Reconciliation of excess 14 C-constrained global CO 2 piston velocity estimates

Oceanic excess radiocarbon data is widely used as a constraint for air–sea gas exchange. However, recent estimates of the global mean piston velocity 〈 k 〉 from Naegler et al., Krakauer et al., Sweeney et al. and Muller et al. differ substantially despite the fact that they all are based on excess radiocarbon data from the GLODAP data base. Here I show that these estimates of 〈 k 〉 can be reconciled if first, the changing oceanic radiocarbon inventory due to net uptake of CO 2 is taken into account; second, if realistic reconstructions of sea surface Δ 14 C are used and third, if 〈 k 〉 is consistently reported with or without normalization to a Schmidt number of 660. These corrections applied, unnormalized estimates of 〈 k 〉 from these studies range between 15.1 and 18.2 cm h -1 . However, none of these estimates can be regarded as the only correct value for 〈 k 〉. I thus propose to use the ‘average’ of the corrected values of 〈 k 〉 presented here (16.5 ± 3.2 cm h -1 ) as the best available estimate of the global mean unnormalized piston velocity 〈 k 〉, resulting in a gross ocean-to-atmosphere CO 2 flux of 76 ± 15 PgC yr -1 for the mid-1990s. DOI: 10.1111/j.1600-0889.2008.00408.x