A severe drought in 2021 — about as dry as 2002 and one of the driest years ever recorded for the region — caused a 22-year drought to surpass the previous mega-drought record in the late 1500s and shows no signs of easing in the near future. future, according to a study published Monday in the journal Nature Climate Change.
The study calculated that 42% of this megadrought can be attributed to anthropogenic climate change.
“Climate change is shifting baseline conditions towards a drier, progressively drier state in the west, meaning that the worst-case scenario continues to get worse,” said study lead author Park Williams, a climatological hydrologist at the University of California, Los Angeles. “This is in line with what people thought in the 1900s as the worst-case scenario. But today I think that we need to prepare even for conditions in the future that are much worse than now.”
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Williams studied soil moisture levels in the West—in a field that includes California, Wyoming, Utah, Nevada, Arizona, most of Oregon and Idaho, most of New Mexico, western Colorado, northern Mexico, and the southwestern corners of Montana and Texas—from using modern measurements and tree rings for estimates that go back to 800. This is about as far as it is possible to reliably estimate the annual rings of trees.
A few years ago, Williams studied the current drought and said that it qualifies as a long and deep “megadrought”, and that the single worst of them was in the 1500s. He believed that the current drought would not surpass that one, because megadroughts usually end after 20 years. And according to him, 2019 was a rainy year, so it looked like the drought in the west was coming to an end.
But in late 2020 and 2021, the region dried up.
Officially, all of California was considered dry from mid-May until the end of 2021, and at least three-quarters of the state experienced its two highest levels of drought from June through Christmas, according to the US Drought Watcher.
“That this drought has just returned to peak drought intensity in late 2020-2021 is a pretty strong statement from this 2000s drought that we are far from over,” Williams said. This drought is now 5% drier than the old record from the 1500s, he says.
The Drought Monitor says 55% of the Western US is in drought, with 13% experiencing the two highest levels of drought.
That mega-drought really began in 2002, Williams says, one of the driest years on record, judging by humidity and tree growth rings.
“I was wondering if we would ever see a year like 2002 in my life, and in fact we saw it 20 years later, in the same drought,” Williams said. Drought levels in 2002 and 2021 were statistically the same, although still trailing 1580, the worst year ever.
According to scientists, climate change as a result of burning fossil fuels leads to an increase in temperature and an increase in evaporation in the air.
Williams used 29 models to create a hypothetical world without anthropogenic warming and then compared it to what happened in real life, a scientifically accepted way to test whether an extreme weather event is caused by climate change. He found that 42% of droughts are directly due to human-induced warming. If not for climate change, the megadrought would have ended sooner because 2005 and 2006 were wet enough to break it, he says.
The study “is an important wake-up call,” said Jonathan Overpeck, dean of the University of Michigan’s environmental department, who was not involved in the study. “Climate change is literally burning the water supply and forests in the southwest, and things could get a lot worse if we don’t stop climate change soon.”
Williams said there is a direct link between drought and heat and the increase in wildfires that have been devastating the West for years. Fires require dry fuel, aided by drought and heat.
Eventually, this mega-drought will end in a fluke of a few good rainy years, Williams said. But then another one starts.
Daniel Swain, a climatologist at UCLA who was not involved in the study, said climate change is likely to make megadrought “a permanent feature of the Colorado River watershed climate in the 21st century.”
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