The southwestern U.S. has at least a 50 percent chance of experiencing a decade-long drought this century thanks to global warming, and the region's chances of a "megadrought" -- one that lasts multiple decades -- lies anywhere from 20 to 50 percent, according to a new study.

Set for release in an upcoming issue of the American Meteorological Society's Journal of Climate by scientists from Cornell University, the University of Arizona and the U.S. Geological Survey, the study urges the western and southwestern states to devise ways to mitigate what it calls "looming long-drought scenarios."

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Toby Ault, a Cornell assistant professor of Earth and Atmospheric Sciences and the study's lead author, said he wasn't optimistic about the Southwest avoiding megadroughts in the coming decades.

"As we add greenhouse gases into the atmosphere -- and we haven't put the brakes on stopping this -- we are weighting the dice for megadrought," he added in a news release.

Most of California already is in "exceptional drought," the most severe drought category, according to the latest report from the U.S. Drought Monitor. Much of the rest of the region -- especially across large swaths of Arizona, New Mexico, Texas and Oklahoma -- are in moderate or severe drought.

Climate scientists don't know how long this drought will continue, Ault said, but added, "with ongoing climate change, this is a glimpse of things to come. It’s a preview of our future."

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The Midwest Dust Bowl of the 1930s -- chronicled perhaps most notably in John Steinbeck's "The Grapes of Wrath" -- lasted anywhere from four to eight years depending on location. What scientists call a megadrought could last three decades or more, and force mass numbers of people to migrate on a scale unheard of for most Americans alive today.

"This will be worse than anything seen during the last 2,000 years and would pose unprecedented challenges to water resources in the region,"Ault said in the news release.

Julia Cole, a professor of geosciences and atmospheric sciences at the Univesity of Arizona and one of the study's co-authors, said the results from this study should prompt the region to take the long view of its future drought risk.

"And the picture is not pretty," she added in a news release. "We hope this opens up new discussions about how to best use and conserve the precious water that we have."

The study, "Assessing the Risk of Persistent Drought Using Climate Model Simulations and Paleoclimate Data," is available online at the American Meteorological Society.

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The Green Bridge passes over full water levels at a section of Lake Oroville near the Bidwell Marina on July 20, 2011, in Oroville, California. (Paul Hames/California Department of Water Resources/Getty Images)