The physics is not as obvious as you suggest. Absorbed energy is also almost instantly reemitted, so some claim that greenhouse gases are in fact transparent. That's not exactly true - while absorption and emission are fast, emission occurs in the random direction, slowing energy transport, as half is effectively reflected back. But details are fine and difficult to estimate.
Better place for this question would be physicsforums.com - search before asking, as there were several long threads on the subject in the last years (check Other sciences/Earth subforum).