The previous correspondent asks the interesting question as to whether the atmosphere is an insulator or a diode (a diode has a preferential direction for the way that it conducts whereas an insulator does not).
The answer is that it behaves much more like a diode than an insulator.
The reason for this is that the predominant radiation from the Sun hitting the Earth is in the form of short wave radiation (eg. light). Much of this is reflected back into space due to the Earth's albedo. The CO2 in the atmosphere does not affect this process. What does not get reflected heats up the Earth. Part of this is then re radiated from the Earth as long wave radiation (eg. infra red). It is the infra red which can be reflected back to Earth due to the effect of greenhouse gasses such as CO2, Methane or water vapour. It is this effect in part which is causing a warming of the Earth. It is the change from short wave to long wave radiation which leads to the 'diode effect'.
Nitrogen, Oxygen and Argon do not play a role in this process.
The correspondent is correct in saying that the model does not take into account other issues such as the Earth's magnetic field, core temperature or many other factors such as the change in the radiation of the Sun or the variation in albedo due to the melting of the ice caps. Full climate models attempt to take account of these and many many other factors. However the advantage of the model above is that it's simplicity (and surprising accuracy) allows us to make predictions of what could happen if certain aspects of the Earth's system (such as the level of CO2) change rapidly.