Flannery, page-49

" it radiates part of this energy back toward space as infrared radiation. This radiation, unlike visible light, tends to be absorbed by the greenhouse gases in the atmosphere, raising its temperature. "

CO2 deflects outgoing radiation like a pinball machine, the radiation bounces around from one molecule to another until it eventually is deflected to outer space or back down to earth.
The CO2 does not get heated in the process, if it did, and it heated the atmosphere, that atmosphere could not warm the earth as you cannot heat a warm body from a cold body.(the 2nd law of thermodynamics)

The warming effect of CO2 is logarithmic, so more and more CO2 have to be added to the atmosphere to have the same effect, CO2 blocks outgoing radiation at a frequency of 15 microns, satellites cannot detect very much IR of this frequency escaping now, it is almost reached saturation point.
Adding increasing amounts of CO2 doesn't add much to the temperature now.