Sea-level rise threatens thousands of homes in California by 2035, especially in cities near San Francisco and Los Angeles, according to an analysis released today.

Chronic flooding by that year imperils nearly 5,000 homes in the Silicon Valley south of San Francisco, a region that’s home to affluent homeowners and an international airport. In the suburbs north of San Francisco, roughly 4,000 homes are at risk, according to the study from the Union of Concerned Scientists.

A decade later, if sea levels rise faster due to accelerated melting of ice sheets, roughly 13,000 homes valued at about $8.6 billion are in danger in the nine counties surrounding the San Francisco Bay Area.

The findings are part of a national report that analyzed coastal regions around the county. It identified ZIP codes most at risk, tallied the number of homes threatened, and calculated the value of those homes and the amount in property taxes that could be lost.

“We know that sea levels are rising and putting our coasts at risk,” said Kristina Dahl, senior climate scientist at UCS. “The near-term threat to our properties—our homes and businesses—is really flying under the radar. And that’s dangerous.”

Researchers looked at areas that have flooded on average 26 times per year. They studied 20 years of data for coastal tide gauges and determined the “chronic inundation” water level. UCS then added sea-level rise projections to existing water levels and developed a chronic inundation point for the time frames of 2035, 2060 and 2099.

The analysis also used digital elevation models from NOAA to estimate the extent of flooding. Researchers then applied data from the real estate site Zillow to calculate the number of homes at risk and their values.

California will not be hit as soon as places like New Jersey or Florida, Dahl said, because the Golden State has beach-side bluffs and other topography that will block water temporarily. Later in the century, however, the impacts are large.

“Even an inch of water can cause significant damage or change the way you use land,” Dahl said. “An inch of water in a basement is something few homeowners would want to deal with 26 times per year.”

The report comes just a few days after a study was published in Nature that showed the West Antarctic ice sheet is melting three times as fast as it did 25 years ago (Climatewire, June 14).

That could wallop California with greater sea rise than the world average. That’s due to a gravitational effect on ocean currents and the way the Earth rotates, according to Helen Fricker, a professor of geophysics at the Scripps Institution of Oceanography at the University of California, San Diego, an author on that study. Three feet of sea-level rise coming from Antarctica would mean it’s 4 feet higher in California.

“We can say, ‘Well, it’s inevitable, and there’s nothing we can do,’” Dahl said. “Or we can say, ‘It’s inevitable, so let’s start planning for it.’ That’s the approach I would hope people would take. The more we take this threat seriously and start taking action now, the better off we’ll be.”

San Francisco airport at risk

The San Francisco Bay Area is particularly endangered because it has low-lying wetland areas that rim the bay, Dahl said. As well, there are a lot of residential areas built on filled-in wetlands. Those areas are “much more at risk than our ocean-facing beaches,” she said.

San Mateo County is one of the most affected places in the Bay Area. By the turn of the century, flooding could affect more than 9,700 homes under an “intermediate scenario,” which includes moderate ice sheet melt and global sea-level rise of 4 feet by 2100, the analysis said.

If there’s more rapid ice sheet loss and global average sea-level rise of 6.6 feet by that year, water inundation would affect nearly 14,000 homes, the analysis said.

In the county, several low-lying wastewater treatment plants that serve multiple cities are at risk with 3 feet of sea-level rise, said Hilary Papendick, climate change program manager for San Mateo County. In addition, San Francisco International Airport, one of the biggest airports in the country, is “very vulnerable,” she said.

“Some of the sea-level rise maps show with just 2 feet of sea-level rise it could be flooded,” Papendick said. The airport authority is “actively working on fixing some of low spots in the levees and coming up with a long-term plan,” she said.

In addition, the county is trying to coordinate adaptation responses between the cities, because a sea wall in one place could just mean flooding in another, said Jasneet Sharma, San Mateo County’s climate resilience specialist.

San Mateo County is one of eight California municipalities suing oil companies for climate-related damages. The suits claim that petroleum products create a public nuisance by worsening sea-level rise. The county’s sea-level rise vulnerability analysis published in March found that an extreme storm could add 42 inches of water on top of expected changes to mean sea-level height.

“Such an event, even with the minimum anticipated sea-level rise, would inundate thousands of acres of County land, breach flood protection infrastructure, and swamp San Francisco International Airport,” the lawsuit said.

Controversy over retreat

Other areas at high risk by the end of the century include Long Beach, Seal Beach, Huntington Beach and Newport Beach, south of Los Angeles. Those areas have wetlands, channels and storm drains that will allow rising water to flow onto land.

“The ocean will find the point of lowest elevation and work its way into the land,” Dahl said. “Sometimes that’s through wetlands; sometimes that’s through a canal; sometimes that’s through a break in the sea wall that you think is protecting you.”

One place likely to get hit hard is Newport Beach’s Balboa Island, located about 40 miles southwest of LA. An affluent enclave built on dredged sand and silt, it sits below high tide level. Rising waters by 2035 will affect nearly 1,000 homes there worth $1.3 billion, under the intermediate sea-level rise scenario.

To deal with current flooding, the city is adding a 9-inch topper to an aging sea wall as a short-term fix. The longer-term plan under development is essentially to lift the island. Homes, streets and sea walls would be raised. It’s controversial, as homeowners would have to pay for their own work (Climatewire, March 29).

“We’re going to be seeing these kinds of conversations play out in hundreds of communities in the coming decades as more and more places start to face these challenges,” Dahl said.

The city of Del Mar, north of San Diego, recently passed a sea-level rise adaptation plan that rejected managed retreat, or removing homes in the ocean’s way as it rises. That came following heavy pressure from residents. The city is at high risk for flooding later this century (ClimateWire, July 21, 2017).

However, Del Mar and other California cities and counties will be forced to confront managed retreat. Municipalities must examine it as part of analyses into coping with sea-level rise, under rules from the California Coastal Commission, a powerful agency that oversees development along 1,100 miles of coastal land (Climatewire, May 23).

Dahl said a more comprehensive look at retreat is needed.

“Given the fact that we can see this risk of sea-level rise growing, it would behoove us as communities, as states, as nations to have a coherent framework that communities can rely on so that they have resources to implement the options that are most desirable,” she said.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at