When I present my global warming talk I state, "in the laboratory CO2 heats the atmosphere, but the real-world atmosphere is far more complex than the one in the laboratory." Here is a great article as to why that is. Note: Not everything in Willis' discussion is literally true (nor does he intend it to be). But, as an illustration of why CO2 doesn't work in the real atmosphere as it does in the lab, it is excellent.
Article is here. It is well-written and easy to read.