Orbitally Forced Radiative Feedbacks as Drivers of Ice-Sheets in a GCM with Observationally Constrained Mixed-Phase Clouds

Periodic variations in the Earth’s orbit alter the seasonal and latitudinal distribution of insolation reaching the Earth’s surface. Over the recent geological past (~2.5 million years) large cyclical shifts in climate have occurred with the Earth shifting from glacial to interglacial states. These climate cycles have been linked to changes in the Earths orbit, in particular the Earth’s obliquity or axial tilt, which has a 41,000-year frequency. The leading hypothesis linking changes in orbit with the glacial-interglacial cycles is that orbitally induced changes in summer insolation drive the fluctuations in ice sheets: during periods of low obliquity seasonality is reduced, leading to warmer winters and cooler summers, this enables more ice to be preserved during the melt season, which is conducive to ice sheet accumulation and vice versa for high obliquity and ice sheet ablation. The exact mechanisms of orbital forcing that drive the the glacial-interglacial cycles are uncertain though. Orbitally mediated changes in insolation and radiative forcing are too small to drive the large fluctuations in ice-sheets that are seen in the geological record and so large climate feedbacks have been invoked to account for the amplified response of the glacial-interglacial cycles. Previous work in which the GFDL climate model CM2.1 was used, isolated the climate response and radiative feedbacks due to changes in obliquity. This work found that while some radiative feedbacks amplified the orbital forcing, cloud feedbacks actually opposed the orbitally mediated changes. When obliquity was low, reductions in cloud liquid and cloud cover increased insolation into the high latitudes, partially opposing the orbital forcing. However, clouds are a large source of uncertainty in climate models and mixed-phase clouds, which are ubiquitous in the polar regions are not realistically represented in many GCMs. Mixed phase clouds contain both a supercooled liquid fraction (SLF) and an ice fraction. SLF in mixed phase clouds is severely underestimated globally in many GCMs. In this work we repeat the idealized obliquity simulations first conducted with CM2.1 using the National Centre for Atmospheric Research model, CESM1. We conduct two sets of simulations; in one set we use the standard out-of-the-box model, in the second set we update the mixed phase cloud parameterization and use observationally constrained SLF to provide a more realistic representation of mixed phase clouds in the model. When the SLF cloud bias is fixed, model sensitivity to orbitally induced forcing increases substantially resulting in increased global and high latitude temperature anomalies. Snow accumulation amount and distribution also indicate larger fluctuations in ice sheet volume would occur when mixed phase clouds are more realistically represented. This work indicates that reducing cloud uncertainties in models may be critical for improving paleoclimate model-data comparisons.