Western mega-drought worsens to become driest in at least 1,200 years


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The mega-drought in the American West got so bad last year that it’s now the driest in at least 1,200 years and is the worst-case climate change scenario playing out live, a new study finds. .

A dramatic drought in 2021 – about as dry as 2002 and one of the driest years on record for the region – pushed the 22-year drought past the previous record holder for mega-droughts at late 1500s and shows no signs of easing in the near future, according to a study published Monday in the journal Nature Climate Change.

The study calculated that 42% of this mega-drought can be attributed to human-induced climate change.

“Climate change is shifting baseline conditions to a drier and progressively drier state in the West, which means the worst-case scenario is only getting worse,” said the study’s lead author, Park Williams, climate hydrologist at UCLA. “It fits perfectly with what people thought in the 1900s as a worst-case scenario. But today I think we have to prepare for even worse future conditions than that.

Williams studied soil moisture levels in the west – a box that includes California, Wyoming, Utah, Nevada, Arizona, most of Oregon and Idaho, a much of New Mexico, western Colorado, northern Mexico, and the southwestern corners of Montana and Texas – using modern measurements and tree rings for estimates dating back to AD 800. That’s about as far as estimates can go reliably with tree rings.

A few years ago, Williams studied the current drought and said that it was called a long and deep “mega-drought” and that the only worst was the one in the 1500s. He thought that the current drought would not exceed that one because the mega-droughts tended to fade after 20 years. And, he said, 2019 was a wet year, so it looked like the drought in the west was coming to an end.

Map showing drought intensity in the western United States.

But the region dried up at the end of 2020 and 2021.

All of California was considered officially drought-prone from mid-May through the end of 2021, and at least three-quarters of the state was in the two highest drought levels from June through Christmas, according to the US Monitor of Drought.

“The fact that this drought has just returned to peak drought intensity from late 2020 to 2021 is a pretty categorical statement from this drought of the 2000s that we’re nowhere near the end,” he said. said Williams. That drought is now 5% drier than the old record of the 1500s, he said.

The Drought Monitor indicates that 55% of the western United States is in drought, with 13% experiencing the two highest drought levels.

This mega-drought really started in 2002, one of the driest years ever, due to humidity and tree rings, Williams said.

“I wondered if we would ever see a year like 2002 again in my lifetime and in fact we saw it 20 years later in the same drought,” Williams said. Drought levels in 2002 and 2021 were a statistical tie, although still behind 1580 for the worst year.

Chart showing changes in global land area affected by severe drought by month since 1950

Chart showing changes in global land area affected by severe drought by month since 1950.

According to scientists, climate change due to the burning of fossil fuels leads to higher temperatures and increased evaporation into the air.

Williams used 29 models to create a hypothetical world with no human-caused warming, then compared it to what happened in real life – the scientifically accepted way to check if an extreme weather event is due to climate change . He found that 42% of drought conditions are directly due to human-caused warming. Without climate change, he said, the mega-drought would have ended early 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 was not part of the study. “Climate change is literally burning the water supply and forests of the southwest, and it could get worse if we don’t stop climate change soon.”

Williams said there is a direct link between the drought and heat and the increase in wildfires that have devastated the West for years. Fires need dry fuel which drought and heat favor.

Eventually, this mega-drought will end with the sheer luck of a few good rainy years, Williams said. But then another will begin.

Daniel Swain, a UCLA climatologist who was not involved in the study, said climate change is likely to make the mega-drought “a permanent feature of the Colorado River watershed climate over the course of the 21st century”.

Study: Warming makes western US mega-drought worse in modern era

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