michael_nielsen

The net effect is for the Earth to have to heat up a little extra, thus producing a little more infrared so that at equilibrium the total amount of energy escaping is the same as the total amount of energy incident.

What I don't understand: the IR that's being blocked by the GHGs is energy which has already been absorbed by the Earth. So this shouldn't change the Earth's overall absorbivity, and I don't see how it could change the Earth's temperature.

Twitter may not be the best medium for this(??) But if someone who understands this well can point me to a good explanation, I'd appreciate it. Thanks!

@AndrewDohertyQu @dabacon @worrydream @patrickc Do you know?

I omitted the technical details, but it's this: the temperature should be set by: incoming energy flux = epsilon sigma T^4, where epsilon is the emissivity, sigma is the Stefan-Boltzmann constant, and T is the temperature.

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.