What Would Happen If Sea Level Rose 10 Feet In California

Picture the Pacific 10 feet higher than it is today. That is the height of a basketball rim, added to every high tide along California's coast. At San Francisco International Airport, planners are designing a wall about 7.6 miles long to wrap around the runways, and it is built for 3.5 feet of rise. Ten feet is almost three times that.

The Bay Area around that airport is expected to take about two-thirds of California's economic damage from rising seas, and the bay is only one stretch of the coast at stake. NOAA's Sea Level Rise Viewer already maps the state's shoreline in half-foot steps up to 10 feet. What those maps leave out, erosion and sinking land, plays a large part in what a 10-foot sea would do here. The first question is how soon it could arrive.

The State's Highest Scenario Reaches 10 Feet in the 2130s

California's Ocean Protection Council sets the sea-level numbers that state agencies and coastal cities plan around, and in its 2024 guidance only the High scenario reaches 10 feet before 2150. Under that scenario, the water at San Francisco stands 6.5 feet above its 2000 level in 2100, 9.9 feet in 2130, and 10.8 feet in 2140, with Los Angeles a few inches behind. The next scenario down leaves San Francisco at 8.1 feet as late as 2150.

The state's 2018 guidance put 10 feet much sooner. It carried an extreme scenario of 10.2 feet by 2100 for San Francisco, based on the possibility of rapid ice loss, and a Coastal Commission draft that year asked cities to evaluate a 10-foot rise for planning and permitting. The 2024 update dropped the extreme scenario as far higher than current science supports, while stating that rises beyond its 2150 range, which reaches 11.9 feet statewide, cannot be ruled out.

With about 6.6 feet of rise plus a 100-year storm, a USGS-led flood model put more than 600,000 Californians and $150 billion in property, equal to more than 6 percent of the state's GDP, inside the flood zone. That scenario stops 3.4 feet short of a 10-foot rise, before any storm arrives.

The Bay's Two Big Airports Sit on Fill at the Waterline

SFO stands largely on bay fill along the bay's western shore, and parts of its ground sit only 7 to 8 feet above the national survey baseline that engineers use for elevations. FEMA already maps the airport inside the 100-year floodplain. The top of the planned wall would stand 16 to 20.2 feet on that same baseline, which puts it roughly 8 to 13 feet above the lowest ground it protects.

Across the bay, Oakland International Airport depends on a dike about 4.5 miles long that runs along its shoreline between Alameda and San Leandro. A rebuild that cost about $30 million raised it 12 inches above the standard height to allow for sea-level rise, and the Port of Oakland expects that work to protect the airport for the next 30 to 35 years. The extra height covers one-tenth of a 10-foot rise.

Foster City Was Built Up While Alviso Sank

On the Peninsula south of SFO, Foster City's developers spread 18 million cubic yards of sand and fill over roughly 3,000 acres of diked marshland, and the city still sits close to sea level behind a levee. In 2018, voters approved $90 million in bonds to raise that levee. The Legislative Analyst's Office found that 2 feet of higher sea level, combined with a 10-year storm surge, the kind with a 10 percent chance of happening in any given year, would bring severe flooding to Foster City.

Alviso, a neighborhood of San Jose at the southern tip of the bay, went the other way. The land sank as groundwater was pumped out from under it, and Alviso now lies about 15 feet below sea level. In 1983, floodwater covered parts of town up to 10 feet deep for weeks. A 10-foot rise would put the bay roughly 25 feet above its streets. Between Alviso and Foster City, former Cargill salt ponds line the shore, and Meta's original 57-acre Menlo Park campus, valued at $2.5 billion in 2016 when it belonged to Facebook, sits beside them at the bay's edge.

Rising Seas Would Lift the Water Table From Below

Under SFO's runways, groundwater already sits about 1 to 4 feet above the survey baseline, only a few feet below the airport's lowest ground, and the regional review of the wall notes that rising seas could cause flooding by pushing that water up. Along the coast, the shallow water table rises with the sea. A study by university and USGS researchers found that with about 3.3 feet of rise, areas flooded from below would reach roughly 165 to 425 feet inland, with low-lying communities around San Francisco Bay most at risk, and Bay Area groundwater mapping found that levees and floodwalls may not protect against it. Sewer, water, and wastewater lines in the region run within six feet of the surface, and groundwater that reaches them can also carry contaminants out of soil that is dry today. Some of that soil sits under industry. A University of California study counted 440 hazardous facilities on the California coast that would face at least one flood a year by 2100 with more than 3 feet of rise, most of them in Alameda, Contra Costa, San Mateo, Orange, and Los Angeles counties.

Animals at the Bay Area's Edge Would Run Out of Room

San Francisco Bay has lost more than 90 percent of its historic wetlands, and the tidal marsh that remains is the only home of the salt marsh harvest mouse. The endangered California Ridgway's rail lives in the bay's marshes too. When the water rises faster than a marsh can build up, the marsh drowns and turns to mudflat. A USGS project found that 96 percent of the marsh area at 12 bay marshes would become mudflat with about 4 feet of rise by the end of the century, and a separate study projected the rail would lose 84 percent of its suitable habitat in the bay by 2100 under high sea-level rise.

