Ocean Carbon Sinks

Blue Carbon

Blue carbon refers to the carbon stored in coastal and marine ecosystems. Scientists have known that coastal vegetation such as mangroves, seagrasses and salt marsh grasses can sequester carbon far more effectively (up to 100 times faster) and more permanently than terrestrial forests. The numbers are staggering:

  1. 83% of the global carbon cycle is circulated through the ocean
  2. Coastal habitats cover less than 2% of the total ocean area
  3. but account for up to half of all carbon sequested in ocean sediments

These ecosystems sequester and store large quantities of blue carbon in both the plants and the sediment below. For example, over 95% of the carbon in seagrass meadows is stored in the soils.

(Source: The Blue Carbon Initiative)

 

Table 1: Efficiency carbon sequestering of coastal vs terrestrial vegetation (Source: The Blue Carbon Project)

 

Mangrove forests are unique, they live in two worlds at once. They thrive in the intertidal areas of sheltered tropical shores, islands and estuaries. They are made up of a variety of salt-tolerant tree and other plant species. Mangrove trees have specially adapted aerial and salt-filtering roots and salt-excreting leaves which enable them to occupy the saline wetlands where other plant life cannot survive. They are found between the latitudes of 32º N and 38º S, along the tropical and subtropical coasts of Africa, Australia, Asia, and the Americas.

Mangrove forests are highly efficient sequesters of carbon. They are often destroyed for development, releasing vast amounts of carbon.

Ecosystem services of Mangroves

Mangroves are estimated to be worth at least US$1.6 billion each year in ecosystem services that support coastal livelihoods and human populations around the world providing a long list of functions:

  • fish habitat –75% of all tropical commercial fish species pass part of their lives in the mangroves, where they encounter:
    • nursery grounds
    • shelter
    • food
  • protection from strong winds & waves – Mangroves’ protective buffer zone helps shield coastlines from storm damage and wave action, minimizing damage to property and losses of life from hurricanes and storms.
  • soil stabilization & erosion protection – The stability mangroves provide is essential for preventing shoreline erosion. By acting as buffers catching materials washed downstream, they help stabilize land elevation by sediment accretion, thereby balancing sediment loss. In regions where these coastal fringe forests have been cleared, tremendous problems of erosion and siltation have arisen.
  • nutrient retention and water quality improvement through filtration of sediments and pollutants – Mangroves have been useful in treating effluent, as the plants absorb excess nitrates and phosphates, thereby preventing contamination of nearshore waters.
  • flood mitigation
  • sequestration of carbon dioxide – Mangroves absorb carbon dioxide and store carbon in their sediments, thereby lessening the impacts of global warming; and
  • protection of associated marine ecosystems – Sea grass beds and coral reefs depend on healthy mangroves to filter sediments and provide nursery grounds for resident species.

Conservation of this coastal vegetation, called “blue carbon” plants is a promising new way to reduce atmospheric CO2.  Carbon is stored in peat below coastal vegetation habitats as they accrete vertically. Because the sediment beneath these habitats is typically anoxic, organic carbon is not broken down and released by microbes. Coastal vegetation also continues to sequester carbon for thousands of years in contrast to forest, where soils can become carbon-saturated relatively quickly. Therefore, carbon offsets based on the protection and restoration of coastal vegetation could be far more cost effective than current approaches focused on trees. Furthermore, there would be enormous ad-on benefits to fisheries, tourism and in limiting coastal erosion from the conservation of blue carbon.

Coastal Managed Realignment