Neglecting Changes in Salinity and Tidal Range Overstates Projected Increases in Greenhouse Gas Emissions from Mangrove Sediments

Abstract

Mangroves are important coastal blue carbon ecosystems, but future sediment GHG emissions remain uncertain because global projections have largely emphasized atmospheric climate drivers, with less attention to coastal hydrology, particularly sea-surface salinity and tidal range. Here, we compiled 810 sediment GHG flux measurements from 294 mangrove sites worldwide and used machine learning to project emissions to 2050, distinguishing climate-driven changes from the additional effects of projected shifts in sea-surface salinity and tidal range. We estimate a mangrove sediment GHG budget of ∼194 Tg CO2e yr–1 across reliably predicted mangrove areas worldwide around 2020. By 2050 under SSP5–8.5, projected changes in salinity and tidal range counteracted 41% of the climate-driven increase in emissions globally, with comparable offset effects among CO2, CH4, and N2O. Hydrological offset effects were strongest at 10–20° latitude in both hemispheres and weakened toward the equator and higher latitudes. Regionally, hydrological offset effects were greatest in East Africa, whereas the Indo-Pacific showed the largest absolute hydrology-driven reductions in emission budgets, largely owing to its extensive mangrove area. These findings highlight the importance of incorporating sea-surface salinity and tidal range into future projections of mangrove sediment GHG emissions under global change.