Permafrost in Switzerland 2004/2005 and 2005/2006

Abstract

The present report covers the period from October 2004 to September 2006. It is the last report of the pilot phase 2000–2006 of the network for permafrost monitoring in Switzerland (PERMOS). At this point, PERMOS includes (a) 11 drill sites (including 22 boreholes and geophysical monitoring at 4 of the sites), (b) 11 surface temperature sites (including measurements in loose debris at 9 sites, in bedrock at 5 sites, and at the bottom of the snow cover (BTS) at 3 sites), and (c) aerial photographs taken by Swisstopo. Winter 2004/2005 was characterized by an early and thin snow cover in the higher alpine regions and an early snow melt in spring. A long phase of above average air temperatures followed, which lasted until July and made summer 2005 the second warmest on record. Winter 2005/2006 started late, had a long lasting thick snow cover, and was followed by a warm summer with the hottest July ever measured. Active layer thicknesses in summer 2004 were similar to those before 2003 at most sites. Hence, thermal changes in the subsurface from the 2003 heat wave were not sustained, although the ice content at the permafrost table may have been permanently modified. The active layer deepened again in several boreholes in summer 2005, reaching values similar to 2003, and remained more or less stable in summer 2006. At ca. 10 m depth, ground temperatures at the drill sites displayed a warming until the beginning of 2005, which results from the preceding heat period in summer 2003. Then, mainly as a result of the snow conditions in winter, a cooling period followed. Since 2005, temperature observations are complemented by electrical resistivity tomography (ERT) measurements at 4 drill sites to monitor physical properties of the ground (e.g., ice content or unfrozen water content). The ERT monitoring strategy is outlined and discussed in this report. Ground surface temperatures (GST) were low in both years of the reporting period, especially during winter. In 2005, GST temporarily dropped to new minima and reached about mean values of the past decade at the end of the reporting period. Rock surface temperatures (RST) were integrated into PERMOS in summer 2004 and are presented and discussed in detail in this report. Temperature values in steep rock clearly display the cold conditions in winter 2004/2005 as well as the warm summer 2006. The chapter on special aspects of permafrost monitoring is dedicated to the observed acceleration of rock glaciers in the past two decades. The destabilization of several landforms shows that permafrost creep conditions in the Alps are changing. In general, both, surface temperatures and ground temperatures in the uppermost meters decreased to pre-2003 conditions at all sites during the reporting period, which is mainly a result of the snow conditions