Climate change has doubled the chance of a California megaflood: Study

California is already known for being vulnerable to natural disasters such as earthquakes, wildfires and even tsunamis, but a new study in the journal Science Advances finds that it is at increased risk of another: a disastrous megaflood that could cause more than $1 trillion in losses and turn low-lying areas into a “vast inland sea.”

The Golden State is currently enduring the worst 20-year drought in at least 1,200 years — an event made more likely due to climate change, as warmer air causes more evaporation. But, the study’s authors note, “Despite the recent prevalence of severe drought, California faces a broadly underappreciated risk of severe floods.”

Just as increased evaporation causes more frequent and severe droughts, it also causes more extreme rainfall when a storm arrives. The paper finds that climate change has doubled the chances of a dramatic flood in California during the next 40 years, and that the risk will continue to increase if average global temperatures keep rising.

An aerial view of a vast expanse of bare lake, showing a road that used to hug the waterline.
Low water levels at Grant Lake, which is fed by now nearly snowless mountains in the Eastern Sierra Nevada, expose an expanded shoreline on Aug. 11 near Lee Vining, Calif. (David McNew/Getty Images)

The researchers used new high-resolution weather modeling and existing climate models to find how often a long series of storms fueled by atmospheric rivers that have occurred about once a century in recent history would occur, now that global average temperatures have risen 1.1 degree Celsius (2 degrees Fahrenheit) since the Industrial Revolution. What they found was that warmer temperatures have doubled the risk of those conditions, so that what was once a 1-in-100-year flood would now occur every 50 years, on average.

"Climate change has probably already doubled the risk of an extremely severe storm sequence in California, like the one in the study," Daniel Swain, a climate scientist at the University of California, Los Angeles, who co-authored the study with Xingying Huang, a climate scientist at the National Center for Atmospheric Research, told NPR. "But each additional degree of warming is going to further increase that risk."

An atmospheric river is a long, narrow band of heavy moisture. Historically, winter atmospheric rivers have led to large snowfalls in the Sierra Nevada mountains. In a warmer climate, however, atmospheric rivers will be stronger because they hold more moisture. With warmer temperatures, more of the precipitation will fall as rain, causing flooding, instead of snow, which melts gradually.

In recent history, the only example of such a flood is the Great Flood of 1862. In December 1861, nearly 15 feet of snow fell in the Sierra Nevada mountain range, and subsequent atmospheric rivers dumped rain for 43 days after that, with the water pooling in valleys. This meant that in the winter of 1862, parts of California were submerged in up to 30 feet of water for weeks, according to CNN. The state capital in Sacramento, “was under 10 feet of debris-filled water for months.”

A dozen flat boats, some propelled by poles, each carrying up to four passengers and in some cases, small dogs, float on a city street in an engraving titled: K. Street, From the Levee. Inundation of the State Capitol, City of Sacrament, 1862, Published by A. Rosenfield, San Francisco.
A lithograph showing K Street in the city of Sacramento, Calif., during the Great Flood of 1862. (A. Rosenfield/WikiCommons)

Buildings were destroyed, including one out of every eight homes, and 4,000 people died. The state lost one-quarter of its economy that year.

No flood that large has happened since then, but river sediment deposits show that in the pre-climate change era, such floods usually happened every 100 to 200 years.

"We find that climate change has already increased the risk of a [1862-like] megaflood scenario in California, but that future climate warming will likely bring about even sharper risk increases," the study’s authors write.

"It's a question of when rather than if [the megaflood] occurs,” Swain told CNN.

But the effects would be far worse, now that California has grown to 39 million residents, with an economy that, if it were a country, would be the world’s fifth-largest.

According to the researchers' modeling, Stockton, Fresno and Los Angeles would be under water and damages could be upward of $1 trillion, potentially the most expensive disaster in world history. Interstate highways in California, such as I-5 and I-80, would probably be shut down for weeks.

A kayak paddles down a flooded street past a pickup truck whose wheels are half-submerged.
A man kayaks down a flooded street in the town of Guerneville, Calif., on Feb. 28, 2019, thanks to floodwaters from the Russian River nearby. (Josh Edelson/AP)

“Every major population center in California would get hit at once — probably parts of Nevada and other adjacent states, too,” Swain said in a UCLA press release.

The increased risk of extreme rainfall due to climate change is not limited to California or the West Coast. The United States recently experienced three extreme rainfalls of the kind that were supposed to only occur once in every 1,000 years: southern Illinois received 12 inches of rain in 12 hours, the St. Louis area 6 to 10 inches of rain in just seven hours, and parts of eastern Kentucky were drenched by 14 inches of rain in two days.

The California Department of Water Resources supported the study with data and funding, as part of an effort to understand and prepare for extreme weather risks exacerbated by climate change. Further research — which will include partnering with state and federal emergency management agencies — will try to determine where the flooding would be worst and how it could be mitigated.

“Modeling extreme weather behavior is crucial to helping all communities understand flood risk even during periods of drought like the one we’re experiencing right now,” Karla Nemeth, director of the California Department of Water Resources, said in a statement.


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