Many of Southern California’s picturesque beaches are in danger of disappearing completely as sea levels rise, according to a new study from the U.S. Geological Survey.

The study predicts that, “with limited human intervention,” 31 to 67 percent of Southern California beaches could become completely eroded by 2100 in the event the ocean rises about 3 to 6.5 feet. (Complete erosion means that the beach would recede up to either cliffs or existing structures along the shore, the study said.)

“Beaches are perhaps the most iconic feature of California, and the potential for losing this identity is real,” said Sean Vitousek, lead author of the study, in a statement. Vitousek was a post-doctoral fellow at the U.S. Geological Survey at the time he conducted the study.

Beaches aren’t just a summertime destination or a powerful tourism driver. The sand between the ocean and the land serves as a protective barrier for the man-made structures onshore, and the loss of beaches “exposes critical infrastructure, businesses, and homes to damage,” Vitousek said.

The good news is that there are steps that can be taken to protect beaches from the erosion that’s on the way. The bad news is that the necessary steps are “massive and costly,” USGS geologist and study coauthor Patrick Barnard said in a statement.

Such steps include increasing the rate at which sand from elsewhere is added to replenish existing beaches. Residents of Malibu’s Broad Beach recently paid $31 million to add 2,000 truckloads of sand to their small section of coastline; the sand is expected to last about 10 years.

The study used a new computer model called CoSMoS (for Coastal Storm Modeling System) to predict the effects of rising sea levels and climate change on beaches. It’s hard to predict shoreline changes, but USGS scientists are confident about their findings because of how accurately the computer model was able to reproduce the historical changes to SoCal beaches that occurred between 1995 and 2010.

LA beaches are already battling erosion. After the El Niño storms of early last year, Cabrillo Beach lost a significant amount of its beach, and its shoreline became rocky.

Santa Monica is restoring a section of its beach with native plant landscaping designed to fight erosion and stave off the effects of rising sea levels.