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Horn, Dr G. Daily Telegraph, 26 March 2006

The important difference between water vapour and carbon dioxide as greenhouse gases is that the former can only build up until the air becomes saturated, ie the humidity reaches 100 per cent (Letter, March 19). Globally, the average moisture content of the atmosphere remains substantially constant, governed mainly by the average air temperature. Carbon dioxide is gaseous at atmospheric temperatures and can only return to earth by slower, less efficient processes such as dissolving in the ocean surface or in raindrops, or by adsorption and photosynthesis by plants. So the CO2 content of the atmosphere has risen by over 30 per cent since the Industrial Revolution. Water vapour contributes significantly to the 'natural' greenhouse effect, but CO2 is thought to have contributed about 70 per cent of the 'enhanced' greenhouse effect due to human activities. Of the remainder, about 24 per cent is due to methane and 6 per cent to nitrous oxide.
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