Modelling rock wall permafrost degradation in the Mont Blanc massif from the LIA to the end of the 21st century

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Magnin, Florence ; Josnin, Jean-Yves ; Ravanel, Ludovic ; Pergaud, Julien ; Pohl, Benjamin ; Deline, Philip (2016)

High alpine rock wall permafrost is extremely sensitive to climate change. Its degradation can trigger rock falls constituting an increasing threat to socio-economical activities of highly frequented areas. Understanding of permafrost evolution is therefore crucial. This study investigates the long-term evolution of permafrost in three vertical cross-sections of rock wall sites between 3160 and 4300&thinsp;m&thinsp;a.s.l. in the Mont Blanc massif, since LIA steady-state conditions to 2100. Simulations are forced with air temperature time series, including two contrasted air temperature scenarios for the 21<sup>st</sup> century representing possible lower and upper boundaries of future climate change according to the most recent models and climate change scenarios. The model outputs for the current period (2010&ndash;2015) are evaluated against borehole temperature measurements and an electrical resistivity transect: permafrost conditions are remarkably well represented. Along the past two decades, permafrost has disappeared into the S-exposed faces up to 3300 m a.s.l., and possibly higher. Warm permafrost (i.e. >&thinsp;&minus;2&thinsp;&deg;C) has extended up to 3300 and 3850&thinsp;m&thinsp;a.s.l. in N and S-exposed faces, respectively. Along the 21<sup>st</sup> century, warm permafrost is likely to extent at least up to 4300&thinsp;m&thinsp;a.s.l. into the S-exposed rock walls, and up to 3850&thinsp;m&thinsp;a.s.l. at depth of the N-exposed faces. In the most pessimistic case, permafrost will disappear at depth of the S-exposed rock walls up to 4300&thinsp;m&thinsp;a.s.l., whereas warm permafrost will extend at depth of the N faces up to 3850&thinsp;m&thinsp;a.s.l., but could disappear at such elevation under the influence of a close S face. The results are site-specific and extrapolation to other sites is limited by the imbrication of the local topographical and transient effects. Shorter time-scale changes are not debatable due to limitations in the modelling approaches and future air temperature scenarios.
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