Atmospheric Weight and Heat

The atmosphere weighs an estimated 5,000 million tons, and about half of this total mass is in the lower layers, within 5km of the Earth's surface. At sea level the average atmospheric pressure is 1.05kg/cm² (or 1,013 millibars) – that is, the weight of air above each square centimeter is 1.05kg. The pressure (and density) of the atmosphere decreases with increasing altitude; at a height of 5km the average pressure is 500 millibars – about half that at sea level – and at 16km above the ground it is only 100 millibars.

Variations in pressure are also caused by temperature changes. The chief source of heat is solar radiation, although little heat comes directly from the Sun's short wavelength radiation. Of the radiation that reaches the outer atmosphere, only about 46 per cent reaches the Earth's surface, most of the rest having been scatteredor reflected back into space. At the surface, however, solar radiation is absorbed (thereby heating the surface) then reradiated in the form of longer wavelength radiation. It is this long-wavelength radiation that is absorbed by the carbon dioxide, water vaporand clouds in the lower atmosphere, creating the greenhouse effect. Hence the atmosphere is heated principally from below and, as a result, temperatures decrease with increasing altitude in the lower part of the atmosphere.

Heating by long-wavelength radiation near ground level makes the air expand so that it becomes less dense than the overlying cold air. As a result, the warm air tends to rise, leaving behind an area of comparatively low pressure. This contrasts with cold, dense air, which tends to sink, creating relatively high air pressure.

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 315 ppm and by 1980 it had increased still further, to 338 ppm. 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 570 ppm could, according to one calculation, raise global temperatures by an average of 3 °C, which could have unforeseen and possibly disastrous ecological consequences.