California, Here Dry Come!

California’s current three-year drought seems terrible, but everything is relative. The moist, lush California we’re used to represents a moment in time—a golden age for the Golden State. A millennium ago, its droughts were longer, drier, and more damaging than anything the United States has seen in its short lifetime. And those massive droughts are about to come back.

According to a study released yesterday in the journal Science Advances, the coming decades will bring droughts that are significantly worse than the so-called megadroughts of the Medieval Warm Period (900–1300 CE). You can call the coming disasters gigadroughts or teradroughts, in honor of Silicon Valley. Whatever you call them, though, California and the rest of the Southwest is about to get parched.

We need some perspective to understand what’s about to happen. Many reporters refer to the current California drought as the worst on record, but that's not exactly true. We have excellent evidence from tree rings, bones, and dead vegetation of the droughts that baked the Southwest and Central Plains during the Medieval Warm Period. Why don’t those count in the record-keeping?

In years when the soil is dry, trees grow slowly, so their annual rings are narrower. By examining cross sections of long-lived trees, climatologists can construct a reliable record of precipitation patterns. Other scientists have used carbon dating to trace tree stumps found underwater or in western seasonal flood zones to the Medieval Warm Period, suggesting that those areas were much drier at the time. The award for most creative approach goes to Vanderbilt University's Tom Dillehay, whose novel approach of counting up bones proved that bison were significantly less plentiful during the droughts that prevailed a millennium ago.

These megadroughts had extreme effects on Native Americans. Economies slowed, as seen through reduced obsidian mining and long-distance trade. Without adequate water supplies, the region's cliff cities, whose residents relied on irrigated farming, were abandoned. The droughts likely increased intertribal warfare and may have eventually brought down the ancient southwestern Pueblo civilization.

These graphs illustrate the magnitude of the medieval droughts. The red, orange, and yellow bars indicate lower-than-normal moisture. Notice in the left column that medieval Americans (I realize they didn’t call themselves that) suffered through decades of barely interrupted drought. PDSI, by the way, stands for the Palmer Drought Severity Index (no relation). The bar graphs on the right represent the most significant droughts of the past two centuries. They are shorter and less severe, and most are regularly interrupted by wet years.

We’re about to go back to the future. A team of climate scientists, led by Benjamin Cook of NASA and Columbia University, ran computer models to determine how carbon pollution would affect drought severity and intensity in the Southwest. They saw drought, drought, and more drought.

“I was honestly surprised at just how dry the future is likely to be,” said coauthor Toby Ault of Cornell University.

You know a result is bad when it surprises a climatologist—they spend their careers trying to warn us about how awful the climate is about to get. Here’s the key graph from their findings:

In the years between 1000 and 1300 C.E., see how the graph spends most of the time slightly below the moisture balance line? Those are the megadroughts of the Medieval Warm Period—the ones that destroyed entire civilizations. The right side of the graphs—where the line nosedives—shows the models that Cook and his colleagues created for rest of the 21st century. They all point to a future significantly drier than anything we’ve seen in the past millennium. When those droughts come, they really will be the worst on record.

This is just one study, and it does contrast somewhat with the work of the Intergovernmental Panel on Climate Change, which found that drought predictions were difficult to make with certainty. However, Cook and his team ran multiple models and tested the results by tinkering with their assumptions. The data are remarkably consistent.

“I look at these future megadroughts like a slow-moving natural disaster,” says Ault. “We have to put megadroughts into the same category as other natural disasters that can be dealt with through risk management.”

Risk management? Hmm. Hey, California, how open are you to relocation?