Carbon Dioxide

In one respect, the composition of the atmosphere has been changing in the last 200 years. Scientists have estimated that the concentration of carbon dioxide in the atmosphere before the Industrial Revolution was between 275 and 285 parts per million (ppm); by 1958 it had risen to 315ppm and by 1980 it had increased still further, to 338ppm. This increase is a result of human disturbance of the carbon cycle by the burning of fossil fuels and the destruction of forests.

Moreover, the proportion of carbon dioxide in the air is continuing to increase. This continual increase has become a matter of concern because although carbon dioxide allows short-wavelength radiation from the Sun through to the Earth's sur­face, it absorbs some of the longer wavelength radiation that is re-radiated by the surface (water vapour and clouds also have this absorptive effect), giving rise to the "greenhouse effect". Thus carbon dioxide prevents the loss of radiation from the Earth, and the greater the amount of this gas the warmer it will become.

An extreme example of the greenhouse effect occurs on Venus, where carbon dioxide makes up 95 per cent of the atmosphere and the average surface temperature is about 475°C. On Earth an increase of the car­bon dioxide concentration to 570ppm could, according to one calculation, raise global temperatures by an average of 3°C, which could have unforeseen and possibly disastrous ecological consequences.