The Earth maintains its equilibrium temperature through a delicate balance between the incoming solar energy it absorbs and the outgoing infra-red energy that it emits and some of which escapes into space. Greenhouse gases (water vapour, carbon dioxide, methane and others) allow solar radiation to pass through the Earth’s atmosphere almost unimpeded but they absorb the infra-red radiation from the Earth’s surface and then re-radiate some of it back to the Earth. This natural greenhouse effect keeps the surface temperature about 33°C warmer than it would otherwise be - warm enough to sustain life. Since the industrial revolution, the concentration of CO2, one of the major greenhouse gases, in the atmosphere has increased significantly. This has contributed to the enhanced greenhouse effect known as ‘global warming’.
(UNEP, Global Environmental Outlook)