The reduced pressure will reduce the effects of conduction and convection. Heat will not be transported as easily from the surface to the upper troposphere and the atmosphere would be more stratified. At the surface the net effect of these changes is warming. A molecule absorbing an infrared photon will be less likely to be thermalized via collisions with a sea of non-absorbing molecules. Radiative transfer will dominate and simpler models of the atmosphere will work better. The spectral line widths of absorbing molecules remaining will narrow (less collision induced broadening) and hence more photons will escape directly to space without suffering absorption and re-emission along the way. The net effect of changing the spectral profile is cooling. It is unlikely that the competing effects are the same size. I think the reduction of convection is the larger effect, but have not verified this by calculation.

Edit: My best guess is that the warming from suppression of convection is 10% of the effect of GHG concentration and the cooling from line narrowing is <1% of the effect of GHG concentration. I have more confidence in the line narrowing estimate.

If you were to remove all of the non-greenhouse gases from the atmosphere, the water would boil, as the total pressure of such gases of carbon dioxide and methane would be close to being a vacuum. As a result, there would be much more water vapor in the atmosphere, and the temperature would be several degrees warmer.



Yes, they would.



The only way to prevent water vapor from increasing would be to remove not only the oceans, but the ice and most of the fresh water. Earth's albedo and a lot of other stuff would change.

edit

I suppose that if you leave just enough oxygen, ntirogen and argon to prevent the water from bioling and the ice from subliming, you could still remove most of the non-greenhouse gases. You could probably reduce atmospheric pressure to as low as 1.7psi. At that pressure, water boils at 120 degrees F.

http://www.dofmaster.com/steam.html

That would still remove 88% of the atmosphere.

The greenhouse gases are sort of like tires on a car, they "grip" inbound and outbound infrared light. But all the mass of the atmosphere stores heat energy.

So if you did not have the mass, then you'd not be able to moderate the day/night temperature swings as much.

That would mean less atmospheric mass, so it would be more like our moon, large swings in temperature between daytime and night and generally much colder,

A stupid question though.

I would not care to explore that avenue. Some yoyo would try to prove it. I am glad that God has the answers and I don't have to work my mind on such useless garbage.

Who would be around to measure it ?

If, hypothetically, you sucked all of the oxygen, all of the nitrogen, all of the argon, and so on out of the Earth's atmosphere, and left only the water vapor, ozone, CO2, methane, chlorofluorocarbons, N2O, and so on, would that change the temperature of the planet? Ignore the effects that removing those gasses would have on the concentration of the remainder, assume that whatever force removed all of the non-greenhouse gasses kept all of the greenhouse gasses at their present levels. Also ignore any direct effect of the removal, albedo effects, and so on. I'm just looking for the impact on atmospheric heat retention.

That is, would greenhouse gasses react "normally" if they didn't have a larger mass of non-greenhouse gasses to bounce off of? Or would the planet become either slightly (or a lot) warmer or slightly (or a lot) cooler?