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A Disaster “Larger Than Any in World History” is Coming for California, Scientists Warn

It’s happened before. It will happen again. But this time it’ll be much worse.



Megaflood California

A ‘megadrought’ may currently be the most significant hazard across California at the moment amid the ongoing risk of earthquakes and wildfires. However in a new study, UCLA scientists warn that another more serious catastrophe—dubbed a ‘megaflood’—is on the horizon for California.

According to an alarming study published on Friday (August 12) in the peer-reviewed journal Science Advances, climate change is dramatically increasing the likelihood of a megaflood across the state of California in the future, which would likely result in multiple cities under water and millions of displaced people.

Their research found that a severe storm lasting for an entire month could bring feet of rain to hundreds of miles of California—with some areas receiving more than 100 inches of rain.

The loop below, which displays the movement of water vapor and the probable buildup of precipitation at selected time slices during the course of the 30-day scenario, was developed by Xingying Huang, one of the authors of the study.

The inevitable disaster is being referred to as California’s “second Big One”—a reference to the catastrophic earthquake expected to strike the state any day now—but this new analysis found that the losses caused by a megaflood would outweigh those caused by the major earthquake “by a wide margin.”

According to the findings of the study, the likelihood of a future megaflood as well as their severity are both significantly increased with each degree of heat caused by global warming.

In fact, the scientists concluded that such catastrophic floods are now twice as likely to occur.

The risk of a California megastorm event increases approximately linearly with global warming. Climate change to date has already doubled the risk relative to a century ago.

Dr. Daniel Swain, a climate scientist at UCLA and co-author of the study, said in a news release that in a future scenario where a megaflood occurs on a hotter planet Earth, “the storm sequence is bigger in almost every respect.”

“There’s more rain overall, more intense rainfall on an hourly basis and stronger wind,” Swain added.

According to Swain, statewide floods affecting all of California like this have occurred around once per century or two over the course of the last thousand years. He believes the present risk of such a catastrophic disaster has been grossly underestimated.

“Societally, from a public policy and climate adaptation infrastructure building perspective we are falling behind,” warned Dr. Swain.

“Our goal in doing this work is to get ahead of the curve as much as we can when it comes to the risk of megaflood,” he said.

“We know that eventually it will happen and that climate change is upping the odds.”

The Great Flood of 1862 in California happened before global warming was a thing, measuring up to 300 miles long and 60 miles across.

Lithograph depicting deep inundation in Sacramento during the Great Flood of 1862, which turned much of the Central Valley into a temporary inland sea.

However, the same storm in 2022 would—according to the study—uproot 5–10 million people, cut off the state’s main roadways for months with significant economic repercussions, and submerge a significant number of major Central Valley cities, in addition to portions of Los Angeles.

Overview of the ARkStorm Scenario

The research is an expansion of the “ArkStorm scenario” that was published in 2010, which got its name from the atmospheric rivers that would fuel the severe storm, resulting in mass flooding of biblical proportions.

This new study is only the first stage of an initiative referred to as ArkStorm 2.0, which aims to revisit that scenario further.

According to the research conducted at UCLA, a catastrophic flood on the scale of the one that occurred in 1862 would result in a loss of at least one trillion dollars.

“Parts of cities such as Sacramento, Stockton, Fresno and Los Angeles would be under water even with today’s extensive collection of reservoirs, levees and bypasses. It is estimated that it would be a $1 trillion disaster, larger than any in world history,” the news release said.

Swain noted in the release that many Californians have forgotten about the severe flooding threat because of the attention that has been paid to the drought and the wildfires.

“There is potential for bad wildfires every year in California, but a lot of years go by when there’s no major flood news. People forget about it,” Swain explained.

The scientists used both new high-resolution weather models and existing climate models to compare two extreme situations: one that would occur approximately once every century in the recent historical climate, and another that would occur in the projected climate of 2081-2100.

Both scenarios were analyzed using state-of-the-art computer modeling software.

Comparison of composite atmospheric river intensity during ARkHist (left) and ARkFuture (right).
30-day cumulative simulated precipitation during ARkHist and ARkFuture scenarios.
ARkFuture and ARkHist cumulative precipitation time series (statewide California average) compared to the most extreme events observed historically (since 1951).

Each of these scenarios featured a serious of storms that were fed by atmospheric rivers over the period of one month.

Ribbons of water vapor make up atmospheric rivers, which can be found stretching across thousands of miles from the tropics to the western United States. They are responsible for providing the fuel for the enormous rain and snowstorms that can bring floods throughout the West Coast. Their width ranges from 250 to 375 miles.

According to the National Oceanic and Atmospheric Administration (NOAA), these kind of occurrences—despite being beneficial for water supplies—can wreak havoc on transportation, bring about lethal mudslides, and cause devastating damage to both life and property.

Numerous studies now suggest the effects of climate change will cause atmospheric rivers to become hotter, more powerful, and more frequent.

“Nobody could possibly argue we didn’t see this coming if and when it hits,” Swain said.

“There’s still potentially time to do something about it before things go haywire.”

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