A NEW STOMATAL-BASED METHOD FOR RECONSTRUCTING ATMOSPHERIC CO2

Estimates of atmospheric CO₂ concentration for the Phanerozoic Eon are based on a variety of proxies, but quantitative reconstructions for the pre-Cenozoic come overwhelmingly from one approach, the carbon isotopic composition (δ¹³C) of pedogenic carbonate. One proxy for reconstructing CO₂ during the late Cretaceous and Cenozoic relies on an empirical relationship between stomatal distributions and CO₂, but estimates are usually unbounded above ~500-1000 ppm and are limited to only a handful of calibrated species. Here, we calculate atmospheric CO₂ using a well-validated equation for leaf gas exchange, with key variables obtained directly from the δ¹³C and stomatal anatomy of fossil leaves. This mechanistic approach avoids the unbounded errors in extrapolative methods and is applicable to most fossil material bearing stomata, therefore opening up much of the 400 m.y. paleobotanical record for analysis. Our new approach, validated against ice cores and direct measurements, produces CO₂ estimates that are less than 1000 ppm for most of the post-Devonian, coincident with the appearance and global proliferation of forests. Uncertainties, obtained from Monte Carlo simulations, are typically less than for CO₂ estimates from other approaches. These results provide critical new empirical support for the emerging view that large (~2000-3000 ppm), long-term swings in CO₂ do not characterize the post-Devonian and that Earth’s long-term climate sensitivity to CO₂ is greater than originally thought.