What is an atmospheric river?
The biggest freshwater rivers on Earth don’t flow along the planet’s surface.
Instead, they surge and whip through the atmosphere thousands of feet above our heads, carrying 2½ times the amount of water that gushes through the Amazon River at any given time.
They’re called atmospheric rivers, or, more aptly, rivers in the sky.
These rivers are capable of burying Sacramento under 30 feet of water.
A research team led by Sasha Gershunov at Scripps Institution of Oceanography at UC San Diego published a new study on atmospheric rivers in Nature Scientific Reports this week that places atmospheric rivers under scrutiny as the driving cause behind California’s increasingly extreme, infrequent bouts of precipitation.
Gershunov’s team used 16 global climate models to analyze the expanding role of atmospheric rivers as contributors to precipitation in California. The results show that atmospheric rivers are getting stronger and wetter, and catastrophic events like the Great Flood of 1862 could happen again.
“In 1862, Sacramento was underwater,” Gershunov said. “It was most certainly due to an atmospheric river.”
In 1861, Northern California became the focal point for two consecutive atmospheric rivers that surged into the Sierra Nevada, melting snow at disastrous rates. By 1862, a catastrophic flood swept through the Central Valley, augmented by two rainstorms, creating an inland sea that was 300 miles long and 60 miles wide.
It rained for 45 days straight, according to a film produced by the U.S. Geological Survey. Thousands of cattle drowned, and vineyards and homes were washed away. The state went bankrupt. The American River near Auburn rose 35 feet, submerging towns.
No place was more affected than Sacramento, however.
Situated at the confluence of the American and Sacramento rivers, the city went under 30 feet of water.
Leland Stanford, the governor-elect, had to row a boat to his inauguration in January 1862.
“Nearly every house and farm over this immense region is gone. America has never seen such desolation by a flood,” wrote botanist William Brewer of the California Division of Mines and Geology in 1862.
According to a team led by scientist Dale Cox at the U.S. Geological Survey, California is due for another megaflood.
“Absolutely, it could happen again,” Gershunov said. “The probability of it happening is increasing with stronger atmospheric rivers.”
Indeed, these rivers in the sky are becoming wetter and stronger.
Around Valentine’s Day this year, California was inundated by record-breaking moisture. On Feb. 13, some cities in Northern California received more snow than Boston saw all winter. The next day, Palm Springs saw more than eight months’ worth of rain in one day. In San Diego, the city’s dusty streets became slick with mud, and sinkholes and mudslides ravaged the formerly dry landscape.
The culprit? One of the largest atmospheric rivers ever recorded, 300 miles wide and 1,000 miles long. All signs point toward even stronger atmospheric rivers, rendering California more vulnerable to a megaflood.
Gershunov said that atmospheric rivers are gradually overtaking storms as the main contributor to California’s fickle precipitation.
Currently, atmospheric rivers are responsible for 40 to 50 percent of total precipitation, according to Gershunov. This number is rapidly expanding, however.
“They’re becoming more potent,” Gershunov said. “They’re becoming wetter, longer, and fatter, so when more moisture gets slammed against the mountains, you get more extreme precipitation events.”
Atmospheric rivers are wetter than ever because warming global temperatures increase their water-holding capacity. When global wind currents push these rivers westward from the subtropics, they slam into the Sierra, unleashing as much water as an East Coast hurricane over three days, Gershunov said.
Everything points to another megaflood, named ARkStorm, short for “atmospheric river 1,000,” by the USGS.
According to the USGS, the ARkStorm is a hypothetical but scientifically realistic megastorm, modeled on past megafloods in California. Analysis of sediment in the Bay Area, Santa Barbara basin, Sacramento Valley and Klamath Mountain region revealed that six floods even more catastrophic than the one in 1862 occurred in 212, 440, 603, 1029, 1418 and 1605, according to the USGS.
“A storm of this scale is inevitable,” the USGS said in a harrowing short film on ARkStorm published in 2011.
According to the film, such a megaflood could flood Sacramento, the Delta, Los Angeles, Orange County and San Diego. Ten feet of rain would fall, with landslides crushing homes, highways and buildings. Levees and dams would be overwhelmed, and property damage would approach $400 billion. This figure is three times more than the predicted damage costs of a mega-earthquake.
After nearly a month of constant rain, the storm would subside, leaving California wholly unable to grapple with the aftereffects. Only 12 percent of Californian properties carry flood insurance, according to the USGS, so millions of homeowners would be unable to rebuild. Parts of California would be left underwater for months.
Anne Wein, a disaster scientist on the ARkStorm team at USGS, warned that atmospheric rivers could be just as dangerous as hurricanes, if not more so.
“In our scenario, we found a million people in the flood zone, which is comparable to Hurricane Katrina,” Wein said.
“They’re like our versions of hurricanes in a sense,” Gershunov said.
To prepare for such a catastrophe, Wein urges Sacramento residents to know evacuation routes ahead of time. This can be crucial to survival, according to Wein.
“We saw that many people don’t own vehicles in Sacramento,” Wein said. “Just knowing what the evacuation plan is for your neighborhood and city is pretty critical ahead of time.”
Although the megaflood would cause widespread catastrophe, Gershunov said that the accumulated damage incurred from smaller, yearly floods due to atmospheric rivers are also a concern.
“It’s not just these extreme floods or megaflood, it’s also the regular floods that we get almost every year in California, mostly driven by atmospheric rivers,” Gershunov said. “That’s clear from flood insurance data.”
In the coming weeks, Gershunov will publish a study on the yearly economic impact of such atmospheric river-driven minor floods on California.
For now, Gershunov’s team continues to examine atmospheric rivers in an attempt to understand their powerful impact on California’s increasingly chaotic weather patterns.
“They are so ephemeral and variable,” Gershunov said.