Like an ax slowly chopping at the trunk of a massive tree, waves driven by sea-level rise will hack away the base of cliffs on the Southern California coast at an accelerated pace, a recent study says, increasing land erosion that could topple some bluffs and thousands of homes sitting atop them.
California officials from Santa Barbara to San Diego will face an awful choice as the sea rises, the U.S. Geological Survey study says: save public beaches enjoyed by millions, or close them off with boulders and concrete walls to armor the shore and stop the waves in a bid to save homes.
The study predicts coastal land loss on an unimaginable scale over the remaining century, up to 135 feet beyond the existing shoreline. “For the highest sea-level rise scenario, taking an average cliff height of more than 25 meters, the total cliff volume loss would be more than 300 million meters by 2100,” it says.
One of the study’s authors, Patrick Barnard, a USGS research geologist, tried to explain the issue in a way that laypeople can understand. “It’s a huge volume of material,” he said. “We place this in a context of dump truck loads. It would be 30 million dump trucks full of material that will be eroded from the cliffs.” The trucks would stretch around the globe multiple times, he said.
The USGS undertook the study to inform the state’s public planners and policymakers of possible effects of climate change, which is causing the seas to rise. The analysis focuses on Southern California, but future studies will examine possible effects on the state's central and northern coasts as well.
In the San Francisco area, officials have already retreated from some parts of the coast, removing homes from cliffs that have eroded and areas that have flooded. San Francisco is taking steps to move the Great Highway away from Ocean Beach because erosion is eating away the earth beneath it. Houses and apartments in Pacifica, south of the city, were declared uninhabitable as cliffs that supported them gave way to erosion.
The study was published last month in the Journal of Geophysical Research. It predicts that by the end of the century, erosion in Southern California will double from the rates observed between 1930 and 2010, depending on how high the seas rise, as waves pound cliffs more frequently.
Barnard, who co-authored the study with fellow USGS researchers Patrick W. Limber and Sean Vitousek, along with Li Erikson of the University of Illinois at Chicago, acknowledged that the research was limited in the way it made predictions.
For the study, they combined five computer models into one, the Coastal Storm Modeling System. CosMoS, as it is called, “simulates changes in local coastal topography through the 21st century,” predicting shoreline change, ocean energy and flooding scenarios, according to a statement announcing the study.
But it does not account for the different textures along the nearly 300-mile coast between Santa Barbara County and San Diego County, how some of the coast has been modified, or how humans will alter the coast during the century.
According to the statement's synopsis of the study, “Without the supply of sand from eroding cliffs, beaches in southern California may not survive rising sea levels — and bluff-top development may not withstand the forecast 62 to 135 feet cliff recession.” As a result, the authors wrote, “managers could be faced with the difficult decision between prioritizing private cliff-top property or public beaches” when they allow or ban hard shore protections.
“Beaches are perhaps the most iconic feature of California, and the potential for losing this identity is real,” Vitousek, the study’s lead author, said in the statement. He was a postdoctoral fellow at the USGS when this study was conducted and is now a professor in the department of civil and materials engineering at the University of Illinois at Chicago. “The effect of California losing its beaches is not just a matter of affecting the tourism economy. Losing the protecting swath of beach sand between us and the pounding surf exposes critical infrastructure, businesses and homes to damage.”