Insights from an ensemble inverse method to quantify basal friction uncertainties for ice-sheet models - Part 1: Greenland ice sheet.

Basal friction is one of the most uncertain processes in ice-sheet models used to project future sea level rise, yet friction parameters remain poorly constrained by observations. Here, we present the Greenland Ice Sheet Ensemble Friction Inversion (GrISEFI), a dataset quantifying basal shear stress and its uncertainty at the Greenland scale using an ensemble approach to inverse methods. This ensemble accounts for uncertainties in ice rheology, friction exponents, regularization parameters, and velocity observations, generating 40 plausible friction states rather than a single deterministic solution. Our spatially-resolved basal shear stress fields reveal systematic patterns controlled by bed geometry and surface mass balance: low values occur within ablation area for bed below sea level, while elevated values characterize regions upstream of the melt limit. Analysis of flow-lines and cross-sections of different Greenland Ice sheet glaciers (land-terminating, tidewater, and ice-shelf glaciers) demonstrates that spatial variations in basal shear stress are primarily controlled by effective pressure changes, validating the importance of subglacial hydrology in friction parameterization. We show that effective pressure parameterizations based on distance from the terminus and melt onset better capture tidewater glacier behavior than commonly-used height-above-flotation approaches. Constraining friction parameters through our ensemble, we find that basal shear stress exhibits substantially weaker velocity dependence than commonly assumed in ice-sheet models, challenging widespread modeling assumptions. These findings have direct implications for how models represent the coupling between basal processes and ice dynamics in future sea level projections.