Geoscience Reference
In-Depth Information
variations of climate of which we have geological evidence. That changes of the
required amount must certainly have occurred at various epochs of past time is also
certain. On the other hand whilst Lyell presented a very strong argument against
these changes having been the sole cause of those variations of climate of which
there is such clear evidence, he further stated that they may have always been an
important and perhaps an essential collateral agent in their production. His two
'ideal' maps illustrating conditions of the earth which, by geographical changes
alone, would bring about a perpetual summer or an almost universal winter provide
models for later reconstructions of past climates (Wallace
1895
).
Harmer continued that among the various geographical hypotheses which had
been suggested concerning the probable effect of the closing of the gap between
Greenland and Europe upon the North Atlantic Drift that of Alfred R. Wallace
(1823-1913) should also be especially mentioned. Although Harmer had referred
to this idea in his 1901 paper he believed it would be valuable to re-consider this
concept in greater detail, especially from a meteorological standpoint.
Wallace was the founder of biogeography and his enduring legacy in this
subject is the 'Wallace Line', a faunal barrier separating the species of Asia from
those of Australia. This invisible boundary catalyzed Wallace's theory of evolu-
tion and prompted Charles Darwin to articulate his own theory.
Wallace had stated that despite certain criticisms made about the views of Lyell
whose fundamental theory of climatic change was based on changes in the dis-
tribution of land and sea, especially in polar and equatorial regions, he (Wallace)
believed that Lyell was substantially correct and that the global maps included in
his Principles of Geology showed 'a condition of the earth which, by geographical
changes alone, would bring about a perpetual summer or an almost universal
winter' (Wallace 1880).
As Harmer explained in his 1925 paper, the North Atlantic region is an area of
paramount interest and importance to glaciologists since the more striking phe-
nomena of the Ice Age had been grouped around it:
The physiographical conditions of the North Atlantic basin are unique. There only in the
Northern Hemisphere does uninterrupted communication exist between the equatorial and
polar oceans. Through the more easterly portions of the great channel, 600 miles wide,
which separates Greenland from the Eurasian continent, warm surface currents from the
tropics carry their ameliorating influence to the north of the Arctic Circle, while to the west
of Iceland cold water flows southward through the Denmark Strait, and from the Davis
Strait by means of the Labrador Current towards North America. But this is not all; the
existence of this channel offers an unobstructed passage northeastwards for the Atlantic
cyclones, many of which find their way, especially in winter, by this route to the North Cape
and beyond, the result being that the British Isles and the Norwegian coast are at that season
under the prevalent influence of warm southwesterly winds, while Labrador and New
England are flooded by aerial currents from the ice sheet of Greenland, and the still colder
regions of the Arctic Archipelago, over which the average winter temperature ranges from
-20 to -30 F. The aerial are possibly of more direct importance than the ocean currents,
as they are certainly more rapid in their action. Water from the Labrador Current is carried
eastward by the westerly winds. Northward movement of water (the Irminger Current)
takes place west of Iceland and west of Greenland. The Norwegian current of the Gulf
Stream is by far the largest and melts an enormous amount of ice (Harmer
1925
).
