The hot and dry conditions that have gripped the southwestern United States since 2000 account for the driest 22-year period on record stretching back 1,200 years, according to a study published Monday in the journal Nature Climate Change.
What’s more, this new research finds that although there would have been a drought anyway, climate change made it considerably worse.
Previous research indicated the current drought was the driest stretch since the 1500s. The new study, however, incorporated data from 2021, a particularly dry year, which led the current 22-year period to exceed the intense drought in the 1500s, researchers who studied tree ring data found.
The hotter and drier climate noted by researchers has fueled wildfires, caused problems for farmers and ranchers whose livelihood depends on water for irrigation, and resulted in lower flows on the Colorado River — as well as other rivers and streams — which provides drinking water to more than 36 million people.
“What’s super intriguing is the headline finding that this is the driest 22-year period since at least 800,” Brad Udall, a senior water and climate research scientist at Colorado State University’s Colorado Water Institute, said. “That’s stunning.”
Udall was part of the peer-review team for the current study, which used a bank of tree ring data to reconstruct soil moisture conditions for June, July, and August during the past 1,200 years. The database, maintained by NOAA, contains information about both dead and live trees. There’s a good correlation, Udall said, between tree ring widths and soil moisture; wider rings indicate years that received more precipitation.
“This is one of the first papers that ties soil moisture to the impacts of this drought,” Udall said.
Although snowpack on April 1 was either at or slightly below 100 percent of average in each of the past two years, Udall said, the spring runoff was significantly lower than expected. “Increasingly, these really warm June, July, August periods have been draining soil moisture,” Udall said. “This explains how serious the current conditions are and why we’ve been getting such terrible runoff out of reasonably good snowpack.”
Spring runoff is critical to irrigated agriculture, domestic water supplies, and the recreation industry in Colorado and across the West. Flows in the Colorado River basin, an area that includes the Colorado River and all the rivers and streams that feed into it, have been hard hit. In 2000, the reservoir system in the basin was 95% full; as of fall 2021, the reservoirs were at 39% capacity, the lowest levels on record, according to the Department of Interior.
“It makes it a little easier to believe that we’ve got a real problem with soil moisture here,” Udall said. “And it’s likely we’re going to continue to see these crummy years of runoff even if we have OK snowpack.”
“The dramatic trend of hotter and drier conditions across the southwest means we have to work harder and faster on solutions that meet the moment,” said Bart Miller, healthy rivers program director at the nonprofit Western Resource Advocates. “It’s urgent that we ramp up conservation and more flexible water management to protect our rivers and our communities.”
The new study estimated that climate change driven by human causes increased the severity of the drought conditions from 2000 to 2021 by 42%. In fact, without the impacts of climate change, according to the study, “2000-2021 would not even be classified as a single extended drought event.” The drought is also likely to continue, according to the study. When factoring in climate change, the current drought lasted through a 30th year in 75% of the soil moisture simulations the researchers conducted.
Udall said drought might not be the right term anymore.
“We’ve been calling it a drought,” he said, “but a number of scientists have been saying this is something else.” Udall has pitched aridification as a new way to categorize the hot and dry conditions impacting the Colorado River basin and rest of the southwest.
“What it really means is the long-term warming and drying of the American West. Not every year is warmer and drier but the overall trend is in that direction,” Udall said. “My sense is that this drought is here to stay; it seems very clear to me we’re on that path.”