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Abstract. Mountain permafrost is sensitive to climate change and isexpected to gradually degrade in response to the ongoing atmospheric warmingtrend. Long-term monitoring of the permafrost thermal state is a key task,but problematic where temperatures are close to 0 ∘C because theenergy exchange is then dominantly related to latent heat effects associatedwith phase change (ice–water), rather than ground warming or cooling.Consequently, it is difficult to detect significant spatio-temporalvariations in ground properties (e.g. ice–water ratio) that occur during thefreezing–thawing process with point scale temperature monitoring alone.Hence, electrical methods have become popular in permafrost investigationsas the resistivities of ice and water differ by several orders of magnitude,theoretically allowing a clear distinction between frozen and unfrozenground. In this study we present an assessment of mountain permafrostevolution using long-term electrical resistivity tomography monitoring(ERTM) from a network of permanent sites in the central Alps. The timeseries consist of more than 1000 datasets from six sites, whereresistivities have been measured on a regular basis for up to 20 years.We identify systematic sources of error and apply automatic filteringprocedures during data processing. In order to constrain the interpretationof the results, we analyse inversion results and long-term resistivitychanges in comparison with existing borehole temperature time series. Ourresults show that the resistivity dataset provides valuable insights at themelting point, where temperature changes stagnate due to latent heateffects. The longest time series (19 years) demonstrates a prominentpermafrost degradation trend, but degradation is also detectable in shortertime series (about a decade) at most sites. In spite of the wide range ofmorphological, climatological, and geological differences between the sites,the observed inter-annual resistivity changes and long-term tendencies aresimilar for all sites of the network.

Mountain permafrost degradation documented through a network of permanent electrical resistivity tomography sites