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Last year, the US West’s megadrought heightened, becoming the driest in at least 1,200 years, a new study found.

A dramatic drought in 2021, about as dry as 2002 and one of the driest years ever recorded for the region, pushed the 22-year drought past the previous record-holder for megadroughts in the late 1500s, according to a study in the journal Nature Climate Change.

Moreover, the study calculated that 42% of this megadrought can be attributed to human-caused climate change.

“Climate change is changing the baseline conditions toward a drier, gradually drier state in the West and that means the worst-case scenario keeps getting worse,” said study lead author Park Williams, a climate hydrologist at UCLA. “This is right in line with what people were thinking of in the 1900s as a worst-case scenario. But today I think we need to be even preparing for conditions in the future that are far worse than this.”

Using modern measurements and tree rings for estimations that go back to the year 800, Williams studied soil moisture levels in the West. 

A few years ago, Williams saw that 2019 was a wet year so it looked like the western drought might be coming to an end. But the region dried up in late 2020 and 2021.

California in official drought 

California was considered in official drought from mid-May until the end of 2021, and at least three-quarters of the state was at the highest two drought levels from June through Christmas, according to the US drought monitor.

“For this drought to have just cranked up back to maximum drought intensity in late 2020 through 2021 is a quite emphatic statement by this 2000s drought saying that we’re nowhere close to the end,” Williams said. This drought is now 5% drier than the old record from the 1500s, he said.

The monitor read that 55% of the US West is in drought with 13% experiencing the two highest drought levels. This megadrought really kicked off in 2002, one of the driest years ever, based on humidity and tree rings, Williams said.

He added, “I was wondering if we’d ever see a year like 2002 again in my life, and in fact, we saw it 20 years later, within the same drought,” Because of burning fossil fuels, climate change is bringing hotter temperatures and increasing evaporation in the air, scientists say.

Hypothetical world without humans

Williams used 29 models to create a hypothetical world with no human-caused warming, then compared what happened in real life to see if an extreme weather event was caused by climate change, which is the scientifically accepted way to determine if an extreme weather event is caused by climate change.

He discovered that human-caused warming is directly responsible for 42% of the drought conditions. The megadrought would have ended sooner if it hadn't been for climate change, he claimed, because 2005 and 2006 would have been wet enough to break it.

The study “is an important wake-up call,” said Jonathan Overpeck, dean of environment at the University of Michigan, who wasn’t part of the study. “Climate change is literally baking the water supply and forests of the Southwest, and it could get a whole lot worse if we don’t halt climate change soon.”

Williams said there is a direct link between drought and heat and the increased wildfires that have been devastating the West for years. Fires need dry fuel that drought and heat promote.

Eventually, this megadrought will end by sheer luck of a few good rainy years, Williams said. But then another one will start.

Daniel Swain, a UCLA climate scientist who wasn’t involved in the study, said climate change is likely to make megadrought “a permanent feature of the climate of the Colorado River watershed during the 21st century.”