My best guess is that some of each ratio change (13/12 and 14/12) is natural and some is from fossil fuel burning. In the case of 14/12, one natural factor is high solar activity in the late 20th century. But most of the rise is fossil fuels. For 13/12, some of the ratio change is natural because some natural sources mimic the fossil fuel ratio (e.g. deep ocean). Dr Spencer also has some thoughts on 13/12 http://wattsupwiththat.com/2008/01/28/spencer-pt2-more-co2-peculiarities-the-c13c12-isotope-ratio/ answered by Englebeen who says that vegetation and fossil fuels show a similar ratio (due to growing plants preference for C12).
Thanks for the info. I knew that the C14/C12 ratio had dropped. The change in C13/C12 ratio I find a bit difficult to swallow as anthropogenic. But I do think that the ratios are far more accessible than some of the other climate data that is presented as fact.