For years, scientists have said that atmospheric rivers can make or break the water supply of California’s thirsty cities and farms.
Over the past two winters, a steady succession of these giant “rivers in the sky” has dumped record-breaking and drought-wrecking precipitation across the state, while simultaneously causing catastrophic flooding, landslides and dangerous snowstorms.
But now new research has found that these recent atmospheric rivers pale in comparison to some of the monster storms that ravaged old California — a sobering revelation that has some experts suspecting that the state could be visited by such cataclysmic storms again.
“Our findings show that atmospheric river activity is greater than what has occurred since instrumental records began,” said Clarke Knight, a research geographer at the US Geological Survey and the lead author of the study that – for the first time – examines the atmospheric details river activity dating back 3,200 years. year. “This is important because it suggests the latent potential of our area to experience storms beyond the storms we saw today.”
Read more: How California’s storms are expected to become more extreme due to climate change
Although few people had even heard of atmospheric rivers just a few decades ago, research into the giant vapor trails has proven critical to California’s water planning and public safety.
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 refilled the once-dry Tulare Lake. The findings also raise the bar for government efforts to harvest rainwater, as climate change causes more precipitation to fall in the form of rain instead of snow, ushering in a new era of more frequent and prolonged droughts.
Knight and her fellow researchers came to their conclusion after analyzing ancient mud layers from Leonard Lake in Mendocino County. The team was able to determine when more sediment had been pushed into the lake, indicating periods of increased precipitation.
Then, using atmospheric river data from the past 60 years, the researchers found a “strong correlation” between their sediment findings and modern storms, allowing them to model that connection through the rest of the mud layers to reconstruct historical atmospheric river activity, Knight . said. Their research was published on Thursday in the journal Nature.
The study provides the most historical context yet for the state’s rainfall variability, finding that the region “consistently recorded extreme precipitation over a 3,200-year period.”
Knight said this new hydrological data can better support climate modeling and projections, creating a historical record 20 times longer than what is available.
Although the team’s research focused on Northern California — where the state tends to have the most atmospheric rivers — she says it’s reasonable to conclude that the southern half of the state would have seen similarly extreme rainfall in the ancient climate. , given the widespread effects of high atmospheric pressure. rivers.
Read more: Satellite photos show California turning green and snowy after winter storms
Previous research has shown that the average atmospheric river carries more than twice as much power as the Amazon. The prospect of even bigger storms hitting California is worrying, experts say.
Daniel Swain, a UCLA climatologist who was not involved in the USGS study, said the paper provides “direct physical evidence” of atmospheric river activity more extreme than anything seen in California’s recent history — well beyond the Great Deluge of 1862, which reshaped the state. landscape.
He said the research “re-highlights the dangers of assuming that the extremes we saw in the 20th century are representative of the types of extremes possible in this part of the world.”
“It’s an indication that even though we haven’t had to deal with climate change, we still have to be careful about the risks that extremes bring, because we know that the climate system can… bring big, bad things to us to throw. periodically,” Swain said. “I don’t find that reassuring at all.”
The continued rise in global average temperatures due to humanity’s burning of fossil fuels also threatens to worsen the situation.
“Adding energy to the system through greenhouse gas emissions is basically like shaking a can of soda… and adding a little more energy to the system, which allows these extremes to be a little more extreme,” says Cody Poulsen, a graduate student. researcher at the Scripps Institution of Oceanography’s Center for Western Weather and Water Extremes, who was also not involved in the Nature study.
Swain has posited that every degree increase in global temperatures increases the risk of an ‘ARkStorm scenario’ – a once-in-a-thousand-year megaflood. But these new USGS findings may indicate that the worst-case scenario modeling is not extreme enough, he said.
For a state that has suffered more frequent and severe droughts, the last two wet winters have proven to be a rare reward. However, many Californians may be surprised to learn that these two wet seasons fall within the realm of natural variability. They may also be surprised to learn that this year has produced more atmospheric rivers than the previous year, causing much more damage and disruption.
Recently, researchers confirmed that 51 atmospheric rivers affected the West Coast during the 2023-2024 rainy season – significantly more than the 38 atmospheric rivers that affected during the 2022-2023 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 through March, compared to 31 during last year’s rainy season, said Chad Hecht, a center meteorologist.
But while there were more atmospheric rivers this rainy season, there were fewer storms that were strong or extreme on the center’s power scale compared to the season before.
“It’s not about the quantity, it’s about the quality,” Hecht said.
For example, between October 2022 and March 2023, twelve strong, extreme or exceptional atmospheric river storms hit California. These more severe storms often bring news-making rain and snow. This season, however, the state only recorded five.
“If you compare it to last year,… this [water] There were some strong storms this year, but it’s a lot weaker,” Hecht said. “But the abundance of weak to moderate storms [atmospheric rivers] has helped us somewhat to stay on track to reach that normal level [precipitation levels].”
As of this month, records for both statewide precipitation and snowpack in the Sierra Nevada were about 105% of average for this time of year — which Hecht called shockingly close to average.
“This year has been abnormally normal,” Hecht said. “We like to talk about California as the land of extremes, where it’s either extremely dry or extremely wet. This year was abnormal because it was pretty close to normal through April 1,” the date that typically marks the end of the wet season. California highlights.
However, according to the California Water Watch, Southern California has experienced an anomalous water year, with annual rainfall over 140% of average in many coastal areas.
Hecht said a strong, slow-moving atmospheric river had an outsized influence on rainfall in the region in early February, and he noted that many areas were also hit by thunderstorms during what he called “overly productive” weak atmospheric river storms.
The systems are not typically accompanied by thunderstorms, but this season there were several systems that locally produced historic rainfall and flash flooding in several areas, including San Diego and Oxnard.
Read more: Rain showers that are ‘much, much more than normal’ could set a record in LA if the wet weather continues
Hecht said it’s not immediately clear why so many atmospheric rivers experienced thunderstorms this season, but he said higher ocean surface temperatures — a hallmark of the El Niño weather pattern — could have boosted the unstable convective pattern.
While many water measurements to date point to an overall average water year, federal officials recently issued a major disaster declaration for nine provinces following February’s deadly atmospheric river storms.
Knowing that further extreme rainfall is possible, Swain hopes government officials can better prepare for emergencies, or at least better understand the potential risks.
“If we don’t assess risk correctly to begin with… it’s incredibly difficult to have an accurate discussion about the costs and benefits of any given intervention,” Swain said.
But he noted that climate change is still expected to push these natural extremes further.
“It’s fair to interpret the 20th century as kind of lucky in California, in the sense that we haven’t seen anything worse… just because of random, natural variability,” Swain said. “The 21st century? It’s a loaded die.”
This story originally appeared in the Los Angeles Times.