Ok, although this question arises out of the global warming debate, this is a question purely for physicists and not intended to branch into that particular debate.
We are told that LWIR from the Earth to the atmosphere is absorbed by greenhouse gasses and then re-radiated (in the form of backradiation) partially to the surface of the planet, "...thus warming the surface...".
My specific question is this:
How exactly can the re-radiated LWIR from a cooler atmosphere warm the original source of its absorbed energy?
Edit: I am editing this question as several people have stated this is a duplicate question. I disagree. The other question quoted three alternatives and some answers stated the 2nd Law of Thermodynamics was not compromised. Please re-read my question: can the backradiated energy possibly warm a surface which was the original source of that energy? In other words, how can the surface heat itself further when there is no other energy source? I hope this clarifies, and thanks to all that have answered.