A small, closed greenhouse is a lot simpler than the entire Earth.
Did you read the science?
What if we add bugs to the greenhouse?
Increasing the CO2 increases the carbon/nitrogen ratio in the plant matter, making it less nutritious for herbivorous predators, which then must eat more, and it is detrimental to the plants.
The record for the PETM is very clear.
Your "CO2 is plant food argument is a nice example of the "fallacy of exclusion" in which a *single* cause and effect relationship is presented as the only cause and effect relationship amongst *many* that may be at work in a system. This is a rhetorical trick, not a scientific argument.
If you want to talk science above a comic book level, please read the original sources.
One is posted above.
A person with your obvious education and intelligence (and as attested by others) should be perfectly capable of reading and understanding this.
Then we can discuss the original sources as skeptic to skeptic.