The Greenland ice sheet (GrIS) has been losing mass in the last several decades, and is currently contributing around 0.7 mm sea level equivalent (SLE) yr-1 to global mean sea level rise (SLR). As ice sheets are integral parts of the Earth system, it is important to gain process-level understanding of GrIS mass loss. This paper presents an idealized high-forcing simulation of 350 years with the Community Earth System Model version 2.1 (CESM2.1) including interactively coupled, dynamic GrIS with the Community Ice Sheet Model v2.1 (CISM2.1). From pre-industrial levels (287 ppmv), the CO2 concentration is increased by 1% yr-1 till quadrupling (1140 ppmv) is reached in year 140. After this, the forcing is kept constant. Global mean temperature anomaly of 5.2 K and 8.5 K is simulated by years 131–150 and 331-150, respectively. The North Atlantic Meridional Overturning Circulation strongly declines, starting before GrIS runoff substantially increases. The projected GrIS contribution to global mean SLR is 107 mm SLE by year 150, and 1140 mm SLE by year 350. The accelerated mass loss is driven by the SMB. Increased long-wave radiation from the warmer atmosphere induces an initial slow SMB decline. An acceleration in SMB decline occurs after the ablation areas have expanded enough to trigger the ice-albedo feedback. Thereafter, short-wave radiation becomes an increasingly important contributor to the melt energy. The turbulent heat fluxes further enhance melt and the refreezing capacity becomes saturated. The global mean temperature anomaly at the start of the accelerated SMB decline is 4.2 K.

The Greenland Ice Sheet (GrIS) mass balance is examined with an Earth system/ice sheet model that interactively couples the GrIS to the land and atmosphere. The simulation runs from 1850 to 2100, with historical and SSP5-8.5 forcing. By mid-21st century, the cumulative contribution to global mean sea level rise (SLR) is 23 mm. Over the second half of the 21st century, the surface mass balance becomes negative in all drainage basins, and an additional 86 mm of SLR is contributed. The annual mean GrIS mass loss in the last two decades is 2.7 mm sea level equivalent (SLE) yr-1. Strong decrease in SMB (3.1 mm SLE yr-1) is counteracted by a reduction in ice discharge from thinning and retreat of outlet glaciers. The southern GrIS drainage basins contribute 73% of the mass loss by mid-century. This decreases to 55% by 2100, as surface runoff in the northern basins strongly increases.