Réf. Gruber & al. 2004a - A

Référence bibliographique complète
GRUBER, S. HOELZLE, M. and HAEBERLI, W. Permafrost thaw and destabilization of Alpine rock walls in the hot summer of 2003. Geophysical Research Letters, 2004, vol 31, 4 p.

Abstract:
Exceptional rockfall occurred throughout the Alps during the unusually hot summer of 2003. It is likely related to the fast thermal reaction of the subsurface of steep rock slopes and a corresponding destabilization of ice-filled discontinuities. This suggests that rockfall may be a direct and unexpectedly fast impact of climate change. Based upon four measurements in Alpine rock faces, we present model simulations illustrating the distribution and degradation of permafrost where the summer of 2003 has resulted in extreme thaw. We argue that hotter summers predicted by climate models for the coming decades will result in reduced stability of many alpine rock walls.

Mots-clés
Hydrology, frozen ground, global change, geomorphology and weathering, global change, impact phenomena.

Organismes / Contacts
Glaciology and Geomorphodynamics Group, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland. stgruber@geo.unizh.ch

(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
Temperature Permafrost Mass movement Rock Falls

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland Swiss Alps 14 data logger in steep Alpine rock faces   2600-4500m asl 2002

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

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

 


(2) - Impacts du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
The temperature distribution and evolution in steep rock was largely unknown until now, even though, permafrost thaw in rock faces maybe equally consequential (in terms of natural hazards and geotechnical consequences for infrastructure) as that in debris-covered slopes. Additionally, the thermal response of rock faces to individual extreme events is fast compared to debris slopes that are often insulated by blocky surfaces [Harris and Pedersen, 1998] and have a high ice-content.
Modélisations
The modeled thaw of 2003 exceeds the maximum of all previous years (one dimensional energy balance model verified with the data measured at the 14 sites). The summer thaw of 2003 (modelised) is 10–50 cm deeper than the maximum of the 21 previous years for the active permafrost zone. The immediate response of the permafrost active layer to increase of temperature has been observed during the 2003 summer.

In northern slopes, the depth of thaw is mainly controlled by the influence of air temperature (mostly via long-wave radiation) on surface temperatures, whereas southern slopes additionally receive high amounts of shortwave radiation. As a consequence, southern slopes exhibit greater inter-annual variability of thaw depth, larger pre- 2003 maxima and, therefore, a smaller 2003 anomaly.
Hypothèses
The 2003 thaw depth is likely to exceed previous maxima even on time scales of centuries, considering the pronounced recent global and hemispheric temperature rise inferred from instrumental records and proxy data and its influence on Alpine ground temperatures

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Sensitivity of permafrost temperature to air surface temperature
Temperature measures between summer 2001 and summer 2002 provided by 14 loggers (depth of 10 cm and frequency of 1 measure / 2 hours) have been used to developp an energy-balance model. Using this model, rock temperatures for different aspects and elevations were calculated from 1982 to 2003. Daily rock temperatures for a slope of 70°, seven elevations from 2000–5000 m and eight aspects (N, NE, . . .. W, NW) were simulated based on meteorological data from Jungfraujoch from 01/1982 to 12/2003. Oct.–Dec. 2003 were not yet available at the time of calculation and substituted by 2002 data.

(3) - Impacts du changement climatique sur l'aléa
Reconstitutions
 
Observations
Most of the rock falls took place between June and August [2003] when the depth of thaw was not at is maximum but when the heat flux in the ground at somewhat shallower depths was greatest.

The increased thaw during the summer of 2003 far outweighs the direct effect that gradually rising temperatures have on rock wall stability in the uppermost meters. The extreme frequency of rockfall in the Alps during 2003 [Keller, 2003; Schirmeier 2003] corroborates this finding.

The observed domination of events in northern slopes can be explained by the strong effect of 2003 as well as the greater extent of perennially frozen northern slopes.
Modélisations
 
Hypothèses
After the immediate response, such as the one observed during the 2003 summer, a delayed response would take place. The temperature profile within the permafrost would become disturbed and the lower boundary of the permafrost layer will rise (final response), both possibly causing large and deep-seated instabilities delayed by decades or centuries.

Therefore, following the projected rise in mean annual and summer temperatures during the 21st century the locations, magnitudes and frequencies of rock wall instabilities are likely to develop beyond the ranges of historic variability.

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

(4) - Remarques générales

 


(5) - Syntèses et préconisations
 

Références cités :

HARRIS, S. A., and D. E. PEDERSEN, Thermal regimes beneath coarse blocky material. Permafrost Periglacial Processes, 1998, vol 9, p 107–120.

KELLER, F. (2003), Kurzbericht über die Steinschlagereignisse im heissen Sommer 2003 im Bergell (Project report on rock fall 2003 to the Kanton Graubünden), report, Inst. für Tourismus und Landschaft Acad. Engiadina, Samedan, Switzerland.

SCHIRMEIER, Q. (2003), Alpine thaw breaks ice over permafrost’s role, Nature, 424, 712. [Fiche Biblio]