Another atmospheric river has soaked California. What role has climate change played?

The second atmospheric river to hit the West Coast in as many weeks is slowly moving out of Southern California. But not before dumping more than 9 inches of rain over 24 hours in some areas near Los Angeles. Streets are flooded in Santa Barbara and Los Angeles; creeks are raging like rivers; and rainfall records in Los Angeles County are nearing all-time records.

At least three people were reported to have died due to storm-related injuries caused by fallen trees. As of Tuesday evening, the severe weather had knocked out power for more than 100,000 Californians.

And while the downpour appears to be easing, the destruction and devastation is ongoing, Los Angeles Mayor Karen Bass said in a press conference Tuesday evening.

"Angelenos should know that even though the rain may ease up a bit today, this storm continues. And that means we still need Angelenos to take precautions and to stay informed during this time," Bass said.

In all, city agencies have responded to 475 mudslides, 390 fallen trees, at least a dozen structure fires, and multiple successful water rescues.

In Santa Barbara, a cliffside collapsed on Tuesday morning, temporarily displacing residents of a multi-unit seaside apartment building near the University of California, Santa Barbara.

East of Los Angeles, a massive mudslide wiped out a hillside in Hacienda Heights and La Habra. KTLA reported about 20 homes were damaged by mud, rain and debris flow.

The deluge is also threatening Southern California's homeless population, many of whom are struggling to find dry shelter.

One unidentified pregnant woman who is believed to be unhoused, was rescued by Anaheim Fire and Rescue officials on Tuesday morning.

City spokesman Mike Lyster told NPR that the woman was found trapped in a storm drain that feeds into the Santa Ana River.

"It looks like she was trying to stay out of the rain and couldn't get out" when the water rose, Lyster said, noting that Anaheim has gotten about a third of its typical annual rainfall in the last 48 hours.

The woman was eventually freed, seemingly unharmed, but, Lyster said the incident shows the dangers of "moving anywhere near the storm drains or the river itself."

Meantime, public health officials are urging people to stay out of several beaches after millions of gallons of sewage spilled into the rivers that flow into the ocean. The cause of the sewage spills is under investigation.

Atmospheric rivers are well-known weather phenomena along the West Coast. Several make landfall each winter, routinely delivering a hefty chunk of the area's annual precipitation. But the intensity of recent atmospheric rivers is almost certainly affected by human-caused climate change, says Daniel Swain, a climate scientist at the University of California, Los Angeles.

Climate change has made the ocean's surface warmer, and during an El Niño year like this one, sea water is even hotter. The extra heat helps water evaporate into the air, where winds concentrate it into long, narrow bands flowing from west to east across the Pacific, like a river in the sky, Swain says. An atmospheric river can hold as much as 15 times as much water as the Mississippi River.

Human-driven climate change has primed the atmosphere to hold more of that water. Atmospheric temperatures have risen about 2 degrees Fahrenheit (just over 1 degree Celsius) since the late 1800s, when people started burning massive volumes of fossil fuels. The atmosphere can hold about 4% more water for every degree Fahrenheit warmer it gets. When that moist air hits mountains on the California coast and gets pushed upwards, the air cools and its water gets squeezed out, like from a sponge.

Swain estimates those sky-rivers can carry and deliver about 5 to 15% more precipitation now than they would have in a world untouched by climate change.

That might not sound like a lot, but it can—and does—increase the chances of triggering catastrophic flooding, Swain says.

In 2017, a series of atmospheric rivers slammed into Northern California, dropping nearly 20 inches of rain across the upstream watershed in less than a week. The rainfall fell in two pulses, one after another, filling a reservoir and overtopping the Oroville dam, causing catastrophic flooding to communities downstream.

The back-to-back atmospheric rivers that drove the Oroville floods highlighted a growing risk, says Allison Michaelis, an atmospheric river expert at Northern Illinois University and the lead of a study on the Oroville event. "With these atmospheric rivers occurring in succession, it doesn't leave a lot of recovery time in between these precipitation events. So it can turn what would have been a beneficial storm into a more hazardous situation," she says.

It's not yet clear if or how climate change is affecting those groups of storms—"families," as one study calls them.

It's also too early to say exactly how much more likely or intense climate change made the current storms on the West Coast. But "in general, we can expect them to all be intensified to some degree" by human-driven climate change, Michaelis says.

Scientists also don't yet know if climate change is affecting how often atmospheric rivers form, or where they go. And climate change doesn't mean that "every single atmospheric river storm that we are going to experience in the next couple of years will be bigger than every other storm" in history, says Samantha Stevenson, an atmospheric and climate scientist at the University of California, Santa Barbara.

But West Coast communities do need to "be prepared in general for dealing with these extremes now," says Stevenson. "Because we know that they're a feature of the climate and their impacts are only going to get worse."

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