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Impacts du changement climatique sur l’activité des avalanches dans les Alpes : apports de la dendrogéomorphologie pour la reconstitution spatiotemporelle de l’activité des avalanches dans un contexte de changements environnementaux dans les hautes vallées du Guil et du Rhône

Abstract : For the 20th century, high mountain areas, such as the Alps, have undergone a significant warming with temperature increase twice as much as the global average. Such warming strongly alters the cryosphere components. It induces, for example, a shift from solid to liquid precipitation, more frequent and more intense snowmelt phases or a strong decrease in the amount and duration of snow cover, especially at the elevation of the snowrain transition. In the future, climate models forecast that snow depth will be significantly reduced and that snow properties such as snow stability will be modified. These changes in snow cover characteristics and amounts are expected to induce significant changes in snow avalanches activity. At the same time, afforestation induced by the abandonment of agriculture and grazing, the democratization of winter recreation activities and the urban sprawl on the slopes have strongly modified the mountain landscapes since the mid-18th century as well as the exposition of individuals to snow avalanches. In this context, a precise documentation of past snow avalanche activity is crucial to decipher and to understand the impacts of the undergoing climate warming on the snow avalanche activity. To date, this documentation usually relies on historical chronicles or systematic observations. However, the firsts are often discontinuous and focused on catastrophic events. The seconds are limited to the second half of the 20th century thus precluding a comparison from climatically distinct period. On forested paths, the dendrogeomorphic approach is theoretically a reliable approach complement to historical archives and series of systematic observation to infer past snow avalanche activity. Yet, so far, the robustness of this approach has been poorly questioned and no regional chronology, crucial to disentangle potential interferences between snow avalanche activity, climate fluctuations and socio-economic changes, has been developed in the Alps. In this PhD thesis proposes new methodological frameworks to (1) detect avalanche events from tree-ring series, (2) remove non-stationarities related to the decreasing number of trees over time in the reconstruction and (3) aggregate locals reconstructions in regional chronologies. Based on these development, homogenized multicentennial regional chronologies developed (4) for 10 paths of the Goms Valley (Valais canton, Swiss Alps, 1880-2014) and (5) 11 paths from the Queyras Massif (French Alps, 1560-2016) are confronted to climatic fluctuations and land use changes. At Goms, the absence of clear climatic signal in the regional chronology evidence the interference with local nonstationarities and question the need for a sampling strategy at the regional scale to create a robust chronology. In the Queyras massif, the strong decrease of avalanche activity observed over the 20th century is attributed to global warming and to the afforestation process.
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Adrien Favillier. Impacts du changement climatique sur l’activité des avalanches dans les Alpes : apports de la dendrogéomorphologie pour la reconstitution spatiotemporelle de l’activité des avalanches dans un contexte de changements environnementaux dans les hautes vallées du Guil et du Rhône. Géographie. Université Clermont Auvergne, 2019. Français. ⟨NNT : 2019CLFAL024⟩. ⟨tel-02869391⟩

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