The increase in 12C in the atmosphere is, in my opinion, weak evidence that the annual increases are driven only by fossil fuel sources. The atmosphere can’t tell ‘anthropogenic’ carbon dioxide from natural carbon dioxide. It seems unlikely that a source that represents only about 4% of the total flux is going to drive the system. The oceans sequester the vast majority of the carbon. One would expect that warming oceans (from whatever forcing) would increase the rate of out-gassing in mid-latitudes, and decrease the rate of extraction at high-latitudes. It seems more reasonable to me that, in a world with warming oceans, there would be a shift in the relative amounts of carbon in the oceans and the atmosphere. That would be the case even in the absence of any anthropogenic carbon.