I did a back of the envelope calculation or maybe better a guesstimate on this awhile back. At perfect conversion efficiency you could turn emitted CO2 back to carbon using 15-20 average sized nuclear power plants. If you realistically get 10% overall efficiency then maybe you need 200 of them. All that assumes you can actually build chemical cells that don't need continuous replacement. It also just compensates for current emissions, and doesn't turn the clock back. My numbers could be very wrong in either direction.

My takeaway was just like losing weight. It's more effective to eat less than to exercise. Here, it's bound to be better to burn less.

This was an electrochemical approach however. It's possible some bio guy will figure it out using photosynthesis.