Impact of permafrost changes on the 2025 Nesthorn-Birchgletscher hazard cascade

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Abstract
In late May 2025, a series of large rock failures from Kleines Nesthorn in the Swiss Lötschental (Lötschen valley) fell directly onto the Birchgletscher (Birch Glacier), loading the latter with around 4 million m3 of rock. On May 28, following several days of acceleration, Birchgletscher collapsed in its entirety, claiming one life and causing the near-total destruction of the historic village of Blatten (which had been fully evacuated prior to the event). Totaling more than 9 million m3 of rock and glacier ice (with a ratio of about 3:1), the rock-ice avalanche dammed the river Lonza and led to the formation of a lake that damaged additional parts of the village.The rock failures that initiated the hazard cascade originated from the north-east face of Kleines Nesthorn, a formerly 3336 meter tall peak. Like all areas at this elevation, this face is expected to have experienced important ground temperature changes in the past decades. In this contribution, we ask how these changes may have affected the observed hazard cascade. We analyzed climatic changes (air temperature and precipitation) and snow cover, simulated permafrost changes, and assessed the geologic preconditioning and failure kinematics. We then attempt to propagate these changes through the hazard cascade to understand their possible impact on the rock slope failures. To do so, we employed a combination of data from weather stations, satellite data, climate models and numerical modeling. Our findings demonstrate the complexity of the Nesthorn-Birchgletscher hazard cascade and highlight challenges and research gaps for assessing the role of permafrost on large rock slope failures in a changing climate.