Prof Claes Johnson has proved in Computational Blackbody Radiation* that energy in radiation only gets converted to thermal energy if the peak frequency of the radiation from the source is above the peak frequency of the radiation from the target.

This essentially provides a mechanism which explains why the Second Law of Thermodynamics also applies for radiative heat transfer, as it does for heat transferred by conduction.

There seems no plausible alternative explanation for the observed Second Law, so I suggest we all heed what Johnson has deduced mathematically, being as he is, a Professor of Applied Mathematics.

It is not the net radiative flux (or even its direction) which determines whether (and in which direction) thermal energy is transferred. For example, if the emissivity of two bodies is very different, there can be more radiative flux from the cooler one. But all that flux will be scattered by the warmer one and not converted to thermal energy. Only the flux from the warmer one (no matter how weak) will be converted to thermal energy in the cooler one. This “ensures” that the Second Law is valid in all cases because it depends
on peak frequency which is proportional to absolute temperature – see http://en.wikipedia.org/wiki/Wien's_displacement_law

Thus the IPCC “backradiation” cannot affect the temperature of the surface and there can be no atmospheric radiative greenhouse effect.

* http://climate-change-theory.com/RadiationAbsorption.html