michael_nielsen

I tried to write out an explanation of how the heating due to greenhouse gases works, and realized there's a big hole in my understanding. Can someone who understand the detailed physics help or point me to a really clear and thorough explanation?

A very simple model (neglecting many effects, but should still work) is as follows: solar radiation comes in. Some is reflected off clouds, while some passes through the atmosphere. Some reflects immediately back from the Earth, and passes back through the atmosphere.

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.