The present report on permafrost in the Swiss Alps covers the four hydrological years 2014/2015 to 2017/2018. The state of permafrost in the Swiss Alps and its changes are observed within the Swiss Permafrost Monitoring Network (PERMOS) based on ground temperature measurements near the surface and at depth, changes in ice content determined from geophysical surveys, and creep velocities of rock glaciers derived from terrestrial geodetic surveys.

The report covers the warmest 4-year period of hydrological years ever recorded since measurements began in 1864. In contrast, the winter conditions during the reporting period were both, extremely poor and rich in snow. There was very little and late snow in winter 2016/2017 (in some regions already in winter 2015/2016), whereas in winter 2017/2018 the snow arrived rather early and snow heights were far above average.

Ground surface temperature (GST) measurements reflect atmospheric conditions and the duration and thickness of the snow cover. They have been continously high since 2009 for all regions and sites. At locations that are influenced by a winter snow cover, mean annual GST were higher than during the previous reporting period but they did not reach the maxima from 2003. The active layer thickness increased for most sites during the reporting period with many new record values observed in summer 2015 and 2018. The ground temperatures measured in boreholes at 10 and 20 m depth show that the warming trend that started in the year 2009 was interrupted at the snow-influenced field sites following the snow poor winter 2017 (in some regions already after winter 2016). The warming trend resumed in the uppermost metres after the hot summer 2018. At depth, however, the cooling is still visible and the warm conditions have not yet arrived.

Electrical resistivities measured within the permafrost layer at five sites showed a similar overall decreasing trend since the beginning of the measurements, except at Les Attelas where no clear trend was observed. Unlike the ground temperatures and the rock glacier creep velocities, no significant interruption of the trend was observed in the electrical resistivities following the snow-poor winter 2016/2017. Terrestrial geodetic surveys of rock glaciers show a common general behaviour: permafrost creep velocities continuously increased during the years 2007 until 2015 and decreased in 2016 and 2017, which is consistent with the decrease in permafrost temperatures after the snow-poor winter 2016/2017.

In summary, all elements observed within the PERMOS network during the reporting period 2014/2015– 2017/2018 show a warming trend since the year 2009, which was temporarily interrupted at snow-influenced sites following the snow-poor winter 2016/2017, despite the constantly above-average air temperatures. However, the continuously decreasing electrical resistivities point to a persisting loss in ground ice and continuously warming permafrost conditions. Continuously warm permafrost conditions were observed at sites with temperatures close to 0 °C and at locations in steep and snow-free rock walls.