Northwest floods offer sneak peek of hotter climate's toll

Climate scientists say December’s back-to-back atmospheric rivers and extreme flooding offer a sneak peek into our warmer future.

The ocean-crossing storms known as atmospheric rivers are nothing new: They deliver precipitation up and down the West Coast every winter and are the source of close to half of Western Washington and Western Oregon’s precipitation from November to April. But climate scientists expect them to grow more powerful, arrive more frequently, and last longer as Earth’s climate keeps warming.

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“The science is clear that floods are going to become larger and more frequent in the future,” Washington State Climatologist Guillaume Mauger said.

About 78,000 people have been ordered to evacuate from Skagit County, and roughly 20,000 more in other Western Washington counties as of Thursday afternoon, according to the Washington Military Department.

In addition to dislocating communities and destroying property, the Skagit River’s extreme flooding is expected to take a large toll on fish in Puget Sound’s most important salmon river.

Mauger said climate change could bring extreme, formerly once-a-century floods to the Skagit River four times more often, as soon as the 2040s.

A warmer atmosphere can hold — and dump — more moisture, and more of that precipitation is likely to fall as rain, not snow.

“If there's less falling as snow and more falling as rain, that means there's more water that can end up directly in the rivers and contribute to flooding,” Mauger said.

“The frequency, magnitude and duration of atmospheric rivers making landfall along the North American West Coast are projected to increase,” scientists with the United Nations Intergovernmental Panel on Climate Change have found. That intergovernmental science panel estimates that for every Celsius degree of human-caused warming, extreme storms over land are expected to dump 7% more precipitation.

Though scientists are forecasting higher-intensity atmospheric rivers hitting the Northwest, they have not detected such a trend to date.

Mauger said neither of the latest atmospheric rivers delivered record-breaking amounts of rain, but the two arriving back to back overwhelmed the region’s rivers.

How much damage a storm does depends not just on its size, but on what humans have done to prepare for it.

Avoiding development on floodplains and leaving forests, wetlands, and other unpaved areas nearby can help rain soak into the ground instead of heading straight into a river.

“We need trees in place to help soak up and slow down this precipitation to help prevent downstream flooding,” said Meade Krosby, senior scientist with the University of Washington’s Climate Impacts Group.

Even paved, built-up areas can be designed to be more sponge-like, Krosby said.

“Things like rain gardens, green roofs, permeable pavement — anything that can help absorb and slow down and spread out the water before it overwhelms our storm water systems and causes flooding,” she said.

Such efforts can help protect natural systems as well as modern infrastructure from flooding.

Extreme river flows can scour away fish eggs from their underwater gravel nests and blast young salmon downriver before they are ready to migrate. The firehose-like flows can also smother salmon eggs in sediment, a process biologists call “entombment.”

Currently in the Skagit River basin, coho and chum are actively spawning, while Chinook and pink salmon have mostly completed their spawning, and their eggs are incubating in river-bottom nests.

“Chinook and to some extent pink salmon are the most at risk from floods of this magnitude because they predominantly spawn in the mainstem river,” fisheries researcher Mike LeMoine with the Skagit River System Cooperative, a project of the Swinomish and Sauk-Suiattle Tribes, said in an email.

“Following the 2021 floods, we documented the lowest abundance of out-migrating Chinook salmon ever recorded,” LeMoine said.

RELATED: Salmon, rivers hit hard by recent Washington floods

In one watershed to the south, pink and Chinook salmon eggs are incubating in the main channels of the Stillaguamish River.

“This timing/location of spawning puts their eggs right in the path of these large floods, which scour and entomb the developing embryos,” Stillaguamish Tribe of Indians biologist Jason Griffith said in an email.

On the flip side, Griffith said, some coho salmon use high water to gain access to habitats in tributary streams they otherwise could not reach.

Ultimately, tackling the root cause of climate change — emissions of heat-trapping gases from fossil fuels and other sources — is essential to minimize the human and biological toll of flooding in the long term.

“If we don't do that, atmospheric rivers are going to keep growing stronger, delivering amounts of rain and precipitation,” Krosby said. “It's going to overwhelm our ability to respond.”