That Swiss glacier collapse? It wasn’t a one-off » Yale Climate Connections

Never in the history of the Swiss Alps has a glacier caused such destruction as the one that collapsed in stunning fashion on May 28. After a few days of ominous advances, coupled with rockslides piling on top, the front section of the Birch Glacier, or Birchgletscher, hurtled into the Lötschental Valley at speeds of up to 200 kilometers per hour (120 mph). Some 20 million metric tons of rock, ice, and water reached the valley in just 40 seconds.

Most of the evacuated village of Blatten (pop. 300) was buried in a debris cone up to 100 meters (328 feet) deep. One person was missing and presumed dead.

“In terms of the volume and the extent of the damage caused, the collapse of the Birch Glacier is unprecedented for the Swiss Alps,” reported the Swiss university ETH Zurich in a news release updated on June 4.

Although the glacier’s full-on collapse wasn’t a certainty until it happened, there were plenty of warning signs that helped prevent large-scale loss of life. A major landslide from Nesthorn Mountain (Kleines Nesthorn) on May 17 rolled to within 500 meters (1,640 feet) of the Lonza River, prompting the evacuation of parts of Blatten. Within days, hundreds of cows, sheep, and rabbits were being ferried out of the danger zone.

The disaster in Blatten was no typical landslide or avalanche. Lately, there’s been growing interest among glaciologists in a number of small but high-impact cases in which large rockslides falling atop glaciers have led to a complex chain of events, culminating in what’s been dubbed “sudden large-volume detachments of low-angle mountain glaciers.”

These dramatic breakoffs vary enormously in their locations, evolution, and impact, but scientists have found some key themes. And as a human-altered climate transforms alpine landscapes — including the permafrost within rocky slopes as well as glaciers themselves — it appears to be laying the groundwork for more such events.

Anatomy of a glacier’s collapse

Most glaciers around the world are in retreat, their forward edges disintegrating and their depths thinning out as the climate warms. Yet unlike any other glacier in Switzerland, the Birch Glacier had been advancing and thickening toward its front edge. It’s a particular paradox because alpine regions are warming more quickly than the worldwide average. Switzerland has warmed about 2.9°C since the late 1800s, roughly twice the global pace, and glaciers across the Alps lost an estimated 10% of their mass in 2022 and 2023 alone.

What seems to have helped engorge the front section of the Birch Glacier was, oddly, a series of rockfalls over the past few years from Nesthorn Mountain. The resulting layers of rock and sediment appear to have distorted the glacier’s front section and insulated the snow underneath. Normally, a thin layer of dust or pollution will help enhance glacial melting by absorbing more heat than the brighter snow and ice it obscures. But once a layer of rock or sediment is more than a few centimeters thick, as with Birch Glacier, an insulating effect outweighs the absorption and helps reduce net melting. Thus, the doomed frontmost part of the glacier was gaining mass for years even as the upper sections continued to thin out. 

From 2017 to 2023, the glacier’s front edge gained up to 15 meters (50 feet) of thickness, according to ETH Zurich. The section that eventually collapsed, about a third of Birch Glacier, had advanced by around 50 meters (165 feet) over the past six years.

Starting on May 17, as the upstream mountain shed millions of cubic meters of rock, huge deposits fell atop the lower Birch Glacier. The added weight and the resulting compression and warming appear to have riddled the glacier with meltwater (up to 1 centimeter or 0.4 inch). Rains later that month only added to the load. And once the glacier started accelerating, the friction on its base caused additional warming and produced an estimated 0.5 cm (0.2 in) of meltwater per day.

A new category of avalanches

Events like the Birch Glacier collapse aren’t as freakish as we once might have assumed. A 2021 paper in The Cryosphere led by Andreas Kääb of the University of Oslo analyzed 20 events, most of them since 1990, grouped under the heading of sudden large-volume detachments of low-angle mountain glaciers. Quite a few of these cases surpass in scope the estimated 9.3 million cubic meters and 20 million metric tons of debris produced in the Birch Glacier disaster.

“Hardly recognized so far, giant catastrophic detachments of glaciers are a rare but great potential for loss of lives and massive damage in mountain regions,” wrote Kääb and coauthors.

These events are a subset of rock-ice avalanches (mixtures of rock and ice), which can emerge from a much broader set of conditions unrelated to climate change. The most notorious rock-ice avalanche in modern history was triggered by a 7.9-magnitude earthquake off the coast of Peru on May 31, 1970. A massive rockslide from the Huascarán mountain careened into a large glacier and eventually into saturated soils, creating a devilish mudflow perhaps 10 times the size of the Birch Glacier collapse. Traveling at hard-to-fathom speeds up to 435 km/hr (270 mph), the mudflow consumed the city of Yungay and several nearby villages, with fatality estimates running from 7,000 to 20,000 (apart from many thousands more killed elsewhere by the earthquake).

