Southern California’s beaches are an essential part of the state’s identity. The sandy, blond shorelines are like Hollywood or the towering redwoods—iconic. They are also an important piece of California’s more than $40-billion annual coastal and ocean economy.

But scientists have bad news: Without human intervention, many of the region’s beautiful beaches may disappear by 2100 as sea levels rise. If the Golden State wants to save its golden shores, it will have to add sand to them—and lots of it.

This troubling conclusion comes from a project to understand how climate change might affect the SoCal coast. Researchers from the University of Illinois at Chicago and the U.S. Geological Survey built a forecasting model for the region’s shoreline and published their results in a recent paper in the Journal of Geophysical Research: Earth Surface. The team examined 500 kilometers of SoCal coast, extending from the Mexican border to Point Conception, just north of Santa Barbara—home to 18 million residents as well as extensive infrastructure. “It’s the most urbanized part of the west coast, so it was an optimal place to assess,” says study co-author Patrick Barnard, a coastal geologist with USGS. The region’s beaches differ significantly, ranging from the wide Baywatch-esque shores in the Los Angeles area to narrow strips of sand in places like Santa Barbara. And they’re backstopped by a wide variety of features—estuaries, cliffs, river mouths, public and private infrastructure and more.

Barnard and his team predicted how SoCal’s shores would evolve from 2010 through 2100 by modeling the factors that influence beaches—estimates for sea level rise as well as wave and storm behavior and predicted climate change patterns if the world eventually stabilizes its greenhouse gas emissions by mid-century, then starts reducing them. The researchers chose their range of sea level–rise projections based on what is most likely to happen to the west coast, according to dozens of regional and global studies. They also took 15 years of historic data on how southern California’s beaches had changed and used that information to tune the model for the individual transects of beach, each 100 meters long. “That gave us confidence to project how the beaches will behave in the future,” Barnard explains, because it allowed the model to account for variations in features like sand-grain size and beach slope among the different beaches, along with dynamics such as sediment supply from rivers, dredging and past human additions of sand.

The model revealed a dramatic picture: Without drastic intervention a huge portion of the sandy shores will likely vanish soon. “Roughly a third to two thirds of the beaches will effectively disappear by the end of the century,” with 0.93 to 2.0 meters of sea level–rise, Barnard says. Although wave conditions influence beach erosion in the short term, sea level rise becomes the dominant eroding force in the long term. This is a huge problem not only because beaches support shoreline life and attract tourists but also because they protect coastal communities from flooding and storms. “Beaches are the first line of defense because they absorb the energy from storms,” he explains.

Climate change is not the only human impact here. If people had not built heavily along the shoreline, the beaches would just naturally migrate inland as the ocean rises. Bernard notes people have put the beaches under serious pressure because “we’re probably not going to let the beach move past a certain point.” In those many cases, he says, “we’ll have to add sand.”

California has added sand to its beaches for decades—for instance, about 1.3 million cubic yards of sand is placed every five to seven years at Surfside–Sunset in Orange County. Since 2000 San Diego has twice pumped about 1.5 million to two million cubic yards of sand from offshore onto beaches throughout the county, and it has performed a number of smaller replenishments during that time as well. These “nourishment” projects, as they are called, usually average out to about $8 to $10 per cubic yard of sand, says Lesley Ewing, a senior coastal engineer with the California Coastal Commission. The problem is, Barnard and his team had already assumed that recent rates of sand addition would continue. Far more beyond that amount will be needed to keep SoCal’s beaches from disappearing.

The researchers do not know exactly how much sand will be required, but they are working with the commission to determine the amount. “My sense is that it’s an order of magnitude larger—you might need 10 times the amount of sand than what’s been placed before to maintain beaches,” Barnard says. “It’s going to take a much larger effort.” He estimates billions more dollars will be necessary. The state will have to pump the sand from offshore or truck it from inland sources like riverbeds and quarries. Both options are expensive and, ironically, can harm the surrounding ecosystem.

Even then some beaches will succumb to sea level rise. “It might not be reasonable to try to keep every beach that exists now, because we don’t have enough sand to do that into perpetuity,” Ewing says. “We’re still going to have some of those nice California beaches but some of the smaller ones will be lost.” According to the study, many popular beaches are at risk in places such as San Diego, Malibu and much of Santa Barbara. Communities may decide to surrender some of their coastline development in favor of saving the beaches and letting them migrate inland. Ewing thinks this type of managed retreat will become more common as people start to understand the onerous cost of relentless nourishment.

Either way, Barnard says, “we’re going to have to do massive interventions if we want to maintain the safety and vitality of these coastal communities.”