Southwest Drought Rivals Those of Centuries Ago, Thanks to Climate Change ~ NYT

The drought that has gripped the American Southwest since 2000 is as bad as or worse than droughts in the region over the past 1,200 years, a new study finds. 

Credit…Rick Wilking/Reuters

ALBUQUERQUE — 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.

Dr. Williams and his colleagues reconstructed drought conditions in the Southwest for every year back to 800 A.D., using tree growth as a proxy for soil moisture content. Examining nearly 1,600 tree-ring records, they found four periods of more than two decades each during which soil moisture content was far below the baseline for the entire 1,200 years, indicating severe drought conditions of lack of precipitation and increased dryness. One of these megadroughts, in the 13th century, lasted more than 90 years.

Their analysis showed that, as measured by soil moisture content, the current drought is more severe than three of the ancient ones. Only one in the late 1500s was worse, and not by much, the researchers said.

“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.”

Water temperatures in the tropical Pacific Ocean are thought to play a large role in determining drought conditions in the Southwest, both now and in the past. Cooler sea-surface temperatures — what is commonly called La Niña — affect the jet stream, which pushes storms approaching Western North America to the north, leaving the Southwest drier.

But 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.

“As of Year 5 this was looking like the beginning of a megadrought,” Dr. Williams said. “It’s developing right along the same path.”

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.

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.

Among other things, the rapid growth of the region’s population in the 20th century, and the agreements to divide water resources like the Colorado River, came at a time when water was more abundant than it is now and is likely to be in the future. “It’s become clearer and clearer that the 1900s were a bad time to develop, if you were going to develop all-out and use every last drop of water available,” Dr. Williams said.

But the finding also makes attributing the current drought to climate change more complicated, he said.

“How much could this just be a return to the long-term average?” he said. “Maybe there was something going on that we don’t understand that caused the last 400 years to be so much wetter.”

“All of this is just to say there’s still a lot of mysteries out there,” Dr. Williams said. “We don’t have the whole story of Southwestern hydroclimate nailed down.”

Henry Fountain specializes in the science of climate change and its impacts. He has been writing about science for The Times for more than 20 years and has traveled to the Arctic and Antarctica.


The western U.S. is locked in the grips of the first human-caused megadrought, study finds ~ The Washington Post

Only one drought in the past 1,200 years comes close to the ongoing, global warming-driven event

Water levels at the Ward Creek Reservoir in Grand Mesa, Colo., have gone down in recent years because of persistent drought conditions. (Carolyn Van Houten/The Washington Post)

April 16 at 2:00 PM

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.”

Cook said the researchers analyzed climate models for the region, which showed warming trends and changes in precipitation. They compared soil moisture with and without global warming-induced trends, “and we were able to determine that 30 to 50 percent of the current drought is attributable to climate change.”

That conclusion is a first, says Jonathan Overpeck, a climate researcher at the University of Michigan who did not participate in the new study.

“They are the first to show conclusively that we’re experiencing our nation’s first megadrought of the instrumental era,” he said via email.

“The real take home,” Overpeck said, “is that the Southwest is being baked by the burning of fossil fuels and other human activities, and the future implications are dire if we don’t stop climate change.”

Looking for a megadrought, only to find it’s already here

Fire crews look for remaining hot spots left over by the Getty Fire, which destroyed a dozen homes, during the early morning hours in Brentwood, Calif., in October. (Michael Robinson Chavez/The Washington Post)

In 2015, Cook took part in a study that predicted a megadrought would grip the American Southwest starting about 2050 and persist for 35 years, with a few wet years to break long dry spells.

At the time, California was experiencing a severe four-year drought. As it dragged past a fifth year, Cook and others asked a question: “Are these changes in drought patterns that we expected in the future already beginning to happen?”

They then set out to answer that question.

For the study, the authors started from scratch, analyzing tree-ring data rather than relying solely on archived information. They took apart calculations already in the record and modified them where needed. Not only was a megadrought happening, they concluded, it had been in progress since the turn of the century.

The same year that Cook and other researchers published their first study, a Stanford University scientist, Noah Diffenbaugh, led a separate study that said rising temperatures and significant declines in snow and rainfall will parch California for years to come.

Diffenbaugh, a professor and senior fellow who studies the Southwest, had also analyzed data showing that the region was becoming hotter and drier. He said Thursday’s study, which he was not involved with, is a breakthrough because of its comparison of droughts in the past two decades to those in the previous thousand years.

“Placing the two-decade period in the whole region in the context of the last millennium is very striking, very powerful,” Diffenbaugh said. “We can conclude that without the warming, this period would not have produced such a severe, regionwide drought.”

What this means for the West

Valerie Trouet, a researcher at the Laboratory of Tree-Ring Research at the University of Arizona, says that the megadroughts of the past brought about major societal impacts, particularly those that persisted for decades.

For example, the megadrought seen in the late 800s is thought to have instigated the downfall of the Mayan civilization. The severe drought in the 16th century may have contributed to the Chichimeca War in Mexico, during which Native Americans and European settlers fought for decades.

“All of these past megadroughts have had severe impacts,” Trouet said in an interview. “We can expect there to be societal impacts now, too.”

These effects may not be as devastating in the future, however. Modern humans have more ways to adapt, Cook said.

“There are a lot of things we can do about it. People in the West are dealing with this drought in a number of ways,” Cook said.

California has already provided a model for living in a warmer and drier region, although it has involved sacrifice at times. Amid its drought in 2015, the state took aggressive steps to preserve water and limit wildfires on thirsty land with varying success. Former governor Jerry Brown (D) imposed the first water restrictions in state historyand declared that watering lawns was going to be “a thing of the past” in California.

Water utilities essentially rationed supply, telling residents to dramatically cut the minutes they showered to no longer than 12 and all but mandating more efficient machines for laundry and dish washing. Utilities encouraged homeowners to purchase new appliances with rebates subsidized by the state, water bills spiked and penalties were imposed on any household that went over their limits. Neighbors spied on neighbors who washed cars, watered grass and sprayed driveways, all outlawed.

A swimming pool contrasts with the drought-dried landscape in East Porterville, Calif., in 2015. With access to water limited, the pool’s owner doesn’t drain it, instead using chemicals to keep it clean. (Bonnie Jo Mount/The Washington Post)

East Porterville, Calif., in the Central Valley became a town without water. A church set up a shower trailer so residents whose wells went dry could wash. The state placed water tanks outside homes so their toilets would flush. Laundering clothes, washing hands and brushing teeth became luxuries.

The drought and the drive to save water had environmental consequences, as well. It resulted in the death of trees that improved air quality, provided animal habitats and beautified urban areas across California. Urban trees joined about 12.5 million wild trees that died in dry California forests during 2015’s drought, according to the U.S. Forest Service.

Such serious drought effects happened with only about 1 degree Celsius of warming since the industrial revolution, Diffenbaugh said. “The impacts we’ve already seen from one degree of warming really highlights the intensification of what’s coming,” he said.


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