H20 is about 1-4% of the atmosphere and is the most important GHG, depending on location and weather, and absorbs most of the LWR. CO2 is responsible for about 4-9% of the GHG effect, so it seems to me that the 99% O2/N2/H20 combo is a much bigger player than the 0.036% CO2, although CO2 packs a wallop on a molecule per molecule basis.