I will attempt to explain backscattering, but first let's choose the best climate model by the following arguments:
300 scientists in the IPCC say .... so what, 1000 scientists say they are idiots .... we are just coming out of an ice age ... sez who? ... most of the heat is lost through convection ... well duh ... invalid! you are violating the 2nd law ... am not ... are too ... am not ... the ideal gas law explains the earth and all the planets too ... buncha crap ... you are just a pinko tree-hugging socialist ... well you are being duped by big oil companies.
Now that we have chosen the climate model, we no doubt agree on this:
The sun warms the earth by a wide band of wavelengths, from IR to UV, not including some IR greenhouse bands that have been blocked at the top of the atmosphere. The earth is too cold to radiate anything but a broad band of long wavelength IR with a wide range of spectra that depend on the local landscape or seascape. A large part of that earth IR escapes through the atmosphere out to space, to the extent that on average, the energy in is roughly equal to the energy out.
Lets look at the atomic physics effects of CO2 on that model.
The wide band of IR that the earth tries to radiate to space includes the wavelengths that the CO2 blocked. This is because the warm earth no longer "remembers" how it got warm, so the amount radiated is solely dependent on the temperature and emissivity of the local landscape. The earth radiation will include CO2 bands, and those bands will be captured by the CO2 in the atmosphere and backscatter back to earth. In short, any earth radiation in the CO2 bands will be sent back to earth. Because of that, the earth can't send that particular band of heat radiation to space, and so the earth won't be as cold as it would otherwise, and the atmosphere won't be as hot.
This explanation only used atomic physics, moreover the outcome did not violate thermodynamics and shows how the earth can get warmer with CO2. There is much more to the story. This is only meant to explain backscattering in terms of the atomic physics and not thermodynamics.
