As I understand it CO2 is such a good absorber of IR that (even at pre industrialisation levels of C02) it only takes about 25 meters of the atmosphere to scatter most of the IR radiated by the earth (
http://clivebest.com/blog/?p=1169 ). This means energy is only radiated into space from high up in the atmosphere, where the air and CO2 is thin enough.
Its been suggested that adding CO2 raises the effective height from which IR energy is radiated. Unfortunately the higher you go the colder it gets and IR radiation is proportional to the fourth power of temperature (T^4).
If less energy is radiated into space the temperature of the planet must increase until the energy lost from the earth once again matches that arriving from the sun.