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 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, as this Drought Monitor map shows. 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 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."

What causes megadroughts?

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)

A more recent study reached the same conclusion but tried to answer bigger questions: What causes megadroughts and what factors control their timing? Lead author Nathan Steiger and colleagues at Columbia’s Earth Institute looked at climate models to find out why the 9th to 16th centuries experienced such droughts, but not since. They found that cooling ocean surface temperatures in the Pacific, warming surface temperatures in the Atlantic and "radiative forcing" were the causes.

Radiative forcing or climate forcing is the underlying concept behind the greenhouse effect, as MIT explains:

The concept of radiative forcing is fairly straightforward. Energy is constantly flowing into the atmosphere in the form of sunlight that always shines on half of the Earth’s surface. Some of this sunlight (about 30 percent) is reflected back to space and the rest is absorbed by the planet. And like any warm object sitting in cold surroundings — and space is a very cold place — some energy is always radiating back out into space as invisible infrared light. Subtract the energy flowing out from the energy flowing in, and if the number is anything other than zero, there has to be some warming (or cooling, if the number is negative) going on.

That science matters because it offers a clear warning for today, when global warming is increasing and these same ocean temperature patterns are occurring. Their work was published in Science Advances.

"Both a warm Atlantic and a cold Pacific change where storms go," Steiger told Vice. "They both result in fewer storms going to the Southwest."

And fewer storms means less rain in a region known to be dry and that gets roughly 70% of its rain during the late summer monsoon season.

Worse than the Dust Bowl

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_lrg Low water marks on the dam at Elephant Butte, on the Rio Grande, near El Paso Texas. (Photo: Curt Teich & Co., Courtesy of Special Collections, University of Houston Libraries/Wikimedia Commons)

"For the southwestern U.S., I'm not optimistic about avoiding real megadroughts," says Ault, who worked on the study published in the Journal of Climate 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%, 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."

Editor's note: This story has been updated since it was first published in September 2014.

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

Southwest may see 'megadrought' this century
A megadrought, which can last more than three decades, is increasingly likely in the region due to climate change.