404 G. R. Wieland — Polar Climate in Time. 



Orbital Eccentricity. — At times of high eccentricity of the 

 earth's orbit, secondary cUmatic changes are set up which have 

 been the subject of profound discussion ever since the publi- 

 cation of James Croll's " Climate and Time," in which he 

 contends strenuously that periods of high eccentricity are 

 causes of climatic change sufficientl}^ great to produce a glacial 

 epoch in the hemisphere passing through aphelion winter. 



Let us recall that according to Croll's theory the causes of 

 glacial periods are physical, not astronomical. That is to say, 

 solar radiation remaining approximately constant, orbital eccen- 

 tricity does not in itself produce any diminution in the sum 

 total of solar heat received each year by the entire surface of 

 the globe, or at any latitude. This follows from three causes ; 

 1st, by Kepler's second law the radius vector sweeps over 

 equal areas in equal periods of time ; 2d, the amount of 

 heat received is inversely as the square of the distance from 

 the sun ; and 3d, the orbit of the earth is constant in the 

 long run as regards its onajor axis and the length of the year, 

 the increase of heat with eccentricity being, according to Sir 

 John F. Herschel, less than '005 of the total solar heat received 

 now, an amount too slight to markedly influence climate. 



When eccentricity is high, therefore, summer is shorter 

 and hotter and winter longer and colder, or vice versa^ depend- 

 ing on the change in perihelion and aphelion due to the pre- 

 cession of the equinoxes, but the actual yearly amount of solar 

 heat received will be fairly constant for any given latitude. 



The argument of Croll is that the true causes of glaciation 

 following eccentricital variations result from the entailed dis- 

 turbances in the ocean currents, the winds, the rain and snow- 

 fall, atmospheric humidity, and (iirst suggested by Newcomb) 

 increased radiation of heat into space, as temperature rises 

 during the short perihelion summer.^ 



In addition, A.R. Wallacef has pointed out that these climatic 

 factors have been profoundly affected in the polar regions by 

 changes in the land masses resulting in much modification in 

 the warm and cold currents, and, especially, altitude and the 

 winter storage of cold, which is not offset by any similar 

 storage of summer heat. I^ow to go into this question at great 

 length, nor can it be fully discussed short of this, is hardly 

 within the scope of the present thesis. And to announce a 

 mere opinion is of course not argument. But it does seem to 

 me that the contention of Croll ( pp. 57-66 of " Climate and 

 Time" ) that the low temperatures of the Antarctic regions are 

 due in considerable measure to the cumulative cooling effect of 



* On Some Points in Climatology. A Eejoinder to Mr. Croll. This Journal, 

 Yol. xxviii, No. 157, 1884. 

 f Island Life. 



