Climate
Geology

Glacier collapse recently buried a Swiss village – here’s what scientists say triggered the event

Posted on: 2025-06-12

On 28 May 2025, the quiet village of Blatten in the Swiss Alps was buried under millions of tons of rock debris and ice after a glacier collapsed from the mountains above. This devastating rock-ice avalanche stemming from the Birch Glacier (or Birchgletscher) is unprecedented for the Swiss Alps both in terms of the dynamics of collapse and its devastating impacts, according to experts.

Figure 1 shows the striking aftermath of this event. Though most of its 300 inhabitants were able to evacuate, almost every home was completely buried, and one person is still missing.

Figure 1 – Town of Blatten shown before and after (move slider from left to right) the rock-ice avalanche that buried the village. Source: Swiss Topo Rapid Mapping

As climate change is already altering our planet in many ways^[1], people online began asking, ‘did climate change play a role in this disaster?’

Scientific evidence clearly shows that global warming is leading to an overall loss of glaciers. The evidence for this is overwhelming, and the reason is clear. Put succinctly by scientists from the International Cryosphere Climate Initiative:

“We can’t negotiate with the melting point of ice.”

The planet is warming, accelerating the loss of glaciers globally. Though scientists have a solid understanding of this process, assessing how climate change influenced a single glacier collapse is more challenging, as we will later explain.

In light of this, Science Feedback contacted five glaciologists who conduct research in Switzerland to understand why and how this event unfolded, and what we can expect in the future under climate change.

Main Takeaways:

Blatten, a small town in the Swiss Alps, was buried under millions of tons of rock debris and ice after the Birch Glacier (Birchgletscher) collapsed from the mountains above.
The main trigger for the glacier’s collapse was an accumulation of rocks on the glacier’s surface following a partial collapse of the Kleines Nesthorn mountain peak above it. These rocks put excess weight on the glacier, which scientists believe triggered its collapse.
Scientists were already monitoring the glacier as a hazard for decades, but the instability of Kleines Nesthorn progressed quickly over two weeks, with residents evacuated a week prior to collapse.
Scientists think that climate change likely played a role in glacier collapse – mainly by helping to destabilize the adjacent mountain side – but further investigation is needed and underway.
No matter how much it contributed to this event, climate change is projected to continue melting glaciers in the coming century, leading to hazards in alpine regions.

A high mountain peak partially collapsed, triggering rock avalanches onto the Birch Glacier leading to its collapse – climate change effects not ruled out

As we said above, the big question on many people’s minds was ‘did climate change play a role in this disaster?’ For example, one post on X with over 100,000 views called the event a ‘climate catastrophe’, while one of the top comments on that post asked ‘how exactly is this related to climate?’.

Given that scientific evidence shows that climate change is accelerating glacier loss^[1,2], it’s no surprise many are wondering how such changes might give rise to disasters.

While this is a valid question – and one that scientists are already investigating – it’s important to note that climate change did not act in isolation. Ignoring that fact – either by saying climate change had no role at all or was the only cause – can fuel misinformation.

With that in mind, let’s instead look at three separate questions: 1. What changes normally happen in alpine environments, 2. What sequence of events happened prior to the glacier collapse, and 3. What role might climate change have played in the event? We will explore each question below.

What changes normally happen in alpine environments?

The peaks of the Swiss Alps have been tectonically uplifted to high elevations, some over 4,000 meters (~13,000 ft). Such high alpine environments undergo constant changes due to harsh weather conditions and steep terrain.

With steep terrain, harsh weather conditions, and the constant pull of gravity, the mountains are slowly but steadily being worn down and broken apart through weathering, and their materials transported away through erosion. As any alpine mountaineer will tell you, if you spend time in these environments, it’s only a matter of time until you hear or see rockfalls careening down the steep mountain sides.

From a basic physics standpoint, materials – including both rock and ice – move down a mountain when the driving forces which pull them down (primarily gravity) exceed the resisting forces holding them up (like friction). The same principle explains why a car will roll down a hill due to gravity (driving force) unless its brakes apply friction (resisting force).

So, at a fundamental level, to understand why Birch glacier collapsed, scientists are investigating why the balance between these forces changed: namely, why the resisting forces were no longer enough to keep the ice and debris from collapsing.

