The Role of Falling Ice Radiative effects on Climate Projections over Arctic under Global Warming

Most global climate models, such as CMIP5 models, ignore the falling ice (snow) radiativeeffects (FIREs). Extended from our previous works, we explore the impacts of FIREs on the 29 geographical distribution changes of sea ice concentration (SIC), sea ice thickness and skin 30 temperature (Ts) under 1% per year increase of atmospheric CO2 concentration. We perform a pair 31 of 140-yr experiments including FIREs (SoN) and excluding FIREs (NoS) using CESM1-CAM5. 32 These two simulations are compared with each other and against CMIP5 multi-model mean 33 without FIREs (CMIP5-NoS). 34The results show that the changes of SIC, thickness and radiation fields in NoS minus SoN 35 largely match the changes between CMIP5-NoS and SoN in winter but less so in summer and 36 annual mean. Both NoS and CMIP5-NoS simulate less downward longwave and net radiative 37 warming (~20—30 W m-2), resulting in colder Ts over Arctic and adjacent lands (~5—8 K colder) 38 and stronger meridional temperature gradient, leading to more SIC and thicker sea ice (~30—40 39 cm) over the Arctic ocean. The inclusion of FIREs produces stronger changes in geographic 40 patterns and magnitudes of Ts, SIC and thickness from the first to middle 20 yrs but less from the 41 middle to last 20 yrs. The SIC and thickness changes in SoN are associated with warmer Ts, 42 increasing downward surface longwave warming and thus net warming relative to NoS and 43 CMIP5-NoS. With FIREs, the model shows faster warming-driven sea ice retreats and thinning 44 over the entire Arctic ocean, resulting in a sea ice-free Arctic ocean 30 yrs earlier, as well as much 45 warmer Ts (up to 5 K) over the adjacent lands than in NoS case.