Reading a CO₂ signal from fossil stomata

The inverse relationship between atmospheric CO₂ and the stomatal index (proportion of epidermal cells that are stomata) of vascular land plant leaves has led to the use of fossil plant cuticles for determining ancient levels of CO₂ . In contemporary plants the stomatal index repeatedly shows a lower sensitivity atmospheric CO₂ levels above 340 ppm in the short term. These observations demonstrate that the phenotypic response is nonlinear and may place constraints on estimating higher-than-present palaeo-CO₂ levels in this way. We review a range of evidence to investigate the nature of this nonlinearity. Our new data, from fossil Ginkgo cuticles, suggest that the genotypic response of fossil Ginkgo closely tracks the phenotypic response seen in CO₂ enrichment experiments. Reconstructed atmospheric CO₂ values from fossil Ginkgo cuticles compare well with the stomatal ratio method of obtaining a quantitative CO₂ signal from extinct fossil plants, and independent geochemical modelling studies of the long-term carbon cycle. Although there is self-consistency between palaeobiological and geochemical CO₂ estimates, it should be recognized that the nonlinear response is a limitation of the stomatal approach to estimating high palaeo-CO₂ levels.