In his recent press blitz, NASA's James Hansen tries to tie extreme weather events, such the current drought affecting much of the central U.S., to anthropogenic global warming. But the real world argues otherwise.
Hansen is quite adept at timing global warming pronouncements with extreme weather events. Recall that it was during a similar hot, dry period back in the summer of 1988 that Hansen first testified to Congress that global warming from human greenhouse gas emissions was impacting current weather events-testimony which many credit as giving rise to the global-warming-is-going-to-be-bad movement. But then, as now, the tie-in between weather events and human changes to the atmospheric greenhouse effect is tenuous at best, and tie-ins to specific events are ill-supported and ill-advised. In the best case, the anthropogenic emissions-driven rise in global temperatures has a small ancillary impact on a specific extreme weather event, but in the vast majority of the cases, its role is nugatory and undetectable.
Such is the case with the impact of global warming on U.S. drought.
In his recent op-ed and article in the Proceedings of the National Academy of Sciences (recall that the peer review process at PNAS is more like watered down "pal review") Hansen clearly states that U.S. droughts are being enhanced by global warming.
Hansen has a lot of weight in his corner on this one, because the standard measure of drought-the Palmer Drought Severity Index (PDSI)-has temperature incorporated into its equation. The higher the temperature, the greater the potential evaporation term, and thus the tendency towards drier conditions. But (obviously) precipitation amount is also included in the calculation of the PDSI. More precipitation pushes the index away from drought conditions. So just because temperature is on the rise does not mean that droughts must become more frequent-changes in precipitation could intervene and counteract the influence of a temperature increase.