Life in the ocean has the potential to help to prevent global warming, according to a report published today.

Marine plant life sucks 2 billion tonnes of carbon dioxide from the atmosphere every year, but most of the plankton responsible never reaches the seabed to become a permanent carbon store.

Mangrove forests, salt marshes and seagrass beds are a different matter. Although together they cover less than 1 per cent of the world’s seabed, they lock away well over half of all carbon to be buried in the ocean floor. They are estimated to store 1,650 million tonnes of carbon dioxide every year — nearly half of global transport emissions — making them one of the most intense carbon sinks on Earth.

Their capacity to absorb the emissions is under threat, however: the habitats are being lost at a rate of up to 7 per cent a year, up to 15 times faster than the tropical rainforests. A third have already been lost.

Halting their destruction could be one of the easiest ways of reducing future emissions, says report, Blue Carbon, a UN collaboration.

With 50 per cent of the world’s population living within 65 miles of the sea, human pressures on nearshore waters are powerful. Since the 1940s, parts of Asia have lost up to 90 per cent of their mangrove forests, robbing both spawning fish and local people of sanctuary from storms.

The salt marshes near estuaries and deltas have suffered a similar fate as they are drained to make room for development. Rich in animal life, they harbour a huge biomass of carbon-fixing vegetation. Seagrass beds often raise the level of the seabed by up to three metres as they bury mats of dead grass but turbid water is threatening their access to sunlight.

“We already know that marine ecosystems are multi-trillion-dollar assets linked to sectors such as tourism, coastal defence, fisheries and water purification services. Now it is emerging that they are natural allies against climate change,” said Achin Steiner, UN Under-Secretary General.

The potential contribution of blue carbon sinks has been ignored up to now, says the report, which was a collaboration between the United Nations Environment Programme, the Food and Agriculture Organisation and Unesco. Accurate figures for the extent of these habitats are hard to obtain, and may be more than twice the lower estimates used in the report.

“The carbon burial capacity of marine vegetated habitats is phenomenal, 180 times greater than the average burial rate in the open ocean,” say the authors. As a result they lock away between 50 and 70 per cent of the organic carbon in the ocean.

To protect them the authors suggest that a Blue Carbon Fund be launched to help developing nations to protect the habitats. Oceanic carbon sinks should also be traded in the same fashion as terrestrial forests, they say. Together with the UN’s scheme to reduce deforestation, they could deliver up to 25 per cent of emission reductions needed to keep global warming below 2C (3.5F).

Christian Nellemann, the editor of the report said:“On current trends they [ecosystems] may be all largely lost within a couple of decades.”

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