seaweed carbon – A climate-fix idea—growing seaweed to pull carbon dioxide from the air—faces fresh scientific warnings. New modelling suggests seaweed cultivation could strip nutrients from phytoplankton, shrink marine food webs, and in some scenarios even increase atmospheri
The pitch sounded clean enough: grow seaweed, capture carbon, sink it deep, and slow the planet’s warming. Tens of millions of dollars have gone into that promise. But new research is pushing back hard. warning that the very chemistry that makes seaweed a potential climate tool could also make it fail—at least at the scale people are imagining.
The Paris Agreement goal of limiting global warming to 2°C is why carbon dioxide removal (CDR) is being discussed so urgently. according to the UN. Seaweed has been framed as a cheap way to do it. Yet the science now points to an uncomfortable trade-off: seaweed can deprive phytoplankton of nutrients. and phytoplankton are themselves major players in moving carbon into the deep ocean.
The stakes are not abstract. A 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. It ran out of financing and closed last year.
The push did not end. A Dutch company. Kelp Blue. has raised at least $2 million to expand the seaweed it is currently growing to produce sustainable agricultural fertiliser in Namibia. Kelp Blue says small particles of its seaweed may break off and drift into the depths. and that this could eventually “sequester and offset” up to 500 million tonnes of CO2 per year.
But the new studies—focused on what happens when seaweed farming scales up—suggest that this kind of promise may not survive contact with ocean ecology.
“ It could backfire locally,” says Manon Berger 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.”
To understand why, the research points to where seaweed lives and what it feeds on. Except for sargassum, macroalgae species live near the coast, where nutrients are plentiful. During photosynthesis they consume carbon dissolved in seawater, helping the ocean absorb more CO2 from the atmosphere.
After that, the story turns. Marine organisms and microbes digest or decompose most of that seaweed, emitting an estimated nine-tenths of its carbon. For more carbon to stay out of the atmosphere. seaweed would have to be grown or transported further offshore. where it could be baled or otherwise sunk to the deep sea.
Offshore, though, the nutrient problem becomes sharper. Berger and colleagues modelled cultivation of 20 billion tonnes of seaweed per year across waters up to 200 nautical miles from coastlines. They found seaweed would quickly start depleting nitrogen, phosphorus and iron in the water. After 25 years, growth would have declined 95 per cent. And the modelling suggests that diminished conditions would also reduce global phytoplankton growth by as much as 8 per cent.
In some scenarios, the studies say, seaweed cultivation could still remove billions of tonnes of CO2. But depending on what species 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.
Berger’s model also narrows the places where seaweed might be effective without strongly reducing phytoplankton. Patches off Senegal and southern Australia—about 0.05 per cent of the ocean—are the only places suggested to flourish without significantly decreasing phytoplankton.
“If you have only a few very specific locations, you can’t grow enough seaweed to have a gigatonne of removal,” Berger says.
A second study adds another layer: even if seaweed farming could be boosted with iron. the price could be paid elsewhere in the ecosystem. Andrew Yool at the UK National Oceanography Centre and his colleagues modelled what would happen if seaweed cultivation areas were fertilised with iron. In their scenarios, up to 40 billion tonnes of CO2 could be removed each year. Yet the same modelling says that would also halve the plankton in the ocean. with dire consequences for fish that eat them.
“You’re robbing the surface ocean of nutrients… and transferring those to depth,” Yool says. “Essentially, you’re curtailing or slowly strangling the natural ecosystem.”
The physical logistics also surface in the modelling. To farm and sink enough seaweed 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 the ocean wasn’t fertilised with iron, the studies warn that the seaweed carbon removal would not fully compensate for plankton loss—leading to an increase in 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 Chelsey Baker at the UK National Oceanography Centre, another member of the team.
The human story behind these studies is a reminder that climate solutions don’t live in spreadsheets alone. They move through ecosystems, through supply chains, through funding rounds—and, eventually, through the atmosphere they’re meant to protect.
seaweed carbon dioxide removal CDR kelp cultivation phytoplankton ocean nutrients nitrogen phosphorus iron Paris Agreement Running Tide Kelp Blue climate mitigation