Our atmosphere consists of a mixture of gases including nitrogen, oxygen and water vapour.
Sunlight passes through the atmosphere, warming the earth's surface.
In turn, the land and oceans release heat, or infrared radiation, into the atmosphere, thus balancing the incoming energy. Water vapour, carbon dioxide and some of the other trace gases can absorb part of this radiation, allowing it to warm the lower atmosphere, while the remainder is emitted to space. This absorption of heat, which keeps the surface of our planet warm enough to sustain us, is called the greenhouse effect. About three-quarters of the natural greenhouse effect is due to water vapour. The next most significant greenhouse gas is carbon dioxide.