ALBERT

Description: The present manuscript compares the last two glacial inceptions, Marine Isotope Stage 5 (MIS5, 125–115 kyr BP) and MIS7 (236–229 kyr BP) with the aim to detect the relative impact of external forcing (orbitals and GHG) and ice-albedo feedbacks on the ice sheets growth and distribution in the Northern Hemisphere high latitudes. In order to investigate the differences between those two states, we combine atmosphere-ocean coupled model experiments and off-line ice-sheet-model simulations. In particular, we use a low resolution coupled Atmosphere-Ocean-Sea-ice general circulation model to simulate the mean climate of the four time periods associated with the inception states of MIS5 and MIS7 (i.e. 236, 229, 125 and 115 kyr BP). The four mean states are then use to force a 3-D thermodynamical ice sheet model by means of two types of ice sheet experiments, i.e., steady-state and transient experiments. Our results show that steady-state ice experiments underestimate the ice volume at both 229 kyr BP and 115 kyr BP. On the other hand, the simulated pre-inception ice distributions at 236 kyr BP and 125 kyr BP are in good agreement with observations indicating that during these periods feedbacks associated with external forcing dominate over other processes. However, if proper ice-elevation and albedo feedbacks are not taken into consideration, the evolution towards glacial inception in terms of ice volume and extent is hardly simulated. The experimental setup chosen allows us to conclude that, depending on the mean background climate state, the effect of model biases on climate are more important during a cold inception, such as MIS7, than during a warm inception, such as MIS5. The last results suggest to be cautious when tuning and calibrating Earth System Models on a specific time period, mainly for the purpose of ice sheet-climate coupling.