The deep landslides beneath multimillion-dollar homes in Rancho Palos Verdes moved at an almost glacial pace, until they stopped.
This affluent Southern California coastal city about 30 miles south of Los Angeles has long drawn 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 about a few feet per year. Recently, after intense winter rains, the pace and magnitude of the movement has increased.
This past weekend saw a drastic acceleration, with devastating consequences.
Homes now lie unevenly spread across the deformed ground, roads are buckled and power has been cut off to more than 200 homes. On Tuesday, Gov. Gavin Newsom declared a state of emergency in the city.
The sight of luxury oceanfront homes teetering over cliffs or partially swallowed by the ground is not unfamiliar in this part of the US. Landslides destroy homes, claim lives and leave communities fearing for their future.
However, scientists warn they will become more common as the climate crisis brings heavier rainfall and more powerful storms, which will change the landscape.
According to Alexander Handwerger, a landslide scientist at NASA's Jet Propulsion Laboratory, landslides depend on three factors: the slope, the rock type and the climate.
Rancho Palos Verdes sits on a volcanic ash layer, deposited approximately 10 to 15 million years ago, that slopes down to the Pacific Ocean coastline.
"It's weathered into a kind of clay mineral that can expand and become slippery when it gets wet," said Gary Griggs, a professor of earth and planetary sciences at the University of California, Santa Cruz.
Several factors can trigger landslides, including earthquakes and human activities. But rainfall is one of the most common.
When it rains, water seeps into the ground and seeps into the layers below. There it can reduce the suction and friction that hold the grains of soil or rock together, causing the soil to weaken and shift.
Slopes are always trying to reach a stable angle, which depends on the climate they're in, said Dave Petley, an earth scientist at the University of Hull in England. If the climate changes and rainfall becomes heavier, the slope "may now be too steep to be stable, so there will be a landslide or series of landslides to find a new, stable angle," he told CNN.
In California, the changing climate is forcing the landscape to respond.
Over the past two years, atmospheric rivers - long plumes of water in the air flowing into the country from the tropics - have lashed the state with rain.
In February of this year, an atmospheric river dumped record amounts of rain in Southern California, triggering hundreds of mudslides and leaving at least nine people dead.
Rain has eroded the cliffs; a stark photo shows a cluster of Dana Point mansions nearly collapsing onto the debris-strewn beach.
Scientists have found clear links between the climate crisis and heavier rainfall. A warmer atmosphere can hold more moisture, meaning more intense rain or snow, and warmer oceans mean more powerful storms.
Climate projections indicate that California will experience less frequent but more intense rainfall in the future, mainly from atmospheric rivers, which are expected to become more powerful as the Earth warms.
The risk of landslides is clear, said Handwerger, who published a study on the topic in 2022. "We looked at the entire state and we see that in years that are wetter than average, landslides accelerate."
The climate crisis also brings other landslide risks. Sea level rise and storm surges are eroding cliffs. Hotter, drier summers are increasing the frequency and severity of wildfires, making the landscape vulnerable to mudslides, Griggs said.
The mudslides that tore through Montecito in 2018, killing 23 people, followed the Thomas Fire, the largest wildfire in California history at the time, which destroyed trees and vegetation.
Landslides are a global phenomenon and scientists are identifying landslide risks caused by climate change around 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 the southern Indian state of Kerala killed at least 150 people, with the rains made at least 10% heavier by the climate crisis, according to a scientific analysis.
Climate change is not the only factor increasing the risk of landslides; human behavior also has an impact.
Cutting slopes to level land for building houses or roads can weaken both the slopes and the mountainsides, making both unstable, said Ugur Öztürk, a landslide scientist at the University of Potsdam and the German Research Center for Earth Sciences (GFZ).
So does adding too much water to the soil, Griggs said. In Southern California, "people wanted to pretend they were in the tropics," he said, "and planted a lot of landscaping that needed a lot of water."
Deforestation is another factor. Tree and plant roots hold the soil together, and ripping them out can destabilize the ground, says Petley of the University of Hull.
But, he added, "climate change is the key."
For the people 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 subside, or if they can save their homes from destruction.
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