Seawater Intrusion in the Observed Grounding Zone of Petermann Glacier Causes Extensive Retreat.

Understanding grounding line dynamics is critical for projecting glacier evolution and sea level rise. Observations from satellite radar interferometry reveal rapid grounding line migration forced by oceanic tides that are several kilometers larger than predicted by hydrostatic equilibrium, indicating the transition from grounded to floating ice is more complex than previously thought. Recent studies suggest seawater intrusion beneath grounded ice may play a role in driving rapid ice loss. Here, we investigate its impact on the evolution of Petermann Glacier, Greenland, using an ice sheet model. We compare model results with observed changes in grounding line position, velocity, and ice elevation between 2010 and 2022. We match the observed retreat, speed up, and thinning using 3‐km‐long seawater intrusion that drive peak ice melt rates of 50 m/yr; but we cannot obtain the same agreement without seawater intrusion. Including seawater intrusion in glacier modeling will increase the sensitivity to ocean warming. Plain Language Summary: Relatively warm seawater melts marine‐terminating glaciers from below. Recent observations suggest that seawater flows below grounded ice at high tide. The presence of seawater at this boundary, referred to as seawater intrusion, has the potential to increase glacier mass loss. We test this hypothesis on Petermann Glacier, Greenland, using an ice sheet flow model. We run the model to reconstruct the glacier's behavior from 2010 to 2022 with and without seawater intrusion. We compare the results with satellite observations of velocity, grounding line position, and ice thinning. When we use enhanced ice melt rates from kilometer‐scale seawater intrusion, we match the observed retreat, speed up, and thinning. When we do not, the model fails to replicate the observations. Seawater intrusion may play a critical role in glacier evolution. Adding this process to ice flow models will increase their sensitivity to ocean warming and projections of ice mass loss and sea level rise. Key Points: Ice melt caused by seawater intrusion in the grounding zone explains the observed grounding line retreat of Petermann GlacierWithout seawater intrusion in the grounding zone, we do not replicate the full extent of observed retreatIncluding seawater intrusion in the grounding zone increases glacier mass loss [ABSTRACT FROM AUTHOR]