Winter sensitivity of glacial states to orbits and ice sheet heights in CESM1.2

Abstract. The changing climate system between icehouse and greenhouse states during the Quaternary Period were dramatic, yet the magnitude of these changes are still uncertain due to unconstrained ice sheets and a lack of transition mechanisms. In this study, we investigate the individual and combined impact of ice sheet heights, orbital configurations, and greenhouse gas changes for a range of Quaternary climate states to assess the linearity or non-linearity of the climate system’s response. To this end, we conduct two sets of sensitivity experiments: a series on the Preindustrial (PI) climate and experiments on Quaternary glacial states. First, modifying the PI conditions with respect to orbit, Greenland ice sheet height, and greenhouse gasses, we find that the climate system’s response to the individual factors do not superimpose on the combined response of the climate system. Thus, already under PI conditions the climate system responds in a non-linear fashion. Second, our results on Quaternary glacial states show that changing ice sheet height is the primary cause of changes in climate systems, regardless of orbit. But the subtle regional effects that orbit has are not always explained by ice sheet height changes. As for the PI simulations we find a strong, non-linear behavior in combined orbital-ice sheet height effect, where the response of the atmospheric circulation plays an important role. Therefore, orbit, ice sheets, and greenhouse gasses evolve through time by specific pathways and imply a theoretical constraint on the real climate state. As the spatial and temporal resolution of the Quaternary proxy data improves, combined with these modeled climates, we expect to generate substantial constraints on the number of realistic Quaternary climate states.