260 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



Turning now to the solar h.ypothcsis, we find that the only serious objection to it is 

 that we do not possess any direct evidence that the intensity of solar radiations has varied 

 greatly in past times. In the nature of the case, no such evidence can ever be forthcoming. 

 We do know, however, that during the few decades since measiu-ements have been 

 possible, it has been proved beyond question that the intensity of solar radiation actually 

 varies, although the amount of variation may be small. Moreover, historic records of sun- 

 spots tell us that the sun's activity has varied in the past, and there are indications that 

 at certain periods the number or size of the sun-spots was greater than recently. When we 

 compare solar changes with variations in the earth's climate during the past few decades, 

 we find that the two agree in manj- different ways. We also find that if allowance is made 

 for the unequal distribution of the sun's insolation over the earth's surface and for the 

 effect of volcanic dust in shutting out the sun's heat, many of the apparent disagreements 

 between solar and terrestrial phenomena disappear. If we suppose that in the past the 

 sun's variations were like those of the present, but on a larger scale, we find an explanation 

 of climatic change which appears to satisfy all the requirements. We may suppose that 

 at certain times the sun was stimulated to unusual activity, just as it is to-day when we 

 have periods of unusually numerous sun-spots, but to a greater degree. This stimulation 

 would cause an increase in all the phenomena associated with sun-spots. If the latest con- 

 clusions of Hale and others are correct, the sun-spots are violent perturbations of a cyclonic 

 character, whereby material from the lower part of the sun's atmosphere is carried into 

 the upper part. This process, according to an interesting suggestion made by Humphreys 

 and quoted in Professor Schuchert's portion of this volume, might so increase the density 

 of the solar atmosphere that an appreciable share of the sun's radiant energy would be 

 prevented from escaping into space. This would produce the same effect as the presence 

 of dust in the earth's atmosphere, and would cause the earth's climate to become cool. 

 The degree of cooling upon the earth, and upon the other planets, where the same result 

 would ensue, would depend upon the amount of material in the solar atmosjjhere. The 

 sun itself might conceivably be hotter than before, and probably would be, although the 

 effect upon outside bodies would be diminished. The material thrown into the solar 

 atmosphere would presumably be in the most comminuted form, perhaps molecular, and 

 part might even escape into space. The remainder, however, would graduallj- fall back 

 toward its place of origin. Thus the solar atmosphere would become clearer, although 

 the process would take a long time. As the atmosphere grew clearer, more and more of 

 the sun's radiation would escape into space and the earth would become correspondingly 

 warmer. If the sun were actually hotter than normal, the clearing of its atmosphere would 

 give rise to an interglacial epoch characterized by unusual warmth or by aridity. This 

 would last until renewed solar activity caused the ejection of more material and the solar 

 atmosphere once more became dense, thus causing another glacial epoch. The processes 

 here suggested, together with those discussed in the following paragraph, would cause cold 

 climates to develop rapidly and pass away more slowly. This would correspond with the 

 conclusions of geology, and would also agree with the changes of the last 3,000 years in 

 California, where the curve of the sequoia usually rises more rapidly than it falls. 



At this point we must consider what would be happening to the earth's crust at such a 

 time of unusual activity in the sun. According to Professor Schuchert the time when 

 the sun would be filling its atmosphere with ejected material, and thus preparing the 

 way for a cool period upon the earth, would be likely to be a period of pronounced crustal 

 deformation. In other words, a stimulation of the earth's interior appears to take place at 

 the same time that our hypothetical stimulation of the sun takes place. The stimulation 

 of the earth causes crustal deformation, the upheaval of continents and mountains, the 

 formation of barriers between adjoining portions of the sea, and a general change in the 

 oceanic and atmospheric circulation with a consequent readjustment of climate. It gen- 



