The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 160

Dryas cool period was of sufficient strength for both Swiss and New Zealand alpine
snowlines to drop to about 300 m below the lowest levels reached in the subsequent
Holocene. In short, there was linkage of effects to the southern hemisphere from
the northern, but was there hemispheric linkage regarding the cause of the Younger
Dryas?
What did trigger the Younger Dryas? It has been suggested that the early melting
of the North American Laurentide ice sheet added fresh water to the Gulf Stream in
the Gulf of Mexico as the presence of the remaining ice sheet to the north prevented
the meltwater discharging into the North Atlantic. The latter would, as we shall see,
most likely disrupt the Broecker thermohaline circulation. Conversely, the addition
of fresh water further south in the Gulf of Mexico could have enhanced the Broecker
circulation and so helped the transportation of heat from the tropics via the Gulf
Stream. This could perhaps have been a significant additional factor in pulling the
planet so rapidly out of the last glacial. The consensus today is that the thermohaline
circulation was disrupted, so causing the cooling of the Younger Dryas. Although
there is still some debate as to exactly where the fresh water came from that did the
disrupting, geological evidence clearly shows that there was flooding south of the
Laurentide ice sheet. Indeed, there is some computer-model evidence to suggest that
about half of the northern hemisphere meltwater came from the Laurentide sheet and
that here the largest pulse (and even if the models are wrong, certainly a significant
one) came, not via the Mississippi or Hudson routes, but via the Fram Straits off
the east of northern Greenland (Tarasov and Peltier, 2005). (We will return to this
shortly.)
But are changes in the North Atlantic circulation the only possible causes for the
end of the short Younger Dryas cool period? Could a marine methane release, such
as happened in the depth of the last glacial, be a cause? The answer is probably not.
Terrestrial and marine biological sources of methane both have the same amount of
hydrogen (four atoms per methane molecule) but some of these hydrogen atoms are
in the form of the deuterium isotope. The amount of deuterium from marine and
terrestrial sources of methane differs markedly: marine methane has more deuterium.
Analysis of the 2 H/ 1 H ratio (deuterium to normal hydrogen) of air trapped in the
Greenland ice sheet suggests that the marine methane clathrates were stable during
these particular warming periods and did not contribute to this episode of climate
change (Sowers, 2006). (This may perhaps be because methane releases from clath-
rates earlier in the glacial took place when the sea level - hence stabilising pressure
on the clathrates - was lower. At the time of the Younger Dryas sea levels were
already rising and so even though the planet was warming clathrate stability would
have benefited from the extra pressure of water.)
If, as it seems, the Younger Dryas was caused by some North Atlantic disruption
of the thermohaline circulation then the part of the Earth most significantly affected
would be north-west Europe and the British Isles, which would no longer be warmed
by the Gulf Stream. Here we do find that in England's Lake District the glaciers
returned (or started growing again) so quickly that living trees were caught in their
advance.
Conversely, the Antarctic Cold Reversal, due to it starting earlier, cannot be entirely
explained by any disruption in the North Atlantic of the thermohaline circulation