## 2 Morphodynamics of the High Alpine Mountains  
### Glacial Retreat and Associated Glaciological and Geomorphological Processes  
Glaciers currently cover around 2000 km\({}^{2}\) in the Alps [29], compared with 4474 km\({}^{2}\) in 1850 [30]. This considerable loss of surface area since the end of the Little Ice Age (LIA) also affects French glaciers: they occupied 544 km\({}^{2}\) in the Ecrins, Vanoise and Mont-Blanc massifs, compared with 264 km\({}^{2}\) in 2006/09, i.e., a loss of 51% [31]. Between 1900 and 2011, Alpine glaciers lost 45% of their surface area and around 50% of their volume [32]. We are witnessing a \"historically unprecedented global glacier decline in the early 21 st century\" [33] worldwide and in the Alps in particular. This acceleration in glacier retreat is the result of a now very negative glacier mass balance in the French Alps [34] and in the Alps as a whole [30].  
Glacial retreat can be accompanied by disturbances in the flow, stability, thermal regime, and drainage of glaciers and can modify the conditions of the slopes surrounding them [35]. When the front of glaciers with a temperate base (i.e., with a temperature close to the melting point) dominates a slope break following its retreat, serac avalanches are released [36; 37]. The fronts of cold-based glaciers, on the other hand, are stuck to the bedrock due to the presence of permafrost at high altitudes. Ice avalanches from cold-based glaciers are the result of a mechanical break caused by an accumulation far greater than the ablation [23; 38]. The current rise in air temperature is causing the temperature of these glaciers to rise. For example, it rose by 1.5 \({}^{\circ}\)C at a depth of 60 m (the neve/ice boundary) on the Col du Dome (4245 m a.s.l.) in the MBM between 1994 and 2010 [39]. While the temperature of the ice beneath the hanging front of the Taconnaz glacier (3415 m a.s.l.) was still between \(-4\) and \(-3\)\({}^{\circ}\)C from a depth of 40 to 80 m in 2012, the stability of this warming glacier is under threat over the next few decades [24]. The changes in topography, thermal regime and glacial drainage associated with glacial retreat can create intra-glacial pockets of water and pro-, juxta- or supra-glacial lakes that can possibly drain rapidly [40; 41].  
Finally, the loss of glacier thickness is part of the paraglacial dynamic that has been underway since the end of the LIA and which has accelerated since the 1990s. The reduction or even disappearance of the mechanical support provided by the ice to a rockface or moraine, combined with changes in their thermal regime, can lead to their destabilisation at highly variable timescales, resulting in deformations, collapses or landslides [35]. For example, the 166 m post-LIA lowering in the surface of the Pilatte glacier (Ecrins massif) and its acceleration since the 2000s have destabilised a volume of 400,000 m\({}^{3}\) in the rocky ridge on which the refuge is built, which has gradually cracked since 1985 [42]. As for moraines destabilised by glacial retreat, there are many examples of this in the MBM (e.g., Bionnassay, Tacul, Mer de Glace, Miage, Brenva) and elsewhere in the Alps, sometimes with recurrent episodes of remarkable dimensions (several thousand of m\({}^{3}\)), as on the Belvedere glacier [35], or combined with the permafrost degradation, as on the Tortin glacier in Valais Alps [43].