Observations

The longest, high-resolution time series of borehole temperatures is available from ice-rich, slowly creeping permafrost with an active layer consisting of coarse blocks in the active Murtel rock glacier (Fig. 14.4). The overall trend observed since 1987 is permafrost warming by about 0.4°C per decade at 10 m depth, and roughly twice as much for the summer temperatures in the active layer. Winter temperatures strongly depend on winter snow conditions rather than on atmospheric temperatures alone. As a consequence, permafrost temperatures remained stable or

0 m 20 TO 40 to 60 to

Fig. 14.3 Tomographies derived from repeated (24 August and 8 September 2006) P-wave refraction seismics on the E—W trending rock crest "Steintalli" (3,150 m above sea level) between Matter- and Turtmann Valleys, Switzerland. Dark red colours correspond to partly frozen rock sections, purple mostly to the deeply frozen permafrost core without residual water in pores. It appears that in delayed response to cool August temperatures, the frozen rock core develops towards the north face and cools inside. Simultaneously, the snow cornice on top (20-30 m) melts and gives way due to thermal heat conduction from the surface (source: M. Krautblatter 2007)

Fig. 14.3 Tomographies derived from repeated (24 August and 8 September 2006) P-wave refraction seismics on the E—W trending rock crest "Steintalli" (3,150 m above sea level) between Matter- and Turtmann Valleys, Switzerland. Dark red colours correspond to partly frozen rock sections, purple mostly to the deeply frozen permafrost core without residual water in pores. It appears that in delayed response to cool August temperatures, the frozen rock core develops towards the north face and cools inside. Simultaneously, the snow cornice on top (20-30 m) melts and gives way due to thermal heat conduction from the surface (source: M. Krautblatter 2007)

even decreased during the past decade with above-average high winter air temperatures but relatively thin snow cover. This example clearly illustrates the complexity of the atmosphere/permafrost coupling under such — quite characteristic — highmountain conditions: snow as a "nervous", hardly predictable interface will continue to cause large uncertainties about future developments in such cases (Lutschg et al. 2003; Lutschg and Haeberli 2005), making continued monitoring indispensable with regard to improved future knowledge.

The PACE borehole temperatures exhibit clear indications of a century-long warming of permafrost within bedrock; however, conclusions can only be drawn on the basis of 4D modelling. Nevertheless, preliminary interpretation of the documented thermal anomalies with respect to an assumed steady-state profile in homogenous

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Year (December)

Fig. 14.4 Borehole temperatures at a depth of 11.6 m in the ice-rich permafrost of the active rock glacier Murtèl/Corvatsch, Grisons Alps, Switzerland. The overall trend is ground warming by about 0.4°C per decade, but inter-annual variations are large and snow-cover effects important

Fig. 14.5 Borehole temperatures and recent warming. a Ground temperature profiles in permafrost at Janssonhaugen (Svalbard, 102 m deep, 78°10'N, 16°28'E, 270 m above sea level), Tarfalaryggen (Sweden, 100 m deep, 67°55'N, 18°38'E, 1550 m a.s.l.), and Juvvassh0e (Norway, 129 m deep, 61°40'N, 08°22'E, 1894 m a.s.l.), recorded on 22 April 2005. b Profiles of reduced temperature anomalies; data are obtained by subtracting temperatures for assumed steady state conditions from measured temperatures for depths at which annual fluctuations are negligible. Steady-state temperatures were estimated by extrapolating the thermal gradient measured in the lowermost part of the borehole, which is assumed to be unaffected by recent warming trends. Reproduced from Isaksen et al. (2007)

Fig. 14.5 Borehole temperatures and recent warming. a Ground temperature profiles in permafrost at Janssonhaugen (Svalbard, 102 m deep, 78°10'N, 16°28'E, 270 m above sea level), Tarfalaryggen (Sweden, 100 m deep, 67°55'N, 18°38'E, 1550 m a.s.l.), and Juvvassh0e (Norway, 129 m deep, 61°40'N, 08°22'E, 1894 m a.s.l.), recorded on 22 April 2005. b Profiles of reduced temperature anomalies; data are obtained by subtracting temperatures for assumed steady state conditions from measured temperatures for depths at which annual fluctuations are negligible. Steady-state temperatures were estimated by extrapolating the thermal gradient measured in the lowermost part of the borehole, which is assumed to be unaffected by recent warming trends. Reproduced from Isaksen et al. (2007)

bedrock of simplified half-space geometry (Harris and Haeberli 2003, Harris et al. 2003) together with first detailed analyses of time-dependent temperature changes at depth leave little doubt that temperature rise in mountain permafrost over the past century has taken place at a continental scale and at a rate which is comparable to atmospheric warming ca. (0.5 to 1.5°C per century), creating a marked thermal anomaly

Fig. 14.6 Development in surface geometry and crevasse patterns due to strongly accelerated flow of an active rock glacier in the Turtmann Valley, Swiss Alps. Orthoimages of 20 August1975, 20 August 1993 (aerial photographs taken by Swisstopo) and 28 September 2001 (HRSC-A survey). From Roer (2007)

down to depths of 50-70m (Figure 15.6; Isaksen et al. 2007) which will continue to penetrate to greater depths. With the record warm winter 2006/2007, the outer parts of steep walls mantling mountain peaks in the European Alps may, in fact, have heated up to levels without precedent during the past millennia since at least the Upper Holocene.

Continued observation is also necessary to better understand the striking large-scale phenomenon of recently accelerated permafrost creep (Kaab et al. 2006; Delaloye et al. 2008), with in places the formation of deep crevasses indicating destabilization of large volumes of ice-rich debris (Fig. 14.6; Roer 2007).

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