So I'm going to use this blog entry to review the climate changes in Austrailia during the late Pleistocene. I realise I’ve use other blog entries to do this already but I’ve found a few more which have been recently published and I thought they would be good to re-enforce the climate hypothesis!
Cohen et al (2010) carried out a study on the modern Lake Mega- Frome, which is the coalescence, (the merged) Lakes Frome, Blanche, Callabonna and Gregory lakes in souther central Australia. The last time these lakes were connected was at 50-47ka, this final disconnection shows a shift in climate to a more arid and dry climate, which coincided with the arrival of the first humans and the demise of the megafaunal species. So this study shows that acidification could have started as long as 50ka ago, and continued until at least 20ka when the majority of the extinctions took place and the LGM occurred. The transitional shift in climate must have had a massive impact on the species already inhabiting Australia. This supports the climate hypothesis, although humans arrived at the same time they would have had a much smaller impact than the environment that changed vegetation composition, habitat patterns, fire regimes and overall environmental thresholds.
Another study I found is by Mooney et al (2011). His study compiled 223 sedimentary charcoal records from Australasia from the late Quarternary period. Fire regimes reflect the overall climatic conditions, with colder climates characterised by less burning biomass burning and vice versa. The records seem to show a clear correlation with the Greenland Interstadials (warm periods) and Greendland stadials (cold periods). Most importantly in these records, there is no change in correlation with the first arrival of humans in Australia 40-50ka ago. There was also an especially low biomass during the Last Glacial Maximum (LGM), with an expansion of drought-tolerant vegetation. During the LGM, there must have been a reduction in fuel availability and severely cold. Dry and arid conditions. This has been supported by a study by Hesse et al (2004) who found there to be stronger winds deduced from dune activity, also demonstrating more arid conditions. This study supports the idea that climate changed significantly during the LGM and the period leading up to it. It also show that fire regimes were controlled by climate and not humans, which I think also shows that the climate had the overriding force in the extinctions of the late Pleistocene, and not human populations.
The last study I found was by Murphy et al (2011). This study specifically focuses on the giant flightless bird, the Genyornis newtoni (don’t worry I’ve never heard of it either!), which disappeared 45-50ka ago. The study found that the preserved eggshells of the extinct emu from Lake Eyre showed an abrupt dietary shift from tropical grasses to temperate grasses and woody browse. This has been interpreted as indicating an ‘ecosystem collapse’. However, this study test the hypothesis that actually it was due to a gradual climatic transition. After re-analysing preserved egg shells from the past 140,000 years, it was found there was no evidence of a sudden climatic shift, but actually a gradual shift from 80-30 ka ago. So this study overrides the theory that a dramtic shift in climate was caused by landscape burning by humans, but that in fact, there was a gradual shift in climate to more arid and dry conditions. This study obviously supports the fact that there was a gradual aridification, changing the environment and its parameters gradually until individual survival thresholds of existing species were reached and were unable to survive any longer.
So I think these studies demonstrate there was a gradual but lethal shift in the climate during the late Pleistocene. The aridification got so severe that the species could no longer survive. The human ‘overkill’ hypothesis just isn’t enough to explain such a large scale extinction of so many species, it just isn’t realistic. Mainly because there simply weren’t enough humans to wipe out so many species, also because there was such a prolonged co-existence of humans and megafaunal species and there is little or no evidence for human impact on the now extinct species.