Model of Atmosphere with CO2 shows Small Emissivity

Computed transmittance of atmosphere with CO2 with main resonance at wave number 667. Notice that the atmosphere is effectively transparent, except in a small interval around 667. The effect of CO2 is thus small.

Here is a another argument indicating that the effect of the atmospheric trace gas CO2 on the radiation balance of the Earth is small.

We recall the model of blackbody radiation studied on Computational Blackbody Radiation as a collection of oscillators with small damping with equal oscillator internal energy T representing temperature, with oscillator resonance frequencies n varying from 1 to a cut-off set at T and each oscillator radiating

• E_n = gamma T n^2    

where gamma is a universal constant, which is Planck's law. Summing over n from 1 to T, we obtain the total radiance

• E = sum_n gamma T n^2 = sigma T^4 

which is Stefan-Boltzmann's law with sigma = gamma/3. In the case of only one resonance frequency n = T, the radiance would be reduced to  

• e = gamma T T^2 = gamma T^3 ~ E/T

with the reduction factor 1/T. 

The radiance of an atmosphere which is fully opaque over the entire spectrum would radiate E, while an atmosphere opaque only for a specific frequency near cut-off T, would radiate e ~ E/T with a reduction factor  1/T. 

We conclude that the emissivity of transparent atmosphere with a trace gas like CO2 with only a few isolated resonances, would scale like 1/T and thus be small as soon as T is bigger than say 100 K.  

We thus find theoretical evidence from a basic model that the emissivity of the Earth's atmosphere with the trace gas CO2 would be small, and thus that CO2 would have little effect on the Earth's radiation balance.

Note that the sparseness of CO2 as a trace gas, gets expressed as a spareseness of absorption spectrum rather than small mass fraction because of the universality of blackbody radiation as being independent of the mass of the oscillators.