Climate-Driven #Megadrought Is Emerging in Western U.S., Says Study — Columbia University

Here’s the release from Columbia University (Kevin Krajik):

With the western United States and northern Mexico suffering an ever-lengthening string of dry years starting in 2000, scientists have been warning for some time that climate change may be pushing the region toward an extreme long-term drought worse than any in recorded history. A new study says the time has arrived: a megadrought as bad or worse than anything even from known prehistory is very likely in progress, and warming climate is playing a key role. The study, based on modern weather observations, 1,200 years of tree-ring data and dozens of climate models, appears this week in the leading journal Science.

“Earlier studies were largely model projections of the future,” said lead author Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. “We’re no longer looking at projections, but at where we are now. We now have enough observations of current drought and tree-ring records of past drought to say that we’re on the same trajectory as the worst prehistoric droughts.”

Reliable modern observations date only to about 1900, but tree rings have allowed scientists to infer yearly soil moisture for centuries before humans began influencing climate. Among other things, previous research has tied catastrophic naturally driven droughts recorded in tree rings to upheavals among indigenous Medieval-era civilizations in the Southwest. The new study is the most up-to-date and comprehensive long-term analysis. It covers an area stretching across nine U.S. states from Oregon and Montana down through California and New Mexico, and part of northern Mexico.

Areas of southwestern North America affected by drought in the early 2000s; darker colors are more intense. Yellow box shows the study area. (Adapted from Williams et al., Science, 2020)

Using rings from many thousands of trees, the researchers charted dozens of droughts across the region, starting in 800 AD. Four stand out as so-called megadroughts, with extreme aridity lasting decades: the late 800s, mid-1100s, the 1200s, and the late 1500s. After 1600, there were other droughts, but none on this scale.

The team then compared the ancient megadroughts to soil moisture records calculated from observed weather in the 19 years from 2000 to 2018. Their conclusion: as measured against the worst 19-year increments within the previous episodes, the current drought is already outdoing the three earliest ones. The fourth, which spanned 1575 to 1603, may have been the worst of all — but the difference is slight enough to be within the range of uncertainty. Furthermore, the current drought is affecting wider areas more consistently than any of the earlier ones — a fingerprint of global warming, say the researchers. All of the ancient droughts lasted longer than 19 years — the one that started in the 1200s ran nearly a century — but all began on a similar path to to what is showing up now, they say.

Nature drove the ancient droughts, and still plays a strong role today. A study last year led by Lamont’s Nathan Steiger showed that among other things, unusually cool periodic conditions over the tropical Pacific Ocean (commonly called La Niña) during the previous megadroughts pushed storm tracks further north, and starved the region of precipitation. Such conditions, and possibly other natural factors, appear to have also cut precipitation in recent years. However, with global warming proceeding, the authors say that average temperatures since 2000 have been pushed 1.2 degrees C (2.2 F) above what they would have been otherwise. Because hotter air tends to hold more moisture, that moisture is being pulled from the ground. This has intensified drying of soils already starved of precipitation.

Nature drove the ancient droughts, and still plays a strong role today. A study last year led by Lamont’s Nathan Steiger showed that among other things, unusually cool periodic conditions over the tropical Pacific Ocean (commonly called La Niña) during the previous megadroughts pushed storm tracks further north, and starved the region of precipitation. Such conditions, and possibly other natural factors, appear to have also cut precipitation in recent years. However, with global warming proceeding, the authors say that average temperatures since 2000 have been pushed 1.2 degrees C (2.2 F) above what they would have been otherwise. Because hotter air tends to hold more moisture, that moisture is being pulled from the ground. This has intensified drying of soils already starved of precipitation.

Varying soil moisture in southwestern North America, 800-2018. The straight horizontal center line indicates average moisture; blue line at bottom shows 2000-2018 mean. Green bars indicate abnormally wet periods, pink ones abnormally dry. The fluctuating red moisture line is based on tree-ring data until it converts to blue at the start of modern instrumental observations. (Adapted from Williams et al., Science, 2020)

All told, the researchers say that rising temperatures are responsible for about half the pace and severity of the current drought. If this overall warming were subtracted from the equation, the current drought would rank as the 11th worst detected — bad, but nowhere near what it has developed into.

