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The Cryosphere An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/tc-2016-132
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
07 Jul 2016
Review status
A revision of this discussion paper was accepted for the journal The Cryosphere (TC) and is expected to appear here in due course.
Modelling rock wall permafrost degradation in the Mont Blanc massif from the LIA to the end of the 21st century
Florence Magnin1, Jean-Yves Josnin1, Ludovic Ravanel1, Julien Pergaud2, Benjamin Pohl2, and Philip Deline1 1EDYTEM Lab, Université Savoie Mont Blanc, CNRS, 73376 Le Bourget du Lac, France
2Centre de Recherches de Climatologie, Biogéosciences, Université de Bourgogne Franche-Comté, CNRS, Dijon, France
Abstract. 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 m 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 21st 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 (2010–2015) 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. > −2 °C) has extended up to 3300 and 3850 m a.s.l. in N and S-exposed faces, respectively. Along the 21st century, warm permafrost is likely to extent at least up to 4300 m a.s.l. into the S-exposed rock walls, and up to 3850 m 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 m a.s.l., whereas warm permafrost will extend at depth of the N faces up to 3850 m 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.

Citation: Magnin, F., Josnin, J.-Y., Ravanel, L., Pergaud, J., Pohl, B., and Deline, P.: Modelling rock wall permafrost degradation in the Mont Blanc massif from the LIA to the end of the 21st century, The Cryosphere Discuss., https://doi.org/10.5194/tc-2016-132, in review, 2016.
Florence Magnin et al.
Florence Magnin et al.

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Short summary
Permafrost degradation in high mountain rock walls provokes their destabilisation, constituting a threat for human activities. In the Mont Blanc massif, more than 700 rock falls have been inventoried in the recent years (2003, 2007–2015). Understanding of permafrost evolution is thus crucial to sustain this densely populated area. This study investigates the changes in rock wall permafrost from 1850 to the recent period, and possible optimistic or pessimistic evolutions during the 21st century.
Permafrost degradation in high mountain rock walls provokes their destabilisation, constituting...
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