• Energy Research

Abstract Cryospheric long-term timeseries get increasingly important. To document climate-related effects on long-term viscous creep of ice-rich mountain permafrost, we investigated timeseries (1995–2022) of geodetically-derived Rock Glacier Velocity (RGV), i.e. spatially averaged interannual velocity timeseries related to a rock glacier (RG) unit or part of it. We considered 50 RGV from 43 RGs spatially covering the entire European Alps. Eight of these RGs are destabilized. Results show that RGV are distinctly variable ranging from 0.04 to 6.23 m a−1. Acceleration and deceleration at many RGs are highly correlated with similar behaviour over 2.5 decades for 15 timeseries. In addition to a general long-term, warming-induced trend of increasing velocities, three main phases of distinct acceleration (2000–2004, 2008–2015, 2018–2020), interrupted by deceleration or steady state conditions, were identified. The evolution is attributed to climate forcing and underlines the significance of RGV as a product of the Essential Climate Variable (ECV) permafrost. We show that RGV data are valuable as climate indicators, but such data should always be assessed critically considering changing local factors (geomorphic, thermal, hydrologic) and monitoring approaches. To extract a climate signal, larger RGV ensembles should be analysed. Criteria for selecting new RGV-sites are proposed.

Climate-related permafrost is widespread in cold mountains and heavily affects slope stability. As a subsurface phenomenon, however, it is often still absent in the perception of key partners concerning the discussion and anticipation of long-term impacts on high mountain regions from continued global warming. Outreach and knowledge transfer, therefore, play a key role. Long-term observations of permafrost temperatures measured in boreholes can be used to convey answers and key messages concerning thermal conditions in a spatio-temporal context, related environmental conditions, affected depth ranges, and impacts of warming and degradation on slope stability.The 35-year Murtèl-Corvatsch time series of borehole temperatures from which data is available since 1987, is used here as an example. Today, mountain permafrost is well documented and understood regarding involved processes, as well as its occurrence in space and evolution in time. Thermal anomalies caused by global warming already now reach about 100 meters depth, thereby reducing the ground ice content, causing accelerated creep of ice-rich frozen talus/debris (so-called “rock glaciers”) and reducing the stability of large frozen bedrock masses at steep icy faces and peaks.

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This paper reviews and analyses the past 20 years of change and variability of European mountain permafrost in response to climate change based on time series of ground temperatures along a south–north transect of deep boreholes from Sierra Nevada in Spain (37°N) to Svalbard (78°N), established between 1998 and 2000 during the EU-funded PACE (Permafrost and Climate in Europe) project. In Sierra Nevada (at the Veleta Peak), no permafrost is encountered. All other boreholes are drilled in permafrost. Results show that permafrost warmed at all sites down to depths of 50 m or more. The warming at a 20 m depth varied between 1.5 °C on Svalbard and 0.4 °C in the Alps. Warming rates tend to be less pronounced in the warm permafrost boreholes, which is partly due to latent heat effects at more ice-rich sites with ground temperatures close to 0 °C. At most sites, the air temperature at 2 m height showed a smaller increase than the near-ground-surface temperature, leading to an increase of surface offsets (SOs). The active layer thickness (ALT) increased at all sites between c. 10% and 200% with respect to the start of the study period, with the largest changes observed in the European Alps. Multi-temporal electrical resistivity tomography (ERT) carried out at six sites showed a decrease in electrical resistivity, independently supporting our conclusion of ground ice degradation and higher unfrozen water content.