Geoscience Reference
In-Depth Information
increased temperature gradient. At present the latter is, in the Northern Hemi-
sphere, much steeper during the winter than during the summer months. For
example, in January the difference between the average temperature of the North
Pole and the Equator is 115 F(-35 F in the first case and 80 F in the second); in
July it is only 55 F (35 F and 90 F, respectively).
It seems that, like today, storm tracks in the Pleistocene epoch may have been
determined by the prevalent position of the anticyclones of that period and especially
by those high pressure systems which may have prevailed over the region occupied
by great ice sheets. Moreover, it seems possible, that the difference in the temper-
ature gradients of the Pleistocene epoch and the present day may not have been as
great as that now existing between those of January and July, and that the increased
steepness may have been local rather than general. Over the European ice sheet, for
example, the gradient may not have been greater than that which now prevails in
winter over Greenland and the Arctic Ocean. Harmer continued:
Dealing, however, with the maximum glaciation of Europe it would appear that it was over
the region between the southern edge of the European ice and the Mediterranean that the
crowding together of the isotherms might have been greatest, and it is there that the
phenomenal erosion of the Pliocene and post-Pliocene strata has taken place. The sharp
manner in which the glaciers descending the southern valleys of the Alps were cut off
when reaching the great plain of Piedmont shows, I think, that the climate of Northern
Italy at this epoch was not severe. There would then have been a strongly marked thermal
gradient between Spitzbergen [Svalbard] and the Mediterranean, similar to that which now
exists in North America, but as before pointed out, its steepest part would lie to the south
of the European ice-sheet.
Under such circumstances, storm-tracks would have probably crossed the Atlantic in
winter, moving eastwards as they do now, but their course northwards being blocked, their
greatest fury would be expended over Central and Southern Europe, resulting in an
increased accumulation of snow upon the mountain regions of the Pyrenees and the Alps,
and torrential rains over the lower ground. The influence of the ice-sheet, close at hand,
would cause snow to fall upon the mountain regions during the summer months at ele-
vations where precipitation now takes place in the form of rain.
In a review of the characteristics of the oceanic and atmospheric circulations in the
Northern Hemisphere, Harmer stated that the chief difference between the North
Atlantic and North Pacific Oceans is that the former is open to the north, while the
latter is practically closed to the north, since the Behring Strait is too narrow and
shallow to allow the passage of an appreciable current. Hence in the North Atlantic
the North Atlantic Drift as a broad extension of the Gulf Stream flows north-
eastward into the Arctic Ocean, while in the Pacific Ocean the Kuroshio or Japanese
Current is forced to turn south-eastward along the coast of North America. This
dissimilarity causes differences in the pressure distribution together with differ-
ences in both oceanic and atmospheric circulations which combine to give Western
Europe a more genial climate than the west of North America.
From this comparison Harmer concluded that the closing of the channel between
Greenland and Europe would bring about changes in the oceanic and atmospheric
circulations which would suffice to cause a glacial epoch in Europe. The diversion
of storm tracks and consequent alteration in the direction of the prevailing winds are
