The Southwest United States is currently facing its worst megadrought of the past 1,200 years. According to a recent study from The University of Texas at Austin, the drought could continue at least until the end of the century, if not longer.
Although some scientists anticipate that natural climate variability will bring relief, new research suggests that ongoing warming could be disrupting the natural rhythm of an important climate cycle that brings needed rain in the region.
Much like the seven-year El Niño and La Niña climate patterns, the Pacific Decadal Oscillation (PDO) has been a dependable ocean climate cycle that alternately brings long phases of drought and rains to the Southwest U.S. every 20 to 30 years. However, a study published in Nature Geoscience that analyzed the area's climate record going back for millennia suggests that this is not necessarily the case.
Researchers found that during the last period of hemispheric warming some 6,000 years ago, the Pacific Decadal Oscillation was forced out of rhythm, leading to a drought that lasted for thousands of years. Now, as the world warms under the effects of climate change, it appears to be happening again. Researchers came to this conclusion when they compared simulations of the ancient warming with climate projections for the future.
The research was led by doctoral student Victoria Todd and Associate Professor Timothy Shanahan at UT's Jackson School of Geosciences. They began by digging into the past — literally. Todd analyzed lake sediment cores collected in the Rocky Mountains, which preserve traces of ancient climate conditions. What she found was striking: about 6,000 to 9,000 years ago, the region went through a major drought, far worse than scientists had previously assumed.
By studying the chemical makeup of the sediments, Todd estimated that precipitation dropped by around 20%, mostly due to a sharp decline in the winter precipitation — the kind that feeds major rivers like the Colorado that runs through Arizona and Nevada. To break this down, she built a statistical model to separate the impacts of summer and winter moisture.
But the size of the drought didn't match what existing climate models predicted for the mid-Holocene. That changed when her coauthors ran simulations that included ancient shifts in vegetation across the globe. This tweak to the models showed a much drier Southwest — aligning closer with Todd's findings.
This swell of plant growth across North Africa, Canada and Eurasia darkened the Earth's surface, causing it to absorb more of the sun's rays, warming the Earth. This appears to have triggered a long-lasting shift in ocean and atmospheric patterns over the North Pacific, resembling the drought phase of the Pacific Decadal Oscillation — the cause of the current megadrought in the Southwest. Except that in the past this drought didn't stick around for 20 or 30 years — it dominated for thousands of years.
"This is a drought related to the winter precipitation collapse," Todd said. "Then the question is well, 'What causes the precipitation to change?' That's where the PDO comes in."
By definition, the Pacific Decadal Oscillation is a naturally occurring cycle that flips every 20 to 30 years, bringing shifts in precipitation across the western U.S. But the study shows that under certain warming conditions, the atmosphere and ocean can lock into a persistent drought-phase — and stay there.
To test if something similar could happen in the future, Todd and Shanahan teamed up with colleagues from the University of Colorado to explore climate model projections for the 21st century. When they averaged results from large sets of simulations, they saw the same kind of long-term North Pacific response and steady declines in winter precipitation.
"If global temperatures keep rising, our models suggest the Southwest could remain in a drought-dominated regime through at least 2100," Shanahan said.
That has major implications for water resources — especially the Colorado River in the Southwest, which has already been in steady decline for decades.
"Many people still expect the Colorado River to bounce back," Shanahan said. "But our findings suggest it may not. Water managers need to start planning for the possibility that this drought isn't just a rough patch — it could be the new reality."
