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A Thousand Years of Precipitation in the Southwestern USA
Ni, F., Cavazos, T., Hughes, M.K., Comrie, A.C. and Funkhouser, G.  2002.  Cool-season precipitation in the southwestern USA since AD 1000: Comparison of linear and nonlinear techniques for reconstruction.  International Journal of Climatology 22: 1645-1662.

What was done
The authors developed a 1000-year history of cool-season (November-April) precipitation for each climate division in Arizona and New Mexico, USA, from a network of 19 tree-ring chronologies.

What was learned
With respect to drought, the authors say that "sustained dry periods comparable to the 1950s drought" occurred in "the late 1000s, the mid 1100s, 1570-97, 1664-70, the 1740s, the 1770s, and the late 1800s."  They also note that the 1950s drought "was large in scale and severity, but it only lasted from approximately 1950 to 1956," whereas the "16th century mega drought" lasted more than four times longer.

With respect to the opposite of drought, the authors say that "several wet periods comparable to the wet conditions seen in the early 1900s and after 1976" occurred in "1108-20, 1195-1204, 1330-45, the 1610s, and the early 1800s."  They also note that "the most persistent and extreme wet interval occurred in the 1330s."

With respect to causes of the different precipitation extremes, the authors say that "the 1950s drought corresponds to La Niņa/-PDO and the opposite polarity corresponds to the post-1976 wet period," leading them "to hypothesize that the prominent shifts seen in the 1000 year reconstructions in Arizona and New Mexico may also be linked to strong shifts of the coupled ENSO-PDO system," where PDO is the Pacific Decadal Oscillation.

What it means
For the particular part of the world covered by this study, there appears to be nothing unusual about the extremes of both wetness and dryness experienced during the 20th century.  This observation appears to be at odds with the climate-alarmist claim that global warming should produce extremes of both types, since they describe the 20th century as having experienced unprecedented warming compared to the entire 900 years that preceded it.

Reviewed 19 February 2003