The past 22 years in a drought-stricken portion of North America has been the driest stretch for the region at least since the year Charlemagne was crowned emperor of the Holy Roman Empire, researchers say.
Published Feb. 16, 2022 12:21 PM EST | Updated Feb. 17, 2022 12:57 PM EST
The pulse of the Colorado River has long served as a gauge of the American southwest's water supply, and a new study has found that the intensity of the current megadrought is a threat to its health not seen in over a millennium.
The western United States and parts of northern Mexico are experiencing their driest period in at least 1,200 years, according to a recent peer-reviewed study published in the journal Nature Climate Change.
The scientists used tree-ring records from thousands of trees at hundreds of sites to reconstruct summer soil moisture across the centuries, said Park Williams, one of the authors of the study. Some of the rings were able to give researchers a look at soil moisture dating back to 800 A.D. -- the year Charlemagne was crowned as the first emperor of the Holy Roman Empire.
Earlier research had previously cited 2000 through 2018 as the second-driest period of time for southwestern North America since 800 A.D., with the driest period at that point being a multi-decade megadrought in the late-1500s. That held until the exceptional severity of the drought from 2020-2021, which contributed to record-low water levels, a water shortage declaration at Lake Mead and a depletion in the snowpack.
FILE - This Tuesday, April 16, 2013 file photo shows a bathtub ring marking the high water line as a recreational boat approaches Hoover Dam along Black Canyon on Lake Mead near Boulder City, Nev. A two-decade-long dry spell that has parched much of the western United States is turning into one of the deepest megadroughts in the region in more than 1,200 years, and about half of this historic drought can be blamed on man-made global warming, according to a study released Thursday, April 16, 2020 in the journal Science. (AP Photo/Julie Jacobson)
Despite the active monsoon season (mid-June through September) during 2021, the U.S. Drought Monitor classified more than 68% of the western U.S. as under extreme or exceptional drought for nearly all of July through October, the study highlighted. In fact, 2021 was a key year in overtaking the drought in the 1500s.
"Exceptionally dry soil in 2021 was critical for the current drought to escalate and overtake the 1500s megadrought as the period with the highest 22-year mean severity," the authors wrote.
After "exceptional drought severity" in 2021, about 19% of which was attributed to human-caused climate trends, 2000-2021 was the driest 22-year period since at least 800 A.D., the report found.
Mean water-year precipitation across southwestern North America from 2000 to 2021 was 8.3% below the 1950-1999 average. Meanwhile, the temperature was 0.91 C (1.6 F) above average. Over the course of the 22 years, only 2002 was drier than 2021, highlighting the aridity of the latter outside of the active monsoon season.
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"The fact that the 2021 drought was comparable to that of 2002 is especially remarkable given high precipitation totals in summer 2021 across much of southern SWN," the authors wrote.
That also isn't to say that a megadrought can't have a wet year. For the 2000-2021 drought, 18 of those years were drier than average, and all of the reconstructed megadroughts had occasional wet years, Williams said. However, only two megadroughts contained 22-year periods with more dry years than the current drought.
Both 2002 and 2021 were most likely drier than any other year in nearly three centuries, ranking respectively as the 11th and 12th driest years from 800 to 2021. The most severe reconstructed drought year was 1580. The most recent year drier than 2002 was 1729 -- the same year Johann Sebastian Bach's "St. Matthew Passion" premiered in Leipzig, the city of Baltimore was founded and North Carolina became a royal colony.
A car crosses Enterprise Bridge over Lake Oroville's dry banks on May 23, 2021, in Oroville, Calif. The American West's megadrought deepened so much last year that it is now the driest it has been in at least 1,200 years and a worst-case scenario playing out live, a new study finds. (AP Photo/Noah Berger, File)
The history of drought in the Southwest is long-running, but, the authors of the study stressed, human activity has played a large role in the severity and length of the current drought.
"The turn-of-the-twenty-first-century drought would not be on a megadrought trajectory in terms of severity or duration without ACC [anthropogenic climate change]," the authors wrote.
When looking at the period from 2000 to 2021 as a whole, human-caused climate trends accounted for 42% of the soil moisture anomaly, similar to the 46% previously found for 2000-2018, the report found. About 19% of the "exceptional drought severity" in 2021 that pushed this megadrought into its current ranking was attributed to human-caused climate trends. The Southwest didn't go without feeling the crushing grip of the intense drought either.
In late 2021, The Salt Lake Tribune changed the outline of the Great Salt Lake in its maps to reflect the cumulative shrinkage over the years, partly due to the impacts of climate change and the ongoing drought. The newspaper's Managing Editor Grant Burningham told AccuWeather National Reporter Bill Wadell that, as it stood at the time, the lake was "in a death spiral."
Satellite images from Google Earth show how the Great Salt Lake has changed over time. (Google Earth/Landsat/Copernicus)
Farther southwest, the federal government declared the first water shortage at Lake Mead during the middle of August 2021 as water levels dropped at Hoover Dam. In June, the water levels had dropped to a then-record-breaking 1,071.56 feet above sea level. Compared to where the water levels had been during 2000, they had fallen by roughly 140 feet -- nearly the height of the Statue of Liberty.
During the summer of 2021, Stephen Wilcox reminisced to AccuWeather National Reporter Tony Laubach about how the sound of water crashing over the edge of the Hoover Dam had once been thunderous in 1983 when the water had been plentiful after an intense El Niño pattern.
"Had the sequence of wet-dry years occurred as observed but without the human-caused drying trend, we estimate that the 2000s would have still been dry, but not on the same level as the worst last millennium's megadroughts," Williams said. "In fact, without human-caused climate trends, 2000-2021 would probably not even classify as a single drought event due to what would have been wet conditions in 2004-2005."
While it's currently the wet season for California, the impacts of the drought can be seen through the loss of snowpack that occurred in recent weeks.
The U.C. Berkeley Central Sierra Snow Lab recorded about a 1.5-inch (roughly 5%) water loss from the snowpack at the lab over the course of a week, coinciding with the warm spell that drove record-high temperatures across California last week. The temperature reaching a high of 57 degrees preceded the biggest drop in the snowpack. The lab lost about 0.5 of an inch of snow-water equivalent per day due to high temperatures and winds.
Snow cover data from the California Department of Water Resources on Tuesday, Feb. 15, showed that the statewide percent of normal snowpack had declined from roughly 160% of the average in December to 72% of the average for Feb. 15, indicating that mid-winter melting has occurred and that any precipitation that has fallen as snow has not been enough to result in any new accumulation. The statewide snowpack had been at 93% of normal as of Feb. 1.
The researchers added that the 22-year-long drought is likely to continue through a 23rd year, matching the duration of the drought from the late 1500s, which was the shortest of the reconstructed megadroughts. Simulations from the study suggested a 93% chance that the drought would continue at least into its 23rd year. When looking at the possibility of the drought surviving through the 30th year -- the duration of the longest megadroughts in the reconstruction -- it lasts in 75% of the simulations.
"Regardless of whether this drought ends in one year or 10, tree-ring records tell us that naturally occurring wet-dry variations will continue," Williams said. "These variations will be super-imposed on an increasingly dry baseline state, however, making future megadroughts increasingly likely."
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