Aren’t photons the vector for energy input to earth?
Photons do not emit energy but rather, carry stored kinetic energy that has the potential to give or TAKE AWAY heat so, to answer your question, they ARE an energy vector but would think they are not the fuel for the Carbon Cycle since they get scattered by particles and water vapour....which leads me to think they are part of the cocktail, but not the fuel in itself. Here is an excellent visual and summarized source for interactions of radiation with the atmosphere > https://www.nrcan.gc.ca/maps-tools-publications/satellite-imagery-air-photos/remote-sensing-tutorials/introduction/interactions-atmosphere/14635 

I believe you have asked an excellent question and look forward to reading the subsequent responses.