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The Upper Colorado River Basin (USA) Super-Megadrought of the Mid-1100s
Meko, D.M., Woodhouse, C.A., Baisan, C.A., Knight, T., Lukas, J.J., Hughes, M.K. and Salzer, M.W. 2007. Medieval drought in the upper Colorado River Basin. Geophysical Research Letters 34: 10.1029/2007GL029988.

What was done
Using a newly developed network of tree-ring sites located within the Upper Colorado River Basin (UCRB) of the western United States - which consists of tree-ring samples from living trees, augmented by similar samples obtained from logs and dead standing trees (remnant wood) - the authors extended the record of reconstructed annual flows of the Colorado River at Lee Ferry, Arizona, into the Medieval Climate Anomaly (MCA), during which period they say that "epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America."

What was learned
"The most prominent feature of the smoothed long-term reconstruction," in the words of Meko et al., "is the major period of low flow in the mid-1100s," which "25-year running mean occurred in AD 1130-1154." For this level of smoothing, they say that "conditions in the mid-1100s in the UCRB were even drier than during the extremely widespread late-1500s North American mega-drought (e.g., Stahle et al., 2000)." For comparison, for example, they state that "if 'normal' is defined as the observed mean annual flow for 1906-2004, the anomalous flow for AD 1130-1154 was less than 84% of normal," whereas "the lowest 25-year mean of observed flows (1953-1977) was 87% of normal," noting further that the 80% confidence band of their data "suggests a greater than 10% probability that the true mean for AD 1130-1154 was as low as 79% of normal."

Additionally, the seven scientists report that "a detailed view of the time series of annual reconstructed flow reveals that the mid-1100s is characterized by a series of multi-year low-flow pluses imbedded in a generally dry 62-year period (1118-1179)," and that "the key drought signature is a stretch of 13 consecutive years of below normal flow (1143-1155)," noting that "in no other period of the reconstruction was flow below normal for more than 10 consecutive years, and the longest stretch of consecutive dry years in the reconstruction for the modern instrumental period (post 1905) was just 5 years."

What it means
One of the major tenets of Al Gore's "climate crisis," as articulated in his testimony of 21 March 2007 to the U.S. Senate, is that "droughts are becoming longer and more intense" in response to global warming. On the one hand, we could say this claim is refuted by the late-1500s North American megadrought described by Stahle et al. (2000), which occurred during the Little Ice Age. On the other hand, we could say it is confirmed by the super-megadrought described by Meko et al., which occurred during the Medieval Warm Period. But if this latter route is taken, the temperature-drought correlation claimed by Gore suggests that the Medieval Warm Period was likely much warmer than the Current Warm Period (a concept climate alarmists absolutely abhor), which has seen nothing even remotely similar to the mid-1100s drought. In terms of drought extremes, therefore, any way one looks at this aspect of Al Gore's climate crisis claim, it rings mighty hollow throughout much of North America.

Stahle, D.W., Cook, E.R., Cleaveland, M.K., Therrell, M.D., Meko, D.M., Grissino-Mayer, H.D., Watson, E. and Luckman, B.H. 2000. Tree-ring data document 16th century megadrought over North America. EOS, Transactions, American Geophysical Union 81: 121-125.

Reviewed 10 October 2007