Diverging runoff drives uncertainty in Antarctic surface mass balance projections under a high emission scenario

Abstract. Three recent downscalings of CESM with MAR, RACMO, and HIRHAM under SSP5-8.5 produce consistent contemporary Antarctic surface mass balance (SMB), but diverge strongly by 2100, especially over ice shelves. HIRHAM simulates a large SMB decline driven by strong runoff increases, MAR a moderate decrease, while RACMO maintains near balance. These differences mainly reflect contrasting melt–albedo feedbacks, present-day melt and refreezing levels, and a persistent 1–2 °C temperature offset between MAR and RACMO. CESM shows a decline similar in magnitude to MAR, with high melt partly compensated by extensive refreezing. Over grounded ice, all models project increased SMB from higher snowfall, though runoff still drives their spread. Despite shared boundary conditions and similar contemporary SMB, model behavior diverges, and CESM’s integrated results resemble MAR's despite its coarse resolution. Performance differences in present-day melt suggest that uncertainty estimates should account for model skill, motivating Bayesian treatment of multi-model ensembles.