Spread in flow law parameters creates a large uncertainty in ice loss in flowline setup and simulations of the Greenland Ice Sheet

Abstract:

Acceleration of the flow of ice drives mass losses in both the Antarctic and the Greenland Ice Sheet. The projections of possible future sea-level rise rely on numerical ice-sheet models, which solve the physics of ice flow, melt, and calving. While major advancements have been made by the ice-sheet modeling community in addressing several of the related uncertainties, the flow law, which is at the center of most process-based ice-sheet models, is not in the focus of the current scientific debate. However, recent studies show that the flow law parameters are highly uncertain and might be different from the widely accepted standard values.

Here, we use an idealized flowline setup to investigate how these uncertainties in the flow law translate into uncertainties in flow-driven mass loss. In order to disentangle the effect of future warming on the ice flow from other effects, we perform a suite of experiments with the Parallel Ice Sheet Model (PISM), deliberately excluding changes in the surface mass balance. We find that changes in the flow parameters within the observed range can lead up to a doubling of the flow-driven mass loss within the first centuries of warming, compared to standard parameters. The spread of ice loss due to the uncertainty in flow parameters is of the same order of magnitude as the increase in mass loss due to surface warming. It is likely that this uncertainty carries over to realistic three-dimensional simulations of Greenland and Antarctica. In simulations of the Greenland Ice Sheet we find that the uncertainty in the flow-law parameters alone can lead up to a threefold increase in flow-driven ice loss within this century.

These results suggest, that the uncertainty of the flow law should be considered in sea-level projections until the parameters are better constrained.