To do so, scientists must construct a solid understanding of how the state of the glacier and its surroundings changed prior to collapse. From there, they can look for potential links to climate change – a connection which they think is likely, as we will detail in a later section.

What events led to the glacier collapse?

Science Feedback contacted several Swiss glaciologists to learn more about what happened prior to the collapse of the Birch Glacier. In addition to sharing their own insights, they also recommended this fact sheet released by ETH Zürich for trustworthy information on the event.

One of the first key points is that the glacier did not go unnoticed as a hazard before collapse. In fact, it’s been observed over recent decades as a potential hazard given some unusual movements and changes it exhibited, like the glacier’s snout thickening while the upper reaches of the glacier thinned.

But even when a glacier is monitored, unforeseen events can occur. One such event – which triggered those that followed – was a partial collapse of the Kleines Nesthorn peak, which deposited rocks on the top of the glacier (Figure 2). The instability that led to the partial collapse of Kleines Nesthorn was observed over the preceding two weeks. These early observations allowed the majority of Blatten’s residents to evacuate on 19 May 2025 after receiving evacuation orders.

**Figure 2 –** Aftermath of the partial collapse of the Kleines Nesthorn peak (where the rock pile in the central part of the photo is descending from) at ~3,340 meters or ~10,960 feet in Valais, Switzerland. Rocks from the avalanche are shown piled on the Birch Glacier in the center of the photo (taken 23 May 2025, five days before the glacier collapsed). Adapted from source: KEYSTONE / Jean-Christophe Bott via ETH Zürich

The geology in this region played a critical role in this event. As explained by Dr. Martin Funk, Emeritus Professor at ETH Zürich, who studied glacier changes and associated hazards in Switzerland for over 30 years:

Martin Funk

Professor Emeritus, ETH Zürich

“To evaluate potential linkages between this event and anthropogenic climate change, it is necessary to first consider the regional geological context.”

Funk explains that the rock of Kleines Nesthorn is crosscut by a network of fractures – which act as zones of structural weakness – and has been deformed by tectonic forces which have both ‘squished’ and ‘cracked’ these rocks.

The presence of these features – particularly on a steep slope like that of Kleines Nesthorn – increases the likelihood of rock collapse. Thus, geology in the region of collapse should not be ignored in these discussions, as these events are more likely to occur in areas with certain rock types and characteristics.

But the rock avalanches are just the first part of the story; most rock avalanches in the Alps do not cause this type of disaster. In fact, the ETH Zürich fact sheet notes that the scale of this event is unprecedented in the Swiss Alps. Painting a more complete picture of the event, Funk explains:

Martin Funk

Professor Emeritus, ETH Zürich

“On 28 May 2025, the lower reach of the Birchgletscher underwent a sudden structural failure. This event was initiated by the rapid accumulation of supraglacial debris [debris on top of the glacier], originating from a sequence of rock avalanches descending from the Kleines Nesthorn—a subsidiary peak located immediately below the Bietschhorn. The cumulative load, estimated at approximately 12 million tonnes, exceeded the glacier’s structural threshold, resulting in the detachment and downslope mobilization of both ice and debris. The proximate cause of the collapse was therefore the excessive surface loading that developed over a short timescale.”

This main cause was also confirmed by Dr. Olivier Gagliardini, Professor at the University of Grenoble Alpes (UGA), who helped with the ETH Zürich fact sheet:

Olivier Gagliardini

Professor, University Grenoble Alpes

“the glacier collapse is mostly due to the overload from the rocks. The loading has been relatively progressive, until the last rock was too much. Then, the rocks and the glacier formed an avalanche that reached the village in a few seconds.”

Now that we better understand what happened, we come to the final question: did climate change influence these events? We posed this question to several glaciologists and paired their insights with details from the ETH Zürich fact sheet to summarize what is known thus far.

What role might climate change have played in the event?

Firstly, it is important to note that there is no way to evaluate the event without considering climate change. This is because the event occurred during a time where humans are unequivocally altering Earth’s climate^[1]. Because of this – and the fact that Earth’s climate system is interconnected – no region is exempt from the influence of climate change.

Funk explained to Science Feedback that although scientists have a strong understanding that climate change can decrease slope stability in alpine environments, attributing climate change to a single event remains challenging. Mainly because we can’t run a ‘control’ experiment to see what would happen without human-caused global warming.

