Scientists have warned that a scientifically plausible 'biblical flood' scenario, known as ARkStorm 2.0, could turn vast stretches of California into an inland sea during any winter. The disaster would involve a relentless chain of extreme storms unlike anything the state has experienced in nearly 200 years.
Researchers say they cannot predict when such a storm might strike. It could arrive this year or remain decades away, and no such storm is currently forming.
The scenario envisions multiple atmospheric rivers slamming into the West Coast over roughly 30 days, dumping extraordinary amounts of rain and snow across California. Floodwater could reach 20 feet in some communities, force more than 1.5 million people to evacuate, and leave populated areas between San Francisco and Los Angeles submerged. The resulting destruction could exceed $1 trillion, potentially making it one of the costliest natural disasters in global history.
California's Central Valley would face the greatest danger, though flooding, landslides, power failures and transportation disruptions could affect every county in the state.
"It was always a when, not if," Dr Daniel Swain, a UCLA climate scientist who coauthored the ARkStorm 2.0 study, told BBC Science Focus. "Before global warming, that 'when' might have been centuries away. Now it's quite likely to be within my own lifetime."
What is ARkStorm?
ARkStorm stands for "Atmospheric River 1,000-year Storm" and was created to help officials prepare for an extreme but realistic weather disaster. Atmospheric rivers are long, narrow corridors of water vapor that flow through the sky from the tropics toward higher latitudes.
When they reach California and collide with coastal mountains or the Sierra Nevada, the vapor condenses and falls as heavy rain or snow. These systems are not inherently destructive and provide up to half of California's annual precipitation, making them essential to the state's water supply. But when one stalls, or several arrive in rapid succession, they can trigger catastrophic flooding, landslides and reservoir overflows. The strongest atmospheric rivers can carry roughly 15 times as much water as the Mississippi River carries at its mouth.
The original ARkStorm scenario was developed by the US Geological Survey and a team of federal, state and academic experts in 2010.
Echoes of the Great Flood of 1861-62
The scenario was modeled partly on California's Great Flood of 1861-62, when storms battered the state for roughly 45 days and turned the Central Valley into a lake nearly 300 miles long. Towns, farms and roads disappeared beneath the water, and the state capital was temporarily moved from Sacramento to San Francisco. Geological evidence indicates six storms more severe than the 1861-62 disaster struck California over the past 1,800 years, showing that flood was not an isolated event.
Researchers updated the scenario in 2022 to examine how a similar sequence would play out in a warmer climate. Swain described the first major change as the "expanding atmospheric sponge effect." For every 1.8 degrees Fahrenheit of warming, the atmosphere can hold about seven percent more water vapor, giving storms additional fuel.
"You can essentially just imagine an ordinary kitchen sponge that grows progressively larger with warming," Swain said. "It has the capacity to absorb much more water."
A rising snowline adds to the danger
The second danger comes from a rising snowline in the Sierra Nevada. Precipitation that once would have stayed frozen in the mountains increasingly falls as rain, while warm rainfall can melt existing snow and send additional water racing downhill.
"You get increased precipitation intensity, but also a large shift from snow to rain," Swain said. The combined "double whammy" could increase runoff in parts of the southern Sierra Nevada by between 200 and 400 percent compared with historical megastorms.
The water would funnel into the Sacramento and San Joaquin river systems before spreading through the Central Valley, home to millions of people and some of the country's most productive farmland. "The San Joaquin Valley in particular really stands out to me as being of great concern," Swain said, noting the region also holds critical infrastructure supplying water to Southern California.
Aging defenses and a slow response
Hundreds of miles of aging levees, canals and pumping stations protect the valley, but many were built for a climate in which more winter precipitation stayed locked in mountain snow. The USGS has warned that California's existing flood defenses were not designed to withstand the water volumes envisioned in an ARkStorm. About one-quarter of buildings in the state could experience some flooding, and millions of property owners may lack adequate insurance to rebuild.
Researchers estimate climate change has already more than doubled the likelihood of a storm sequence capable of producing a megaflood compared with the early 20th century. Without major emissions reductions, events once expected every two centuries could occur three times per century by around 2060.
California is strengthening levees, restoring floodplains and improving emergency planning through the Central Valley Flood Protection Plan, which could cost up to $30 billion over 30 years. Officials are also developing high-resolution maps showing how floodwater would move through communities and interact with dams, levees and highways. But experts warned the work is moving too slowly.
"There's a lot more that needs to be done. We are not prepared," said Dr Christine Albano of the Desert Research Institute.
Unlike an earthquake, an approaching megastorm could be detected days or potentially weeks in advance, giving authorities time to order evacuations and move equipment. That warning would offer California its best chance to prevent an extreme weather event from becoming an unprecedented human catastrophe.
"Statistically, it could happen this year, or it might not happen for 70 years, if we get lucky," Swain said.
