Paper: Rapid intensification of the emerging southwestern North American #megadrought in 2020–2021 — Nature #ClimateChange #ActOnClimate

Ranking and time evolution of summer (June–August) drought severity as indicated by negative 0–200 cm soil moisture anomalies. Maps show how gridded summer drought severity in each year from 2000–2021 ranked among all years 1901–2021, where low (brown) means low soil moisture and therefore high drought severity. Yellow boxes bound the southwestern North America (SWNA) study region. Time series shows standardized anomalies (σ) of the SWNA regionally averaged soil moisture record relative to a 1950–1999 baseline. Black time series shows annual values and the red time series shows the 22-year running mean, with values displayed on the final year of each 22-year window. Geographic boundaries in maps were accessed through Matlab 2020a.

Click the link to access the paper at Nature Climate Change (A. Park Williams, Benjamin I. Cook & Jason E. Smerdon). Here’s the abstract:

A previous reconstruction back to 800 CE indicated that the 2000–2018 soil moisture deficit in southwestern North America was exceeded during one megadrought in the late-1500s. Here, we show that after exceptional drought severity in 2021, ~19% of which is attributable to anthropogenic climate trends, 2000–2021 was the driest 22-yr period since at least 800. This drought will very likely persist through 2022, matching the duration of the late-1500s megadrought.

Salvage crew at St. Thomas, Nevada during Lake Mead first fill, 1938 via University of Nevada Las Vegas

Click the link to view an article on The Los Angeles Times (Ian James). Here’s an excerpt:

In their research, the scientists examined major droughts in southwestern North America back to the year 800 and determined that the region’s desiccation so far this century has surpassed the severity of a megadrought in the late 1500s, making it the driest 22-year stretch on record. The authors of the study also concluded that dry conditions will likely continue through this year and, judging from the past, may persist for years.

The researchers found the current drought wouldn’t be nearly as severe without global warming. They estimated that 42% of the drought’s severity is attributable to higher temperatures caused by greenhouse gases accumulating in the atmosphere…

Williams and his colleagues compared the current drought to seven other megadroughts between the 800s and 1500s that lasted between 23 years and 30 years.

Concentric rings of various widths mark the annual growth of trees. Photo by Peter Brown, Rocky Mountain Tree-Ring Research. Photo credit: NOAA

They used ancient records of these droughts captured in the growth rings of trees.

Wood cores extracted from thousands of trees enabled the scientists to reconstruct the soil moisture centuries ago. They used data from trees at about 1,600 sites across the region, from Montana to California to northern Mexico…

Some scientists describe the trend in the West as “aridification” and say the region must prepare for the drying to continue as temperatures continue to climb.

Brad Udall: Here’s the latest version of my 4-Panel plot thru Water Year (Oct-Sep) of 2021 of the Colorado River big reservoirs, natural flows, precipitation, and temperature. Data (PRISM) goes back or 1906 (or 1935 for reservoirs.) This updates previous work with

Click the link to read an article from The New York Times (Henry Fountain). Here’s an excerpt:

But exceptional conditions in the summer of 2021, when about two-thirds of the West was in extreme drought, “really pushed it over the top,” said A. Park Williams, a climate scientist at the University of California, Los Angeles, who led an analysis using tree ring data to gauge drought. As a result, 2000-21 is the driest 22-year period since 800 A.D., which is as far back as the data goes…

Julie Cole, a climate scientist at the University of Michigan who was not involved in the research, said that while the findings were not surprising, “the study just makes clear how unusual the current conditions are.”

Dr. Cole said the study also confirms the role of temperature, more than precipitation, in driving exceptional droughts. Precipitation amounts can go up and down over time and can vary regionally, she said. But as human activities continue to pump greenhouse gases into the atmosphere, temperatures are more generally rising.

As they do “the air is basically more capable of pulling the water out of the soil, out of vegetation, out of crops, out of forests,” Dr. Cole said. “And it makes for drought conditions to be much more extreme.”


Although there is no uniform definition, a megadrought is generally considered to be one that is both severe and long, on the order of several decades. But even in a megadrought there can be periods when wet conditions prevail. It’s just that there are not enough consecutive wet years to end the drought.

That has been the case in the current Western drought, during which there have been several wet years, most notably 2005…

Several previous megadroughts in the 1,200-year record lasted as long as 30 years, according to the researchers. Their analysis concluded that it is likely that the current drought will last that long. If it does, Dr. Williams said, it is almost certain that it will be drier than any previous 30-year period…

Monsoon rains in the desert Southwest last summer had offered hope that the drought might come to an end, as did heavy rain and snow in California from the fall into December.

But January produced record-dry conditions across much of the West, Dr. Williams said, and so far February has been dry as well. Reservoirs that a few months ago were at above-normal levels for the time of year are now below normal again, and mountain snowpack is also suffering. Seasonal forecasts also suggest the dryness will continue.

Lake Powell boat ramp at Page, Arizona, December 17, 2021. Photo credit: Allen Best/Big Pivots

Click the link to read an article on National Public Radio (Nathan Rott). Here’s an excerpt:

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.”


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…

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…

“We actually have to change our relationship with water,” [Park Williams said.]

