In Europe there is a region called "Karst" (in German anyway), that used to be full of florishing green woods. Now it is a sort of desert, and "Karst" is a synonym for an unfriendly, dry and barren landscape.

How did it get like that? Romans felled all the trees for ship building, many centuries ago.

I know this does not relate directly to the CO2 discussion (which also seems a bit simplistic to me), but driving through that landscape as a kid left a deep impression on me. Surely if the ancient Romans could change the environment, humans can affect the environment in the 21st century. It would be ridiculous to ascertain that humans have no effect on the environment at all, even if we don't understand the precise chain of effects.