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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

Let us next examine some of the variations in the atmospheric carbon dioxide amount in past geological epochs and their correlation with the climate as deduced from the geologic record. It is interesting that a large number of these climatic variations can be explained simply and naturally by the carbon dioxide theory.

During the last glacial epoch of perhaps a million years’ duration, four distinct periods of glaciation separated by warmer interglacial periods have long been recognized. Recently Wiseman has studied the sediments of the deep ocean floor and has found evidence for ten distinct temperature minima with the last 620,000 years. It appears that a fundamental property of a glacial epoch is to have a climate that is continually fluctuating. The glaciers advance and then recede and repeat the cycle several times before the end of the glacial epoch. No other theory of climatic change seems able to explain in a simple and straightforward manner these continual oscillations in climate during a million-year epoch of glaciation.

In order to understand these oscillations let us consider the figure where the equilibrium pressure of the carbon dioxide in the atmosphere is plotted against the total amount of carbon dioxide in the atmosphere-ocean system. These curves were calculated as described above with the additional assumption that the average temperature varies as predicted by the carbon dioxide theory. Curves are shown when the oceans have 0.90, 0.95, and 1.00 times their present volume in order to allow for the face that the ocean volume decreases during a period of glaciation.

The present value for the carbon dioxide pressure (3 x 10-4 atmosphere) and the total amount of carbon dioxide in the atmosphere-ocean system (1.32 x 1014 tons) is marked with the letter “P” in the figure. Let us suppose that a million years ago the carbon dioxide balance was upset and that the total amount of carbon dioxide in the atmosphere-ocean system was reduced 7 per cent to 1.23 x 1014 and that it remained fixed at this new lower value throughout the ensuing glacial period. Let us further assume that if the average temperature should fall 3.8 degrees that great ice sheets would again form and cover sizable portions of the continents. With the reduced carbon dioxide amount the atmosphere-ocean system would finally come to equilibrium at the point “G” in the figure. The new atmospheric carbon dioxide pressure would be 1.5 x 10-4 atmosphere. This would reduce the temperature by 3.8 degrees according to this theory; this would be sufficient to start a period of glaciation.

Let us assume in agreement with the estimates of glacial authorities that the glaciers contain about 5 per cent of the water of the oceans when the ice sheets have reached their maximum development. Since only small amounts of carbonates are held permanently in glacial ice, the loss of this water by the oceans means that oceans now contain too much carbonate for their reduced volume. They release that carbon dioxide, thus increasing the amount in the atmosphere.

The atmosphere-ocean system again reaches equilibrium at the point “N” of the figure some tens of thousands of years later. This point represents the equilibrium conditions when the ocean volume is 95 per cent of its present value and the atmospheric carbon dioxide pressure is 2.5 x 10-4 atmosphere. However, when the carbon dioxide pressure reaches this value, the average surface temperature rises to virtually its present value. It is then too warm to maintain the glaciers and they start to melt. This process probably takes thousands of years, but finally the oceans return to their original volume. Now the oceans do not contain sufficient carbonates for their increased volume; the atmosphere-ocean system is no longer in equilibrium. The oceans absorb additional carbon dioxide from the atmosphere until after tens of thousands of years the system is again near equilibrium at the point “G” in the figure. This cycle continues indefinitely as along as the total carbon dioxide amount in the atmosphere-ocean system remains fixed at 1.23 x 1014 tons. The period for one complete cycle depends on the rate of circulation of the oceans, but may be very roughly estimated as 50,000 years or more.

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