Eighteen thousand years ago, give or take, the continental shelf 30 miles off the coast of San Francisco was exposed as a wide, flat coastal plain with an extensive system of dunes and river valleys connecting the Farallon Islands to the continental United States, which didn’t go by that name. Farther north, salt waters from the Pacific Ocean stretched all the way inland to present-day Sacramento.
Rising sea levels have continued to erode the shoreline ever since, as human beings established a society they would eventually call California. Some of our biggest cities were built in this 10 miles of dynamic coastline.
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“We often think of the California coast as this high-relief terrain with active tectonics, which it is, but we’ve put millions of people in the lowest parts of the entire state, like the San Diego Bay, the San Francisco Bay, and the Los Angeles Basin,” says Patrick Barnard, the research director of the climate impacts and coastal processes team at USGS Pacific Coastal and Marine Science Center in Santa Cruz.
As global warming causes ocean waters to expand and rise, those low-lying regions across California’s coast are particularly vulnerable to flooding. According to Barnard, we can expect at least one foot of sea level rise by the end of the century on our current trajectory. By 2050, sea levels are expected to rise as much as nine and a half inches in California, according to a report released Tuesday by the National Oceanic and Atmospheric Administration. The report also found that regardless of trends in future global greenhouse gas emissions, sea level rise across the U.S. is expected to increase as much over the next 30 years as it has over the past century.
The study updates 2017 projections to include more-recent trends in global warming and the melting of polar ice sheets but does not account for El Niño–Southern Oscillation ocean dynamics, which also affect sea level rise.
More-extreme projections estimate three feet of rising waters as soon as 2060 and six feet by 2100. For comparison, sea levels have risen just eight inches in the past 100 years of relative stability.
These projections are dependent on global greenhouse gas emissions, the frequency of strong “100-year” storms, and the continued melting of the polar ice caps. Current best estimates suggest that the upper extremity of these projections would affect 600,000 Californians living along the coast and cause more than $200 billion in property damage, which would have an economic impact similar to that of Hurricane Katrina in 2005.
Engineered defenses like seawalls have historically been the go-to strategy for California to fight rising sea levels, but armored defense strategies can lead to passive erosion and push unwanted floodwaters into neighboring communities instead. Beach replenishment, in which tons of sand are pumped into eroded regions to restore beaches, has also been deployed along some parts of the coast, but this can be expensive, short-lasting, and at times harmful to nearby ecosystems.
In Southern California, 38 percent of San Diego, Orange, Los Angeles, and Ventura Counties are lined with seawalls, breakwaters, or riprap, says Gary Griggs, distinguished professor of earth sciences at UC Santa Cruz. Researchers estimate that by 2100 two-thirds of Southern California’s beaches could be lost to erosion as a consequence of sea level rise.
“In the absence of developments, the shoreline would keep moving inland, we’d still have a beach, and that’s what’s been happening since the last Ice Age ended 20,000 years ago,” Griggs says. “Once you put a wall in, or a railroad, or a building, the shoreline can’t move back and we basically drown or flood the beach.”
The Bay Area is expected to face two-thirds of the state’s coastal flooding exposure over the next century. East Palo Alto in San Mateo County, called “ground zero” for sea level rise, is already seeing the effects of rising waters.
East Palo Alto is a diverse community with many low-income residents. Some can’t afford flood insurance and fear that more-frequent floods will displace them, says Violet Wulf-Saena, founder and executive director of Climate Resilient Communities, a Palo Alto–based group that works closely with the county to build climate preparedness. In 1998, historic rains flooded 1,100 homes in Palo Alto, Menlo Park, and East Palo Alto, which held floodwaters like a bathtub. The flood displaced 500 residents and caused $40 million in property damage across the three cities.
Rising sea levels also drive higher coastal groundwater levels, which threaten to release pollutants buried in the soil and compromise septic systems and building foundations. This too disproportionately affects low-income communities, which are five times more likely than the general population to live within one kilometer of wastewater treatment plants or other toxic sites at risk of flooding by 2050.
Climate solutions like reducing emissions with electric cars or energy-superefficient homes have often “left a lot of our population out, especially low-income families and communities of color,” says Wulf-Saena. Because of this, Wulf-Saena’s work is focused on including San Mateo County residents in creating solutions. One of her recent pilot projects involves constructing 25 rain gardens in the community, built by the community.
“We need to look at this regionally because what is done in one community affects the others,” Wulf-Saena says. “It’s such a huge problem, it cannot be done alone.”
Managed retreat in regions threatened by sea level rise has been met with resistance in cities like Pacifica, and some residents along the coast may not have the financial means to relocate or to elevate their homes to protect against flooding.
The San Mateo County Flood and Sea Level Rise Resiliency District, also referred to as OneShoreline, was born out of 2019 state legislation to oversee flood-mitigation projects and build resilience among the region’s shores. The idea is to protect existing infrastructure for 10 feet of sea level rise through a mix of nature-based and engineered solutions, and then to retrofit structures down the line as needed for higher sea levels, says OneShoreline CEO Len Materman.
OneShoreline coordinates with organizations like the San Francisco Bay Conservation and Development Commission (BCDC) and the San Francisquito Creek Joint Powers Authority to reengineer parts of the region’s shoreline with horizontal levees and augment marshlands to protect the area from floods. Altogether, a series of projects has been undertaken to protect the region: Adapting to Rising Tides (ART Bay Area), the Strategy to Advance Flood protection, Ecosystems and Recreation along San Francisco Bay (SAFER Bay), and Bay Adapt, to name a few.
What California’s coasts will look like with rising sea levels depends on the success of regional programs like these, as well as how industry and each of us reduce emissions globally by midcentury.
“We’re kind of built on this knife’s edge to be able to withstand a little bit of sea level rise without having to respond drastically,” says USGS’s Barnard. “But then once you get past 25 to 50 centimeters [of sea level rise], that’s when things start to go south in a hurry without management action.”•