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The choice of the best basal friction law to use in ice-sheet models remains a source of uncertainty in projections of sea level. The parameters in commonly used friction laws can produce a broad range of behavior and are poorly constrained. Here we use a time series of elevation and speed data to examine the simulated transient response of Pine Island Glacier, Antarctica, to a loss of basal traction as its grounding line retreats. We evaluate a variety of friction laws, which produces a diversity of responses, to determine which best reproduces the observed speedup when forced with the observed thinning. Forms of the commonly used power law friction provide much larger model-data disagreement than less commonly used regularized Coulomb friction in which cavitation effects yield an upper bound on basal friction. Thus, adoption of such friction laws could substantially improve the fidelity of large-scale simulations to determine future sea level.

Plain Language Summary Although much effort has gone into improving numerical simulations of ice-sheet behavior for sea-level projection, the best choice for the friction law that governs the sliding of ice over its bed remains poorly known. As a consequence, a wide range of friction laws is used by the ice-sheet modeling community. Here we take advantage of several remotely sensed data sets to model the changes in speed of Pine Island Glacier, Antarctica, over nearly two decades, using a variety of existing friction laws. We find that the behavior of this glacier is simulated far more faithfully relative to observations when a "regularized Coulomb" friction law is used. This type of friction law accounts for the formation of pressurized basal water pockets (cavitation) that occur when ice slides rapidly over its bed, which limits friction in a way that may amplify ice-sheet instability. By contrast, low-order exponents in more traditional power law friction parameterizations yield unbounded basal friction as speed increases, placing a much stronger brake on unstable ice-sheet behavior. Thus, these results make a strong case for adopting bounded regularized Coulomb friction laws in place of the more commonly used unbounded friction laws.