Climate

Southwest And Great Plains Could Face Worst ‘Megadroughts’ In Modern History, Study Finds

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Entrance to a new housing development in Arizona.

A new study that builds upon recent research predicts that in the second half of the 21st century, the Southwest and Great Plains will face climate change-induced drought conditions worse than anything in “ancient or modern” times.

While other studies have predicted that the West will get hotter and drier due to climate change, this is the first to posit that the drying will exceed even the worst conditions of the distant past. According to the findings, future droughts in both regions will be more severe than even the hottest, driest megadroughts of the 12th and 13th centuries, which are believed to have contributed to the fall of ancient Native American civilizations that inhabited the Southwest, such as the Pueblo Indians.

“We are the first to do this kind of quantitative comparison between the projections and the distant past, and the story is a bit bleak,” said Jason E. Smerdon, a co-author and climate scientist at the Lamont-Doherty Earth Observatory, part of the Earth Institute at Columbia University. “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.”

The study, “Unprecedented 21st-Century Drought Risk in the American Southwest and Central Plains,” was published in the inaugural edition of the new online journal Science Advances, produced by the American Association for the Advancement of Science. It includes research by scientists at NASA and Cornell and Columbia universities.

Using an extensive drought history based on tree ring samples, the researchers used 17 different climate models to determine the outcome of two different GHG emissions trajectories: a “business as usual” projection in which GHGs continued to rise steadily and a scenario in which emissions were moderated. They found that the impacts of atmospheric CO2 concentrations on drying in the Southwest and Great Plains emerges as a “strong signal” across the majority of the models, which is consistent with many previous studies. Nearly all the models showed “coherent and robust” drying caused by both reduced precipitation and warming temperatures.

“I was honestly surprised at just how dry the future is likely to be,” said Toby Ault, an assistant professor in the department of earth and atmospheric sciences at Cornell University. “I look at these future megadroughts like a slow moving natural disaster. We have to put megadroughts into the same category as other natural disasters that can be dealt with through risk management.”

While future megadroughts are worrisome specters, climate change is already having an impact on the climate of the Southwest, as was discussed in last year’s National Climate Assessment. By the time the potential for these unprecedented megadroughts arrives in mid-century, the population of the fast-growing Southwest is expected to increase from 56 million people to 94 million; this will further stress a region already struggles to allocate water to meet the demand of communities, industries and ecosystems.

Benjamin Cook, of NASA and Columbia University’s Lamont Doherty Earth Observatory, said that this new study shows how droughts like California’s current four-year dry spell could extend for decades.

“In our study, (the area) where we saw the most severe drying does include California, that drying over California is predominantly driven by an increase in temperature and an increase in evapotranspiration,” said Cook. “Expect longer and more severe droughts.”

The “tiny piece of optimism” in all this comes from Ault, who said that since the records are based on tree rings it means the megadroughts of the past weren’t “so bad as to kill off all the trees.”