De‐Tuning Albedo Parameters in a Coupled Climate Ice Sheet Model to Simulate the North American Ice Sheet at the Last Glacial Maximum.

The Last Glacial Maximum extent of the North American Ice Sheets is well constrained empirically but has proven to be challenging to simulate with coupled Climate‐Ice Sheet models. Coupled Climate‐Ice Sheet models are often too computationally expensive to sufficiently explore uncertainty in input parameters, and it is unlikely that values calibrated to reproduce modern ice sheets will reproduce the known extent of the ice at the Last Glacial Maximum. To address this, we run an ensemble with a coupled Climate‐Ice Sheet model (FAMOUS‐ice), simulating the final stages of growth of the last North American Ice Sheets' maximum extent. Using this large ensemble approach, we explore the influence of numerous uncertain ice sheet albedo, ice sheet dynamics, atmospheric, and oceanic parameters on the ice sheet extent. We find that ice sheet albedo parameters determine the majority of uncertainty when simulating the Last Glacial Maximum North American Ice Sheets. Importantly, different albedo parameters are needed to produce a good match to the Last Glacial Maximum North American Ice Sheets than have previously been used to model the contemporary Greenland Ice Sheet due to differences in cloud cover over ablation zones. Thus, calibrating coupled climate‐ice sheet models on one ice sheet may produce strong biases when the model is applied to a new domain. Plain Language Summary: At the peak of the last ice age, an ice sheet covered much of North America. The extent of this ice sheet is well‐understood after decades of intensive data collection, but producing a computer simulation of the ice sheet which matches our observations has been a challenge. This is partly because of uncertainty about the "correct" model set‐up to create the best simulation, and partly because the computer models used in the simulations require large computing resources. In this paper, we present a series of simulations of the North American Ice Sheet at the peak of the last ice age using a fast‐running computer model in which the atmosphere and ice sheets interact. We run hundreds of simulations to tackle the uncertainty about the optimum values for unknown input parameters. We find that the model's representation of how reflective the ice sheet surface is has the most impact on the size and shape of the simulated ice sheet. Importantly, the parameter values that produce the best simulations of modern‐day Greenland produce poor simulations of the North American Ice Sheets during the last ice age, calling into question whether the parameters chosen for modern Greenland are appropriate from simulating ice sheets under different conditions. Key Points: Simulating the Last Glacial Maximum North American Ice Sheets successfully requires de‐tuning the coupled Climate‐Ice Sheet model FAMOUS‐iceAfter running a large ensemble, we identify multiple acceptable simulations of the LGM North American Ice SheetThe LGM North American cloud cover revealed overtuned albedo parameters to compensate for modern Greenland cloud biases [ABSTRACT FROM AUTHOR]