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.

Read more: Gimbert F., A. Gilbert, O.  Gagliardini, C.  Vincent and L. Moreau, 2021.  Do existing theories explain seasonal to multi-decadal changes in glacier basal sliding speed? Geophysical Research Letters,  48, e2021GL092858. https://doi.org/10.1029/2021GL092858