Large parts of the U.S. are in for a drought of epic proportions in the second half of this century, scientists warn in a new study that provides the highest degree of certainty yet on the impact of global warming on water supplies in the region.

The chances of a 35-year or longer "megadrought" striking the Southwest and central Great Plains by 2100 are above 80 percent if the world stays on its current trajectory of greenhouse gas emissions, scientists from NASA, Columbia University, and Cornell University report in a study published Thursday in the new open-access journal Science Advances.

If countries reduce their emissions to current "middle of the road" targets, the chances of a megadrought hitting the Great Plains drop to between 60 and 70 percent. But they remain nearly 80 percent for the Southwest.

That's because rising temperatures spurred by the greenhouse effect result in more evaporation and less precipitation for the region, which is already relatively dry. (Read "Drying of the West" in National Geographic magazine.)

"Even at the middle-of-the-road scenario, we see enough warming and drying to push us past the worst droughts experienced in the region since the medieval era," said Benjamin Cook, the study's lead author and a scientist at NASA's Goddard Institute for Space Studies in New York.

Why It Matters

Drought often has significant impacts on agriculture, ecosystems, and city water supplies. "We see some of those impacts going on now in California," said Cook, referring to the ongoing drought that is the worst in that state's recorded history.

In fact, 11 of the past 14 years have seen drought in much of the American West, from California across to Texas and Oklahoma, according to the U.S. Drought Monitor. (Learn about the resulting groundwater drilling boom.)

In their study, Cook's team used 17 computer models of droughts and three models of soil moisture to predict the likelihood of dryness over the next century. After they found a high degree of agreement among the models, they applied them to data gathered from tree rings going back to about the year 1000.

They found that the megadrought that struck the region in the 1100s and 1200s—which has been tied to the decline of the ancient Pueblo peoples, or Anasazi, of the Colorado Plateau—was likely not as severe as the one expected in the near future.

"Even when selecting for the worst megadrought-dominated period, the 21st-century projections make the megadroughts seem like quaint walks through the Garden of Eden," study co-author Jason E. Smerdon of Columbia University's Lamont-Doherty Earth Observatory said in a statement.

The Big Picture

The megadrought predicted for the U.S. seems to be part of a "northward creeping of desert bands" in other subtropical regions, especially in the Mediterranean and southern Africa, said Tom Painter, a snow and drought scientist with NASA's Jet Propulsion Laboratory in Pasadena, California, who was not involved with the study. That shift is clearly related to changes in air circulation caused by global warming, but the precise mechanics are "fuzzy," Painter said. (Learn more about Painter's work.)

The new study is the latest in a series over the past decade highlighting the challenge facing people in the American West, where strategies for coping with drought, such as irrigation and water conservation, have a long history. "The real challenge is whether we can take strategies we have now and apply them to the more severe droughts that are likely in the future," Cook said. (Learn about efforts to restore the Colorado River ecosystem.)

"Over the past year, water managers and the public have started paying more attention to the possibility of a megadrought," said Painter. "Water demand has passed supply in some areas. Throwing 30 years of drought on top of that means we're going to have to change the way we live out here."