An anonymous reader writes: Droughts in the western U.S. have been bad recently, but not as bad as they could be. Researchers from NASA, Cornell, and Columbia are now warning that if we don't slow the rate at which we produce greenhouse gases, then we're dramatically increasing our odds of a drought that lasts upwards of three decades. "The scientists were interested in megadroughts that took place between 1100 and 1300 in North America. These medieval-period droughts, on a year-to-year basis, were no worse than droughts seen in the recent past. But they lasted, in some cases, 30 to 50 years. When these past megadroughts are compared side-by-side with computer model projections of the 21st century, both the moderate and business-as-usual emissions scenarios are drier, and the risk of droughts lasting 30 years or longer increases significantly."