Thallus of filamentous algae underwater in the ocean, algal bloom
Toxic bloom: adding iron to the ocean stimulates the growth of algae, which can absorb carbon dioxide © Alamy

Next summer, if all goes to plan, shiploads of iron-rich dust will be distributed over the Pacific Ocean off the coast of Alaska, as well as the Atlantic off New England. And it is being done for two good reasons: to stimulate the growth of marine life, and pull carbon dioxide out of the atmosphere.

Although this idea of ocean fertilisation — adding iron, a nutrient in short supply across many of the world’s seas — dates back more than 30 years, until now, there have been few attempts to put it into practice.

Public, political and financial support has been held back by fears of unanticipated adverse consequences. There is also a more general antipathy to fighting climate change through technical fixes and geoengineering rather than solving the fundamental problem that human activities generate far too much CO₂ and other greenhouse gases.

However, growing alarm about the threat posed by global warming is reviving interest in carbon-removal technologies. Earlier this year, the Intergovernmental Panel on Climate Change report highlighted for the first time the importance of removing CO₂ from the atmosphere, such as by planting trees or carbon capture and storage.

This renewed interest has also given a new lease of life to ocean-based methods of carbon dioxide removal (CDR). While fertilisation to stimulate the growth of green phytoplankton (microscopic algae), which absorb CO₂ in nutrient-poor waters, will be used in the Alaska and New England projects, there are other methods. One is to counter the growing acidity of seawater as it absorbs CO₂ by adding alkaline minerals, enhancing its absorptive capacity.

Earlier this year, a report by the US National Academies of Sciences, Engineering, and Medicine advocated a programme of CDR research, development and evaluation at a cost of hundreds of millions of dollars from public and private sources.

Russ George, the Alaska-based environmental entrepreneur behind the projects in his state and New England, calls them Ocean Pasture Restoration — to emphasise what he sees as a drive to bring marine ecosystems back to health, as well as to capture millions of tonnes of CO₂ in the sea.

He says climate change and human activities have diminished phytoplankton populations — the base of many natural food chains — across many oceans through nutrient depletion. One factor, he says, is that less iron-rich dust is being blown off the Earth’s landmasses into the seas.

George points to a small-scale experiment in 2012 when iron dust was spread over 5,000 square miles of the Gulf of Alaska. “Our work produced the largest catches of salmon in Alaskan history over the following four years,” he says, though others suggest natural factors, rather than the fertilisation, were responsible for the blossoming of marine life then.

Opponents worry that ocean fertilisation might feed too much algal growth, leading to toxic blooms. The National Academies report said there was little data to support this concern, but did not entirely dismiss the possibility.

But proponents of the technology respond that harmful algal blooms follow excessive run-off of agricultural nitrogen and phosphorus fertilisers into inshore waters — and are not relevant to applying iron further out to sea.

Phil Williamson, an expert on the ocean environment at the University of East Anglia, says: “I wouldn’t raise a big red flag over toxic algal blooms. That is not my big worry about ocean fertilisation. My concern is that, if you increase productivity in one area of the ocean, you might decrease it somewhere else.”

The biggest obstacle, though, in his view, is public and political acceptance. “Many countries still dislike ocean fertilisation vehemently and say: ‘Don’t go there, don’t even think about messing up the ocean’,” he points out.

The National Academies report agrees that “social and regulatory acceptability is likely to be a barrier to many ocean carbon dioxide removal approaches”, while noting that no international agreement imposes a legally binding ban on the practice.

However, proponents of “blue carbon” approaches aimed at increasing oceans’ uptake of CO₂ insist that the tide of opinion is turning in their favour.

In the US, a bipartisan group of congress representatives recently introduced the Ocean Restoration Research and Development Act of 2022, which would authorise the spending of $33mn a year from 2023 to 2027 on research and development of technologies — such as adding alkaline minerals, or iron, to the seas.

“Ocean restoration provides a unique opportunity to sequester tens of millions of tonnes of blue carbon while simultaneously helping restore fisheries,” says Todd Johnston, head of policy for the conservative environmental group ConservAmerica. “This initiative could be a game changer when it comes to our efforts to address climate change and improve the health of our oceans.”

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