Both the atmosphere and the oceans continuously exchange carbon dioxide with rocks and with living organisms [see illustration on pages 44–45]. They gain carbon dioxide from the volcanic activity that releases gases from the earth's interior and from the respiration and decay of organisms; they lose carbon dioxide to the weathering of rock and the photosynthesis of plants. As these processes change pace, the content of carbon dioxide in the atmosphere also changes, shifting the radiation balance and raising or lowering the earth's temperature.
Of course during any particular geologic era other factors may influence climate. Nonetheless let us examine some of the known facts of geological history and see how many can be explained in terms of variation in the carbon dioxide content of the atmosphere.
Studies of rock strata reveal that for the past billion years most of the world has had a tropical climate. Every 250 million years or so this tropical spell is broken by relatively short glacial periods which bury a substantial portion of the earth under ice sheets. These cool periods last several million years, during which the glaciers retreat and advance many times as the temperature rises and falls. During the last 620,000 years of the current glacial epoch, for example, deep ocean sediments show 10 distinct temperature cycles. The carbon dioxide theory may well account for these temperature fluctuations.
A decline in the carbon dioxide concentration in the atmosphere–ocean system—and a period of decline in worldwide temperature—may be induced by a number of developments. The rate of volcanic activity could slow down as the rate of rock weathering increased, or an especially flourishing mantle of vegetation could take up huge quantities of carbon dioxide and form new coal beds and other organic deposits in marshy areas. After a geologically short time, the adjustment of the atmosphere–ocean equilibrium to the leaner supply of carbon dioxide could bring the atmospheric concentration down to .015 per cent, half its present value. Calculations show that a 50-per cent decrease in the amount of carbon dioxide in the air will lower the average temperature of the earth 6.9 degrees Fahrenheit.
We can be reasonably sure that such a sharp drop in temperature would cause glaciers to spread across the earth. As the ice sheets grow, the oceans shrink; at the height of glacial periods ice sheets contain 5 to 10 per cent of the oceans' waters. The glaciers contain little carbon dioxide, however, because ice can hold very small amounts of carbonates compared to the same volume of sea water. The shrunken oceans thus accumulate an excess of carbon dioxide which they must release to the atmosphere in order to return to equilibrium. And so the cycle draws to a close: As carbon dioxide returns to the atmosphere, the earth's temperature rises and the ice melts away. The oceans fill to their former levels, reabsorb the carbon dioxide they had released, and a new glacial epoch begins.
So long as the total amount of carbon dioxide in the atmosphere-ocean system does not change, such a cycle of temperature oscillation will tend to repeat itself. The period of the complete cycle would be determined primarily by the time required for an ice sheet to form, grow to maximum size and melt away. Estimates indicate that this should take about 50,000 years, in agreement with the observed time for the cycle. Other time factors in the cycle, such as the period required for the ocean–atmosphere system to come to equilibrium after a change in its carbon dioxide concentration, are probably much shorter. The system never quite reaches equilibrium, however, because the freezing and melting of glaciers is out of phase with the fluctuation of carbon dioxide in the atmosphere. Glaciers are slow to form and slow to melt, so for thousands of years during the earth's recovery from an ice age the cold winds from melting glaciers continue to chill the earth.