There's a reason why the two largest reservoirs in the U.S. are ?? ?????? ??????? ???????? ???? ?????? 40 ???wavering around half full, a far cry from the bountiful, wet days of decades past. It's why water managers in the Southwest have agreed to mandatory water cuts, and why millions of annual visitors to the Hoover Dam can't ignore the telltale white ring around Lake Mead, revealing where the water once reached during more plentiful times.

Much of Western America is mired in a historically unprecedented 19-year drought, though there have been wet spells within the persistent dry period. But this isn't a normal drought. Previous research has suggested the Southwest might be in a bonafide megadrought— a fuzzy term referencing the most severe and enduring of droughts over the last millennium.

Now, a study published Thursday in the journal Science providesevidence that this parched period (covering nine U.S. states from Oregon down to California and New Mexico) is among the worst droughts to hit the region in some 1,200 years — and the relentlessly warming climate is a major reason why.

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"This current drought is on par with the megadroughts of the Medieval Era," said Benjamin Cook, a research scientist at Columbia University’s Lamont-Doherty Earth Observatory and an author of the study.

"The reason this drought is so severe and approaching the magnitude of megadroughts of 1,000 years ago is largely due to climate change," Cook said.

Among the dozens of droughts between 800 CE through the present day, only four previous events reached the scale of major megadroughts — including one that may have pushed the flourishing Chaco Culture to abandon their great houses and intricate societies. In this latest analysis, the 21st century drought ranks second in dryness, only outmatched by the last profoundly dry era in the late 1500s.

But without human-caused climate change, which has so far warmed the region by over two degrees Fahrenheit since the 19th century, this drought would have only ranked as the eleventh worst. Importantly, there would have still naturally been a drought, as dry spells come and go in the American Southwest.

But it wouldn't have been mega.

"The majority of droughts happen naturally," explained Flavio Lehner, a climate scientist at the National Center for Atmospheric Research who had no role in the research. "But this drought is being made worse by a human contribution."

"It would have seemed much more normal without climate change," said Cook. "With climate change, it pushed this drought into an exceptional event."

Southwest drought over May 31 - June 7, 2018. Credit: Nasa Soil moisture over the last 1,200 years. Blue line at bottom shows 200-2018 mean, as a comparison. Credit: Adapted from Williams et al., Science, 2020

The whole planet is warming, not just the American Southwest. With unchecked carbon emissions, atmospheric levels of the potent heat-trapping gas carbon dioxide have skyrocketed to their highest levels in at least800,000 years, but more likely millions of years. Nineteen of the last 20 years are now the warmest on record.

To grasp the severity of the current 19-year drought, the researchers analyzed over 1,500 tree ring records which represent tree growth going back 1,200 years. Tree rings preserve a natural history of how wet and dry the climate used to be, and in this study the climate researchers used this ancient evidence to gauge the levels of moisture in the summer soil.

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"The Southwest is a terrific region to be doing this kind of analysis," said Valerie Trouet, a paleoclimatologist and associate professor in the Laboratory of Tree-Ring Research at the University of Arizona who was not involved in the study, noting desert trees are especially sensitive to temperature.

"They're very good recorders of past drought," Trouet said. Southwestern trees don't grow as well during hot, dry summers, and they leave clear proof of this in their tree rings.

"This is just the beginning."

Today, the exceptional drying out of the Southwest is unquestionable. There have certainly been some years of good snowpack in the mountains that feed into the main water artery of the Southwest, the Colorado River, explained Brad Udall, a senior water and climate research scientist at Colorado State University. But hotter temperatures also drive drought by drying out the ground. That's a major reason why there's been less water flowing through the Colorado River for the past two decades.

"It's the high temperatures that push us into this unprecedented drought situation," said Udall, who had no involvement in the research.

"This is our future," he added. "The drying of the American West."

To understand how these higher temperatures influenced today's drought, the researchers also used over 30 climate models to simulate how much rising greenhouse gases (like CO2) in the atmosphere impacted the current 19-year dry spell. "It'sstate-of-the-art science," noted the National Center for Atmospheric Research's Lehner. Taken together, these simulations of an atmosphere disturbed by humans show climate change is responsible for between 30 to 50 percent of the current drought. So in an unchanged world, the drought wouldn't have been nearly as severe.

"That's a pretty large component of this event," said Cook.

The current extreme drought has big implications for the future. Indeed, in the years or decades ahead the West may experience wetter-than-normal years — which might temporarily relieve western states from their ever-increasing water woes. But, overall, it will get warmer this century, meaning more parched years overall and, critically, more severe future droughts.

"They will be more severe," said Trouet. "This is just the beginning."

"Betting on wet years to bail us out is a real terrible strategy," emphasized Udall.

A tourist overlooks the Hoover Dam in 2015. Credit: Jae C Hong /AP / Shutterstock Drought in Western states in the early 2000s. Credit: Adapted from Williams et al., Science, 2020 / Columbia University

The better strategy, according to many economists, political scientists, and climate experts, is slashing carbon emissions enough to significantly limitfuture warming. This will limit the severity of drought, even if civilization doesn't meet the immensely ambitious climate targets of capping Earth's warming at 1.5 degrees Celsius (or 2.7 degrees Fahrenheit) above 19th century levels. (We're probably not going to meet this big climate goal).

Already, the West is feeling the heat. A 1.6 million-acre national forest in New Mexico temporarily closed in 2018 due to exceptionally dry conditions and high fire risk. Nearly 200 dead horses were found decomposing around a dried-up Arizona watering holing that same year. Wildfires are surging. Many of the first widescale mandatory cutbacks on water use have begun.

"We’re living through something very serious," said Udall, who has previously published research on the reduction in water flow through the Colorado River — a consequence of warmer climes.

It's unknown how long this megadrought will last. But it's well-known that the problem is exacerbated by human activity. Hoping it will end isn't a viable solution, emphasize climate experts. It will take exceptional climate leadership and smart water management to avoid ever-worsening droughts and water shortages in the coming years.

"The losing move is to put your head in the sand and pretend it’s not happening," said Cook.

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