In other words, Funk explains that if we had an ‘replica’ of Earth that had carried on without human influence, we could compare the two to see if the glacier collapse occurred or how it played out on each planet. But unfortunately, we don’t. However, scientists can still investigate this in other ways. As explained in the ETH Zürich fact sheet:

“While long-term geological preconditioning certainly played a role in the failure of Kleines Nesthorn, and while the event was the result of several compound elements that require further analysis, there are several indications suggesting that climate change played a role in the processes that led to the collapse and the subsequent cascading event.”

As explained to Science Feedback by Dr. Mylène Jacquemart, Professor of Glaciology at ETH Zürich, who helped write the ETH Zürich fact sheet:

Mylène Jacquemart

Professor of Glaciology, ETH Zürich

“In this type of hazard cascade, there are several different processes involved. First, the destabilization of the mountain slope next to the glacier, then the collapse of the glacier under the weight of the landslide debris, followed by the damming of the river and an increased sediment load downstream. Climate change could have contributed to the first part of this hazard chain – namely the destabilization of the mountainside. The slope that failed was identified as having warm permafrost, which may have affected its stability. However, additional studies will be necessary to determine what role permafrost and warming played in this case.”

Although glaciers look static, they actually slowly move down mountain valleys, carving and steepening the adjacent rock. As glaciers melt and shrink, they can pull away from the steepened mountain slopes they were in contact with, providing less support (i.e., ‘debuttressing’ the slopes). As explained to Science Feedback by Dr. Christian Huggel, Professor at University of Zürich – who studies climate change impacts in the alps:

Christian Huggel

Professor, University of Zürich

“Birch glacier lost a lot of mass since the 1980’s (due to anthropogenic warming) which resulted in stress-release (debuttressing) at the toe of the northern slopes of Kleines Nesthorn, a common source of rock slope failures in (para-)glacial environments.

Most of the rock mass of Kleines Nesthorn are in permafrost conditions, with temperatures a few degrees below 0°C, and into great depth (probably 100 m or more). Atmospheric warming of the past decades has significantly affected mountain permafrost, and in steep rock slopes this affects slope stability in various ways (e.g. more water percolating and refreezing in cleft systems, reducing shear strength).

Compared with conditions in the 1980’s the steep slopes of Kleines Nesthorn have virtually lost all seasonal and perennial snow and firn which leads to an enhanced penetration of atmospheric warming into the bedrock.

Considering all these processes it would be absurd, ignorant or dishonest to state that anthropogenic warming has not played any role in the ice-rock avalanche disaster in Blatten.”

Switzerland has already seen significant changes under global warming. As explained in the ETH Zürich fact sheet, “Regional warming in Switzerland has been about twice as large than the mean observed global warming, reaching a long-term warming of 2.9°C in 2024”. It also notes that recent warming has led to significant glacier melting and permafrost thawing across Switzerland. In fact, from the early 1980s to 2016, Switzerland lost roughly 43% of its glacier volume, and this loss has been accelerating^[3].

What future hazards can we expect from glaciers under climate change?

Regardless of what investigations tell us about this specific event, climate change is projected to continue melting glaciers, leading to hazards in alpine regions. The Intergovernmental Panel on Climate Change (IPCC) explains: “Glacier, snow and permafrost decline has altered the frequency, magnitude and location of most related natural hazards”^[4].

Specifically, glacier melt and permafrost thaw have helped destabilize mountain slopes. The IPCC projects that this will continue, and that “Resulting landslides and floods, and cascading events, will also emerge where there is no record of previous events”^[4].

To learn more, Science Feedback contacted Dr. Horst Macguth, Senior Researcher at the University of Fribourg, who explained:

Horst Machguth

Senior Researcher, University of Fribourg

“Climate change most and foremost leads to the disappearance of glaciers. The glaciers in the Alps are currently disappearing at a rapid pace, in some recent years up to 4 or 5 % of the Swiss glacier volume was lost, per year. Given the current rate of warming, it is likely that within 50 years most Alpine glaciers have disappeared. The remaining glaciers will be at the highest elevations, such as Monte Rosa or the Bernese Alps. Small glaciers such as the Birchgletscher will have mostly disappeared. […] disappearing glaciers lead to other types of hazards. For example, with glaciers melting away, there will be many new lakes forming in troughs of the terrain formerly covered in ice. These lakes can be a hazard as they might have weak dams. Furthermore, slopes can fail when a valley is not filled with ice any more […] If such rock falls hit a lake, devastating floods can result.”

