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A New Model to Construct Ice Stream Surface Elevation Profiles and Calculate Contributions to Sea-Level Rise


Sea-level rise is a problem that affects regions worldwide - from the marshlands of the San Francisco Bay Area to the farmlands in coastal Bangladesh. Three-dimensional ice sheet models are the principle tools to evaluate mass loss from ice sheets that contribute to sea-level rise. We recognize that given the current limitations in representing the full extent of dynamical processes that affect ice sheet mass loss in 3-D ice sheet models, we cannot make reliable forecasts of sea-level rise from melting polar land ice. Thus, we take a completely different approach to gaining insight about the potential effects of climate change-induced perturbations on ice sheets.

We build a flowline model that resolves the fast-flowing portions of ice sheets (i.e., ice streams). We express the dynamics along the flowline with (a) vertical shear deformation, (b) horizontal shear deformation, and (c) basal slip. Knowledge accumulated from prior force balance analyses performed on some polar ice streams allows us to form relations between (a) and (c), and between (a) and (c) combined and (b). Based on these relationships, we numerically construct surface elevation profiles along flowlines centered on ten select ice streams in Greenland and Antarctica, by prescribing three climate change-induced perturbations: grounding line retreat, ice stream widening, and surface mass balance increase. Comparing these constructed profiles to the current observed ones allows us to quantify the effect of these perturbations on the various characteristics that these ten ice streams possess.

Pine Island Glacier, which flows over a long overdeepening, will lose more than half of its stored ice volume that is contributable to sea-level rise before it reaches a possible steady state. Recovery Ice Stream, with its slippery base, long stretch of streaming-flow, and longest flowline among those we examined, loses the most mass (812 km3/km width). Jutulstraumen, which has little room to widen and a short stretch of streaming-flow, experiences more mass gain due to surface mass balance increase than mass loss due to grounding line retreat and widening. The broad range of ice streams and their diverse responses to prescribed perturbations is a convincing message that an accurate assessment of the contribution of ice sheets to future sea-level rise can only be obtained by raising the resolution of models to resolve the fast-flowing features and looking at their mass changes individually over time.

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