Master thesis defense by Laurent Lindpointner

Title: Incorporating a dynamical Greenland Ice Sheet into a Global Climate Model System

Abstract: The Greenland ice sheet is projected to continually lose mass at an accelerating pace during the 21st century. Being the second largest single ice mass on Earth and the largest in the Northern Hemisphere, this has implications both for future sea level rise and the surrounding climate system. In order to realistically represent the changing Greenland ice sheet in a global climate model system, twoway coupled climate model ice sheet model systems are needed. In this project, a dynamical ice sheet model for the Greenland ice sheet was coupled to a global climate model and emerging consequences investigated.

The interface for coupling the climate model ECEarth 3 and the ice sheet model PISM v1.2, which was developed in this project, is described in detail. The interface offers two different ways of forcing the PISM ice sheet model: one with surface forcing generated by ECEarth 3 and another with surface forcing generated by the downscaled energy balance model CISSEMBEL, which itself uses atmospheric forcing data from ECEarth 3.

CISSEMBEL experiments are performed and resulting surface mass and energy fluxes compared with standalone ECEarth 3 model data. The experiments use atmospheric forcing from 29 year ECEarth 3 experiments: two preindustrial historical and two SSP58.5 scenario experiments. One of each uses a constant albedo over glaciated areas and one uses a variable albedo scheme, allowing albedo-melt feedback. Using the variable albedo scheme, CISSEMBEL generates 218% and 238% more melt than ECEarth 3 in the preindustrial and scenario experiments, respectively.

Ice sheet initial states for the coupled system experiments are generated with PISM initialization experiments under preindustrial surface forcing. Separate initial state ice sheets are generated for the two different forcing methods in the coupled system. The resulting ice sheet based on surface forcing from ECEarth 3 has 15.0% more mass and covers a 28.0% larger area than the observed present-day Greenland ice sheet. The ice sheet forced by CISSEMBEL data has 8.5% less mass but covers a 5.2% larger area than the observed present-day Greenland ice sheet. Spatial differences in surface mass balance forcing from the two models are the key factor in determining the resulting ice sheet geometries.

Finally, two 80 year abrupt 4xCO2 scenario experiments for testing the fully coupled system are performed. In one, PISM is forced with surface forcing data from ECEarth 3 and in the other with surface forcing data from CISSEMBEL. The impact of the coupling on the two meter air temperature, surface mass and surface energy fluxes in ECEarth 3 is assessed by comparison to a 50 year ECEarth 3 standalone experiment of the same scenario.

The Greenland ice sheet has an increasingly negative mass balance throughout both coupled experiments. The ECEarth 3 forced ice sheet has an average ice discharge contribution of 18.1% to the total mass balance while for the CISSEMBEL forced ice sheet this fraction is 4.7%. The ECEarth 3 and CISSEMBEL forced ice sheets lose 5.7% and 6.1% of their mass, respectively. This equals respective cumulative contributions to global mean sea level rise of 473 mm and 406 mm.

Supervisor: Marianne Sloth Madsen, Aslak Grinsted and Shuting Yang
Censor: Sebastian Bjerregaard Simonsen (DTU)