The last glacial cycle transiently simulated with a coupled climate-ice sheet model

Abstract

Understanding climate variability from millennial to glacial-interglacial timescales remains challenging due to the complex and non-linear feedbacks between ice, ocean, and atmosphere. Although the ever-increasing number of reconstructions has helped to form compelling hypotheses for the evolution of ocean and atmosphere circulation or ice sheet extent over the last glacial cycle, climate models, required for systematically testing these hypotheses, struggle to dynamically and comprehensively simulate such long time periods as a result of the large computational costs. Here, we therefore coupled a dynamical ice sheet model to the Bern3D Earth system model of intermediate complexity, that allows for simulating multiple glacial-interglacial cycles in reasonable time. To test the fully-coupled model, we explore the climate evolution over the entire last glacial cycle in a transient simulation forced by the orbital configuration and greenhouse gas and aerosol concentrations. We are able to simulate Global Mean Surface Temperature (GMST) in fair agreement with reconstructions exhibiting a gradual cooling trend since the last interglacial that is interrupted by two more rapid cooling events during the early Marine Isotope Stage (MIS) 4 and Last Glacial Maximum (LGM). The glacial-interglacial GMST and mean ocean temperature differences are 5 °C and 1.6 °C, respectively. Ice volume shows pronounced variability on orbital timescales mirroring northern hemispheric summer insolation. From early MIS3 to the LGM ice volume roughly doubles in good agreement with recent sea-level reconstructions. The Atlantic overturning circulation shows larger variability during the relatively warm MIS5 than during the cooler MIS3, however we note that Dansgaard-Oeschger events are not intrinsically simulated in our setup. At the LGM the Atlantic overturning has a strength of about 14 Sv, which is a reduction by about one quarter compared to the pre-industrial. We thus demonstrate that the new coupled model is able to realistically simulate glacial-interglacial cycles, which allows as to systematically investigate the sensitivities to parameters such as equilibrium climate sensitivity or aerosol radiative forcing during the last glacial cycle.