HARRIS, C., VONDER MÜHLL, D., ISAKEN, K., HAEBERLI, W., SOLLID, J.L., KING, L., HOLMLUND, P., DRAMIS, F., GUGLIELMIN, M., PALACIOS D., Warming permafrost in European mountains.Global and Planetary Change, 2003, 39, 215 –225.

Meteorological stations such as St. Bernhard 2472 m (Switzerland) [Boehm et al., 2001] and Silandro at 720 m (Valtelina, Italy) indicate significant increases in 20th century atmospheric temperatures at high elevations within the Alps.

The PACE mountain permafrost borehole network offers the prospect of long-term monitoring as part of the Global Terrestrial Network for Permafrost (GTN-P) of the Global Climate Observing System (GCOS).

The first data sets presented show thermal gradients consistent with 20th century surface warming. Relief and aspect lead to greater variability between the Swiss and Italian Alpine boreholes than between those in Scandinavia and Svalbard. Observed warm-side deviation of ground temperatures at both 20 and 40 m is greater in the polar latitude Svalbard borehole than the more southerly boreholes in the network, suggesting greater warming at higher latitudes, as predicted by Global Climate Models (Houghton et al., 1996). First approximations of surface warming range from 0.5 °C at Stockhorn, Switzerland and Juvvasshøe, Norway to 1.0 °C at Janssonhaugen in Svalbard. A 15-year record of permafrost temperatures from the 58 m deep Murtèl–Corvatsch in Switzerland shows a warming trend, but with large amplitude interannual variations that reflect early winter snow thickness and duration rather than mean air temperatures. The significance of snow cover to the thermal regime of the upper permafrost boundary must therefore be carefully considered in interpreting future thermal inversion modelling of the permafrost geothermal profiles.

The longest continuous series of temperature measurements within European mountain permafrost is from the 58 m deep borehole at Murtèl–Corvatsch (Engadin, Switzerland) which was drilled in 1987 through slowly creeping ice-rich debris (Vonder Mühll et al., 1998; Haeberli et al., 1988; Vonder Mühll, 2001). Rapid warming of the uppermost 25 m of permafrost was observed between 1987 and 1994. The mean annual ground surface temperature is estimated to have increased from -3.3 °C (1988) to -2.3 °C (1994). However, low snowfall in December and January in the winter of 1994–1995, followed by moderate snow fall in 1995–1996 caused intense cooling of the ground and permafrost temperatures returned to values similar to those in 1987. Early winter snow was thin in 1998–1999 and winter ground temperatures remained low in 1999, 2000 and 2001. Mean permafrost temperature at a depth of 11 m rose and fell by 1.0 °C and at 20 m by 0.4 °C during the 15-year observation period, the dominant variable being snow cover rather than mean atmospheric temperatures. Thus, interpretation of inversion modelling based on the PACE borehole profiles must take account of the complex relationship between the ground surface and atmospheric temperatures, particularly the strong modulating affect of snow conditions. Overall, permafrost warming during the 15 years of observation at Murtèl–Corvatsch was about 0.6 °C at 11.6 m (within the depth range of strong seasonal temperature variability) and 0.2 °C at 20 m (below the depth of seasonal variation), indicating continued if not accelerated warming in recent years.

Permafrost has been identified as one of six cryospheric indicators of global climate change within the monitoring framework of the WMO Global Climate Observing System (GCOS) (Cihlar et al., 1997; Burgess et al., 2000; Harris et al., 2001a,b). Permafrost reacts sensitively to changes in atmospheric temperature (e.g. Anisimov and Nelson, 1996 [...]).

Permafrost temperature profiles show near-surface warm-side deviation from linear, with thermal gradients increasing with depth. [...] Borehole thermal gradients may be influenced by four major factors; regional geothermal heat flux, variation in lithology with depth, ground surface topography (governing the shape of the local upper thermal boundary), and past changes in ground surface temperatures.[...] In the Alps, the high relief results in heat flow less than the Swiss average of 85 mW/m² (Bodmer and Rybach, 1984). Bedrock lithology influences thermal diffusivity, but in all PACE boreholes, little variation in lithology was observed during drilling. Three-dimensional modelling of the temperature field is necessary to assess the influence of topography. However, initial data suggest that at the PACE permafrost borehole sites, the final factor, past changes in ground surface temperature, is highly significant with respect to depth-related changes in the thermal gradient.