michael_nielsen

Since anything absorbed by the GHGs has already been absorbed by the Earth, the absorptivity (and thus the emissivity) shouldn't be changed by the GHGs, and so I don't see how T can be changed by the GHGs.

The temperature of the earth at the ground is not the same as the temperature of the upper atmosphere, where the IR has its last chance to radiate from Earth. Analogy: your clothes radiate at a lower temperature than your skin. Your clothes make the air above your skin warmer.

Good point. So you're saying the temp of the Earth+atmosphere system is determined by the Stefan-Boltzmann relation, but the Earth may be quite a bit warmer.

In more detail, the temperature of the Earth+atmosphere system isn't changed by GHGs (since the absorptivity and thus emissivity isn't changed, and ignoring the role of water vapour in setting emissivity). But the ground temperature may well be.

Though true, this is not the resolution. Even in the simple S-B picture, the temperature *does* go up, because the incoming flux is all inward flux, not just incident flux. Inward flux goes up from GHG, raising T

Remember, it's an equilibrium relation

I'm talking about SB for the combined Earth+atmosphere system - the point that was confusing me is that the emissivity for that system wasn't changed.

Just to be clear: I understand the points in your last two tweets, and agree - that wasn't where my confusion lay. Thanks!