236 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



the temporary filling of the air with dust from volcanoes, and the cooling of parts of the 

 ocean by an unusual number of icebergs are a few of the many agencies which combine to 

 cause the weather of one year to differ from that of another. It is not only conceivable, 

 but highly probable, that these agencies account for a large share of the variations which 

 are visible within an ordinary human hfetime. Many meteorologists of the highest 

 standing regard them as a sufficient explanation of all changes since the end of the glacial 

 period. These meteorologists, however, assume a definite end of the glacial period, and 

 their theory was framed before the trees of California had disclosed such strong evidence of 

 post-glacial and, especially, of historic fluctuations. 



Unfortunately it is impossible to disprove or prove their ^'iew. At the present time we 

 are only beginning to determine whether the difTerence between the weather of one year and 

 another is due solely to meteorological causes, such as are mentioned above, or partly to 

 them and partly to solar or other unknown agencies. The most that we can do is to 

 consider the probabilities, and they are bound to seem different to different observers. 

 The earth's atmosphere is extremely sensitive and mobile. The slightest change in temper- 

 ature, pressure, or other conditions sets it in motion. Because of this mobility, however, 

 there is an equally prompt tendency to correct any departure from equilibiium as quickly 

 as it is formed. In summer the full development of the great areas of continental low- 

 pressure, and of the accompanying monsoons or other in-blowing winds, reaches its full 

 development only about a month after the sun reaches its most northern point. Therefore 

 it would seem as if any accidental departures of the earth's chmate due to purely meteoro- 

 logical causes must speedily reach their limit, and then disappear as the atmosphere 

 attempts to regain its equihbrium. So long as we knew of no climatic changes between 

 those of 35-year cycles and those of glacial epochs, the purely meteorological explanation of 

 present phenomena was commonly accepted as sufficient. The changes indicated by the 

 California trees, however, seem to put a strain on this explanation. They appear to demand 

 not only that something should have caused changes which seem to be of as great intensity 

 as those which now differentiate one year from another, but that the atmosphere, in spite 

 of its instability, should have been held for a century or two at one or the other of these 

 extremes. This may be possible, but in many ways it seems improbable. Some other 

 cause seems needed thus to change the condition of the atmosphere and then prevent 

 it from swinging back to its old state of equihbrium. 



Coming now to the sun as a possible cause of climatic variations both large and small, 

 the case is quite difTerent. The solar hypothesis is not confronted by the same difficulties 

 as any of the others. From the work of Abbott, Fowie, and others we know, as a matter 

 of observation, that the radiation of the sun varies to a degree which is easily measurable 

 with the pyrheliometer, but the variation is quite irregular in duration and still more so in 

 ampUtude, some maxima being several times as important as others. The difficulty with 

 the solar theory is that it is avowedly indefinite. The sun has indeed been proved to 

 be a variable star, but the observed variations are of only slight magnitude and duration. 

 We can, of course, assume that in the past it has varied on a larger scale than at present, 

 but no one has yet been able to point to more than the most shadowy indications that 

 this is the case. Moreover, meteorologists are not as yet whoUy agreed as to what would 

 be the effect of an increase in the intensity of the sun's radiation. Some think that it would 

 simply warm the earth and produce a mild climate, the change being especially marked 

 far toward the poles. Others think that its effect would be felt chiefly at the equator, and 

 that thereby the circulation of the atmosphere would be so accelerated and cloudiness in 

 non-equatorial regions would be so increased as to bring on glaciation. 



Without attempting to chscuss this matter, let us briefly see what ground there is for 

 thinking that present changes in the intensity of the sun's radiation are actually connected 

 with present variations of chmate. In the first place, it is ahnost universally agreed that 



