Response of alpine ground temperatures to a rising atmospheric 0 °C isotherm in the period 1955-2021.

Ground temperatures in alpine terrain vary considerably over short distances, particularly due to differences in elevation and incoming short-wave radiation but also snow and surface conditions. To allow for direct cross-site, interregional, and cross-disciplinary comparisons, we introduce a parameter called the potential 0 °C isotherm (PZDI) in the ground. This parameter represents an aspect- and elevation independent geo-projection of ground temperatures. It was calculated for several ground temperature datasets from the (sub-)permafrost zone in the European Alps. We analyse the reaction of the PZDI at different depths to long-term changes in atmospheric temperature, represented by the atmospheric 0 °C isotherm (AZDI). The close correspondence of PZDI and AZDI mainly provided two new insights: Based on 15 boreholes, the PZDI/AZDI interaction allowed a depth-dependent analysis of the magnitude and temporal evolution of the disequilibrium between atmosphere and ground and the potential heat transfer processes involved. Moreover, it allowed a reconstruction of the development of ground temperatures in the European Alps during the second half of the 20th century, adding 50 years to the longest existing time series measured in ice-poor permafrost. This extension into the past reveals a major warming at the end of the 1980s, which occurred before the start of the available permafrost temperature observations. In the 1955-2021 period, the 5-year running mean of the AZDI rose by 400 m, while the PZDI at 15 m depth rose by about 300 m. Projecting this warming into a permafrost distribution map shows a decline in shallow, ice-poor permafrost area by roughly 60 % since the 1980s. We expect the PZDI to follow the AZDI and to increase by another 100 m in the coming decade and up to at least 14 m depth. This would increase the area with loss of shallow permafrost to approximately 80 % since the 1980s.

Permafrost is warming at a global scale.

Permafrost warming has the potential to amplify global climate change, because when frozen sediments thaw it unlocks soil organic carbon. Yet to date, no globally consistent assessment of permafrost temperature change has been compiled. Here we use a global data set of permafrost temperature time series from the Global Terrestrial Network for Permafrost to evaluate temperature change across permafrost regions for the period since the International Polar Year (2007-2009). During the reference decade between 2007 and 2016, ground temperature near the depth of zero annual amplitude in the continuous permafrost zone increased by 0.39 ± 0.15 °C. Over the same period, discontinuous permafrost warmed by 0.20 ± 0.10 °C. Permafrost in mountains warmed by 0.19 ± 0.05 °C and in Antarctica by 0.37 ± 0.10 °C. Globally, permafrost temperature increased by 0.29 ± 0.12 °C. The observed trend follows the Arctic amplification of air temperature increase in the Northern Hemisphere. In the discontinuous zone, however, ground warming occurred due to increased snow thickness while air temperature remained statistically unchanged.