Role of Model Initialisation for Projections of 21st Century Greenland Ice Sheet Mass Loss

Abstract:

Model simulations of the Greenland ice sheet contribution to 21st-century sea level rise are performed with a state-of-the-art ice sheet model (PISM). The climate forcing fields are obtained from the EU FP7 project ice2sea, in which three regional climate models are used for dynamically downscaling two scenarios (A1B and E1) from two general circulation models (ECHAM5 and HadCM3). To assess the sensitivity of the projections to the model initial state, four initialisation methods are applied. In these experiments, the simulated contribution to sea level rise by the year 2100 ranges from an equivalent of 0.2 to 6.8 cm. The largest uncertainties arise from different formulations of the regional climate models (0.8--3.9 cm) and applied scenarios (0.65--1.9 cm), but an important source of uncertainty is the initialisation method (0.1-0.8 cm). These model simulations do not account for the recently observed acceleration of ice streams and consequent thinning rates, the changing ice discharge that may result from the spatial and temporal variability of ocean forcing, or the feedback occurring between ice sheet elevation changes and climate forcing. Thus the results should be considered the lower limit of Greenland ice sheet contributions to sea level rise, until such processes have been integrated into large-scale ice sheet models.