Do Existing Theories Explain Seasonal to Multi‐Decadal Changes in Glacier Basal Sliding Speed?. Issue 15 (9th August 2021) - Gimbert, F.
Abstract: Theoretical descriptions of ice‐bed friction beneath glaciers and ice sheets are key to predict changes in sea level. Applicability of these theories at the natural landscape scale and over long periods has however not been tested. Here we test hard bed friction laws by analyzing a unique data set of in‐situ basal sliding measurements collected over three decades under an Alpine glacier enduring large changes in geometry. We report many observational features that are in striking agreement with theoretical predictions. However, we also observe an undocumented behavior where the basal stress state stabilizes near Iken's limit under meltwater input, which suggests the basal effective pressure is primarily set by bed shear stress rather than by water input and drainage specifics as commonly thought. As a result, long‐term changes in year‐averaged sliding velocities follow a simple power law scaling with bed shear stress as opposed to more complex pressure‐dependent relationships. Plain Language Summary: Basal sliding is an important component of glacier motion. However, our knowledge of the physics that controls basal sliding is incomplete. This causes large uncertainties in the contribution to sea‐level rise predicted for ice sheets over the coming century. Here, we test our understanding of basal sliding against particularly unique observations, made via a rotating bicycle wheel that has been continuously measuring glacier basal motion over three decades within excavated tunnels under the Argentière Glacier in the French Alps. Due to stress changes from significant glacier thinning over the multi‐decadal period we are able to establish an observationally derived sliding law and compare it with expectations from theory. We report many observational features that are in striking agreement with theoretical predictions from glacier sliding over bedrock beds. However, we also observe an undocumented behavior of stress stabilization during the melting period at a specific stress state known as the Iken's limit. This behavior causes long term sliding velocities to follow a simple power law scaling with bed shear stress. This finding has the potential of strongly simplifying and reducing uncertainty on predicting glaciers response to climate change. Key Points: We use unique basal sliding observations collected over three decades to test basal friction theories The observations show striking agreement with theory, although we report an unexpected stress stabilization near Iken's limit Stress stabilization causes basal effective pressure to scale with bed shear stress, and long‐term sliding speeds to follow a simple law