Potential faster Arctic sea ice retreat triggered by snowflakes' greenhouse effect

Abstract. Recent Arctic sea ice retreat has been quicker than in mostgeneral circulation model (GCM) simulations. Internal variability may haveamplified the observed retreat in recent years, but reliable attribution andprojection requires accurate representation of relevant physics. Most currentGCMs do not fully represent falling ice radiative effects (FIREs), and here weshow that the small set of Coupled Model Intercomparison Project Phase 5(CMIP5) models that include FIREs tend to show faster observed retreat. Weinvestigate this using controlled simulations with the CESM1-CAM5 model.Under 1pctCO2 simulations, including FIREs results in the first occurrence ofan “ice-free” Arctic (monthly mean extent 1 × 10 6  km 2 ) at550 ppm  CO 2 , compared with 680 ppm otherwise. Over60–90 ∘  N oceans, snowflakes reduce downward surface shortwaveradiation and increase downward surface longwave radiation, improvingagreement with the satellite-based CERES EBAF-Surface dataset. We proposethat snowflakesu0027 equivalent greenhouse effect reduces the mean sea icethickness, resulting in a thinner pack whose retreat is more easily triggeredby global warming. This is supported by the CESM1-CAM5 surface fluxes and areduced initial thickness in perennial sea ice regions by approximately0.3 m when FIREs are included. This explanation does not apply across theCMIP5 ensemble in which inter-model variation in the simulation of otherprocesses likely dominates. Regardless, we show that FIRE can substantiallychange Arctic sea ice projections and propose that better including fallingice radiative effects in models is a high priority.