A mention of California might usually 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 just how bad it gets. . 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 has happened before — most recently in 1862 — and, if history is any indication, we’re overdue for another, according to a new paper published Friday in Advances in Science which seeks to shed light on the underlying danger. “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 get ahead of it.” In such a case, some in the Sierra Nevada could end up with 40 or 50 feet of snow, and most of California’s major highways would be washed out or inaccessible. Swain is already working with emergency management officials and the National Weather Service, explaining that it’s not a matter of whether a major flood will occur — it’s a matter of when. “It already happened in 1862, and it probably happened about 5 times per millennium before that,” he said. “On a human scale, 100 or 200 years sounds like a long time. But these are fairly regular occurrences.” What drives the massive, devastating rainfall across the country His work built on the work of other scientists, who examined layers of sediment along the coastline to determine how often megaflood events occurred. They found evidence of extreme freshwater runoff, washing soil and rocky material into the sea. These layers of dirt were buried under years of sand. The depth of each layer, as well as the size of the pebbles and material it contains, provides insight into the severity of past floods. “It hasn’t happened in recent memory, so it’s a little bit ‘unknown, out of mind,’” Swain said. “But [California is] an area that is in the perfect area … in climatic and geographical context’. On 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 major flood in California, you would need a near-stationary low-pressure area in the northeast Pacific that would drive a series of high-quality atmospheric rivers to the California coastline. “These would be atmospheric river families,” Swain said. “You get one of those semi-persistent ones [dips in the jet stream] over the Pacific Northeast that oscillates for a few weeks and allows winter storm after winter storm across the Pacific Northeast into California.” The paper warns of “extraordinary implications” and says such an episode could “[transform] the inland valleys of the Sacramento and San Joaquin into a temporary but vast inland sea nearly 300 miles long and [inundate] much of the more densely populated coastal plain in present-day Los Angeles and Orange counties.” The effects of a month of storms could be devastating, but Swain notes that advance warning is likely. “This is something we would see three to five days out and I would hope a week and maybe even 2 weeks out with a possible forecast type,” Swain said. “We’d have a decent warning for that.” Atmospheric rivers flooding the west coast are rated on a scale of 1 to 5 like hurricanes Swain’s simulations showed that the chances of a megaflood event occurring are much greater during an El Niño winter compared to a La Niña. El Niño is a large-scale atmosphere-ocean chain reaction pattern that can dominate the atmosphere for several years at a time and usually begins with warmer-than-normal warm 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 in El Niño years,” he said. The influence of human-caused climate change also plays a role: Swain says it raises the cap on a large megaflood event. “We have many scenarios. The future is much longer, consistent with [climate change],” he said. “In the historical scenario, the smallest, some parts of the Sierra Nevada see 50 to 60 inches of equivalent precipitation … but in the future event, some parts see 70 to 80 and some 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 in just a month.” An independent study published in Scientific Reports on Friday concluded that human-induced 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 capacity 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 deck is stacked to favor a great event. “Humidity is not the limiting factor in California,” Swain said. “There is enough moisture around even in drought years. Absence is lack of mechanism. It’s the lack of storms rather than moisture.” While they can’t say when the next major flood will hit California, forecasters are confident it will happen again. There is a 0.5 to 1.0 percent chance of occurring in any given year. Swain said one goal of his job is to push officials to prepare. He proposed working with the National Oceanic and Atmospheric Administration to “run through simulations as real tabletop disaster scenarios on the ground.” “We’re going to work down to where the points of failure would really be, because one of the things we want to do is get ahead of the curve,” he said.
