Usually plants are limited by things other than CO2. I have heard of experiments where someone tried to make plants grow faster by feeding them extra CO2, and what I usually hear the result being is unsuccessful.
Although I don't doubt that there are some things that would grow faster and/or better if we had more CO2 in our atmosphere. And I'm sure there are farmers and prospective farmers in parts of Canada and other cold places that would be glad to have a longer growing season from global warming.
The main complaint of "greenhouse gases" is that they make the world warmer, and volcanoes make the world cooler since they spew particles that block sunlight.
- Don Klipstein ( snipped-for-privacy@misty.com)