Atmospheric rivers could pound California with more extreme rain

For years, scientists have argued that atmospheric rivers can increase or destroy the water supplies of thirsty California cities and farms.

Over the past two winters, a steady succession of these “giant rivers in the sky” have dumped record, drought-busting rainfall across the state, while simultaneously causing catastrophic flooding, landslides and dangerous blizzards .

But now, new research has shown that these recent atmospheric rivers pale in comparison to some of the monster storms that hit ancient California — a sobering revelation that suggests to some experts the state could be hit again by such cataclysmic storms.

“Our results show that atmospheric activity in rivers exceeds what has occurred since instrumental record keeping began,” said Clarke Knight, a U.S. Geological Survey research geographer and lead author of the study. which detailed – for the first time – the atmospheric activity of rivers dating back 3,200 years. years. “This is important because it suggests the latent potential for our region to experience storms beyond those we saw today.”

Although few people had heard of atmospheric rivers until just a few decades ago, research into the gigantic vapor trails has proven essential to water planning and public safety in California.

The study’s findings do not bode well for a state whose flood infrastructure was severely strained last year, when a train of atmospheric rivers breached numerous levees, inundated communities and filled the formerly dry Tulare Lake again. The findings also up the ante for the state’s efforts to capture stormwater, as climate change drives more precipitation as rain rather than snow and sets the stage for a new era of more frequent droughts and more prolonged.

Knight and his fellow researchers came to their conclusion after analyzing ancient mud layers from Lake Leonard in Mendocino County. The team was able to determine when more sediment had been pushed into the lake, indicating periods of higher precipitation.

Then, using data on atmospheric rivers over the past 60 years, the researchers discovered a “strong correlation” between their sediment findings and modern storms, allowing them to model this connection across the rest of the mud layers. to reconstruct the historical atmospheric activity of rivers, Knight. said. Their research was published Thursday in the journal Nature.

The research provides the most historical context yet for precipitation variability in the state and found that the region “has consistently experienced extreme precipitation over a 3,200-year period.”

Knight said this new hydrology data can better inform climate modeling and projections, providing a historical record 20 times longer than what was available.

Although the team’s research focused on Northern California — where the state typically experiences the most atmospheric rivers — she said it was fair to conclude that the southern half of the state would have experienced similarly extreme precipitation in its ancient climate, given the widespread effects of great atmospheric changes. rivers.

Previous research has shown that the average atmospheric flow of a river carries more than twice the flow of the Amazon River. The prospect of even bigger storms hitting California is concerning, experts say.

Daniel Swain, a UCLA climatologist who was not involved in the USGS study, said the paper provides “direct physical evidence” of more extreme riverine atmospheric activity than anything that has been studied. observed in California’s recent history – well beyond the Great Flood of 1862, which reconfigured the state’s shape. landscape.

He said the research “reemphasizes the danger of assuming that the extremes we saw in the 20th century are representative of the kinds of extremes possible in this part of the world.”

“It shows that, even if we didn’t have to deal with climate change, we should remain cautious about the risks posed by extremes, because we know that the climate system… can throw big, bad things at us. periodically,” Swain said. “I don’t find that reassuring at all.”

The continued rise in global average temperature due to humanity’s burning of fossil fuels also threatens to exacerbate the situation.

“Adding energy into the system via greenhouse gas emissions is like shaking the soda can…and adding a little more energy into the system, which allows these extremes to occur. “be a little more extreme,” said Cody Poulsen, a graduate student. researcher at the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, who was also not involved in the Nature study.

Swain posited that each degree of global temperature increase increases the risk of an “ARkStorm scenario” – a megaflood that only occurs once every thousand years. But these new USGS findings could indicate that the worst-case scenario modeling isn’t extreme enough, he said.

A child walks under the Dana Point waterfall, which only happens during heavy rains, in March 2023.

(Allen J. Schaben / Los Angeles Times)

For a state grappling with more frequent and severe droughts, the past two wet winters have been a rare blessing. However, many Californians might be surprised to learn that these two wet seasons fall within the realm of natural variability. They might also be surprised to learn that this year the flow of atmospheric rivers was greater than the previous year, causing far more damage and disruption.

Recently, researchers confirmed that 51 atmospheric rivers hit the West Coast during the 2023-2024 rainy season, far more than the 38 atmospheric rivers that hit the 2022-23 rainy season, according to new data from the Center for Western Weather and Water Extremes. .

In California specifically, 44 atmospheric rivers made landfall from October to March, compared to 31 during last year’s rainy season, said Chad Hecht, the center’s meteorologist.

But even though there were more atmospheric rivers this rainy season, fewer storms were strong or extreme on the central strength scale compared to the previous season.

“It’s not the quantity that matters, it’s the quality,” Hecht said.

For example, 12 strong, extreme, or exceptional atmospheric river storms hit California between October 2022 and March 2023. These more severe storms tend to bring news-making rain and snow. This season, however, the state has recorded just five.

“If you compare to last year, this year (on the water) has had some strong storms, but it’s much weaker,” Hecht said. “But the abundance of low to moderate atmospheric rivers has kind of helped us stay on course to get to that normal (precipitation levels).”

As of this month, record statewide precipitation and snowpack in the Sierra Nevada stood at about 105 percent of average for this time of year — which Hecht called ‘surprisingly close to average.

“This year has been abnormally normal,” Hecht said. “We like to say that California is the land of extremes, where the climate is either extremely dry or extremely humid. This year was abnormal because it was pretty close to normal until April 1, which is usually the end of the rainy season in California.

However, Southern California had a more abnormal water year, with annual precipitation well above 140% of average in many coastal areas, according to the California Water Watch.

Hecht said a strong, slow-moving atmospheric river in early February had an outsized effect on the region’s precipitation, and he noted that many areas were also hit by thunderstorms during what he called weak atmospheric river storms “too productive”.

The systems are not typically accompanied by thunderstorms, but several systems have been this season, causing locally historic rainfall and flash flooding in several areas, including San Diego and Oxnard.

Hecht said it was not immediately clear why so many atmospheric rivers this season included thunderstorms, but he said higher ocean surface temperatures – a signature of the El Niño weather pattern – could have contribute to stimulating the instability of the convective model.

Although many water measurements so far indicate an overall average water year, federal officials recently issued a major disaster declaration for nine counties after February’s deadly atmospheric river storms.

Knowing that more extreme rainfall is possible, Swain said he hopes state officials can better prepare for emergencies, or at least better understand the possible risks.

“If we don’t properly estimate risk at the outset…it’s terribly difficult to have an accurate discussion about the costs and benefits of a particular intervention,” Swain said.

But he noted that climate change is expected to extend these natural extremes even further.

“It’s reasonable to interpret the 20th century as being kind of lucky in California, in the sense that we haven’t seen something worse… just because of random, natural variability,” said Swain. “The 21st century? This is a very loaded die.