michael_nielsen

But the Earth also absorbs some of the radiation, heating the Earth. At equilibrium that energy is later re-radiated. Crucially, that's at infrared frequencies, where greenhouse gases make the atmosphere somewhat opaque

Intuitively, the GHGs makes the atmosphere a little like a one-way "blanket", allowing some energy through (at optical and UV frequencies), but making it harder for the infrared radiation to get back out again.

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?

Here's my attempt. ENERGY IN = ENERGY OUT. Model OUT as Stefan-Boltzman, so proportional to T^4. Except some of the "OUT" doesn't make it out due to atmospheric absorption of IR. So T has to increase to get the equation to work.

Maybe another way to say it is, the Stefan Boltzmann law for gray bodies can only be true if the output spectrum follows Planck's law.