Uncertainties in forecast surface mass balance outweigh uncertainties in basal sliding descriptions for 21st Century mass loss from three major Greenland outlet glaciers

Abstract. The Greenland Ice Sheet contributed 10.6 mm to global sea level rise between 1992 and 2018 and is projected to be the largest glacial contributor to sea level rise by 2100. Here, we assess the relative importance of two major sources of uncertainty in 21st century ice loss projections: 1) the choice of sliding law and 2) the surface mass balance (SMB) forecast. Specifically, we used the ice flow model Úa to conduct an ensemble of runs for 48 combinations of sliding law and SMB forecast for three major Greenland outlet glaciers with differing characteristics (Kangerlussuaq [KG], Humboldt [HU] and Petermann [PG] glaciers) and evaluated how the importance of these uncertainties varied between the study glaciers. Overall, our results show that SMB forecasts were responsible for 4.45 mm of the variability in sea level rise by 2100, compared to 0.33 mm SLE due to sliding law. HU had the largest absolute contribution to sea level rise and the largest range (2.16 to 7.96 mm SLE), followed by PG (0.84 and 5.42 mm SLE), and these glaciers showed similar patterns of ice loss across the SMB forecasts and sliding laws. KG had the lowest range and absolute values (-0.60 to 3.45 mm SLE) of sea level rise and the magnitude of mass loss by SMB forecast differed markedly from HU and PG. Our results highlight SMB forecasts as a key focus for improving estimates of Greenland’s contribution to 21st century sea level rise.