This outsized role in the climate system \u2013 and, more generally, the fact it covers most of the planet \u2013 makes it a tempting target for those who want to place a finger on the scales to tilt the Earth\u2019s systems against warming.<\/p>\n\n\n\n
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 \u201chothouse\u201d event.<\/p>\n\n\n\n
\u201cThis is how the Earth responds on a 100,000-year time scale,\u201d she says. The question for geoengineers: \u201cCan we make it a 100-year time scale so it\u2019s useful for humans?\u201d<\/p>\n\n\n\n
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.<\/p>\n\n\n\n While geoengineering via carbon dioxide removal has been criticised for distracting effort from emissions reductions, supporters cite the Intergovernmental Panel on Climate Change\u2019s 2022 conclusion<\/a> that carbon dioxide removal is \u201cunavoidable if net-zero CO2 or GHG [greenhouse gas] emissions are to be achieved\u201d.<\/p>\n\n\n\n Raven says: \u201cAll 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\u2019t stop putting it in the air.\u201d<\/p>\n\n\n\n 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.<\/p>\n\n\n\n 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.<\/p>\n\n\n\n This has led to a voluntary carbon market \u2013 or markets\u2013 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.<\/p>\n\n\n\n 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<\/a>.<\/p>\n\n\n\n Currently there are no marine projects that have been given Puro\u2019s vote of carbon confidence \u2013 though that is about to change.<\/p>\n\n\n\n In January, Puro.earth unveiled<\/a> its methodology for how carbon could be removed from the atmosphere via \u201cocean storage of biomass\u201d. It is also looking at certifying carbon directly captured from the air and stored in the ocean.<\/p>\n\n\n\n The latter has several relatively complex<\/a> steps, involving applying electricity to water and then going through various chemical processes to store carbon in ocean waters. Puro summarises<\/a> the former in a sentence: \u201cThe methodology focuses on depositing sustainable biomass into permanently oxygen-depleted (anoxic) ocean basins where natural seafloor conditions allow for durable CO2 storage.\u201d<\/p>\n\n\n\n Marianne Tikkanen, co-founder of Puro.earth, says: \u201cFrom the beginning, we\u2019ve understood that the ocean has good potential for removals, but for a methodology, a good level of technology maturity is also needed.\u201d<\/p>\n\n\n\n The methodology, she claims, makes it easier for a market to evolve around such carbon credits. For biomass sinking, there is a narrow scope. \u201cThat scope makes the environmental impacts very contained, and it is a safe and conservative way to start exploiting the ocean\u2019s removal capacity,\u201d says Tikkanen.<\/p>\n\n\n\n She adds: \u201cOur mission is to make it economical to remove carbon. It\u2019s 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.\u201d<\/p>\n\n\n\n The methodology is currently scheduled for public release in June<\/a>. <\/p>\n\n\n\n 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 \u201ctiny\u201d.<\/p>\n\n\n\n \u201cAt current prices, it\u2019s 20 bucks worth of CO2 removal,\u201d says Raven, who is the company\u2019s chief science officer. \u201cIt is a large physical amount, but you can picture a tonne as being about a meter and a half cubed.\u201d<\/p>\n\n\n\n On the other side of the Earth, another company is looking to a different oxygen-free sea.<\/p>\n\n\n\n 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<\/a>, \u201cthe optimal environment allowing affordable, environmentally safe, gigaton-scale carbon removal in this decade\u201d.<\/p>\n\n\n\n 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.<\/p>\n\n\n\n \u201c100 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,\u201d says Amar.<\/p>\n\n\n\n The question of how quickly they can scale up is a crucial one for marine carbon dioxide removal (mCDR) companies \u2013 both so they can have a real impact on mitigating climate change, and so they can sell carbon credits and remain solvent.<\/p>\n\n\n\n \u201cI don\u2019t think anyone\u2019s going to leap into doing a million tonnes [of biomass deposition] overnight by any means,\u201d 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).<\/p>\n\n\n\n \u201cI think they all need to do trials. And even if you look through the Puro.earth protocol, there\u2019s a lot of work they would need to do before they could go to any scale where they could potentially claim significant carbon credits.\u201d<\/p>\n\n\n\n Sinking biomass may be \u201crelatively straightforward\u201d 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.<\/p>\n\n\n\n The depths of the Black Sea are devoid of oxygen and saturated with hydrogen sulphide \u2013 a state known as euxinic<\/a>. 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<\/a>, their hulls protected from the voracious creatures that would devour them in most other seas.<\/p>\n\n\n\n This has often led to characterisation of these depths as a \u201cdead zone\u201d. But there is life here.<\/p>\n\n\n\n In a 2021<\/a> paper, researchers described how small invertebrates seem to have evolved to live under the challenging conditions at the bottom of the Black Sea.<\/p>\n\n\n\n And in 2019, another team demonstrated the presence<\/a> of \u201cdeep red fluorescence\u201d in the anoxic waters caused by bacteria. The lead author of that research, Cristiana Callieri, told Dialogue Earth she was \u201cabsolutely against\u201d plans to sink biomass into the area. She says environments such as the Black Sea \u201care a reservoir of high, but as yet unexplored microbial biodiversity\u201d, and the microbiomes \u201cwould be destroyed by a catastrophic event such as throwing biomass into the sea\u201d.<\/p>\n\n\n\n \u201cOne cannot for a good cause destroy an ecosystem by upsetting the existing balance,\u201d says Callieri, who is a marine researcher at the CNR-IRSA Water Research Institute in Verbania, Italy.<\/p>\n\n\n\n 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.<\/p>\n\n\n\n There is also an admission that the lack of what biologists call charismatic megafauna \u2013 your whales, sharks, etc \u2013 in anoxic zones brings more public acceptance. Disruption of microbial communities might be an easier sell to the public \u2013 and regulators \u2013 than the kind of animals that appear in children\u2019s books about the ocean. The top question on Carboniferous\u2019s FAQ page<\/a>, for example, is: \u201cWill the biomass bonk whales on the head when it\u2019s transported to the basin?\u201d. (Answer: no, and \u201cwe think this question should extend to all ocean animals\u201d.)<\/p>\n\n\n\n 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.<\/p>\n\n\n\n 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 \u201cforestry residue\u201d 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. <\/p>\n\n\n\nGetting standardised<\/h2>\n\n\n\n
![\"Men](\"https:\/\/dialogue.earth\/content\/uploads\/2025\/05\/sugarcane-bagasse_Alamy_2XCBWD5.jpg\")
<\/div>The players: Carboniferous and Rewind<\/h2>\n\n\n\n
It\u2019s 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<\/blockquote>Marianne\u00a0Tikkanen, co-founder of Puro.earth<\/cite><\/div><\/div>\n\n\n\n
Environmental concerns<\/h2>\n\n\n\n
One cannot for a good cause destroy an ecosystem by upsetting the existing balance<\/blockquote>Cristiana Callieri, marine researcher at the CNR-IRSA Water Research Institute<\/cite><\/div><\/div>\n\n\n\n
The ex-player: Running Tide<\/h2>\n\n\n\n