Asymmetric bubble-mediated gas transfer enhances global ocean CO2 uptake

Abstract

Sea-air carbon dioxide (CO2) flux is typically estimated from the product of the gas transfer velocity (K) and the CO2 fugacity difference between the ocean surface and atmosphere. Total gas exchange comprises interfacial transfer across the unbroken surface and bubble-mediated transfer from wave breaking. While interfacial transfer is symmetric for invasion and evasion, bubble-mediated transfer theoretically favours invasion due to hydrostatic pressure, though field evidence has been lacking. Here we provide direct field evidence of this asymmetry and develop an asymmetric flux equation. Applying the asymmetric equation reduces bias in K, and increases global oceanic CO2 uptake by 0.3-0.4 Pg C yr-1 (~15% on average from 1991 to 2020) relative to conventional estimates. Further evasion data are needed to better quantify the asymmetry factor. Our study suggests that the ocean may have absorbed more CO2 than previously thought, and the asymmetric equation should be used for future CO2 flux assessments.