Budgeting sinks and sources of CO₂ in the coastal ocean: Diversity of ecosystems counts

Air‐water CO 2 fluxes were up‐scaled to take into account the latitudinal and ecosystem diversity of the coastal ocean, based on an exhaustive literature survey. Marginal seas at high and temperate latitudes act as sinks of CO 2 from the atmosphere, in contrast to subtropical and tropical marginal seas that act as sources of CO 2 to the atmosphere. Overall, marginal seas act as a strong sink of CO 2 of about −0.45 Pg C yr −1 . This sink could be almost fully compensated by the emission of CO 2 from the ensemble of near‐shore coastal ecosystems of about 0.40 Pg C yr −1 . Although this value is subject to large uncertainty, it stresses the importance of the diversity of ecosystems, in particular near‐shore systems, when integrating CO 2 fluxes at global scale in the coastal ocean.