Why growing seaweed isn’t a climate solution

Growing seaweed has long been seen as a way to slow climate change. But this approach is mistaken, a new study argues, as the loss of seaweed emits far more carbon than what is being absorbed by attempts to grow it.

Seaweeds absorb an estimated 3.7 billion tonnes of carbon dioxide through photosynthesis every year, drawing it from seawater, which then draws more CO2 from the air above. They mostly grow near the coast, where they are eventually digested or decomposed, emitting carbon. But fragments of fronds, as well as sugars and carbohydrates from the seaweeds that dissolve in seawater, can sink into the deep sea, sequestering a small fraction of the CO2 as “blue carbon” for thousands of years.

Scientists and start-ups have suggested that seaweed growth, especially kelp, could be expanded to take up more CO2, something the UN has said we will need to do on a massive scale to eventually reverse dangerous levels of global warming.

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But there are concerns that cultivating huge amounts of seaweed could backfire and diminish growth of phytoplankton, another important carbon sink. And even the kelp forests we currently have, which are estimated to cover around 640,000 square kilometres worldwide – an area slightly larger than France – are dying off as ocean temperatures reach record highs.

Now, Karen Filbee-Dexter at the University of Western Australia and her colleagues have concluded that, with some 60 per cent of kelp forests in decline, these ecosystems could eventually begin emitting more CO2 than they absorb. Researchers, as well as governments and companies trying to offset their emissions, need to focus on preserving rather than growing seaweed, they suggest.

“Instead of asking how much carbon we can remove with seaweeds, we should be asking how much carbon we can stop from being released as these ecosystems are damaged by climate change,” says Filbee-Dexter.

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Some studies have investigated the possibility of removing a billion tonnes of carbon per year with seaweed, but there are already signs that this may not be feasible. For instance, the US carbon removal start-up Running Tide, which raised $70 million to cultivate kelp on wooden buoys that would gradually sink into the deep sea, had difficulty growing kelp in the open ocean and shut down last year.

Other projects have tried to bring back lost kelp forests. An experiment in northern Norway restored 70 hectares of kelp by poisoning seaweed-eating sea urchins, which can multiply when overfishing and hunting remove their predators.

While kelp restoration like this is a win for biodiversity, it only draws down about 50,000 tonnes of CO2 per year. Meanwhile, global kelp loss is emitting an estimated 14 million tonnes of CO2 per year.

The study by Filbee-Dexter and her colleagues also notes that preserving kelp ecosystems is at least eight times cheaper per hectare than restoring them, yet only 2 per cent of kelp forests lie in protected areas.

Firms are also cultivating seaweed to make nori for sushi, animal feed and thickeners for products like ice cream and toothpaste. Fragments of seaweed still drift away and sink into the deep sea, and aquaculture companies like Kelp Blue have promised to eventually sequester and offset hundreds of millions of tonnes of CO2.

Seaweed farming now takes up 4000 square kilometres of ocean. But that’s only about a third of the kelp forest cover lost each year. And after factoring in emissions associated with farming operations – like boat fuel and plastic lines – nine out of 11 seaweed farms were found to emit more CO2 than they sequestered in a study published earlier this year.

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“Seaweed restoration and farming is unlikely to be a major climate change solution,” Filbee-Dexter says. “But that doesn’t mean it isn’t worthwhile.”

Restoration can improve biodiversity, and farming can make low-carbon alternatives to fossil-fuel products like fertiliser, she notes.

Pamela Walsh at Queen’s University Belfast in the UK, however, says scientists don’t have a solid enough understanding of how much carbon seaweed sequesters to write it off yet. Macroalgae restoration, conservation and farming can all still be climate solutions, even if their impact has often been exaggerated in the past, she argues.

“Blue carbon is part of the puzzle,” she says. “It can make a contribution. The question is how that is determined and validated.”

PLOS Biology DOI: 10.1371/journal.pbio.3003949