Now we know for sure: Human-caused warming played a critical role in creating the mega-drought that's been drying up the American Southwest for the last two decades.
The drought that struck the Southwestern U.S. from 2000 to 2019 was the second-driest period in the area since at least 800 C.E., reducing rivers to a trickle and causing widespread water rationing. That wasn't all because of man-made climate change, but a new paper published Thursday in the journal Science concludes that human activity played a significant role in making it worse.
Park Williams, a researcher with the Lamont-Doherty Earth Observatory at Columbia University and a lead author of the study, said researchers found human activity accounted for 47% of the drought's severity. That has potentially devastating implications for future dry spells.
Williams' paper is part of a larger Science special report on drought that conveys the costs of extreme lack of rainfall in both money and lives: 3,000 deaths and $250 billion in the U.S. alone between 1980 and 2020. In an overview of drought science, Toby Ault of Cornell University reaches back to the Old Testament ("the ground is chapt, for there was no rain in the earth") to illustrate what the near future might hold. And it will only get worse. "Droughts of the future may eclipse those of past centuries in their duration, severity, and frequency," he writes.
In the southwestern U.S., the severest and longest lasting droughts are believed to be linked to the oscillations of La Nina, the chillier ocean atmospheric occurrence that brings drier conditions to North America. The challenge tackled by researchers in the case of the most recent mega-drought was to distinguish the effects of human activity on weather patterns from weather patterns themselves.
By analyzing 1,200 years of tree-ring data, the researchers were able to estimate soil moisture levels during each of those years. Combined with hydrological modeling and statistical analysis, that enabled them to ascertain the lengths and causes of droughts in the American Southwest going back more than a millennium. The end result: they determined that what might have been a moderate dryness event caused by natural variability was instead made epic by human-caused climate change.
"This means warming continues to load the dice toward drought," says Williams. "The random good luck and bad luck with the natural variability of ocean currents is now superimposed on top of an ever dryer future."
While the American Southwest faced numerous episodes of dryness during the period they analyzed, the researchers found only four mega-droughts. The most devastating occurred in the 16th century and had profound and lasting effects on the region's ecology and inhabitants. David Stahle, a professor of geosciences at the University of Arkansas, credits a mega-drought, in part, with the abandonment of the Salinas Pueblo, now part of New Mexico and a "hemorrhagic fevers that killed half of the native population" of post-conquest Mexico.
Two other papers discuss the implications of ongoing droughts on trees and water systems. "Water insecurity," a problem that afflicts much of the developing world, is now on the rise in the United States, especially in rural and impoverished communities. Reduced water availability not only affects the quantity of water communities can rely on, but also the quality, as it exacerbates factors like saltwater intrusion and microbial growth. Forests, meanwhile, face a dire future. Many people -- scientists among them -- have argued that more CO2 in the atmosphere would benefit trees. But no arboreal species, the paper's authors conclude, can withstand desiccation, especially when combined with extreme heat.
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