What the professor seems to not know is this: 

1) The marginal effectiveness of CO2 as a greenhouse gas asymptotically approaches zero as the atmospheric density increases. And it's close to that now (basic Calculus applied to the relevant equation.) 

2) More CO2 => more plants

nostr:note155y5f3yjekkra5j05dp4gxakxz8v4u455tfd2elgxz6jjckf96lspjp5pa 

 You also pointed out the other day that cloud cover as a moderating factor is omitted from climate models