Reply on RC1

Abstract

During the Last Glacial Cycle (LGC), ice sheets covered large parts of Eurasia and North America which resulted in ~120 meters of sea level change. Ice sheet – climate interactions have considerable influence on temperature and precipitation patterns, and therefore need to be included when simulating this time period. Ideally, ice sheet – climate interactions are simulated by a high-resolution earth system model. While these models are capable of simulating climates at a certain point in time, such as the Pre-Industrial (PI) or the Last Glacial Maximum (LGM; 21,000 years ago), a full glacial cycle is currently unfeasible as it requires a too large amount of computation time. Nevertheless, ice-sheet models require forcing that captures the gradual change in climate over time to calculate the accumulation and melt of ice and its effect on ice sheet extent and volume changes. Here we simulate the LGC using an ice sheet model forced by LGM and PI climates. The gradual change in climate is modelled by transiently interpolating between pre-calculated results from a climate model for the LGM and the PI. To assess the influence of ice sheet – climate interactions, we use two different interpolation methods: The climate matrix method, which includes these interactions, and the glacial index method, which does not. To investigate the sensitivity of the results to the prescribed climate forcing, we use the output of several models that are part of the Paleoclimate Modelling Intercomparison Project Phase III (PMIP3). In these simulations, ice volume is prescribed and the climate is reconstructed. Here we test those models by using their climate to drive an ice sheet model over the LGC. We find that the differences caused by the climate forcing exceeds the differences caused by the interpolation method. Some GCMs produced unrealistic LGM volumes and only four resulted in reasonable ice sheets with LGM Northern Hemisphere sea level contribution ranging between 74 – 113 meters with respect to the present day. The glacial index and climate matrix methods result in similar ice volumes at LGM but yield a different ice evolution with different ice domes during the inception phase of the glacial cycle, and different sea-level rates during the deglaciation phase. The temperature-albedo feedback is the main cause of differences between the glacial index and climate matrix methods.