Dican, I read your posts and i agree that plants will grow fine without the additional CO2 but they will grow even better with it. Although, i still haven't purchased a CO2 set up yet, so i'm not sure on how much better they will grow.
But as for global warming and greenhouse gases, I'm not sure you have the full picture here. Global warming is not caused mearly by CO2 and the amount of CO2 a grower (or for ALL growers combined) uses that doesn't get consumed by the plants and goes out into the atmosphere is tiny. Especially when compared to what the industiral world is emitting daily. For example, In that recent Iceland Volcano eruption there were predicted to be about 150,000 tons of CO2 released PER DAY by the volcano. That's a lot of CO2. BUT, that number was dwarfed by the amount of CO2 emissions NOT being released due to all the airplanes being grounded. The amount of grounded airlines would have emitted about 345,000 tons of CO2 PER DAY.
So with all that said adding 1200 parts per million of CO2 to a grow room is doing nothing to aid global warming.