Reply on RC2

Abstract

In the last decades, great effort has been made to reconstruct the Laurentide Ice Sheet (LIS) during the Last Glacial Maximum (LGM, ca. 21,000 years before present, 21 kyr ago). Uncertainties underlying its modelling have led to large differences in fundamental features such as its maximum elevation, extension and total volume. However, the uncertainty in ice dynamics and thus in ice extension, volume and ice-stream stability remains large. We herein use a higher-order three-dimensional ice-sheet model to simulate the LIS under LGM boundary conditions for a number of basal friction formulations of varying complexity. Their consequences on the Laurentide ice streams, configuration, extension and volume are explicitly quantified. Total volume and ice extent generally reach a constant equilibrium value that falls close to prior LIS reconstructions. Simulations exhibit high sensitivity to the dependency of the basal shear stress on the sliding velocity. In particular, a regularized-Coulomb formulation appears to be the best choice in terms of ice volume and ice-stream realism. Notable differences are found when the stress balance is thermomechanically coupled: the LIS volume is lower than for a purely mechanical friction scenario and the base remains colder. Thermomechanical coupling is fundamental for producing rapid ice streaming, yet it leads to a similar distribution of ice overall.