News•August 29, 2014

Climate Change Ups Odds of a Southwest Megadrought

If you think the drought in California is bad, you ain’t seen nothing yet. New research indicates that climate change is giving a boost to the odds of long-term drought across the Southwest.

The research, published Thursday in the Journal of Climate, puts the chances of a megadrought lasting 35 years or longer at up to 50 percent in the region. It would be a drought of epic proportions that would wreak havoc on the region’s already tenuous water supply for its growing population.

Low water levels on Lake Mead, a major source of water for western states that's fed by the Colorado River.
Credit: Raquel Baranow/Flickr

“It’s been recognized for awhile now that during climate change, because of rising temperatures, a lot of the Southwest dries out, gets less average precipitation,” said Toby Ault, the study’s lead author and Cornell-based climate researcher. “The novelty of this research was to just try and use those predictions of the future to estimate the risk of prolonged drought, to translate what those predictions of long term drying meant for megadrought.”

The impetus for the study was to assess the odds of consecutive dry years. That’s because even in an overall drier climate, an occasional wet year or two are still expected and could break up or at least temporarily alleviate long-term drought. While that’s still a possibility, what Ault found was that climate change is still likely to increase the odds of long-lasting drought.

If current greenhouse gas emissions trends continue, the odds of a megadrought hitting some parts of the Southwest is a 50-50 proposition. And the odds of a decade-long drought – like the Dust Bowl of the 1930s or Southwest drought of the 1950s – are around 90 percent, meaning it’s near certain parts of the Southwest will deal with substantial drought impacts at some point in the next century due to climate change.RELATEDEpic Drought in West is Literally Moving Mountains
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There’s also a 5-10 percent chance that parts of the region could see a state of “permanent” megadrought lasting 50 years or longer under the highest-warming scenario, a greenhouse gas emissions path we’re currently on.

“Even without climate change, there would be some risk of megadrought even if we weren’t warming up the planet. But because of climate change and drying predicted from climate change, that weights the dice toward making these things more likely,” Ault said.

The estimates for all three types of drought could be even higher because Ault and his colleagues only considered precipitation and not temperature, which is expected to rise and further dry out the region.

A graphic showing the risk of megadrought in the Southwest as well as the rest of the western U.S. over the 21st Century.
Click the image to enlarge. Credit: Ault et al, 2014

To get the estimates, researchers compared projected shifts in precipitation under different climate change scenarios with past river flow and drought data. That includes data from tree rings, which provide a glimpse of drought going back 1,000 years and includes a megadrought that gripped the region in the 1100s that likely contributed to the downfall of the Anasazi culture in the region.

Kevin Trenberth, a senior research scientist at the National Center for Atmospheric Research who didn’t contribute to the research, said the study’s use of historical data helped fill in some of the gaps models have in replicating water storage and flow.

Jason Smerdon, a scientist at Lamont-Doherty Earth Observatory also not affiliated with the study, agreed.

“Such hybridizations of model and observational evidence can thus improve future risk assessments, and in this case, the authors estimate that drought projections informed by paleoclimatic and observational evidence significantly increase the estimated risks of prolonged droughts in the future,” Smerdon said.

The past impacts of drought offer a cautionary tale. During the megadrought in the 1100s, the Colorado River’s flow dropped to 85 percent of its 20th Century average. That would be like losing almost the entire allocation of water the Colorado provides for the state of Arizona, one of eight states that rely on the river. Given that the Southwest has 3 of the 10 fastest-growing states in the country, that would be a huge issue for water managers and city planners.

Talking about severe drought in 2014 brings California immediately to mind. Nearly 60 percent of the state is in the worst state of drought listed by the Drought Monitor, leading to water restrictions, wildfires and economic and job losses. There hasn’t been any research concretely linking the drought with climate change, but it does provide some perspective on what future droughts could look like.

“This (current drought) is kind of worse than the things we looked at in terms of intensity, but it’s not nearly as long as multidecadal or megadrought. I think it’s an important window into what we expect from climate change. What’s happening now, whether or not its driven by global warming, is a glimpse of the shape of things to come,” Ault said

The California drought is likely to cost the state $2 billion by the end of the year. The severe drought that overtook the nation in 2012 was also disastrous, causing an estimated $30 billion across 22 states. So preparing for future impacts is of utmost importance, a challenge Ault thinks water managers are up for.

“I do feel a sense of optimism in the sense that this is a natural hazard in the Southwest – and it appears to be a very important one under climate change – but because we know this and because we’re a very adaptive and sophisticated species, I’m confident we can find ways of managing that risk and even thriving,” he said.