It's not just the atmosphere and the oceans that are heating up. An ever-denser blanket of greenhouse gases is also sending warmer air and water deeper into the planet's rocky bones.
In the mountains of Switzerland, scientists have measured startling temperature increases, with jumps of as much as half a degree Celsius in just a decade 20 feet deep into the rocks. On Svalbard, an Arctic island north of Norway, similar warming has been measured more than 100 feet deep in the permafrost.
Tracking these changes is critical to assessing growing threats to people, said Bjørn Samset, research director at the CICERO climate research center in Oslo.
The warming, combined with other climate effects like extreme rainfall, is speeding up some basic geological processes. Softer rocks and soils that used to stay frozen most of the year, like permafrost, are thawing and eroding faster. Seemingly monolithic slabs of solid granite are peeling off mountainsides like the layers of an onion. And powerful rock glaciers—wide swaths of slow-moving ice and rock rubble that can pulverize granite—are speeding up, in one case from 40 feet per year to 226 feet per year.
In Norway, which just recorded its hottest and driest summer on record, climate scientists are now warily watching the first big storm of the rainy season roll in from the North Atlantic, with torrential rains forecast.
After a summer of melting, some of the steep rock walls along the glacier-carved fjords could be primed for collapse as rainwater percolates downward, increasing the pressure in existing faults and cracks. As a result, towns, roads and power lines along the coast face a growing double threat from falling rocks and from tsunamis triggered by massive amounts of debris crashing into the sea.
"We have an overall increase in extreme rainfall, which is a trigger for rockslides," Samset said. "The attitude here is, we've seen everything, we can handle anything, but I think we are really vulnerable in some areas."
Thawing Permafrost Can Destabilize the Land
The effects of heat-trapping pollution are showing up in the high peaks of the Alps. In Austria, a historic mountain weather and climate observatory nearly tumbled from the craggy summit of the Sonnblick peak as global warming thawed permafrost and melted ice that had held the rocks together.
A team of scientists monitoring the Swiss peaks above Andermatt say they've detected some warming as deep as 300 feet into the rocks. In a few places, some giant rock slabs have moved more than 4 feet in a year, a possible warning sign of a large collapse.
Continued warming and the thawing of permafrost also increases the chances for big mudslides and debris flows. As more and more of the previously frozen soil thaws, it can easily be set into motion by rain, said climate scientist Ketil Isaksen of the Norwegian Meteorological Institute.
At one test site on the Arctic island of Svalbard, permafrost 130 feet below the surface has warmed by about half a degree Celsius since 1998, and Isaksen said the warm-up reached new extremes this summer.
"This will probably affect slope stability, especially on steep slopes," he said. "When the water comes, you see an increase in the movement of the whole slope, and at a certain point the whole slope will fail."
Glacier Loss and the Accelerating Effect of Water
Along with the melting permafrost, another force is at work. Steep rock walls that were once buttressed by glacial ice several hundred feet thick have been laid bare as the glaciers receded. With nothing to hold them back, gravity pulls the rock toward the valley floors.
The rocky peaks are reacting to long-term climate change, said Marcia Phillips, lead permafrost researcher at the Swiss Federal Institute for Snow and Avalanche Research (WSL).
"These hot summers have an effect right near the surface of the rock and can cause smaller rockfall events of several thousand cubic meters, which is not small if you're a mountaineer. Anywhere above 2,500 meters elevation we're seeing crumbling rock walls," she said.
"The very large events—over a million cubic meters—are the result of thousands of years of freezing and thawing. They are mainly a geological problem, and the permafrost (thaw) accelerates the whole thing," she said.
Large amounts of water could trigger an area to collapse, she said. "Water definitely has an accelerating effect. It rapidly conveys heat into the ground, and very high pressures on the rock joints."
Basically, global warming is dissolving the glue that holds mountains together, and that could result in unexpected hazards for mountain communities, said Perry Barthelt, an avalanche expert with the WSL.
One of the biggest concerns is large-scale slides made up of different materials—snow, rocks, mud and water—all mixed together, he said.
In one recent event, the approach to Gotthard Tunnel, a major European north-south transit route, was closed after a large slide that started as a wet snow avalanche and tuned into a debris flow as it picked up material from the ground.
"These are cascading processes—snow avalanches turning into debris flows, ice avalanches turning into a rock avalanches. We're getting flow types that you can't define to one particular category," Barthelt said.
Alaska and the Canadian Arctic are facing similar hazards, as documented in a recent study linking one of the largest tsunamis on record with global warming. A 633-foot-high wave that surged down a remote fjord in Alaska in 2015 was caused by 180 million tons of rock falling into the water. The massive quantities of unstable loose rock had been exposed as the Tyndall Glacier retreated over the past 50 years.
Understanding Climate Change Can Help Locate High-Risk Areas
Water is the key ingredient, said Austrian hydrologist Thomas Thaler, who studies threats to mountain communities.
"For us it's very important to know if climate change has an impact on rainfall, because that has the most influence on landslides and debris flows," he said. "Climate change will increase the magnitude, not only the frequency, of these events."
Long-term data from high mountain stations on rainfall is sparse, but the assumption is that the rainfall regime will change as the climate warms. And the bottom line is that development pressure in parts of the Alps means many houses have been built in vulnerable areas.
But assessing and communicating the risks is complicated, because even if they become more frequent with global warming, large and destructive mass movement events are still going to be pretty rare, he said.
On-site monitoring is expensive and difficult to maintain, so it's often focused on known hazard areas rather than trying to detect new threats. One of the most promising avenues for warning people could come from satellite-based sensors, which can detect slow movements that can be precursors to sudden catastrophic events.
"It's a creepy hazard," he said. "You never know when it starts and when it ends."