This was something fun. I remember a questions in one of my first exames in university; what would happen to the plants if CO2 was increasing? My answer was wrong and I was told that more CO2 was good for the plants. This thing have annoyed me since that. Ok. Ok let's say that's the fact, plants like it- they use it and got to grow more, if plants use it- then an increase in the atmosphere should not be visible, right? And it is the surplus that lays around the earth as a cover and cause global warming. The surplus is also the cause of ocean acidification, lowering in pH, which is the other CO2 problem, leading to use-up of alkalic composions as in mountain rocks and coral reefs, which leads to a decline of production in the ocean.
Take a look at the CO2 is green homepage and make a consideration yourself.