The last 25 years have been the driest in centuries in the American West

Three years ago, climate researchers shocked drought-weary Californians when they revealed that the American West was experiencing its driest 22-year period in 1,200 years — and that this severe megadrought was being intensified by global warming.

Now a UCLA climate scientist has reexamined the data and found that even after two wet winters, the past 25 years are likely still the driest quarter-century since A.D. 800.

“Dryness still trumps humidity, by far,” said UCLA professor Park Williams.

The latest climate data show that the years since 2000 in western North America (from Montana to California and northern Mexico) have been slightly drier on average than a similar megadrought in the late 16th century.

Williams shared her findings with the Los Angeles Times, providing an update to her widely cited 2022 report. studywhich he co-wrote with scientists at Columbia University's Lamont-Doherty Earth Observatory.

The new findings reveal that even the unusually wet conditions that have drenched the West since early 2023 pale in comparison to the long stretch of mostly dry years over the previous 23 years.

And that drought hasn’t been caused by natural cycles alone. Williams and his colleagues have estimated that a significant portion of the drought’s severity — about 40 percent — is attributable to warming from the burning of fossil fuels and rising levels of greenhouse gases. The warming that has occurred in the region — an increase of more than 2.5 degrees Fahrenheit since record-keeping began more than a century ago — has intensified dry conditions, making the latest megadrought significantly more severe than it would be without climate change.

But are we still in a megadrought? How will we know when it is over?

Williams said those questions will take some time to answer and that conclusions will only become clearer in retrospect.

“Based on the definition of megadrought that we have been using, which involves looking at the past 10 years to see if dry or wet conditions prevailed, we can only see the end of a megadrought in retrospect,” Williams said. “If the next few years are on average wet, that will mark the end of the megadrought. If they are dry, the megadrought will continue.”

A boat sails through Lake Powell on the Arizona-Utah border in 2021. The reservoir, the second-largest on the Colorado River, has declined dramatically over the past 25 years.

(Luis Sinco/Los Angeles Times)

Williams and his colleagues track drought severity using a 10-year moving average of summer soil moisture across western North America.

They compare this century's drought and other megadroughts using ancient records captured in tree growth rings. Wood cores taken from thousands of trees provide data from about 1,600 sites across the region, allowing scientists to reconstruct soil moisture centuries ago.

A comparable megadrought occurred between 1571 and 1593, and ended after 23 years. Williams said his latest review of data through June shows the past 25 years, compared with the late 16th century, have been “slightly drier.”

“It’s important to recognize that even the megadroughts we looked at in our tree-ring reconstruction had extremely wet years, wet years like 2023,” Williams said. “Megadroughts can have short breaks.”

Whether this megadrought will continue or subside will become clear over the next year or two, he said.

If wetter-than-average conditions continue, he said, it could be the case that the megadrought is already over after 23 years by 2023. On the other hand, it could be the case that the rest of 2024 turns out to be drier than average and is followed by more dry years, in which case the megadrought would still be ongoing.

A visitor looks out over the Colorado River and Canyonlands National Park at Dead Horse Point near Moab, Utah.

(Luis Sinco/Los Angeles Times)

Williams said his research shows that much of the drought severity has been driven by the West's extreme natural variability, which he likens to a yo-yo swinging from wet to dry. But these variations are now overlaid on a drying trend with climate change, he said, a “shifting baseline” that is making droughts more severe and longer lasting.

Williams said the megadrought since 2000 would likely not be comparable to the long droughts of centuries ago if not for warmer temperatures brought on by human-caused climate change.

“We don’t know whether the next 10 years will be a sequence of good luck or bad luck,” Williams said. “But we do know, based on climate models, math and logic, that as the atmosphere continues to warm, the odds of the next 10 years being drier than average will be greater than they were in the past century.”

Scientists and policy experts widely agree that adapting to climate-change-driven aridification in the Western United States will require major changes in how limited water supplies are managed for farms, cities, and the environment.

“Regardless of what happens in the next few years, which will depend largely on the randomness of the climate, as the atmosphere continues to warm, we should expect our water supply to continue to degrade,” Williams said. “A warmer atmosphere is a thirstier atmosphere, and without a compensating increase in precipitation — which hasn’t happened — humans and ecosystems will be left with less water.”

This will require continued focus on curbing excessive and unsustainable use of river and aquifer water, he said. “Even during periods of good fortune and wetness, we cannot forget that the long-term average tends to be drier.”

Williams said the data suggests that by 2100, the region will likely have experienced one or two additional megadroughts, which could be even more severe.

Looking ahead, however, the biggest source of uncertainty in climate projections is how people will respond to addressing climate change.

“We who burn fossil fuels have enormous ability to control the climate for the rest of this century. The climate of the 2090s is very sensitive to what we do with fossil fuels over the next 20 to 30 years,” Williams said. “We need to reduce carbon emissions to stabilize the climate.”