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A mention of California may generally 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 one, according to a new paper published Friday in Advances in Science which aims to shed light on the danger that lurks.
“This risk is increasing and it was already underestimated,” Daniel Swain, one of the study’s two authors and a professor of atmospheric sciences at the University of California, Los Angeles, said in an interview. “We want to move forward.”
In this event, some in the Sierra Nevada could end up with 40 to 50 feet of snow, and most major roads in California would be washed out or inaccessible.
Swain is already working with emergency management officials and the National Weather Service, explaining that it is not a question of whether a mega flood will happen, it’s just a matter of when.
“It’s already happened in 1862, and it’s probably happened about 5 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 dirt were buried under years of sand. The depth of each layer, as well as the size of the pebbles and the material contained, provides insight into the severity of past flood events.
“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, you would need a near-stationary area of low pressure in the Pacific Northeast, which would drag a series of upper-level atmospheric rivers to the California coast.
“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 newspaper warns of “extraordinary impacts” and reports that this episode could “[transform] the interior of the 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 impacts of a month of soaking storms could be disastrous, but Swain notes that it is 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 an El Niño winter than during a La Niña. 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 warmer-than-normal sea surface temperatures in the eastern tropical Pacific.
“When you look at the top eight monthly rainfall totals in simulations, eight out of eight occurred during El Niño years,” he said.
The influence of human-caused climate change also plays a role: Swain says it raises the ceiling on a large megaflood event.
“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 that they see 100 in a thirty 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 in Scientific Reports Friday 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 the air can dry out the landscape more quickly, hence California’s prolonged drought, but in the event of rain, the canopy stacks up 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.”
While they can’t say when California’s next megaflood will occur, forecasters are confident it will happen again. 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 tablet in disaster scenarios on land.”
“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.