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[1] Biological and geochemical CO 2 proxies provide critical constraints on understanding the role of atmospheric CO 2 in driving climate change during Earth history. et al., 1994) is extended and calibrated using our experimental results. The potential for fossil liverworts to record past CO 2 changes is investigated by analyzing the d 13 C of specimens collected from Alexander Island, Antarctica dating to the ''greenhouse'' world of the mid-Cretaceous. Our analysis and isotopic model yield mid-Cretaceous CO 2 concentrations of 1000-1400 ppm, in general agreement with independent proxy data and long-term carbon cycle models. The exceptionally long evolutionary history of bryophytes offers the possibility of reconstructing CO 2 concentrations back to the midOrdovician, pre-dating all currently used quantitative CO 2 proxies.