The earth has a natural greenhouse effect due to the greenhouse gases present in the atmosphere. A greenhouse gas is a gas that is capable, due to its molecular structure, of absorbing infra-red radiation. The natural greenhouse gases are water vapour (H2O), carbon dioxide (CO2), methane (CH4), ozone (O3) and nitrous oxide (N2O). These gases are released from natural sources or produced by secondary reactions in the atmosphere. The process of the natural greenhouse effect is outlined below:-
The sun (the earths's source of heat) emits solar radiation, mainly short wave visible and ultra-violet radiation. As this radiation travels towards earth, 25% is absorbed by the atmosphere and another 25% is reflected by clouds back into space. The remaining radiation travels uninterupted to the earth where it is absorbed, heating the earth's surface. The earth releases a lot of the energy it receives from the sun back into space. However, as the earth is a lot cooler than the sun the radiation re-emitted has less energy so has a longer wavelength. Therefore the radiation is emitted as invisible infra-red radiation. The greenhouse gases in the atmosphere absorb the infra-red radiation trapping it, thus warming the atmosphere and the earth. This natural process keeps the earth at an average 15°C instead of -18°C (the termperature of the moon which has no atmosphere). The diagram below summarises this process:-
On the next page we will talk about the enhanced greenhouse effect and greenhouse gases.
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