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Australia's "Big Dry" or "Millennium Drought" in New Perspective

Paper Reviewed
Barr, C., Tibby, J., Gell, P., Tyler, J., Zawadzki, A. and Jacobsen, G.E. 2014. Climate variability in south-eastern Australia over the last 1500 years inferred from the high-resolution diatom records of two crater lakes. Quaternary Science Reviews 95: 115-131.

Between 1997 and 2009, southeastern Australia experienced the most persistent drought since the start of the 20th century. Variously called the "Big Dry" by Ummenhofer et al. (2009) or the "Millennium Drought" by Bond et al. (2008), this protracted dry spell was said by Gergis et al. (2011) to be one of the most severe droughts on record, due to the effects of reduced rainfall being "exacerbated by elevated temperatures," as noted by Nicholls (2004) and Murphy and Timbal (2008). And in light of this information, many people were persuaded to believe that the rainfall decline across this part of the Australian continent was caused by climate change that was brought about by the ongoing rise in the atmosphere's CO2 concentration. But was this really the case?

In a study that sheds some much needed light on the subject, Barr et al. (2014) studied two crater lakes in southeast Australia, where fluctuations in lake-water conductivity - which is a proxy for effective moisture - were reconstructed at sub-decadal resolution over the past 1500 years, based on a statistically-robust diatom-conductivity transfer function. And at both of these sites, the data they obtained implied the existence of a multi-decadal drought, commencing ca 650 AD, which discovery led them to conclude that "the Big Dry was, in the context of the last 1500 years, a relatively minor event."

Another thing we would hasten to add, in this regard, is the fact that the 650 AD multi-decadal drought occurred at a time when the atmosphere's CO2 concentration was something on the order of only 275 ppm, compared to today's much higher value of approximately 400 ppm, which further suggests that the air's CO2 content at the time of the recent "Millennium Drought" had essentially nothing to do with either its magnitude or its duration.

References
Bond, N.R., Lake, P.S. and Arthington, A.H. 2008. The impacts of drought on freshwater ecosystems: an Australian perspective. Hydrobiologia 600: 3-16.

Gergis, J., Gallant, A.J.E., Braganza, K., Karoly, D.J., Allen, K., Cullen, L., D'Arrigo, R., Goodwin, I., Grierson, P. and McGregor, S. 2011. On the long-term context of the 1997-2009 'Big Dry' in south-Eastern Australia: insights from a 206-year multi-proxy rainfall reconstruction. Climatic Change 111: 923-944.

Murphy, B.F. and Timbal, B. 2008. A review of recent climate variability and climate change in southeastern Australia. International Journal of Climatology 28: 859-879.

Nicholls, N. 2004. The changing nature of Australian droughts. Climatic Change 63: 323-336.

Ummenhofer, C., England, M., McIntosh, P., Meyers, G., Pook, M., Risbey, J., Gupta, A. and Taschetto, A. 2009. What causes southeast Australia's worst droughts. Geophysical Research Letters 36: 10.1029/2008GL036801.

Posted 11 November 2014