What was done
The authors developed a 1200-year history of drought for the western half of the United States and adjacent parts of Canada and Mexico (hereafter the "West"), based on annually-resolved tree-ring records of summer-season Palmer Drought Severity Index that were derived for 103 points on a 2.5°x2.5° grid, 68 of which grid points (66% of them) possessed reconstructions that extended back to AD 800.

What was learned
The resulting reconstruction, in the words of Cook et al., revealed "some remarkable earlier increases in aridity that dwarf the comparatively short-duration current drought in the 'West'."  Interestingly, they report that "the four driest epochs, centered on AD 936, 1034, 1150 and 1253, all occur during a ~400 year interval of overall elevated aridity from AD 900 to 1300," which they say is "broadly consistent with the Medieval Warm Period."

What it means
Commenting on their findings, the five scientists say "the overall coincidence between our megadrought epoch and the Medieval Warm Period suggests that anomalously warm climate conditions during that time may have contributed to the development of more frequent and persistent droughts in the 'West'," as well as the megadrought that was discovered by Rein et al. (2004) to have occurred in Peru at about the same time (AD 800-1250).  After citing nine other studies that provide independent evidence of significant drought during this time period for various sub-regions of the "West," they thus warn that "any trend toward warmer temperatures in the future could lead to a serious long-term increase in aridity over western North America," noting that "future droughts in the 'West' of similar duration to those seen prior to AD 1300 would be disastrous."

We certainly agree with Cook et al.'s analysis, noting that such an unfortunate fate could well befall the "West," even in the absence of CO2-induced global warming; for the millennial-scale oscillation of climate that brought the world the Medieval Warm Period (which was obviously not CO2-induced) could well be repeating itself in the current initial stages of the Modern Warm Period.  Coincidently, we note that if the association between warmth and drought in the "West" is robust, it suggests that temperatures of the latter part of the 20th century are still significantly less than those experienced during large segments of the Medieval Warm Period over much of western North America.

Reference
Rein, B., Luckge, A. and Sirocko, F.  2004.  A major Holocene ENSO anomaly during the Medieval period.  Geophysical Research Letters 31: 10.1029/2004GL020161.