California's 'Big One' Might Not Be a Quake

Californians have learned to live under the threat of huge earthquakes, megadrought, and wildfires. They may have to contend with another horrifying scenario: a devastating megaflood more powerful than anything anyone alive on Earth has seen. Thanks to climate change, the threat from a megaflood like the one that hit the state during the winter of 1861-62, drowning communities in minutes, is increasing, researchers say. What California would typically expect every couple hundred years could now come once every 50 years, per a new study in Science Advances. And it could shrink to once every 30 years if global average temperatures climb another 1.8 degrees Fahrenheit, reports the New York Times. As CNN notes, a warmer atmosphere can hold more water vapor.

It's predicted that at some point, "from the vast expanse of tropical air around the Equator, atmospheric currents will pluck out a long tendril of water vapor and funnel it toward the West Coast," the Times reports. This massive vapor plume "will be carrying so much water that if you converted it all to liquid, its flow would be about 26 times what the Mississippi River discharges into the Gulf of Mexico at any given moment." And when it hits California's mountains, it will launch a month of precipitation across the state. Some areas would see more than 8 feet of precipitation in the month, according to a model of the projected climate at the end of this century, per a release.

"We got kind of lucky to avoid it in the 20th century," Swain tells the Times. He doesn't expect the same to hold for the 21st. The good news is that it won't come as a surprise. Forecasters are now about to identify "incoming atmospheric rivers" as much as a week in advance, although the specifics on where they'll hit and how hard aren't always clear. The bad news is that California is not at all prepared for a flood of this size, which could cause up to $1 trillion in damage—or 5 times the cost of Hurricane Katrina—and displace millions of people. "Ultimately, one of our goals is not just to understand these events scientifically, but it's also to help California prepare for them," Swain tells CNN. The Times notes state officials are indeed mapping out potential scenarios and emergency responses based on the latest findings. (More California stories.)