Blue carbon is the term for carbon captured by the world’s ocean and coastal ecosystems, with policy interest in the potential to mitigate climate change while achieving co-benefits, such as coastal protection and fisheries enhancement. The marine carbon cycle constitutes 83% of the global carbon cycle, with marine plants, such as phytoplankton, taking up atmospheric CO2 through photosynthesis, some of which is sequestered when marine life dies and buried on the seabed. More is known about the rates of carbon uptake by coastal ecosystems, and seagrass meadows, mangroves, and salt marshes. It has been estimated these ecosystems represent 50% of all carbon sequestered in marine sediments, but the sequestration potential of other marine ecosystems remains less well-understood. Marine sediments cover an area greater than all other habitats on Earth combined, and these benthic systems may have the potential to capture more carbon. For example, if less fish biomass were extracted from the oceans or if disturbance of the seabed through activities, such as trawling, were reduced.

A range of nations have included coastal wetlands in their nature-based solution mitigation activities within their National Determined Contributions since 2016, but significant uncertainties remain in relation to other marine nature-based solutions, such as the extent to which seaweeds play a significant role in the capture of carbon in marine ecosystems. There is consensus that conserving and restoring coastal ecosystems would create a globally significant carbon sink, but they continue to be degraded by economic activities at a rapid rate, reducing carbons stocks and increasing CO2 emissions and will be affected by sea level rise and ocean warming in coming decades. For example, the UK’s historical seagrass meadows may have stored 11.5 megatonnes (Mt) of carbon but have been reduced to 8,493 ha storing only 0.9 Mt of carbon. Saltmarshes are also important coastal fringe ecosystems with high carbon sequestration rates that have undergone decline in the UK and globally.

A POSTnote on this subject will summarise the evidence on the extent to which these ecosystems become carbon sources if damaged or degraded, the options for marine nature-based solutions that sequester significant amounts of carbon and the extent of knowledges gaps in relation to these different possibilities.

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