The natural greenhouse effect of atmospheric oxygen (O2) and nitrogen (N2)

The effect of collision-induced absorption by molecular oxygen (O-2) and nitrogen (N-2) on the outgoing longwave radiation (OLR) of the Earth's atmosphere has been quantified. We have found that on global average under clear-sky conditions the OLR is reduced due to O-2 by 0.11 Wm(-2) and due to N-2 by 0.17 Wm(-2). Together this amounts to 15% of the OLR-reduction caused by CH4 at present atmospheric concentrations. Over Antarctica the combined effect of O-2 and N-2 increases on average to about 38% of CH4 with single values reaching up to 80%. This is explained by less interference of H2O spectral bands on the absorption features of O-2 and N-2 for dry atmospheric conditions.