Groundwater releases CO₂ to diverse global coastal ecosystems

Coastal ecosystems play a major role in marine carbon budgets, but substantial uncertainties remain in the sources and fluxes of coastal carbon dioxide (CO₂). Here, we assess when, where, and how submarine groundwater discharge (SGD) releases CO₂ to shallow coastal ecosystems. Time-series observations of dissolved CO₂ and radon (²²²Rn, a natural groundwater tracer) across 40 coastal systems from 14 countries revealed large SGD-derived CO₂ fluxes. The mean groundwater partial pressure of CO₂ was 35 times higher than surface seawater. The mean SGD-derived CO₂ flux was 148 ± 226 millimoles per square meter per day (mmol m^-2 day^-1), resulting in a mean water-air CO₂ flux of 80 ± 133 mmol m^-2 day^-1. Tidal rather than diel cycles drove CO₂ enrichment in most ecosystems. Tidally driven SGD was the primary CO₂ source in mangroves, salt marshes, tidal flats, estuaries, and canals. Overall, we expand current knowledge of marine carbon cycles by demonstrating SGD as an important source of CO₂ that requires inclusion in coastal carbon budgets.