How are atmospheric rivers affected by climate change?

The second atmospheric river to hit the West Coast in as many weeks has stalled over Southern California, 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.

The storm isn't over yet. Areas east and south of Los Angeles could see several more inches of rainfall by Tuesday. That includes San Diego, which was inundated a few weeks ago by a different storm.

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."