Bob Henson
Published: April 16, 2020
Warming temperatures from human-produced climate change have exacerbated an otherwise moderate drought in the western United States and northwestern Mexico, leading to the worst two decades of drying in more than 400 years, argues a paper published in the journal Science on Thursday.
The researchers found that the drying from 2000 to 2018 was on par with the driest 19-year periods found in tree-ring records over the last 1,200 years.
What’s more, they add, this region could already be in the type of megadrought that can last for decades.
“We now have enough observations of current drought and tree-ring records of past drought to say that we’re on the same trajectory as the worst prehistoric droughts,” said lead author Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, in an LDEO news release. The team behind the study includes scientists from NOAA, NASA and four universities.
Areas of southwestern North America affected by drought in the early 2000s; darker colors are more intense. Yellow box shows the study area.
Tree-ring data shows that megadroughts occurred across the study region (Southwestern North America, or SWNA, including most of the western U.S. and far northwestern Mexico) in the late 800s, mid-1100s, 1200s and late 1500s.
The study area includes California, Nevada, Utah, Arizona, eastern New Mexico, eastern Colorado and most of Oregon, Idaho and Wyoming.
Soil moisture – which hinges on rainfall and snowfall, as well as weather conditions – was found to be lower in 2000-2018 than in any 19-year span during those megadroughts, except for a period in the late 1500s that was slightly lower. Without climate change, the soil moisture in 2000-2018 would have ranked as the 11th-lowest in 1,200 years – still serious, but not on the same level.
Soil moisture over the last 1,200 years in southwestern North America, as reconstructed from tree-ring records (red) and weather observations (blue). Shown are the running averages for overlapping 19-year periods. The pink and green shading shows the driest and wettest periods, respectively.
“Anthropogenic [human-caused] warming was critical for placing drought on a trajectory consistent with the most severe droughts,” the authors wrote. They add that “anthropogenic global warming and its drying influence in SWNA are likely still in their infancy.”
Megadroughts of the past have been linked to La Niña-type patterns of cooling in the tropical Pacific, as well as warming in the Atlantic. “The tree-ring record serves as an ominous reminder that natural climate variability can drive SWNA megadroughts that are as severe and longer than the 21st-century drought thus far,” said the researchers.
Expectations for the future may be warped by the moist conditions that prevailed across southwestern North America in the 1900s, the wettest century of any in the study’s 1,200-year database.
“The 20th century gave us an overly optimistic view of how much water is potentially available,” said coauthor Benjamin Cook (LDEO/NASA). “It goes to show that studies like this are not just about ancient history. They’re about problems that are already here.”
According to Jeff Lukas, a research scientist at Western Water Assessment (University of Colorado Boulder), "This study tells us that the West needs to prepare for a warmer and drier future that is likely to be punctuated by droughts that are almost unimaginable by 20th-century standards – the standards to which our infrastructure and other critical systems were built.”
Although the western U.S. – like most of the nation – was much wetter than average in 2019, it is now heading into the late spring and summer with widespread below-average snowpack and water storage yet again. As of April 6, Lake Powell was at 48% of full capacity and is predicted to end the current water year this autumn at 54% of capacity.
Lake Mead, which provides water and power to Las Vegas and surrounding areas, was at 44% of capacity. That is the highest it’s been in six years; the water level had its lowest five-year average on record from 2015 to 2019.
A view from March 30, 2015, of a section of Lake Powell near Big Water, Utah, that had previously been underwater. The Colorado River Basin supplies water to 40 million people in seven Western states.Calculating the Push from Climate Change Toward Possible Megadrought
Comparing the period 2000-2018 to simulations from 31 climate models, the team found that high temperatures and low humidity related to climate change accounted for nearly half of the 2000-2018 soil drying. Natural variations in weather and climate – the type expected to produce drought – were responsible for the other half of the drying.
The study’s climate models estimated that long-term global warming pushed temperatures in the 2000-2018 period about 1.2 degrees Celsius (2.1 degrees Fahrenheit) higher than they otherwise would have been.
Even though there was a modest drop in precipitation during 2000-2018, climate change wasn't responsible for that, the study found. Instead, it was the heating and drying of the atmosphere – closely linked to climate change – that made the drought impacts so much worse.
The model average for 2000-2018 showed that the annual evaporation demand, or the amount of moisture the atmosphere was “trying” to remove from the landscape, was boosted by climate change by a large amount: 59 millimeters (2.32 inches). In contrast, the effect of climate change on precipitation itself was a comparatively minor boost of 6 millimeters (0.24 inches) to the low values that prevailed.
Although drought is often simplified as a lack of rainfall, much of the impact occurs when high temperatures and low humidity parch the landscape, pulling moisture from trees and other vegetation. Such drying can set the stage for catastrophic wildfires, crop failures and other consequences.
The period from 2000-2018 included some intensely dry years, as well as several wet ones. Throughout the period, the impact on ecosystems and hydrology from “hot droughts” became more and more evident. In California, record heat exacerbated an intense drought from 2011 to 2015. When wetter years returned to California in the late 2010s, they boosted the growth of vegetation that served as fuel for wildfire during subsequent periods of record heat and drought.
In 2017 and 2018, California experienced massive wildfires, including the Camp Fire in November 2018 – the deadliest and most destructive in state history.
A resident watches as the Cave Fire burns a hillside near homes in Santa Barbara, California, early on Nov. 26, 2019.
Daniel Swain, a scientist with the University of California, Los Angeles and the National Center for Atmospheric Research, has researched the “hot drought” phenomenon in California. “I think this is interesting work and is indeed consistent with quite a bit of recent research,” Swain said of the new study in an email.
“It's becoming increasingly clear that large declines in precipitation are not needed to produce more severe droughts due to climate change – the warming temperatures themselves can achieve that effect on their own,” Swain added. “This means drought may intensify in a warming world even in places where overall precipitation changes little.”
According to lead author Williams, “Because the background is getting warmer, the dice are increasingly loaded toward longer and more severe droughts. We may get lucky, and natural variability will bring more precipitation for a while. But going forward, we’ll need more and more good luck to break out of drought, and less and less bad luck to go back into drought.”
Williams said it is conceivable the region could stay arid for centuries. “That’s not my prediction right now, but it’s possible,” he said.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
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