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The Worst Droughts in a Thousand Years Are Still to Come

Computer models combined with 1,000 years of climate data indicate that greenhouse gases will bring severe droughts to the Southwest and Central Plains by the end of the century.
Folsom Dam in Northern California.* (Photo: Robert Couse-Baker/Flickr)

Folsom Dam in Northern California.* (Photo: Robert Couse-Baker/Flickr)

When it comes to water shortage, it seems the worst is yet to come. A new climate analysis indicates that by the end of the century, the United States Southwest and Central Plains regions are likely to experience drought conditions worse than any in the last millennium. These impending conditions could pose "a major adaptation challenge" for humans in a rapidly changing climate.

"We're talking about megadrought risk," says co-author of the study and Columbia University professor Toby Ault—an 80 percent chance or more of decades-long droughts before the end of the century.

Though it's well established that droughts and other extreme climate events are likely to become more intense over the next century, Ault, along with climatologists Benjamin Cook and Jason Smerdon note that the Americas are no stranger to massive droughts, like the Medieval "megadrought" between roughly 1,100 and 1,300 C.E. and the Little Ice Age that followed several centuries later. This raises a vital question: Compared to those events, how bad will the coming droughts be?

"Our results point to a remarkably drier future that falls far outside the contemporary experience of natural and human systems in Western North America," the climatologists write.

Pretty bad, Cook says: "What we find is pretty clear. After 2050 ... the Southwest and Great Plains will be drier than at any time" in the last thousand years, and "these future changes that we're seeing are likely to be more persistent than past megadroughts."

Cook, Ault, and Smerdon began their study by calibrating a set of 17 computer-simulation climate models from the Coupled Model Intercomparison Project on data from 1930 to 1990. They then used the calibrated models to project soil moisture and drought severity through the end of the 21st century. While details varied between models, the big picture was consistent and troubling: The Southwest and Central Plains will dry out, droughts in the Southwest will be more severe than in the Central Plains, and increased levels of greenhouse gases are to blame.

To put the coming drought in historical context, the team drew on the North American Drought Atlas, which uses tree-ring data to estimate soil moisture conditions in the U.S. going back more than a millennium. Droughts in the second half of the century, the simulations predict, will likely be the worst of the past thousand years, meeting or exceeding the Medieval megadrought, even if carbon emissions are cut in half immediately and remain there over the next century.

"Our results point to a remarkably drier future that falls far outside the contemporary experience of natural and human systems in Western North America," the climatologists write in Science Advances, a new journal that launches this week. That future will be compounded by increased temperatures and groundwater reserves that are already being depleted, "presenting a major adaptation challenge for managing ecological and [human] water needs in the region."

As bad as things will get, however, there will be some water, and Cook says he remains optimistic about "coping with megadrought." The situation could be an opportunity for the Southwest, which is already dealing with drought conditions, to take on a leadership role in developing policies to mitigate climate change, he says.

*UPDATE — February 13, 2015: An earlier photo identified the lake shown as Lake Hume in California, when in fact that was Lake Hume in Australia.