Slide 2 in this series.
CO2 like other IRGs can absorb a IR photon at specific wavelengths. Once absorbed the molecule vibrates like the doors on a car opening and closing automatically. This is the nature of the quantum mechanics of CO2.
However this photon can be released into thermal energy or with a release of another photon by either a collision or a re-radiation event. What we find amazing is that this phenomena although studied have not been tested and statistics vs temp and density taken experimentally.
NEXT: The photon energy or incoming spectra coming into earth's atmosphere is reasonably well understood, although its variability is not.
The links below are to the pages or slides in this section, which covers the science of CO2.