A relentless series of ‘rivers in the sky’ is creating extreme conditions across the state, but a role for climate change is unclear
Not again! Earlier this week, California was battered by heavy rain, strong winds and thick snow — the latest in a seemingly unending procession of strong storms. Wild weather has afflicted the previously drought-stricken state for three months, resulting in devastating floods, paralysing blizzards and dozens of deaths. Data released Thursday show that the snowpack is the biggest on record. Nature spoke to atmospheric and climate scientists about what’s driving the surge in wet weather and what the state could look like in a warmer future.
Why are so many storms hitting California?
California’s recent parade of storms is driven by atmospheric rivers — long, narrow plumes of moist air that travel from the tropics to higher latitudes. When these ‘rivers in the sky’ sweep over mountainous regions they condense into clouds that produce heavy rain and snow, says Allison Michaelis, an atmospheric scientist at Northern Illinois University in DeKalb.
An atmospheric river can ferry enormous amounts of water vapour; some discharge more than twice as much water as the Amazon River1. In the western United States, atmospheric rivers contribute up to half of the region’s annual rain and snow. Since last November, 31 atmospheric rivers have hit California, more than half of which ranged from moderate to extreme, according to data from the Scripps Institution of Oceanography in La Jolla, California.
Although back-to-back atmospheric rivers are not unheard of, they make a significant impact, says Michaelis. “What might have typically been a more beneficial event could turn potentially hazardous if it comes on the heels of another system.”
How much snow is there?
In the Sierra Nevada mountain range in eastern California, the season is the snowiest since 1952, says Andrew Schwartz, an atmospheric scientist who leads the University of California, Berkeley’s Central Sierra Snow Lab in Donner Pass. “It’s just dumping snow,” he says. A total of 18 metres of snow has fallen at the lab this season, nearly double the yearly average. And statewide, the snow’s water content — the amount of water that would result if the snow were melted — is roughly double the average, says Schwartz.
The conditions have brought welcome relief after the three driest years on record in California, allowing the rollback of ‘exceptional’ and ‘extreme’ drought designations for the first time since 2020, according to the US National Oceanic and Atmospheric Administration’s US spring outlook. But capturing and storing water released as the thick snowpack begins to melt can be a race against time, says Tom Corringham, a research economist at Scripps. If the snow melts too quickly, the excess water ends up in the ocean instead of being stored and distributed to where it’s needed most, he says. “That’s not ideal for water management.”
Is climate change playing a part?
As the atmosphere warms, atmospheric rivers are likely to become more frequent and hold more moisture, and that will result in heavy downpours of rain and snow, says Schwartz. He notes that California is swinging between wet and dry periods that are more extreme than in the past. “While this variability has always existed, it’s becoming amplified due to climate change,” he says.
Kim Reid, a climate scientist at Monash University in Melbourne, Australia, says that more work needs to be done to understand how climate change will affect jet streams and other systems that influence the direction of atmospheric rivers. If atmospheric rivers shift by a few degrees latitude, they could become more common in some regions and rarer in others, she says. (Nature)
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