Flooding events have wreaked havoc across the country this summer.
Heavy rains in late July devastated eastern Kentucky, causing at least 39 deaths and millions of dollars in damage. Unprecedented rainfall caused flash flooding that stranded 1,000 people in Death Valley National Park earlier this month.
In Northern California, amidst efforts to contain the McKinney Fire, intense rainfall triggered flash floods and debris flows - fast-moving slurries of rock, mud and sediment - which temporarily trapped firefighters.
More of this flooding may be on the way for the western United States as the planet continues to warm up. Atmospheric rivers - long, narrow regions of the atmosphere that transport water vapor from the tropics - drive storms that can produce extreme precipitation, causing 84% of flood damages in the western United States.
If greenhouse gas emissions remain high, a new study proposes that average flood damages each year due to atmospheric rivers could triple. The analyses included California, Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming.
As storms intensify, destruction could amount to $3.2 billion annually by the 2090s, compared to an estimated $1 billion each year a century earlier. The team - based out of Scripps Institution of Oceanography at UC San Diego - estimates that in the 2020s, atmospheric river storms are totalling $1.6 billion per year in flood damages.
"It's really useful to put a monetary value on what we are expected to see with these projected climate changes," said Alison Michaelis, an atmospheric scientist at Northern Illinois University, who was not involved in the study. The values could help inform decision makers as they prepare for future floods.
The researchers also identified counties in the western U.S. projected to show the biggest increases in flood damages by the end of the century. Four of the top five are in California or on the California-Nevada border.
The study authors used 40 years of flood insurance data to develop statistical models linking features of atmospheric rivers to flood damages. They ran these models through 16 global climate simulations under intermediate and high greenhouse gas emissions scenarios.
The result was unanimous: all the analyses predicted increased damages.
The report chalks up these additional flood damages to stronger atmospheric events, which carry more water vapor and last longer. That means more precipitation and the potential for more severe floods.
"Even if you have a slight shift towards these more extreme storms, that can create a really significant increase in damages," said study author Tom Corringham, a Scripps climate economist.
The place with the biggest jump in flood damages? Sonoma County.
The study estimated that annual damages due to atmospheric rivers would increase by approximately $283 million per year by the end of the century, from about $185 million in the 1990s to a projected $468 million in the 2090s. This North Bay county is particularly at risk because towns like Guerneville are along the Russian River.
"It's really prone to extreme flooding," Corringham said.
The study's simulations didn't include the potential damages of a rare, atmospheric river-induced megaflood, which could trigger even higher costs.
Northern California is already familiar with the toll of atmospheric rivers. In February 2017, an atmospheric river event pummeled the region and damaged the Oroville Dam's spillways. Fears of catastrophic failure prompted the evacuation of 188,000 people from the Feather River Basin. Damage repairs totaled over $1 billion.
"I grew up as a Northern California kid, so I have a real close connection to all four of those regions," said Alan Rhoades, a hydroclimate research scientist at Lawrence Berkeley National Laboratory, who was not involved in the work. "That [result] was striking to me."
Some steps are being taken to manage flood risk in California. The Department of Water Resources is putting money towards improving radar coverage in the Bay Area, to give people more time to prepare before flood damages occur. Projects at both local and state levels are underway to improve the resiliency of communities to floods. Widening creeks and reintroducing natural meander, for example, will enable waterways to absorb water better during storm events.
Hardened flood defenses and improved community resilience will be critical in a world with increasingly intense atmospheric rivers.
"California by and large is doing a good job in working to improve flood protection across the state," Corringham said. But the most meaningful way to prevent future flood damage is to address climate change, he said. Even moderate reduction of greenhouse gas emissions could discount the cost of future flood damages by 30%.
"All of these solutions that we've talked about are really just bandaids," he said. "What we really need to focus on is the reduction in greenhouse gas emissions and the transition to renewable energy."
Jack Lee is a San Francisco Chronicle staff writer. Email: jack.lee@sfchronicle.com