Drought has scorched western North America for the better part of two decades, withering crops, draining rivers and fueling fires. Scientists now warn that this trend could be just the beginning of an extended megadrought that ranks among the very worst of the past 1,200 years and would be unlike anything known in recorded history.
As with past megadroughts, the current event is driven largely by natural variations in climate. But unlike prehistoric megadroughts, it’s happening during an era of climate change that the authors say is responsible for nearly half of its destructive impact.
“No matter which way you slice it, the clear indication is that the current drought ranks right up there with the worst in more than a thousand years, and there’s a human influence on this of at least 30 percent and possibly as much as 50 percent in terms of its severity,” says Jason Smerdon, a paleoclimatologist at Columbia University’s Lamont Doherty Earth Observatory who co-authored the new research published today in Science. “It would have been a bad drought without anthropogenic warming, but not a contender to rival these really heavyweight droughts that occurred during the Medieval Era.”
Megadroughts, by definition, are occasional events of unusual severity lasting for at least 20 years. During the past 1,200 years, four major megadroughts occurred in the American West: during the 800s, the mid-1100s, the 1200s, and the late 1500s.
Some evidence suggests these events upended life in the West. For example, no one is certain what circumstances led the Anasazi people to abandon their cliff dwellings at Chaco Canyon during the 12th century and Mesa Verde during the late 13th century, but researchers have long theorized that megadroughts corresponding to those periods drove their inhabitants to seek reliable sources of water. The worst known drought of the entire 1,200-year period, in the 16th century, may have helped to amplify the devastating epidemics of cocoliztli in Mexico, which killed perhaps half of the indigenous population. Theories suggest drought weakened a malnourished population, or that conditions became ideal for the disease to spread widely among rodent hosts.
"There's always been the prospect that by chance we could have one of these droughts in the West, but we haven't had one since the late 1500s," Smerdon says.
The evidence was already alarming. A 2016 study by some of the same researchers tried to model the probabilities that a megadrought of 35 years or longer would occur by 2100 if global climate change continued unabated, and put that probability at 90 percent.
Now the new research reveals that the period of drought between 2000 and 2018 was the second driest of all 19-year periods in the past 1,200 years. “Suddenly, looking at the data since 2000, they’re definitely suggesting that we are currently on a megadrought trajectory,” Smerdon says. And while 20 years is a long time to live with drought, the megadroughts recorded in the paleorecord lasted far longer, like 50 or even 90 years.
An extended megadrought isn’t inevitable. Complex climate variations that brought some wetter years during the past two decades, and that ended past megadrought events, could reemerge. For example, La Niña conditions, when the Pacific Ocean cools, tend to correlate with big droughts in the American West by pushing storms north of the region. Warm-water El Niño conditions can bring precipitation and drought relief. But the warmer temperatures brought on by climate change make it all the more difficult for a drought to dissipate naturally.
“The fact that the climate system was capable of producing those droughts in the past provides pretty strong evidence that similar droughts could occur in the future,” says Connie Woodhouse, a climate scientist at the University of Arizona not involved in the research. “However, with the increasing temperatures, the impacts of future droughts will be greater than those that occurred under cooler temperatures.” In fact, she notes, the new study shows that this drought wouldn’t have been nearly as bad if not for anthropogenic climate change.
Since the early 20th century, scientists have known that ancient trees hold clues to past climate. Good years are reflected in wide growth rings, while narrow rings mark lean and dry years. Clusters of narrow rings show prolonged periods of drought.
The study’s lead author, Park Williams, a bioclimatologist at Columbia University’s Lamont Doherty Earth Observatory, says that when scientists first analyzed rings, they were shocked at the tale the trees told. “These megadroughts looked nothing like what modern society in the 1920s had ever dealt with. At that point these Medieval megadroughts started being talked about almost mythically,” he says.
Through the 1990s, Williams says, scientists and water managers tended to talk of these droughts only as remarkable events from prehistory because nothing like then had ever been seen in modern times. “The conclusion now, that we may be actually converging on one of these events, is really something else.”
Williams’s group reached its conclusion by poring over thousands of tree and wood samples from across the region, from Oregon and Montana to Mexico. They reconstructed a record of drought conditions from the year 800 A.D. to the present and compared the current drought to the worst 19-year periods within that long historic record. The current drought hasn’t persisted as long as the notable megadroughts, one of which stretched over almost the entire 13th century. However, Smerdon says, “this particular drought could go toe to toe with the worst megadroughts of the past over any 19-year interval that we were able to characterize.”
The team employed 31 climate models to estimate how evidence from the past, combined with the facts of the current drought, might translate into future projections. They conclude that the biggest factor in amplifying the current drought into a megadrought of historic (or prehistoric) significance is a warming Earth. Using 120 years of weather data and 31 different climate models the study suggests that the region’s average temperature has risen over the past two decades by 2.2 F (1.2C) compared to what would have been likely without anthropomorphic warming. Warmer temperatures mean more evaporation, which reduces soil moisture levels and exacerbates drought.
Jonathan Overpeck, a climate scientist at the University of Michigan not involved in the study, lauds the group’s work on this front. “They document the impact that anthropogenic climate change has played in amplifying what might have been a modest drought into what instead has become the first true multi-decadal megadrought to hit the United States.”
Overpeck adds that while the study period has ended, the drought continues. The nation’s two biggest reservoirs, Lake Powell (Utah and Arizona) and Lake Mead (Nevada and Arizona), remain less than half-full. He says the study highlights that soil moisture, like river flows, is dramatically declining in the Southwest. “This new work makes clear that if climate change is left unchecked, a large region of the country will continue to be slammed by ever-worsening droughts into the future,” Overpeck says.
Williams says we still can’t be certain exactly where today’s drought ranks among the millennium’s very worst. But to debate that matter would be to miss the point.
“There’s no getting away from the basic conclusion that this drought that we’re in now is definitely contending, in severity, to be one of the worst megadroughts of the last millennium—and climate change did contribute in some important way to making it worse.”