| Summary: | Understanding grounding line dynamics is critical for projecting glacier evolution and sea level rise. Recent observations from satellite radar interferometry reveal rapid grounding line migration forced by oceanic tides that are several kilometers larger than predicted by hydrostatic equilibrium alone, indicating that the transition from grounded to floating ice is more complex than previously thought. Recent studies suggest that seawater intrusions beneath grounded ice may play a role in glacier dynamics. Here, we investigate their impact on the evolution of Petermann Glacier, Greenland, using an ice sheet model. We compare the model results with observed changes in grounding line position, velocity, and ice elevation between 2010 and 2022. If we exclude seawater intrusions, the model requires anomalously high melt rates to replicate the retreat. Conversely, we match the observed retreat with 3-km-long seawater intrusions with a maximum ice shelf melt rate of 50~m/yr, consistent with observations. We also obtain more realistic glacier speedup and ice thinning when including seawater intrusions in the model. We conclude that seawater intrusions play a critical role in the dynamics of Petermann Glacier. Including them in glacier flow models will make glaciers more sensitive to ocean warming and increase projections of sea level rise. Funding provided by: National Aeronautics and Space Administration ROR ID: https://ror.org/027ka1x80 Award Number: 80NSSC20K1618
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