Jacquemart similarly explained to Science Feedback:

Mylène Jacquemart

Professor of Glaciology, ETH Zürich

“There are a number of ways by which climate change poses risks for glaciers in the future. First and foremost, rising temperatures threaten glaciers fundamentally – if we continue on our current emissions pathway, there will be very little ice mass in the European Alps in 2100. This loss of ice and snow threatens our water supply, our ecology, and our tourism.

Many glaciers will disappear regardless of future greenhouse gas emissions due to what is called ‘committed loss’^[1]. We are ‘locked in’ to a certain amount of glacier loss, because even if we stopped emitting today, a significant portion of past emissions would remain in our atmosphere and continue raising global temperatures. However, projections show that limiting future warming – through reducing emissions – can significantly limit glacier loss^[1,5].

See the Scientists’ Feedback section below for a more in-depth review of climate change impacts on this event.

Scientists’ Feedback

Questions from Science Feedback:

Are you aware of any attribution studies completed or underway to assess how the Birch Glacier collapse in Switzerland could be tied to climate change? Is such attribution possible for an event at this scale (a single glacier collapse)?
What are the ways in which climate change could have contributed to this type of glacier collapse – or the events leading up to it – if any?
What types of problems does climate change pose for glaciers in the future – are these types of disasters or others expected?

Mylène Jacquemart

Professor of Glaciology, ETH Zürich

1. There are no concluded studies assessing how the Birch Glacier collapse could be tied to climate change, but this question will be intensely studied over the next weeks and months and we will likely have a lot more clarity then. In general, it is tricky to do such an attribution for any individual extreme event.

Early on, studies tried to understand the connection between climate change and slope instabilities, but did so largely with a few single events looking various triggering conditions, but this isn’t truly attribution (refer to e.g. these references, or contact the main author directly):

Huggel et al. (2012) Is climate change responsible for changing landslide activity in high mountains?. Earth Surface Processes and Landforms.^[6]
Huggel et al. (2010) Recent and future warm extreme events and high-mountain slope stability. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.^[7]

More comprehensive studies have recently been published, that take this a bit further (see below). The clearest signal currently emerges for rock fall events (so much magnitude than what we saw at Blatten. A study from the Glacier Bay NP looked more closely at large events, showing that there number is rising, but also that we have to look really carefully to understand how many events happened when satellite monitoring was not as good as it is today (observation bias).

2. In this type of hazard cascade, there are several different processes involved. First, the destabilization of the mountain slope next to the glacier, then the collapse of the glacier under the weight of the landslide debris, followed by the damming of the river and an increased sediment load downstream. Climate change could have contributed to the first part of this hazard chain – namely the destabilization of the mountainside. The slope that failed was identified as having warm permafrost, which may have affected its stability. However, additional studies will be necessary to determine what role permafrost and warming played in this case.

3. There are a number of ways by which climate change poses risks for glaciers in the future. First and foremost, rising temperatures threaten glaciers fundamentally – if we continue on our current emissions pathway, there will be very little ice mass in the European Alps in 2100. This loss of ice and snow threatens our water supply, our ecology, and our tourism.

Glaciers can also change internally, namely their ice temperature can change (glacier ice can be at the melting point or quite a few degrees below). If ice warms to the melting point, the glacier can slide over the rock more easily and create larger ice avalanches. Somewhat unintuitively, small glaciers can also cool as a consequence of climate change (because the loss of snow cover means that meltwater runs off the glacier quickly and doesn’t refreeze in the snow where it releases its latent heat (the energy that was required to produce the meltwater). In such “cold” ice (i.e., ice below the melting point) water can get trapped, possibly creating a risk of water pocket outburst floods. We don’t expect that either of these mechanisms played a role in the Blatten event, however.

