Not all carboniferous fuels produced are burned (conversion to plastics and other products).
That being said, I did see in a scholarly study that the “gross greenhouse gas production” of either Krakatu, Toba, or Tambora, was more than the total output of all the fossil fuels we will ever extract and burn.
The exact method by which they came to that conclusion I do not know, but I believe it had to do with ice, and ocean floor cores dating back to about 1.1 million years.
True
But if a high estimate based on coal mined and oil pumped were to show that it is impossible for the rise in CO2 concentration over the same period of time to have been caused exclusively by man, it would make an interesting problem for the AGW theory.
The question is, could the nearly 0.01% increase since 1900 be caused by a high estimate of the carbon burned?
Knowing that carbon is also returned to the land and oceans, if the amount of carbon burned is less than or equal to a 0.01% increase in atmospheric concentration, then at least some of the rise must be caused by other sources. If it is much lower, then it would be pretty clear that man's influence is small.