Disappearing glaciers and emerging landscapes: new opportunities or rising risks?

Global warming has a major impact on the cryosphere. Mountains worldwide are witnessing the inexorable loss of glaciers, the degradation of permafrost, and a growing frequency of droughts and extreme precipitation events. Whereas many eyes are pointed to the loss of beauty, biodiversity, and freshwater resources, as well as to the catastrophic collapses of mountain slopes around the globe, only little attention is deserved to the new emerging post-glacial landscapes. A narrow belt of land remains bare for the time required to vegetation to adapt and migrate to higher elevations. This bare land is often characterized by fine sand and big boulders, unconsolidated debris, and over-steepened slopes that easily become unstable and can generate large amount of sediment which, if mobilized, may threaten the downstream valleys.

The paraglacial adjustment may require hundreds of years to reach equilibrium. In the classical model, the peak of sediment yield is expected to come immediately after the onset of the deglaciation, to then gradually decline towards a long-term equilibrium. Yet, climatic or anthropogenic perturbations can significantly modify the expected decline in sediment yield over time. Increased frequency of extreme precipitation events, associated with shifts in snowfall cover and with the increase of unconsolidated sediment, may create unprecedented conditions where enormous amount of sediment may be available to be mobilized and transported to the valley bottoms. Quantifying these volumes, and especially understanding the peaks around the expected sediment-transport curve, is fundamental for the communities living in high mountain areas, for river system management, and for the mitigation of risks associated with debris-flood events. In this context, it becomes essential to understand: 1) where are we along the paraglacial adjustment curve, and especially, 2) what frequency and intensity should be expected for the climate-induced peaks in sediment yield.

In the Sulden/Solda catchment (South Tirol, Italy) ongoing investigations aim to address these questions. New cosmogenic data allows to estimate average sediment production over centennial timescales, whereas modern digital elevation models allow to quantify recent average values. Interestingly, preliminary data indicates that long-term and modern averages are very similar, rising a new set of questions rather than answering those posed above. Are we still on the rising limb of the paraglacial curve? Or did sediment yield decline and are we witnessing the effects of global warming? And how do extreme precipitation events enter in the picture? Hopefully, by May, some of these questions will have been answered, and it will be a pleasure to discuss them at EGU.