Proponents of modifying the ocean to battle climate change have rising hopes for their biomass sinking plans, but critics fear the environmental impacts
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Entrepreneurial companies think sinking crop waste in oxygen-depleted waters could lock up carbon and help fight with climate change. The Black Sea is one location on their lists (Image: NASA / Dembinsky Photo Associates / Alamy)
Scientists and entrepreneurs from across the world gathered in Bucharest earlier this year to discuss a temptingly simple idea to battle climate change.
Gathering up plant scraps left over from agriculture and sinking them into the depths of the ocean could, they believe, take carbon that nature has locked up in plants and put it where it cannot escape.
Proponents believe the simplicity of the concept sidesteps some of the issues that have dogged other attempts to take geoengineering into the ocean. They are hoping a new standard defining how they should operate could fuel interest in this previously niche form of carbon removal. If it does, huge bundles of crop waste could one day be jettisoned from vessels in the Black Sea and the Gulf of Mexico, and money could flow to those doing it.
But significant questions remain about the environmental impact and how long carbon stored in this way will remain trapped. Opponents say geoengineering in general is a dangerous distraction from tackling climate change, and that the environmental impact of specific concepts – including this one – are often understated or underexplored in the rush to make money via the market for carbon credits.
The ocean for geoengineering
The ocean takes up somewhere between a quarter and a third of the current carbon dioxide emissions from fossil fuel burning and other sources, carbon dioxide that could otherwise be in the atmosphere, warming the planet. It has absorbed about 90% of the excess heat produced by these human activities too.
This outsized role in the climate system – and, more generally, the fact it covers most of the planet – makes it a tempting target for those who want to place a finger on the scales to tilt the Earth’s systems against warming.
Morgan Raven works on the carbon cycle at the University of California, Santa Barbara, in the US. She notes that studies of Cretaceous rocks show large deposits of organic matter in the ocean helped the Earth cool during a past “hothouse” event.
“This is how the Earth responds on a 100,000-year time scale,” she says. The question for geoengineers: “Can we make it a 100-year time scale so it’s useful for humans?”
Raven now works with Carboniferous, a company looking to sink agricultural waste, such as straw stalks and leaves, into the ocean. The concept was recently given the title Marine Anoxic Carbon Storage, or Macs, at that workshop in Romania.
While geoengineering via carbon dioxide removal has been criticised for distracting effort from emissions reductions, supporters cite the Intergovernmental Panel on Climate Change’s 2022 conclusion that carbon dioxide removal is “unavoidable if net-zero CO2 or GHG [greenhouse gas] emissions are to be achieved”.
Raven says: “All of us are frustrated and disappointed with the fact that we are completely failing to control our emissions, and that is the big lever that has to happen. There are acute effects, [such as] some of these threshold changes in the system that are coming at us, and so I see this as a sort of emergency response tool to avoid some of those. But obviously, none of this matters if we don’t stop putting it in the air.”
But the ocean has proven a difficult place for would-be geoengineers. Operating at sea is, in general, harder than working on land, and there are complex and sometimes overlapping jurisdictions and regulators governing what can and cannot be done there.
Companies looking to undertake geoengineering need money. To-date, that has generally meant selling carbon credits to companies looking to go green by compensating for carbon emissions they are responsible for.
This has led to a voluntary carbon market – or markets– where companies claiming to have reduced the amount of carbon in the atmosphere sell carbon credits to companies that worry they have increased it. To try and make these trades more robust, various businesses have sprung up offering standards for such activity.
One of them is Puro.earth. It issues what it calls Corcs (CO2 Removal Certificates) for each tonne of carbon removed from the atmosphere and sequestered safely. These are given out to firms that meet its standard in five terrestrial forms of carbon dioxide removal, including the production of biochar.
Currently there are no marine projects that have been given Puro’s vote of carbon confidence – though that is about to change.
