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mild winters and cool summers, the latter favoring survival of snow/ice through
the summer. Changes in external radiative forcing then bring about additional
responses and feedbacks that foster ice sheet growth and decay, notably in the car-
bon cycle. Orbital forcing can hence be viewed as the “pacemaker” of the Ice Ages
(Hayes et al., 1976 ). Despite the strong explanatory power of the Milankovitch
theory, a number of unresolved issues nevertheless remain, both with respect to the
relative roles of eccentricity, obliquity and precession in explaining the onset and
termination of glaciations and attendant climate feedback processes (e.g., Wunsch,
2004 ).
As changes in solar radiation alter the latitudinal temperature gradient, they can
modify the strength and characteristics of the atmospheric circulation. This will
alter patterns of moisture transport and precipitation. A trigger for glaciation may
have been an increase in the meridional temperature gradient leading to enhanced
poleward moisture transport (Kukla et al., 2002b ). This idea was first suggested
by J. Tyndall ( 1872 ). As cool Northern Hemisphere summers favor the survival of
snow/ice cover through the melt season, this can invoke ice-albedo feedbacks and
other processes to promote more ice growth, also affecting the circulation. Barry
( 1966 ) calculated atmospheric moisture fluxes into northeastern Canada and exam-
ine the synoptic patterns that might favor ice buildup.
Along the lines of the ice-albedo feedbacks, J.D. Ives (Ives, 1957 , 1962 ; Ives,
Andrews, and Barry, 1975 ) proposed the idea of “instantaneous glacierization,” by
which a small cooling leads to persistence of a snow cover in critical areas at the end
of summer. Ives focused on the Labrador-Ungava plateau and Baffin Island, where
modest cooling would bring large areas of the plateau above the snowline, so that
glaciation could proceed quickly through ice-albedo feedbacks. Given its summer
warmth and low precipitation, the buildup of ice over Keewatin (Canada) poses spe-
cial difficulty (Brinkman and Barry, 1972 ; Bromwich, Toracinta, and Wang, 2002 ).
A different idea, proposed by R. Flint ( 1943 , is known as “highland origin and
windward growth”. Flint argued that the Laurentide Ice Sheet was initiated in the
coastal mountains of Labrador-Ungava and Baffin Island. Glaciers formed in these
areas were nourished by precipitation on their windward flanks and advanced into
western lowlands to ultimately attain ice sheet proportions. Lowering of the snow-
line initiated the growth of small cirque glaciers in the mountains of Scandinavia.
These glaciers expanded, coalesced, and advanced eastward onto Russia. Growth
on the seaward flank was checked by calving into the ocean. G. Denton and T.
Hughes ( 1981 ) applied a marine ice transgression hypothesis to explain the devel-
opment of ice sheets with large marine-based components (e.g., the Barents Ice
Sheet). In this model, glacierization was triggered by an extension of permanent
sea ice into interisland channels and embayments. This resulted in the formation of
extensive fast ice (see Chapter 7 ). Albedo increased, dropping the snow line to sea
level. The ice thickened and eventually ground itself to the ocean bottom, allow-
ing development of a marine ice dome. As discussed by R. Souchez ( 1997 ), these
different hypotheses are not mutually exclusive and have been applied with some
success to understanding the geological traces of the last Pleistocene ice sheets. The
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