CLIMATIC CYCLES. 247 



CLIMATIC CYCLES. 



Nature. The general nature of climatic cycles as well as their universal 

 occurrence and fundamental importance is summed up in the following state- 

 ment (Plant Succession, 329): 



"It is here assumed that all climatic changes recur in cycles of the most 

 various intensity and duration. In fact, this seems to be established for 

 historic times by Huntington and for geologic times by the studies of glacial 

 periods which have made possible the table compiled by Schuchert. The 

 cyclic nature of climatic changes has been strongly insisted upon by Hunting- 

 ton: 'The considerations which have just been set forth have led to a third 

 hypothesis, that of pulsatory climatic changes. According to this, the earth's 

 climate is not stable, nor does it change uniformly in one direction. It appears 

 to fluctuate back aftd forth not only in the Uttle waves that we see from year 

 to year and decade to decade, but also in much larger ones, which take 

 hundreds of years or even thousands. These in turn seem to merge into and 

 be imposed upon the greater waves which form glacial stages, glacial epochs, 

 and glacial periods.' 



" Climatic changes, then, are assumed to be always related in cycles. No 

 change stands out as a separate event; it is correlated with a similar event 

 which has preceded it, and one that has followed or will follow it, from which 

 it is separated by a dissimilar interval. Climate may thus be hkened to a 

 flowing stream which rises and falls in response to certain causes. It is not 

 a series of detached events, but an organic whole in which each part bears 

 some relation to the other parts. Considering climate as a continuous pro- 

 cess, it follows that we must recognize changes or variations of climate only 

 as phases or points of a particular climatic cycle, which lose their meaning and 

 value unless they are considered in connection with the cycle itself. It is in 

 this sense that changes and variations are spoken of in the following pages, 

 where the cycle is regarded as the climatic unit. " 



Ignoring the familiar cycle of the year, there is more or less conclusive evi- 

 dence of cycles of 2.5, 11, 22, 35, 50, 100, 400, and 1,000 years, approximately. 

 In addition, there are the great geological cycles of unknown duration, which 

 are discussed at some length in "Plant Succession" (337). 



The 11-year cycle. The best^known and most significant of climatic cy- 

 cles for the present day is the 11-year cycle and its multiples. So far as its 

 relation to tree growth, and hence to vegetation, is concerned, our knowledge 

 of this cycle is due chiefly to Douglass (1909, 1914, 1919), though Huntington 

 (1914) and Kapteyn (1914) have had a share in establishing the certainty of 

 this relation. The effect of cycles upon succession, and consequently upon 

 indicator communities and crop production, has been pointed out by Clements 

 (1916: 342; 1917: 304; 1918: 295). The relation of the 11-year cycle to changes 

 in native vegetation and to variations in plant production has received con- 

 stant study since 1914. It has proved so universal and fundamental as to 

 warrant its being made the basic feature of production systems in the arid 

 West (fig. 13). 



The 11-year cycle is known also as the sun-spot climatic cycle, owing to the 

 striking correspondence with the sun-spot period. The correlation of the 

 dry and wet phases of cUmate and of the variations of tree growth with the 

 sun-spot cycle is often so exact as to warrant the assumption of a causal 

 relation between the two. Such a relation has not yet been established, how- 