Martin Funk

Professor Emeritus, ETH Zürich

“On the Interaction Between the Birchgletscher Collapse and Climate Change

On 28 May 2025, the lower reach of the Birchgletscher underwent a sudden structural failure. This event was initiated by the rapid accumulation of supraglacial debris [debris on top], originating from a sequence of rock avalanches descending from the Kleines Nesthorn—a subsidiary peak located immediately below the Bietschhorn. The cumulative load, estimated at approximately 12 million tonnes, exceeded the glacier’s structural threshold, resulting in the detachment and downslope mobilization of both ice and debris. The proximate cause of the collapse was therefore the excessive surface loading that developed over a short timescale.

Importantly, this discussion on the interaction between glacier collapse and climate change is limited to evaluating the destabilization of the Kleines Nesthorn rock slope. The glacier collapse would not have occurred in the absence of the Birchgletscher itself, which served as the transport medium for the mobilized debris.

To evaluate potential linkages between this event and anthropogenic climate change, it is necessary to first consider the regional geological context.

The Kleines Nesthorn is situated within the crystalline basement complex (Altkristallin) of the Aar Massif, one of the external crystalline massifs of the Central Alps. The lithology is characterized by numerous bands of amphibolite and schist that have experienced pronounced ductile and brittle deformation. As a result, the rock mass exhibits significant mechanical heterogeneity and is crosscut by a dense network of inherited fractures and discontinuities.

The long-term destabilization of steep alpine rock slopes (like Kleines Nesthorn) is governed by a suite of interacting processes, including mechanical and chemical weathering, permafrost degradation, ice segregation (formation of ice lenses in fractures), and gravitational loading. These processes act synergistically to reduce cohesion along pre-existing discontinuities and facilitate time-dependent (creep) deformation. The net effect is a progressive reduction in rock mass integrity and slope stability—an evolution increasingly observed across high-relief mountain environments globally.

Empirical observations at the Kleines Nesthorn are consistent with this interpretation. Recurrent small-scale rockfalls, documented over multiple decades, suggest that slope degradation has been ongoing and is not a recent development.

Climate change, particularly in the form of rising mean air temperatures and altered precipitation regimes, plays an important amplifying role. Elevated temperatures enhance chemical weathering rates and accelerate the degradation of permafrost. Permafrost serves as a stabilizing agent by occupying fractures with ice, thereby increasing the mechanical cohesion of the near surface rock mass. Thawing of this ice diminishes cohesion and promotes failure.

However, this stabilizing influence is confined to the shallow subsurface. At greater depths—as is the case at the Kleines Nesthorn—the mechanical contribution of ice is limited due to high lithostatic pressures. In such settings, elevated water pressures resulting from increased meltwater infiltration become the dominant destabilizing mechanism, promoting fracture propagation through hydraulic pressurization and hydrofracturing.

In conclusion, progressive atmospheric warming likely contributed to the long-term destabilization of the Kleines Nesthorn rock slope, ultimately culminating in the 2025 Birchgletscher collapse. However, the precise timing of the initial onset of instability remains uncertain, and the ability to predict future collapse events remains limited. Although a causal relationship between climate change and increased slope instability in alpine environments is strongly supported by both empirical observations and theoretical frameworks, definitive attribution of individual events remains challenging. The absence of a controlled counterfactual—i.e., an Earth system evolving without anthropogenic influence—precludes absolute causal inference for specific occurrences.”

Horst Machguth

Senior Researcher, University of Fribourg

“1. I am unaware of any study being underway about the potential link between the detachment of the Birchgletscher (together with the large mass of rock sitting on top of it) and climate change. I have seen that colleagues from ETH Zurich have published a statement where they discuss these potential links [see here]. Maybe they are planning a study, it could be worth asking them.

2. I find the discussion on potential impact of climate change in the above cited Factsheet very good. I might add three points:

(i) As is well known, global glacier retreat is caused by anthropocentric climate change. The melting back and thinning of the lower Birchgletscher (I refer to long-term thinning which is much larger than the recent slight thickening of the glacier mentioned in the factsheet) could also have led to the unstable slope losing its support by the glacier ice. This could have contributed to the eventual failure of the slope of the Klein Nesthorn. This loss in support when a glacier disappears can be responsible for otherwise weak valley slopes failing. For example, it has been observed two decades ago at the Unterer Grindelwaldgletscher [see here].

