So we have the surprising logical conclusion that GHGs should actually facilitate atmospheric cooling by permitting upward radiation which could not otherwise occur. (newclimatemodel.com)
James Shearer's comment about «If the emissivity of the layer is low, most of the additional upward radiation can pass right through the layer without warming it» is one of those it depends things. (realclimate.org)
When upward radiation close to the right frequency hits a CO2 molecule, it can excite the vibrational mode at that frequency. (realclimate.org)