Cloud Feedbacks and Glacial-Interglacial Climate Variations

Abstract:

The recurrent ice ages of the Quaternary period (i.e., the past ~2.6 million years) have been linked to slow changes in the shape and orientation of Earth’s orbit and the tilt of its axis. But these orbital variations produce only very small changes in Earth’s global and annual mean radiation budget, implying that climate feedback mechanisms are essential to the large glacial-interglacial climate variations that are documented in the paleoclimate record. We examine the radiative feedbacks operating in simulations of the response of climate to orbital forcing with the GFDL CM2.1 coupled atmosphere-ocean model. When evaluated on an annual and global mean basis, such feedbacks are an order of magnitude larger that the direct radiative forcing that results from changes in orbital parameters. Cloud feedbacks are of particular interest, as they represent a net global cooling effect when the tilt of Earth’s axis is reduced and a warming effect when perihelion is shifted from summer to winter. At high northern latitudes in summer, where ice sheet growth originated during incipient glacial periods, cloud feedbacks oppose the direct effect of orbital forcing that was first identified by Milankovitch as the driver of glacial-interglacial climate variations. In contrast, the effects of growing ice sheets may induce a cloud feedback response that leads to cooling, as suggested by an analysis of simulations of the climate of the last glacial maximum made in conjunction with the Coupled Model Intercomparison Project Phase 5 (CMIP5).