Climate change made the current megadrought worse, study of tree ring data says. CSU researcher who reviewed the work says the conditions aren’t likely to improve

Chris Outcalt


Feb 14, 2022

Blue Mesa Reservoir in Gunnison County shows the effect of a water draw down on October 29, 2021. The reservoir has lowered because water from it is being released downstream to increase the volume of water available to downstream users that rely on the Colorado River. Blue Mesa is fed by the Gunnison River, one of the Colorado River’s largest tributaries. (Dean Krakel, Special to The Colorado Sun)

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. 

Tree ring specimens, collected from avalanche debris above Silverton on June 2, 2019, are stacked before being taken for analysis by dendrochronologists trying to plot the relationship between climate and avalanche cycles in Colorado. The work is intended to inform planning for such things as building codes and zoning in Colorado’s high country as the climate changes. (Nina Riggio, Special to The Colorado Sun)

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.” 


Study finds Western megadrought is the worst in 1,200 years

February 14, 2022


LISTEN· 3:59

Shrunk reservoirs. Depleted aquifers. Low rivers. Raging wildfires. It’s no secret that the Western U.S. is in a severe drought. New research published Monday shows just how extreme the situation has become.

The Western U.S. and northern Mexico are experiencing their driest period in at least 1,200 years, according to the new study, published in the journal Nature Climate Change. The last comparable — though not as severe — multidecade megadrought occurred in the 1500s, when the West was still largely inhabited by Native American tribes.

Today, the region is home to tens of millions of people, massive agricultural centers and some of the fastest-growing cities in the U.S. — all in an area where there’s less water available than there was in the past, partially due to human-caused climate change.

“We have a society that’s relying on there being the amount of water there was in the 1900s,” said the study’s lead author, Park Williams, a bioclimatologist at the University of California, Los Angeles. “But now with the number of water molecules available to us declining, it really is time for us to get real about how much water there is for us to use.”

Williams looked at tree ring data from thousands of sites to conduct the research. The researchers sampled data collected from live trees, dead trees and wood beams preserved at Native American archeological sites. The tree rings gave Williams insight into drought events dating back to A.D. 800, around the time Charlemagne was being crowned emperor of Rome.

He identified four other megadroughts in that time period, the most notable being a 23-year drought that ended in the late 1500s. There were hopes during a wet 2019 that the current megadrought was following a similar pattern, Williams said.

“And then from summer 2020 through all of 2021, it was just exceptionally dry across the West … indicating that this drought is nowhere near done.”

Western water managers were again hopeful for a change at the beginning of this winter. In December, California’s Sierra Nevada had record-breaking snowfall, and big snowstorms blanketed the northern Rockies. But a hot, dry start to the year has since dropped snowpack levels to below average in many places.

Lake Mead and Lake Powell, the country’s two largest reservoirs, are filled at only about one-third of their total capacity. Communities, ranchers and farmers have depleted groundwater stores to meet demands.

Federal water managers declared the first-ever water shortage along the Colorado River last year, triggering cuts to some of the river’s 40 million users. It was a recognition “that the hydrology that was planned for years ago — but we hoped we would never see — is here,” said Bureau of Reclamation Commissioner Camille Touton.

“The Colorado River Basin no longer has the privilege of time,” said Kim Mitchell, senior water policy adviser at Western Resource Advocates, an environmental nonprofit, after hearing about the new research. “It’s imperative for water managers in the West to incorporate a smaller [Colorado] River into future operations and pull out all the stops in scaling up basin-wide conservation. Incremental solutions just won’t be enough.”

Existing management guidelines for the Colorado River are set to expire in 2026. The seven states that draw from the watershed are negotiating with the federal government, Native American tribes and Mexico over what future management should look like.

Last December, Nevada, Arizona and California agreed to take less water from the Colorado River in an effort to prop up Lake Mead, and more cuts could follow.

“This is a wake-up call for everyone,” Adel Hagekhalil, general water manager for the Metropolitan Water District of Southern California, told KUNC. “For all of us. We are facing a new normal when it comes to climate change.”

Williams, the study author, said roughly one-fifth of the current megadrought can be attributed to human-caused climate change. Greenhouse gas emissions are warming the world, speeding evaporation and disrupting weather patterns.

He described water patterns in the West as a yo-yo — sometimes high, sometimes low. Climate change has put that yo-yo on an escalator heading down, he said, “and we cannot let ourselves get tricked by a few wet years into giving up on the progress we’ve been making.”

“We actually have to change our relationship with water.”

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s