CNN — The deep landslides beneath the multimillion-dollar homes in Rancho Palos Verdes moved at an almost glacial pace, until they didn’t.
This affluent coastal city in Southern California, around 30 miles south of Los Angeles, has long enticed people with its Pacific Ocean views and lush greenery. But it sits atop a complex of slow-moving landslides that have been active since the 1950s, causing the land to shift by roughly a few feet a year. Recently, after intense winter rain, the pace and scale of movement has increased.
Last weekend, a drastic acceleration brought devastating consequences.
Homes now lie sprawled unevenly across distorted ground, roads have buckled and power has been shut off to more than 200 households. On Tuesday, Gov. Gavin Newsom declared a state of emergency in the city.
The sight of luxurious, ocean-front houses teetering precariously over cliff tops or part-swallowed by the ground is not unfamiliar in this part of the US. Landslides destroy homes, take lives and leave communities fearing for their future.
But scientists warn they are set to become more frequent as the climate crisis fuels heavier rainfall and more powerful storms, reshaping landscapes.
Landslides depend on three factors: the slope, the rock type and the climate, said Alexander Handwerger, a landslide scientist at NASA’s Jet Propulsion Laboratory.
Rancho Palos Verdes sits on top of a volcanic ash bed, laid down about 10 to 15 million years ago, that slopes down to the Pacific shoreline.
“It has weathered to a type of clay mineral that can expand and get slippery when it gets wet,” said Gary Griggs, distinguished professor of earth and planetary sciences at the University of California Santa Cruz.
A variety of factors can trigger landslides, including earthquakes and human activities. But rainfall is one of the most common.
As it rains, water seeps into the ground, percolating into the layers below. There, it can reduce the suction and friction holding together grains of soil or rock, causing the ground to weaken and shift.
Slopes are always trying to reach a stable angle, which depends on what kind of climate they are in, said Dave Petley, an earth scientist at the University of Hull in England. If the climate changes, and rainfall becomes heavier, the slope “might now be too steep to be stable, so it will suffer a landslide or a series of landslides to find a new, stable angle,” he told CNN.
In California the changing climate is forcing the landscape to respond.
For the past two years, atmospheric rivers — long plumes of water in the sky that sweep in from the tropics — have lashed the state with rain.
This February, an atmospheric river dumped record amounts of rain across southern California, triggering hundreds of mudslides and leaving at least nine people dead.
Rain ate away at cliffs; one stark image shows a small cluster of mansions in Dana Point perilously close to tumbling onto the rubble-strewn beach below.
Scientists have found clear links between the climate crisis and heavier rain. A warmer atmosphere can hold more moisture, meaning more intense rain or snow when it falls, and hotter oceans fuel more powerful storms.
In California, climate projections suggest the state will experience less frequent but more intense rainfall in the future, especially from atmospheric rivers, which are expected to become more potent as the world warms.
The risk for landslides is clear, said Handwerger, who published a study on the topic in 2022. “We’ve looked all across the state, and we see that in years that are wetter than average, the landslides speed up.”
The climate crisis raises other landslide risks too. Sea level rise and storm surge are eating away at cliffs. Hotter, drier summers are increasing the frequency and severity of wildfires, leaving the landscape vulnerable to mudslides, Griggs said.
The 2018 mudflows that tore through Montecito, killing 23 people, followed the Thomas Fire, at the time the largest wildfire in California’s history, which incinerated trees and plants.
Landslides are of course a global phenomenon, and scientists are identifying climate change-fueled landslide risks across the world.
Cyclone Gabriel in New Zealand triggered more than 140,000 mapped landslides — and possibly more than 800,000 in total, researchers believe.
In July, a landslide triggered by heavy monsoon rains in India’s southern state of Kerala killed at least 150 people. The rainfall was made at least 10% heavier by the climate crisis, according to a scientific analysis.
Climate change is not the only factor increasing the likelihood of landslides; human behavior has an impact too.
Cutting into slopes to flatten areas for houses or roads can weaken them and mountain-sides, making both unstable, said Ugur Öztürk, a landslide scientist at the University of Potsdam and the GFZ German Research Centre for Geosciences.
As can adding too much water to the ground, Griggs said. In Southern California, “people wanted to pretend they lived in the tropics,” he said, “and planted a lot of landscaping that required lots of watering.”
Deforestation is another factor. Tree and plant roots hold the soil together and ripping them out can destabilize the ground, the University of Hull’s Petley said.
But, he added, “climate change is key.”
For those living in Rancho Palos Verdes, where the ground is now moving up to 12 inches a week, the future of their community hangs in the balance. It’s not clear when the movement will slow, or whether they can save their homes from being torn apart.
