3. The Greenhouse Effect

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The earth's atmosphere is a mixture of gases but it is dominated by Nitrogen and Oxygen which together make up 99% of air. Despite the abundance of these two gases it is the 'minor' constituents, particularly carbon dioxide and water vapour that are responsible for the natural green house effect. Without this process the average temperature of the earth would be below zero (about 30 degrees Celcius cooler than it is at present).

Greenhouse gases have the peculiar property of absorbing radiation emitted by the earth (longwave radiation) and not that emitted by the sun (shortwave radiation). While solar energy is mainly either absorbed by the earth's surface or reflected, the earth's energy is recycled causing the temperature to rise. Only a small proportion of the earth's radiation is not absorbed by the atmosphere and passes through it, into space. This 'gap' in the ability of the atmosphere to absorb is the atmospheric window. Atmospheric scientists believe that increasing the amount of greenhouse gases will cause this window to narrow so that more earth radiation is recycled and the temperature will rise still further.