The American West is well on its way into one of the worst mega-droughts on record, a new study warns, a dry period that could last for centuries and spread from Oregon and Montana, through the Four Corners and into West Texas and northern Mexico.

Several other mega-droughts, generally defined as dry periods that last 20 years or more, have been documented in the West going back to about 800 A.D. In the new study, the researchers, using an extensive tree-ring history, compared recent climate data with conditions during the historic mega-droughts.

They found that in this century, global warming is tipping the climate scale toward an unwelcome rerun, with dry conditions persisting far longer than at any other time since Europeans colonized and developed the region. The study was published online Thursday and appears in the April 17 issue of the journal Science.

Human-caused global warming is responsible for about half the severity of the emerging mega-drought in western North America, said Jason Smerdon, a climate researcher at Columbia University in New York and a co-author of the new study.

“What we’ve identified as the culprit is the increased drying from the warming. The reality is that the drying from global warming is going to continue,” he said. “We’re on a trajectory in keeping with the worst mega-droughts of the past millennia.”

The ancient droughts in the West were caused by natural climate cycles that shifted the path of snow and rainstorms. But human-caused global warming is responsible for about 47 percent of the severity of the 21st century drought by sucking moisture out of the soil and plants, the study found.

The regional drought caused by global warming is plain to see throughout the West in the United States. River flows are dwindling, reservoirs holding years worth of water supplies for cities and farms have emptied faster than a bathtub through an open drain, bugs and fires have destroyed millions of acres of forests, and dangerous dust storms are on the rise.

A similar scenario is unfolding in South America, especially in central Chile, a region with a climate similar to that in western North America. Parts of the Andes Mountains and foothills down to the coast have been parched by an unprecedented 10-year dry spell that has cut some river flows by up to 80 percent.

In both areas, research shows, global warming could make the droughts worse than any in at least several thousand years, drying up the ground and shifting regional weather patterns toward drier conditions.

This is bad news for modern civilizations that have developed in the last 500 years, during which they enjoyed an unusually stable and wet climate. Assumptions about water availability based on that era are not realistic, said climate scientist Edward Cook, another co-author of the study who works at Columbia University’s Lamont-Doherty Earth Observatory.

The impacts of a long-lasting drought in the West could affect adjacent regions. A 2019 study showed that dry conditions in upwind areas may be intensifying agricultural droughts. With west winds prevailing across North America, hot and dry conditions in the Southwest could reduce the amount of atmospheric moisture available to produce rainfall farther east, in Oklahoma and Texas, for example. The study found that such drought linkages accounted for 62 percent of the precipitation deficit during the 2012 Midwest drought.

In Chile, the current drought, with rainfall deficits of 20 to 40 percent, started in 2010, said René Garreaud, a climate researcher at the Universidad de Chile. Common threads run through the research on the two hemispheres because the climate system is globally linked. Large-scale changes in the tropical Pacific affect both regions, Garreaud said.

He added, “Southwestern North America and central Chile share a semi-arid, drought-prone climate. Our decade-long dry spell is unprecedented not only in the historical record but also in a millennial perspective, according to our tree-ring based rainfall reconstructions.”

The soil-drying effect of global warming is a factor in the Chilean mega-drought, but the biggest driver is a persistent lack of winter rainfall. In a 2019 study, Garreaud and other researchers found a clear global warming fingerprint on the extended dry weather pattern. The fingerprint leads to the southwestern Pacific Ocean, one of the fastest-warming ocean regions in the world.

That temperature increase may be shifting the path of rain and snowstorms away from central Chile. And human-caused global warming is also pushing the Antarctic version of the jet stream poleward, contributing to the intensity and longevity of Chile’s mega-drought, he said.

“All that has dramatic consequences in potable water provision for rural areas, but not so much for large cities, creating some social conflicts at the local level. Less water in the rivers also mean less nutrients reaching the ocean,” which could affect important coastal fisheries, he said, noting a 2018 study that explored links between drought and ocean ecosystems. Like the North American West, Chile has also seen extreme wildfires during the current drought.

In North and in South America, researchers have identified natural climate cycles as key drivers of historic mega-droughts. The most important are a combination of a warm North Atlantic Ocean and cooler-than average conditions in the eastern tropical Pacific Ocean, as well as decreased solar and volcanic activity.

“Arid periods over the last several millennia have dwarfed anything we’ve seen so far,” Smerdon said. And when the soil-drying effect of human-caused warming is added into the climate equation, the outlook is not good. Previous studies by Columbia University researchers predicted that the 21st century has a 90 percent chance of seeing a drought that lasts 25 years or longer.

He said that prospect will require people to rethink how to manage resources.

“On a regional level, this means being more proactive about water management,” Smerdon said. “There are things we can do if you recognize that the West will probably be much drier. You can start thinking about transitioning to less water intensive crops, or about beef production, which is incredibly water intensive.”

Other features “that go part and parcel with these droughts are things like forest fires and beetle infestations,” he added, noting that there were also impacts to winter recreation and tourism, with less snow for skiing and water for rafting.

Smerdon said he’s also concerned that the drought impacts are being underestimated because of an over-reliance on groundwater as a temporary buffer to the decline of river flows, and the drop of reservoir water levels. If you look at simultaneous droughts in North and South America, he said, you could also anticipate potential impacts to global food supply networks, as both regions are important for agricultural production.

The only real long-term solution is to halt greenhouse gas pollution, he said.

“It’s like with the coronavirus pandemic, we have to flatten the curve of global warming. We do that by removing the emissions.”

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