trapping of part of the absorbed energy and a tendency to warm the planetary surface. Water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) are the primary greenhouse gases in the EarthÕs atmosphere.