Supposedly the CO2 produced by burning fossil fuels has a lower C13 isotope component than naturally occurring CO2. The argument goes that the ratio of the C13 to C12 isotope has been dropping therefore the increase in the CO2 must be due to anthropogenic sources however that may not be the case Manmade CO2 ?
In addition we know from the ice cores that CO2 increases lag temperature increases by approximately 800 years ( Not vice versa as Gore claimed in his ridiculous movie). Perhaps not co-incidentally the Medieval Warm period was reaching its zenith approximately 800 years ago.
Also there are other views on whether or not all of the CO2 increases are man made. An alternative View of CO2
Yes, there would be a rise in CO2 now due to the MWP centuries ago. But the deep ocean warming behind that process would not be more than 1C and the CO2 release would not be more than 10ppm (probably half of that). So MWP warming cannot explain the current CO2 rise of 120ppm and rising.