Rivers in the sky: Why California is flooding

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Global warming is amplifying droughts and floods

During the past several years alone, California has seen an epic drought and record warm summers followed by an unexpectedly prolific and destructive rainy season. Experts say this should be seen through the prism of global warming, otherwise infrastructure planners may not learn the lessons needed to better fortify the extreme weather-prone state.

Over the longer-term, precipitation extremes in many parts of the globe have become more frequent and intense, studies have shown.

In the coming years, this whiplash may only get worse as human-caused global warming amplifies both droughts (by raising temperatures and increasing the loss of surface moisture), and floods (by increasing the incidence of heavy precipitation events). 

Michael Dettinger, a research hydrologist with the U.S. Geological Survey in Nevada, said the climate whiplash during the past few years, going from a paucity of precipitation to plenty, is a preview of what's to come. 

"The combination of this winter and the recent hot-drought form a pretty reasonable analog for thinking about the challenges to come," he said in an email. 

However, many uncertainties remain in how regional precipitation patterns may shift in a warming world, particularly in the Southwest U.S., where many computer models show an increasing likelihood of drought in the future. 

Scientists are still studying how global warming may alter atmospheric rivers themselves, although there is agreement that as temperatures rise, more heavy precipitation is likely to fall as rain in higher elevations instead of snow, which would cause more strain on water infrastructure like the Oroville Dam.

Dettinger and other experts said sudden swings from drought to flood may be the new norm, which will require a fundamental rethinking of infrastructure design for new dams, levees, bridges and other critical infrastructure across the West and the rest of the country.

"The current situation in California —specifically, the dramatic swing from extreme drought to water overabundance and flooding — is indeed a preview of California's likely climate future," said Daniel Swain, a climate scientist at UCLA. 

"There is now quite a bit of evidence that future droughts here will be warmer and more intense, yet will be interrupted by increasingly powerful 'atmospheric river' storms capable of causing destructive flooding." 

Some research shows that extremely prolific atmospheric rivers could become far more common — perhaps twice as common as they are now — in parts of California by the end of the century, though this is still a matter of scientific debate. 

Most atmospheric river events are not of the same powerful intensity as the one that struck the state over the weekend, with pineapple express storms considered a subset of overall atmospheric river events. According to NOAA, a total of 42 atmospheric river events hit California between 1997 and 2006. 

Swain and Dettinger both say there is an increased expectation for more frequent and potentially stronger atmospheric river events in the future. 

"One important point that the present situation highlights: while climate change sometimes leads to "unprecedented" weather extremes that have no historical precedent, it even more commonly increases the risk of extreme weather events at the upper end of the historical intensity spectrum," Swain said.

Dettinger's work supports this conclusion, too. 

"... What has really been going on is that we’ve just had so many [atmospheric river events] arrive in such a short time this winter. This is actually in keeping with one of the early findings re: West Coast atmospheric rivers and climate change… that the numbers of atmospheric rivers will increase faster than their (average) intensities," he said. 

This is also worrisome, since reconstructions of historic flood events —like the Great Flood of 1862 — as well as a simulation of what would be a devastating flood both involved many weak to moderate atmospheric river events hitting in rapid succession; the same thing that's happening now. 

Noah Diffenbough, a climate scientist at Stanford University, says California's wet winter demonstrates how we're already hitting the limits of water infrastructure that was designed in a completely different climate. 

“We know that the climate is different now than when our water infrastructure in the West was designed and built,” Diffenbough said.  

He said adapting to climate change means building a more resilient, less risk-prone water management system in California. "We’re already incurring great costs for the climate that we have now,” he said.

“Most of what we would do to protect ourselves now would help us be prepared for the future.”