California’s seaside cliffs — which make up roughly 70% of the state’s iconic coastline — are rapidly crumbling into the sea, according to a new study by University of California, San Diego.

Much of the coastal cliff erosion was concentrated in hotspots in Northern and Central California, specifically Humboldt, San Francisco, San Mateo and Monterey counties. They included areas retreating more than 30 feet or even 70 feet per year, in what one researcher called a “significant hazard to the coastal community.”

“The cliffs are not stable. They erode and collapse over time,” said Danghan Xie, lead author of the study, said in a university post. “We’ve seen many sad news stories about fatal cliff landslides.”

Researchers highlighted the example of the Lost Coast Headlands in Humboldt County, where the cliff top eroded by 22.3 meters, or about 73 feet, per year between 2016 and 2023. There was a landslide there of over 1.4 million cubic yards of rock and soil.

“Seventy feet per year is a lot, over the seven-year study period that is more than the length of a football field,” said Adam Young, a coastal geomorphologist at the university.

Cliff top retreat rates of 10 feet or more per year also ocurred on the coasts at the King Range, Redwood National Park, Point Reyes National Seashore, Big Sur, Vandenberg Space Force Base, Palos Verdes and more.

Statewide, 65% of cliffs showed a measurable change.

Researchers conducted their study using lidar imagery of the coast in 2016 and 2023 to compare and contrast. Using that method, researchers found that across the entire coast, cliff tops eroded approximately 1.6 inches per year in California.

The conclusions may have some timely necessity given a historically strong El Niño may churn out stronger-than-usual waves and tides that could cause more erosion.

Scientists at UC San Diego have created a California Coastal Cliff Erosion Viewer website for the public to see the erosion rate along any segment of the coast. The site is expected to update potentially twice a year with more lidar data.

Knowing where and the rate at which cliffs are crumbling can be crucial for public safety, experts said — particularly with landslides.

Repeated landslides in Northern California often block off portions of Highway 1 along the coast, the San Francisco Chronicle reported, causing repair costs and even deaths.

For example, a landslide in Humboldt County last year killed a contractor working for the state transportation department.