Uncertainty in ice sheet modelling affects centennial and longer time-scale projections of the Greenland Ice Sheet's (GrIS) sea level contribution. One source of this uncertainty is the interaction between the ice sheet’s evolution and the Earth’s deformation in response to changes in the applied surface loading as the ice sheet waxes and wanes. Such deformation, Glacial Isostatic Adjustment (GIA), causes a vertical displacement of the ice sheet’s surface which affects temperature dependent precipitation and ice ablation. The significance of this feedback mechanism depends on the magnitude of GIA which is dependent on the mantle’s viscosity. Studies find that the sub-Greenland mantle viscosity varies in three dimensions which leads to a spatially varying rate of GIA. Results obtained with an ice sheet model coupled with an Earth model including 3D variations in sub-Greenland viscosity illustrate its significant effect on the GrIS’ modelled evolution by affecting both elevation-melt and elevation-precipitation feedback mechanisms.