There's a Good Chance the Southwest Will See a 35-Year Megadrought This Century

There's an 80 percent chance the region will be hit with a decade-long drought, and a 20-50 percent chance it will weather a 35-year megadrought.

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Aug 26 2014, 7:35pm

Image: Fikret Onal/Flickr

This probably isn't what residents of the parched American Southwest want to hear right now, but there's a good chance that the region is headed for a decades-long megadrought. 

As if climatologists' forecasts for a warming world weren't dire enough, a new paper published in the American Meteorological Society concludes that current climate models "underestimate the risk of future persistent droughts."

The study, spearheaded by Cornell's Toby R. Ault, suggests that there is an 80 percent chance the region will be hit with a decade-long drought by the end of the century, a 20-50 percent chance it will weather a 35 year megadrought, and that the prospect of a severe dry spell afflicting the region for half a century is, I quote, "non-negligible." 

"The change in human activity in greenhouse gases and global warming provokes a change in the hydrological cycle, which dries out everything, and that just weighs the dice towards more likely megadroughts, really strikingly so," Ault told me in a phone interview. His team's work looks to the paleoclimate record and current climate models to predict the likelihood of future drought.

"There's evidence form the paleoclimate record—tree rings, lakes, caves, other types of geological records and biological records of the past—that both decadal droughts and megadroughts have happened in the past," Ault said. Climate change threatens to usher in many different kinds of 'mega' events—mega heat waves, mega methane release, and, of course, megadrought. 

"Now, a real megadrought hasn't happened in the American Southwest in a long time. Decadal droughts have happened, but the kinds of megadroughts that we're looking for... a 30-year interval that had devastating effects on the region, and from which the region is still recovering from today," he said. "That scale, almost a generation of droughts, we haven't seen in the West or really anywhere in the US in this century or the last."

Now, with a little help from global warming, that's likelier to change.

"Decadal droughts, if we look in the past, tend to happen once or twice per century, so that looks down do a 50 percent risk in any given half-century," Ault said. "So, if we didn't change the climate, we could say with a lot of confidence that the risk is about 50 percent. But with climate change, that risk looks like it's closer to 80 percent."

Climate change, then, increases the risk of longterm drought events—and very longterm drought events that span 35 years or more. Ault calls these events "multidecadal megadroughts." 

The risk of decade-long drought throughout the US, according to Ault's models.

"Multidecadal megadrought is extremely rare, happening maybe once every 500 or 1000 years," he said. "But, with climate change, the risk of that kind of event goes up to 20, 30, perhaps even 50 percent, in some parts of the Southwest. So, just changing the average background conditions seems to elevate the risk for a long drought substantially." 

Those background conditions involve an intensifying of the hydrological cycle as a result of climate change—it's one of the great paradoxical curses of climate change that, essentially, dry regions grow drier, and wet ones get wetter.

"Most of the deserts of the world are in this band of air that is already subsiding, and during climate change because of human activity, you increase the intensity of that cycle," Ault said, "which dries out the deserts and really dries out the edges of those regions, like the US Southwest, which are already on the edge of this drying, really see the effects of climate change in their local hydrological cycle."

Other scientists have speculated that the Southwest may actually already be in the midst of a 13 year megadrought, though thus far it doesn't meet Ault's criteria. If it continues, however, it very well may. To explain the possibility that the US may fall victim to megadrought, he enlists a metaphor popularized by the pioneering climate scientist James Hansen, that of 'loaded' dice. 

"I think the dice are loaded now. We're moving in to a century where these types of prolonged events are expected to become more frequent and more likely, even if we don't see this once evolve into a megadrought, the risk is still higher than it was a century ago," he said. "I think that is really important, because it gives us a sneak preview of the future."

Even if the droughts currently sweeping the Southwest weren't caused by climate change, he says, they're exactly what we should expect to see more of down the line. We're bearing witness to the serious ramifications of such drought right now: Water shortages and wildfires. Agriculture suffers. It's so dry the ground is literally rising

Some have become so desperate they're turning to water witches and dowsers in last ditch efforts to score some water. If droughts become megadroughts, they may turn to riskier alternatives, like geoengineering and cloud seeding.

If anything, Ault said, his work shows that we need to start planning for a water-starved century.

"We can start using these extreme events, extreme weather that's happening right now, to get ready for what's going to happen in the future," he said. "We shouldn't ignore this stuff and pretend that it's going to go away."