Future Greenland Melt on Multi-Century and Multi-Millennial Time Scales assessed with the Community Ice Sheet Model version 2.1

Abstract

The Greenland Ice Sheet is the world’s second largest ice sheet, storing an equivalent of 7.3 meters of sea level rise. Due to climate change, the Greenland ice sheet is currently losing mass at an accelerated rate. Ice sheet models are used to project long term melt of the ice sheet, which are often forced by output from climate models. Most of the multi-millennium time scale ice sheet simulations conducted in the past used SMB calculations based on empirical relationships between melt and temperature (Positive-Degree Day schemes). In this thesis, I address the question of the future evolution of the Greenland ice sheet by means of an ice sheet model forced with an elevation dependent SMB field that accounts for the energy available for melt. This work focuses on key variables such as ice thickness, ice area, velocity and contribution to eustatic sea level rise, and assesses the reversibility of the mass loss. For this thesis, I performed uncoupled CISM2.1 simulations which were forced by the elevation- SMB field from a coupled CESM-CISM simulation. The coupled simulation used to force the ice sheet has a length of 160 years and a CO2 concentration that is increased with 1% per year from pre-industrial levels and capped at 4 times CO2. Time segments with 2x, 3x and 4x pre-industrial CO2 concentrations of this CESM-CISM run were used to force the ice sheet on multi-millennium time-scales. In addition, a Recovery from 4x CO2 was conducted in which the pre-industrial forcing from a coupled CESM-CISM simulation is re-introduced after 55% mass loss. The 2x, 3x and 4x CO2 scenarios resulted in a cumulative sea level rise of 0.49 m, 3.0 m, and 8.2 m by year 4,000. The 2x CO2 scenario resulted in limited retreat and stability within 4,000 years. No stability of the ice sheet was attained by year 8,000 in the 3x CO2 simulation, with a final Mass Balance of -108.8 Gt/yr (0.30 ± mm/yr). The 4x CO2 simulation resulted in the complete deglaciation of the ice sheet within 3,000 years. Despite the lower initial topography compared to the pre-industrial ice sheet, the Recovery from 4x CO2 simulation resulted into expansion of the ice sheet. Within 4,000 years, the mass increased from 46% to 67% relative to the pre-industrial ice sheet.