Outside/Inbox: Is climate change making landslides more common?
Editor's note: This episode was first published in February 2022.
Every other Friday on Morning Edition, the Outside/In team answers a question from a listener about the natural world.
This week, Phil from Maine submitted this question: “My wife and I were on a hike and noticed a landslide that had probably happened in recent years. How often do they occur, why do they happen, and any notable events? Will climate change make landslides happen more often?”
Whoa, that’s a lot of questions! To help organize this one, I’m going to break down what I’ve learned into three basic facts about landslides.
Fact No. 1: They’re more common than you think.
The term "landslide" is kind of a catchall, and there are lots of different types. For example, a “rockfall” is a type of landslide where rocks tumble down a slope or cliff. (Here is a terrifying example.) A “rockslide,” by contrast, is when a section of rock separates and slides down a slope, sort of like a layer of snow might give way during an avalanche.
One of the types you’re most used to seeing on the news is a mudslide (technically called a “debris flow”) where water-laden soil and debris suddenly liquefy and form rivers of mud.
But even the slow tumble of stone down an eroded trail can qualify as a landslide! In other words, landslides can move at a rate of inches per year, or as fast as a city bus — and they’re happening all the time.
“They happen often; they happen in all 50 states,” says Corina Cerovski-Darriau, a researcher with the U.S. Geological Survey’s Landslide Disaster Assistance Team. “Annually they cause billions of dollars of damage, and kill on order of 25 to 50 people, unfortunately, each year in the U.S. alone.”
Fact No. 2: Landslides are often “secondary disasters.”
So why do landslides happen? First of all, because of gravity. Take a slope between 25 and 40 degrees, put some vaguely round rocks on it and you’re liable to see some tumble down over time.
But many landslides are what you might call a “secondary disaster.”
“Landslides are typically triggered by water — be that rainfall, or changes in river or lake levels,” says Cerovski-Darriau. And there are other triggers as well: “earthquakes, volcanoes, and then humans.”
Humans can trigger landslides through road-building and other construction projects, or during mining operations. But again, many of the most widespread and destructive landslides in recent history are tied to precipitation.
In 2014, a month and a half of abnormally heavy rains saturated an already landslide prone hillside in Oso, Washington, and triggered the deadliest debris flow in recent American history. The mudslide traveled about half a mile in under a minute, killing 43 people. According to Cerovski-Darriau, there was so much debris it would have covered “700 football fields 10 feet deep.”
Another example of the secondary disaster concept came in 2017, when Hurricane Maria triggered over 70,000 landslides in Puerto Rico. The torrential rains, the mountainous topography of the island, and the road infrastructure (Puerto Rico has some of the highest “road density” in the world) were all contributing factors.
Rainfall is such a big factor, you can see it in the data — a global database of deadly landslide events from 2004 to 2016 shows a very predictable and steady spike of landslides during annual cycles associated with rainfall, especially in East and South Asia.
But fire is a part of the equation, too. Out-of-control wildfires can strip land of the vegetation that roots soil into place, paving the way for more mudslides during the rainy season.
Fact No. 3: Landslides and climate are linked… but it’s complicated.
Extreme weather events and global precipitation are on the rise. So it goes without saying that our warming climate is changing the nature of global landslide risk, too.
This winter, California has seen hundreds of landslides caused by heavy rains from the “atmospheric river” (and exacerbated by wildfire damage). Given the current trends, it would be fair to predict an increase in landslides there in the coming years.
But there are other areas that may see decreased risk from landslides because of climate change too.
“On the flip side,” Cerovski-Darriau says, “these prolonged droughts might decrease the risk from deep-seated landslides.”
She says some of the biggest landslides, caused by rising lake or groundwater levels, may take place less frequently.
Because of these regional variations, it’s hard to say if climate change is leading to an increase in overall global landslide frequency; at least according to the global landslide database I mentioned earlier.
But one thing it does show is a clear increase in deadly human-triggered landslides from mining and construction.
One way or another, the ground is literally shifting beneath our feet.
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