Inorganic carbon exports from coastal wetlands can offset part of blue carbon systems' CO₂ removal

Many studies have regarded coastal wetland-derived total alkalinity (TA) as a durable pathway for atmospheric carbon dioxide (CO₂) removal, mainly owing to the formation of bicarbonate during the anaerobic mineralization of biogenic organic matter in wetlands and the subsequent lateral export as TA from intertidal wetlands to the coastal ocean. However, this perspective may overestimate the contribution of wetland lateral carbon export to coastal atmospheric CO₂ removal because these ecosystems commonly also export equal or larger amounts of dissolved inorganic carbon (DIC) alongside TA. Using a dataset of TA and DIC exports from global saltmarshes and mangroves, we show that wetland-derived lateral inorganic carbon export lowers ratios or differences between TA and DIC in coastal waters, thus elevating coastal ocean CO₂ levels and reducing its net CO₂ uptake from the atmosphere by a median of 0.033-0.048 Pg C a^-1 (at least 15% of the total net uptake, 1Pg = 10¹⁵ g). This reduction is larger than carbon burial in sediments of global saltmarshes and mangroves (0.025 Pg C a^-1). Our results clarify the role of wetland lateral carbon exports in coastal CO₂ uptake and highlight that ocean alkalinity enhancement strategies must increase TA relative to DIC, not merely add TA, to achieve durable CO₂ removal.