Coupling an ice sheet model to EC-Earth

Abstract

The evolution of ice sheets is determined by climate and the climate is affected by the presence of the ice sheet. For climate models, this means that a coupling of ice sheets and climate is needed in order to understand both. However, the difference in time scales in which both evolve poses a difficulty for coupling the two. Ice sheets have a response time of millenia, while climate models typically assess climate problems on a much shorter scale. Usually, this issue is solved by prescribing the ice sheets in climate models, but on intermediate timescales it is not a priori obvious whether ice sheet coupling can be neglected.

In this project, a one-way coupling from the ESM EC-Earth to the GRICE Greenland ice sheet model is achieved. As surface mass balance (SMB) is not available as output from EC-Earth data, a method of calculating the SMB from available EC-Earth output data is presented. Here, the temperature and precipitation fields from EC-Earth, which were obtained from a historic run (1850-2000) and the RCP4.5 radiative forcing scenario (2000-2100) are used to force the SMB over the period 1850-2100. For a past-glacial climate forcing required to spin-up the model, a global temperature record was used as a perturbation.

SMB is calculated by separating between accumulation and run-off, using EC-Earth data for accumulation and parameterizing the run-off as a height-dependent perturbation of a reference field, using a modification of the SMB gradient method from Helsen et al. (2012). A climate perturbation is introduced by modifying the surface height with temperature. Following this method, a reconstruction of present-day climate is presented, as well as projections for the evolution of the Greenland ice sheet in the 21st century. The model predicts a best estimate for global sea-level rise of 79 mm, with a range between 61 and 105 mm. This is within the range of 0-17 cm found by Graversen et al. (2011).