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.

Something I don't understand is whether the effect is due to the *fraction* of CO2 in the atmosphere or the *amount* of CO2 in the atmosphere. I've been wondering this after reading that the atmosphere may have been thicker in the past (http://pubs.acs.org/subscribe/archive/ci/30/i12/html/12learn.html …)

I've read - reliably, I think - that it depends on concentration on different altitudes.

@redblobgames and @michael_nielsen , I found this article useful. The amount of CO2 matters because it broadens the absorption bands and raises the altitude of the surface of last scattering, which has a lower temperature and lower emitting power. https://skepticalscience.com/saturated-co2-effect-advanced.htm …

There are many subtle or technical aspects to the whole problem, and I'm a little surprised I don't hear more conversations like these. I spent days mulling over these things before finding that article and a related figure of the fine-grained absorption bands.