What seems to distinguish the newly recognized subset of rock-ice avalanches is the role of various factors in lubricating the bases of glaciers that lie at relatively low angles. While a typical snow avalanche tumbles down slopes of 30 to 45 degrees, many of the collapsed glacier sections analyzed by Kääb and colleagues sat atop gentler slopes of 10 to 20 degrees. Among the major events shown in the figure below:

• Alaska: Flat Creek, Alaska (2013 and 2015)
• Chile: Aparejo (1980) and Tinguiririca (1994 and 2007)
• Argentina: Leñas (2007)
• Russia/Georgia border area: Devdorak (18th and 19th centuries) and Kolka (1902 and 2002)
• Tajikistan: Rasht (2017 and 2019)
• Tibet: Aru (2016), Amney Machen (2004, 2007, 2016, and 2019), Sedongpu (2018), and Zelunglung (1950, 1968, and 1984)
• Mongolia: Tsambaragav (1988)

The deadliest such glacial detachment in recent years was the collapse of part of Kolka Glacier, in the Caucasus Mountains near the Russia-Georgia border, on September 20, 2002. A volume of rock similar to that atop Birch Glacier helped force a section of Kolka Glacier some 40 times larger than the lower Birch Glacier (close to 130 million cubic meters) to detach. As it raced off the 13-degree slope and pushed more than 16 km (10 mi) downstream, the rock-and-ice flow buried a village and caused at least 125 deaths.

Compared to this case and others analyzed in The Crysophere, the lower Birch Glacier moved off a steeper slope — around 28 degrees.

“That does not mean the Birch Glacier case is totally different from the cases described by us in 2021!” said lead author Kääb in an email. “We face a spectrum of the same processes that combine in different ways in the individual disasters. For all practical means, the result is very much the same: a highly mobile large-volume avalanche consisting of rock, ice, snow, water, mud, and whatever it takes up on its way.”

The climate link(s) to glacial detachments

The tendrils of a warming climate appear to be goosing glacier detachments in several ways, as analyzed by Kääb and colleagues. When meltwater or high-altitude rainfall shows up in places where it’s been rare or absent in modern times, it can add instability to rock and glacier structures. Compromised permafrost can help destabilize rock walls above glaciers. And as seems likely in the Birch Glacier case, heavy rock deposits can increase stress near a glacier’s base and induce basal melting and sudden advance and/or collapse.

Four years since his 2021 paper, said Kääb, “I am 1749486357 more convinced that climate change is able to contribute to glacier detachments and rock-ice avalanches. It is hard to prove for individual cases that climate change was the main cause. But it is also clear that climate change can in different ways facilitate these events.”

Thanks to the intensive monitoring carried out for years beforehand, and even closer scrutiny in the days leading up to its collapse, the Birch Glacier case could prove to be a gold mine for research as well as an exemplar for life-saving alerts.

“Switzerland is/was lucky to have the resources to set up an extensive monitoring system as soon as the severity of this situation was clear. There are few mountainous regions in the world that have that same capacity,” Jane Walden, a doctoral student in glaciology at ETH Zurich and WSL Sion, said in an email.

As for technology, “ground-based radar systems are the best choice to monitor the movements of a moving slope or glacier due to the fact that they work in inclement weather, but they are also very expensive and eventually struggle with very fast motion,” Walden said. GPS sensors and other simpler and less costly instruments may be better suited for hard-to-reach areas, but they can also be vulnerable: One such GPS device was destroyed on Nesthorn Mountain prior to the Birth Glacier collapse.

Mylène Jacquemart, a glaciologist at ETH Zurich and WSL Sion, is among those who’ve been joining forces to analyze rock-ice avalanches. She points to the critical need to understand the multilayered thermal structure within glaciers that can lead to speedups and detachments.

“Having a section of cold-based ice (frozen to the bed, no liquid water present) which is down-glacier from warm-based ice (below the pressure-melting point, and where liquid water is typically present) can result in high englacial water pressure and could lead to large breakoff events, though more research is needed to determine if and how the thermal regime is relevant to the Blatten case,” Jacquemart said.

In terms of slope-related hazards, she added, “We are seeing an increase in rockfall and landslides in formerly glacierized regions, so it’s critical to understand how the rapidly changing cryospheric environment is impacting the stability of the rock slopes adjacent to the glacier and what factors may lead to a slope’s destabilization or failure.”

The lower Birch Glacier is now gone, and the valley below appears safe from major rock-ice falls for the time being. But it’s possible other spots around the world, including the area near Kolka Glacier, may not be off the hook even after they’ve endured a glacial collapse.

“Reloading could absolutely be a problem,” Kääb warned. “The only advantage we have now is that we know that these glaciers have the potential to detach. It’s important that these events don’t get forgotten.”

Jeff Masters contributed to this post.