332 



PAST CLIMATES AND CLIMAXES. 



nental and perhaps of oceanic climates, and in them may be realized the 

 opportunity of predicting the character of a season months or a year or two 

 beforehand. He has compared the curve of the monthly departures from 

 mean temperature at Arequipa with the curve of the variations of the solar 

 constant from 1902 to 1907, and reaches the conclusion that the pleions of the 

 tropics are caused by changes in the solar constant. Temperate regions with 

 a more complex climate show a similar correspondence, but with irregularities 

 and delay. 



The periodicity of this pleion cycle is well shown in figure 29. Its relation 

 to plant growth is indicated by Arctowski's charts of the com crop of the 

 United States for 1901, 1906, 1908, and 1909. It seems probable that a similar 

 effect must be shown by native vegetation, and perhaps recorded in the growth 

 of perennial plants, but investigation has not yet been directed to these points. 



1900 1901 1902 1903 1804 



1906 1907 1908 1909 1910 



__''Mauritius (Indian Ocean) 



Tananarive (Madagascar) 

 ^V, BuIawayo(S. Africa) 



Fio. 29. — Monthly departures of temperatures in south equatorial 

 regions, showing agreement. After Arctowski. 



The 11-year sun-spot cycle. — Of all climatic cycles, this is the one most 

 studied, and hence best established. A large nimiber of investigators have 

 placed its existence and its relation to climate and vegetation beyond doubt, 

 though Koppen, Newcomb, Hann, and others have questioned its efficiency 

 in terrestrial climates. In spite of the conclusions of the investigators just 

 mentioned, their own figures place the existence of a harmony between earth 

 temperatures and sun-spots beyond doubt. Moreover, in spite of some con- 

 tradictory results, Meldrum, Lockyer, and Pettersson seem to have established 

 a similar harmony for rainfall ; Meldrum, Poey, and Wolf for tropical cyclones; 

 and Bigelow, Kullmer, and Huntington for cyclonic areas. Even more signifi- 

 cant is the evidence drawn from the growth of trees by Douglass and Hunting- 

 ton, since the annual ring is an integration of climatic effects. Their results 

 will be discussed later, but it should be emphasized here that they establish a 

 basic and probably imiversal relation between sun-spot cycles and the growth 

 of trees beyond question. Indeed, there is no other method which promises 

 such far-reaching quantitative results as to the climates of historic, and 

 perhaps even of geologic, times. 



