UNESCO’s marine world heritage sites store five billion tonnes of blue carbon

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What’s happening? Three world heritage listed marine sites in Australia – the Great Barrier Reef, Shark Bay and the Ningaloo coast – store over two billion tonnes of carbon dioxide (CO2) in their seagrass meadows, coastal mangroves and tidal marshes, according to a report from UNESCO. The agency has calculated the blue carbon stored in its 50 such sites, and estimated they collectively house around five billion tonnes of CO2 and other greenhouse gases. The sites in Australia store almost 40% of this total.

Why does this matter? Alongside forests and other land-based ecosystems, which more commonly come to mind when thinking about carbon sinks, blue carbon ecosystems are increasingly recognised as having a key role to play in tackling climate change.

UNESCO’s report indicates that marine world heritage sites, despite covering less than 1% of the ocean’s area, represent 15% of total blue carbon assets. Such marine habitats store carbon accumulated over thousands of years, which is locked into sediments. They also sequester more carbon per unit area than terrestrial forests – and at a faster rate. Coastal habitats are particularly important, representing less than 2% of ocean area but accounting for around half the carbon sequestered in ocean sediments.

The Great Barrier Reef, which has a conservation outlook status of “critical”, holds more blue carbon than any other of UNESCO’s sites, demonstrating the importance of protecting its seagrass meadows as well as its corals. UNESCO estimates the reef’s seagrass meadows hold one billion megagrams of organic carbon, or 11% of the world’s total. Its tidal marsh area holds a similar amount.

Action is needed to protect the reef and its corals. The Australian government recently ranked the marine environment along the reef’s coastline at a “D” grade, though improvements in water quality were recorded at some Australian land catchment areas with run-off that affects the reef – such as Cape York and Fitzroy – due to improved agricultural practices. Alongside scientific efforts to protect the reef ecosystem, market mechanisms are being employed to improve water run-off, such as the HSBC-backed “Reef Credits” market which works in a similar fashion to carbon credits by assigning a value to improving the quality of water flowing into the reef.

A further recent study looking at blue carbon has quantified that, alongside its negative biodiversity implications, the practice of bottom trawling is responsible for “one billion tonnes of underwater emissions” each year. This is comparable to Germany’s CO2 emissions, and has also been likened to the emissions output of the aviation industry. The study estimates Croatia’s bottom-trawling emissions, for example, at 23 million tonnes a year – an equivalent amount to the country’s recorded greenhouse gas inventory.

Not all the CO2 released from seafloor sediments enters the atmosphere, but that which stays in the ocean causes acidification. The researchers state that increasing the status of Marine Protected Areas (MPAs) and banning industrial fishing in 3.6% of the ocean would cut 90% of the risk of carbon disturbance from bottom trawling. This can be applied to the UK where, according to the Marine Conservation Society, trawlers operate in all but one of the country’s MPAs. Recently, however, the UK government has proposed to outlaw bottom trawling in four MPAs including the Dogger Bank special conservation area – a key breeding ground for cod, whiting and sand eels.

Alongside protecting existing stores of blue carbon, seeding new ones is equally important. Also, in the UK, one million seagrass seeds are to be planted off the Welsh coast to create a 20,000 sq m meadow as part of one of the largest-ever projects to restore seagrass habitats.

Sources: The Guardian, UNESCO: Custodian’s of the Globe’s Blue Carbon Assets