Placeholder while loading item actions
A mention of California might normally conjure up images of wildfires and droughts, but scientists say the Golden State is also the site of extreme once-in-a-century “megafloods,” and climate change could amplify how much point one gets bad.
The idea seems inconceivable: a month-long storm dumping 30 inches of rain on San Francisco and up to 100 inches of rain and/or melted snow on the mountains. But it’s happened before, most recently in 1862, and if history is any indicator, we’re in for another, according to research published Friday in Science Advances that aims to shed light on the lurking danger.
“This risk is increasing and it was already underestimated,” said Daniel Swain, one of the study’s two authors and a climate scientist at the University of California, Los Angeles. “We want to move forward.”
In this event, some in the Sierra Nevada could end up with 25 to 34 feet of snow, and most major roads in California would be washed out or become impassable.
Swain is working with emergency management officials and the National Weather Service, explaining that it’s not a matter of if a megaflood will happen, but when..
“It’s already happened in 1862, and it’s probably happened about five times per millennium before that,” he said. “On human time scales, 100 or 200 years sounds like a long time. But these are fairly common occurrences.”
What is driving the massive and destructive rains across the country
Their paper built on the work of other scientists, who examined sediment layers along the coast to determine how often megafloods occurred. They found evidence of extreme freshwater runoff, which washed soil and stony materials into the sea. Those layers of material were buried under years of sand. The depth of the layers and the sizes of the pebbles and other materials contained within them provide insight into the severity of past floods.
“It hasn’t happened in recent memory, so it’s kind of ‘out of sight, out of mind,'” Swain said. “But [California is] a region that is in the perfect zone… in a climatological and geographical context”.
Along the west coast, there are usually atmospheric rivers, or currents of moisture-rich air in the mid-atmosphere with connections to the deep tropics. For a California megaflood to occur, you would need a near-stationary area of low pressure in the Pacific Northeast, which would drag a succession of upper-level atmospheric rivers to the California coast.
Videos posted on social media on October 24, 2021 showed storm damage and flooding in California and Oregon as an “atmospheric river” hit the region. (Video: The Washington Post)
“These would be atmospheric river families,” Swain said. “You have one of those semi-persistents [dips in the jet stream] in the Pacific Northeast that moves for several weeks and allows winter storm after winter storm to cross the Pacific Northeast into California.
The document warns of “extraordinary impacts” and reports that such an episode could transform “the inland Sacramento and San Joaquin valleys into a temporary but vast inland sea nearly 300 miles long and [inundate] much of the now densely populated coastal plain of present-day Los Angeles and Orange counties.”
The effects of a month of soaking storms could be disastrous, but Swain points out that it’s possible to have advance warning.
“This is something we would see three to five days out, and hopefully a week and maybe even two weeks out, with a probabilistic kind of prediction,” Swain said. “We’ll have a decent amount of notice for that.”
Atmospheric rivers that permeate the West Coast are rated 1-5 like hurricanes.
Swain’s simulations showed that the odds of a megaflood occurring are much greater in El Niño-dominated winters than in La Niña-influenced winters. El Niño is a large-scale chain-reaction atmosphere-ocean pattern that can dominate the atmosphere for several years at a time, and typically begins with above-normal sea surface temperatures in the eastern tropical Pacific.
“When we look at the top eight monthly precipitation totals in simulations, eight out of eight occurred during El Niño years,” Swain said.
The influence of human-caused climate change also plays a role: Swain says it raises the ceiling on a megaflood.
“We have several scenarios. The future is much bigger, consistent with [climate change],” he said. “In the historical scenario, the least, certain parts of the Sierra Nevada see 50 to 60 inches of liquid-equivalent precipitation … but in the future, some places see 70 to 80 and some see 100 in a 30 day period. . Even places like San Francisco and Sacramento could see 20 to 30 inches of rain, and that’s just in one month.”
An independent study published Friday in Scientific Reports concluded that human-caused climate change will intensify atmospheric rivers and could double or triple their economic damage in the western United States by the 2090s.
A warmer atmosphere has a greater ability to store moisture. In the absence of storms, this means that the air can dry out the landscape more quickly, hence the prolonged California drought, but if there is rain, the canopy is stacked to favor an exceptional event.
“Humidity is not the limiting factor in California,” Swain said. “There is a lot of moisture even in drought years. The absence is a lack of mechanism. It is the lack of storms rather than moisture.”
Alan Rhoades, who is an expert on atmospheric rivers and was not involved in either study, said the research highlights “the importance of not forgetting about large flood events, which are also central to the history of California.”
“The main concern is how much climate change will alter the frequency of these events and how much it will fuel and amplify the impacts of the next record. [atmospheric river] event,” Rhoades, a hydroclimate research scientist at Lawrence Berkeley National Laboratory, wrote in an email.
He added that compared to previous megafloods in the late 19th century, “California has greatly expanded its rural, urban and agricultural sprawl, which could lead to greater potential for loss of life and property.”
Although researchers can’t say when California’s next megaflood will occur, forecasters are confident it will happen. There is a 0.5 to 1.0 percent chance of it happening in a given year.
Swain said one of the goals of his work is to push officials to prepare. He suggested working with the National Oceanic and Atmospheric Administration to “run simulations like a real tableau in land-based disaster scenarios.”
“We’re going to work out where the failure points would really be, because one of the things we want to do is get ahead of the curve,” he said.