Instantaneous Radiative Effect of Surface Longwave Spectral Emissivity in a Snowball Earth Simulation

Abstract

Spectrally dependent emission by the surface (i.e., surface spectral emissivity) is commonly ignored by current climate models. Surface spectral emissivity matters more in cold and dry environments than in hot and humid environments. Recent modeling studies confirmed that, for current climate simulations, this process affects the polar climate more than the extra-polar climate. As for the Snowball Earth, a period characterized by global polar-like conditions of extreme cold and low humidity, including surface spectral emissivity could alter the simulated global radiation budget. This, in turn, could affect the simulated climate of the Snowball Earth. Here, we use an aqua-planet slab-ocean simulation of Snowball Earth by the ICON model to perform offline radiative transfer calculations to quantify such impact on the outgoing longwave radiation (OLR). The offline radiative transfer model is used to compute the clear-sky OLR for two surfaces that would be present in the extremely cold simulation: ice and snow. Compared to the results with assumed blackbody surface, the global mean OLR decreases by 2.9 and 1.0 W m⁻² for ice and snow surfaces, respectively. The impact of surface spectral emissivity on the OLR is strongest at the equator and weakens toward the poles, presenting a noticeable meridional gradient. Effects of surface emissivity are also larger during the summer than the winter. The radiative effects of this often-neglected process would be small for a snow-covered globe but could be important for climate states with exposed ice, particularly Jormungand states, as well as simulations of other cold and dry climates.