Unstable slopes and warming peaks: scientists race to understand Blatten collapse

Professor Johan Gaume from ETH Zurich demonstrates his 3D simulation tool that accurately modelled the ten-million-cubic-metre rock-ice release in Blatten earlier this year. Michael Buholzer / Keystone

The catastrophic collapse of the Birch Glacier that wiped out the Swiss mountain village of Blatten in May has triggered an unprecedented scientific effort to understand how a mountainside could break up so violently – and what might come next.

Driving up the Lötschental valley, the pastoral calm of lime-green pastures and copper-yellow forests gives way to a raw, brown wound: the vast debris field where Blatten once stood.

On May 28, the Birch Glacier above Blatten collapsed under the huge weight of rocks from the crumbling Kleines Nesthorn mountain. In just 40 seconds, over nine million cubic metres of rock, mud, ice and debris thundered down the slope at 200 km/h, engulfing the village. Remarkably, all 300 residents had been evacuated in time. One man, 64, remains missing.

Today, Blatten municipality is mostly off limitsExternal link due to ongoing dangers. The Kleines Nesthorn continues to move — up to 10cm a day in summer — though winter has slowed the creep. A second collapse of the same magnitude is no longer possible, as the Birch Glacier has mostly vanished. But the danger level remains high.

“The breaking-up of the [remaining] hanging glacier resulting in an ice avalanche, debris flows from the couloir or a new landslide from an unstable part of the Kleines Nesthorn: these are all still possible and could reach the valley floor,” Guillaume Bulle-Favre, head of the Valais natural disasters serviceExternal link, told Swissinfo.

The biggest risks are linked to the huge debris cone, which is over 100 metres high in places, and the possible formation of a new lake if the Lonza River is blocked again.

View of the rock debris that accumulated on the Birch Glacier on May 23, 2025, ahead of its collapse five days later. Keystone / Jean-Christophe Bott

Blatten dominated discussions at an international landslide conferenceExternal link held last month in Lausanne, where over 60 experts gathered to understand how such an event unfolded — and how to anticipate the next one.

Numerous researchers, especially in Europe, are studying the case of Blatten and are collaborating in a non-competitive manner, explained Christophe Lambiel, a professor at the Faculty of Geosciences and Environment at the University of Lausanne:External link “Everyone wants to understand what happened, but not necessarily be the first.” Using simulationsExternal link and seismic analysisExternal link, for example, groups have been studying the unprecedented event to better comprehend the dynamics and the growing threat of multi-hazard cascades External linklinked to permafrost thaw and glacier destabilisation.

Six months on, certain patterns have emerged, say scientists. The three largest Alpine landslides of the past 20 years — Piz Cengalo (2017), Piz Scerscen (2024) and now Blatten (2025) — all involved rockfalls onto glaciers that transformed into massive rock-ice avalanches and debris flows.

“This is a major concern for the very densely populated regions like the European Alps. There is a huge potential for damage because they can transport the sediment and ice very far down the valley. In the context of climate change, with permafrost degradation and glacier retreat in steep faces, such cases may increase in the future,” Lambiel told Swissinfo.

Whether Blatten can be attributed to climate change remains a central, unresolved question. Some scientists argue the link is evident. University of Zurich scientist Christian Huggel believes climate change played a key role in Blatten. “Of course, the geology, especially the layering and composition of the rock, is the key factor in such an event,” he told a conference in Innsbruck, Austria, in SeptemberExternal link. But Huggel believes that without climate warming, the Blatten landslide would have happened centuries later, if at all.

Others are more cautious. A July factsheetExternal link from the federal technology institute ETH Zurich concluded it was “quite likely” that warming was a relevant factor, noting that the unstable rock zone lies within permafrost, which is sensitive to rising temperatures.

Switzerland has warmed 2.9° degrees Celsius since pre-industrial times — about twice the global average — leading to widespread glacier loss, altered snowfall patterns and thawing permafrost.

Rockfalls are increasing as snowmelt and permafrost thaw worsen, but scientists say it’s still unclear whether larger rockslides are becoming more frequent, and it’s hard to make reliable statements about very big, rare events as data is uneven.

