Carbon Fluxes in the Coastal Ocean: Synthesis, Boundary Processes, and Future Trends

This review examines the current understanding of the global coastal ocean carbon cycle and provides a new quantitative synthesis of air-sea CO2 exchange. This reanalysis yields an estimate for the globally integrated coastal ocean CO2 flux of -0.25 +/- 0.05 Pg C year(-1), with polar and subpolar regions accounting for most of the CO2 removal (>90%). A framework that classifies river-dominated ocean margin (RiOMar) and ocean-dominated margin (OceMar) systems is used to conceptualize coastal carbon cycle processes. The carbon dynamics in three contrasting case study regions, the Baltic Sea, the Mid-Atlantic Bight, and the South China Sea, are compared in terms of the spatiotemporal variability of surface pCO(2). Ocean carbon models that range from box models to three-dimensional coupled circulation-biogeochemical models are reviewed in terms of the ability to simulate key processes and project future changes in different continental shelf regions. Common unresolved challenges remain for implementation of these models across RiOMar and OceMar systems. The long-term trends in coastal ocean carbon fluxes for different coastal systems under anthropogenic stress that are emerging in observations and numerical simulations are highlighted. Knowledge gaps in projecting future perturbations associated with before and after net-zero CO2 emissions in the context of concurrent changes in the land-ocean-atmosphere coupled system pose a key challenge. A new synthesis yields an estimate for a globally integrated coastal ocean carbon sink of -0.25 Pg C year(-1), with greater than 90% of atmospheric CO2 removal occurring in polar and subpolar regions. The sustained coastal and open ocean carbon sink is vital in mitigating climate change and meeting the target set by the Paris Agreement. Uncertainties in the future coastal ocean carbon cycle are associated with concurrent trends and changes in the land-ocean-atmosphere coupled system. The major gaps and challenges identified for current coastal ocean carbon research have important implications for climate and sustainability policies.