Scientists had already found that the Southwestern United States were at great risk of experiencing a significant megadrought (in this case meaning drought conditions that last for over 35 years) before the end of the 21st century. But a new study published in Science Advances added some grim context to those predictions.

Columbia University climate scientists Jason Smerdon and Benjamin Cook, and Cornell University's Toby Ault were co-authors on the study. They took data from tree rings and other environmental records of climate from the Southwest and compared them to the projections of 17 different climate models that look at precipitation and soil moisture. When they made the comparison between past and future, they found that all the models agreed: the next big megadrought is coming, and it will be way worse than anything we've seen in over 1,000 years--including droughts that have been credited with wiping out civilizations.

"We are the first to do this kind of quantitative comparison between the projections and the distant past, and the story is a bit bleak," Smerdon said in a press release. "Even when selecting for the worst megadrought-dominated period, the 21st century projections make the megadroughts seem like quaint walks through the Garden of Eden."

Those past megadroughts hit what is now the Southwestern United States particularly hard in the late 13th century, and are widely attributed to the decline of the Ancestral Pueblo culture. While those droughts were bad enough, they were caused by natural climate variations that evened out over time. The climate models that the researchers looked at show that unlike the past megadroughts, increasing temperatures due to higher levels of greenhouse gases mean that megadroughts of the future will be way worse—and could continue for a very long time.

“My feeling is that it is a shift towards a new normal,” Smerdon told Popular Science. “The statistics of drought in the future are towards a more arid state that is a consequence of these CO2 increases.”

The researchers used models that looked at a number of very complex systems. “These models are trying to simulate all the processes that we think are relevant in the climate system,” says Cook. Precipitation is incredibly important, but so is soil moisture, and temperature. The researchers found that for most of these areas, precipitation will decline during a megadrought. Weirdly, though, a few places might see more precipitation during a megadrought. Unfortunately, rising temperatures in those spots mean that even with the rain, moisture in the soil will be evaporated faster than it is now, and the overall effect will still tend towards drought.

So what is the future going to look like, other than very, very dry? Communities living in areas with consistently less water will have to adjust, and policymakers may have to take a hard look at how water is allocated in the future, particularly with regard to agricultural areas. After all, both plants and livestock need water, and, of course, humans need both food and water to survive.

“Water security is food security, and if we can manage water security then we can manage food security in the future,” Ault said in a press conference.

The one bright point in the study is that it predicts what will happen if greenhouse gas emissions continue on their current upward trend. But that isn't inevitable, says Smerdon. “The fact is that it's based on decisions we make in the future.”