“It doesn’t matter if this is exactly the worst drought ever,” said coauthor Benjamin Cook, who is affiliated with Lamont and the Goddard Institute for Space Studies. “What matters is that it has been made much worse than it would have been because of climate change.” Since temperatures are projected to keep rising, it is likely the drought will continue for the foreseeable future; or fade briefly only to return, say the researchers.

“Because the background is getting warmer, the dice are increasingly loaded toward longer and more severe droughts,” said Williams. “We may get lucky, and natural variability will bring more precipitation for a while. But going forward, we’ll need more and more good luck to break out of drought, and less and less bad luck to go back into drought.” Williams said it is conceivable the region could stay arid for centuries. “That’s not my prediction right now, but it’s possible,” he said.

Lamont climatologist Richard Seager was one of the first to predict, in a 2007 paper, that climate change might eventually push the region into a more arid climate during the 21st century; he speculated at the time that the process might already be underway. By 2015, when 11 of the past 14 years had seen drought, Benjamin Cook led a followup study projecting that warming climate would cause the catastrophic natural droughts of prehistory to be repeated by the latter 21st century. A 2016 study coauthored by several Lamont scientist reinforced those findings. Now, says Cook, it looks like they may have underestimated. “It’s already happening,” he said.

The effects are palpable. The mighty reservoirs of Lake Mead and Lake Powell along the Colorado River, which supply agriculture around the region, have shrunk dramatically. Insect outbreaks are ravaging dried-out forests. Wildfires in California and across wider areas of the U.S. West are growing in area. While 2019 was a relatively wet year, leading to hope that things might be easing up, early indications show that 2020 is already on a track for resumed aridity.

In the Catalina Mountains in southern Arizona, forests struggle to keep up with recent increases in drought and wildfire activity, which are expected to continue due to human-caused climate change. (Park Williams/Lamont-Doherty Earth Observatory)

“There is no reason to believe that the sort of natural variability documented in the paleoclimatic record will not continue into the future, but the difference is that droughts will occur under warmer temperatures,” said Connie Woodhouse, a climate scientist at the University of Arizona who was not involved in the study. “These warmer conditions will exacerbate droughts, making them more severe, longer, and more widespread than they would have been otherwise.”

Angeline Pendergrass, a staff scientist at the U.S. National Center for Atmospheric Research, said that she thinks it is too early to say whether the region is at the cusp of a true megadrought, because the study confirms that natural weather swings are still playing a strong role. That said, “even though natural variability will always play a large role in drought, climate change makes it worse,” she said.

Tucked into the researchers’ data: the 20th century was the wettest century in the entire 1200-year record. It was during that time that population boomed, and that has continued. “The 20th century gave us an overly optimistic view of how much water is potentially available,” said Cook. “It goes to show that studies like this are not just about ancient history. They’re about problems that are already here.”

The study was also coauthored by Edward Cook, Jason Smerdon, Kasey Bolles and Seung Baek, all of Lamont-Doherty Earth Observatory; John Abatzaglou of the University of Idaho; and Andrew Badger and Ben Livneh of the University of Colorado Boulder.

From Inside Climate News (Bob Berwyn):

Warmer temperatures and shifting storm tracks are drying up vast stretches of land in North and South America.

The American West is well on its way into one of the worst megadroughts on record, a new study warns, a dry period that could last for centuries and spread from Oregon and Montana, through the Four Corners and into West Texas and northern Mexico.

Several other megadroughts, generally defined as dry periods that last 20 years or more, have been documented in the West going back to about 800 A.D. In the study, the researchers, using an extensive tree-ring history, compared recent climate data with conditions during the historic megadroughts.

They found that in this century, global warming is tipping the climate scale toward an unwelcome rerun, with dry conditions persisting far longer than at any other time since Europeans colonized and developed the region. The study was published online Thursday and appears in the April 17 issue of the journal Science.

Human-caused global warming is responsible for about half the severity of the emerging megadrought in western North America, said Jason Smerdon, a Columbia University climate researcher and a co-author of the new research.

“What we’ve identified as the culprit is the increased drying from the warming. The reality is that the drying from global warming is going to continue,” he said. “We’re on a trajectory in keeping with the worst megadroughts of the past millennia.”

