Risk of catastrophic ‘mega-flood’ in California has doubled due to global warming, researchers say

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Even today, as California grapples with a severe drought, global warming has doubled the likelihood of weather patterns triggering a deluge as devastating as the Great Flood of 1862, according to a UCLA study released Friday.

In that flood 160 years ago, 30 straight days of rain triggered monster floods that roared across much of the state and changed the course of the Los Angeles River, moving its mouth from Venice to Long Beach. .

If a similar storm were to occur today, the study said, up to 10 million people would be displaced, major interstate highways such as Highways 5 and 80 would be closed for months, and population centers such than Stockton, Fresno and parts of Los Angeles. would be overwhelmed – a $1 trillion disaster bigger than any in the history of the world.

It would also likely be “larger in almost every way” than what scientists called the 1862 “ARKStorm scenario,” said climatologist Daniel Swain, co-author of the study published Friday in the journal Science Advances.

“There is more rain overall, more intense precipitation on an hourly basis and stronger wind,” he said.

The paper is the latest research paper describing the whiplash effects of a warming planet, where rising temperatures allow the atmosphere to absorb and store more and more moisture. This atmospheric “thirst” can result either in extreme dryness and aridity, or in a massive spill 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 ‘mega-flood’ increases as global temperature rises. It also simulated how a long string of atmospheric river-fed storms over the course of a month in the predicted climate of 2081-2100 would affect parts of California locally. They found that some locations would receive over 100 inches of precipitation.

Graphic illustration showing the moisture plume in an atmospheric river rising above the Coast Mountains and Sierra Nevada, dropping rain and snow.

(Paul Duginski/Los Angeles Times)

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

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

Increased runoff could trigger massive landslides and debris flows, especially in hilly and mountainous areas stripped of vegetation by wildfires.

“Whip shifts” in extreme weather could also challenge the stability of California’s massive array of aging dams and levees, exposing major cities to flooding.

The study also found that further increases in the risk of “mega-flooding” are likely with each additional degree of global warming this century.

The ARKStorm is also known as “the other Big One” after the nickname of a major earthquake expected along the San Andreas Fault.

But unlike an earthquake, an ARKStorm event would result in a disaster area spanning thousands of square miles, complicating emergency response efforts and triggering economic and supply chain blockages that would be felt across the country. worldwide.

Researchers are now working in conjunction with the California Department of Water Resources to develop maps showing where flooding could be worst and preparedness strategies to reduce potential loss of life and property.

Some of their initial proposals, however, are bound to generate tension between flood risk management plans and water conservation projects.

More frequent cycles of droughts and deluges fed by atmospheric rivers – Pacific-based storms that span hundreds of miles across – will present both problems and opportunities for coastal reservoir managers. west, balancing water storage and flood control mandates, said Alexander Gershunov, a climatologist. at the Scripps Institution for Oceanography in San Diego which is not affiliated with the study.

Indeed, he said, “California will increasingly have to rely on atmospheric rivers and potentially hazardous floodwaters for the generation of water resources in a warming climate.”

According to the UCLA study, preparing for massive flooding “could mean preemptively draining water from reservoirs, allowing water to inundate floodplains, and diverting water away from population centers.”

Earlier this week, however, Governor Gavin Newsom called on state agencies to start preparing for a warmer, drier future with strategies that include expanding water storage and recycling capacity. .

Indeed, he said, new data indicates that California will lose 10% of its water supply by 2040.

“Holding back as much water as possible is a good decision,” Swain said, “except it might collide with the need to prepare for catastrophic flooding.”

Currently, people are focused on the risk of wildfires, disease outbreaks and earthquakes, Swain said. “But catastrophic flooding is a risk that has quietly, but steadily increased in the background.”

“Eventually it will come back to bite us,” he said.

This story originally appeared in Los Angeles Times.

Teresa H. Sadler