Figure 1: Typical mid-latitude TOA emission of radiation from earth. (original graph is from NASA )
Below the tropopause water vapor (WV) increases from an average of about 10,000 ppmv (1%, 4% near the equator) at ground level to, because of the low temperature (about negative 50 °C), 32 ppmv at the tropopause (about 10 km altitude). Radiation from WV molecules below about wavenumber 600/cm can only be absorbed by other WV molecules. This huge WV molecule population gradient means that much of the outward directed radiation from WV molecules will make it all the way to space, especially from near the tropopause. This is demonstrated by the ‘hash’ observed in this wavenumber range measured at the top-of-atmosphere as shown on Figure 1.
The redirection mechanism is radiation energy that is absorbed by CO2 below the tropopause is shared with all molecules (thermalization) and emitted to space by WV molecules.
At high altitude (above about 20 km), the remaining energy is conducted from non-ghg molecules to ghg molecules for radiation towards space. For lack of a better term, call the conduction of energy from non-ghg to ghg molecules and radiated from them reverse-thermalization.
Ghg in the warmed air can emit photons only at a limited number of wavelengths (or wavenumbers) characteristic for each molecule species. Furthermore, all theoretically possible wavenumbers are not equally likely.
Most of the photons emitted by the water vapor molecules are at wavelengths different from the comparatively narrow band that CO2 molecules can absorb. Effectively, much of the terrestrial thermal radiation energy absorbed by CO2 (and other non-condensing ghg) is thermalized, redirected to, and radiated to space from water vapor.
At very high altitudes, temperature, molecule spacing and time between collisions increases to where reverse-thermalization to CO2 (and O3) molecules becomes significant as does radiation from them to space.