This was actually done in the days before the ice core measurements were available: Both Freyer in Germany and Roger Francey in Australia derived the atmospheric 13C history over the last 300 years from tree rings of selected trees. But as Willi Brand rightly points out, the variable fractionation makes it difficult to extract the atmospheric 13C signal. Therefore the 13C records of Freyer and Francey were quite different, which lead to heated debates at the time.