You initial premise is faulty, i.e. a net change in energy being necessary. CO2 stores energy - as heat - that’s why it warms. There is no net change, only an energy transfer.
Actually, my premise is exactly correct. In order for global warming to occur, the atmospheric net energy content must increase. I have never seen anyone describe a mechanism by which CO2 stores energy. All I have seen is that CO2 absorbs in the IR, and emits in the IR--the process, which is known as fluorescence, happens almost instantaneously and is therefore not a storage mechanism.
On the other hand, I saw where another FReeper pointed out that water has a built-in energy storage mechanism--when it undergoes phase changes, a large amount of energy is absorbed or released depending on the direction of the phase change. In that respect, water can and does strongly affect atmospheric energy content. The only phase change CO2 undergoes occurs at about -80 C (give or take a few degrees). Above that temperature, I do not see any great capacity for CO2 to store energy.