Abstract
Abstract. Recent Arctic sea ice retreat has been quicker than in most general circulation model (GCM) simulations. Internal variability may have amplified the observed retreat in recent years, but reliable attribution and projection requires accurate representation of relevant physics. Most current GCMs do not fully represent falling ice radiative effects (FIREs), and here we show that the small set of Coupled Model Intercomparison Project Phase 5 (CMIP5) models that include FIREs tend to show faster observed retreat. We investigate this using controlled simulations with the CESM1-CAM5 model. Under 1pctCO2 simulations, including FIREs results in the first occurrence of an “ice-free” Arctic (monthly mean extent <1×106 km2) at 550 ppm CO2, compared with 680 ppm otherwise. Over 60–90∘ N oceans, snowflakes reduce downward surface shortwave radiation and increase downward surface longwave radiation, improving agreement with the satellite-based CERES EBAF-Surface dataset. We propose that snowflakes' equivalent greenhouse effect reduces the mean sea ice thickness, resulting in a thinner pack whose retreat is more easily triggered by global warming. This is supported by the CESM1-CAM5 surface fluxes and a reduced initial thickness in perennial sea ice regions by approximately 0.3 m when FIREs are included. This explanation does not apply across the CMIP5 ensemble in which inter-model variation in the simulation of other processes likely dominates. Regardless, we show that FIRE can substantially change Arctic sea ice projections and propose that better including falling ice radiative effects in models is a high priority.
Highlights
The Arctic region is undergoing pronounced change, becoming warmer and wetter (Boisvert and Stroeve, 2015) while its land ice melts (Jacob et al, 2012; Kjeldsen et al, 2015) and spring arrives weeks earlier than in the 1990s (Post et al, 2018)
Differences in sea ice, ocean and atmosphere schemes may drive changes that confound detection of falling ice radiative effects (FIREs)-driven sea ice effects across the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble; our analysis of controlled CESM1-CAM5 simulations in which the only difference is the inclusion of FIREs allows a direct comparison
Our conclusion that FIRE drives faster retreat is likely robust to internal variability. These simulations show that falling ice radiative effects could lead to much greater Arctic sea ice retreat when the system is forced under global warming and support the inclusion of FIRE in future modelling efforts
Summary
The Arctic region is undergoing pronounced change, becoming warmer and wetter (Boisvert and Stroeve, 2015) while its land ice melts (Jacob et al, 2012; Kjeldsen et al, 2015) and spring arrives weeks earlier than in the 1990s (Post et al, 2018). It is possible that increased LW↓ from falling ice radiative effects (FIREs) may not have a substantial effect on winter sea ice extent but may restrict its thickness This should favour faster retreat in sea ice cover during both a typical summer melt season and long-term warming. Differences in sea ice, ocean and atmosphere schemes may drive changes that confound detection of FIRE-driven sea ice effects across the CMIP5 ensemble; our analysis of controlled CESM1-CAM5 simulations in which the only difference is the inclusion of FIREs allows a direct comparison In these simulations our analysis ignores coupled dynamical responses in favour of studying the surface radiative flux terms that provide a direct test of our hypothesis. The paper is structured as follows: Sect. 2 lists the data and methodology, Sect. 3 reports on the simulated and observed sea ice changes, Sect. 4 looks at the simulated and observed surface radiative fluxes, Sect. 5 synthesises and discusses the results and their limitations, and Sect. 6 concludes the paper
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
