CLIMATE SINCE THE LATE CRETACEOUS 75 



climate: (1) composition of the atmosphere and (2) amount and 

 kind of solar radiation. Without doubt there have been changes in 

 both of the fundamental controls during geologic time. In addition, 

 it is important to note that changes in solar radiation can also affect 

 the composition of the atmosphere by changing the amount of both 

 water vapor and carbon dioxide. Thus the two controls are not inde- 

 pendent. As far as we know now the two constituents of the atmos- 

 phere most concerned with world climate are carbon dioxide and 

 water, although the concentration of oxygen and ozone aloft may 

 also play a part. Carbon dioxide and water both act to absorb earth 

 radiation while they are comparatively transparent to short-wave 

 solar radiation. Thus the surface of the earth is blanketed by these 

 gases and, as a consequence, is much warmer than it would be 

 without them, and daily fluctuations are much reduced. Recent 

 studies of the absorption bands of carbon dioxide indicate that it 

 can have a rather large effect independently of the presence of water 

 vapor. Plass (1956) brought up to date the climatic effects of changes 

 in the concentration of carbon dioxide. However, he did not explain 

 convincingly the role of this gas in initiating world wide climatic 

 changes, such as the beginning or ending of the Glacial periods. 

 It is only necessary to recall again the climatic changes between the 

 spring of 1956 and of 1957 to realize that circulation patterns can 

 vary remarkably from one year to another without changes in the 

 composition of the atmosphere. We still have much to learn con- 

 cerning the true causes of these year-to-year fluctuations, let alone 

 those of longer periods. The problem of the function of carbon 

 dioxide as an atmospheric climatic control is most complex, and 

 several important questions remain unanswered. Removal of carbon 

 dioxide from the atmosphere by natural causes might lower world 

 temperatures. Removal is accomplished by lime-secreting organisms, 

 by the weathering of the rocks, and by the deposition of oil, coal, 

 and the like. When we consider the huge limestone deposits of the 

 Paleozoic and the Cretaceous, together with the accompanying coal 

 deposits, we wonder if the concentration of carbon dioxide in the 

 atmosphere has not been tremendously reduced since pre-Cambrian 

 days. If the role of carbon dioxide in the atmosphere is of climatic 

 importance, there must be some natural means of replenishing the 

 atmospheric supply. The glaciation of the late Paleozoic might be 

 a natural consequence of the removal of the gas in the formation of 