Getting standardised
In January, Puro.earth unveiled its methodology for how carbon could be removed from the atmosphere via “ocean storage of biomass”. It is also looking at certifying carbon directly captured from the air and stored in the ocean.
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Agriculture can produce significant amounts of waste plant matter. Sinking this biomass could potentially stop the carbon it contains being released to the atmosphere as carbon dioxide as it rots (Image: James Wakibia / Sipa US / Alamy)
The latter has several relatively complex steps, involving applying electricity to water and then going through various chemical processes to store carbon in ocean waters. Puro summarises the former in a sentence: “The methodology focuses on depositing sustainable biomass into permanently oxygen-depleted (anoxic) ocean basins where natural seafloor conditions allow for durable CO2 storage.”
Marianne Tikkanen, co-founder of Puro.earth, says: “From the beginning, we’ve understood that the ocean has good potential for removals, but for a methodology, a good level of technology maturity is also needed.”
The methodology, she claims, makes it easier for a market to evolve around such carbon credits. For biomass sinking, there is a narrow scope. “That scope makes the environmental impacts very contained, and it is a safe and conservative way to start exploiting the ocean’s removal capacity,” says Tikkanen.
She adds: “Our mission is to make it economical to remove carbon. It’s clear that a new industry needs to be born where there are parties who pay for carbon removal to happen, and then there are parties who can make it happen.”
The methodology is currently scheduled for public release in June.
The players: Carboniferous and Rewind
Carboniferous is one of the companies placed to take advantage of this certification. It is targeting the Orca Basin in the Gulf of Mexico. If it receives permission from the US government, it hopes to run a trial depositing 20 tonnes of biomass there, an amount Raven describes as “tiny”.
“At current prices, it’s 20 bucks worth of CO2 removal,” says Raven, who is the company’s chief science officer. “It is a large physical amount, but you can picture a tonne as being about a meter and a half cubed.”
On the other side of the Earth, another company is looking to a different oxygen-free sea.
Rewind wants to drop crop waste 2 kilometres into the Black Sea, which has famously anoxic depths trapped below a more oxygenated surface layer. It is, the company says, “the optimal environment allowing affordable, environmentally safe, gigaton-scale carbon removal in this decade”.
Ram Amar, its CEO, says the company has applied for a permit for a 100-tonne trial in the Black Sea from Romania. He adds that the Danube River is thought to carry about 100 tonnes of organic matter into the Black Sea every hour.
It’s clear that a new industry needs to be born where there are parties who pay for carbon removal to happen, and then there are parties who can make it happen
- Marianne Tikkanen, co-founder of Puro.earth
“100 tonnes is really nanoscale in the Black Sea, but we hope that it will still allow us to measure very localised changes in the water chemistry and biology that we could use to forecast what would happen when we go to the next level of scale: 1,000 tonnes, and then 10,000 tonnes,” says Amar.
The question of how quickly they can scale up is a crucial one for marine carbon dioxide removal (mCDR) companies – both so they can have a real impact on mitigating climate change, and so they can sell carbon credits and remain solvent.
“I don’t think anyone’s going to leap into doing a million tonnes [of biomass deposition] overnight by any means,” says Chris Vivian, an independent consultant and co-chair of a working group on mCDR at the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (Gesamp).
“I think they all need to do trials. And even if you look through the Puro.earth protocol, there’s a lot of work they would need to do before they could go to any scale where they could potentially claim significant carbon credits.”
Sinking biomass may be “relatively straightforward” compared to some mCDR options, but monitoring, reporting and verification are still big issues, Vivian notes. He also cites the need for public acceptance, and wider public knowledge in order to obtain that.
Environmental concerns
The depths of the Black Sea are devoid of oxygen and saturated with hydrogen sulphide – a state known as euxinic. These waters are unfriendly enough to traditional marine life that wooden ships that sank to the bottom thousands of years ago have remained remarkably preserved, their hulls protected from the voracious creatures that would devour them in most other seas.
This has often led to characterisation of these depths as a “dead zone”. But there is life here.
