Almaden Reservoir, California

2013 was California's driest year on record, and it may foreshadow even drier decades to come. (Photo: Justin Sullivan/Getty Images)

The Southwestern U.S. is no stranger to droughts, but it may soon dry out more than it has in thousands of years. Thanks to man-made climate change, the region's chances of a decade-long drought are now at least 50 percent, according to a new study, while its odds of a "megadrought" — which can last more than three decades — range from 20 to 50 percent over the next century.

California is already three years into its worst drought in generations, and patches of extreme drought also fester in other Western states from Oregon to Texas. Some scientists even say dryness across the U.S. West already classifies as a megadrought. But today's dry spells are nothing compared with what's on the way, warns Cornell University geoscientist Toby Ault, who led the new research.

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

Not even the 1930s Dust Bowl, which lasted up to eight years, qualified as a true megadrought. These multi-decade disasters have struck around the world throughout history, though, leaving behind evidence in tree rings and sediments. A severe one developed along the Colorado River in the 1150s, for example, and some in southwestern North America have reportedly lasted 50 years.

Megadroughts occur naturally, but like the Dust Bowl, they're also susceptible to human influence. As humanity's greenhouse gas emissions fuel global warming, many natural climate cycles are expected to grow more exaggerated, resulting in more strong storms and hotter, more relentless droughts.

Elephant Butte Reservoir

Bathtub rings were exposed in Elephant Butte Reservoir during the 2012 Texas drought. (Photo: climate.gov)

"For the southwestern U.S., I'm not optimistic about avoiding real megadroughts," says Ault, who worked on the new study with researchers from the U.S. Geological Survey and the University of Arizona. "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."

Finding that even top computer models didn't capture some low-frequency hydroclimate quirks, Ault and his colleagues devised a way to assess the risk of a megadrought over the next century using models as well as paleoclimate data. While other models peg that risk at less than 50 percent for the U.S. Southwest, the new study suggests it's higher, and "may be higher than 90% in certain areas."

The Southwest also faces a 20 to 50 percent chance of a 35-year megadrought within 100 years, according to the study. And under the most severe warming scenario, the odds of a drought persisting for 50 years range from 5 to 10 percent, a risk the researchers call "non-negligible."

Since heat-trapping carbon dioxide lingers in the sky for centuries, some climate change is inevitable. The U.S. West needs to prepare for long-term droughts with adaptation plans, the study's authors write, especially in places where population growth already strains water supplies. Drought is a big reason why climate change is forecast to wreak havoc with agriculture worldwide, a danger illustrated for millions of Americans recently by dry spells in California, Texas and other states.

It's unclear how long current droughts across the Western U.S. will continue, Ault adds, but "with ongoing climate change, this is a glimpse of things to come. It's a preview of our future."

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.