ALBUQUERQUE — The megadrought in the American Southwest has become so severe it is now the region’s driest two decades in at least 1,200 years, scientists said Monday, and climate change is largely responsible.
The drought, which began in 2000 and has reduced water supplies, devastated farmers and ranchers and helped fuel wildfires across the region, was previously considered the worst in 500 years, according to the researchers.
But exceptional circumstances in the summer of 2021, when about two-thirds of the West was in extreme drought, “really pushed it over the top,” said A. Park Williams, a climate scientist at the University of California, Los Angeles, who led an analysis. using tree ring data to measure drought. As a result, 2000-2021 is the driest 22-year period since 800 AD, which is as far back as the data goes.
The analysis also showed that human-induced warming played an important role in making the current drought so extreme.
There would have been a drought regardless of climate change, said Dr. Williams. “But the severity of it would have been only about 60 percent of what it was.”
Julie Cole, a climate scientist at the University of Michigan who was not involved in the study, said that while the findings weren’t surprising, “the study just makes it clear how unusual current conditions are.”
dr. Cole said the study also confirms the role of temperature, more than precipitation, in causing exceptional droughts. Precipitation amounts can go up and down over time and can vary regionally, she said. But as human activities continue to pump greenhouse gases into the atmosphere, temperatures generally rise.
While they do, “the air is basically better able to pull the water out of the ground, out of vegetation, out of crops, out of forests,” said Dr. Cole. “And it makes the drought much more extreme.”
While there is no uniform definition, a megadrought is generally considered to be severe and prolonged, on the order of decades. But even in a mega-drought, there can be periods of wet conditions. Only there aren’t enough consecutive wet years to end the drought.
Such has been the case in the current Western drought, which has seen several wet years, most notably 2005. The study, published in the journal Nature Climate Change, found that climate change was responsible for the continuation of the current drought after that year.
“By our calculations, it’s a little extra drought in the average background conditions due to human-induced climate change that actually kept 2005 from ending the drought,” said Dr. Williams.
Climate change also makes it more likely that the drought will continue, the study finds. “This 22-year drought is still in full swing,” said Dr. Williams, “and it’s very, very likely that this drought will last 23 years.”
Several previous megadroughts in the 1,200-year record lasted as long as 30 years, according to the researchers. Their analysis concluded that it is likely that the current drought will last that long. If so, said Dr. Williams, it is almost certain that it will be drier than any previous 30-year period.
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Tree rings are an annual measure of growth – wider in wet years, thinner in dry years. Using observational climate data from the past century, researchers have been able to closely link tree ring width to soil moisture content, which is a common measure of drought. They then applied that latitude-moisture relationship to data from much older trees. The result “is an almost perfect record of soil moisture” over 12 centuries in the Southwest,” said Dr. Williams.
Using that record, the researchers determined that last summer was the second driest in the past 300 years, with only 2002, in the early years of the current drought, drier.
Last summer’s monsoon rains in the desert southwest had offered hopes that the drought would end, as did heavy rain and snow in California from fall through December.
But January brought record dry conditions across much of the West, said Dr. Williams, and so far February has also been dry. Reservoirs that were above normal levels for the time of year a few months ago are now below normal levels, and the snow cover in the mountains is also suffering. Seasonal forecasts also suggest the drought will continue.
“This year could get wet,” said Dr. Williams, “but the dice are getting more and more loaded as this year turns into an abnormally dry year.”
Samantha Stevenson, a climate modeler at the University of California, Santa Barbara who was not involved in the study, said the research shows what the projections show — that the Southwest, like some other parts of the world, is even more parched. is becoming .
It’s not getting drier everywhere, she said. “But in the western US it certainly is. This is mainly due to the warming of the land surface, with some contribution from precipitation changes.”
†We are shifting more or less into unprecedented times compared to anything we’ve seen in the past hundreds of years,” she added.