A sea-volution for carbon offsets?

What’s happening? UK-based start-up Seafields plans to establish farms in the Atlantic Ocean, growing sargassum seaweed that can be used to capture and sequester CO2. The algae will absorb CO2 during its growth, after which the plant can be harvested, formed into bales and sunk to the bottom of the sea where the CO2 will be retained. Sargassum’s vigorous growth rate has led to its invasion of large areas of the Caribbean coast. To address this, Seafields aims to build dedicated farms in areas where it would not ordinarily survive to prevent it from becoming an invasive species elsewhere. (inews)

Why does this matter? Along with large-scale emissions cuts and advancing CO2-capturing technologies, nature-based solutions have a role to play in climate-change mitigation.

Land-based carbon sequestration is more widely recognised when thinking of such projects, but we can also find an alternative solution in the sea – where interest in leveraging the ocean’s carbon sink potential is growing.

The role of the ocean as a natural carbon sink is immense, given it covers around 70% of the Earth’s surface. Ocean ecosystems have been proven to be more efficient in absorbing CO2 compared to terrestrial forests, with marine habitats including seagrass, salt marshes and coastal mangrove habitats able to store up to 10 times more CO2. Large marine organisms also represent carbon stores.

Ocean-based solutions will may additionally contribute to reducing the emissions gap – the difference between current emissions pledges and those consistent with the 1.5C temperature trajectory – by up to 21%.

Why seaweed? Other than carbon sequestration, adding seaweed to the ocean can offer other benefits, such as improving fish populations and encouraging biodiversity. Seafields plans to sequester at least one billion tonnes of CO2 from the atmosphere each year by 2025 while selling carbon credits in the voluntary carbon market.

How will they ensure the seaweed grown does not become problematic? Sargassum is invasive in some regions, including Mexico and the Caribbean coast, causing issues for marine ecosystems and local communities.

Seafields’ farms will be located in the South Atlantic gyres, where sargassum cannot naturally survive, reducing the chances of introducing the invasive seaweed elsewhere. There are still other uncertainties, however, such as how the baled sargassum will impact marine life when it sinks to the seabed.

Why can’t we rely on land-based CO2 offsets alone? We’ve previously looked at problems which can arise from poorly planned reforestation campaigns, such as using non-native tree species and schemes which prioritise single species plantations and their impacts on local ecosystems. And without efforts to diversify land-use, there also isn’t enough land available to offset the emissions on the scale required.

Lateral thought – Ways to repurpose sargassum are being explored in areas struggling with excessive natural build-up. For instance, converting the seaweed into bio-oil which could then be processed into fuel and fertiliser. From a circular lens, sargassum can be repurposed into a range of functional biomaterials, including paper pulp and biodegradable resin pellets. More broadly, the seaweed market is rapidly growing for applications ranging from electricity to low-emission livestock feed.

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