The county in California most at risk from sea level rise is San Mateo, with nearly 100,000 people — half residents of color — living just three feet above the high-tide line. If climate models prove correct, rising seas threaten billions of dollars of homes and businesses, and hundreds of contaminated sites could harm residents if flooded.

“We’re at ground zero for the state, so it’s our responsibility to act,” said Len Materman, who leads the San Mateo County Flood and Sea Level Rise Resiliency District, or OneShoreline.

Sea levels along the California coastline, including the San Francisco Bay, could rise 7 to 21 inches by the year 2050, depending on how much and how quickly the world's countries manage to cut carbon dioxide emissions. By the end of the century, with little drop in emissions, seas could rise by as much as 6.5 feet, according to a national study released last week. That’s from the climate emergency alone; storms, king tides and sinking land add inches to those estimates.

While the 2050 numbers are a little lower than those in a similar 2017 report, it’s nonetheless distressing news for Black, Latino, Middle Eastern and Asian communities that ring the San Francisco Bay. Places like the Alviso neighborhood in San Jose, Richmond, East Palo Alto, Marin City and Bayview-Hunters Point already flood yearly during king tides or big storms. In most of these places, Bay Area scientists believe rising groundwater could push up legacy contamination in the soil, harming people's health.

“The Bay Area is definitely a hot spot,” said Rachel Morello-Frosch. Frosch is co-leading a UC Berkeley-UCLA project called Toxic Tides, which maps contaminated sites, sea level rise and the communities most at risk.

“When you think about the coast, people think about Malibu, mansions and people living by beaches,” she said. “We want to focus on ... not only knowing about the facilities that are at risk but the communities that live nearby.”

One area of concern is just north of the Golden Gate Bridge. Near Sausalito, tiny Marin City lies adjacent to Highway 101 and the bay, and is the only predominantly Black community in Marin County. Many residents there are the direct descendants of Black people who settled in the city in the 1940s to help build ships for World War II. All these years later, residents are beginning to test for remnants of legacy contamination throughout the community.

During atmospheric rivers in October 2021, heavy rain flooded the single entrance into Marin City with multiple feet of water.

“We had to walk through floodwaters that are filled with toxins from the groundwater that's actually mixing with the sewer water,” said Chinaka Green, a Marin City resident who said she disposed of her wet clothes because of the contaminated water.

Her mother, Terrie Green, director of Marin City Climate Resilience and Health Justice, along with a group of teenage climate activists, are appealing to the state for action on flooding. They say the county won’t invest in the unincorporated area because of the color of residents' skin.

“They want the Black and Brown people out of here,” she said.

And when thinking about multiple feet of sea level rise by the end of the century, Green says that kind of flooding already happens during storms.

“But what attention are we getting?” she said of the lack of long-term planning to protect Marin City from worse flooding.

With communities of color already on the line because of existing inequities, the new federal projections show just how important it is to plan for the full range of projected sea level rise.

“Make no mistake, sea level rise is upon us,” said Nicole LeBoeuf, the National Oceanic and Atmospheric Administration's National Ocean Service director, during a webinar on the updated sea level rise outcomes. “We do recognize that there are communities around our coastlines that are more vulnerable to these kinds of impacts because of their history and being underserved to begin with.”

The new national report forecasts frequent flooding on the California coastline due to sea level rise of as much as 21 inches by 2050, 6.5 feet by the end of the century, and more than 12 feet by 2150. If emissions continue to escalate, some sea level rise scientists worry these upper limits could become a reality.

“Emissions do matter,” said Susheel Adusumilli, a postdoctoral researcher who studies sea level rise and changes in ice sheets at the Scripps Institution of Oceanography. “If you have high emissions, then it's just going to be a widespread massive impact on communities in coastal areas.”

The report, led by the NOAA, NASA, the U.S. Geological Survey and other federal agencies, updates federal sea level rise projections from 2017. It underscores how sea level rise brought on by human-caused climate change cannot be ignored and demands an all-hands-on-deck approach to preparing for encroaching seas.

Using data from tide gauges, satellites and computer modeling, the authors were able to project sea level rise with greater certainty up to 2050 than they'd been able to do before, and have extended their projections long into the future. The two leading causes of the rising tides are directly related to the continued burning of fossil fuels: Seas rise as ice sheets and glaciers melt and because ocean water expands as it warms.

“If we keep emissions down, you start to take some of those very rapid, high-impact sea level processes off the table,” said Ben Hamlington, one of the report's authors and a scientist at NASA’s Jet Propulsion Laboratory in Southern California. “There’s still quite a bit of uncertainty about the melting of the ice sheets. They could really play a big role in those higher-end scenarios here in California.”

What Hamlington says is significant about this update is that tidal gauge observations almost mirror the intermediate levels of sea level rise of nearly 10 inches on the West Coast by 2050. That suggests the intermediate sea level rise projections, rather than the higher ones, may be more accurate for California. But after the mid-century mark, he says, the “uncertainty range blows up.”

Since real-time tidal gauge observations closely track with climate models, Mark Merrifield, director of the Center for Climate Change Impacts and Adaptation at the Scripps Institution, says it's critical to take swift action on climate change.

