Friction at the bed does not control fast glacier flow

Abstract

The largest uncertainty in the ice sheet models used to predict future sea level rise originates from our limited understanding of processes at the ice/bed interface. Near glacier termini, where basal sliding controls ice flow, most predictive ice sheet models use a parameterization of sliding that has been theoretically derived for glacier flow over a hard bed. We find that this sliding relation does not apply to the 140 Greenland glaciers that we analyzed. There is no relationship between basal sliding and frictional stress at the glacier bed, contrary to theoretical predictions. There is a strong relationship between sliding speed and net pressure at the glacier bed. This latter finding is in agreement with earlier observations of mountain glaciers that have been largely overlooked by the glaciological community.

Regularized Coulomb Friction Laws for Ice Sheet Sliding: Application to Pine Island Glacier, Antarctica

“…Although our study is confined to PIG, it is useful to consider the results in the context of other glaciers. For instance, a recent study suggests that τ b exerts no control on glacier speed, finding instead that u b ~ N q with q ≈ − 0.5 (Stearns & Van der Veen, ). In actuality, what their results show is that τ b is insensitive to u b for fast flowing glaciers in Greenland, which is fully consistent with regularized Coulomb friction.…”

“…The notion that speed depends solely on N is based on several flawed arguments (Stearns & Van der Veen, ). First, in a regression of the speeds of 140 Greenlandic glaciers against τ b , they found no apparent relationship between the two, which was taken to indicate that τ b exerts no control.…”

Future Evolution of Greenland's Marine‐Terminating Outlet Glaciers

“…In part, there is large uncertainty because future human behavior and climate policy is unknown. There also remains a debate about the physics that dominate important processes contributing to dynamic changes, particularly for outlet glaciers that connect to the ocean (Minchew et al, ; Pattyn, ; Stearns & van der Veen, ; Stearns & van der Veen, ; Sutherland et al, ). In addition, while most of the outlet glacier dynamic response occurs in the terminal zone (at the ocean boundary), contemporary terminus change is proven long‐lasting, ultimately affecting the full ice sheet, and is responsible for much of the expected sea‐level rise from Greenland (Felikson et al, ; Howat et al, ; Price et al, ).…”

“…Little direct observation of bed characteristics exists for the GrIS as a whole, with even fewer in outlet glacier regions owing to the difficulty, and expense, in obtaining such observations. The basal regime beneath outlet glaciers is expected to be thawed (MacGregor et al, ), although the exact relationship between bed conditions and ice flow remains elusive (Minchew et al, ; Stearns & van der Veen, ; Stearns & van der Veen, ). Borehole observations for a land‐terminating region near the ice sheet edge confirm the presence of liquid water at the base of the ice sheet at high‐pressure conditions, enabling fast ice motion by reducing effective pressure at the bed (Andrews et al, ).…”

Comment on “Friction at the bed does not control fast glacier flow”

“…S tearns and van der Veen (1) seek to address two important problems in glaciology: understanding the physics of glacier sliding and the parameterization of sliding in ice-flow models. Focusing on fast-flowing Greenland outlet glaciers, the authors combine observations of glacier geometry and surface velocity to infer the shear stress at the bed and the influence of subglacial water pressure on ice flow.…”