How Climate Change Influenced the Blatten Landslide

A whole series of factors need to come together for a landslide to occur. This summer, in the canton of Valais, a large chunk of the peak of the Kleines Nesthorn mountain crumbled and crashed onto the Birch glacier below. The glacier was unable to carry the extra load and eventually collapsed. Masses of rock and ice thundered into the valley, forming a dam that turned the Lonza river into a small lake. Most of the village of Blatten was buried under the debris, while another part was flooded.

It is rare for an entire glacier to collapse, according to Christian Huggel. The UZH glaciologist specializes in climate change attribution, which links specific events to climate change factors. He is one of the main authors of the relevant chapters in the two latest reports of the Intergovernmental Panel on Climate Change (IPCC).

“Climate change played a key role at the Nesthorn,” says Huggel. “Of course, the geology, especially the layering and composition of the rock, is the key factor in such an event,” he explains. But without the warming brought about by climate change, Huggel believes the landslide in the Valais would either not have occurred at all, or only centuries later.

The UZH glaciologist lists three factors that are clearly linked to rising temperatures and were decisive in the Blatten event: the melting Birch glacier, the warming of permafrost in the rock, and the decrease in mountains’ snow, firn ice and ice cover during summer.

The heavy mass of glaciers pushes against the slopes of mountains, exerting a stabilizing force. When a glacier melts, this counterpressure disappears, which results in the slope becoming less stable and more likely to crumble or collapse.

Images of the Kleines Nesthorn and Birch Glacier in 2022 (left) and September 1980 (right). The decline in snow cover is clearly visible. (Images: Swisstopo)

In addition, the Kleines Nesthorn is in a permafrost zone. Modeling by UZH glaciologist Wilfried Häberli has shown that rising temperatures extend deep below the surface. This can, for example, cause the ice in rock crevices to melt or allow more meltwater to seep into the rock. If it then refreezes in winter or at night, the expanding ice can fracture and destabilize the rock.

“We can’t say precisely which processes are taking place inside the rock at the Kleines Nesthorn, as they’re highly complex,” says Huggel. What is clear, however, is that thawing permafrost is undermining the stability of the rock, adds the glaciologist.

In the past, the slopes of the Nesthorn mountain were covered with snow and firn far more heavily and for much longer than today. A comparison of aerial photos taken in the 1980s with images from today shows that there is hardly any snow or ice on the mountain in summer. “Without this cover, heat can penetrate even more deeply into the rock, and the surface begins to crack,” explains Huggel.

All three factors – the loss of stabilizing pressure from glacial retreat, the warming of the ground, and reduced snow and firn cover – came together at the Kleines Nesthorn and the Birch glacier. Combined, they contributed to a large amount of rock and debris falling onto the glacier in a very short time, which ultimately led to its complete collapse.

Back in 2002, the collapse of the Kolka glacier in the Caucasus mountain range drew international attention, when more than a hundred people lost their lives in an avalanche of ice and debris. At the time, it was considered a one-off event, says Huggel. But since then, several similar cases have been observed and analyzed scientifically around the world.

“This has given us a much better understanding of the processes that can lead to a collapse,” the researcher explains. A glacier collapse of this magnitude had never been observed in the Alps. Until it happened in Blatten, such an event was unthinkable for many people.

Even if events of this magnitude are unlikely to occur in the Alps more frequently over the coming years, Huggel believes it is important for people to take the links to climate change seriously. “Some voices tend to see the landslide as a rare event that is not directly linked to climate change,” he says.

And yet, Switzerland has seen a substantial rise in temperatures since the 1980s. “It is twice as high as the global average,” Huggel says. This means the country is far more heavily affected by climate change than many people assume. As a scientist, Huggel is able to point out the consequences of rising temperatures and support our society and policymakers adapt to climate change.