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The southwestern mega-drought is the driest period in over 1,200 years

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The extreme drought that has plagued the American West for more than two decades is now believed to be the driest 22-year period in at least 1,200 years, and scientists have found that this mega-drought is being exacerbated by human warming of the planet .

In their research, scientists examined major droughts in southwestern North America dating back to 800 AD and found that the region’s drying out this century was comparable to the severity of a late 16th-century megadrought. The study’s authors also concluded that the drought is likely to continue this year and could last for years after the past.

The researchers found that the current drought wouldn’t be nearly as bad if it weren’t for global warming. They estimated that 42% of the drought’s severity was due to higher temperatures caused by greenhouse gases accumulating in the atmosphere.

“The results are really concerning because they show that the drought conditions we’re facing now are significantly worse due to climate change,” said Park Williams, a UCLA climate scientist and lead author of the study. “But there is also still a lot of room for drought conditions to worsen.”

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

They used old records of these droughts, recorded in tree growth rings.

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

The study, published Monday in the journal Nature Climate Change, adds to a growing body of research showing the American West faces major challenges as fossil fuel burning continues to drive temperatures higher and amplify the trend toward dehydration .

Williams was part of a team that published a similar study in 2020. At that time, they noted that the drought since 2000 was the second worst after the mega drought of the late 1600s. With widespread heat and drought over the past two years, the current drought has crossed that extreme mark.

Some scientists are describing the trend in the west as “aridification,” and say the region needs to brace itself for continued desiccation if temperatures continue to rise.

Williams said the West is prone to extreme dry-to-wet swings, like a yo-yo going up and down, but those swings are now being “overlaid by a serious drying trend” due to climate change.

“The cubes were loaded so heavily for drying,” he said.

The average temperature in southwestern North America since 2000 was 1.6 degrees Fahrenheit warmer than the average for the past 50 years, the researchers said. Warmer temperatures have exacerbated the drought by increasing evaporation, drying out soils and reducing water flow in streams and rivers.

Higher temperatures make the atmosphere thirstier and dry out soil and vegetation, much like “our houseplants dry out when we turn on the heat,” Williams said.

The scientists pointed out that in the 2020 and 2021 water years, the flow of the Colorado River shrank to the lowest two-year average in more than a century of records.

The river supplies water to seven states, from Wyoming to California and northern Mexico. But it has been chronically overused, and the drought has made the problems worse. Last year, the two largest reservoirs, Lake Mead and Lake Powell, sank to their lowest levels on record.

“We need to understand that the water balance of the West is changing rapidly beneath our feet,” Williams said. “We need to be prepared for a much drier future and not rely so much on the hope that when it gets wet again we can just go back to normal water management.”

The hot, dry years have severely affected water supplies and landscapes throughout California and the West. California’s reservoirs have been sinking over the past two years. In Utah, the Great Salt Lake has dropped to a record low. Extreme heat has contributed to explosive wildfires. And in the Mojave Desert, scientists have attributed the sharp decline in bird populations to hotter, drier conditions caused by climate change.

Of course, even without climate change, the past two decades would have been a “bad luck” for the region, Williams said. But without the impact of climate change, he said, “this drought wouldn’t even come close to the worst mega-drought.”

Some of the long droughts were those from 1213 to 1237 and from 1271 to 1300. During this century, the indigenous people who lived and farmed in the villages of the Four Corners region are said to have abandoned their cliff-side homes because of the drought.

The scientists examined data collected over decades by hundreds of other researchers who extracted wood cores by drilling into long-lived trees such as Douglas fir, Piñon pine, ponderosa pine and blue oak.

They found that the current drought spanned two years — 2002 and 2021 — among the driest in the past 1,200 years. And with last year’s increase in drought, Williams said, those 22 years have already averaged drier than most prolonged mega-droughts.

The late 15th century drought ended abruptly after 23 years as wet conditions swept the region. But the current drought shows no signs of abating.

According to the US Drought Monitor website, 96% of the western US is now unusually dry or worse, and 88% of the region is experiencing drought.

The scientists predicted that the drought will most likely last at least this year. They considered a hypothetical future scenario based on soil moisture in all 40-year periods over the past 1,200 years, and then overlaid the same amount of climate change-related drying that has occurred in recent years. They found that the drought lasted at least a 23rd year in 94% of their simulations. And in 75% of the simulations, the drought lasted 30 years.

“When it’s in a very depleted state, it takes a long time to refill the bucket,” Williams said. “It would take exceptional luck to end this drought in the next few years. In the last 1,200 years of data available to us, there have been few examples of this kind of happiness.”

Williams co-authored the study with researchers Benjamin Cook and Jason Smerdon of Columbia University’s Lamont-Doherty Earth Observatory. They used 29 climate models to estimate the impact of higher temperatures triggered by climate change.

When they analyzed how the drought would have evolved without climate change, they found that the region would have emerged from drought in the wet years of 2005 and 2006 and then drought again in 2007, Williams said.

The scientists used a 10-year average to assess long-term trends, so a single wet year like 2019 wasn’t enough to end the streak of mostly dry years.

The research focused on the entire region, with differences depending on the area. While drought was most extreme in areas from Arizona to the Rocky Mountains, the study showed that much of California experienced one of the driest 22-year periods, although not the absolute driest.

Williams said the research should serve as a warning that drying could get much worse in the years and decades to come.

“The great mega-droughts of the last millennium happened without climate change,” Williams said. If such mega-droughts return, they will “occur in a world where the atmosphere is also artificially warmer due to human-caused climate change, which would be absolutely catastrophic.”

Isla Simpson, a climate scientist at the National Center for Atmospheric Research who was not involved in the study, said she thinks the methods are sound and the results made an important contribution to the science to date.

“It’s really useful to have this update considering how tough the last two years have been,” Simpson said.

She said the current drought was partly due to low rainfall, but it was really the effects of higher temperatures that had worsened drying and was a “very clear signal of climate change”.

“We’re out of the 20th century climate in terms of temperature, which will affect evaporation and soil moisture,” Simpson said. There will still be the natural variability from dry to wet, she added, “but we’re seeing that variability now within this long-term desiccation due to anthropogenic climate change that will exacerbate events.”

Williams said the research shows real problems with chronic overexploitation of water sources like the Colorado River, which have fueled the growth of cities from Los Angeles to Phoenix over the past century. He said widespread depletion of groundwater was another symptom of overexploitation of the region’s critical water reserves.

A lot of people in the west might not feel like they’re going through a mega-drought, he said, because “we have all these buffers in our system now, like groundwater and big reservoirs.”

“But we’re using these backstops so fast right now that we’re taking a real risk that in 10 or 20 years, those backstops won’t be there for us,” he said, “if either this event isn’t over or if it is The next mega drought has already begun.”

https://www.latimes.com/environment/story/2022-02-14/western-megadrought-driest-in-1200-years The southwestern mega-drought is the driest period in over 1,200 years

Tom Vazquez

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