Carbon Dioxide and the Climate
A 1956 American Scientist article explores climate change; two contemporary commentaries illuminate its relevance to the present
The explanation of the time lag in terms of the carbon dioxide theory is that large quantities of carbon dioxide are removed from the atmosphere by the increased weathering after a period of major mountain building. After some millions of years, the carbon dioxide content of the atmosphere is reduced sufficiently to bring on a period of glaciation. From estimates of the increased weathering that occurs after the uplift of a mountain range, it is found that the time lag is of the order of a million years.
However, during an epoch of mountain building greatly increased amounts of carbon dioxide must be released from the interior of the Earth into the atmosphere through volcanic vents and hot springs. Additional millions of years are required to use up this additional carbon dioxide by the process of weathering. Thus the actual time interval between the onset of an epoch of mountain building and the ensuing glaciation can be considerably greater than a million years, if large additional quantities of carbon dioxide are released from the interior of the Earth. Indeed, if these amounts are very large, weathering would be unable to reduce the atmospheric carbon dioxide content to a sufficiently low level to cause a glacial period. In fact some periods of mountain building have not been followed by extensive glaciation. Such theories of glacial change as the variation in the amount of volcanic dust in the atmosphere and the change in the average elevation of the lands have found it difficult to explain why the glaciers do not form immediately after the uplift of a major mountain range.
During the geological history of the Earth the amount of carbon dioxide lost from the atmosphere in the formation of coal, oil, and other organic deposits has varied widely. This loss is relatively minor today. On the other hand it would be especially large during a period such as the Carboniferous when there were extensive marshes and shallow seas. At the end of the Carboniferous the atmospheric carbon dioxide content may have been reduced to a very low level because of the tremendous quantities that had been used in the newly formed coal and oil deposits. It is perhaps significant that the glaciation at the end of the Carboniferous may have been the most severe in the Earth’s history.
Radiocarbon dating indicates that recent changes in climate have been contemporaneous in both hemispheres. In the last fifty years virtually all known glaciers in both hemispheres have been retreating. According to the carbon dioxide theory, such changes in climate should occur at the same time in both hemispheres. The exchange of air between the two hemispheres is relatively rapid. Even if the atmospheric carbon dioxide content should increase suddenly in one hemisphere through a variation of some factor that enters into the carbon dioxide balance, the amount in the two hemispheres should again be equal in a relatively short interval on the geological time scale, perhaps no more than a few decades. It should be mentioned that it is possible to have glaciation in one hemisphere and not the other even though the atmospheric carbon dioxide amounts are the same. If one hemisphere has extensive mountain ranges and the other is relatively flat, glaciers could spread from the mountainous region of one hemisphere whereas they would be unable to form on the more level land of the hemisphere at the same average temperature.
The carbon dioxide theory has given plausible explanations for the beginning of a glacial period and of the climatic oscillations that occur during a glacial period. What increases the total carbon dioxide amount sufficiently to bring a glacial period to an end? One possibility is that the rock weathering is slowly reduced because of the increasing flatness of the land. In addition extensive glaciation probably reduces the rate of weathering for the fraction of the land surface that is covered by the glaciers. Thus, as the loss of carbon dioxide from the atmosphere for weathering decreases as a glacial epoch nears its end, the amount of atmospheric carbon dioxide slowly increases until finally the surface temperature is too high to allow further growth of the glaciers. An extensive period of mountain building has occurred at intervals of roughly 250,000,000 years during the Earth’s history and a glacial period has followed in each case during the time interval when sufficient carbon dioxide was removed from the atmosphere.