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By Mike McRae 1 December 2016 4 min read

Hinchinbrook Island channel and mangroves as seen from lookout near Cardwell, Queensland. Image: Willem van Aken.

Among the devastating statistics from Australia’s 2009 Black Saturday bushfires was the release into the atmosphere of 13.5 million tonnes of carbon dioxide, tripling Australia’s usual annual bushfire emissions.

When carbon is held back from entering the atmosphere for more than a century, we consider it ‘sequestered’. Trees are good at sequestering carbon but fire can quickly undo all the good.

One place where plants can sequester carbon without fear of fire is under water. It’s what is known as ‘blue carbon’.

Mangroves, seagrass and saltmarshes

While the sequestering of carbon on dry land is well studied, ecologists are only beginning to understand the significance of the role of aquatic ecosystems in sequestering carbon. But three standout performers have been identified.

mangrove shoots poking above water surface
Mangrove shoots emerging from the water at Cairns, Queensland. Image: Christine Painter.

“Blue carbon ecosystems refer to tidal marshes, mangroves and seagrass,” explains Dr Oscar Serrano from Edith Cowen University’s Centre for Ecosystems Management. The role of seaweed in sequestering carbon is also being considered, he says.

Seagrass, mangroves and saltmarsh, which take up only 2% of the world's seabed area, are responsible for 50% of the carbon that is captured and stored in ocean sediments.

A single hectare of seagrass can store twice the amount of carbon that a hectare of tropical forest can store. A single hectare of intact tidal marshland can offset the emissions of nearly 500 cars on American roads.

Yet it’s the mangroves that really seem to be the kings of carbon capture. “On a unit area basis, mangroves are certainly the ecosystems that have the largest potential to sequester CO2. For instance, one square meter of mangrove ecosystem could sequester 10 times more CO2 than tidal marshes and seagrasses,” says Dr Serrano.

Locating the planet’s blue carbon ecosystems

Quantifying the ability of these blue carbon ecosystems to lock up carbon is just half the story. To effectively understand their contributions, we also need to know how much of them we have on the planet.

Globally, blue ecosystems are estimated to cover an area equal to just 6% of the world’s tropical forests. Yet, up to 19% of the world’s annual emissions are accounted for by the vegetation lost through the destruction of our wetland ecosystems. That’s half a billion tonnes of CO2 released into the atmosphere every year.

Aerial imaging could provide a detailed map of mangroves and tidal marshes, says Dr Serrano, but seagrass meadows are harder to map, being hidden beneath the waves. Given their potential impact, this is no small problem: “In Australia, for example, seagrasses are estimated to be the ecosystems with the largest capacity for carbon dioxide sequestration because they occupy a larger area than other blue carbon ecosystems,” he says.

Do not disturb

The sediment held in place by coastal vegetation is rich in carbon which, left alone, can sit there for thousands of years. If this ‘ocean soil’ is disturbed, its organic matter can be exposed to oxygen, helping microbes release the locked-up carbon back to the atmosphere.

“Coastal vegetated ecosystems in Australia have been affected by development over the last century,” says Dr Serrano. “Their disturbance can result in the loss of their CO2 sequestration capacity, but most importantly, to the release of their millenary carbon stores back to the atmosphere.”

Accounting for blue carbon

The risks and uncertainties make a sound case for bringing together experts and authorities to improve our understanding of blue carbon. As a nation girt by sea, Australia holds a particular interest in driving collaboration on blue carbon. To date, we have yet to include carbon from our surrounding coastal environments in our carbon accounting processes. This is set to change.

Seagrass. California, Channel Islands. Image: Claire Fackler/NOAA Photo Library

For the past five years, our Coastal Carbon Cluster project, which includes eight partner universities, has been collecting and distributing information on Australia’s coastal carbon. CSIRO’s Dr Jeff Baldock says the collaboration will help define Australia’s inclusion of blue carbon within frameworks such as the National Greenhouse Gas Inventory and the Emissions Reduction Fund.

“As a result of the cluster’s research, new datasets are becoming available to help support development of carbon accounting approaches within blue carbon ecosystems. Gaps remain, but we are now in a better position,” says Dr Baldock.

Collaborating to protect and restore blue carbon hotspots

In 2015, at the Global Landscapes Forum in Paris, the Australian Government announced its establishment of the International Blue Carbon Partnership. Behind the initiative is a concern about the gaps between policy and the science of carbon cycling through our coastal ecosystems.

Patrick Suckling is the Australian Ambassador for the Environment: “The challenge is how we can harness the science, help get national and regional policy settings right, and unlock the finance. This requires a multi-sectoral response,” he says.

Those sectors are spread across government, non-government bodies, and academia and can build awareness in the international community, share expertise and upscale practical action. The partnership brings together the scientific expertise of organisations such as CSIRO and the University of Queensland with the policy expertise of governments such as the US and Indonesia, and the practical project experience of NGOs such as Conservation International. The intention is to foster projects that prioritise the protection and restoration of blue carbon ‘hotspots’.

“The partnership and supporting members are investigating how they can better support and implement change through the upscaling of current efforts and highlighting the need for change,” says Patrick Suckling.

We’ve long valued our coastal ecosystems for their intrinsic beauty, their integral role in biodiversity, and the natural resources they provide. Now Australia is leading the way by adding carbon to the long list of ‘true blue’ reasons why the world must protect its mangroves, its seagrass meadows and its saltmarshes.

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