How dry is it?

Notice to the Southwestern United States: the drought may worsen and last much longer than anticipated.

A new study by the University of Arizona, Cornell University, and the U.S. Geological Survey is disquieting. The researchers looked at the deep historical record, such as tree rings and the latest climate-change models to estimate the likelihood of major droughts in the Southwest over the next century.

"The results are as soothing as a thick wool sweater on a mid-summer desert hike," says the publication Mother Jones.

The odds of a decade-long drought are at least 80 percent, say the researchers. Worse: the chances of a mega-drought, or one lasting 35 or more years, are between 20 percent and 50 percent. The prospects of a 50-year mega-drought — one worse than anything in the past 2000 years — are 5 percent to 10 percent. Among cities most likely affected are San Diego, Phoenix, Tucson, and Albuquerque.

The Mother Jones article also cites an early-August study of the Colorado River by the University of California-Irvine and NASA researchers. Conclusion: 40 million people, especially from San Diego, Los Angeles, Las Vegas, Phoenix, and Tucson, rely on the river's water.

Over the past decade, the region's aquifers have undergone a much larger than expected drawdown. There is little prospect that these aquifers will refill anytime soon, according to the study.