Stable carbon isotope studies on marine and terrestrial organic and inorganic carbon provide a means for detecting global climate change and for reconstructing past concentrations of atmospheric CO2. Comparison between the CO2 estimates reconstructed from carbon isotope studies for the past 150Ma show good agreement with the predictions of a long-term carbon-cycle model based on mass-balance studies. Further, the CO2 estimates from these sources over the entire Phanerozoic show agreement with the fossil record of leaf stomatal density change—a feature inversely related to the concentration of atmospheric CO2. Isotopic studies on temporal sequences of fossilized terrestrial organic matter have contributed to palaeoecological studies on shifts in the dominance of plants with the C4 photosynthetic pathway in ecosystems and historical changes in the metabolic processes of leaves of individual species. The long-term perspective offered by these studies provides critical information for assessing the responses of biological systems to future global environmental change.