The drought, which began in 2000 and has reduced water
supplies, devastated farmers and ranchers and helped fuel wildfires across the
region, had previously been considered the worst in 500 years, according to the
researchers. But exceptional conditions 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 gauge drought. As a
result, 2000-21 is the driest 22-year period since AD 800, which is as far back
as the data goes. The analysis also showed that human-caused warming played a
major role in making the current drought so extreme. There would have been a drought regardless of climate
change, Williams said. “But its severity would have been only about 60% of what it
was,” he said. Julie Cole, a climate scientist at the University of
Michigan who was not involved in the research, said that while the findings
were not surprising, “the study just makes clear how unusual the current
conditions are.” Cole said the study also confirms the role of temperature,
more than precipitation, in driving 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
are more generally rising. As they do “the air is basically more capable of pulling the
water out of the soil, out of vegetation, out of crops, out of forests,” Cole
said. “And it makes for drought conditions to be much more extreme.” Although there is no uniform definition, a megadrought is
generally considered to be one that is both severe and long, on the order of
several decades. But even in a megadrought there can be periods when wet
conditions prevail. It’s just that there are not enough consecutive wet years
to end the drought. That has been the case in the current Western drought,
during which there have been several wet years, most notably 2005. The study,
which was published in the journal Nature Climate Change, determined that
climate change was responsible for the continuation of the current drought
after that year. “By our calculations, it’s a little bit of extra dryness in
the background average conditions due to human-caused climate change that
basically kept 2005 from ending the drought event,” Williams said. Climate change also makes it more likely that the drought
will continue, the study found. “This drought at 22 years is still in full swing,” Williams
said, “and it is very, very likely that this drought will survive to 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 it
does, Williams said, it is almost certain that it will be drier than any
previous 30-year period. Tree rings are a year-by-year measure of growth — wider in
wet years, thinner in dry ones. Using observational climate data over the past
century, researchers have been able to closely link tree ring width to moisture
content in the soil, which is a common measure of drought. Then they have
applied that width-moisture relationship to data from much older trees. The
result “is an almost perfect record of soil moisture” over 12 centuries in the
Southwest,” Williams said. 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, being drier. Monsoon rains in the desert Southwest last summer had
offered hope that the drought might come to an end, as did heavy rain and snow
in California from the fall into December. But January produced record-dry conditions across much of
the West, Williams said, and so far February has been dry as well. Reservoirs
that a few months ago were at above-normal levels for the time of year are now
below normal again, and mountain snowpack is also suffering. Seasonal forecasts
also suggest the dryness will continue. “This year could end up being wet,” Williams said, “but the
dice are increasingly loaded toward this year playing out to be an abnormally
dry year.” Samantha Stevenson, a climate modeller at the University of
California, Santa Barbara, who was not involved in the study, said the research
shows the same thing that projections show — that the Southwest, like some
other parts of the world, is becoming even more parched. Not everywhere is becoming increasingly arid, she said. “But in the Western US it is for sure," Stevenson said.
"And that’s primarily because of the warming of the land surface, with
some contribution from precipitation changes as well.” “We’re sort of shifting into basically unprecedented times
relative to anything we’ve seen in the last several hundred years,” she added. © 2022 The New York Times Company