The Greenland-Ice-Sheet evolution over the last 24,000 years: insights from model simulations evaluated against ice-extent markers

Abstract. Continental ice sheets possess a long-term memory that is stored within both the geometry and thermal properties of ice. In Greenland, this causes a disequilibrium between the present-day ice sheet and current climate, as the ice sheet is still adjusting to past changes that occurred over millennial timescales. Data-consistent modelling of the paleo Greenland-Ice-Sheet evolution is thus important for improving model initialisation procedures used in future ice sheet projection experiments. Additionally, open questions remain regarding the ice sheet’s former volume, extent, flux, internal flow dynamics, thermal conditions, and how such properties varied in space since the last glaciation. Here, we conduct a modelling experiment that aims to produce simulations in agreement with empirical data on the extent and timing of the ice sheet’s margin positions over the last 24,000 years. Due to large uncertainties in ice-sheet model parameters and boundary conditions, we apply a perturbed parameter ensemble approach and run 100 ice-sheet-wide simulations at 5 x 5 km horizontal resolution using the Parallel Ice Sheet Model. Our simulations are forced by paleo-climate and ocean simulations of the isotope-enabled Community Earth System Model. Using quantitative model-data comparison and the newly developed Greenland-wide reconstruction of former ice margin retreat (PaleoGrIS 1.0), we scored each simulation’s fit across Greenland from 24,000 years ago until 1850 AD. The resulting ensemble and best-scoring simulations provide insights related to the dynamics, causes and spatial heterogeneities of the local LGM, Late-glacial, and Holocene evolution of the Greenland Ice Sheet. We for instance find that between 16 and 14 thousand years ago, the ice sheet lost most of its ice grounded on the continental shelf. This marine-sector demise, associated with up to seven times greater mass loss rates than observed today, was predominantly caused by ocean warming while air temperatures possibly remained too cold to generate surface melt. We specifically detail and showcase results from our model-data comparison procedures, including regional heterogeneities in model-data fit and the sensitivity of model-data agreement scores to certain parameter configurations, that will likely prove useful for others working on paleo-ice-sheet modelling experiments. Finally, we report on the remaining model-data misfits in ice extent, here found to be largest in northern, northeastern, and central-eastern Greenland, and discuss possible causes for such spatial heterogeneity in model-data agreement.