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A serious weakness of the burning hypothesis stems from the sedimentary charcoal
records obtained from 224 sites in and around Australia extending back to 70 ka,
which show peak burning during wetter phases in both Australia and Indonesia and
no increase in burning following the first arrival of humans on the continent at around
50 ka (Mooney et al., 2011 ). These authors rejected earlier arguments that humanly
induced changes in fire regime and vegetation cover could have triggered regional
climate change (Miller et al., 2005 ). On the contrary, they argue for a strong climatic
control over biomass burning, with maximum burning during warm, wet intervals with
the greatest plant cover and minimum burning during cold, dry intervals with least
plant cover. In their statistical analysis of peaks and troughs in charcoal abundance,
they discerned widespread millennial-scale fluctuations which have more in common
with the Dansgaard-Oeschger climatic events recorded in Greenland ice cores (see
Chapter 3 ) than they do with the record of climatic fluctuations preserved in the
EPICA ice core record from Antarctica. However, they noted that the radiocarbon
dating of many charcoal-bearing sites is not of adequate precision to demonstrate
such a correlation with any real confidence. They also found significant geographical
variation in the pattern of biomass burning in southern Australia during the Holocene.
A fourth working hypothesis may need to be considered and thoroughly tested
(Williams et al., 1998 ; Wroe and Field, 2006 ). Because the extinct species of mega-
fauna (such as Procoptodon , Sthenurus and Diprotodon ) were primarily browsers
rather than grazers, it is equally possible that a progressive and long-term increase in
savanna grassland at the expense of forest and woodland may have contributed to a
long-term decline in the numbers of browsing species, increasing their vulnerability to
even minor impacts on their habitat, whether they were caused by humans or climate
or both. There is growing evidence that the final demise of the megafauna occurred
during a very dry interval. Van der Kaars et al. ( 2010 ) examined the pollen record
from a deep-sea core off the coast of south-west Sumatra and found that the vegetation
was most open during MIS 3, between around 52 and 43 ka, identifying this period
as the driest of the last glacial, with the onset of cooler conditions starting at around
52 ka. Cohen et al. ( 2010b ) showed that Lake Eyre was connected to Lake Frome for
the last time at 50-47 ka, with a major shift to aridity thereafter, coincident with the
demise of the megafauna and the arrival of humans.
It may be time to reconsider the impact of changes in climate. The evidence from
desert lakes may help resolve whether the demise of the megafauna in Australia was
a result of human impact (direct or indirect) or of climatic change. We know that
the arrival of humans in Australia about 45 ka ago seems to coincide with a wave of
faunal extinctions, so it might seem that humans were the cause of these extinctions.
However, until recently, our knowledge of the climate at this time was sketchy. This
lacuna is now being remedied. Cohen et al. ( 2010b ) have shown that aridity set in
soon after 45 ka in central Australia. Before then, the climate across the continent was
much wetter and lakes were greatly expanded. Lake Frome immediately east of the
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