“If we assume the worst-case scenarios … we're going to be facing a sea level rise problem here that will far outpace what we've been dealing with in the past,” he said. “There's literally no end in sight if we keep going on the same trajectory.”

The 2050 predictions are slightly lower than those of a few years ago, and that’s because science has improved. However, researchers note that it all depends on carbon dioxide emissions and local factors like subsidence, storm surge, waves and groundwater levels.

“In some cases, it's going actually to inundate and flood low-lying zones,” said Merrifield. “Places that have been built on reclaimed land, and the water tables going up with the sea level, are areas that are going to be particularly vulnerable as time goes on.”

Low-lying communities around the lip of the San Francisco Bay, like East Palo Alto on the peninsula, are already vulnerable to nuisance flooding from rain, king tides and contaminated groundwater.

East Palo Alto sits within a federally designated flood zone. According to projections, in 10 years or so, up to two-thirds of the land within city limits may regularly flood. By mid-century, those areas could be frequently underwater during high tides. Flooding of that magnitude would stress major flex points for the entire Bay Area, such as Highway 101 and the Dumbarton Bridge. That would burden many residents already dealing with inequities like homelessness, joblessness or poverty.

“If you were to get to know 100 families in East Palo Alto, maybe 50 out of 100 already are right at that point at which savings are so low that ... a flood event ... could be that tipping point,” said Derek Ouyang, a program manager and lecturer at the Stanford Future Bay Initiative, who works with community leaders in the city.

For some in East Palo Alto, flooding and climate change are threatening their homes for a second time. Climate refugees here from the Pacific Islands have already fled rising seas, only to face similar threats in a new country several thousand miles away. Appollonia Grey 'Uhilamoelangi of Samoa, known as Mama Dee in her East Palo Alto community, founded 'Anamatangi Polynesian Voices as a bridge between the city and its Polynesian residents.

“The last two floods over here, the question is, where was God?” she said. “Don't get me wrong, I believe in prayers. But I lived through so many disasters.”

Residents in East Palo Alto — and commuters across the Dumbarton Bridge — will be protected, in part, once a new, high levee is built, separating a portion of the city from a creek nearby that connects to the bay.

Creating flood protection for existing communities like East Palo Alto is essential because the new federal report found that moderate flooding, which already happens during king tides or storms, will likely arise 10 times more often by mid-century than it does today.

While sea level rise predictions have become more definitive up to 2050, Zack Wasserman, chair of the San Francisco Bay Conservation and Development Commission, or BCDC, says the forecasts only confirm what the agency already knows: A slow-moving disaster is coming.

“The difference in potential damage between 7 feet and 10 feet [past the year 2100] has some significance, but today, we're not prepared for either,” he said of the more extreme climate models.

Wasserman says the slight adjustment in the near term gives BCDC a little more time to prepare and involve more agencies, cities and counties into a Bay Area-wide plan.

“This report just demonstrates the need for us to continue our efforts and, to some extent, to accelerate our efforts,” he said.

Jessica Fain, director of planning for the agency, says she’s glad the projections extend to 2150. This allows her team to plan even further into the future, which is vital because the state agency is spearheading a regional sea level rise adaptation plan, called Bay Adapt.

“It’s 70 years away, the lifetime of a person who was born today,” she said. “So, having these further-out numbers to think about is really valuable.”

California sea level planners are taking the new update seriously. Kelsey Ducklow, a coastal resilience coordinator at the California Coastal Commission, says it will likely take a year to incorporate the recent federal data into the state’s climate plans.

“Having more confidence in what the sea level rise impacts are going to be over the next 30 years gives more confidence about the actions that we can take,” Ducklow said of the more secure levels of sea level rise projected by 2050.

But she admits that any projects in play in California — highways, homes, buildings — have life spans beyond the 2050 time frame and are why it’s essential to plan for the more extreme projections after mid-century.

Susheel Adusumilli, from the Scripps Institution, is collecting data to rework the state’s 2018 sea level rise guidance. He says rising tides could be worse for some regions of California, like Foster City in San Mateo County, where he says the land is sinking.

He also says that the California update, to come in 2023, needs to detail how Black, Middle Eastern, Latino and Asian communities will suffer economically because of rising tides.

“California's a rich state, and if California adapts to sea level rise in an equitable way … it will be heard around the world,” he said.

Sea level rise planners in San Mateo County are readying the entire shoreline — from East Palo Alto to Brisbane — for 10 feet of extra water above today’s high tide.

This level of protection goes well above the new federal predictions.

“It's an aggressive number so that in this century, we’re not going to see overtopping if we pick that number,” said Materman of OneShoreline.

To protect the hundreds of thousands of people, tech giants and infrastructure from San Mateo County that support the entire Bay Area, Materman says choosing not to be conservative when planning for rising tides is a no-brainer.

“Forty years from now, I don't want people to look back on our agency and say, ‘Oh, you trusted a report back in 2022, which under-assumed what the damage would be. So, now we have to go in and raise everything,’” he said. “That's not what we're about.”