NO. 6 PAST CLIMATE OF NORTH POLAR REGION HERKY 23 



Nor must it 1)e lost sight of that at those times in the past when 

 certain groups were varied and abundant, as were the seed ferns in 

 the Paleozoic or the cycads in the Mesozoic, they were quite likely 

 to have shown the features of dominant organisms, both plant and 

 animal, and to have occupied more environmental niches than the 

 depleted survivors of the cycad phylum do at the present time. 



In Newfoundland and western Labrador the larch {Larix anicri- 

 cana), the balsam poplar {Populus balsamifera) , the paper birch 

 (Befula papynfera), and the balsam {Abies hahamea) fail to reach 

 the Straits of Belle Isle (52°) whereas they all extend far above 

 Latitude 60° in Alaska and the first crosses the Arctic Circle. Podo- 

 carpus just fails to reach the Tropic of Cancer in Cuba. A Chilean 

 species reaches 42° South Latitude in Chile. The northern limit of 

 forests crosses the Arctic Circle in Alaska and reaches 70° North 

 Latitude in Norway, the latter 20° north of the tree line on the 

 Atlantic coast of North America. 



EXPLANATION OF PAST ARCTIC CLIMATES 



It is perhaps fatuous to point out that climate, either present or 

 past, depends upon a variety of factors, both cosmic and terrestrial. 

 Of the former the only one that is of practical importance is solar— 

 that is, radiant energy from the sun, since it is inconceivable that 

 other heavenly bodies or the introduction of kinetic energy by meteor- 

 ites exert any appreciable effect. 



The amount of solar energy reaching the earth depends upon the 

 sun's activity, which is variable ; on the distance of the earth from the 

 sun, which is also variable ; and more practically in so far as terrestrial 

 climates are concerned, on the condition of the earth's atmosphere, 

 especially with respect to the amount of ozone, water vapor, carbon 

 dioxide, and dust present, all of which again are variable. The lati- 

 tude, determining the angle of incidence of the sun's rays, is an obvi- 

 ous factor, as is also the geographic pattern and the topography, in- 

 cluding altitude under the latter. The geography determines whether 

 the sun's energy falls on the land or the water, it determines the 

 temperature gradient between the equator and the poles and the con- 

 sequent force of the planetary winds and ocean currents, and in less 

 obvious ways is of the greatest significance, as the following illus- 

 tration will make clear. 



The North and South Equatorial currents in the Atlantic are so 

 situated that the South Equatorial, the stronger and the larger of the 

 two. is divided by Cape San Roque into a larger, northern or Guiana 

 current ; and a smaller, southern or Brazil current. Some authors, 



