The odds of a potentially devastating Southwestern “megadrought” due to human-caused climate change are as high as 50 percent in this century, a new study finds.

The chances of a megadrought lasting more than 35 years are 10 to 50 percent, says the study, in which University of Arizona researchers played key roles. The highest risks are in parts of Southeastern Arizona and in southwest Texas, they say.

Those chances are a lot higher than many other researchers have thought.

Moreover, if the current drought plaguing Tucson and Southern Arizona lasts another 15 years, “this would almost certainly constitute a megadrought” — the kind that typically strikes a particular place once or twice in 1,000 years, says the study’s lead author.

“What’s happening right now in California and much of the Southwest, that’s a sneak preview of what we expect from the future,” said Toby Ault, a Cornell University researcher who began the report while working on his doctorate at the UA. “We’re not saying that what’s happening now (with drought) is because of climate change. What I’m saying is that as the Earth’s temperatures rise, the likelihood of megadrought gets higher, and we expect this kind of event gets more and more common.”

The report says that computer models used to make predictions of future droughts, including those done by the Intergovernmental Panel on Climate Change, “critically underestimate” the risks of future droughts lasting longer than a decade.

A megadrought combined with continued growth and “business-as-usual water use” would wreak havoc on the Southwest’s already fragile water supplies, says Julia Cole, a UA researcher who co-authored the study.

But there are ways to adjust to all of these problems, including more realistic water pricing strategies, water conservation, and taking action to reduce greenhouse-gas emissions and limit climate change, said Cole, a UA professor of atmospheric and geologic sciences. Jonathan Overpeck, director of the UA’s Institute of the Environment, also co-authored the study.

“We’re incredibly adaptive, adaptable creatures. As Americans we don’t give up easily,” lead author Ault said last week in an interview. “I’m very optimistic there are solutions out there that we can plan and make them work.”

At the same time, Ault said he’s not optimistic about this region’s chances of avoiding megadroughts entirely, although he held out hope that “we might just get lucky with two fortuitious El Niño years” that could refill the region’s now-sinking reservoirs. He emphasized that a megadrought isn’t inevitable.

“As we add greenhouse gases into the atmosphere — and we haven’t put the brakes on stopping this — we are weighting the dice for megadrought,” Ault said.

Dust Bowl revisited

Specifically, the new study, to be published in the American Meteorological Society’s Journal of Climate, says:

  • The chance of a drought lasting at least a decade in the Southwest in this century is 60 to 80 percent, and could be higher than 90 percent in some areas. Other current computer models have put such a risk at less than 50 percent. Droughts of this magnitude struck parts of the region during the Dust Bowl era of the 1930s and again in the 1950s.
  • About a 30 percent risk exists for a megadrought of more than 35 years in most of Arizona and New Mexico, and parts of Southern California and West
  • Texas
  • . The earlier computer models had put the risk of a multi-decade megadrought at less than 1 percent.
  • The risk of a drought lasting longer than 50 years — which the study calls a permanent megadrought — is 5 to 10 percent. That, like the other forecasts, assumes the most severe global warming scenario in which growth continues in global greenhouse gas emissions until the end of this century. Total greenhouse gas emissions in the U.S. and other Western countries have dropped in recent years but global emissions have continued rising as China and other developing countries industrialize.
  • On a more positive note for Tucson and other Southwestern cities dependent on Colorado River supplies, the report finds lesser risks of a prolonged megadrought in Colorado — the source of much of the river’s water — than in Arizona, New Mexico and Southern California.
Tricky forecasts

Locally, since 1994, rainfall measured at Tucson International Airport has exceeded the current annual average of 11.59 inches only five years out of 20. It’s exceeded the historical annual average of 12.17 inches, which was lowered in 2010, four times in that period.

“If conditions remain as dry or drier than they have been during the last 20 years for an additional 15, this would almost certainly constitute megadrought,” Ault said.

But to understand if this drought will last another one, two or 10 years, researchers would need to be able to make predictions that are now on the cutting edge of science, he said.

“It’s really a hot topic. We don’t have the skill at the time scales of one to 10 years to say that this drought will continue,” said Ault, who attended graduate school at UA from 2004 to 2011 and has worked on this report since 2007.

If a prolonged megadrought occurred, it would cause both socioeconomic and ecological problems with unprecedented consequences in modern times, the study said.

While there’s been real debate among the researchers as to when the Southwest last had a prolonged megadrought, some studies have said one happened during the 16th century. There’s general agreement that one occurred during the 1150s and beyond, Ault said. The Sahel region of Africa, a narrow band of land south of the Sahara, experienced a 30-year megadrought from the 1970s through the 1990s.

Guided by tree rings

While no one can say for sure that the Southwest’s current drought is caused by global warming, many researchers agree that climate change aggravates the risk of drought, in part because more heat means more evaporation of water.

In areas such as Colorado that are important water sources for the region, data-monitoring has shown that warmer weather means more precipitation falls as rain compared to snow, meaning less snowpack to form river runoff, Cole said. Also, more dust storms in the Colorado Rockies are blanketing snowpack, absorbing more of the sun’s rays and causing snowpack to melt early, reducing spring runoff levels.

All this has been known for some time by those who study climate change.

But this study’s conclusion that earlier computer models underestimated the risks of prolonged drought comes partly because it made much more use than did past studies of paleoclimactic data such as tree rings and lake and cave sediments, the UA’s Cole said.

Past studies, by contrast, tended to look mainly at past trends in rainfall based on historical written records, and projected how climate change could affect them, Cole and Ault said. “The novelty of this project is to take those trends and try to turn them into tangible numbers of megadrought risk,” Ault said.

Bad or worse

Also, the new study looked only at hundreds of years of rainfall records — not past temperatures — to make its risk estimates, the researchers noted. Because warmer weather can aggravate a drought, “the view of risk presented here is quite conservative,” the report said.

The Star contacted three climate researchers, not involved in this study, who at times have drawn conclusions at odds with these findings. None had anything critical to say about the new study.

Jason Smerdon, a Columbia University researcher was not willing to say that his studies are at odds with the new Ault-UA paper because they looked at different things and used different data sampling methods.

“The bottom line is that I think there are still some open questions about what the models and the historical data are telling us about the potential risks into the future,” said Smerdon, an associate research professor at Columbia’s Lamont-Doherty Earth Observatory.

Generally, however, it breaks down to a question of bad or worse, he said.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. Follow him on Twitter @tonydavis987