Modeling the 21st-century response of Greenland's Zachariæ Isstrøm and Nioghalvfjerdsfjorden glaciers to atmosphere–ocean forcing and friction laws

Recent rapid retreat of outlet glaciers in Northeast Greenland has raised concerns over their future stability and sea-level-rise potential. However, large uncertainties remain in long-term projections due to limited constraints on basal friction and external forcings. Here, we use the Ice Sheet System Model (ISSM) to simulate the century-scale evolution (2018–2100) of Zachariæ Isstrøm (ZI) and Nioghalvfjerdsfjorden (79N) glaciers under a suite of atmosphere–ocean forcing scenarios, with a systematic evaluation of how different basal friction laws and parameterizations influence ice-dynamic responses. ZI Glacier exhibits limited retreat (20–25 km) under the most extreme ocean forcing (frontal melt = 6 m/d; basal melt = 90–0 m per year from 450 to 100 m b.s.l.) and remains pinned on a subglacial ridge about 25 km inland from its 2018 terminus, even under the strongest climate scenarios (MIROC5_RCP8.5, MPI-ESM_SSP5-8.5). By contrast, 79N Glacier experiences more sustained ice-shelf thinning and grounding-line retreat, with local thickness reductions approaching 200 m in some experiments. Both frontal sides of the 79N ice shelf show partial detachment even under low-emission scenarios (MIROC5_RCP2.6, MPI-ESM_SSP1-2.6), indicating an elevated risk of structural collapse. Among the tested forcings, atmospheric warming produces the largest total ice-mass loss, yielding a combined sea-level contribution of ∼3 mm by 2100 under high-emission scenarios, while ocean thermal forcing and basal friction exert comparatively smaller effects. Despite this dominant atmospheric influence, our results highlight that grounding-zone basal conditions critically modulate glacier sensitivity and remain a major source of uncertainty in Greenland's sea-level-rise projections.