Researchers and start-ups are exploring a variety of ways of locking up carbon in the sea, from seaweed farming to fertilising the oceans with iron – but we know little about the implications.
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| Farmed kelp could capture and store large amounts of carbon Running Tide |
The researchers behind the idea insisted that, in theory, adding huge amounts of baking soda would do our seas no harm – in fact, it could have a beneficial effect by reversing acidification of marine waters.
It seems to be something of a trend. Growing numbers of scientists, entrepreneurs and big businesses are eyeing the oceans as a potential depository for unwanted carbon, and baking soda isn’t the only game in town.
There is no shortage of radical ideas for so-called ocean carbon dioxide removal (ocean CDR), from adding iron to “fertilise” ocean waters and boost algae growth, to restoring natural seagrass meadows and farming seaweed that would then be sunk into the deep ocean.
Already the world’s oceans act as a carbon sink, absorbing about 31 per cent of the CO2 we emit into the atmosphere. Raising that figure is the fastest, most scalable way of reversing climate change, ocean CDR proponents argue.
But how much do we know about the science, ethics and legal implications of using the oceans as a carbon dumping ground?
Seaweed-based storage
There is one type of ocean CDR that is advancing faster than almost any other, and that is seaweed farming. In recent years, a flurry of start-ups and research projects have been launched to investigate whether growing seaweed, then sinking it into the deep sea, could be an effective way of capturing and storing carbon.
Running Tide, for example, is a US start-up that will start field trials off the coast of Iceland this year. The plan is to set thousands of buoys that will function as tiny floating kelp farms adrift in the ocean. Eventually the buoys dissolve and the seaweed that grows on them sinks to the deep ocean, storing the trapped carbon there.
Meanwhile, UK firm Seafields aims to establish a huge farm for sargassum seaweed in the south Atlantic, with trials already under way in the Caribbean nation of Saint Vincent and the Grenadines. Once the seaweed reaches maturity, it will be baled and sunk in the deep ocean.
Big businesses are also eyeing the opportunity. Canadian firm Lundin and Norwegian oil outfit Equinor are among the companies investing in a new project to establish seaweed-based carbon capture off the coast of Norway. Once the seaweed is grown, the plan is either to sink it into the deep ocean or turn it into biochar, a carbon-rich solid that can be buried underground, says Jorunn Skjermo at Sintef, a research agency working on the project. The first pilot farm will be set up this autumn in Norway, with researchers from Sintef monitoring it around the clock to see how it performs.
Most of the commercial incentive for these schemes lies in their potential to produce carbon offset credits, which can be sold to firms that want to offset the impact of their own operations.
But credible carbon offsetting depends on whether schemes actually deliver the carbon reductions they promise. It is an issue that has dogged the wider carbon offset industry for years, and when it comes to seaweed-based CDR, the benefits are still highly uncertain.
Scientific uncertainties
At the moment, it isn’t clear how effective growing and sinking seaweed is at capturing carbon. Some of the seaweed will break away from the farm in strong currents for example, allowing it to leak out of the system; some of it may wash up on land and release carbon back into the air.
Unlike biochar, it is uncertain whether seaweed sunk into the deep ocean will prove an effective, long-term carbon store. “We know how much of the carbon will end up as biochar,” says Skjermo. “For this deposition solution, we know a lot less.”
For her, the Norway pilot project is about getting answers: “This is about testing and doing research, so that we know that we are not causing any harm,” she says. But if all goes to plan, a radical scale-up is on the horizon. Around 20,000 square kilometres of seaweed farming would harvest enough carbon to offset Norway’s annual emissions, according to Sintef.
But other researchers are wary. A 2022 study by scientists at the National Oceanography Centre in Southampton, UK, warned that some of the carbon sunk in the deep ocean may not remain locked away as long as first thought. Only 66 per cent of the carbon that reaches 1000 metres below the ocean surface stays trapped for 100 years, according to their research.
Meanwhile, there are fears the mass cultivation of seaweed could disrupt the delicate ecological balance of oceans, with seaweed farms outcompeting other phytoplankton communities for nutrients. In a 2022 paper, Philip Boyd at the University of Tasmania, Australia, and his colleagues warned large-scale seaweed plantations in the open ocean would “probably affect offshore ecosystems through a range of biological threats, including altered ocean chemistry and changed microbial physiology and ecology”.
Growing a lot of seaweed, therefore, may come with ecological risks, and there is no guarantee it can make a meaningful dent in atmospheric CO2 concentrations. Frithjof Kuepper at the University of Aberdeen, UK, “has doubts” that seaweed will be able to trap and store carbon on the scale necessary to make a global impact on climate change. “Research should definitely happen, because we don’t know how much seaweed contributes to global carbon drawdown,” he says. “I don’t want to go as far as saying whether this is a solution or not, because we don’t have the figures.”
A race against time
Firms such as Running Tide are backed by millions of dollars from investors and are under pressure to prove that ocean-based carbon removal is a bona fide climate win.
But that investment is a drop in the ocean compared with the funding needed to prove the worth of such approaches. For seaweed-based CDR alone, it will cost $1 billion in controlled field trials to adequately verify the carbon benefits and ecological impacts, says Ocean Visions, an organisation that researches CO2 storage in the oceans.
It is money the world has no choice but to spend, according to Brad Ack, its CEO. “The whole field is nascent, emerging,” he says. “All of the science is still unsettled. We need a massive acceleration in testing and development in order to answer a whole host of remaining questions, and we need to do it very quickly.”
The timeline of climate change and the need to cut emissions rapidly this decade to stop runaway warming mean ocean carbon storage needs to be urgently investigated alongside all other carbon capture and emission reduction measures, says Ack. The latest IPCC report, released last week, made clear that emissions must fall rapidly this decade to keep alive any hope of limiting warming to 1.5°C. That might mean reducing CO2 concentrations in any way possible, even if the solution isn’t perfect, he says.
“We are in the realm of least-bad alternatives,” says Ack. “We’re already past the point of dangerous change on the planet. Now the question is, what are the least bad measures we can take to slow down and ultimately reverse these problems? And the oceans have to be part of the question.”
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