So if we kept adding this weak greenhouse gas indefinitely in our hypothetical Moon experiment, could we expect the day side of the Moon to continue growing cooler and cooler?
I wouldn't want to even guess without thinking about it a long time. I am not even sure my analysis is correct, I just assumed that if somehow by magic we could create a moon with a CO2 atmosphere that the concentration profile with altitude would be a nearly a horizontal line because of such a low gravitational field. With regard to the Earth, I am not even sure that during daytime the effect of CO2 on our Earth is net cooling it might be very slightly net warming because the concentration of CO2 at low altitudes is higher. The point is that on Earth any net cooling or warming during the day due to CO2 is going to be small compared to all the major cooling and warming mechanisms going on. At night, there is no question that the greenhouse effect plays an important role. But our hypothetical moon atmosphere is very different. It has only one component. There I think unlimited cooling during the day if we kept adding CO2 to the atmosphere is impossible. For one thing the CO2 is going to condense and covert to the solid phase at some point on the dark side and become microcrystalline particles.
I think it is far more important to recognize that CO2's greenhouse effect on Earth is only going to be significant at night when there is no scattering or absorption cooling at higher altitudes. On the other hand, we don't need to do a complicated experiment to know the non-condensing greenhouse gases have a weaker effect compared to that of clouds in a night sky. We only need to spend two nights in the desert. One night with a clear cloudless sky; the other with an uncharacteristically overcast sky.
I wish I could correctly model the Earth, Sun and Moon and accurately predict future Earth temperature what would happen in each scenario. If I could, I'd invite you to join me in Stockholm and we'd share a bottle of Champagne.
One of the interesting, to me, things I learned from thinking about your pesky questions, is how important high altitude scattering is as a shielding mechanism. Jem has been harping on this for a long time, but I just assumed that in the case of CO2 it probably wasn't as important as the greenhouse contribution. Now I'm not so sure that during the day there isn't a net cooling effect from CO2 (not at night of course). The problem is I don't know exactly the relative scattering contribution of the various gases --if I wasn't so lazy, I'd look that up. They are all going to scatter. There is mostly nitrogen, so that's got to be a big contributor. Probably blue light, right. The sky is blue, there is way more nitrogen, so that's why I suppose we say blue scatters more than yellow or red which will be scattered by lessor, and larger atmospheric components. (I'm guessing obviously, but the wavelengths scattered are determined by molecular size.. N2 is smaller than CO2.) It is pretty evident from looking at that solar spectrum graph I posted that scattering plays a big role in cutting down the amount of shorter wavelength solar emr reaching the Earth. It is a much bigger role than I had realized.
