How accurate is the estimation of anthropogenic carbon in the ocean? An evaluation of the Delta C* method

The Delta C* method of Gruber et al. ( 1996) is widely used to estimate the distribution of anthropogenic carbon in the ocean; however, as yet, no thorough assessment of its accuracy has been made. Here we provide a critical re- assessment of the method and determine its accuracy by applying it to synthetic data from a global ocean biogeochemistry model, for which we know the " true'' anthropogenic CO2 distribution. Our results indicate that the Delta C* method tends to overestimate anthropogenic carbon in relatively young waters but underestimate it in older waters. Main sources of these biases are ( 1) the time evolution of the air- sea CO2 disequilibrium, which is not properly accounted for in the Delta C* method, ( 2) a pCFC ventilation age bias that arises from mixing, and ( 3) errors in identifying the different end- member water types. We largely support the findings of Hall et al. ( 2004), who have also identified the first two bias sources. An extrapolation of the errors that we quantified on a number of representative isopycnals to the global ocean suggests a positive bias of about 7% in the Delta C*- derived global anthropogenic CO2 inventory. The magnitude of this bias is within the previously estimated 20% uncertainty of the method, but regional biases can be larger. Finally, we propose two improvements to the Delta C* method in order to account for the evolution of air-sea CO2 disequilibrium and the ventilation age mixing bias.