Inferring the concentration of anthropogenic carbon in the ocean from tracers

We present a technique to infer concentrations of anthropogenic carbon in the ocean from observable tracers and illustrate the technique using synthetic data from a simple model. In contrast to several recent studies, the technique makes no assumptions about transport being dominated by bulk advection and does not require separation of the small anthropogenic signal from the large and variable natural carbon cycle. Mixing is included naturally and implicitly by using observable tracers in combination to estimate the distributions of transit times from the surface to interior points. The time‐varying signal of anthropogenic carbon in surface waters is propagated directly into the interior by the transit time distributions (TTDs) without having to consider background natural carbon. The TTD technique provides estimates of anthropogenic carbon, as simulated directly in the model, that are more accurate than techniques relying on single tracer “ages” (e.g., CFC age) to represent transport. In general, the TTD technique works best when at least two tracers are used in combination, and the tracers have significantly different timescales in either their surface temporal variation or radioactive decay. Possibilities are a CFC or CCl 4 in combination with natural Δ 14 C or 39 Ar. However, even for a CFC alone the TTD technique results in less bias for anthropogenic carbon estimates than use of a CFC age.