That's not entirely correct Mark. We can calculate, roughly, how much radiative forcing comes from a doubling of CO2 and how that might effect the energy balance of the plant. We still don't know what the actual effect of that extra energy will be, it might just all end up at the bottom of the ocean or aerosols in the atmosphere may completely negate any effect whatsoever, heck, we don't even know what role clouds actually play in the whole picture or cosmic rays or a thousand other climatic variables. Ask any climate scientist worth their salt they will tell you the picture is far from complete.
http://notrickszone.com/2018/03/22/200-non-hockey-stick-graphs-published-since-2017-invalidate-claims-of-unprecedented-global-scale-warming/#sthash.XKr6pqTD.dpbs