CAUSES OF CLIMATIC CHANGES. 327 



center of the continent is less stormy at sun-spot maxima than at minima, 

 while southern Canada and a large area southward and oceanward are more 

 stormy. If the splitting went further, it would result in a boreal storm-belt of 

 great severity and a subtropical belt of minor severity, a condition which 

 apparently obtained during the geological past. 



The cyclonic solar hypothesis. — ^Newcomb, Koppen, Hann, and others have 

 proved that there is a close relation between changes of temperature in tropical 

 regions and the sun-spot cycle. Arctowski has shown that in areas where the 

 climate is under direct solar control the temperatiu^e shows synchronous 

 departm-es from the mean, which are probably due to the sun. Where the 

 variations of temperature in different regions show discrepancies or contra- 

 dictions, they are perhaps to be explained by the transport of heat by cvurents 

 of water and of air, or by the action of volcanic dust. The measured varia- 

 tions in the solar constant indicate that they are not the chief cause of differ- 

 ences in earth temperatures, and clearly suggest a cyclonic hypothesis of solar 

 action in place of the caloric hypothesis. This suggestion is strongly supported 

 by the harmony between the number of sun-spots and the number of cyclonic 

 storms in the tropics, as well as that between sun-spots and the growth of trees 

 in northern Germany. In North America, Kullmer has pointed out that the 

 total number of storms increases dming sun-spot maxima, and that in both 

 Europe and America there is a distinct shifting of the zone of storminess in 

 agreement with the sun-spot cycles. Hence, Huntington concludes that this 

 evidence warrants replacing the old caloric hypothesis with the cyclonic 

 hypothesis of the action of the variations in the sun as the main factor in 

 producing changes of climate. 



Since variations in the sun do not seem to be directly reflected in terrestrial 

 temperatures, it is necessary to determine the probable effect of variations in 

 storminess upon temperature. The rise of warm air in the center of a low or 

 cyclonic area results in a loss of heat from the earth's surface. The greater 

 storminess of periods of many sun-spots causes a corresponding increase in 

 the rise of warm air and in the loss of heat from the lowermost portion of the 

 atmosphere. As the rising warmer air comes from the tropics, the restdt is 

 to lower the temperatiu-e of the latter between the storm-belts of the two 

 hemispheres. Such a cooling of the tropics must occur whenever the storms 

 increase, regardless of solar radiation. In the storm-belt itself, air rushes in 

 from both north and south, but the cold air flows beneath the warm air, and 

 the effect is to lower the surface temperatures. Polar regions are regions of 

 high pressure, and hence of descending air which has been cooled by remaining 

 at high altitudes for a long time. As a consequence, polar temperatures would 

 seem to change Httle, except as a result of variations in the amoujit of heat 

 received from the sun. Hence it appears that in boreal, temperate, and 

 tropical regions, changes in storminess must change the mean temperature of 

 the atmosphere at low levels, even without any change in the amount of heat 

 received from the sun. 



Relative value of causes. — It seems certain that deformation, changes in the 

 Sim, and volcanic dust constitute the causes of climatic change. While their 

 causal relation to climate appears to be beyond doubt, their relative importance 

 is less evident. With respect to climates of the present and the immediate past 

 deformation is negligible as an existing process, though the consequences of 

 past deformations are plainly evident in the differentiation of regional climates. 

 While the work of Newcomb, Koppen, and Abbot in particular seems to prove 

 the inability of solar variations to produce efficient changes of temperature 



