Is global warming behind the drought in Southwest?

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TUCSON, Ariz. -- The Southwest's current drought could be the start of the Dust Bowl-like future that some scientists have already predicted will come from human-caused warming.Or, it could just be another in the long line of natural, cyclical droughts in this region dating back 1,000 years.But one of the nation's leading climate scientists, the University of Arizona's Nobel Prize-sharing Jonathan Overpeck, says he's coming to believe there's "a real likelihood" the drought is caused by global warming. New research at the UA backs that up.Some other scientists disagree, even as leading climate experts generally agree that someday, global warming will make the Southwest drier.This drought is already known as the region's worst in more than a century and one of the worst in the past 500 years.The dispute has implications for Tucson's water supply. The drought has already shrunk water levels in the Colorado River, which furnishes Central Arizona Project (CAP) water that's pumped to Tucson.If those believing this drought is caused by warming are right, major shortages could occur much sooner than expected on the river, says Connie Woodhouse, a University of Arizona geography professor who has studied tree-ring records to understand river-runoff patterns.Various studies have already warned that a drying climate could ultimately reduce spring runoff in the Colorado by 6 percent to 45 percent, meaning less water for the CAP.Until now, federal officials have predicted that shortages could start sometime after 2010 but won't be serious enough to cut deliveries to Tucson and Phoenix for another 20 years or more. Before then, non-Indian farmers in Central Arizona would take the big hit.The leading proponent of the theory that this drought isn't natural says it could have been caused not just by warmer weather but also by the hole in the atmosphere's ozone layer that has been linked to past use of aerosol sprays. The key compounds in those sprays are illegal now, but the ozone hole may not heal for 70 to 80 years."Whether it's manmade global warming or a manmade ozone hole, either way, it's more drought for us," says UA climatologist Joellen Russell. She acknowledges that her opinion is based on "cutting edge" research, some still unpublished, and that the controversy may not be settled for decades.Other scientists, however, say that while the current drought is consistent with what they would expect from global warming, it's too early to blame warming.One reason some scientists hesitate to blame this drought on warming is that the Southwest has had large "megadroughts" in the past, as shown in detailed tree-ring records studied by UA's Tree Ring Research Laboratory.The worst megadrought came for six decades in the 1100s, when the Colorado River's flow was significantly below normal for 25 years, says David Meko, an associate professor at the lab.All do agree that warmer weather aggravates the effects of drought.Russell's theory rests on several points. She says:-- Jet streams have moved slightly toward the North and South poles over 25 to 30 years, probably because of global warming and the hole in the ozone layer. Jet streams in the upper atmosphere mark boundaries between hot and cold air and affect U.S. rainfall patterns.-- This movement contributed to the current drought. That would match the regular pattern where jet streams move north in the winter, and drier, warmer weather dominates in the Southwest.-- The jet stream's poleward shifts are caused by global warming or the ozone hole.If it is a permanent change in the jet stream due to warming, "you'd have drought a lot more often and you'd never really recover from a particular drought," although there would still be some wet years, Russell says.(E-mail Tony Davis at tdavis(at)azstarnet.com.)(Distributed by Scripps Howard News Service, www.scrippsnews.com.)