My response to your water / food coloring analogy you made the other day (see post 7).
I will freely and fully admit that I have no ability to calculate carbon fluxes from stable isotope carbon ratios. I can only provide numbers from people that purport to know how to do it with "box models" and the like. My analogy was primarily intended to show that in a world with nearly equal natural input and output fluxes of CO2 to the atmosphere, the addition of fossil fuel CO2 is entirely responsible for the increase -- as seen. And I've noted (a few times now to drrocket, and elsewhere) that if there were no human activities, the net CO2 flux would be out of the atmosphere. THEN (on a very long, long time-scale) we might have to worry about another cool period.