Climate change decreases groundwater carbon discharges in global tidal wetlands

Ocean acidification is a grand challenge to sustainable ocean management. Tidal wetlands are distributed from the polar to the tropical domain and provide unique ecosystem services such as groundwater carbon export. The carbonate and bicarbonate of exported dissolved carbon represent total alkalinity and buffer ocean acidification. However, the magnitude and variability of the dissolved carbon discharge from groundwater to the coastal ocean are poorly understood. Here, we estimate groundwater dissolved carbon and total alkalinity discharge by combining data from 337 locations in tidal wetlands. The average annual groundwater total alkalinity and dissolved carbon discharge in global tidal wetlands reach 16.2 (0.1–77.9) and 20.1 (0.1–96.6) Tmol, respectively, of which 83.4% is exported as dissolved inorganic carbon (DIC). Future groundwater DIC discharge decreases by 9.02%–28.91% due to increasing evapotranspiration, changing rainfall regimes, and relative sea level rise. Our study suggests that tidal wetlands export over 30% of terrestrial dissolved carbon to the coastal ocean.