Was atmospheric CO2 capped at 1000 ppm over the past 300 million years?

Atmospheric carbon dioxide concentration has shifted dynamically over the Phanerozoic according to mass balance models and the majority of proxy estimates. A new paleo-CO2 proxy method underpinned by mechanistic understanding of plant stomatal, isotopic and photosynthetic responses to CO2 has provocatively claimed that maximum paleoatmospheric CO2 was capped at 1000 ppm for the majority of the past 300 million years. Here we evaluate the robustness of the new paleo-proxy CO2 model by testing its sensitivity to initial parameterization and to scaling factors employed to estimate paleophysiological function from anatomical and morphological traits. A series of sensitivity analyses find that the model is robust to modification in some of the constants employed, such as CO2 compensation point and mesophyll conductance, resulting in variability in paleo-CO2 estimates which are already accounted for in the error propagation of the model. We demonstrate high sensitivity in the model to key input parameters such as initial fossil plant assimilation rate, termed A(0) and scaling factors used to estimate stomatal conductance from measurements of fossil stomata. Incorrect parameterization of A(0) has resulted in under estimation of pCO(2) by as much as 600 ppm. Despite these uncertainties, our analysis highlights that the new mechanistic paleo-CO2 proxy of Franks et al. (2014) has significant potential to derive robust and more accurate CO2 estimates from fossil plant stomata, as long as parameterization of A0 is strongly justified with species appropriate morphological and anatomical data. We highlight methods that can be used to improve current estimates of fossil plant assimilation rates, reduce uncertainty associated with implementation of the Franks et al. (2014) model and importantly add to understanding of patterns of plant productivity over the Phanerozoic, for which there currently is no consensus. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).