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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

2010-01PlassFB.jpgClick to Enlarge ImageOne further objection has been raised to the carbon dioxide theory: the atmosphere is completely opaque at the center of the carbon dioxide band and therefore there is no change in the absorption as the carbon dioxide amount varies. This is entirely true for a spectral interval about one micron wide on either side of the center of the carbon dioxide band. However, the argument neglects the hundreds of spectral lines from carbon dioxide that are outside this interval of complete absorption. The change in absorption for a given variation in carbon dioxide amount is greatest for a spectral interval that is only partially opaque; the temperature variation at the surface of the Earth is determined by the change in absorption of such intervals.

Thus there does not seem to be a fundamental objection to the carbon dioxide theory of climate change. Further the temperature changes given by the theory for reasonable variations in the carbon dioxide amount are more than enough to cause noticeable changes in the climate. It is not usually appreciated that very small changes in the average temperature can have an appreciable influence on the climate. For example, various authorities estimate that, if the average temperature should decrease from 1.5 to 8 degrees, the glaciers would again form over an appreciable fraction of the Earth’s surface. Similarly a rise in the average temperature of perhaps only 4 degrees would bring a tropical climate to most of the Earth’s surface.

Before discussing in detail the carbon dioxide theory of climatic change it is first necessary to study the various factors that enter into the carbon dioxide balance, including the exchange of carbon dioxide between the oceans and the atmosphere.

The largest loss of carbon dioxide from the atmosphere is due to the process of photosynthesis which uses about 60 x 109 tons per year. In a steady state precisely the same amount of carbon dioxide is returned to the atmosphere each year by all the processes of respiration and decay of plants and animals, provided only that none is permanently lost in the form of new coal, oil, and other organic deposits. At the present time, at least, this loss is very small (0.01 x 109 tons per year) and can be neglected for all practical purposes. If this steady state of absorption and emission of carbon dioxide by the organic world is disturbed, for example, by a sudden increase of carbon dioxide in the atmosphere, it is known that the amount used in photosynthesis would then increase. However, after a very few years the processes of decay and respiration would also have increased. Since an average carbon atom that has been used in photosynthesis returns to the atmosphere from the biosphere within about 10 years, and virtually all of the carbon atoms return within 250 years it follows that the factors influencing the carbon dioxide balance from the organic world would again be in balance in a very few years.




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