The ancient droughts in the West were caused by natural climate cycles that shifted the path of snow and rainstorms. But human-caused global warming is responsible for about 47 percent of the severity of the 21st century drought by sucking moisture out of the soil and plants, the study found.

The regional drought caused by global warming is plain to see throughout the West in the United States. River flows are dwindling, reservoirs holding years worth of water supplies for cities and farms have emptied faster than a bathtub through an open drain, bugs and fires have destroyed millions of acres of forests, and dangerous dust storms are on the rise.

A similar scenario is unfolding in South America, especially in central Chile, a region with a climate similar to that in western North America. Parts of the Andes Mountains and foothills down to the coast have been parched by an unprecedented 10-year dry spell that has cut some river flows by up to 80 percent.

In both areas, research shows, global warming could make the droughts worse than any in at least several thousand years, drying up the ground and shifting regional weather patterns toward drier conditions. This is bad news for modern civilizations that have developed in the last 500 years, during which they enjoyed an unusually stable and wet climate. And assumptions about water availability based on that era are not realistic, said climate scientist Edward Cook, another co-author on the study who is also with the Lamont-Doherty Earth Observatory.

The impacts of a long-lasting drought in the West could also affect adjacent regions. A 2019 study showed that dry conditions in upwind areas may be intensifying agricultural droughts. With west winds prevailing across North America, hot and dry conditions in the Southwest could reduce the amount of atmospheric moisture available to produce rainfall farther east, in Oklahoma and Texas, for example. The study found that such drought linkages accounted for 62 percent of the precipitation deficit during the 2012 Midwest drought…

In North and in South America, researchers have identified natural climate cycles as key drivers of historic megadroughts. The most important are a combination of a warm North Atlantic Ocean and cooler-than average conditions in the eastern tropical Pacific Ocean, as well as decreased solar and volcanic activity.

“Arid periods over the last several millennia have dwarfed anything we’ve seen so far,” Smerdon said. And when the soil-drying effect of human-caused warming is added into the climate equation, the outlook is not good. Previous studies by Columbia University researchers predicted that the 21st century has a 90 percent chance of seeing a drought that lasts 25 years or longer.

He said that prospect will require people to rethink how to manage resources.

“On a regional level, this means being more proactive about water management,” Smerdon said. “There are things we can do if you recognize that the West will probably be much drier. You can start thinking about transitioning to less water intensive crops, or about beef production, which is incredibly water intensive.”

Other features “that go part and parcel with these droughts are things like forest fires and beetle infestations,” he added, noting that there were also impacts to winter recreation and tourism, with less snow for skiing and water for rafting.

Smerdon said he’s also concerned that the drought impacts are being underestimated because of an over-reliance on groundwater as a temporary buffer to the decline of river flows, and the drop of reservoir water levels. If you look at simultaneous droughts in North and South America, he said, you could also anticipate potential impacts to global food supply networks, as both regions are important for agricultural production.

The only real long-term solution is to halt greenhouse gas pollution, he said.

Photo of Lake Powell in extreme drought conditions by Andy Pernick, Bureau of Reclamation, via Flickr creative commons

From The Washington Post (Andrew Freedman and Darryl Fears):

A vast region of the western United States, extending from California, Arizona and New Mexico north to Oregon and Idaho, is in the grips of the first climate change-induced megadrought observed in the past 1,200 years, a study shows. The finding means the phenomenon is no longer a threat for millions to worry about in the future, but is already here.

The megadrought has emerged while thirsty, expanding cities are on a collision course with the water demands of farmers and with environmental interests, posing nightmare scenarios for water managers in fast-growing states.

A megadrought is broadly defined as a severe drought that occurs across a broad region for a long duration, typically multiple decades.

Unlike historical megadroughts triggered by natural climate cycles, emissions of heat-trapping gases from human activities have contributed to the current one, the study finds. Warming temperatures and increasing evaporation, along with earlier spring snowmelt, have pushed the Southwest into its second-worst drought in more than a millennium of observations.

