The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 161

that led to the Younger Dryas. The Antarctic Cold Reversal could be explained by
Antarctic melt as the Earth left the LGM behind. This could disrupt part of the
Broecker thermohaline circulation in the southern hemisphere. If this happened it
would still weaken the overall global thermohaline circulation. Because the northern
hemisphere had large land-based ice sheets outside of the Arctic Circle (unlike
the Antarctic Circle) these would have taken longer to react to coming out of the
LGM. If the global thermohaline circulation had already been part-disrupted by the
Antarctic Cold Reversal then it would be more susceptible to subsequent northern
hemisphere disruption. Again the lesson to learn is that changes in ocean circulation
(and atmospheric circulation for that matter) have a profound effect on the climate.
As discussed in the previous section, while the pacemaker of the glacial-interglacial
cycles is the change in Milankovitch insolation, the vagaries of warming and cooling
within glacials were driven either by changes in greenhouse gases and or changes
in ocean or atmospheric circulation and in particular the Broecker thermohaline
circulation. Indeed, Milankovitch insolation in July at 60 ◦ N had already peaked
following the LGM by the time of the Younger Dryas and remained at a high value
throughout it, and for a time into the early Holocene. We have already noted how
greenhouse gases parallel each other in both northern and southern hemispheres,
which means that the likely candidate primarily causing the northern hemisphere's
Younger Dryas return to glacial conditions was connected with atmospheric and/or
ocean circulation and not greenhouse climate forcing. Having said this, there was
a noticeable dip in both carbon dioxide and methane concentrations at the time of
the Younger Dryas, but it is likely that this was the result of a biotic response to the
cooling rather than a cause of it.
Yet if it was the melting of the northern hemisphere ice caps that caused the Younger
Dryas temporary cooling, why did this happen suddenly and not gradually as the ice
caps melted? Furthermore, why was there not cooling of a similar degree in the
southern hemisphere? Here, the geography of the northern and southern hemispheres
is markedly different. A key factor is that in the northern hemisphere the landmasses
of North America, Europe and Siberia enabled large ice sheets to form during the
LGM. Conversely the southern hemisphere's ice sheet was centred on Antarctica and
the Antarctic continental shelf and not on land at lower latitudes as in the north.
Unlike their marine counterparts, terrestrial ice sheets push (terminal) moraine in
front of them and as the ice retreats these can act as dams. Local terrestrial geography
can also serve to constrain meltwater. So it is likely that as the Earth warmed, leaving
behind it the depth of the last glacial, the melting Laurentide ice sheet created a
volume of inland fresh water. It is this water that ultimately discharged into the salty
northern Atlantic, probably in a pulse, thus switching off the Broecker thermohaline
circulation.
This hypothesis is worth further consideration because, as we shall see, there was at
least one other pulse of meltwater likely to have disrupted the Broecker circulation. So
is there evidence for such a reservoir of fresh water? Indeed there is. The geological
evidence for large lakes off of the Laurentide ice sheet at the end of the last glacial is
clear and there were also numerous smaller bodies of water. Two lakes in particular
were bigger than Canada's present Great Lakes and so they represented a considerable
volume. Indeed, it is likely that the majority of freshwater fish in Canada and the