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Sorry California, Melting Snowpacks To Unleash More Epic Floods

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“When all of that snow starts melting, we have a major challenge on our hands.”

Sometime in January, as a string of “atmospheric rivers” from deep in the South Pacific pummeled California, that query began hitting my phone in the form of texts, emails, and tweets. In a chapter of 2020 book Perilous Bounty (excerpted here) I had written about a Biblical-scale flood that had submerged much of the state after the ferociously wet winter of 1861-62, turning the entire Central Valley—a 50 mile-wide flatland that runs about 400 miles between the coastal mountain ranges and the Sierra Nevada—into an inland sea. After the waters receded, I noted, the valley gradually emerged as one of the global epicenters of agriculture, now providing about a quarter of our food supply, including 40 percent of the nation’s fruits and nuts as well as more milk than Wisconsin.

When you hear about valley’s water problems these days, the topic is typically drought. The first months of 2023 pointed a spotlight on the other pole that characterizes California’s chaotic weather regime: untimely overabundance. While media coverage will likely fade as California’s rainy season tapers off, flood risk doesn’t end when the storms do. The Sierra Nevada snowpack, the fickle annual endowment that hydrates most of the state’s farm and cities as it melts, holds a big portion of this season’s precipitation high in the air in frozen form. Daniel Swain, a climate scientist in the Institute of the Environment and Sustainability at the University of California, Los Angeles, and a fellow at the National Center for Atmospheric Research, calls it a “loaded spring” that could yet flood a substantial portion of the valley.


While the Great Flood of 1862 has largely vanished from popular memory, the threat of its recurrence quietly haunts the Golden State. In recent decades, scientists have looked at the fossil record and concluded that over the past millennium, a megaflood at least as severe as the 1862 catastrophe visits California on average every 100 to 200 years—a chilling finding, given that it’s been 161 years since the last one. In 2010, a major US Geological Survey modelling study found that even a megastorm less fierce that the 1861-62 one would cause $725 billion in damage in modern California, by far the costliest natural disaster in US history. Urban areas from Sacramento to parts of Los Angeles and Orange counties would be underwater, as would “thousands of square miles” of farmland.

When you layer climate change onto this historical pattern, things look yet more dire. As the tropical Pacific Ocean warms, more seawater evaporates, feeding ever bigger atmospheric rivers streaking toward the California coast. In a 2018 paper, a team led by Swain found that warmer temperatures increases the likelihood 1862-scale events “more than threefold,” meaning a megaflood expected to happen on average every 200 years will now happen every 65 or so. Their conclusion: it’s “more likely than not we will see one by 2060.”

In a paper published last year, this one co-authored with Xingying Huang, Swain found that “megastorm” risk rises significantly with each additional degree of global warming this century. Currently, average global temperatures sit at about 1.1 degrees Celsius above pre-industrial levels. In its most recent report, released in March, the UN Intergovernmental Panel on Climate Change (IPCC) warned that the world will likely reach 1.5 degrees Celsius by the early 2030s—and will keep edging up without fast cuts to global greenhouse gas emissions. Uh-oh.

The Indigenous people were long gone by the early 1920s, when the Boswells and Salyers, cotton farmers chased out of Georgia and Virginia by the boll weevil, showed up in Corcoran. The lake had been halfway drained by all the upstream farmers. The cotton growers dried up the rest. They planted a new plantation. The South, its Black cotton pickers, its Jim Crow, rose up in the West.

In late March, after yet another burst of atmospheric-river storms had slammed California and flooded parts of the Tulare Basin, the southern third of the Central Valley, I got Swain on the phone and posed the Big One question that I, ignorant East Coast journalist, had been getting from friends, colleagues, family, and Twitter acquaintances for months.  

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