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If the Earth receives energy, it also emits it. The diagram above pictures the great exchanges of energy between the Sun, the atmosphere and the terrestrial surface. A proportion of the energy emitted by the ground (in the form of infrared radiation) escapes directly into space but another (the largest, 324 watts/m²) is sent back to the soils because of the presence of water vapour and certain gases (called greenhouse gases) which act like a greenhouse windowpane.

Without the energy contribution of the greenhouse effect, it is estimated that the Earth would no longer emit 390 watts/m², which corresponds to an average temperature of +15°C, but only 235 watts/m².That corresponds to an average temperature of around – 19°C.

The greenhouse effect is thus a phenomenon which is above all else natural, in other words that it is to a very great extent independent of human activity. The greatest proportion of the natural greenhouse effect comes from the actions of water vapour (close to two thirds of heat trapping) and CO2 (close to a third) which come from vegetation, the ground and the oceans. Greenhouse gases of human origin amplify the natural greenhouse effect.

Only a small proportion of the constituents of the atmosphere absorb the infrared radiation coming from the terrestrial surface and play a part in the greenhouse effect phenomenon. These constituents are called greenhouse effect gases. In effect the composition of the dry atmosphere (in other words without water vapour) is the following: nitrogen 78%, oxygen 21%, diverse gases 0.95%, and greenhouse gases make up only 0.05%. For the latter, the composition is the following (besides water vapour): carbon dioxide = CO2 (99.45%); methane = CH4 (0.46%); nitrous oxide = N20 (0.083%), ozone = O3 (0.00026 %), various fluoride gases.