Investigating the surface mass balance of the Laurentide Ice Sheet during the last deglaciation

Abstract. In spite of decades of research, the role of climate feedbacks in the Pleistocene glacial cycles is still not fully understood. Here, we calculate the surface mass balance (SMB) of the Laurentide Ice Sheet (LIS) throughout the last deglaciation using the isotope-enabled transient climate model experiment (iTraCE). A surface energy balance framework is used to calculate yearly melt, and a parameterization of the refreezing of snow melt and liquid precipitation is incorporated. The SMB calculated from iTraCE overestimates the total ice mass loss rate in comparison to the ICE-6G reconstruction from the Last Glacial Maximum (LGM; 21 ka) until about 15–14 ka; subsequently, the fully forced climate model experiment better fits the ICE-6G ice volume loss rate. We find the melt rate for the LIS to be primarily set by the small residual of large net shortwave and longwave radiative fluxes. The melt, and hence the SMB, are very sensitive to small changes in the albedo and downwelling longwave radiation. By increasing albedo by a mere 1.9 % or by decreasing downwelling longwave radiation by only 1.45 % (well within the uncertainty range of these variables), the large overestimation of the rate of mass loss deduced from the SMB compared to reconstructed rates of mass loss from 19–15 ka can be eliminated. The inconsistency of the climate model-derived, offline SMB calculation and the ice mass reconstructions exists irrespective of the role of ablation caused by ice flow, which cannot be calculated using this analysis. The extreme sensitivity of the melt rate suggests that General Circulation Models (GCMs) still struggle to reliably calculate the SMB, presenting a significant roadblock in our attempt to understand the Pleistocene ice ages.