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In Future Droughts, Big Storms Decide California’s Fate

Climate change is expected to make California more dependent on extreme “atmospheric river” storm events, scientists say.

Written by Published on Read time Approx. 3 minutes
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California will become increasingly dependent on the whims of a few big storms to meet water demand, according to research into the effects of climate change.

Already, California is well known for extreme weather events, from the severe drought now gripping the state to the deadly floods and mudslides that can wipe out whole neighborhoods. These are largely triggered by storms called “atmospheric rivers.”

Sometimes compared to a horizontal hurricane, an atmospheric river funnels high-velocity winds that pull huge quantities of tropical moisture across the Pacific Ocean. That moisture is concentrated in a narrow band, like a fire hose, that often targets a relatively small area of California, bringing torrential rain, deep snow – and sometimes dangerous flooding. Such storms are also called “pineapple express” events.

The effects of climate change caused by human greenhouse gas emissions are expected to make such storms more severe in the future, said Michael Dettinger, a climate scientist at the U.S. Geological Survey and Scripps Institution of Oceanography. At the same time, routine storm events – that winter pattern of cold Pacific low-pressure systems every two weeks or so – are expected to become less common.

What that means, he said, is that atmospheric rivers will determine whether California has enough water to survive the climate of the future.

Dettinger’s work is based on analyzing computer models of the state’s future climate, based on different scenarios of greenhouse gas emissions. He spoke on Monday at the California Climate Change Symposium in Sacramento.

“Fundamentally what’s going on is most of the normal storms, the small storms, decline in all the models,” Dettinger said. “The way we’ll get wetter is by having the largest storms get wetter. That’s not exactly how you want to see it develop.”

The reason this happens is that as the atmosphere warms, it becomes capable of holding more water vapor. As a result, atmospheric river storms – already the cause of California’s worst natural disasters – are expected to become even more extreme in the future. Dettinger referred to these as the “hell raiser” atmospheric river events.

Therefore, when California does get those big snow- and rain-producing storms, it is more likely that they will also bring damaging floods, mudslides and wave erosion.

The phenomenon is independent of the El Niño condition, the cyclical warming of the equatorial Pacific Ocean every three to five years, which is not directly connected to climate change. Atmospheric rivers can form no matter what El Niño condition is in effect.

“The biggest atmospheric rivers seem to increase dramatically,” Dettinger said. “There aren’t many. But in current projections, they do tend to increase a lot. The way that our precipitation will change in the future is pretty much dictated by what happens to those large storms.”

He pointed out that, even now, droughts are mostly caused when California misses out on big storms. That has certainly been the tale of the current drought.

Although the state has benefited from several large storms over the past four years, they have often come early in the winter season – a time when precipitation is generally not heavy – and then the storms stopped arriving completely.

One cause of this in the current drought is a persistent ridge of strong high pressure looming off the coast that has blocked Pacific storms from reaching the state. Instead, many have been diverted to the north, bypassing California.

Dubbed the “Ridiculously Resilient Ridge” by Stanford graduate student Daniel Swain, the phenomenon is largely unexplained.

“What we find is that having really persistent ridging like this is unprecedented in our historic record,” said Noah Diffenbaugh, an associate professor of Earth System Science at Stanford University. Swain is one of his students. “It’s a really rare occurrence in the current climate – anywhere from century scale to two millennia. It’s clearly bad luck to get this kind of persistent ridging.”

There are indications, however, that such troublesome ridges may become more likely in the future as climate change intensifies.

“There’s some evidence that global warming has made that bad luck more likely,” said Diffenbaugh, who also spoke at the symposium. “It’s less rare than it would have been without any global warming.”

Photo courtesy by Scripps Institution of Oceanography

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