Long-term ground temperature monitoring, repeated ERT measurements, and historical sources reveal increasing permafrost degradation at a high-mountain pass in Austria (Hochtor, Hohe Tauern Range)

Average European air temperatures in the meteorological summer 2022 (June-August) were 1.3°C higher than normal impacting the cryosphere in the Alps. We studied the long-term warming effects at a high mountain pass landscape in central Austria named Hochtor (2576 m asl, 47.08°N, 12.84°E), Hohe Tauern Range. Archaeological finds along the former travel route over Hochtor suggest that this mountain crossing was already used in prehistoric times. Solifluction processes created the widespread existence of solifluction landforms at the pass and caused the displacement of archaeological finds from their original positions. This archaeological significance has also implications for present periglacial research. We worked on the research question how ongoing climate change caused modifications in the ground thermal regime and subsequently on permafrost and periglacial conditions at this site. The aims were: (1) to analyse ground temperature and permafrost conditions and trends, (2) to evaluate changes of potential frost-related weathering, and (3) to assess the impact of the recent atmospheric warming including the summer 2022 on the ground thermal conditions since the late 19^th century at Hochtor. We used long-term ground temperature data (2010-2022) from three different depths (max. 60 cm), repeated electrical resistivity tomography (ERT) measurements from two years (2019, 2022), and auxiliary data dating back to 1887 (instrumental data) or Roman times (archaeological finds).

Our results indicate that Hochtor changed during the period 2010-2022 from an active permafrost site to an inactive one with a supra-permafrost talik zone in between the seasonally thawing and freezing top layer and the permafrost. A general three-layer structure was quantified for the three 96m-long ERT profiles measured in 2019 at the mountain pass location. The central, 5 to 10 m thick stratum is a lens-like, ice-poor permafrost layer detected in 2019 and confirmed in its existence – although smaller in extent – in 2022. As revealed by time-lapse ERT analyses, a mean annual resistivity decreasing rate of 3.9 to 5.2% yr-1 indicates distinct and profile-wide permafrost degradation at the three profiles. The summers of 2003, 2015, 2019 and 2022 were the four warmest ones in the period 1887-2020. Therefore, resistivity changes between the exceptional warm summers 2019 and 2022 are not the single effect of the summer heatwave of 2022 but must be seen as a long-term signal of permafrost degradation which has increased significantly in the recent past.

Reconstructed ground surface warming between the two normal periods 1891-1920 and 1991-2020 is for annual ground surface temperature 1.8°C and for summer ground surface temperature 2.5°C. Thus, summer warming surpasses annual warming which agrees with previous works and future scenarios. Frost-related weathering and periglacial processes decreased, although to an unknown extent. As we will face a warmer climate during the twenty-first century, we argue that our results suggest rapid ground warming since the 1980s accompanied by permafrost degradation leading within the next decades to permafrost-free conditions at this 2576 m high mountain pass.

Acknowledgement: This work was supported by the Austrian Science Fund (FWF P18304-N10), the European Regional Development Fund (18-1-3-I) and the Hohe Tauern National Park Carinthia.