You make an excellent point with the initial oxygenation of the atmosphere. Yet I would be careful in understating the significance of the impacts of "a trace gas".
The physics of carbon dioxide is very interesting. I don't fully understand it myself though I've read widely on the matter. The heat retention of carbon though is well known... mostly as the release of heat when carbon is oxidised (burned). It is a big deal, if you do a bit of reading on CO2 and other greenhouse gases.
