Réf. Mihajlovic & al. 2008 - P

Référence bibliographique complète
9th International Conference on Permafrost (June 29-July 3, 2008, University of Alaska Fairbanks). Twenty Years of Permafrost Research on the Furggentälti Rock Glaciers, Western Alps, Switzerland. MIHAJLOVIC D., STAUB B., NUSSBAUM A., KRUMMENACHER B., KIENHOLZ H. Fairbanks, 2008, Vol. 2, p. 1209-1214.

Abstract: Since 1988, long-term monitoring of the Furggentälti rock glaciers (Western Alps, Switzerland) has revealed significant changes in process dynamics. The changes include large seasonal and interannual variations of rock glacier activity, superimposed by an exponential increase of overall rock glacier creep velocity. During the monitoring period, the largest of the rock glaciers (located at 2450 m a.s.l.) developed signs of decay, with some parts of the rock glacier becoming inactive and others showing patterns of collapse. Analysis of local climate data suggests a strong and surprisingly low-latency link between rock glacier activity and weather patterns. Even though some of the long-term developments in the kinematics of the Furggentälti rock glaciers are also influenced by other factors, such as topography and process feedback, the short-term response to climate signals points at ever warmer permafrost conditions at the site as the major cause for the changes observed.

Mots-clés
Climate change, ground surface temperature, long-term monitoring, rock glacier activity, Switzerland, warm permafrost.

Organismes / Contact
Institute of Geography, University of Bern, Switzerland
Flotron AG Engineers, Meiringen, Switzerland
Geotest AG, Davos, Switzerland

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
  Permafrost (rock glacier)    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland Western Bernese Alps Furggentälti rock glaciers (46°24.5'N, 7°38'E) North ~2450-2600 m a.s.l. 1988-2007

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
A first photogrammetric assessment of the Furggentälti rock glacier in 1996 (Krummenacher et al. 1998) revealed unusually high surface velocities and a rapid increase during the late eighties and early nineties. Further photogrammetric surveys (Mihajlovic et al. 2003) confirmed this multi-decadal trend, which was also observed in neighboring rock glaciers in the valley. The survey also found indications for a slowdown in surface velocities in the peripheral parts on the left and right side of the rock glacier. The latest data confirm both trends, with increased surface velocities found along the centerline of the rock glacier and a slowdown on the sides. The data have also detected the inactivation of peripheral parts of the rock glacier.

Since 1994, annual terrestrial survey campaigns have uncovered interannual variations in the activity of the rock glacier, showing a clear pattern of thermally induced acceleration and deceleration of surface velocities. The pattern clearly reflects the development of the warming conditions at the site, with some attenuation during 2005 to 2007.

A seasonal activity pattern was detected during a series of repeated survey campaigns from August 1998 to October 1999 (Mihajlovic et al. 2003). After a continuous slowdown during winter, the permafrost creep process accelerates rapidly at the beginning of the Zero Curtain phase in spring, when water infiltrates into the frozen active layer and rock glacier material, triggering an instant warm-up of the thermal conditions there.

During most of the 13 year monitoring period, winter equilibrium temperatures (WEQT) recorded on the Furggentälti rock glacier were within the range of -2.5°C to -1.5°C, with the exception of two distinct cooling events that led to a significantly lower WEQT in early 1996 and 2006. The GST monitoring period (1994 to 2007) contains both positive and negative interannual WEQT fluctuations, which coincide with positive and negative variations of rock glacier activity. In the positive case, atmospheric conditions during the preceding year led to a general increase of ice temperatures, whereas in the negative case an overall cooling occurred.

Comparing different years of GST data by adding temperatures sum for N and L phases only, the relation between thermal “signal” and the thermal “response” becomes obvious. The non-linearity in the relation shows how the thawing of ice in the rock glacier material is acting as a thermal buffer in warmer years with a high energy input. Doubling the thermal signal from, for example, 300 day-degrees C to 600 will not lead to substantial increase of the WEQT (and, therefore, permafrost temperature), as the excess energy input of the warm season is leaving the rock glacier system as melt water. In contrast to this, reducing the thermal signal to 150 day-degrees C leads to a significant drop in permafrost temperature.

The clustered occurrence since the early nineties of winters with only little snow did not lead to a general cooling of the permafrost. At the relatively low altitude of the site (average air temperature 1988–2006 = 0.1°C), the importance of the snow cover acting as a radiation shield during early summer is exceeding the “cooling effect” of the absence of snow during the cooling phase in early winter.
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Rock glacier activity increase seems to be related with air temperature increase and snow cover deacrease.
The Furggentälti valley (46°24.5'N, 7°38'E) is located in the western part of the Bernese Alps, with an altitude of approx. 2450 to 2850 m a.s.l. The focus of interest lies on an approximately 250 m long tongue-shaped rock glacier in the lower western part of the valley. The rock glacier is situated on a slope of about 20° of northern aspect, its front protruding into the bottom of the valley, at an altitude of approx. 2450 m a.s.l.

Initiated in 1988, the long-term monitoring project consists of several measurement programs recording meteorological data, ground temperatures, ground surface temperatures (GST), and rock glacier activity data through aerial and terrestrial survey. The winter equilibrium temperature (WEQT) is used as an estimation for the permafrost temperature at its coldest state in the year. The WEQT is derived from continuously recorded GST, which are measured at several locations on the rock glacier surface.

In order to get a better picture of how different temporal patterns of snow cover occurrence influence permafrost temperature variations, a simple method was developed. The basic idea is to skip from looking at GST time series in fixed intervals to intervals which take into consideration the different states of thermal insulation properties of the snow cover. For this, the annual cycle of snow cover ablation and development is divided into four phases:
- Phase Z: Zero Curtain (springtime), GST shows 0°C;
- Phase N: No Attenuation , during the absence of snow;
- Phase L: Low Attenuation , in this phase, a snow cover of limited depth is present;
- Phase H: High Attenuation, in this phase, the ground surface is covered by a thick and thermally insulating layer of snow.

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètres de l'aléa
Sensibilité du paramètre de l'aléa à des paramètres climatiques et du milieu
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 

(4) - Remarques générales
 

(5) - Préconisations et recomandations
 

Références citées :

Krummenacher, B., Budmiger, K., Mihajlovic, D. & Blank B. 1998. Periglaziale Prozesse und Formen im Furggentälti, Gemmipass. Davos: Mitteilungen des Eidgenössisches Institut für Schnee- und Lawinenforschung Davos Nr. 245.

Mihajlovic, D., Krummenacher B. & Imhof, M. 2003. Developing new methods for monitoring periglacial phenomena. Proceedings of the Eighth International Conference on Permafrost 1: 455-460.