The vast majority of CO2 is naturally produced.
From what? As far as I know, the only production of CO2 in nature comes from animals exhaling and natural forest fires.
Simply calculating the amount of CO2 produced is a herculean task far beyond our capabilities because the world literally runs on CO2
Not really...I imagine global CO2 production closely mimics consumption of oil, coal, and natural gas, which should be pretty easy to get a reasonable CO2 production value from.
plants require CO2 and plants are primary producers. The best we can do is to calculate the increase in CO2 and make some rough calculations based on measurements of isotopes as to what percentage we produce. (Assuming non-preferential uptake anyway, and ignoring that most man-made CO2 production occurs away from most CO2 consumption. And ignoring that C13 concentration has often dropped when CO2 increases - which may well turn out to be an artifact of the proxies used. And ignoring that such events as volcanic eruptions and tectonic activity may also emit old and depleted CO2.)
I don't even really know what you're saying here, my only contribution is the isotopic distribution of C13 is essentially constant, distribution of CO2 released from humans will be pretty evenly distributed due to weather, and I don't think tectonic activity releases a whole lot of CO2. SO2, sure, but not so much CO2.
This gives us an estimate of the excess CO2 that is naturally produced. We can then make a very, very rough estimate about how much is naturally produced by measuring production and consumption in sample environments and projecting the results, but even within those sample environments we're making only a rough estimate because most CO2 produced or consumed is used to maintain life processes, not to, say, add body mass. Nonetheless, we can say with certainty that humans produce a very small percentage of all CO2, and we can say with certainty that one cannot calculate what percentage of CO2 we produce simply by subtracting out what we calculate we produce from what is there to be measured.
And again, I think you miss the point. We can obviate all the of machinations you just described by understanding one very important fact: we can closely model how much anthropogenic CO2 is released based on the fact that we know to a reasonable degree of accuracy how much fossil fuel is burned on an annual basis around the globe. We also know fairly accurately how much CO2 is in the atmosphere. We could probably then extrapolate over the years of the global CO2 concentration to deduce how much of the CO2 we've put in the air and has remained in the air, which will allow us to judge the effects of natural sequestration feedback loops.
We know we're dumping millions of metric tons of CO2 in the atmosphere, that's not up for debate. We also know CO2 is a greenhouse gas, also not up for debate. What we don't know is how much our activity is going to affect the climate...we know it will, but to what degree? And can we develop means to sequester CO2 that will reduce our atmospheric CO2 input?