michael_nielsen

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!

Ah, my mistake! Note that for the whole system, the absorptivity/emissivity does change, for infrared wavelengths.

(GHGs alter absorption of infrared radiation from space. Although not part of the critical dynamics that determine the temperature, that is how absorptivity is defined)