Desert glaciations - Climatic Change in Deserts - page 244

cold events, consistent with the 'bipolar seasaw' hypothesis (Blunier and Brook, 2001 ;
Steig, 2006 ). The authors concluded that both sets of changes (in Greenland and in
Antarctica) reflected a reduction in the overturning of cold surface waters flowing
south from the North Atlantic (EPICA Community Members, 2006 ).

13.12 Conclusion
For mountain glaciers and ice caps to form, two conditions are necessary. The temper-
ature needs to be sufficiently low for precipitation to occur in the form of snow, and
there needs to be sufficient precipitation for the winter snow to persist throughout the
following summer and for this process to continue until the snow at depth is converted
to ice. The elevation at which net ice accumulation within a mountain glacier and net
loss of ice through melting are in long-term balance is termed the equilibrium snow-
line, or the equilibrium line altitude (ELA). In the drier regions of the world, such as
the Tian Shan ranges in Asia, the ELA is highly sensitive to even small changes in
precipitation and will be at much higher elevations during dry climatic intervals than
during wetter phases.
There is growing evidence from glacial moraines, directly dated using cosmogenic
nuclide exposure dating methods, that in many arid and semi-arid areas, the Last
Glacial Maximum at about 21

2 ka was not the time of maximum glacier advance
but was preceded by more extensive glaciation earlier in the Late Pleistocene or even
before then, suggesting an increase in aridity during the Late Pleistocene and perhaps
also during successive glacial cycles. This inference remains a working hypothesis,
but is supported by the evidence from desert lakes outlined in Chapter 11 .
Because temperature is such a critical factor controlling glacier advance and retreat,
too narrow a focus on purely local climatic influences can be unhelpful. Evidence from
the central Andes in South America suggests that at least during the Holocene, the
climatic linkages responsible for the waxing and waning of mountain glaciers may
have been more closely linked to events in the North Atlantic than they were to more
southerly influences. In southern Patagonia, there is also persuasive evidence that the
rapid ice retreat there, which began at 19 ka, was associated with Southern Hemi-
sphere warming triggered by Northern Hemisphere forcing operating via the 'bipolar
seesaw' phenomenon. This phenomenon, discussed in Section 13.11 , simply means
that when temperatures in Greenland and the North Atlantic were cold, temperatures
in the Antarctic and Southern Ocean were relatively warm, and conversely. A further
example of this effect is the terminal Pleistocene ice advance in southern Patagonia
which reached its maximum extent by 14.2 ka, during the Antarctic cold reversal
(14.6-12.8 ka).

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