This photo from December 2021 shows one of the intake towers at Hoover Dam. California, Nevada and Arizona recently penned a deal to keep 500,000 acre-feet of water in Lake Mead to boost the declining reservoir levels.

Click the link to read an article on The Pueblo Chieftain (Seth Borenstein). Here’s an excerpt:

A dramatic drying in 2021 – about as dry as 2002 and one of the driest years ever recorded for the region – pushed the 22-year drought passed the previous record-holder for megadroughts in the late 1500s and shows no signs of easing in the near future, according to a study Monday in the journal Nature Climate Change…

Williams studied soil moisture levels in the West – a box that includes California, Wyoming, Utah, Nevada, Arizona, most of Oregon and Idaho, much of New Mexico, western Colorado, northern Mexico, and the southwest corners Montana and Texas – using modern measurements and tree rings for estimates that go back to the year 800. That’s about as far back as estimates can reliably go with tree rings…

West Drought Monitor map February 8, 2022.

The drought monitor says 55% of the U.S. West is in drought with 13% experiencing the two highest drought levels…

Climate change from the burning of fossil fuels is bringing hotter temperatures and increasing evaporation in the air, scientists say.
Williams used 29 models to create a hypothetical world with no humancaused warming then compared it to what happened in real life – the scientifically accepted way to check if an extreme weather event is due to climate change. He found that 42% of the drought conditions are directly from human-caused warming. Without climate change, he said, the megadrought would have ended early on because 2005 and 2006 would have been wet enough to break it.

The study “is an important wakeup call,” said Jonathan Overpeck, dean of environment at the University of Michigan, who wasn’t part of the study. “Climate change is literally baking the water supply and forests of the Southwest, and it could get a whole lot worse if we don’t halt climate change soon.”

Williams said there is a direct link between drought and heat and the increased wildfires that have been devastating the West for years. Fires need dry fuel that drought and heat promote.

USFS highest risk firesheds January 2022.

Click the link to read an an article from The Washington Post (Diana Leonard). Here’s an excerpt:

The extreme heat and dry conditions of the past few years pushed what was already an epic, decades-long drought in the American West into a historic disaster that bears the unmistakable fingerprints of climate change. The long-running drought, which has persisted since 2000, can now be considered the driest 22-year period of the past 1,200 years, according to a study published Monday in the journal Nature Climate Change…

The double whammy of searing heat and persistent drought in recent years reflects the steady increase in global temperatures brought on by the burning of fossil fuels. The authors attribute 19 percent of the severe 2021 drought, and 42 percent of the extended drought since the 21st century began, to human-caused climate change…

This “background drying” brought on by a warmer atmosphere can dwarf occasional wet or cool periods. For example, the Southwest’s 2021 drought maintained its grip despite robust monsoon rains and record summer precipitation in some areas, in part because of extraordinary heat waves early last summer, and generally above-average temperatures…

The study finds that the 21st century has been substantially drier than the previous five decades, with 8.3 percent less precipitation, and nearly 1 degree Celsius (1.8 Fahrenheit) warmer than the period from 1950 to 1999…

That analysis found that at the current warming trajectory, droughts in drying regions that previously occurred only once every 10 years are now happening about 1.7 times per decade, on average. If the Earth warms 2 degrees Celsius, scientists expect those once-rare events to take place roughly 2½ times per decade, on average…

From Wednesday through Sunday, the National Weather Service in Los Angeles issued the first heat advisory on record during the winter months in Southern California. Scores of record high temperatures were set, from San Diego to San Francisco.

Death Valley soared to 94 degrees on Feb. 11, its highest temperature recorded so early in the season.

Douglas Fir tree rings via the Western Water Assessment

Click the link to read an article on The Guardian (Gabrielle Canon):

Turning up the temperature – the result of human caused warming – has played a big part. Other studies show how the climate crisis “will increasingly enhance the odds of long, widespread and severe megadroughts”, the researchers write. Noting that as the west is now in the midst of the driest 22-year period in knowable history, “this worst-case scenario already appears to be coming to pass”.

The research builds on conclusions from a previous study, also led by Williams, that ranked the period between 2000 and 2018 as the second driest in 12 centuries. The last two incredibly dry years – which were marked by record-setting heatwaves, receding reservoirs, and a rise in dangerously erratic blazes that burned both uncontrollably and unseasonably – were enough to push this period into first…

Looking at moisture levels in soils, the team of climate scientists from UCLA, Nasa, and Columbia University focused on landscapes from Montana to northern Mexico north to south and from the Pacific Ocean to the Rocky Mountains. They analyzed data collected tree ring patterns that offered clues to soil moisture levels throughout the centuries. Rings that appear closer together show the stunted growth patterns occurring during dry times…

According to Jason Smerdon, a climate scientist at the Lamont-Doherty Earth Observatory of Columbia University who worked with Williams on the study, the crisis is a “slow-motion trainwreck”.

“These multi-decade periods of dryness will only increase with the rest of the century,” Smerdon said.

Still, Smerdon cast the conclusions in a more hopeful light. The extreme events taking place right in people’s backyards may spur understanding and action. “Knowing is half the battle,” he says. “We have a lot of challenges in front of us but we all have agency in the face of this. And there are pathways we can take that are much more sustainable and involve much less risk than the burn baby burn approach that we would take if we didn’t do anything.”

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