In terms of the volume and the extent of the damage caused, the collapse of the Birch Glacier that wiped out the village of Blatten, canton Valais, is unprecedented for the Swiss Alps. Garrett Fisher / Keystone

“Processes are interlinked and difficult to disentangle,” says ETH Zurich glaciologist Daniel Farinotti, who hopes to present firmer conclusions about the Blatten disaster next year.

“What can be said is that the local geology, climate, glacier and permafrost all played a role [in the Blatten disaster].”

Permafrost’s role and fragile rock

For Lambiel, however, the role of permafrost degradation on the Kleines Nesthorn remains “an open question”.

Since 2019, the Birch Glacier had advanced about 50 metres — probably driven by repeated rockfalls from the Nesthorn dumping debris onto its surface and pushing it downslope. Meanwhile, parts of the glacier clinging to the mountain’s north face had dwindled.

“The retreat of this glacier combined with permafrost degradation, provoked many rock falls that covered the glacier, pushing it down the valley and destabilising it,” said Lambiel.

>>Watch the collapse of the Birch Glacier above Blatten in southern Switzerland on May 28, 2025.

Did rapid recent permafrost thaw drive the instability?

“Probably yes,” he says, “but we cannot yet say definitively. We need additional data. We especially need modelling of the mechanics of the instability at depth.”

New sensors should provide additional data on the thermal state of the permafrost and its evolution over time, as well as the mechanical behaviour of the rock. The mountain’s geology offers little reassurance: fractured layers of gneiss and amphibolite rock sit beside granite, forming an unstable structure.

ETH Zurich researchers describe it as “prone to fail”, with the slope steepened over millennia by glacier erosion and increasingly exposed due to retreating snow and firn cover.

They say permafrost has also warmed on the Kleines Nesthorn in recent decades. It is therefore possible that ice loss and increased water infiltration could have led to higher pressures and additional stresses in the slope. This in turn could have accelerated the failure of the slope.

Modelling for hazard planning

Amid the flurry of research activity, the Blatten disaster has helped push scientific modelling forward. A 3D simulation toolExternal link developed at ETH Zurich and the WSL Institute for Snow and Avalanche Research can now accurately predict the flow, height, and runout of snow, ice, and rock avalanches. It successfully predicted the runout of a major landslide in Brienz, canton Graubünden, in 2023.

An aerial view on October 28, 2025, shows the partly flooded village of Blatten after recent snowfalls, five months after a landslide destroyed the village. Michael Probst / Keystone / AP

And days before the Blatten collapse, researchers modelled a 10-million-cubic-metre rock-ice release; their projections — 1.2km runout to the southwest side of the valley, 700m to the northeast — closely matched the real disaster.

The team is now working with cantons and engineering firms to deploy the tool for hazard planning across the Alps.

“We are discussing actively with canton Valais to use this new technology for their 80 most risky and hazardous cases. We are also actively working in the Kandersteg area on the Spitzer Stein mountain doing simulations to evaluate the potential runout of a catastrophic rockslide and how it could impact Lake Oeschinen,” Johan Gaume, professor of Alpine Mass Movements at ETH Zurich and the WSL, told Swissinfo.

Improved monitoring

Switzerland monitors about 1,400 glaciers under the national GLAMOS project; a quarter of their volume has vanished since 2015External link. Sixty glaciers, most in Valais, are listed as dangerous. The Birch Glacier has been monitored since 1993.

Thanks to a well-coordinated hazard-management system, Blatten residents were evacuated before the mountain collapsed. The canton’s network includes geologists, 90 local observers, numerous monitoring devices, early-warning tools and a robust evacuation plan.

Since the collapse, new monitoring instruments have been installed locally. University of Zurich scientists have scannedExternal link the debris cone with LiDAR, hyperspectral imaging, and photogrammetry to assess remaining ice and its potential melt risk to downstream residents.

The organisers of the Lausanne conference stressed the “urgent need” to improve slope monitoring and modelling. Even mountainous Switzerland, however, cannot monitor every peak, and all systems have limits.

Lambiel admits that large disasters are hard to predict: “We knew the Kleines Nesthorn was unstable. It was moving for the last ten years, but it can also occur in unknown places like Piz Scerscen last year. Nobody knew that it was unstable.”

Some risky mountains have been identified, and are monitored regularly, he says, but others are unknown. 

“We have to expect surprises in the future… where and when, it’s very difficult to know.” 

Edited by Veronica De Vore/ts