Rockfall from an increasingly unstable mountain slope driven by climate warming

Rockfall in alpine regions is a significant concern, exacerbated by climate warming and permafrost thaw. The increased frequency and magnitude of rockfall events, particularly during recent heatwaves, pose substantial risks to mountain communities and infrastructure. While anecdotal evidence suggests a link between climate change and increased rockfall, a systematic, long-term dataset to quantify this relationship has been lacking. This research addresses this gap by employing a novel approach to reconstruct a continuous rockfall record spanning a century, enabling a comprehensive assessment of the influence of climate warming on rockfall activity. The study focuses on the Täschgufer site in the Swiss Alps, characterized by a steep slope with heavily disintegrated gneissic outcrops situated in permafrost-dominated environments. The unique methodology employed allows for the detailed reconstruction of rockfall events over an extended period, providing invaluable insights into the temporal dynamics of this geomorphic process. This long-term perspective offers crucial information for developing effective mitigation strategies and risk management plans in high-mountain areas, where both the frequency and intensity of hazardous events are expected to intensify with ongoing climate change. The importance of understanding this relationship cannot be overstated, given the increasing vulnerability of populations and critical infrastructure in mountainous regions globally.

Previous research has highlighted the connection between climate change, permafrost degradation, and increased mass movement activity in high-mountain environments. Several studies have noted increased rockfall incidences during recent heatwaves in the Alps, associating these events with the thawing of permafrost. However, the complexities of rockfall triggers, coupled with limited long-term observational data and the inherent challenges of high-elevation climate modeling, have hindered a complete understanding of this relationship. The lack of systematic, multi-decadal rockfall records, often hampered by non-uniform observation rates and biases towards recent and larger events, further complicates the analysis. This scarcity of data underscores the need for long-term monitoring and innovative methodologies to improve our understanding of the link between climate change and rockfall activity.

This study utilizes dendrogeomorphic techniques to reconstruct a century-long (1920-2020) time series of rockfall activity at Täschgufer in the Swiss Alps. European larch (*Larix decidua* Mill.) trees, growing at the forest fringe, were sampled to record the impact of rockfall events. Rockfalls cause injuries and tangential rows of traumatic resin ducts (TRDs) in the trees, which serve as reliable indicators for dating past rockfall events. A total of 375 trees were sampled, with 1450 growth disturbances (GDs) identified. The intra-ring position of GDs enabled dating rockfall events with sub-seasonal precision. A Monte Carlo modeling approach was employed to account for uncertainties in actual rockfall dates, assessing the seasonal occurrence of rockfall over time. To address potential biases caused by changes in sample size and tree distribution over the years, conditional impact probabilities (CIPs) were calculated. The CIP approach incorporates both forest and rockfall characteristics to provide realistic estimates of past rockfall activity. This method accounts for the varying coverage of the slope by trees over time, ensuring a more accurate assessment of rockfall frequency. The reconstructed rockfall activity was correlated with historical air temperature and precipitation data for the region. Various time windows were considered to identify the most significant correlations between rockfall events and temperature variations. Both interannual and decadal trends were examined to assess the long-term impact of climate warming on rockfall frequency. In addition, borehole temperature data from a nearby site, Corvatsch, was used for comparison to support the conclusions of the study.

The analysis reveals a marked increase in rockfall frequency over the past century. A notable increase occurred in the late 1940s to early 1950s, corresponding to the end of the Early Twentieth Century Warming (ETCW). However, a more substantial increase in rockfall activity is observed since the mid-1980s, coinciding with accelerating climate warming. This suggests a stronger link between more recent warming and rockfall incidence. Analysis of the seasonal timing of rockfall events demonstrates a shift towards warmer-season activity. While rockfall was previously concentrated during winter and spring, more recent events increasingly occur in the warmer months, aligning with the increased instability due to permafrost degradation. At the interannual scale, rockfall frequency exhibits a significant positive correlation with summer air temperatures (July-August). This correlation persists after detrending for the accelerating warming and increased activity. Further analysis shows significant correlations between rockfall and summer temperatures during the 1920-1969 period, as well as late spring temperatures since 1970. Decadal trends reveal a strong correlation (r = 0.69, p < 0.01) between warming summer temperatures and increased rockfall activity. The results also point to a potential lag effect between warming temperatures and the occurrence of rockfall, suggesting that several consecutive warm summers may be necessary to destabilize the slope sufficiently to trigger rockfall events. The combination of DBH with the mean impact circles was used to estimate the percentage of the slope covered by trees in any given year or decade allowing the correction of the rockfall impacts and definition of periods for which activity is either over- or underestimated. Eyewitness reports and the construction of rockfall dams in the area corroborate the observed increase in rockfall activity.

The findings strongly suggest a significant link between accelerating climate warming, permafrost degradation, and enhanced rockfall activity in the Swiss Alps. The long-term rockfall record, coupled with the robust statistical correlations with summer temperatures, provides compelling evidence of the impact of climate change on slope stability. The observed shift in rockfall seasonality from winter/spring to warmer months further supports this conclusion. While the study demonstrates a clear correlation between warming temperatures and rockfall, the potential lag effects and the influence of other factors like snow cover and ground heat transport highlight the complexity of the rockfall system. The study’s findings have important implications for risk assessment and mitigation strategies in high-mountain areas. The observed increases in rockfall frequency and the shift in seasonality necessitate the adaptation of current practices to account for the changing risks. The study site, Täschgufer, emerges as a benchmark site for future research, offering a valuable long-term dataset for investigating the impacts of climate change on rockfall dynamics in similar environments.

This study provides compelling evidence of a direct link between climate warming and increased rockfall activity in the Swiss Alps. The century-long rockfall reconstruction, coupled with robust statistical correlations with summer temperatures, highlights the significant impact of climate change on slope stability. The findings demonstrate the need for improved risk assessment and mitigation strategies in high-mountain regions, given the projected increase in rockfall activity under continued warming. Future research should focus on expanding the monitoring efforts, incorporating detailed ground heat flux data, and investigating the lag effects observed in the relationship between temperature and rockfall triggering.

While the study provides a valuable long-term dataset and robust analysis, several limitations should be considered. The rockfall reconstruction is based on a single site, limiting the generalizability of the findings to other high-mountain environments. The study's focus is on individual rockfall events (<10 m³), and larger scale mass wasting events are not directly considered. Although the CIP approach mitigates biases related to sample depth and tree distribution, uncertainties remain due to the limitations of tree-ring analysis. Finally, the absence of local ground heat flux data restricts the ability to fully quantify the impact of permafrost thaw on slope stability.