Constraining global air‐sea gas exchange for CO₂ with recent bomb ¹⁴C measurements

Abstract

The 14 CO 2 released into the stratosphere during bomb testing in the early 1960s provides a global constraint on air‐sea gas exchange of soluble atmospheric gases like CO 2 . Using the most complete database of dissolved inorganic radiocarbon, DI 14 C, available to date and a suite of ocean general circulation models in an inverse mode we recalculate the ocean inventory of bomb‐produced DI 14 C in the global ocean and confirm that there is a 25% decrease from previous estimates using older DI 14 C data sets. Additionally, we find a 33% lower globally averaged gas transfer velocity for CO 2 compared to previous estimates (Wanninkhof, 1992) using the NCEP/NCAR Reanalysis 1 1954–2000 where the global mean winds are 6.9 m s −1 . Unlike some earlier ocean radiocarbon studies, the implied gas transfer velocity finally closes the gap between small‐scale deliberate tracer studies and global‐scale estimates. Additionally, the total inventory of bomb‐produced radiocarbon in the ocean is now in agreement with global budgets based on radiocarbon measurements made in the stratosphere and troposphere. Using the implied relationship between wind speed and gas transfer velocity k s = 0.27〈 u 10 2 〉( Sc /660) −0.5 and standard partial pressure difference climatology of CO 2 we obtain an net air‐sea flux estimate of 1.3 ± 0.5 PgCyr −1 for 1995. After accounting for the carbon transferred from rivers to the deep ocean, our estimate of oceanic uptake (1.8 ± 0.5 PgCyr −1 ) compares well with estimates based on ocean inventories, ocean transport inversions using ocean concentration data, and model simulations.