A review of marine anthropogenic CO₂ definitions: introducing a thermodynamic approach based on observations

Abstract

A review of existing methods that define anthropogenic CO₂ as deduced from total inorganic carbon is presented. A refined approach to define anthropogenic CO₂ is introduced that has a stronger thermodynamic orientation than current methods, and is based on a back-calculation technique by Chen and Millero and Poisson and Chen. Anthropogenic CO₂ results of the new technique are compared with results from the original technique as well as with results of the technique of Gruber et al. The new technique is furthermore applied to three time-separated data sets in the subpolar North Atlantic and shows consistent results with regard to available data quality and anthropogenic CO₂ quantities.

The difference between the new thermodynamic approach and the anthropogenic CO₂ definition of Gruber et al., which is termed mechanistic, is discussed. Here likely changes in the CO₂ solubility pump are a thermodynamic property of this definition, whereas it is a separate phenomenon in the mechanistic definition. The thermodynamic approach is not without caveats, but points to improvements by the synergistic use of model results and those from observations. Future improvements are considered for the initial saturation state of oxygen and CO₂, at the instant the surface water loses contact with the atmosphere and for variations in the Redfield ratio.