Abstract. Mountain permafrost is sensitive to climate change and is
expected to gradually degrade in response to the ongoing atmospheric warming
trend. Long-term monitoring of the permafrost thermal state is a key task,
but problematic where temperatures are close to 0 ∘C because the
energy exchange is then dominantly related to latent heat effects associated
with phase change (ice–water), rather than ground warming or cooling.
Consequently, it is difficult to detect significant spatio-temporal
variations in ground properties (e.g. ice–water ratio) that occur during the
freezing–thawing process with point scale temperature monitoring alone.
Hence, electrical methods have become popular in permafrost investigations
as the resistivities of ice and water differ by several orders of magnitude,
theoretically allowing a clear distinction between frozen and unfrozen
ground. In this study we present an assessment of mountain permafrost
evolution using long-term electrical resistivity tomography monitoring
(ERTM) from a network of permanent sites in the central Alps. The time
series consist of more than 1000 datasets from six sites, where
resistivities have been measured on a regular basis for up to 20 years.
We identify systematic sources of error and apply automatic filtering
procedures during data processing. In order to constrain the interpretation
of the results, we analyse inversion results and long-term resistivity
changes in comparison with existing borehole temperature time series. Our
results show that the resistivity dataset provides valuable insights at the
melting point, where temperature changes stagnate due to latent heat
effects. The longest time series (19 years) demonstrates a prominent
permafrost degradation trend, but degradation is also detectable in shorter
time series (about a decade) at most sites. In spite of the wide range of
morphological, climatological, and geological differences between the sites,
the observed inter-annual resistivity changes and long-term tendencies are
similar for all sites of the network.
Review of “Long-term monitoring of mountain permafrost degradation using an electrical resistivity tomography network” by Mollaret et al.
