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Perfect Droughts of Southern California (USA)
Volume 11, Number 50: 10 December 2008

In introducing their study of perfect drought in Southern California (USA), MacDonald et al. (2008) define the term as "a prolonged drought that affects southern California, the Sacramento River basin and the upper Colorado River basin simultaneously." They note that the instrumental record indicates the occurrence of such droughts throughout the past century, but that they "generally persist for less than five years." That they have occurred at all, however, suggests the possibility of even longer perfect droughts, which could well prove catastrophic for the region. Hence, the three researchers explore the likelihood of such droughts occurring in the future, based on dendrochronological reconstructions of the winter Palmer Drought Severity Index (PDSI) in southern California over the past thousand years, plus the concomitant annual discharges of the Sacramento and Colorado Rivers, under the logical assumption that what has occurred before may well occur again.

So what did they find?

MacDonald et al. report finding that "prolonged perfect droughts (~30-60 years), which produced arid conditions in all three regions simultaneously, developed in the mid-11th century and the mid-12th century during the period of the so-called 'Medieval Climate Anomaly'," which is also widely known as the Medieval Warm Period, leading them to conclude that "prolonged perfect droughts due to natural or anthropogenic changes in radiative forcing, are a clear possibility for the near future." And so they are! Therefore, whether one believes or disbelieves in the global warming theories of Al Gore and James Hansen, the possibility of a long and sustained perfect drought in Southern California is very real; and measures should be taken to prepare for that possibility.

Another conclusion that could be drawn from MacDonald et al.'s findings is that the current warmth of the world is not yet as great as it was during the peak heat of the Medieval Warm Period, else we might have already experienced, or been in the process of experiencing, a multi-decadal perfect drought. That such has not occurred is very encouraging; but it must be remembered that even if the theory of CO2-induced global warming is incorrect or vastly overstated (as we believe it to be), further natural warming could well push the planet's climate over the "tipping point" that initiates such a drought. The fact that the earth has experienced no net warming over the past decade or so is thus a good sign in this regard; but there is no guarantee that the globe may not begin to warm again ... at any time and for whatever reason. Clearly, therefore, planning for a significant perfect drought to possibly occur would appear to be warranted.

Sherwood, Keith and Craig Idso

Reference
MacDonald, G.M., Kremenetski, K.V. and Hidalgo, H.G. 2008. Southern California and the perfect drought: Simultaneous prolonged drought in Southern California and the Sacramento and Colorado River systems. Quaternary International 188: 11-23.