Over in the comments at RealClimate, Dave of ClimateWarning.org pointed to some new research by Aiguo Dai and colleagues at NCAR on the detection of increased drought on a warming earth.

In the December issue of the Journal of Hydrometeorology, Dai et al. report"observational evidence for the increasing risk of droughts as anthropogenic warming progresses and produces both increased temperatures and increased drying." Then in a subsequent paper presented last week, Dai and his colleagues took the results further:

"This depiction of linear trends in the Palmer Drought Severity Index from 1948 to 2002 shows drying (reds and pinks) across much of Canada, Europe, Asia, and Africa and moistening (green) across parts of the United States, Argentina, Scandinavia, and western Australia. (Illustration courtesy Aiguo Dai and the American Meteorological Society.)"

The Palmer index is not the best measure of drought, but it's a good crude index, and the results in the Dai map shown above are striking: significant areas of the globe - especially areas, like Africa, with significantly vulnerable populations - are drying.