The Oroville Dam in northern California is the nation’s tallest dam and it creates the state’s second-largest reservoir. In February of 2017, an atmospheric river dumped a huge amount of snow then a huge amount of rain into the reservoir’s watershed.
A 30-foot wall at the top of the dam, called the weir, nearly gave way to the volumes of water and 188,000 people had to be evacuated.
The intensity of the atmospheric river that threatened the dam was the result of a warming atmosphere, according to a Feb. 9 study.
“Rainfall from the atmospheric river was enhanced by about 11 to 15 percent by climate change, compared to what it would have been in pre-industrial times,” said Alexander Gershunov, a research meteorologist at the Scripps Institution of Oceanography in San Diego.
Gershunov is co-author of an article about that epic event in the journal "Earth’s Future," which he calls the “first study that quantified the influence of global warming on a specific, real, recent, and impactful atmospheric river event.”
Atmospheric rivers are bands of water vapor in the atmosphere that bring up to 50 percent of California’s precipitation. The one that arrived in the Oroville Dam’s watershed came in two “pulses.” The fact that the first one was snow and the second was rain means there was a lot of snowmelt, which made the flooding much worse.
The year 2017 was the most active year on record for atmospheric rivers in California, according to Gershunov.
The ability of air to hold moisture increases with temperature. The "Earth's Future" article's conclusions were based on climate modeling projections, which show that climate change increased the rainfall at that event.
“It illustrates very well the kind of hydroclimate change that we expect to see with climate change in California," Gershunov said. "Specifically, less frequent precipitation but more intense extremes due to atmospheric rivers.”
Climate change means California’s water supply is becoming more dependent on atmospheric rivers, which are becoming more intense. So researchers hope to be better at forecasting them.
“That means longer lead times and better accuracy in terms of their specific target zone,” Gershunov said.
As for preventing more climate change, Gershunov said people need to enter the "next phase in the evolution of humankind" as it relates to our development and use of energy resources.
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