Excess radiocarbon constraints on air‐sea gas exchange and the uptake of CO 2 by the oceans

We re‐assess the constraints that estimates of the global ocean excess radiocarbon inventory (I E ) place on air‐sea gas exchange. We find that the gas exchange scaling parameter a q cannot be constrained by I E alone. Non‐negligible biases in different global wind speed data sets require a careful adaptation of a q to the wind field chosen. Furthermore, a q depends on the spatial and temporal resolution of the wind fields. We develop a new wind speed‐ and inventory‐normalized gas exchange parameter a q N which takes into account these biases and which is easily adaptable to any new estimate of I E . Our study yields an average estimate of a q of 0.32 ± 0.05 for monthly mean winds, lower than the previous estimate (0.39) from Wanninkhof (1992). We calculate a global annual average piston velocity for CO 2 of 16.7 ± 2.9 cm/hr and a gross CO 2 flux between atmosphere and ocean of 73 ± 10 PgC/yr, significantly lower than results from previous studies.