The contribution of mangrove carbon to the coastal ocean in low latitudes was evaluated. Mangrove forests occupy only 2% of the world’s coastal ocean area yet they account for about 5% of net primary production, 12% of ecosystem respiration and about 30% of carbon burial on all continental margins in subtropical and tropical seas. Mangroves also account for nearly one-third of all riverine DIC discharging into low latitude coastal waters. Mangrove forests fix, release and sequester more carbon by area than all other coastal ecosystem types, except perhaps for subtropical and tropical seagrass meadows for which data are lacking. Globally, mangrove waters release to the atmosphere more than 2.5 times (−42.8 Tg C y−1) the amount of CO2 emitted from all other subtropical and tropical coastal waters. The global destruction of the large carbon stocks (956 Mg C ha−1) of mangroves at the current annual rate of about 1% results in an additional annual release of roughly 133 Tg C y−1 to the atmosphere. Mangroves account for only 0.7% of tropical forest area globally, but their destruction currently adds another 10% to global CO2 release from tropical deforestation. Despite considerable uncertainty upscaling small numbers of measurements with large coefficents of variation, authors’ calculations suggest that mangroves are a globally significant contributor to the carbon cycle in low latitude seas, and to greenhouse emissions resulting from tropical deforestation.