How is the mechanism of greenhouse gases interacting with IR radiation?

Question

How does atmospheric CO2 and other Greenhousgases (GHG) affect the incoming (from sun) and outgoing (from earth) radiation. I understand that at certain wavenumbers (or areas of wavenumbers) in the Infrared (IR) these molecules (amongst others) absorb an IR-photon, get a bit "warmer" (means perhaps rotate faster or similar) and after a while reemit another photon with less energy (because the enhanced movement costs a bit of energy) ... resulting in a further warming. The emitted photon either hits another photon or will be going to space or will go back to earth.

Now you have lots of molecules (N) and statistically these three mechanisms lead to a warming of the surface of the earth (let aside scattering etc.).

My question is: 1. is this picture correct 2. when yes how are the different mechanisms in terms of photonic energies

Answer 1

To make things simple you want to compare two atmospheres that are identical, except that one has infinitessimally more of some IR absorber. In the second case, some IR photons that were traveling upwards are absorbed, and thermalized (i.e. the energy is transferred to the surrounding gas molecules, before being re-emitted). The IR photons are basically transferring energy, in this case from the surface (or perhaps a lower level of the atmosphere), to wherever they are absorbed. If the surface is warmer than the atmosphere where our absorber molecule resides, then that layer is on net absorbing more energy than it was in the unperturbed case, so that layer of the atmosphere will warm (with respect to the control case). Eventually once we let the atmosphere respond (but then the two atmospheres have different temperature profiles), we see that there is also more downgoing IR radiation below our absorbed level, so the layers of atmosphere below, and the ground will see a net warming effect.

To think of it in another way, the radiative impedence of the system (surface cum atmosphere) to space is higher with the addition of the greenhouse gases, so given a fixed energy input (absorbed sunlight) the equilibrium temperature will rise. Beyond that first order effect, the details of atmospheric structure and flow begin to change, and then the hard part of the problem begins.