People living in the American Southwest have seen their share of drought. But the West's recent water woes may pale in comparison to what's coming later in the century, researchers say. The Southwest may very well face a decades-long megadrought before the 21st century is out.

A team climate scientists led by Toby Ault, an atmospheric researcher at Cornell University, has just calculated the risk of a Southwestern megadrought occurring by 2100. By "megadrought" they mean an extreme, bone-dry period that can last for more than 35 years. According to Ault's research, if we continue producing greenhouse gases at our current rate, then there's a 70 to 99 percent chance of a megadrought that would stretch across the West, from San Francisco to Boulder, Colorado to the Gulf of California. The scientists' drought analysis was published today in the journal Science Advances.

"We know these megadroughts have happened here before, and we know they can happen again," says Ault. "If we get really aggressive on combating climate change now, we can cut that risk in roughly half."

Megadrought

"Megadrought" may sound like a plot device fabricated for a low-budget disaster movie, but it's certainly not a new concept. "Based on tree rings and various other geological records, we know that multiple megadroughts have occurred in the last 1,000 years," says Ault. In fact, the most recent recorded megadrought in the Southwest happened at the end of the 16th century. "We think it's linked to the fall of the Puebloan civilization, which built cities into the cliff faces of the four corners region" (the intersection of Utah, Colorado, New Mexico and Arizona).

"Those expectations are a lot less relevant to our future."

Megadroughts are created by the same environmental factors that cause the periodic normal droughts people of the West are accustomed to, but they persist unabated for decades and can become disastrous. "In some ways, it's as simple as less rainfall and hotter weather," says Ault. "Basically the risk of a megadrought depends critically on the balance of soil moisture at the soil's surface, and that's a tug-of-war between evaporation from hotter weather and the supply of moisture through precipitation."

The reason Ault's risk assessment gives a pretty wide range—a 70 to 99 percent chance by 2100—is that while Ault and his colleagues are confident about how much climate change might heat the American Southwest, they're less certain exactly how much it will effect rainfall. Generally, most climate scientists agree that climate change will start to push storms in the American Southwest farther north, decreasing the total rainfall the region receives. To deal with this uncertainty, Ault and his colleagues' computer models incorporated a range of rainfall projections. (If rainfall stays the exact same as it is today, the change of a southwestern megadroubt by 2100 is about 90 percent.)

"You have to understand, Ault says, "in some of our models, even when we considered a 30 percent increase of precipitation from what we see today, and the risk of a megadrought still goes up. That was particularly eye-opening."

"If we get really aggressive on combating climate change now, we can cut that risk in roughly half."

Leaving the Station

To, Ault, there are two takeaways from these frightening results. "The first is that we need to seriously revise our expectations surrounding water and water availability. A lot of our current infrastructure in heavily populated areas like Phoenix, Los Angeles, or San Diego are based around expectations of the past. And those expectations are a lot less relevant to our future.

"Secondly, it's not too late to get really serious about climate change, to curtail our production of greenhouse gasses and ameliorate [much of the risk of these megadroughts]," Ault says. "The train is leaving the station, but we still have a little time left to catch it."

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