NOTE ON A POSSIBLE FACTOR IN CHANGES OF 
GEOLOGICAL CLIMATE 
HARLOW SHAPLEY 
Mount Wilson Observatory, Carnegie Institution of Washington 
In a recent discussion of the factors that control variations of 
world climatic conditions, Professor Humphreys has criticized the 
various astronomical hypotheses that attempt to explain the ice 
ages and other features in geological climate.* Several factors, 
formerly given weight, are held responsible only for effects of the 
second order at most. Among the insufficient interpretations he 
would place Croll’s theories which involve the changing elements 
of the earth’s orbit, and the various hypotheses connecting sun 
spots (and other intrinsic changes in solar radiation) with terres- 
trial insolation and the climates of the geologic past. 
The primary factors of climatic control clearly appear to be of 
terrestrial origin—land elevation (and its concomitant factors of 
oceanic and atmospheric distribution and circulation), combined 
with vulcanism. Cosmic factors are evidently not of primary 
importance. The observed connection of climatic phenomena 
with land elevation can by no means be attributed to chance. A 
cosmic non-terrestrial origin for the principal factors of climatic 
control would therefore leave unexplained such significant coin- 
cidences as mountain forming and glaciation, since one could 
hardly propose seriously that cosmic factors are the cause of both 
topographic and climatic change. 
One astronomical possibility that I believe has not been urged 
heretofore appears, however, as a result of recent observation, to 
deserve attention as a potential secondary factor in recorded 
geological climates, and possibly even as a primary agent at some 
prehistoric time. 
tW. J. Humphreys, “Factors of Climatic Control,” Jour. Frank. Inst. 
CLXXXVIII (1919), 775; CLXXXVIII (1920), 63. 
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