Impacts of Surface Mass Balance Uncertainties on Ice Sheet Initialization and Predictions of Sea Level Rise

The initialization of an ice sheet model can involve significant uncertainties that affect projections of sea level rise. Errors in the specification of boundary conditions (bed topography, geothermal fluxes, and surface mass balance) as well as flow physics parameters and parameterizations of bed properties will be amplified and partly compensate each other under the requirement that the model ice sheet resembles the modern ice sheet. Here we quantify uncertainties associated with imperfect knowledge of the pre-industrial surface mass balance using the Community Ice Sheet Model (CISM; Rutt et al. 2009). CISM is a shallow-ice model coupled to global climate within the Community Earth System Model (CESM). It is the only coupled ice sheet-climate model that uses an energy surface mass balance instead of a positive degree day (PDD) scheme. We explore how large uncertainties in surface mass balance must be before they affect future projections of sea level rise and how the calculation of surface mass balance may be adjusted to reduce these uncertainties.