(ii) As you can see when you look at the following map [see here] the slope of the Klein Nesthorn was even partially glaciated in the 1960s (remove the tick at “journey through time – Maps” to see the difference between around 1960 and today). The removal of the glacier cover of the rock wall could have also contributed to the failure of the Klein Nesthorn slope. Once the ice cover of the rock face was removed, the underlying rock can warm to temperatures above the melting point and water can percolate into the rock. However, given that it was only partially glaciated, the subsequent loss might have had only a small influence (if any at all) on the recent slope failure”.

(ii) The slope which has collapsed is subject to permafrost. The following link shows you the simulated permafrost distribution and likelihood for the area of the Birchgletscher and Klein Nesthorn [see here]. Keep in mind this is a model result not measurements, but it indicates that the permafrost situation in the collapsed wall could be patchy or relatively warm, which means relatively close to 0 °C. The wall is likely not in an area where permafrost extends everywhere to great depth (so-called continuous permafrost). As the wall is probably located at the lower margin of continuous permafrost, this also means that its permafrost is vulnerable to the current rapid warming. And melting permafrost is known to destabilize rock faces. However, I do not know at what depth the rock failed. If this was at relatively large depth, then it is less likely that melting permafrost has had a major influence. As mentioned in the factsheet by the ETH, the stability of a rock face is also a question of its geological structure.

3. Climate change most and foremost leads to the disappearance of glaciers. The glaciers in the Alps are currently disappearing at a rapid pace, in some recent years up to 4 or 5 % of the Swiss glacier volume was lost, per year. Given the current rate of warming, it is likely that within 50 years most Alpine glaciers have disappeared. The remaining glaciers will be at the highest elevations, such as Monte Rosa or the Bernese Alps. Small glaciers such as the Birchgletscher will have mostly disappeared. If there are much fewer glaciers, the very combination of processes as we have now experienced, might become less likely. However, disappearing glaciers lead to other types of hazards. For example, with glaciers melting away, there will be many new lakes forming in troughs of the terrain formerly covered in ice. These lakes can be a hazard as they might have weak dams. Furthermore, slopes can fail when a valley is not filled with ice any more (I discussed this process above). If such rock falls hit a lake, devastating floods can result.”

Christian Huggel

Professor, University of Zürich

“Three days after the tragic and shocking ice-rock avalanche disaster in Blatten a few points on the causes and the role of climate change:

Significant amounts of rock debris (ca 3 mill. m3) were deposited on the glacier during the days prior to the avalanche. Somewhat simplified, the stability of a glacier (or rock slope) can be represented by a balance between shear stress and shear strength. Failure can occur when shear stress is increased or shear strength is reduced beyond a critical level. Loading by such large amounts of rock debris significantly increased normal and shear stresses. At the same time, loading increases pressure on the ice which likely resulted in melting of the ice, increase of basal water (pressure) and hence a reduction in shear strength.

The glacier tongue has advanced and shown accelerated flow speed over the past few years, probably as a response to earlier debris loading from Kleines Nesthorn (cf factsheet of our ETH colleagues, link below). However, when a large amount of debris falls onto the glacier in only a few days, the glacier cannot dynamically adjust its geometry, shear stress is abruptly increased and shear strength reduced, such that the downward forces become too high and failure occurs as with the avalanche from 28 May.

This cascade of processes has not been documented in the Alps to my knowledge but in several other mountain regions worldwide (cf paper link below).

With respect to the widely discussed role of climate change: This role is complex and we necessarily need to analyze it in specific detail. A short summary here which factors were likely involved:

Birch glacier lost a lot of mass since the 1980’s (due to anthropogenic warming) which resulted in stress-release (debuttressing) at the toe of the northern slopes of Kleines Nesthorn, a common source of rock slope failures in (para-)glacial environments.

Compared with conditions in the 1980’s the steep slopes of Kleines Nesthorn have virtually lost all seasonal and perennial snow and firn [Figure 3] which leads to an enhanced penetration of atmospheric warming into the bedrock.

Considering all these processes it would be absurd, ignorant or dishonest to state that anthropogenic warming has not played any role in the ice-rock avalanche disaster in Blatten. And whether we like it or not, the discussions about the role of climate change are inherently and inevitably political (and I recognize they are here inconvenient for some political forces). My recommendation is to use very precise wording to try to avoid from mis-understandings or mis-use.