2025.8.23.(六)Samedi 23 août 2025
A megadrought has sapped water supplies, ravaged
farms and ranches, and fueled wildfires across the American Southwest for going
on 25 years. Not in 12 centuries has the region been so dry for so long.
Now comes worse news: Relief might still be
decades away.
According to new findings published in the
journal Nature Geoscience, the dry spell is no mere bout of bad luck.
Instead, it seems to be the result of a
pattern of Pacific Ocean temperatures that is “stuck” because of global warming,
said Victory Todd, a doctoral student in paleoclimatology at the University of
Texas at Austin who led the new research.
That means the drought could continue through 2050, perhaps even 2100 and beyond—effectively,
Todd said, for as long as humans keep heating up the planet.
In their study, Todd and her colleagues set
out to understand a different dry period in the region’s deep past. For clues,
they looked to mud from the bottoms of two lakes in the Rocky Mountains: Stewart
Bog in New Mexico and Hunters Lake in Colorado.
The waxy coating on a planet’s leaves
preserves a chemical signature of the rain and snow that the plant absorbs. So by
analyzing the vegetal remains that had accumulated on the lake beds and become
entombed in layers of sediment, Todd and her colleagues reconstructed how wet
the Rockies had been over the past 14 millennia. They found that winters were
dry for thousands of years in the middle of this period.
Todd and her colleagues ran computer
simulations of the prehistoric climate during this warm time to see what might have
led to such a severe drought in the Southwest. They found that the extra heat
gave rise to something striking in the Pacific: a giant blob of warm water
extending east from Japan and surrounded on three sides by cool water,
including along the West Coast of the United States. The warm blob shifted the
band of winds knowns as the jet stream and deflected storms away from the Southwest.
In the warm world of 6,000 years ago, the
blob stayed put, drying out the Southwest for thousands of years.
And, when Todd and her colleagues ran
simulations of the present-day climate, they found that the blob might be stuck
in place again—only this time, it appears to be because humans are changing the
atmosphere by burning coal, oil and gas.
(Raymond Zhong )
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