The warming trend in Alpine permafrost observed over the past two decades continues and is accentuated in the hydrological year 2019/2020. This is shown by more than 20 years of observations in the framework of the Swiss Permafrost Monitoring Network PERMOS: the ground temperatures measured near the surface and at depth reached record values at most of the sites, as well as the active layer thickness and the rock glacier velocities.

The year 2019/2020 was the warmest year in Switzerland above 1000 m since the start of the measurements. The winter was particularly warm, with temperatures up to 1°C higher than the average of recent years. It was followed by a very warm spring, especially from mid-March to April, and a summer that was marked by two heat waves. At high elevations, the snow arrived early in November 2019, was about average thick during winter and disappeared comparably early in spring.

Ground temperatures measured near the surface in 2019/2020 were as high or even above the record level of the previous extremely warm years 2003, 2015 and 2018. This is the result of high air temperatures together with the early onset of snow, which insulated the ground from cold atmospheric conditions. The warm surface conditions resulted in new record values of active layer thickness (ALT) in all boreholes of the PERMOS Network that could be evaluated. ALT in the year 2020 ranged from 2.8 m (Flüela, GR) to 11 m (Schilthorn, BE). The increase in ALT compared to 2019 amounts to a few centimetres to almost half a metre depending on the location.

The almost continuous very warm surface conditions over three years resulted in increasing permafrost temperatures. The temporary interruption of the warming trend following winter of 2016 and 2017 is definitely over. Temperatures measured at 10 and 20 m depth have reached, and in some cases even exceeded, the previous record values measured in 2015. After more than 20 years of measurements, increasing permafrost temperatures are observed at all sites of the PERMOS Network. At the Stockhorn near Zermatt (VS), the permafrost temperatures at a depth of 20 m increased by about 0.8°C over 20 years, which is comparable to the observations at Murtèl-Corvatsch in the Upper Engadine (+0.5°C/10 years).

The kinematics of rock glaciers (masses of rock debris and ice moving downstream) indirectly reflect the permafrost conditions as the changes in rock glacier velocities follow the permafrost temperatures: when permafrost temperatures increase, rock glaciers generally accelerate. In 2020, rock glaciers accelerated significantly with an average increase in velocities of +21% compared to 2019 and are as high or higher than measured in the record year 2015.

Near-surface and deep temperatures have reached new records at many of the sites, as have active layer depths and rock glacier velocities. All measurements agree that the temporary pause in warming that followed the winter of 2016-2017 is definitely over. The observations in the year 2019/2020 are the result of persistent warm conditions over the past decades and the trends observed today are likely to continue.