Exploring the sensitivity of the Northern Hemisphere ice sheets at the last two glacial maxima to coupled climate-ice sheet model parameters

Abstract. Simulations of past periods are useful for testing the ability of numerical models to simulate ice sheet changes under significantly different climate conditions to present day. This can help improve projections of future sea level rise made by these same models and avoid over-tuning to particular (e.g. modern) stationary climate conditions. The Last Glacial Maximum (LGM; ~21 thousand years ago (ka)) has been extensively used for this purpose since it is relatively well constrained by empirical evidence. However, less is known about the Penultimate Glacial Maximum (PGM; ~140 ka) and why the vast ice sheets covering much of the Northern Hemisphere (NH), differed to the LGM. The answer likely lies, at least in part, in the different orbital configurations between the two periods, and the resulting impact on climate-ice sheet interactions.

Here, we perform and compare the first large ensembles of coupled climate-ice sheet (FAMOUS-BISICLES) simulations of the LGM and PGM to better understand how NH ice sheets interact with the climate and quantify how sensitive the simulations are to the choice of uncertain model inputs, including physical parameter values. Specifically, we vary 12 uncertain parameters that control the model representations of ice sheet albedo, ice dynamics and climate. The ensembles are evaluated against palaeo-evidence of global mean temperature, ice volume and extent to calibrate the model and find combinations of parameters that simultaneously yield plausible ice sheets and climates for both periods. The sensitivity of the North American ice sheet and the Eurasian ice sheet during the LGM and PGM, to each of the 12 parameter values, is explored using Gaussian Process emulators to perform a Sobol sensitivity analysis. From the whole ensemble, we find two simulations that meet our evaluation constraints for the LGM ice sheets. The parameter values that influence the albedo of the ice sheet have the largest influence on the resulting ice sheet volumes, but several other parameters display different sensitivity indices depending on the ice sheet (North American versus Eurasian) and time period (PGM versus LGM). This includes parameters that affect the cloud liquid water, lapse rate, basal sliding and downscaling elevation heights.