US West Facing Record Historic Megadrought Status

Among the most memorable sites in the western United States are the cliff dwellings in national parks like Colorado's Mesa Verde. Constructed by ancestral Pueblo peoples known as the Anasazi. 

These vertiginous caves and homes were carved into the region's cliffs between the late BCE and early AD eras. They were mysteriously abandoned around 1200-1500 AD, though scientists now believe that a years-long megadrought is what made habitation there untenable. That same region is now experiencing another megadrought which may exceed the earlier one which depopulated the region. JL

Scott Johnson reports in ars technica:

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? This drought is the result of higher temperatures which evaporate moisture from the land while decreasing snowpack. This is the second-driest period in the record, beat out only by a megadrought in the late 1500s.

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.)

Enlarge / The drought reconstruction shown as a 22-year running average, with megadrought periods highlighted in red. The blue line and map pair show the contribution of anthropogenic climate change (ACC).

Williams et al./Nature Climate Change

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.

 

Feeling the heat

This drought isn’t the result of strong trends in precipitation but rather the impacts of higher temperatures. A warmer atmosphere can evaporate more moisture from the land while also decreasing the storage of mountain snowpack that feeds Western rivers in the summer months.

The researchers say that the current drought is likely to extend through the end of this year, after which point it would equal the length of the shortest megadroughts at 23 years. (The longest were 30 years.) That isn’t a hunch based on our weather outlook; it's a statistical calculation. The researchers copied a rolling 40-year window of years from the past drought reconstruction and pasted each of them onto the end of 2021 for plausible samples of near-future weather. The current drought reaches 23 years in 94 percent of those stitched-together timelines and hits 30 years in 75 percent of them.

Odds are good that by the end of 2022, the current drought could inarguably be classified among the historical megadroughts, absent some extremely fortuitous precipitation.

The researchers write, “Previous work indicated that continued [anthropogenic climate change] will increasingly enhance the odds of long, widespread, and severe megadroughts returning to [Southwest North America] after a hiatus of >400 [years]. After the driest 22-year period in at least 1,200 [years], which included two of the driest 12 individual years in at least 1,200 [years], this worst-case scenario already appears to be coming to pass.”