A couple of days after the tragic and shocking ice-rock-debris avalanche in Blatten on May 28, here are a few points on the underlying physics of the cascade of processes and the role of climate change.

Large amounts of rock debris – ca. three million cubic meters – 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 this amount of rock debris largely 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.

Imbalance between shear stress and shear strenght

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 (see ETH factsheet). 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.

The effects of anthropogenic warming

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 on 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 (see figure below).

🔶 Most of the rock mass of Kl. Nesthorn are in permafrost conditions, with temperatures a few degrees below 0°C, and into great depth, probably 100 meters 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 Kl. Nesthorn have virtually lost all seasonal and perennial snow and firn which leads to 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 inevitably political – and I recognize that 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.

Christian Huggel, Environment and Climate

Media reports with statements by Christian Huggel (in German and English)