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Risk of catastrophic California 'megaflood' has doubled due to global warming, researchers say

Louis Sahagún, Los Angeles Times on

Published in Science & Technology News

LOS ANGELES — Even today, as California struggles with severe drought, global warming has doubled the likelihood that weather conditions will unleash a deluge as devastating as the Great Flood of 1862, according to a UCLA study released Friday.

In that inundation 160 years ago, 30 consecutive days of rain triggered monster flooding that roared across much of the state and changed the course of the Los Angeles River, relocating its mouth from Venice to Long Beach.

If a similar storm were to happen today, the study says, up to 10 million people would be displaced, major interstate freeways such as Interstates 5 and 80 would be shut down for months, and population centers including Stockton, Fresno and parts of Los Angeles would be submerged — a $1 trillion disaster larger than any in world history.

It would also likely be “bigger in almost every respect” than what scientists have come to call the “ARKStorm scenario” of 1862, said climate scientist Daniel Swain, co-author of the study published Friday in the journal Science Advances.

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

The paper is the latest piece of research to describe the whiplash effects of a heating planet, where increasing temperatures allow the atmosphere to absorb and store more and more moisture. This atmospheric “thirstiness” can result in either extreme drought and aridity or the massive release of water in the form of an atmospheric river.

 

The study used a combination of new, high-resolution weather modeling and existing climate models to learn that the risk of a “megaflood” increases as global temperature rises. It also simulated how a long series of storms fueled by atmospheric rivers over the course of a month in the projected climate of 2081-2100 would affect parts of California at the local level. They found that some locations would get more than 100 inches of precipitation.

Atmospheric rivers bring long narrow plumes of concentrated water vapor lifting over the mountains, producing rain and snow.

On 10,000-foot peaks, which would still be somewhat below freezing despite global warming, “you get 20-foot-plus snow accumulations,” Swain said. “But once you get down to South Lake Tahoe level and lower in elevation, it’s all rain.”

Swain and co-author Xingying Huang project that end-of-the-century storms will generate 200% to 400% more runoff in the Sierra Nevada Mountains due to increased precipitation and more precipitation falling as rain, not snow.

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