"You can calculate the amount of carbon that can be dumped
into the environment before it leads to utter catastrophe,
such as the end of civilization as we know it, or possibly
extinction of the human species. This is a finite number,
on the order of 500 gigatons."
The remainder your post is a very interesting discussion of how to deal with this starting assumption. I wonder if you could expand on how you come to this initial point.
* What specific "utter catastrophe" do you foresee happening after the next 500 GT of carbon is burned (on the order of 50 years at current rates) that could kill the human race?
* What calculations back your conclusion?
I have heard many plausible sounding counter-arguments that you might also address, such as:
* While the direct radiative forcing of CO2, is pretty well understood (1 °C global warming per doubling = "settled science"), the climate sensitivity (ie the impact of the feedbacks) is not. This adds large uncertainties to the projections.
* Climate models have utterly failed to model the current 18 year "pause". If all of the models show more warming than observed, why should we trust the calculations?
* CO2 makes plants grow better, and has been responsible for increased agricultural productivity, thus benefiting humanity (possibly more than the damages from warming).