A new international study in Nature Communications Earth & Environment, led by Rayees Ahmed (Indian Institute of Science), shows that the catastrophic nature of large ice and rock avalanches depends not only on the scale of the hazard, but more importantly on preparedness and policies.
The study compares two recent events: the 2021 Chamoli disaster in the Indian Himalayas, which killed more than 200 people, and the 2025 ice and rock avalanche in Blatten, Switzerland, which buried most of the village but claimed only one victim. Despite similar physical characteristics, the consequences were drastically different. In Switzerland, unusual activity on the slopes was observed and closely monitored in the days before the collapse. Authorities deployed radar and other sensors, confirmed the accelerating movement of the slope and evacuated about 300 residents before the avalanche struck. In contrast, the Chamoli avalanche occurred in a remote valley in the Himalayas without operational monitoring or coordinated warning systems. Satellite observations showed in retrospect that there were signs of instability months before the collapse, but the signals were not noticed or monitored in time.
The study highlights how climate change increases the instability of high mountain slopes as glaciers retreat, permafrost thaws and extreme precipitation intensifies. This can make such sequential hazards more frequent.
"Events like in Chamoli show how devastating ice and rock avalanches can be when warning systems are lacking," says Dr Lander Van Tricht (Vrije Universiteit Brussel / ETH Zurich), co-author of the study. "But the case in Blatten also shows that even very large dangers do not necessarily have to turn into disasters with massive casualties."
"Global warming is destabilising glaciers and steep mountain slopes worldwide," adds Van Tricht. Recent disasters, such as the collapse of the Marmolada glacier in the Italian Alps in 2022 that left 11 dead, illustrate how rapidly changing mountain environments can cause sudden and deadly cryospheric hazards. "We cannot prevent hazards from occurring, but we can drastically reduce their impact by monitoring unstable slopes, responding to early warning signals and ensuring communities know how to act."
The researchers argue that countries in the Himalayas can significantly reduce risks by adopting elements of the Swiss integrated risk management approach. This includes combining satellite monitoring with targeted ground sensors, establishing clear warning thresholds, improving communication networks and strengthening preparedness within local communities.
"Ultimately, the difference between catastrophe and survival is not just the natural hazard itself," says Van Tricht. "It’s about whether societies are prepared when the warning signs appear."
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