540 E. HUNTINGTOX SOLAR HYPOTHESIS OF CLIMATIC CHANGES 



conditions apparentlj^ prevailed in the snbarid or desert portions of the 

 United States and Asia. 



THE SHIFTIXG OF CLIMATIC ZOXES — PENCK'S HYPOTHESIS 



These considerations, and others described in the works already referred 

 to, suggest that climatic changes consist largely of a shifting of the earth's 

 climatic zones, sometimes poleward and sometimes eqnatorward. At 

 times of eqnatorward shifting, polar conditions apparently cause glacia- 

 tion in the northern portions of Xorth America and Europe. In sub- 

 tropical latitudes greater moisture prevails than formerly, for the storms 

 of the temperate zone are shoved eqnatorward of their former courses. 

 Still farther toward the equator the subtropical zone of aridity appears 

 to invade the zone of equatorial rains, while the equatorial zone itself 

 suffers contraction. 



The hypothesis of the shifting of climatic zones, as outlined in the pre- 

 ceding paragraph, needs modification; but before proceeding to that it 

 will be well to turn for a moment to the Glacial period and consider a 

 recent article by Penck.^^ In this article, which appeared almost simul- 

 taneously with "The Climatic Factor," he explains the conditions of cli- 

 mate during the Glacial period by almost exactly the same hypothesis 

 which I have offered in respect to historic changes of climate. Indeed, 

 he apparently means that his hypothesis shall apply not only to glacial 

 climates but to historic times, for near the beginning he speaks as follows : 



"There are . . . many indications that, some centuries after the begin- 

 ning of the Christian era, there was in central Asia a period of extreme dry- 

 ness, which exercised a considerable influence on the conditions of settlement. 

 and even, it would appear, upon migration." [This is the dry period which, 

 in the curves of figures 13 and 14. culminates about 650 A. D.] 



Penck sums up his conclusions as follows (page 288 ff.) : 



"Thus from the New as from the Old World we may draw the following 

 conclusion : On the equatorial side of the Great Desert regions are flat pans 

 occupied with slightly saline or occasionally fresh water, while on the polar 

 side occur strongly saline lakes. . . . On the equatorial side ... of the 

 desert belts we have to do with rising lakes, on the polar side with shrinking 

 pans. Both point to variations in climate, to increase of aridity on the polar 

 side, to increase of humidity on the equatorial side. . . . Very characteristic 

 are the phenomena in the Sahara. In the north there are living dunes consist- 

 ing of bare wind-blown sand ; the dead dunes, however, covered by sparse vege- 

 tation are confined to the south, and these dead dunes stretch beyond the arid 

 region far into the humid zone along the right bank of the Niger. The dunes 



35 A. Penck : The shifting of the climatic belts. Scottish Geographical Magazine, vol. 

 30, 1914, pp. 281-291. 



