The Greenhouse Effect: Trapping IR Energy through Absorption and Re-emission

[John Plant]

Is the mechanism of greenhouse gases trapping energy emitted as Infra Red radiation by the cooling ground either
reflecting back a 50% of the radiated IR energy back to Earth
or/and
the greenhouse gas molecule becoming excited with the extra energy and essentially becoming hotter?
Is it just the first or both?

 

[Keith_McClary]

I would say neither. Many explanations are oversimplified.
I like Eli Rabett's "The Simplest Explanation"

I see that just yesterday he is working on an improved version:
"How Greenhouse Gases Heat the Surface"
where he says:

Energy does not stay in the molecule that absorbs the IR photon, to be re-radiated later. This is not so, it is quickly degenerated to thermal motion (translation, zipping about) via collisions. Thermalization requires about a 10 μs at atmospheric pressure. So where does the emission come from the bunnies ask?

Well, there is a considerable thermal energy at room temperature, and even much lower. True this average energy is low compared to even the lowest vibrational excitation of CO2 (which would be ~1000 K), but it is enough that a small, but significant fraction of CO2 molecules are always found in excited levels which can emit in the IR (about 6% at room temperature).

 

[DrStupid]

Is the mechanism of greenhouse gases trapping energy emitted as Infra Red radiation by the cooling ground either reflecting back a 50% of the radiated IR energy back to Earth or/and the greenhouse gas molecule becoming excited with the extra energy and essentially becoming hotter?

The energy goes back to the ground by absorption and re-emission (not reflection). There is a nice Sankey diagram in the German Wikipedia entry for the greenhouse effect:

[Edit: I just see that FactChecker posted an English version of the diagram in a parallel thread.]