252 



THE CLIMATIC FACTOR AS ILLISTRATED IN ARID AMERICA. 



was omitted from the text until the present time because of the doubt attaching to the 

 whole subject. The curve, however, was allowed to remain. Wlien the smoothed vol- 

 canic curve is compared with the smoothed curve of tree growth, 7' in figure 76, a certain 

 amount of resemblance is seen, but this breaks down at the right-hand end of the curves, 

 and is by no means so pronounced as the similarity between the curves of tree growth 



and sun-spots. Hence it would seem that 

 this method, as well as that of Abbott, 

 Fowle, and Humphreys, leads to the con- 

 clusion that although volcanic dust may 

 at certain times have an important influ- 

 ence upon terrestrial temperature, and thus 

 upon other climatic elements, the effect of 

 solar radiation is much more important. 

 The second method of testing the control of 

 climate by volcanic dust has been used by 

 Professor Schuchert, in Part II of this 

 volume. It is so purely geological that I 

 shall leave it for the next chapter. Mean- 

 while, so far as present conditions are con- 

 cerned, we seem to be led to the conclusion 

 that although the solar hypothesis can not 

 be regarded as proved, and although other 

 factors, such as volcanic dust, have played 

 an important part, changes of the sun seem, 

 on the whole, to explain the known facts 

 better than any other hypothesis yet sug- 

 gested. Present changes in the intensity 

 of the sun's radiation not only seem to be 

 of sufficient amplitude to produce distinct chmatic effects, but the time of their occur- 

 rence seems to be in harmony with the observed variations of climate. The reason this 

 has not hitherto been reaUzed seems to be partly that due allowance has not been made 

 for the fact that the effects of the sun's heat are concentrated on certain portions of the 

 earth's surface and can not reach other places and influence the other meteorological 

 elements without the lapse of an appreciable and variable amount of time. An equally 

 important or even stronger reason seems to be that the function of volcanic dust in mod- 

 ifying terrestrial temperature has only recently been discovered. 



Turning now from the minor cUmatic fluctuations of the present time to the fluctuations 

 of all sizes from glacial epochs downward, let us sum up our conclusions: 



It appears, in the first place, that of the well-established hypotheses of cUmatic changes 

 only the solar and volcanic hyjwtheses invoke causes capable of varying and actually known 

 to vary with sufficient rapidity to cause changes of climate such as the trees of California 

 appear to give evidence of during the past three thousand years. 



In the second place, numerous authorities, including the majority of meteorologists, 

 beheve in the existence of a climatic cycle related to the sun-spot cycle, and the trees of 

 Germany and CaUfornia, as well as the prices of wheat in England, add their quota of 

 evidence to this same effect. 



Thirdly, the sun's radiation is universally acknowledged as the controlling factor of 

 terrestrial climate. It has been proved to vary from one extreme of the sun-spot cycle 

 to another, but the amount of variation is held by so high an authority as Newcomb to be 

 too slight to cause appreciable meteorological phenomena. Nevertheless, a comparison of 

 his results with the conclusions of students of the glacial period suggests that the solar vari- 



KrakaioaTgraweraBogoslof 

 BandS'^an Awoe 



Pel^ Colima 

 Santa Maria 



Fig. 85. — The Relation of Volcanoes, Sun-spots, and 

 Terrestrial Temperature, after Humphreys. 



