Unraveling the physical and biological controls of the global coastal CO2 sink

In spite of considerable efforts using approaches combining observations and modeling, the mechanisms governing the exchange of carbon dioxide (CO₂) at the air-sea interface and the spatio-temporal variability of this exchange in coastal oceans are not yet fully understood. The present study uses simulations performed over the last decades with the global ocean biogeochemical model MOM6-COBALT to quantify the relative contributions of thermal changes, oceanic transport, freshwater fluxes, and biological processes to the spatial and seasonal variability of CO₂ sources and sinks in the coastal ocean worldwide. These results allow identifying five distinct coastal domains each characterized by different behaviors: coastal regions dominated by biological carbon drawdown, regions controlled by vertical transport, influenced by land-derived inputs, regions shaped by intracoastal alongshore currents, and regions with weak CO₂ fluxes. Using the spatial distribution of these behaviors, we propose a new, process-based delineation of the global coastal ocean that reflects the dominance of specific controlling processes for the spatial and seasonal dynamics of the CO₂ exchange at the air-sea interface.

Our results also reveal that the spatiotemporal variability of CO₂ fluxes in coastal regions is primarily driven by exchanges with the open ocean and local intra-coastal processes, while the influence of continental inputs remains confined to specific hotspot areas. In addition, although thermal changes are often associated with seasonal CO₂ variability, their dominance stems from compensating effects between larger non-thermal processes, particularly biological drawdown and vertical transport.

The classification of the global coastal ocean presented in our study provides an updated process-based vision of the complex interplay between physical and biogeochemical drivers of CO₂ exchange at the air-water interface. These findings provide a more comprehensive framework for understanding coastal CO₂ dynamics and their role in the global carbon cycle, offering valuable insights for predicting the responses of coastal regions to both natural and anthropogenic environmental changes.