An aerial view of a flooded neighborhood on Feb. 28, 2019, in Guerneville, California. The Russian River crested over flood stage and inundated the town.

(Justin Sullivan/Getty Images)

At a Glance

Flood damage from atmospheric rivers averaged $1.1 billion annually throughout the West.An AR-triggered flood in February inundated the California town of Guerneville.Global warming will increase the intensity of atmospheric rivers, scientists said.

Atmospheric rivers are causing billions of dollars in damage in the the western United States, according to a new study.

Flooding caused by atmospheric rivers – long, thin plumes of moisture capable of carrying tremendous amounts of water – in western states in the last 40 years, the study by researchers at the and the U.S. Army Corps of Engineers found.

Total flood damage from atmospheric rivers (AR) averaged $1.1 billion annually throughout the West, according to the study published Wednesday in the journal Science Advances.

(MORE: A Blessing and a Curse, Atmospheric Rivers Explained)

A strong AR can carry an amount of water vapor equivalent to as much as 15 times the average flow of liquid water , according to NOAA's Earth System Research Laboratory. One well-known AR is the "Pineapple Express," which brings moisture from the tropics near Hawaii to the West Coast.

ARs can be beneficial. They generate much of the annual precipitation in California and the West, which replenishes the region’s water supply. About half of all ARs over the 40 years in the study caused no insured losses.

However, "If an atmospheric river stalls over land, particularly when that moisture is lifted by mountainous terrain, ," explained weather.com senior meteorologist .

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An AR-triggered flood in February in Northern California's Sonoma County. Some areas received more than 20 inches of rain, and the Russian River overflowed its banks and swamped more than 2,600 homes, businesses and other buildings.

A resident carries his dog as he navigates his boat through a flooded neighborhood on Feb. 28, 2019, in Guerneville, California. The town was inundated when the Russian River crested over flood stage.

(Justin Sullivan/Getty Images)

The new Scripps study says ARs were responsible for more than 99 percent of all flood damage in some coastal areas of Northern California and Oregon. Relatively high proportions of damage were associated with AR activity in much of Arizona, Idaho and western Montana and as far east as 100 degrees west longitude, which runs from North Dakota to Texas, the study said.

And it doesn't take many ARs to see those damage costs skyrocket.

Almost half of all flood damage in the West over the past 40 years – about $23 billion – was caused by just 10 atmospheric rivers, the study found. Last year, researchers developed a scale of AR intensity from category 1 to 5, similar to the categories for hurricane strength. The new study confirmed category 1 and 2 ARs caused negligible damage, while category 4 and 5 storms caused damage in the tens and hundreds of millions of dollars.

"A small number of extreme ARs cause most of the flood damages in the West, and even modest increases in intensity could significantly increase their impacts," Tom Corringham, a Scripps researcher and leader of the study team, said in a statement.

As the climate warms, the study said, extreme ARs will become more intense as they become wetter, longer, and wider. This is already happening in the Pacific Ocean, the study added.

"Modest increases in AR intensity could lead to significant increases in damages," the study said.

That's one more reason that changes in flood mitigation policy and disaster response are necessary, Corringham said.

"This is a reminder that weather and climate matter," he said. "Every step we take now to stabilize the global climate system stands to reduce future adverse impacts on our economy."

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