The protection of organic carbon stored in forests is considered as an important method for mitigating climate change. Like terrestrial ecosystems, coastal ecosystems store large amounts of carbon, and there are initiatives to protect these ‘blue carbon’ stores. Organic carbon stocks in tidal salt marshes and mangroves have been estimated, but uncertainties in the stores of seagrass meadows—some of the most productive ecosystems on Earth—hinder the application of marine carbon conservation schemes. Here, we compile published and unpublished measurements of the organic carbon content of living seagrass biomass and underlying soils in 946 distinct seagrass meadows across the globe. Using only data from sites for which full inventories exist, we estimate that, globally, seagrass ecosystems could store as much as 19.9 Pg organic carbon; according to a more conservative approach, in which we incorporate more data from surface soils and depth-dependent declines in soil carbon stocks, we estimate that the seagrass carbon pool lies between 4.2 and 8.4 Pg carbon. We estimate that present rates of seagrass loss could result in the release of up to 299 Tg carbon per year, assuming that all of the organic carbon in seagrass biomass and the top metre of soils is remineralized.

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This is a contribution of the International Blue Carbon Science Working Group. We thank the contributors of unpublished data to our database, including A. Paytan, W.H. Orem and M. Copertino. Partial support for J.W.F.’s contribution was provided by a Gledden Visiting Senior Fellowship from the Institute of Advanced Studies, University of Western Australia and an Australian National Network in Marine Sciences Visiting Scholar fellowship and by the National Science Foundation through the Florida Coastal Everglades Long-Term Ecological Research programme under Grant No. DBI-0620409. C.M.D and N.M. were financially supported through the MEDEICG project of the Spanish Ministry of Science and Innovation (project ID CTM2009-07013). G.A.K. was supported by NHT II- Caring for our Country funding. M.H. was financially supported by the Danish Natural Science Foundation (09-071369). M.A.M. and O.S. acknowledge the Spanish Ministry of Science and Innovation (MICINN) and the High Council of Scientific Research (CSIC) for financially supporting various pioneering projects to explore the role of P. oceanica as a coastal C sink and a palaeoecological record. D.K.J. acknowledges the Danish National Monitoring and Assessment Programme for the Aquatic and Terrestrial Environment (NOVANA) and colleagues associated with the programme for support. K.J.M. was supported by the National Science Foundation through the Virginia Coast Reserve Long-Term Ecological Research programme under Grant No. 0621014. This is contribution no. 550 from the Southeast Environmental Research Center at Florida International University.

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  1. Department of Biological Sciences and Southeast Environmental Research Center, Marine Science Program, Florida International University, 3000 NE 151 St, North Miami, Florida 33181, USA

    • James W. Fourqurean
  2. Department of Global Change Research. IMEDEA (CSIC-UIB) Institut Mediterrani d’Estudis Avançats, C/ Miguel Marqués 21, 07190 Esporles (Mallorca), Spain

    • Carlos M. Duarte
    •  & Núria Marbà
  3. The UWA Oceans Institute, University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia

    • Carlos M. Duarte
    •  & Gary A. Kendrick
  4. School of Ocean Sciences, College of Natural Sciences, Bangor University, Askew Street, Menai Bridge, LL59 5AB, UK

    • Hilary Kennedy
  5. Institute of Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark

    • Marianne Holmer
  6. Centre for Advanced Studies of Blanes, (CEAB-CSIC), Acceso Cala S. Francesc 14, 17300 Blanes, Spain

    • Miguel Angel Mateo
    •  & Oscar Serrano
  7. Institute of Oceanography, Hellenic Centre for Marine Research, PO Box 2214, 71003, Heraklion—Crete, Greece

    • Eugenia T. Apostolaki
  8. School of Plant Biology, University of Western Australia, Crawley, Western Australia 6009, Australia

    • Gary A. Kendrick
  9. Department of Bioscience, Aarhus University, Vejlsøvej 25, DK-8600 Silkeborg, Denmark

    • Dorte Krause-Jensen
  10. Department of Environmental Sciences, University of Virginia, PO Box 400123, Clark Hall, Charlottesville, Virginia 22904, USA

    • Karen J. McGlathery


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All authors contributed extensively to the work presented in this paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to James W. Fourqurean.

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