Climate and ice-sheet mass balance at the last glacial maximum from the GENESIS version 2 global climate model

Abstract

At the last glacial maximum (LGM) about 21,000 calendar years ago (21 ka BP), the overall mass balance of the Laurentide and Eurasian ice sheets should have been close to zero, since their rate of change of total ice volume was approximately zero at that time. The surface mass balance should have been zero or positive to balance any iceberg/iceshelf discharge and basal melting, but could not have been strongly negative. In principle, this can be tested by global climate model (CGM) simulations with prescribed ice-sheet extents and topography. We describe results from a suite of 21 ka BP simulations using a new global climate model (GENESIS version 2.0.a), with SSTs prescribed from CLIMAP (1981) and predicted by a mixed-layer ocean model, and with ice sheets prescribed from CLIh the ICE-4G (Peltier, 1994) and CLIMAP (1981) reconstructions. In common with previous GCM simulations using mixed-layer oceans, substantial cooling over and around Antarctica occurs due to ‘normal’ GCM dynamics and polar sea-ice feedback, without recourse to changes in thermohaline circulation. We find slightly enhanced cooling and cloudiness over low-latitude land masses compared to over ocean, especially over the Andes, which decreases the disagreement with land-based data and suggests that tropical CLIMAP SSTs may only be ∼1–2°C too warm. The GCM used here is well suited for ice-sheet mass-balance studies because (i) the surface can be represented at a finer resolution than the atmospheric GCM, (ii) an elevation correction accounts for spectral distortions of the atmospheric GCM topography, (iii) a simple post-processing correction for refreezing of meltwater is applied, and (iv) the model's precipitation and mass balances for present-day Greenland and Antarctica are realistic. However, for all reasonable combinations of SSTs and ice-sheet configurations, the predicted annual surface mass balances of the LGM Laurentide and Eurasian ice sheets are implausibly negative. Possible reasons for this discrepancy are discussed, including increased ice-age aerosols, higher CLIMAP-like ice-sheet profiles in the few thousand years preceding LGM, and a surge of the southern Laurentide just before LGM to fleetingly produce the ICE-4G profile at 21 ka BP.