In a 2021 paper, researchers described how small invertebrates seem to have evolved to live under the challenging conditions at the bottom of the Black Sea.
One cannot for a good cause destroy an ecosystem by upsetting the existing balance
- Cristiana Callieri, marine researcher at the CNR-IRSA Water Research Institute
And in 2019, another team demonstrated the presence of “deep red fluorescence” in the anoxic waters caused by bacteria. The lead author of that research, Cristiana Callieri, told Dialogue Earth she was “absolutely against” plans to sink biomass into the area. She says environments such as the Black Sea “are a reservoir of high, but as yet unexplored microbial biodiversity”, and the microbiomes “would be destroyed by a catastrophic event such as throwing biomass into the sea”.
“One cannot for a good cause destroy an ecosystem by upsetting the existing balance,” says Callieri, who is a marine researcher at the CNR-IRSA Water Research Institute in Verbania, Italy.
Proponents insist that environmental impacts can be minimised by targeting bodies of water largely devoid of large life forms, and that the biomass may even serve as food for microbes that can live in these extreme conditions.
There is also an admission that the lack of what biologists call charismatic megafauna – your whales, sharks, etc – in anoxic zones brings more public acceptance. Disruption of microbial communities might be an easier sell to the public – and regulators – than the kind of animals that appear in children’s books about the ocean. The top question on Carboniferous’s FAQ page, for example, is: “Will the biomass bonk whales on the head when it’s transported to the basin?”. (Answer: no, and “we think this question should extend to all ocean animals”.)
Some companies have chosen other areas to deposit biomass into the ocean, and courted controversy in doing so. Perhaps the highest profile is the now-defunct firm Running Tide.
The ex-player: Running Tide
Running Tide was previously a standard bearer for the idea of helping save the planet by sinking plant matter into the seas. Its website touted plans to put buoys, made of “forestry residue” and limestone and decorated with kelp seeds, into the ocean, combining three different technologies for carbon removal into one. It sold carbon credits to major companies including Microsoft.
But last year the company announced via social media that it was shutting down. “Ultimately the contraction in the voluntary carbon market meant that it was impossible to scale up, and without buyers beyond the Microsoft’s [sic] of the world there wasn’t a way to get financing for continued R&D [research and development],” it said.
James Kerry, of the marine conservation NGO OceanCare, is one of those who wants a precautionary approach to all these marine carbon dioxide removal efforts.
“I have not been impressed with any of the mCDR technologies that I’ve looked at. Most of them I think won’t actually work under a full life cycle analysis, but they also carry with them a considerable amount of risk, some of which would only reveal itself upon large-scale deployment,” says Kerry, who is a senior research fellow working on ocean-climate issues with a focus on geoengineering at James Cook University in Australia.
Some techniques simply do not work as advertised, he says. Biomass sinking does initially sequester carbon, he adds, but a 1,000-year time frame for locking it up is “optimistic”, and it is problematic that the Puro.earth draft standard suggests monitoring for only 15 years after deposition.
Other questions also remain to be answered. Critics say the crop residues that might be sunk into the ocean could be better used to return nutrients to soils on land, and question how monitoring could work in practice, as well as the ultimate economics.
But proponents are pushing forward, and believe they can dodge the issues that sank Running Tide.
The key, Rewind CEO Amar tells Dialogue Earth, is balancing the environmental safety and scientific rigour of this more experimental phase with the need to scale up for climate change mitigation.
“The risks that ocean ecosystems bear by inaction is much larger than the pace that we want to increase pilot scales. The Danube does a million tonnes per year [of biomass into the Black Sea], and we can definitely get to that scale with very, very, very small risk within a few years. But if we do nothing, then in 10 years’ time, the oceans can go through a point of no return.”
Kerry is unconvinced. “I tend to call this ocean dumping of biomass, rather than ocean storage of biomass,” he says.
This article was originally published on Dialogue Earth under the Creative Commons BY NC ND licence. Read the original article.
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