Doctoral thesis

The evolution of mountain permafrost in the context of climate change : towards a comprehensive analysis of permafrost monitoring data from the Swiss Alps


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Thèse de doctorat: Université de Fribourg, 2015

English In the Swiss Alps, permafrost occurs discontinuously and commonly has a temperature close to 0 °C. A reduction of Alpine permafrost area and volume is expected in the course of atmospheric warming, but to date, limited evidence is available for Alpine permafrost degradation. Permafrost warming or thaw is accompanied by structural changes in the subsurface, which endanger infrastructure by increasing kinematic activity or slope instability. Changes in the permafrost impact sediment transport to the valley bottom as well as gravitational natural hazards such as rock falls, landslides or debris flows. For these reasons, the quantitative analysis of past and potential future changes in the Alpine permafrost is of great interest and importance. The objective of this PhD project was to investigate observational data from the Swiss Permafrost Monitoring Network PERMOS using an interdisciplinary approach and to develop new methods for the homogenisation and quantitative analysis of long-term monitoring data. The main focus was on assessing changes in the energy fluxes at the ground surface as a function of the snow cover, as well as on evaluating permafrost response to different meteorological conditions and events. This PhD project was part of the research project The Evolution of Mountain Permafrost in Switzerland (TEMPS, 2011-2015), which used combined observational and model-based approaches and aimed at improving the consistency and completeness of permafrost monitoring data. One achievement of this PhD thesis consists of the development of data processing algorithms for filling data gaps in temperature time series and the quantification of resulting uncertainties. Moreover, algorithms for the approximation of the thermal insulation effect of the snow cover based on ground surface temperature (GST) data were developed. This was of particular importance because snow information is usually not available for the points of interest. Furthermore, possibilities for estimating temperature variations at depth based on GST data were evaluated. The information obtained about the propagation of the thermal signal into the ground led to new insights into the temperature dependency of rock glacier creep, which were supported by observational data. Data from more than 20 study sites were made comparable in order to quantify differences at the site- and the regional scale. The GST variability proved to be almost as high at the site scale as at the regional scale. This was explained by heterogeneous topo-climatic conditions as well as by the variable snow cover in the geographic context of the Swiss Alps. The roughness of the terrain played a key role, since it modifies the thermal insulation effect of the snow. Coarse-blocky terrains require more snow to be thermally insulated from the atmosphere and freeze more rapidly compared to smooth ground surfaces. The seasonal GST pattern showed that differences among sites and years were large in early winter, whereas GST were less variable in the summer season. Many locations showed similar snow conditions and therefore similar seasonal and inter-annual GST variations, which could not be explained by variations in air temperature. Although no overall increase in GST was found, the data indicate persistent warm conditions at the ground surface since 2009. Ground temperatures (GT) experienced an overall warming trend down to several tens of m depth over the past 10-25 years. This warming was most distinct in relatively cold permafrost with temperatures below -1 °C. Since the GT at depths between 10-30 m influences the kinematic activity of rock glaciers, the surface deformation rates of the majority of the observed rock glaciers reached maxima between 2013 and 2015. Surface deformation rates quantified by photogrammetry for selected rock glaciers showed an increase in the order of 200-600 % compared to 1990-1995 and 400-800 % compared to 1960-1980. Long-lasting warm conditions at the ground surface were identified to be the cause of the rise in ground temperature and the increased kinematic activity of rock glaciers. Compared with air temperature, where direct effect on the ground is limited to the snow-free period, the snow cover and its onset in early winter had a much greater influence on the heat and energy exchange at the ground surface. After one or two snow-poor winters, permafrost was able to regenerate thermally. Strong ground cooling occurred between 2005 and 2007, which caused a temporary trend reversal in the warming ground temperatures, limiting the effect of the particularly warm air temperatures between June 2006 and May 2007. Since Alpine permafrost is not in equilibrium with the current climatic conditions, recovery periods of efficient winter cooling will probably play a key role for its future evolution and preservation. Overall, the results of this PhD project contribute to an improved process understanding and put observed ground thermal and kinematic phenomena in the context of past and potential future changes of permafrost in the Swiss Alps.
Faculté des sciences et de médecine
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