Sources of uncertainty in Greenland surface mass balance in the 21st century

The surface mass balance (SMB) of the Greenland ice sheet is subject to considerable uncertainties that complicate predictions of sea level rise caused by climate change. We examine the SMB of the Greenland ice sheet in the 21st century with the Bergen Snow Simulator (BESSI) surface energy and mass balance model. To estimate the uncertainty of the SMB, we conduct simulations for four greenhouse gas emission scenarios using the output of a wide range of Earth system models (ESMs) from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to force BESSI. In addition, the uncertainty of the SMB simulation is estimated by using 16 different parameter sets in our SMB model. The median SMB across ESMs and parameter sets, integrated over the ice sheet, decreases over time for every emission scenario. As expected, the decrease in SMB is stronger for higher greenhouse gas emissions. The regional distribution of the resulting SMB shows the most substantial SMB decrease in western Greenland for all ESMs, whereas the differences between the ESMs are most pronounced in the north and around the equilibrium line. Temperature and precipitation are the input variables of the snow model that have the largest influence on the SMB and the largest differences between ESMs. In our ensemble, the range of uncertainty in the SMB is greater than in previous studies that used fewer ESMs as forcing. An analysis of the different sources of uncertainty shows that the uncertainty caused by the different ESMs for a given scenario is larger than the uncertainty caused by the climate scenarios. In comparison, the uncertainty caused by the snow model parameters is negligible, leaving the uncertainty of the ESMs as the main reason for SMB uncertainty.