The study, published in the journal Science on Thursday, compares modern soil moisture data with historical records gleaned from tree rings, and finds that when compared with all droughts seen since the year 800 across western North America, the 19-year drought that began in 2000 and continued through 2018 (this drought is still ongoing, though the study’s data is analyzed through 2018) was worse than almost all other megadroughts in this region.

The researchers, who painstakingly reconstructed soil moisture records from 1,586 tree-ring chronologies to determine drought severity, found only one megadrought that occurred in the late 1500s was more intense.

Historical megadroughts, spanning vast regions and multiple decades, were triggered by natural fluctuations in tropical ocean conditions, such as La Niña, the cyclic cooling of waters in the tropical Pacific.

“The megadrought era seems to be reemerging, but for a different reason than the [past] megadroughts,” said Park Williams, the study’s lead author and a researcher at the Lamont-Doherty Earth Observatory at Columbia University.

Although many areas in the West had a productive wet season in 2019 and some this year, “you can’t go anywhere in the West without having suffered drought on a millennial scale,” Williams said, noting that megadroughts contain relatively wet periods interspersed between parched years.

“I think the important lesson that comes out of this is that climate change is not a future problem,” said Benjamin I. Cook, a NASA climate scientist and co-author of the study. “Climate change is a problem today. The more we look, the more we find this event was worse because of climate change.”

Drought affected Lake Mead via the Mountain Town News

From The New York Times (Henry Fountain):

A severe drought that has gripped the American Southwest since 2000 is as bad as or worse than long-lasting droughts in the region over the past 1,200 years, and climate change has helped make it that way, scientists said Thursday.

The researchers described the current drought, which has helped intensify wildfire seasons and threatened water supplies for people and agriculture, as an “emerging megadrought.” Although 2019 was a relatively wet year, and natural climate variability could bring good luck in the form of more wet years that would end the drought, global warming increases the odds that it will continue.

“We know that this drought has been encouraged by the global warming process,” said Park Williams, a bioclimatologist at Lamont-Doherty Earth Observatory at Columbia University, and lead author of a study published Thursday in Science. “As we go forward in time it’s going to take more and more good luck to pull us out of this.”

While the term megadrought has no strict definition, it is generally considered to be a severe dry period persisting for several decades or longer. Many climate researchers and hydrologists have long thought that a Southwestern megadrought was highly likely. A 2016 study put the probability of one occurring this century at 70 percent or higher…

“Ancient megadroughts have always been seen by water managers as worst-case scenarios,” Dr. Williams said, “and we just have to hope that there’s some kind of protection measure in the climate system that’s not going to allow one of those to repeat itself. And what we’re seeing is that we’re actually right on track for one.”

[…]

since the beginning of the 20th century, when large-scale emissions of heat-trapping gases began, warming has played a role as well. Using 31 computer climate models, the researchers estimated that climate change contributed nearly half to the severity of the current drought.

Put another way, without global warming the current drought would be only of moderate severity rather than one of the worst.

While the natural variability of La Niña conditions continues, Dr. Williams said, “all of that is being superimposed on what appears to be a pretty strong long-term drying trend.”

The current drought has followed a pattern that is similar to the ancient ones, he said. Rather than one or two extremely dry years that would suddenly throw the region into drought, dry conditions have been nearly continuous and the drought has built up over time…

Brad Udall, a water and climate research at Colorado State University who was not involved in the study, said that tying the current drought to a longer-term context “fits what a lot of people have been thinking.”

Dr. Udall said the researchers’ finding that climate change accounted for about half of the drought’s severity was strongly supported by his and others’ recent studies of the shrinking flow of the Colorado River, which attribute about half of the decline to global warming.

“I love the focus on soil moisture,” Dr. Udall said of the new study. “People underappreciate how important soil moisture is.”

Soils have a buffering effect that can allow problems of water scarcity to persist even after a relatively wet year, because soils that are dry from years of drought soak up more water that would normally run into rivers and streams.

The wetter weather in 2019, for example, resulted in a deep mountain snowpack across much of the West. “But it’s increasingly clear we didn’t get the runoff we had expected,” Dr. Udall said.

The latest ancient megadrought the researchers found was the long one in the 16th century. That finding reinforces the widely held idea that conditions were relatively wetter in the Southwest for centuries before the current drought.

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