Landslides destroy millions of homes in California and it's getting worse

CNN
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The deep landslides beneath the multimillion-dollar homes in Rancho Palos Verdes moved at a snail's pace until they stopped.

This affluent Southern California coastal town, about 30 miles south of Los Angeles, has long lured 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 and shift the land by about a few feet each year. Recently, after intense winter rains, the pace and extent of movement have accelerated.

Last weekend, a drastic acceleration had devastating consequences.

Homes are now scattered unevenly on the uneven ground, roads are twisted, and more than 200 homes have had their power cut off. On Tuesday, Governor Gavin Newsom declared a state of emergency in the city.

The sight of luxurious oceanfront homes perched precariously over cliffs or partially submerged under the earth is not uncommon in this part of the United States. Landslides destroy homes, cost lives and leave communities fearful for their future.

But scientists warn that these events will become more frequent in the future as the climate crisis leads to heavier rainfall and more violent storms, changing landscapes.

Landslides depend on three factors: slope gradient, rock type and climate, said Alexander Handwerger, a landslide researcher at NASA's Jet Propulsion Laboratory.

Rancho Palos Verdes sits on a layer of volcanic ash deposited approximately 10 to 15 million years ago that slopes down toward the Pacific coast.

“It's weathered into a type of clay mineral that can expand and become slippery when it gets wet,” said Gary Griggs, distinguished professor of earth and planetary sciences at the University of California, Santa Cruz.

Landslides can be triggered by a variety of factors, including earthquakes and human activities, but the most common is rainfall.

When it rains, water seeps into the soil and penetrates into the layers below. There, it can reduce the suction and friction that holds grains of soil or rock together, causing soil weakening and shifting.

Slopes always try to reach a stable angle, which depends on the climatic situation at the time, said Dave Petley, a geoscientist 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 it will suffer a landslide or a series of landslides to find a new, stable angle,” he told CNN.

In California, climate change is forcing the landscape to respond.

Over the past two years, atmospheric rivers—long plumes of water in the sky that blow in from the tropics—have showered the state with rainfall.

In February, an atmospheric river brought record amounts of rain across Southern California, triggering hundreds of mudslides and killing at least nine people.

Rain has eroded the cliffs; in one haunting image, a small cluster of mansions in Dana Point comes perilously close to collapsing onto the debris-strewn beach below.

Scientists have found clear links between the climate crisis and heavier rainfall. A warmer atmosphere can hold more moisture, meaning it rains or snows more when it does fall, and warmer oceans fuel stronger storms.

Climate forecasts for California indicate that rain will become less frequent but more intense in the state in the future. The precipitation will be caused primarily by atmospheric rivers, the amount of which is expected to increase in intensity as a result of global warming.

The risk of landslides is clear, said Handwerger, who published a study on the subject in 2022. “We looked across the state and see that landslides accelerate in years that are wetter than average.”

The climate crisis is also increasing other landslide risks. Rising sea levels and storm surges are eroding cliffs. Hotter, drier summers are increasing the frequency and severity of wildfires and making the landscape vulnerable to mudslides, Griggs said.

The mudslides that swept through Montecito in 2018, killing 23 people, followed the Thomas Fire, which was then the largest wildfire in California history and destroyed trees and plants.

Landslides are, of course, a global phenomenon, and scientists are noting that climate change is increasing landslide risks worldwide.

Cyclone Gabriel in New Zealand triggered more than 140,000 mapped landslides – and possibly more than 800,000 in total, researchers believe.

In July, at least 150 people died in a landslide in the southern Indian state of Kerala caused by heavy monsoon rains. According to a scientific analysis, the rainfall has increased by at least 10 percent due to the climate crisis.

It is not only climate change that increases the likelihood of landslides; human behavior also has an influence.

Cutting into slopes to level areas for houses or roads can weaken these and the mountain slopes and make both unstable, says Ugur Öztürk, a landslide researcher at the University of Potsdam and the German Research Centre for Geosciences GFZ.

The same goes for putting too much water in the soil, Griggs says. In Southern California, “people wanted to pretend they lived in the tropics,” he says, “and planted a lot of landscape plants that required a lot of water.”

Another factor is deforestation. The roots of trees and plants hold the soil together, and pulling them out can destabilise the soil, says Petley of the University of Hull.

However, he added: “The key is climate change.”

For residents of Rancho Palos Verdes, where the ground is currently moving as much as a foot a week, the future of their community is at stake. It's not clear when the movement will slow or if they will be able to save their homes from destruction.