Scientists have been saying for years that atmospheric rivers could either make or break the water supplies of thirsty California cities and farms.
Over the past two winters, these massive “rivers in the sky” have consistently dumped record-breaking and drought-busting rainfall across the state, while also causing devastating floods, landslides and dangerous snowstorms.
But now, new research has found that these last atmospheric rivers pale in comparison to some of the monster storms that hit ancient California; It’s a thought-provoking statement that has some experts suggesting the state could be visited by such catastrophic storms once again.
“Our findings suggest that atmospheric river activity exceeds anything that has occurred since instrumental record-keeping began,” said Clarke Knight, a U.S. Geological Survey research geographer and lead author of the study, which is the first detailed examination of atmospheric river activity going back 3,200 years. years. “This is important because it shows that our region has the potential to experience storms beyond what we’re seeing today.”
Read more: How are storms in California predicted to become more extreme with climate change?
Although few people had heard of atmospheric rivers just a few decades ago, studies of giant vapor trails have 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, inundating communities and refilling Tulare Lake after it dried out. The findings also boost the state’s efforts to capture rainwater as climate change causes more precipitation to fall as rain instead of snow, ushering in an era of more frequent and prolonged droughts.
Knight and his fellow researchers came to this conclusion after analyzing ancient mud layers from Lake Leonard in Mendocino County. The team was able to determine when more sediment was pushed into the lake, indicating periods of higher rainfall.
Then, using data on atmospheric rivers from the past 60 years, the researchers found a “strong correlation” between the sediment findings and modern storms, allowing them to model the connection between the remaining mud layers to reconstruct historical atmospheric river activity. aforementioned. Their research was published Thursday in the journal Nature.
The study provides the most historical context to date on the state’s precipitation variability and found that the region “has consistently recorded extreme precipitation over a 3,200-year period.”
Knight said this new hydrological data could better inform climate modeling and predictions and provide a historical record 20 times longer than the current one.
Although the team’s research focused on Northern California, where the state typically sees the most atmospheric rivers, he said it’s fair to conclude that the southern half of the state would see similarly extreme precipitation in its ancient climate, given the widespread effects of large atmospheric precipitation. rivers.
Read more: Satellite photos show California turning green and snowy after winter storms
Previous studies have shown that the average atmospheric river carries more than twice the flow of the Amazon River. Experts say bigger storms hitting California are concerning.
UCLA climate scientist Daniel Swain, who was not involved in the USGS study, said the paper provides “direct physical evidence” that atmospheric river activity is far more extreme than anything seen in recent California history. view.
He said the research “re-emphasizes the dangers of assuming that the extremes we saw in the 20th century represent the kinds of extremes possible in this part of the world.”
“This is an indication that even if we don’t have to fight climate change, we still need to be cautious about the risks posed by extremes because we know that the climate system can bring big, bad things to us on a periodic basis,” 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 worsen the problems.
“Adding energy to the system through greenhouse gas emissions is basically like shaking the soda can and adding a little more energy to the system, allowing those extremes to get a little more extreme,” said graduate student Cody Poulsen. Nature is a researcher at the Scripps Institution of Oceanography’s Center for Western Air and Water Extremes who was not involved in the research.
Swain suggested that every degree rise in global temperature increases the risk of an “ARkStorm Scenario,” a once-in-a-millennium megaflood event. But he said these new USGS findings may indicate that worst-case scenario modeling wasn’t extreme enough.
For a state plagued by more frequent and severe drought spells, the last two wet winters have come as a rare blessing. But many Californians may be surprised to learn that these two rainy seasons fall within the scope of natural variability. They may also be surprised to learn that this year provides more atmospheric fluxes than last year, resulting in much more damage and disruption.
Recently, researchers confirmed that 51 atmospheric rivers are hitting the West Coast during the 2023-24 wet season, according to new data from the Center for Western Weather and Water Extremes; This is far more than the 38 atmospheric rivers hit in the 2022-23 rainy season. .
In California specifically, 44 atmospheric rivers make landfall from October to March, compared with 31 during last year’s wet season, said center meteorologist Chad Hecht.
But even though there were more atmospheric rivers this rainy season, fewer storms measured as severe or extreme on the center’s power scale compared to the season before it.
“It’s quality, not quantity, that matters,” Hecht said.
For example, between October 2022 and March 2023, 12 strong, extreme, or exceptional atmospheric river storms hit California. These heavier storms tend to bring news-making rain and snow. But this season the state has recorded only five.
“If you compare it to last year… [water] “There were a few strong storms a year, but much weaker,” Hecht said. “But the abundance of weak to moderate storms [atmospheric rivers] helped us move towards normality [precipitation levels]”
As of this month, records for both statewide precipitation and snowpack in the Sierra Nevada stood at about 105% of average for this time of year; Hecht described it as surprisingly close to average.
“This year has been abnormally normal,” Hecht said. “We like to talk about California being a land of extremes, where it’s either extremely dry or extremely wet. This year was abnormal because it was pretty close to normal until April 1.” The date that typically marks the end of California’s rainy season.
However, Southern California experienced a more abnormal water year, with annual precipitation 140% above average in many coastal areas, according to California Water Watch.
Hecht said a strong, slow-moving atmospheric river had a major impact on precipitation in the region in early February, noting that many areas were also affected by storms during what he called “super-productive” weak atmospheric river storms.
Systems are not usually accompanied by thunderstorms, but some have been experienced this season, causing locally historic rainfall and flash flooding in several areas, including San Diego and Oxnard.
Read more: ‘Much above normal’ rain could break all-time Los Angeles record as wet weather continues
It’s not immediately clear why so many atmospheric rivers contain storms this season, Hecht said, but warmer ocean surface temperatures (a signature of the El Niño weather pattern) may help promote the unstable convective pattern.
While many water measurements point to an overall average water year so far, federal officials recently issued a major disaster declaration for nine counties following February’s deadly atmospheric river storms.
Knowing that extreme precipitation could increase further, Swain said he hopes state officials can better prepare for emergencies or at least better understand potential risks.
“If we don’t accurately estimate the risk at the outset…it is extremely difficult to make an accurate discussion about the costs and benefits of any intervention,” Swain said.
But climate change is still expected to further expand these natural extremes, he said.
“It’s reasonable to interpret the 20th century as meaning that we actually got a little lucky in California, that we didn’t see anything worse … just through random, natural variability,” Swain said. “The 21st century? A heavily loaded dice.”
This story was first published in the Los Angeles Times.