Seaweed's ability to suck huge quantities of carbon out of the air have been questioned, but there are other reasons to grow it.
With its low-lying 2,700 km-long coastline and vulnerability to tropical cyclones, Mozambique is on the frontlines of the climate crisis. In recent years, devastating storms, surge flooding, and droughts - made more intense and frequent by climate change - have affected thousands of people and killed hundreds in the southern African nation.
But could Mozambique also be on the frontlines of mitigating climate change by drawing carbon out of the air?
In 2021, the Oceanographic Institute of Mozambique (InOM) begin an experimental seaweed cultivation project in country's northern region. Supported by funding from Selt Marine Mozambique, a subsidiary of the Tunisia-based seaweed farming and processing company Selt Marine Group, the initiative promises to simultaneously fight climate change, enhance marine biodiversity, and restore fishing activities.
The project began by identifying suitable areas for seaweed farms in the ocean and the most appropriate varieties for cultivation. It then trained around 100 locals - predominantly women - on how to cultivate the plant by fixing equidistant stakes into the ground and attaching seaweed along wires and nets between the posts. This process takes around 5-7 days. Over the following 60 days, the locals then work closely with the InOM team to measure growth, spot signs of disease, and clear harmful algae from the fields twice a week.
The climate hope from the project comes from the fact that growing seaweed can remove atmospheric carbon by converting it into organic biomass. According to the Intergovernmental Panel on Climate Change, 1,000 gigatonnes of carbon dioxide - equivalent nearly half of what humans have emitted since the Industrial Revolution - needs to be removed in this century to achieve the 1.5°C target for global warming.
Seaweed has various advantages over other techniques for sequestering carbon. It grows extremely quickly compared to many other plants. It sequesters up to 20 times more carbon per acre than terrestrial forests and currently covers about 3.4 million square km of ocean. It is not at risk of burning and releasing carbon back into the atmosphere as in the case of trees.
A 2016 paper estimated that seaweed sequesters around 200 million tonnes of carbon dioxide per year. That is roughly equivalent to half the annual emissions of Australia, the world's 14th biggest emitter.
With its vast coastline on the Indian Ocean, there are hopes that Mozambique could become a significant seaweed grower, though clear challenges remain, including from the climate crisis itself.
"If climate change affects the ocean and there is too much carbon in the ocean, it gets overheated and seaweed will start dying," says Archibong Akpan, a climate policy expert at the UN Framework Convention on Climate Change.
Ocean warming could also lead to a rise in seaweed developing ice-ice disease, causing it to bleach like corals. This disease may have affected one of the two species cultivated in the InOM project, according to research assistant Henriques Bustani. "Kappaphycus is not developing properly," he says.
According to Valera Dias, a Mozambican scientist at Eduardo Mondlane University who began experimental seaweed cultivation in Mozambique in 2022 with support from the European Union, growers can take measures to reduce the risk of disease. These include choosing areas for cultivation carefully, cleaning fields regularly, and adopting techniques such as the "bottom system" that allows seaweed to be farmed deeper so it is less frequently exposed to the sun.
Other researchers, however, point to more significant limitations on seaweed's potential to remove large quantities of carbon from the air. A 2023 study, for instance, suggested there is insufficient iron concentration in the open ocean to sustain seaweed growth. Worse still, research from the previous year suggested that seaweed could in fact be a natural carbon source that releases 20 tonnes of carbon per square km each year. This finding came from considering the extra carbon emitted by organisms that gain increased food sources due to plankton washing through the seaweed canopy.
"The idea of using seaweeds to capture carbon as a means of ameliorating levels of atmospheric carbon is not a credible idea," says Craig Johnson, professor of Marine Science at the University of Tasmania's Institute for Marine and Antarctic Studies.
He adds that seaweed's potential to permanently sequester carbon - typically defined as locking it up for at least 100 years - depends on what happens to the plant after cultivation.
"In some cases, detritus from seaweeds ends up in ocean sediments where it can be buried for 100 years," he says. "[But] if it is used for human consumption, then most of the carbon is likely to be respired back into the atmosphere through our metabolic processes."
Certain varieties of seaweed are widely eaten across Asia and are growing in popularity in other parts of the world too. The plant is also used to make various food additives, often thickeners or emulsifiers, that can be found in items like ice-cream and beer as well as certain cosmetic and pharmacological products. The global commercial seaweed market size was valued at $10.66 billion in 2024 and is expected to grow significantly in the coming years.
The InOM project in Mozambique hopes to tap into this market, though it is early days for the initiative. According to Bustani, the group has collected 1.3 tonnes of dried seaweed since they began harvesting in 2023. These quantities are not yet sufficient to start exporting to Tunisia.
In the meantime, however, the locals involved in the project say they have benefited from the additional work for which they receive around $2 per day.
"The income from the work helps me to buy some food for my home," says Muanatruco Rajbo, 30. Trained in May 2022, she says that her other sources of livelihood from farming and selling firewood are not enough to feed her family of four children.
Estefánia Calisto, 33, says that before her training in August 2023, her income from collecting seafood on beaches had dwindled "due to problems with [unusual] rainfall". The extra source of income has provided her with a new lifeline. "With the money I receive from this project I can buy food for my family, clothes, and sometimes school supplies for my children," says the mother-of-eight.
Sign up for free AllAfrica Newsletters
Get the latest in African news delivered straight to your inbox
Bustani adds that the additional seaweed in the sea has also helped provide additional food sources for marine life and enhanced biodiversity.
"The seaweed is a friend of the ecosystem," he says. "It's like a house of fish. With this seaweed farming, we are already seeing some species of fish that were not seen before."
Bustani says high costs of inputs such as stakes, the lack of a local market to buy materials such as tubular nets, and funding challenges are limiting the project's expansion but that he is optimistic about its progress.
Given the questionable potential for seaweed to sequester carbon especially at the levels needed to significantly mitigate climate change, the Mozambique project suggests the potential benefits of cultivation may be much more apparent and provable locally than globally, at least for now.
As Johnson puts it, "there are many good reasons to cultivate seaweed in an aquaculture setting, and seaweed aquaculture has many environmental benefits. However, current evidence would indicate that seaweed is not going to be the saviour of our climate crisis."
Ekpali Saint is a freelance journalist based in Nigeria.
