Thanks!
That was enough to track down the original paper.
Note that the Palmer Drought Severity Index in most of the world is estimated based on rainfall, not actually measured. There are an awful lot of calculation assumptions in this paper, a fact which the paper itself makes no attempt to hide, but which is not transmitted well to the Scientific American article, and I predict will be lost completely when this hits the regular media.
Interestingly, a "related link" was "Drought May Have Brought on Demise of the Maya." Obviously, they should have been driving electric cars and burning less coal.