CYCLE PROBLEM AND LONG-RANGE FORECASTING 137 



annual value. It interferes, however, very little with the prediction of a 

 mean value between each two successive years. 



The Pacific Coast states are especially fortunate in having the longest 

 known dated tree records whose climatic significance has been demonstrated. 

 I refer to the giant sequoias. Sequoias on the high and steep slopes in the 

 upper part of the groves give curves that somewhat resemble rainfall in 

 central and southern California. The Hellmann cycle shows in the general 

 sequoia ring growth curve since 1760. We may look upon California as a 

 very favorable point for early prediction tests. 



Of course, these discussions of American climatic cycles are only part of 

 a world problem. What is happening in north Europe when certain cycle 

 conditions exist in America? What in South America, Africa, and Australia? 

 Some day the whole matter will be examined, as it deserves. Inroads into 

 the subject have been made here and there by various students — Abbot, 

 Alter, Clayton, Arctowski, C. E. P. Brooks, Sir Gilbert Walker, W. J. S. 

 Lockyer, C. F. Brooks and others. 



RELATION TO THE SUN 



It has been stated that climatic cycles resemble cycles found in the sun. 

 This is based directly on the frequency periodograms illustrated in several 

 parts of the last chapter showing that dominating terrestrial cycles have 

 lengths coinciding with lengths of cycles in the sun. Practically the same 

 group of cycle lengths has been found in fossil trees. Coincidence in time 

 of changes in sun and earth has been shown in one case at least (Hellmann 

 cycle in Arizona) but this has not been fully tested in other long records. 



The most difficult part of the physical line of cause and effect between 

 sun and trees is the distribution about the earth by atmosphere and ocean, 

 of the energy received from the sun. Since the earth's energy is derived con- 

 tinuously from the sun by some form of radiation, we are justified in holding 

 the hypothesis that the sun's part in the drama of climate is more than the 

 mere exemplification of certain cycles. The resemblance between their re- 

 spective cycles points directly to the sun's radiation as a cause of terrestrial 

 changes; and once this is admitted, it brings climatic study into close con- 

 tact with astronomy; the study of the sun and its physical characteristics 

 and mechanical operations become of great economic importance to the 

 human race. 



In view of the facts shown by the periodograms above referred to, and 

 from many other reasons for regarding seriously this influence of the sun's 

 energy on the earth, we can form a preliminary climatic hypothesis as an 

 immediate guide in developing the prediction problem along its most favor- 

 able lines. This hypothesis may be expressed in three terms: 



(1) Discontinuous periods in the sunspot cycle and in other solar activi- 

 ties produce through radiation climatic cycles on the earth of corresponding 

 length. These are disturbed in many places by the complexity of the dis- 

 tribution of solar effects about the earth. Conservation and topographic 



