Ian James

The Desert Sun

Scientists have projected that the chances of a decades-long megadrought in the Southwest are increasing dramatically as humanity pumps more carbon dioxide into the atmosphere.

A group of researchers estimated in a new study published Wednesday that if emissions of greenhouse gases continue unchecked, the odds of a monster drought ravaging the region for 35 years or longer this century would be between 70 percent and 99 percent, depending on a range of precipitation scenarios.

On the flipside, the scientists found that if steps are taken to aggressively reduce greenhouse gases, the risks of a decades-long drought could be cut nearly in half.

The findings point to major risks for water supplies in states from California to Utah to New Mexico, as well as the potential for reducing the likelihood of such a drought by combating global warming.

“The more warming we see, the greater the risks,” said Toby Ault, a climate scientist at Cornell University and the study’s lead author.

The team of researchers, which also included scientists from Columbia University’s Lamont-Doherty Earth Observatory and the NASA Goddard Institute for Space Studies, used climate models, soil moisture estimates and data from the Palmer Drought Severity Index to estimate risks.

Ault said the research team focused on separating the influences of rising temperatures and various scenarios of precipitation.

“One of the things that was really surprising that we found is that even if precipitation increases modestly, the risk of megadrought can still go up,” Ault said. “The demand for moisture from the atmosphere is so much higher when the atmosphere is hotter, because the evaporation rates are so much higher. And so that really weights the dice towards making megadroughts more likely.”

The researchers found that under a “business-as-usual” emissions scenario, the risk of a decades-long drought would be 90 percent in the southwestern U.S. if precipitation is unchanged. If there’s a modest increase in precipitation, the region would still face a 70 percent risk of a megadrought by the end of the 21 century. And if precipitation decreases under that warming scenario, the scientists estimated the risk at 99 percent.

If, however, humans reduce greenhouse gas emissions significantly and warming stays below 2 degrees C (3.6 degrees F) in the region, the researchers calculated the chances of a southwestern megadrought this century would decrease to less than 66 percent. If emissions are reduced further, they said, the chances would go down even more.

“Mitigation matters quite a lot for keeping risks relatively low, or low compared to the alternative, which is to just continue down our current path,” Ault said in a telephone interview. At the same time, he said, the impacts of rising temperatures in exacerbating droughts will “force us to rethink our expectations for how much water is going to be available in the Southwest.”

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Previous scientific studies have projected more severe droughts in the Southwest due to rising global temperatures. The latest study, which was published in the journal Science Advances, goes further by calculating the risks of a megadrought.

“I think this is a nice confirmation of what we likely would have guessed would be the case. But they’ve quantified it, which is an important step in the scientific process,” said Abigail Swann, a University of Washington climate scientist and ecologist who wasn’t involved in the research. She said the study also underlines the importance of taking steps to address climate change as quickly as possible.

Scientists’ studies of tree rings have confirmed previous megadroughts centuries ago. A decades-long drought in the 13th century coincided with the collapse of the Ancestral Pueblo people, who left behind ruins nestled in the cliffs of the Four Corners region.

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Ault said the growing risks due to rising temperatures could be compared to a theoretical golf course in which the sand traps represent extreme, long-lasting droughts. As temperatures rise, those sand traps have been expanding and covering more of the golf course, he said, and the chances of hitting a ball into a sand trap are going up.

“We’re basically saying nearly the entire golf course looks like a sand trap during climate change if we don’t do something,” Ault said. “The size of those sand traps has already grown because of a little bit of climate change, and if we continue down the current path, the entire golf course could be replaced by sand traps.”

The Colorado River Basin is in the midst of a 16-year drought, and the levels of Lake Mead and Lake Powell – the nation’s two largest reservoirs – have declined dramatically. The drought in California is entering its sixth year.

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Jonathan Overpeck, a climate scientist at the University of Arizona, said Ault and his colleagues have improved upon previous research by quantitatively estimating risks.

“This is definitely the best there is now in terms of estimating odds of future megadrought,” said Overpeck, who was not involved in the research. He said scientists should be able to further improve on the estimates in the future, “but this is a big step forward.”

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He said the research also has important implications as the United States and other countries move toward the goals laid out in the Paris agreement on climate change.

“We have a choice,” Overpeck said. “If we choose to cut greenhouse gas emissions in accord with the Paris agreement and other nations, we will significantly reduce the odds of a megadrought in the Southwest.”

Ian James can be reached by email atian.james@desertsun.com. Twitter:@TDSIanJames.

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