Reconstructing the 2025 Nesthorn-Birchgletscher hazard cascade

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 at this point was completely evacuated). 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.To reconstruct and understand the physical processes that controlled this remarkable hazard cascade, we used aerial topographic surveys, radar and time-lapse images, direct field observations, eyewitness accounts, meteorological data, and numerical modeling. From these data we 1) determined the precise chronology of the event, including the failure and deposition volumes and geomorphologic event traces; 2) reconstructed the pre-event (1946-2023) history of Kleines Nesthorn and Birchgletscher, including the substantial mass loss of the latter and its recent surge-type acceleration; 3) analyzed the kinematics of the rock instability on Kleines Nesthorn and the resulting rock failures that loaded the glacier; 4) used the 3-D finite element model Elmer/Ice to reconstruct the effect of the rock loading on the force balance of Birchgletscher and its relevance for the observed acceleration and collapse; and 5) processed data from several long-term weather stations, satellite data and climate models to evaluate the relevance of human-caused climate change on Birchgletscher, snow-cover, permafrost and the entire process chain. Our results highlight the complexity of the Nesthorn-Birchgletscher hazard cascade and provide valuable insights for the assessment and management of glacier-related hazards in high mountains.