The effects of increased CO2 are not as simple as may seem at first. We all know that plants use the stuff in photosynthesis, but above a certain level there are other factors that kick in and prevent the plants benefitting further. Something to do with pressure. I need to check up on the details, but I have a friend who has been researching this sort of thing with regard to planetary habitability, and he has checked atmospheric stuff with a colleague in the Met Office.

Penny