A mention of California usually conjures up images of wildfires and droughts, but scientists say the Golden State is also the site of extreme, once-a-century “megafloods” — and that climate change could amplify how bad a person gets.
A month-long storm that dumps 30 inches of rain on San Francisco and up to 100 inches of rain or snowmelt in the mountains seems impossible. But it’s happened before, most recently in 1862, and if history is any indication, we’re too late for another one, says a new paper published in Science Advances on Friday. The paper tries to explain the danger that’s waiting in the shadows.
“This risk is increasing and has already been undervalued,” said Daniel Swain, one of the study’s two authors and a professor of atmospheric sciences at the University of California at Los Angeles. “We want to lead the way.”
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In this case, some parts of the Sierra Nevada could get 25 to 34 feet of snow, and most of California’s major highways would be washed away or become inaccessible.
Swain is working with emergency response workers and the National Weather Service to explain that the question is not if a megaflood will happen, but when.
“It already happened in 1862, and it probably happened about five times a millennium before that,” he said. “On human timescales, 100 or 200 years sounds like a long time. But these are fairly regular occurrences.”
Scientists who looked at sediment layers along the coast to figure out how often megafloods happened were used as a basis for his paper. They found proof that there was a lot of freshwater runoff, which carried soil and rocks out to sea. These layers of stuff were buried for a long time by sand. The depth of the layers and the size of the pebbles and other things in them show how bad the flooding was in the past.
“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 area… in a climatic and geographical context.”
On the west coast, there are usually atmospheric rivers or streams of moisture-rich air in the middle of the atmosphere with connections to the deep tropics. For a California megaflood to happen, you’d need a near-stationary low-pressure zone in the northeastern Pacific Ocean, which would hurl a succession of high-end atmospheric rivers to the California coastline.
“These would be atmospheric river families,” Swain said. “You get one of these semi-persistent [dips in the jet stream] over the northeast Pacific waddling for a few weeks and allowing winter storm after winter storm across the northeast Pacific to California.”
The newspaper warns of “extraordinary consequences” and says that such an event would “turn the Sacramento and San Joaquin valleys into a temporary but vast inland sea nearly 300 miles long and [submerge] much of the densely populated coastal plain in Los Angeles and Orange County.”
The effects of a months-long barrage of drenched storms could be disastrous, but Swain notes it’s possible to get forewarned.
“A probabilistic forecast would tell us this would happen three to five days later, and I hope a week or even two weeks later,” Swain said. “We’d have a good amount of time to prepare for it.”
Like hurricanes, atmospheric rivers that hit the west coast are rated on a scale of 1 to 5.
Swain’s simulations showed that the chance of a megaflood is much higher when El Nio is in charge of the winter than when La Nia is in charge. El Nio is a large-scale chain reaction between the atmosphere and the ocean that can take over the weather for several years in a row. It usually starts with sea surface temperatures that are higher than normal in the eastern tropical Pacific.
“If you look at the top eight monthly precipitation totals in simulations, eight of the eight occurred in El Niño years,” Swain said.
Climate change caused by humans also plays a role. According to Swain, it raises the ceiling of a megaflood.
“There are many possible outcomes. Because of climate change, the next one will be much bigger. In the smaller historical scenarios, some parts of the Sierra Nevada will get 50 to 60 inches of liquid-equivalent rain. In the future, though, some places will see 70 to 80 and in a few cases 100 in 30 days. Even places like San Francisco and Sacramento can get 20 to 30 inches of rain in just one month.
An independent study that was published in Scientific Reports on Friday found that climate change caused by humans will make atmospheric rivers worse and could double or triple the economic damage they cause in the western United States by the 2090s.
When the air is warmer, it can hold on to more water. This means that when there are no storms, the air can dry out the land more quickly, which is why California has been in a drought for so long. However, when it does rain, everything is set up for an extraordinary event.
“Moisture isn’t the limiting factor in California,” Swain said. “Even in the dry years, there is enough moisture. The absence is a lack of mechanism. It is more a lack of storm than moisture.”
Forecasters are sure that the next big flood in California will happen, but they can’t say when. In any given year, it could happen between 0.5 and 1% of the time.
Swain said that getting people to get ready is one of his goals. He suggested working with the National Oceanic and Atmospheric Administration to “play out disaster scenarios on the ground like a real tabletop game.”
“We’ll figure out where the points of failure would be because one of the things we want to do is lead the way,” he said.
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