Concentration of atmospheric CO2: support for the strategy of model implementation outlined in Franks et al. (2014)

Fig. 2. Validation of recommended A 0 values for the Franks et al. (2014) model. To estimate atmospheric CO 2 concentration (c a ) the Franks et al. (2014) model calculates the average operating CO 2 assimilation rate (A n ) for the fossils using a reference CO 2 assimilation rate at ambient atmospheric CO 2 concentration (A 0 ) as one of the input variables. The range of A n calculated for the Phanerozoic in Franks et al. (2014) (yellow symbols with method 1, red symbols with method 2) is similar to that calculated from the model of Franks and Beerling (2009) (termed F&B 2009; blue shaded area) which uses a different methodology to determine A n from prescribed c a values. Close agreement between these two independently determined Phanerozoic A n ranges supports the recommended A 0 values in Franks et al. (2014). To enable comparison, A n from Franks and Beerling (2009) was determined from the modelled upper and lower limits of maximum A n (A n(max) ; see text), and A n from Franks et al. (2014) was adjusted to correspond with the prescribed c a values in Franks and Beerling (2009). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) –

Comment on “Was atmospheric CO2 capped at 1000ppm over the past 300millionyears?” by McElwain J. C. et al.

by Franks P. J., Royer Dana L. (2017)

Peter J. Franks, University of Sydney
Dana Royer Department of Earth and Environmental Sciences Wesleyan University, Middletown, CT.

in  [Palaeogeography, Palaeoclimatology, Palaeoecology 441 (2016) 653–658] –

Palaeogeography Palaeoclimatology Palaeoecology · January 2017
DOI: 10.1016/j.palaeo.2017.01.015 –
Franks et al. (2014) developed a model for estimating the concentration of atmospheric CO2 (ca) that can be applied to most stomata-bearing plant fossils.
In a recent paper, McElwain et al. (2016a) proposed changes to two of the key inputs: mesophyll conductance to CO2 (gm) and CO2 assimilation rate at a known ca (A0). These proposed changes lead to increases in the model-estimates of ca.
Here we show a lack of support for these proposed changes. First, the downward revision in gm is the result of a mathematical error by McElwain et al. (2016a) for describing the relationship between CO2 assimilation rate (An) and gm. Once corrected, values for gm are very similar to the values recommended by Franks et al. (2014).
Second, the proposed ~ 2-fold upward revision of A0 is not supported by data from extant analogs or by hydraulic constraints from fossils.
Moreover, the modelled estimates of An from Franks and Beerling (2009) are in fact most consistent with the modelled An in Franks et al. (2014) using their recommended A0 values, not those proposed by McElwain et al. (2016a).
These results provide further support for the strategy of model implementation outlined in Franks et al. (2014).

Published by

Willem Van Cotthem

Honorary Professor of Botany, University of Ghent (Belgium). Scientific Consultant for Desertification and Sustainable Development.

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