The West’s Megadrought Might Not Let Up for Decades, Study Suggests. Clues from another dry spell 6,000 years ago are helping scientists understand what’s driving the latest one, and why it’s been so unrelenting.

 

Lake Powell in Glen Canyon National Recreation Area in Arizona, where July water levels are only at a third of capacity.Credit...Rebecca Noble/Getty Images

 

By Raymond Zhong

 

 

A megadrought has sapped water supplies, ravaged farms and ranches, and fueled wildfires across the American Southwest for going on 25 years. Not in 12 centuries has the region been so dry for so long.

Now comes worse news: Relief might still be decades away.

According to new findings published in the journal Nature Geoscience, the dry spell is no mere bout of bad luck, no rough patch that could end anytime soon.

Instead, it seems to be the result of a pattern of Pacific Ocean temperatures that is “stuck” because of global warming, said Victoria Todd, a doctoral student in paleoclimatology at the University of Texas at Austin who led the new research.

That means the drought could continue through 2050, perhaps even 2100 and beyond — effectively, Ms. Todd said, for as long as humans keep heating up the planet.

 Even in the arid Southwest, the long, chronic deficit of moisture since the turn of the millennium has exacted a heavy toll. The possibility of more parched decades ahead raises big concerns in a fast-growing region where agriculture and other industries, including computer-chip manufacturing, use lots of water.

 In their study, Ms. Todd and her colleagues set out to understand a different dry period in the region’s deep past. For clues, they looked to mud from the bottoms of two lakes in the Rocky Mountains: Stewart Bog in New Mexico and Hunters Lake in Colorado.

 

The waxy coating on a plant’s leaves preserves a chemical signature of the rain and snow that the plant absorbs. So by analyzing the vegetal remains that had accumulated on the lake beds and become entombed in layers of sediment, Ms. Todd and her colleagues reconstructed how wet the Rockies had been over the past 14 millenniums. They found that winters were dry for thousands of years in the middle of this period.

Scientists have long known that those were warm years for the planet. Earth’s orbit was in a phase that caused more solar radiation to reach the Northern Hemisphere in summer. The radiation melted Arctic sea ice and caused vegetation to flourish in Siberia and the Sahara. These changes darkened the planet’s surface and caused it to absorb more sun, raising temperatures further.

Ms. Todd and her colleagues ran computer simulations of the prehistoric climate during this warm time to see what might have led to such a severe drought in the Southwest. They found that the extra heat gave rise to something striking in the Pacific: a giant blob of warm water extending east from Japan and surrounded on three sides by cool water, including along the West Coast of the United States. The warm blob shifted the band of winds known as the jet stream and deflected storms away from the Southwest.

This kind of pattern isn’t unusual in and of itself: Today, it emerges in the northern Pacific every few decades, alternating with a cold blob that has the opposite effect, namely making the Southwest wetter.

But in the warm world of 6,000 years ago, the blob didn’t alternate, according to Ms. Todd and her colleagues’ simulations. It stayed put, drying out the Southwest for thousands of years.

And, when Ms. Todd and her colleagues ran simulations of the present-day climate, they found that the blob might be stuck in place again — only this time, it appears to be because humans are changing the atmosphere by burning coal, oil and gas.

A. Park Williams, a climate scientist at the University of California, Los Angeles, who researches water in the West, called the new study “thorough” and “convincing.” Still, he noted that researchers’ computer models underestimated how badly the warm blob — or, as scientists prefer to call it, the negative phase of the Pacific Decadal Oscillation — can dry out the Southwest. That means projections of future drought risk in the region are probably underestimates as well, Dr. Williams said.

Human-caused warming is creating conditions that can worsen droughts in many parts of the globe. The warmer air pulls more water out of the soil and vegetation. It causes more precipitation to fall as rain rather than accumulate in the mountains as snow. In the American Southwest, these factors come on top of natural climate fluctuations that have long shaped water availability.

Even so, events like the megadrought raise the possibility that greenhouse warming is starting to overpower certain well-established rhythms and patterns in nature, said Pedro DiNezio, a climate scientist at the University of Colorado Boulder who contributed to the new study.

For instance, El Niño, the cyclical temperature pattern in the tropical Pacific Ocean, typically leads to wetter winters in the Southwest. But that wasn’t the case during the most recent El Niño, from 2023 to last year.

“All these trends are starting to emerge recently that are very unlikely within our understanding of the climate system,” Dr. DiNezio said. These trends start to make sense, he said, only once you account for how much humans are now influencing the climate.