Since the beginning of fossil fuel emissions by mankind the carbon isotopes (13C, 14C) are altered due to the so- called Suess effects. Furthermore, the depletion of 14C due to the emission of radiocarbon-free fossil fuels (14C Suess effect) might lead to similar values in future and past radiocarbon signatures potentially introducing ambiguity in dating. I here test if a similar impact on the stable carbon isotope via the 13C Suess effect might help to distinguish between ancient and future carbon sources. To analyze a wide range of possibilities, I add to future emission scenarios carbon dioxide reduction (CDR) mechanisms, which partly enhance the depletion of atmospheric already caused by the 14C Suess effect. The 13C Suess effect leads to unprecedented depletion in d13C shifting the carbon cycle to a phase space in D14C-d13C, in which the system has not been during the last 50 000 years and therefore the similarity in past and future D14C (the ambiguity in 14C dating) induced by fossil fuels can in most cases be overcome by analyzing 13C. Only for slow changing reservoirs (e.g. deep Indo-Pacific Ocean) or when CDR scenarios are dominated by bioenergy with capture and storage the effect of anthropogenic activities on 13C does not unequivocally identify between past and future carbon cycle changes.