Life in the American Southwest may not be as harsh as life in the Sahara Desert, but water is still scarce. For a growing population, maintaining a water supply with some semblance of sustainability while balancing the various legal rights to the water that is there is hard enough without the unpredictability of year-to-year weather. A drying climate in the Southwest over the coming decades is the last thing anybody needs, but there have been some hints that's what is in store.

Projecting climate change for regions is necessarily less certain than global projections, and the Southwest is no exception. While expecting increasing temperatures is pretty straightforward, climate models simulate a range of possible precipitation changes in the area. This has complicated research into the likelihood of a future “megadrought”—droughts as severe as any we’ve seen in the 20th Century, but lasting a few decades without respite.

A previous study of precipitation estimated a 20 to 50 percent increase in the risk of a megadrought this century if greenhouse gas emissions continue unabated, while another study that focused on soil moisture estimated a much higher risk. That's because precipitation alone can’t tell the whole story here—a warming atmosphere acts like an ever-thirstier sponge, increasing evaporation and transpiration from plants.

Some of the researchers behind those previous studies, led by Cornell’s Toby Ault, found a better way to attack the question. Instead of relying on a limited number of model simulations to calculate probabilities, they generated many randomized, virtual timelines based on tweaking the statistics of historical conditions. This new approach allowed them to calculate meaningful probabilities for the occurrence of megadroughts for any given change in the mean or variability of temperature and precipitation.

That creates a backdrop of hypotheticals—you can basically look up the change in megadrought risk for any possible change in temperature or rainfall. The researchers then laid a large number of climate model simulations of the coming century on top of this backdrop. For example, simulations that produced 4 degrees Celsius warming this century, or a five percent decrease in rainfall, can be seen to correlate with a probability of megadrought.

With rising temps, rising evaporation

Overall, the results are alarming. It seems the Southwest is already near the knife’s edge—it has seen multi-decade droughts in the past, after all. For a business-as-usual emissions scenario, the second half of the 21st century averages 4.5 degrees Celsius warmer than the second half of the 20th Century in the Southwest. If there is no change in precipitation, that equates to about a 90 percent chance of experiencing a megadrought this century.

That's in part because changes in the average of temperature or precipitation were much more important than changes in the variability. Greater variability can keep you out of a multi-decade megadrought by inserting a wet year here and there, but averages have the biggest say over whether you're in a drought in the first place.

Even if precipitation were to increase 10 percent along with the temperature rise, we would still be looking at 70 percent odds of a megadrought. For a decrease in rainfall of a similar magnitude—the most common result in climate model simulations—the risk rises nearly to 100 percent.

It’s also instructive to look at other emissions scenarios where we actually take action to cut greenhouse gas emissions. The most optimistic scenario in the last IPCC report involved drastic actions to limit global warming to below 2 degrees Celsius. Even this change puts the Southwest at much greater risk for megadrought, though.

Only the simulations with the greatest precipitation increase keep the risk down to less than 10 percent—roughly the per-century probability we've seen over the past thousand years. At the other end, the greatest precipitation decreases reach nearly certain odds of a megadrought. Still, it’s a better outlook than you get with higher temperatures.

Drought changes can be difficult to project in areas where such predictions depend on weather systems. Shifts in weather patterns or the statistics of precipitation can be complex—but the fact that evaporation increases with rising temperatures is simple. That’s what controls the story here. Even the substantial uncertainty about precipitation trends in this area can’t do much to muddy the effects of warming on evaporation. As a result, the hotter it gets, the worse the droughts will be in the Southwest.

The researchers note that there are obviously many ways people in the region can adapt to use less water or access new sources in the face of greater drought risk. “However,” they add, “the feasibility, sustainability, and implementation of these measures and the extent to which they could reduce megadrought risk remain critical open questions.

“Historically, megadroughts were extremely rare phenomena occurring only once or twice per millennium,” they write. “According to our analysis of modeled responses to increased [greenhouse gases], these events could become commonplace if climate change goes unabated.”

Open Access at Science Advances, 2016. DOI: 10.1126/sciadv.1600873 (About DOIs).