Enhanced CO₂ uptake of the coastal ocean is dominated by biological carbon fixation

Observational reconstructions indicate a contemporary increase in coastal ocean CO2 uptake. However, the mechanisms and their relative importance in driving this globally intensifying absorption remain unclear. Here we integrate coastal carbon dynamics in a global model via regional grid refinement and enhanced process representation. We find that the increasing coastal CO2 sink is primarily driven by biological responses to climate-induced changes in circulation (36%) and increasing riverine nutrient loads (23%), together exceeding the ocean CO2 solubility pump (41%). The riverine impact is mediated by enhanced export of organic carbon across the shelf break, thereby adding to the carbon enrichment of the open ocean. The contribution of biological carbon fixation increases as the seawater capacity to hold CO2 decreases under continuous climate change and ocean acidification. Our seamless coastal ocean integration advances carbon cycle model realism, which is relevant for addressing impacts of climate change mitigation efforts.