"I have never been able to get an answer from any of the many green warriors on HC regarding the correct amount of CO2 that we and the planet need as an optimum figure"
Interesting there probably is an overall optimal figure as far as humanity is concerned, though different parts/countries of the world probably have a different number based on their location.
I would guess it is somewhere in the range that allowed humanity to drag itself out of the stoneage and thrive as a species. Though stability may be more important than the actual number.
