Evaluating different geothermal heat flow maps as basal boundary conditions during spin up of the Greenland ice sheet

Abstract. There is currently poor scientific agreement whether the ice-bed interface is frozen or thawed beneath approximately one-third of the Greenland ice sheet. This disagreement in basal thermal state results, at least partly, from a diversity of opinion in the subglacial geothermal heat flow basal boundary condition employed in different ice-flow models. Here, we employ seven Greenland geothermal heat flow maps in widespread use to 10,000-year spin ups of the Community Ice Sheet Model (CISM). We perform both a fully unconstrained transient spin up, as well as a nudged spin up that conforms to Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) protocol. Across the seven heat flow maps, and regardless of unconstrained or nudged spin up, the spread in basal ice temperatures exceeds 10 °C over large areas of the ice-bed interface. For a given heat flow map, thawed-bedded ice-sheet area is consistently larger under unconstrained spin ups than nudged spin ups. Under the unconstrained spin up, thawed-bedded area ranges from 33.5 to 60.0 % across the seven heat flow maps. Perhaps counterintuitively, the highest iceberg calving fluxes are associated with the lowest heat flows (and vice versa) for both unconstrained and nudged spin ups. This highlights the direct, and non-trivial, influence of choice of heat flow boundary condition on the simulated equilibrium thermal state of the ice sheet. We suggest that future ice-flow model intercomparisons should employ a range of basal heat flow maps, and limit direct intercomparisons to simulations employing a common heat flow map.