Contrasting the Penultimate and Last Glacial Maxima (140 and 21 ka BP) using coupled climate-ice sheet modelling

Abstract. The configuration of the Northern Hemisphere ice sheets during the Penultimate Glacial Maximum differed to the Last Glacial Maximum. However, the reasons for this are not yet fully understood. These differences likely contributed to the varied deglaciation pathways experienced following the glacial maxima and may have had consequences for the interglacial sea level rise. Therefore, a better understanding of how and why these two glacial maxima differed is crucial for developing the full picture on why the Last Interglacial sea level was up to 9 meters higher than today, and thus may help constrain future sea level rise. To understand the differences between the North American Ice Sheet at the Last and Penultimate Glacial Maxima (21 and 140 ka BP), we perform two perturbed-physics ensembles of 62 simulations using a coupled climate-ice sheet model FAMOUS-ice, in which the North American and Greenland ice sheets are dynamically simulated with the Glimmer ice sheet model. We select six ensemble members that match reconstructed ice extent and volumes at the Last and Penultimate Glacial Maxima. To understand the role of orbit, greenhouse gases and initial conditions on the final ice sheet configurations, we use a factor decomposition technique. This reveals that the initial ice sheet conditions used in the model are extremely important in determining the difference in final ice volumes between both periods due to the large effect of the . In contrast to evidence of a smaller Penultimate North American Ice Sheet, our model shows that the climate boundary conditions at these glacial maxima, if considered in isolation, imply a larger Penultimate Glacial Maximum North American Ice Sheet than at the Last Glacial Maximum, of around 6 meters sea level equivalent. This suggests the growth of the ice sheet prior to the glacial maxima is key in explaining the differences in North American ice volume.