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FEATURE ARTICLE

Carbon Dioxide and the Climate

A 1956 American Scientist article explores climate change; two contemporary commentaries illuminate its relevance to the present

Gilbert N. Plass, James Rodger Fleming, Gavin Schmidt

Variations in the concentration or distribution of any gas that absorbs in the infrared portion of the spectrum can influence the surface temperature in the same manner as we have already discussed for carbon dioxide. Ozone and water vapor are the only two other gases that absorb in this region and also exist in the atmosphere in sufficient quantities to have an appreciable effect. Few suggestions have been made that relate variations in the concentration of these two gases to the climate, since these changes do not seem to be related directly to definite geological factors. However, recent calculations have shown that variations in the distribution of ozone can appreciably change the surface temperature. Normally the ozone concentration has a maximum in the stratosphere with relatively small amounts at lower altitudes. Vertical air currents occasionally bring some of the ozone down from the stratosphere, thus greatly increasing the concentration at lower altitudes. This is sufficient to increase the surface temperature from radiation effect by several degrees.

The relative humidity as a function of altitude is continually changing and a similar effect on the surface temperature exists for water vapor. These relatively rapid variations in temperature are superimposed on those from carbon dioxide alone. The latter variations are relatively constant over long time intervals compared to the former. However, water vapor can also have an effect over long time intervals, since the amount that can be held in the atmosphere decreases very rapidly as the temperature drops. During a glacial period the atmosphere has a smaller capacity to hold water vapor; for this reason the infrared heat energy from the Earths surface can escape more easily to space. Thus the influence of water vapor on the infrared absorption tends to reduce the surface temperature still more once a glacial period has started. The increased cloud amount during such a period also acts to reduce the surface temperature by reflecting the incoming solar radiation back to space. Therefore the temperature decrease during a glacial epoch is probably somewhat greater than is calculated from the carbon dioxide effect alone.

A very large number of different theories of climatic change has been proposed. As more evidence about past climatic change is obtained, each theory has to meet continually more rigorous tests in order to explain the known facts. Each of the major theories of climatic change predicts a different temperature trend during the remainder of this century. A comparison of these predictions with the actual record at the end of the century will provide an important test of these theories.

The variable sun theory predicts that the temperature will decrease for some decades. The maximum of the 80-year period in the sunspot cycle probably occurred in 1947. Thus the total energy received from the sun including the ultraviolet should decrease for some decades when the records are averaged over the shorter periods in the cycle. On the other hand a continued increase in the average temperature could be justified by the variable sun theory only if measurement showed a corresponding increase in the solar constant.

Changes in the average elevation of the continents clearly cannot be used to explain any variations in the climate over a period of a few centuries. However, the volcanic dust theory predicts appreciably lower temperatures for a few years following volcanic activity that throws large quantities of dust into the atmosphere. The last such explosion was when Katmai on the Aleutian Islands erupted in 1912. More volcanic explosions of this kind must occur before sufficient data can be obtained to correlate with the predictions of this theory. At the present time it is entirely possible that volcanic dust creates small perturbations in the climate while the general trend is determined by some other factor.

On the other hand the carbon dioxide theory is the only one that predicts a continually rising average temperature for the remainder of this century because of the accumulation of carbon dioxide in the atmosphere as a result of industrial activity. In fact the temperature rise from this cause may be so large in several centuries that it will present a serious problem to future generations. The removal of vast quantities of carbon dioxide from the atmosphere would be an extremely costly operation. If at the end of this century the average temperature has continued to rise and in addition measurement also shows that the atmospheric carbon dioxide amount has also increased, then it will be firmly established that carbon dioxide is a determining factor in causing climatic change.

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