California is no stranger to wide swings between wet and dry weather. The “atmospheric river” storms that hit the state this winter are part of a system that has long interrupted dry spells with huge bursts of rain. Indeed, they provide between 30 and 50% of all precipitation on the west coast.
The parade of storms that have hit California in recent months have dumped more than 30 trillion gallons of water on the state, filling empty reservoirs for years and burying mountain towns in snow.
But climate change is making these storms much wetter and more intense, increasing the risk of potential flooding in California and other west coast states. This is not just because the air over the Pacific will hold more moisture as sea temperatures rise, bringing giant volumes of rain and snow, but also because warming temperatures over earth will cause more precipitation in the form of rain in the future, which will lead to more dangerous floods.
“There’s a cascading chain of impacts,” said Tom Corringham, a research fellow at UC San Diego’s Scripps Institution of Oceanography. “The harder you push the rivers, the harder and harder you push the flood protection system, you get a kind of impact that increases exponentially. You flood the whole floodplain, or a levee breaks , and that’s where you get the really catastrophic events.
An “atmospheric river” is a long, narrow ribbon of moisture that carries water vapor from the tropics to lands at higher latitudes. One of the best-known examples is the “Pineapple Expresswhich flows east from Hawaii across the Pacific Ocean and makes landfall on the west coast. The term atmospheric river was born in the 1990s and became established because of the high volume of water that these ribbons can contain: only one can move more than twice as much water across the sky as it flows from the mouth of the Amazon, the world’s largest river by volume.
As sea and air temperatures in the Pacific Ocean rise, storms hitting the west coast now retain more moistureleading to longer and more intense rain episodes. At the same time, rainfall from low- and medium-intensity storms began to decline, leaving California to swing on a pendulum between extreme drought and extreme rain. Research suggests that with further warming, atmospheric river events will account for an ever larger share of California’s total water budget, dumping water faster than the state can absorb it.
“Around the world, some places are going to get wetter, and some places are going to get drier, and for California it looks like we’re going to have both,” Corringham said. “There will be longer dry spells, then when the rains come, these events will be more intense. For water management, this is not what you want.