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Of Droughts and Megadroughts in North America
Volume 13, Number 43: 27 October 2010

In an exceptionally perceptive paper published last year in the Journal of Quaternary Science, Cook et al. (2009) wrote that "IPCC Assessment Report 4 model projections suggest that the subtropical dry zones of the world will both dry and expand poleward in the future due to greenhouse warming," and that "the US southwest is particularly vulnerable in this regard and model projections indicate a progressive drying there out to the end of the 21st century." They then wrote that "the USA has been in a state of drought over much of the West for about 10 years now," and that "while severe, this turn of the century drought has not yet clearly exceeded the severity of two exceptional droughts in the 20th century," so that "while the coincidence between the turn of the century drought and projected drying in the Southwest is cause for concern, it is premature to claim that the model projections are correct."

We begin to understand this fact when we compare the "turn of the century drought" with the two "exceptional droughts" that preceded it by a few decades. Based on gridded instrumental Palmer Drought Severity indices for tree ring reconstruction that extend back to 1900, Cook et al. calculated that the turn-of-the-century drought had its greatest Drought Area Index value of 59% in the year 2002, while the Great Plains/Southwest drought covered 62% of the US in its peak year of 1954, and the Dust Bowl drought covered 77% of the US in 1934. In terms of drought duration, on the other hand, things are not quite as clear. Stahle et al. (2007) estimated that the first two droughts lasted for 12 and 14 years, respectively; Seager et al. (2005) estimated them to have lasted for 8 and 10 years; and Andreadis et al. (2005) estimated them to have lasted for 7 and 8 years, yielding means of 9 and 11 years for the two exceptional droughts, which durations are to be compared to 10 or so years for the turn-of-the-century drought, which again makes the latter drought not unprecedented compared to those that occurred earlier in the 20th century.

Real clarity, however, comes when the turn-of-the-century drought is compared to droughts of the prior millennium. Cook et al. write that "perhaps the most famous example is the 'Great Drouth' (sic) of AD 1276-1299 described by A.E. Douglass (1929, 1935)." Yet this 24-year drought was eclipsed by the 38-year drought that was found by Weakley (1965) to have occurred in Nebraska from AD 1276 to 1313, which Cook et al. say "may have been a more prolonged northerly extension of the 'Great Drouth'." But even these multi-decade droughts truly pale in comparison to the "two extraordinary droughts discovered by Stine (1994) in California that lasted more than two centuries before AD 1112 and more than 140 years before AD 1350." And each of these megadroughts, as Cook et al. describe them, occurred, in their words, "in the so-called Medieval Warm Period." And they add that "all of this happened prior to the strong greenhouse gas warming that began with the Industrial Revolution [authors' italics]."

In further ruminating about these facts in the "Conclusions and Recommendations" section of their paper, Cook et al. again state that the medieval megadroughts "occurred without any need for enhanced radiative forcing due to anthropogenic greenhouse gas forcing" -- because, of course, there was none at that time -- and, therefore, they say "there is no guarantee that the response of the climate system to greenhouse gas forcing will result in megadroughts of the kind experienced by North America in the past." And if the world's climate alarmists refuse to acknowledge this possibility and continue to claim that global warming will most assuredly trigger the occurrence of medieval-like megagroughts, they will also have to acknowledge that the Medieval Warm Period of a thousand years ago had to have been much warmer than the Current Warm Period has been to date. But this acknowledgement destroys yet another of their claims, i.e., that the earth is currently warmer than it has been for one (Mann et al., 1999) to two (Mann and Jones, 2003) millennia.

So when consulting the real world on the matter, climate alarmists find themselves positioned squarely between the proverbial rock and a hard place, with nowhere to run, no place to hide, because their contentions are simply untenable.

Sherwood, Keith and Craig Idso

Andreadis, K.M., Clark, E.A., Wood, A.W., Hamlet, A.F. and Lettenmaier, D.P. 2005. Twentieth-century drought in the conterminous United States. Journal of Hydrometeorology 6: 985-1001.

Cook, E.R., Seager, R., Heim Jr., R.R., Vose, R.S., Herweijer, C. and Woodhouse, C. 2009. Megadroughts in North America: placing IPCC projections of hydroclimatic change in a long-term palaeoclimate context. Journal of Quaternary Science 25: 48-61.

Douglass, A.E. 1929. The secret of the Southwest solved with talkative tree rings. National Geographic December: 736-770.

Douglass, A.E. 1935. Dating Pueblo Bonito and other ruins of the Southwest. National Geographic Society Contributed Technical Papers. Pueblo Bonito Series 1: 1-74.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.

Seager, R., Kushnir, Y., Herweijer, C., Naik, N. and Velez, J. 2005. Modeling of tropical forcing of persistent droughts and pluvials over western North America: 1856-2000. Journal of Climate 18: 4068-4091.

Stahle, D.W., Fye, F.K., Cook, E.R. and Griffin, R.D. 2007. Tree-ring reconstructed megadroughts over North America since AD 1300. Climatic Change 83: 133-149.

Stine, S. 1994. Extreme and persistent drought in California and Patagonia during mediaeval time. Nature 369: 546-549.

Weakly, H.E. 1965. Recurrence of drought in the Great Plains during the last 700 years. Agricultural Engineering 46: 85.