Quantifying the uncertainty in Greenland’s contribution to sea level rise due to the bed topography

Uncertainty in the future contribution of the ice sheets to sea level rise associated with different climate forcings has been well studied in the most recent Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). Similarly, the uncertainty due to differences in basal sliding laws or calving laws has also been thoroughly investigated. However, the uncertainty due to the bed topography has not yet been rigorously quantified. The majority of the models within the ISMIP6 ensemble use a single bed topography map, BedMachine Greenland, thereby hindering our ability to better understand how uncertainties in the bed topography affect the overall uncertainty in future projections of sea level rise. To address this, we follow the methodology from Castleman et al. (2022) and create an ensemble of 32 bed topography maps with realistic bed roughness by perturbing the BedMachine bed topography using discrete wavelet decomposition techniques. We update the initial bed topography in ice sheet models from Choi et al., (2021), which provide Greenland-wide, high-resolution, data constrained projections that include calving dynamics, and run projections out to 2300. Though we expect northwest and central west Greenland glaciers to contribute more to sea level rise than other glaciers, we find that models initialized with BedMachine bed topographies tend to overestimate mass loss in these regions. We also find that the addition of bed roughness reduces the future contribution of the ice sheet to sea level rise over the 21st century, but to a lesser extent than the deep, wide subglacial basins of Antarctica. Lastly, we also determine that the uncertainty in the future contribution of the Greenland Ice Sheet due to different climate forcings and the uncertainty due to the bed topography are comparable at the end of this century, however the uncertainty due to climate forcings dominates in the long run.