Southwestern Drought Likely to Continue Through 2100, Research Finds

Climate change is warming the North Pacific Ocean, leading weather patterns that drive drought in the U.S. Southwest to persist decades longer than they have in the recent past.

The drought in the Southwestern U.S. is likely to last for the rest of the 21st century and potentially beyond as global warming shifts the distribution of heat in the Pacific Ocean, according to a study published last week led by researchers at the University of Texas at Austin.

Using sediment cores collected in the Rocky Mountains, paleoclimatology records and climate models, the researchers found warming driven by greenhouse gas emissions can alter patterns of atmospheric and marine heat in the North Pacific Ocean in a way resembling what’s known as the negative phase of the Pacific Decadal Oscillation (PDO), fluctuations in sea surface temperatures that result in decreased winter precipitation in the American Southwest. But in this case, the phenomenon can last far longer than the usual 30-year cycle of the PDO.

“If the sea surface temperature patterns in the North Pacific were just the result of processes related to stochastic [random] variability in the past decade or two, we would have just been extremely unlucky, like a really bad roll of the dice,” said Victoria Todd, the lead author of the study and a Ph.D student in geosciences at UT Austin. “But if, as we hypothesize, this is a forced change in the sea surface temperatures in the North Pacific, this will be sustained into the future, and we need to start looking at this as a shift, instead of just the result of bad luck.”

Currently, the Southwestern U.S. is experiencing a megadrought resulting in the aridification of the landscape, a decades-long drying of the region brought on by climate change and the overconsumption of the region’s water. That’s led to major rivers and their basins, such as the Colorado and Rio Grande rivers, seeing reduced flows and a decline of the water stored in underground aquifers, which is forcing states and communities to reckon with a sharply reduced water supply. Farmers have cut back on the amount of water they use. Cities are searching for new water supplies. And states, tribes and federal agencies are engaging in tense negotiations over how to manage declining resources like the Colorado River going forward. 

“Planners need to consider that this drought, these reductions in winter precipitation, are likely to continue, and plan for that,” said Tim Shanahan, an associate professor at UT Austin’s Jackson School of Geosciences and co-author of the study. 

The research began with decades-old sample cores taken from lakes in the Rocky Mountains. Using modern geochemical techniques, Todd was able analyze drought conditions during the mid-Holocene period 6,000 years ago, a period in Earth’s history when the Northern Pacific warmed and the Southwestern U.S. experienced hundreds of years of drought. 

But the sample cores suggest the drought was much worse than previously thought by scientists. Through a series of climate models, the researchers found vegetation change in the tropics darkened the Earth’s surface so that it absorbed more of the sun’s heat. That led to a warming of the North Pacific that was similar to the PDO that drives drought in the Southwest, but in this case, the drying lasted for centuries. “As soon as we saw that, you know, we started thinking about what’s happening today,” Todd said.

For the past 30 years, the PDO has been in its negative phase, which leads to drought in the Southwest by reducing winter precipitation and the runoff from mountain snowpack that fills many of the region’s rivers and recharges groundwater aquifers. 

Using an ensemble of historical and future climate models forecasting climate and precipitation patterns until 2100, they found the PDO-like negative phase continues through this century. But unlike the mid-Holocene period’s warming, which was brought on by vegetation change, today’s is driven by greenhouse gas emissions. Certain models revealed that the change in the ocean pattern was less about vegetation absorbing solar radiation, Todd said, and more about warming in general. 

The study also revealed that current climate models are underestimating drought conditions, Todd and Shanahan said, and they hope to find better ways to approximate aridity going forward.

Drought that continued until the end of the century would have major implications for water resources in the Southwest and how they are managed. The region currently sustains some of the country’s biggest cities and most productive agricultural areas. 

Brian Richter, president of the water research and education group Sustainable Waters and a water researcher not involved in the study, said the research further proves the drought in the Southwest is more intense than previously thought and is not going away any time soon.

“Doesn’t it suck that every time the science improves, the outlook for the climate and water looks worse?” he said. 

In many ways, Richter said, what people are seeing on the ground is outpacing science. Five years ago, he said, farmers would say they’ve been through droughts before, and this one would soon pass. Now, he said, their tone has changed to “This is a different kind of a drought.”

Cover photo:  A houseboat is docked on Lake Powell in the Glen Canyon National Recreation Area, as the critical Colorado River reservoir sits at only a third of its capacity on July 10 in Page, Ariz. Credit: Rebecca Noble/Getty Images