Seaweed 'forests' can help fight climate change

Farming seaweed, then sinking the mature plants to the bottom of the ocean, could be an effective way to fight warming. So why don’t we do it?

By Todd Woody
Published 29 Aug 2019, 19:14 BST
A mix of kelp, Irish moss, and sea lettuce harvested off the coast of Maine.

A mix of kelp, Irish moss, and sea lettuce harvested off the coast of Maine.

Photograph by Rebecca Hale, Nat Geo Image Collection

As the Amazon burns, there’s growing interest in cultivating forests that absorb planet-warming carbon emissions, but that are fireproof.

That’s because these forests are underwater.

An increasing body of research is documenting the potential of seaweed farming to counter climate change as deforestation decimates rainforests and other crucial carbon sinks. Fast-growing oceanic jungles of kelp and other macroalgae are highly efficient at storing carbon. Seaweed also ameliorates acidification, deoxygenation, and other marine impacts of global warming that threaten the biodiversity of the seas and the source of food and livelihoods for hundreds of millions of people.

“Seaweed is finally having its moment in the spotlight,” says Halley Froehlich, a marine scientist at the University of California, Santa Barbara.

She is the lead author of a new study that for the first time quantifies the global capacity of large-scale seaweed farming to offset terrestrial carbon emissions and maps areas of the ocean suitable for macroalgae cultivation.

Farming seaweed in just 3.8 percent of the federal waters off the California coast—that’s 0.065 percent of the global ocean suitable for growing macroalgae—could neutralise emissions from the state’s $50 billion agriculture industry, according to the paper published Thursday in the journal Current Biology.

Seaweed is currently grown on a small scale for use in food, medicines, and beauty products. The scientists, however, propose the establishment of industrial-size farms to grow seaweed to maturity, harvest it, and then sink it in the deep ocean where the captured carbon dioxide would be entombed for hundreds to thousands of years.

They found that raising macroalgae in just 0.001 percent of seaweed-growing waters worldwide and then burying it at sea could offset the entire carbon emissions of the rapidly growing global aquaculture industry, which supplies half of the world’s seafood. Altogether, 18.5 million square miles of the ocean is suitable for seaweed cultivation, the study concluded.

There’s a catch

“The technology doesn’t yet exist” to sequester seaweed in the deep ocean, notes Froehlich. “Hopefully this paper spurs conversation among engineers and economists about what would it take for the actual mechanisms to be put in place.”

Carlos Duarte, a leading seaweed scientist at the Red Sea Research Center in Saudi Arabia, attended a presentation on the paper’s findings in April.

“The new study adds to previous research and global estimates … to point at seaweed aquaculture as a major avenue to mitigate climate change,” he writes in an email, noting that he had not reviewed the final paper. “My opinion is that the estimates are very conservative and the potential is much higher, if the crop is maintained properly.”

But Duarte opposes sinking seaweed.`

“Seaweed is a very valuable material and there are better ways of using this material, while contributing to mitigating climate change, than disposing of it in the deep sea,” he says.

Indeed, Froelhich and other marine ecologists dub seaweed “charismatic carbon” for the macroalgae’s Swiss army knife-like ability to address a variety of environmental ills, in the ocean and on land.

Beyond seaweed’s potential to counteract acidification and deoxygenation, absorb excess nutrients and provide habitat for marine life in at least 77 countries, seaweed can be processed into biofuel. And research has shown that adding seaweed to livestock feed can reduce potent methane emissions from the burps of cows and other grazing livestock—a significant source of global greenhouse gases—by as much as 70 percent. Seaweed can also be used as an agricultural soil supplement, replacing petroleum-based fertilisers.

“The maths show you that seaweed can be a very effective tool to fight climate change, but it has to be validated by the market,” says Scotty Schmidt, chief executive of Primary Ocean, a Los Angeles company working on a United States government-funded project to develop technologies to deploy large-scale seaweed farms.

“Farming seaweed just for carbon sequestration is not a viable business case at this time as there's barely a carbon market that's willing to accept seaweed offset credits,” he says.

Primary Ocean’s strategy is to extract material from seaweed that can be sold for agricultural use. If a profit could be made from those sales and carbon credits were available, the company could then sequester the macroalgae waste, Schmidt says.

Getting the international carbon credit bean counters to accept seaweed as a legitimate source of greenhouse gas reduction is one of the bigger challenges.

“The science and the demand is already there; the bottleneck is a catalyser that makes the production meet the demand,” says Duarte. “Specifically we need carbon credit protocols that can be used to claim carbon credits from seaweed aquaculture and also regulatory environments that facilitate concessions and licences for seaweed aquaculture.”

China and other Asian nations that produce most of the world’s farmed seaweed are expected to take the lead in establishing macroalgae as a source of “blue carbon.”

“In the U.S. it is likely to be easier to obtain a licence for an oil rig than it is for seaweed farming,” says Duarte.


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