California megadroughts vanished after 1600 but may return soon, study finds

Jared Gilmour

For centuries, California and the rest of the southwestern United States have been spared the multi-decade megadroughts that afflicted the region before 1600. But the Southwest’s lucky streak could soon run out, a new study finds.

Scientists have discovered that unusually cold Pacific Ocean surface temperatures drove the megadroughts that plagued the region in the medieval era — meaning that in future La Niña cold temperature cycles, multi-decade megadroughts could return to California as climate change makes the Southwest more arid, according to findings published Wednesday in Science Advances.

“Because you increase the baseline aridity, in the future when you have a big La Niña, or several of them in a row, it could lead to megadroughts in the American West,” Nathan Steiger, a Columbia University hydroclimatologist and the study’s lead author, said in a statement released by the school.

The researchers wrote that their findings are “the first paleoclimatic support for the prediction that the risk of American Southwest megadroughts will markedly increase with global warming.”

Currently, none of the Golden State is in drought — and the only place listed as so much as “abnormally dry” is the area around San Diego, according to the state’s drought monitor.

But that’s a marked shift from a few years ago, when drought plagued California and much of the American West. By 2013, the Associated Press reported (in an article entitled “Is the West’s dry spell really a megadrought?”) that the West had been mired in drought for 13 years, and that research at an American Geophysical Union meeting revealed the “likelihood is high that this century could see a multi-decade dry spell like nothing else seen for 1,000 years.”

Recent rains ended California’s more than 10-year drought, but that doesn’t mean it couldn’t come roaring back and still be considered part of a broader megadrought, Mashable reports.

“If the next five years are dry again then you can treat it as one 24-year drought,” said Flavio Lehner, a National Center for Atmospheric Research climate scientist who wasn’t involved in the study, according to Mashable.

Steiger and his team completed the new study by reconstructing aquatic climate data and ocean surface temperatures from the last two millennia. That helped the researchers pinpoint three causes of the region’s string of megadroughts from 800 and 1600 — first, that the earth was absorbing more heat than it reflected back to space; second, that cool Pacific La Niña events were more common and stronger; and third, that conditions were warm in the Atlantic Ocean, according to the Columbia news release on their findings.

Scientists estimate centuries-old weather and climate by looking at proxy data in lake sediments, tree rings and more, USA Today reports.

“Having paleoclimatic evidence shows you what happened in the past,” Steiger said, according to VICE. “It helps verify projections that say the American Southwest is almost assured to have a megadrought in the next few decades.”

Predicting exactly when those decades-long dry spells will strike is more tricky.

“It certainly looks like with warmer temperatures that we’re going to see more megadroughts,” said Peter Fawcett, a University of New Mexico paleoclimatologist uninvolved in the study, according to National Geographic. “But we can’t predict precisely how many or how big they’ll be.”