Isn't that pretty obvious? It's like forcing the pigs to eat too much potatoes - they will grow faster, if a person will eat too much he'll get fat. So more Carbon Dioxide = more greeneries
Yes, but. When the tropical rain forests eat the carbon dioxide, they exhale oxygen. More oxygen means that we can burn more fossil fuels, thereby making more carbon dioxide. Neat, eh? We can drive our cars further and longer from the old (fossil fuel) rain forests, and the present rain forests give us oxygen, and ultimately convert themselves (over a long period of time) into fossil fuels so that the next generations can drive their cars because of the oxygen future generation rain forests exhale.
Neat, right?