Scott K. Johnson – February 14, 2022
About half of the contiguous US is currently experiencing moderate to extreme drought—including almost all of the West. That shouldn’t come as a surprise, as widely pervasive drought has been present for quite a while now in this region, where major reservoirs like Lake Powell and Lake Mead are hovering around all-time low-water levels. But how does this ongoing drought compare to the past? After all, the region is no stranger to dry stretches.
A 2020 paper examined the 2000-2018 data in the context of a tree ring reconstruction going back to the year 800 and stretching from Southern California to Wyoming. That team found that this was likely the second-driest period in the record, beat out only by a megadrought in the late 1500s.
At the time, the paper’s authors guessed that good precipitation in 2019 would be enough to end the extended drought. But instead, a particularly wicked 2021 kept the drought alive. As a result, three of those researchers—UCLA’s Park Williams and NASA’s Benjamin Cook and Jason Smerdon—decided to update the numbers through 2021.
The most mega
The term “megadrought” isn’t some sensationalist moniker from bad television; it’s a term for the handful of two- to three-decade Southwestern droughts in the past millennium or so—some with history-defining impacts on the civilizations who lived there at the time.
With the analysis updated, 2000-2021 ranks as the driest such 22-year period in the data going back to the year 800. The megadrought years of 1571-1592 slip into second place. (Though the error bars on the two periods overlap.)
Between 2000 and 2021, 18 of those years saw soil moisture below the long-term average. Only two past megadroughts meet that level of consistent dryness. The years 2002 and 2021 rank as the 11th and 12th driest years in the whole record—and it has been three centuries since a drier year occurred. And although some megadroughts were focused in a particular region, the drought of the last two decades has been widespread across the West. That’s what you would expect to see from a drought driven more by global warming than by precipitation patterns.
To estimate the contribution of climate change, the researchers repeated their analysis of climate models run with and without warming temperatures. Their previous work had estimated that 46 percent of the severity of the 2000-2018 drought was due to human-caused warming. For 2000-2021, they get a similar answer of 42 percent despite using a new generation of climate model simulations.