Reply on RC2

Abstract

We have estimated the air–sea CO2 fluxes (fgCO2) over the global ocean from the open sea to the continental shelves. Fluxes and associated uncertainty were computed from an ensemble-based reconstruction of CO2 sea surface partial pressure (pCO2) maps trained with observations from the Surface Ocean CO2 Atlas v2020 database. The ensemble mean (which is the best estimate provided by the approach) fits independent data well and a broad agreement between the spatial distribution of model-data differences and the ensemble standard deviations (which are our model uncertainty estimate) is seen. The space-time varying uncertainty fields identify oceanic regions where improvements in data reconstruction and extensions of the observational network are needed. Poor reconstructions of pCO2 are primarily found over the coasts and/or in regions with sparse observations, while fgCO2 estimates with largest uncertainty are observed over the open Southern Ocean (44° S southward), the subpolar regions, the Indian gyre, and upwelling systems. Our estimate of the global net sink for the period 1985–2019 is 1.643 ± 0.125 PgC yr−1 including 0.150 ± 0.010 PgC yr−1 for the coastal net sink. Results suggest that the open ocean Subtropical Pacific (between 18° N–49° N) has the strongest CO2 sink (0.485 ± 0.014 PgC yr−1) among the basins of the world, followed by the open ocean sub-basins in the Southern hemisphere. The coastal Subpolar Atlantic (between 49° N–76° N) is the most significant coastal net sink, amounting to one third of the total coastal uptake; the northern Pacific continental shelves (north of 18° N) are the next contributors. The Equatorial Pacific (between 18° S–18° N) is the predominant source emitting 0.523 ± 0.016 PgC yr−1 of CO2 back to the atmosphere. Based on the mean flux density per unit area, the most intense CO2 drawdown is, however, observed over the Arctic (76° N poleward) followed by the Subpolar Atlantic and Subtropical Pacific for both open ocean and coastal sectors. The mean efflux density over the Equatorial Pacific remains the highest, but similar densities can also be found along other strong upwelling systems in the equatorial band.