What was done
The authors used "an expanded grid of tree-ring reconstructions of the summer Palmer drought severity indices (PDSI; Cook et al., 2004) covering the United States, southern Canada, and most of Mexico to examine the timing, intensity, and spatial distribution of decadal to multidecadal moisture regimes over North America."

What was learned
To date, during the Current Warm Period, Stahle et al. state that "the Dust Bowl drought of the 1930s and the Southwestern drought of the 1950s were the two most intense and prolonged droughts to impact North America," citing the studies of Worster (1979), Diaz (1983) and Fye et al. (2003). During the Little Ice Age, on the other hand, they report the occurrence of three megadroughts, which they define as "very large-scale drought[s] more severe and sustained than any witnessed during the period of instrumental weather observations (e.g., Stahle et al., 2000)." However, they report that "much stronger and more persistent droughts have been reconstructed with tree rings and other proxies over North America during the Medieval era (e.g., Stine, 1994; Laird et al., 2003; Cook et al., 2004)." In fact, these latter megadroughts were so phenomenal that they refer to them as "no-analog Medieval megadroughts."

What it means
Climate alarmists typically claim that CO2-induced global warming will result in more severe droughts. However, the much more severe and sustained megadroughts of the Little Ice Age appear to render their claim somewhat dubious. On the other hand, the still more severe and sustained no-analogue megadroughts of the Medieval Warm Period would appear to bolster their contention. However, the incredibly more severe droughts of that period - if they were indeed related to high global air temperatures (which we believe they were) - would suggest that it is not nearly as warm currently as it was during the Medieval Warm Period, when there was far less CO2 in the air than there is today; and these observations undercut the climate alarmists' more fundamental claim that the historical rise in the air's CO2 content has been responsible for what they describe as unprecedented 20th-century global warming that has taken earth's mean air temperature to a height that is unprecedented over the past two millennia.

Clearly, Al Gore and his ilk can't have it both ways. If global warming does indeed lead to more severe droughts, it must not yet be nearly as warm as it was back in the core centuries of the Middle Ages, which really takes the wind out of the sails of those who claim that today's warmth is both unprecedented and primarily CO2-induced.

References
Cook, E.R., Woodhouse, C., Eakin, C.M., Meko, D.M. and Stahle, D.W. 2004. Long-term aridity changes in the western United States. Science 306: 1015-1018.

Diaz, H.F. 1983. Some aspects of major dry and wet periods in the contiguous United States, 1895-1981. Journal of Climate and Applied Meteorology 22: 3-16.

Fye, F.K., Stahle, D.W. and Cook, E.R. 2003. Paleoclimatic analogs to 20th century moisture regimes across the USA. Bulletin of the American Meteorological Society 84: 901-909.

Laird, K.R., Cumming, B.F., Wunsam, S., Rusak, J.A., Oglesby, R.J., Fritz, S.C. and Leavitt, P.R. 2003. Lake sediments record large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia. Proceedings of the National Academy of Sciences USA 100: 2483-2488.

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: 212, 225.

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

Worster, D. 1979. Dust Bowl: The Southern Plains in the 1930s. Oxford University Press.