The mechanism here proposed to explain the cycle of glaciation does not depend in any way upon the particular numbers assumed for illustrative purposes. Such oscillations will occur whenever the temperature during one phase of the cycle falls low enough to cause ice sheets to grow and during another phase rises high enough to cause them to melt. A change in the comparatively small volume of carbon dioxide in the atmosphere provides ample leeway to swing the temperature past either extreme. The oscillation is reinforced by the accompanying change in the earth's humidity. A colder atmosphere holds less water vapor, and so further reduces the atmospheric absorption of infrared radiation emitted by the earth's surface. At the same time, however, the earth's cloud cover thickens and precipitation increases despite the reduction in the water-vapor burden of the atmosphere. The top of a cloud is cooled by the radiation of heat into space; when there is less carbon dioxide in the atmosphere, cloud tops lose more heat energy and thus become colder. With a steeper temperature gradient there is increased convection within the cloud. The result is larger clouds and more precipitation. Moreover, since the cloud cover reflects the sun's visible radiation back into space, less solar energy reaches the earth, and the temperature falls still lower.
The geological record indicates that the huge capacity of the biosphere to store and turn over carbon dioxide has also had its effect upon climatic change. We know that plants borrow 60 billion tons of carbon dioxide yearly for photosynthesis. Under present conditions the organic world repays nearly all of this debt each year via respiration and decay. The formation of new fossil fuel deposits withholds at most only 100 million tons of carbon dioxide, or less than .2 per cent of the annual amount used for photosynthesis. At one time, however, the withdrawals were much larger. During the Carboniferous period, when most of the coal and oil deposits were formed, about 1014 tons of carbon dioxide were withdrawn from the atmosphere–ocean system. This staggering loss must have dropped the earth's temperature to chilly levels indeed; it is not surprising that the gigantic glaciers that moved across the earth after this period were perhaps the most extensive in history.
The present capacity of plants to consume carbon dioxide in photosynthesis gives us an interesting clue to the carbon dioxide content of the atmosphere in bygone ages. Plants are almost perfectly adapted to the spectral range and intensity of the light they receive, yet they grow far more rapidly and luxuriantly in an atmosphere that contains five to 10 times the present carbon dioxide concentration; in fact, florists sometimes release tankfuls of carbon dioxide in greenhouses to promote plant growth. The present carbon dioxide concentration in the atmosphere must therefore be unusually low. Apparently plant evolution was keyed to some much higher concentration in the atmosphere of the geologic past. This hypothesis is also supported by the known fact that the earth's climate was warmer during most of geologic time; presumably the atmosphere then contained a much higher percentage of carbon dioxide.
Much of the carbon dioxide in the atmospheres of past geologic epochs now lies buried in the carbon dioxide reservoir of the earth itself. The earth's hot springs and volcanoes pour about 100 million tons of carbon dioxide back into the atmosphere per year. The earth in turn recaptures approximately the same amount each year by the weathering of rocks. But this equilibrium is upset during periods of mountain-building. In fact, the carbon dioxide theory provides an essential link to explain the timing of the last two glacial epochs with respect to the mountain-building periods that preceded them.
At least several million years intervened between the climax of these mountain-building episodes and the formation of the great ice sheets. If glaciation was brought on only by the elevation of the land or by the slight darkening of the sky with the dust of volcanoes, there should have been no great time lag before the onset of the glaciers, But these upheavals exposed large quantities of igneous rock to the chemical action of the minute amounts of atmospheric carbon dioxide dissolved in the rain water that washed over them, Over millions of years the weathering of the rock trapped vast quantities of carbon dioxide from the air. With the atmospheric concentration reduced sufficiently, the temperature fell, permitting the young mountains to provide natural birthplaces for the glaciers that then crept across the earth.