A warning from ancient tree rings: The Americas are prone to droughts

[sman]

A warning from ancient tree rings: The Americas are prone to catastrophic, simultaneous droughts

"https://www.sciencemag.org/news/2019/12/warning-ancient-tree-rings-americas-are-prone-catastrophic-simultaneous-droughts"

For 10 years, central Chile has been gripped by unrelenting drought. With 30% less rainfall than normal, verdant landscapes have withered, reservoirs are low, and more than 100,000 farm animals have died. The dry spell has lasted so long that researchers are calling it a “megadrought,” rivaling dry stretches centuries ago. It’s not so different from the decadelong drought that California, some 8000 kilometers away, endured until this year.

By analyzing tree ring records, scientists have now found evidence that such tandem droughts are more than a coincidence: They are surprisingly common over the past 1200 years, and they may often share a common cause—an abnormally cool state of the eastern Pacific Ocean known as La Niña. “We did not expect there to be as much coherence as we see,” says Nathan Steiger, a paleoclimatologist at Columbia University who presented the work this month at a meeting of the American Geophysical Union. “They just happen together.” The results suggest that, in the future, extreme aridity could strike all along the Americas’ western coast.

Evidence for synchronous, hemispherewide droughts first emerged in a 1994 study in Nature, which documented dead tree stumps in the middle of lakes and rivers in both Patagonia and California’s Sierra Nevada. For trees to grow in stream- and lakebeds, the droughts must have lasted for decades, and at least one of these megadroughts seemed to have hit both continents simultaneously.