Surface Expression of Low Basal Friction Upstream of Antarctic Grounding Lines

Ice sheets leave contact with the bed at grounding lines, beyond which floating ice shelves experience no friction at their base. In places where basal friction begins to decrease upstream of the grounding line, ice sheets respond more strongly to climate forcing. However, the spatial extent of zones of low grounding line friction is poorly constrained by observations. Here, we use a steady-state model of marine-terminating ice stream flow to show that the location where basal friction begins to weaken upstream of the grounding line is accompanied by a prominent surface slope break. We then use observations of grounding zone features around the Antarctic Ice Sheet derived from ICESat-2 laser altimetry to find the displacement between grounding line locations determined from SAR flexure measurements and such surface slope break points. We find widespread evidence of decreasing friction hundreds to thousands of meters upstream of grounding lines around the Antarctic Ice Sheet, indicating that grounding lines may be more sensitive to forcing than typically assumed in ice sheet models where friction does not decrease upstream of the grounding line. We suggest that such an observational approach should be used to parameterize grounding line friction interpolation schemes in ice sheet models.