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Tens of millions of dollars have been invested in growing seaweed to absorb carbon dioxide and slow climate change. But due to unwanted side effects, this technique could fail to significantly decrease the CO2 in the atmosphere, and it might even increase it.

Carbon dioxide removal (CDR) will be needed to meet the Paris Agreement goal of limiting global warming to 2°C, to the UN, and many have hoped seaweed could be a cheap way to do that. The US start-up Running Tide raised $70 million to grow seaweed on pucks of wood that would eventually become sodden and sink to the deep sea, sequestering the carbon, but it ran out of financing and last year.

Dutch company Kelp Blue has raised at least $2 million to expand the seaweed that it is currently growing to produce sustainable agricultural fertiliser in Namibia. Because small particles of this seaweed may break off and drift into the depths, it it could eventually “sequester and offset” up to 500 million tonnes of CO2 per year.

But a global seaweed-cultivation programme could in many places rob nutrients from phytoplankton, which also sequester carbon when they die and sink to the depths, two studies have found.

“It could backfire locally,” says at the University of Bern, Switzerland, who worked on one of the studies. “In some places, you’d actually reduce how much carbon the ocean takes up. The potential is extremely limited, with large ecological consequences.”

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Except for sargassum, macroalgae species live near the coast, where nutrients are plentiful. During photosynthesis, they consume carbon dissolved in seawater, allowing the ocean to absorb more CO2 from the atmosphere.

Marine organisms and microbes eventually most of that seaweed, emitting an estimated nine-tenths of its carbon. To sequester more carbon, seaweed would have to be grown or transported further offshore, where it could be baled or otherwise sunk to the deep sea.

But nutrients are scarce in the open ocean, and most research before now hasn’t examined how the lack of iron could limit seaweed growth. Berger and her colleagues modelled the cultivation of 20 billion tonnes of seaweed per year across waters up to 200 nautical miles from coastlines.

They found the seaweed would quickly start depleting nitrogen, phosphorus and iron in the water, and after 25 years, its growth would have declined 95 per cent. Moreover, this would diminish global phytoplankton growth by as much as 8 per cent.

In some scenarios, seaweed cultivation could still remove billions of tonnes of CO2. But depending on what species of seaweed are grown and how much nutrients they consume, it could also increase the amount of carbon in the atmosphere by half a tonne for every tonne of seaweed carbon grown.

Patches off Senegal and southern Australia, about 0.05 per cent of the ocean, are the only places seaweed could flourish without significantly decreasing phytoplankton, the model suggests.

“If you have only a few very specific locations, you can’t grow enough seaweed to have a gigatonne of removal,” says Berger.

In another study, at the UK National Oceanography Centre and his colleagues modelled what would happen if seaweed-cultivation areas were fertilised with iron, finding that up to 40 billion tonnes of CO2 could be removed each year. But that would also halve the plankton in the ocean, with dire consequences for the fish that eat them.

“You’re robbing the surface ocean of nutrients… and transferring those to depth,” says Yool. “Essentially, you’re curtailing or slowly strangling the natural ecosystem.”

Furthermore, such seaweed cultivation and sinking would require setting up cages or other frameworks across 14 per cent of the ocean surface, largely in the nutrient-rich but stormy seas of the Southern Ocean and northern Pacific and Atlantic.

And if all this ocean wasn’t fertilised with iron, the seaweed carbon removal wouldn’t fully compensate for the plankton loss, increasing CO2 in the atmosphere by up to 700 million tonnes per year.

“You can’t just grow macroalgae and assume that you’re going to be undertaking CDR if you’re not accounting for offsetting phytoplankton growth,” says at the UK National Oceanography Centre, another member of the team.