On the Marin County coast, Point Reyes holds the beaches where elephant seals haul out. A study of those beaches projected that about two-thirds of the potential habitat for northern elephant seals would be underwater by 2099 with about 4.6 feet of rise. The National Park Service warns that rising seas could also take nesting habitat from western snowy plovers at Point Reyes, where the park counted 22 to 40 breeding birds a year between 2015 and 2021.

The Delta's Farm Islands Already Sit Below Sea Level

Inland from the bay, the Sacramento and San Joaquin rivers meet in a 738,000-acre maze of channels and islands spread across six counties. Many of the islands lie 10 to 25 feet below sea level, some deeper, and about 1,100 miles of levees keep the channels off the fields. Rising sea level widens that difference in height between the water and the island floors, according to the Delta Stewardship Council's plan.

The channels stay fresh because enough river water flows out through the Delta to hold the bay's salt back. As the sea rises, more fresh water is needed to keep that salt from moving upstream, and if levees on the western islands failed, salt water could be drawn deep into the Delta.

Salt in those channels would threaten water that much of California depends on. Two sets of large pumps draw from the Delta and move water south, some of it as far as San Diego, and the California Department of Water Resources says saltwater intrusion from sea-level rise puts the water used by 27 million Californians at risk. About 4.5 million acres of farmland receive at least part of their water from the Delta.

Long Beach's Older Piers Stand on Former Marsh

Down the coast in 2014, a storm surge tore three large holes in the breakwater at the Port of Long Beach and halted shipping at two of its terminals. Together with the neighboring Port of Los Angeles, the port handles more than 30 percent of U.S. container imports. Its older piers, S, D, and A, are lower-lying because they were built on former marshland. Protecting a single electrical substation on Pier S was estimated at about $65,000 with sandbags, or $10.4 million to relocate it.

The city's waterfront neighborhoods of Naples, Belmont Shore, and the Peninsula sit at an elevation near zero, and some 1,983 homes in Long Beach, worth $1.3 billion, are at risk from rising seas. Coastal geologist Gary Griggs of UC Santa Cruz has said that "you only need 2 to 5 feet for it to be a disaster." A 10-foot rise is twice the top of that range.

Beach Towns Would Flood From Their Harbors

Many of Southern California's lowest beach towns face water on their inland side as well, through harbors and back bays that reach behind the sand. In Los Angeles County, the area around Marina del Rey, including Venice, is among the places expected to be hit hardest. A 2019 assessment for a Venice project found that 6.6 feet of rise would flood a very large area of the neighborhood.

In Orange County, the Coastal Commission expects tides to begin flooding properties in Huntington Harbour by 2030, with widespread inundation there by 2050 and across northern Huntington Beach by 2100. Pacific Coast Highway could see tidal flooding as early as 2030. In Newport Beach, Balboa Island and the Balboa Peninsula already flood during the highest water levels. Southern California's salt marshes are also home to the Belding's savannah sparrow, and a USGS study found that under its highest scenario, no suitable breeding or foraging habitat would remain at any of six estuaries in the region by 2100.

Imperial Beach, at the Mexican border, has water on three sides and about 26,000 residents in 4.5 square miles. With 3.3 feet of rise, nearly 20 miles of its roads could be closed by flooding at times.

Beaches Would Erode Back to the Cliffs and Seawalls

At Del Mar in San Diego County, the dry sand above high tide is expected to disappear with just 1 to 2 feet of rise. Statewide, a 2023 USGS study modeled California's beaches under rises of about 10 inches up to roughly 9.8 feet and found that 24 to 75 percent of them could be completely eroded by 2100 with little human intervention, with the top of that range tied to the extreme, low-probability 9.8-foot case. A beach normally shifts inland as the water rises, but about 30 percent of Southern California's shoreline sits behind some form of seawall, so on those stretches the sand narrows in place until little is left above the tide.

Trains and Highway 1 Are Already Moving Inland

Above those beaches, rail service through San Clemente stopped for about six weeks in 2025 so crews could work on track at immediate risk from landslides and cliff erosion. The line is the LOSSAN corridor, the second-busiest intercity rail corridor in the country, which carried about 1.5 million Pacific Surfliner riders in 2023. In Del Mar, where the tracks run along eroding bluffs, emergency work to shore them up was expected to cost $10.5 million in 2021. The Coastal Commission has said the tracks need to move inland, and regional planners are studying a tunnel beneath the town.

Caltrans counts about 160 miles of state highway centerline exposed to 6 feet of sea-level rise, 94 of them in the Bay Area, and that total leaves out roads in the Delta. In San Luis Obispo County, the agency has already moved a section of Highway 1 up to 475 feet inland.

A 10-foot sea would reach California by a different path at almost every stop. Around the bay, it would press against a planned wall built for 3.5 feet of rise and a dike raised by one, and under SFO, Foster City's diked marsh, and sunken Alviso it would also come up through the ground. In Venice, Huntington Harbour, and Balboa Island, it would arrive through the harbors while the beaches in front of them narrowed against seawalls. The state's highest scenario puts that sea in the 2130s. For the rise the Bay Area plans around, its regional plan estimates that seawalls, levees, marshes, and other projects would cost $110 billion, against $230 billion in losses if the region did nothing. In the Delta, the measure of a 10-foot sea would be salt, pressing up the same channels that carry water to pumps serving farms and cities as far away as San Diego.