The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 164

around 8470 years ago (Rohling and Palike, 2005) from the North American glacial
lakes of Agassiz and Ojibway, until then held back by the ice, disrupted the Broecker
thermohaline circulation by freshening the cold, salty water in the North Atlantic.
Again, as with the Younger Dryas (discussed in the previous section), this prevented
the water there from sinking, one of the processes driving the global conveyor. It
subsequently cooled the temperature in central Greenland by 4-8 ◦ C and regionally in
the surrounding North Atlantic sea and land by 1.5-3 ◦ C for very roughly a century
around 8200 years ago, again in a similar (albeit lesser) climate response as with the
Younger Dryas. The graph in Figure 4.12b shows Greenland ice core conductivity,
an indicator of dust trapped in the ice, which in turn reflects precipitation in that
part of the northern hemisphere, and in turn very loosely reflects temperature (the
cooler the air the less moisture it can hold). Because of the length of this chain of
interpretation, some caution is needed: the usefulness of dust in ice cores is more
that it illustrates how fast the regional climate can flip (see section 4.6.2). What is
known is that this early Holocene climate blip does show up in a number of northern
hemisphere climate proxies around 8200 years ago. While not as great a return to more
cooler glacial times as was the Younger Dryas around 12 000 years ago, the 8200-
and 4200-year events do highlight the need for concern as to what might happen if
the Broecker thermohaline conveyor became disrupted, as some hypothesise it might
with Greenland melt due to current and near future global warming. Yet this northern
hemisphere event is comparatively trivial to the changes in the Earth system that took
place between its glacial and interglacial modes.
The warming since the LGM, although triggered by change driven by Milankovitch
orbital variation in the Sun's energy reaching the northern hemisphere, was amplified
by changes in the carbon cycle. Among these was considerable growth in peri-Arctic
peatland. Glen MacDonald, David Beilman and colleagues reported in 2006 their 14 C
radioactive isotope dating of these peatlands. This revealed that the development of
the current circumarctic peatlands began 16 500 years ago and expanded explosively
between 12 000 and 8000 years ago in concert with high summer insolation and
increasing temperatures. Their rapid development contributed to the sustained peak
in methane and modest decline of carbon dioxide during the early Holocene, and likely
contributed to methane and carbon dioxide fluctuations during earlier interglacial. So,
as the Earth left the last glacial and entered the current interglacial the peri-Arctic
peatlands grew. This would have sequestered (drawn down) carbon in the form of
carbon dioxide from the atmosphere. This sequestering of atmospheric carbon would
have lowered the global temperature or dampened the warming taking place, but
some of this carbon returned to the atmosphere as methane. Methane is a stronger
greenhouse gas than carbon dioxide (see Table 1.2) and this made up for the cooling
effect of the carbon dioxide drawdown.
Some 10 000 years ago Milankovitch forcing peaked (as determined by the 60 ◦ N
July insolation). However, inertia within the planet's climatic system, and other for-
cing factors combined, delayed the peak in the planet's climate until 6000 years ago:
the peak of the Holocene climatic maximum. Whereas the climate below latitudes of
about 40 ◦ was only a little warmer (mostly about 0.5 ◦ C or less) than today (the early
21st century), the climate at higher latitudes was roughly 1.5 ◦ C above the Holocene
average. Further, at extremely high latitudes over much of the North Pole and