The prolonged mega-drought that has hit the southwestern United States is so severe that the past two decades have become the driest period in the region in at least 1,200 years, scientists said on Monday. And climate change is the main culprit.
The drought, which began in 2000, reduced water supplies, devastated farmers and ranchers and helped fuel fires across the region, previously ranked as the worst in 500 years, according to the researchers.
But the exceptional conditions of the summer of 2021, when about two-thirds of the American West experienced extreme drought, “really worsened the situation,” said A. Park Williams, a climate scientist at the University of California, Los Angeles, who led an analysis. based on tree rings to assess drought severity. As a result of the analysis, 2000-2021 was ranked as the driest 22-year period since the year 800, which is the oldest period for which data are available.
The analysis also showed that warming caused by human activities played a major role in making the current drought so extreme.
There would have been a drought regardless of climate change, according to Williams.
“But its severity would have been only 60% of what we actually had,” he said.
Julie Cole, a climate scientist at the University of Michigan who was not involved in the research, said that while the findings are not surprising, “the study serves to make even clearer how unusual current conditions are.”
Cole said the study also confirms that the role of temperature is more important than that of precipitation in causing exceptional droughts. The volume of precipitation can rise and fall over time and vary regionally, she said. But because human activities continue to pump greenhouse gases into the atmosphere, temperatures are always rising, in general terms.
And as they do, “the air basically becomes more capable of pulling water out of the soil, the vegetation, the crops, the forests,” Cole said. “And it makes drought conditions even more extreme.”
Although there is no uniform definition, a drought is considered a megadrought when it combines severity and a long duration, on the order of a few decades. But even in a mega-drought there may be periods when damp conditions prevail. The problem is that there aren’t enough consecutive wet years to end the drought.
This has been the case in the current drought in the West American region, during which there were several years of humidity, especially 2005. The study, published in the scientific publication Nature Climate Change, determined that the change in climate was responsible for the continuation of the current drought after of that year.
Climate change also makes drought more likely to continue, the study added.
“This drought has been going on for 22 years and it continues in full force,” Williams said. “And it’s very, very likely that she will survive for at least 23 years.”
Several megadroughts lasted up to 30 years in the 1,200 years of available records, the researchers say. Their analysis concludes that the current drought is likely to last at least as long. If that happens, Williams said, it will almost certainly be drier than any other comparable 30-year period.
The year may end up being wet, but the odds are increasing that this year will prove unusually dry.
Tree rings allow you to measure their growth year by year and are wider in wet years and narrower in dry years. Using weather observation data over the last hundred years, the researchers were able to tightly link the width of tree rings to the soil’s moisture content, which serves as a common indicator of drought. They then applied the relationship between ring width and soil moisture to data obtained from much older trees. The result is a “near-perfect record of soil moisture in the southwestern United States” over the past 12 centuries, Williams said.
Using this record, the researchers determined that last year’s summer was the second driest in the last 300 years and that only 2002, at the beginning of the current drought, recorded less humidity.
Last summer’s monsoon rains in the southwest region offered hope that the drought could end, as did the rains and snow in California in the fall and December.
But January produced a record lack of moisture across much of the west, according to Williams, and so far February has also been dry. Reservoirs that until a few months ago were above the normal level for that time of year have returned to below normal, and the snow cover of the mountains is also suffering. Seasonal forecasts also indicate that the drought will continue.
“It could be that the year ends up being wet, but the odds are increasing that this year will prove to be unusually dry,” Williams said.
Samantha Stevenson, who develops climate models at the University of California at Santa Barbara and was not involved in the study, said the research shows the same thing projections do: that the southwestern United States, like other parts of the world, is increasingly more parched.
Not all places are becoming increasingly arid, she said.
“But in the western United States, this is definitely happening,” Stevenson said. “And primarily because of the warming of the earth’s surface, with some contribution also from changes in precipitation.”
“We’re heading into a period that’s unprecedented for anything we’ve seen in the last few hundred years,” she added.
The New York Times, translated by Paulo Migliacci
