Although the team’s research focused on Northern California — where the state typically sees the the most atmospheric rivers — she said it’s fair to conclude that the southern half of the state would have seen similarly extreme rainfall in its ancient climate given the widespread effects of large atmospheric rivers.

Previous research has shown that the average atmospheric river transports more than twice the flow of the Amazon River. The prospect of even larger storms hitting California is a concerning one, experts say.

Daniel Swain, a UCLA climatologist who was not involved in the USGS study, said the paper provides “direct physical evidence” of atmospheric river activity more extreme than anything seen in recent California history — well beyond the Great Flood of 1862, which reconfigured the state’s landscape.

He said the research “re-emphasizes the perils of assuming that the extremes we saw in the 20th century are representative of the kinds of extremes that are possible in this part of the world.”

“It’s an indication that — even if we didn’t have to contend with climate change — we should still be circumspect about the risks that are posed by extremes because we know that the climate system ... can throw big, bad things at us periodically,” Swain said. “I don’t find that at all reassuring.”

The continuing climb of global average temperature due to humanity’s burning of fossil fuels also threatens to exacerbate matters.

“Adding energy into the system through greenhouse gas emissions is basically like shaking the soda can ... and adding a little bit more energy into the system, allowing these extremes to be a little bit more extreme,” said Cody Poulsen, a graduate student researcher at the Scripps Institution of Oceanography’s Center for Western Weather and Water Extremes, who also was not involved in the Nature study.

Swain has posited that every degree increase in global temperature increases the risk of an “ ARkStorm Scenario” — a once-in-a-thousand-years megaflood event. But these new USGS findings may indicate that worst-case-scenario modeling isn’t extreme enough, he said.

For a state that is grappling with more frequent and severe periods of drought, the last two wet winters have come as a rare bounty. However, many Californians may be surprised to learn that these two wet seasons fall within the realm of natural variability. They may also be surprised to learn that this year has delivered more atmospheric rivers than the previous year, which caused far more damage and disruption.

Recently, researchers confirmed that 51 atmospheric rivers hit the West Coast during the 2023-24 rainy season — significantly more than the 38 atmospheric rivers that hit during the 2022-23 rainy season, according to new data from the Center for Western Weather and Water Extremes.

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