Can we fight climate change by sinking carbon into the sea?

Two Israeli companies are betting that by trapping biomass deep underwater, they can keep gigatons of CO2 out of the atmosphere.
By Alison F. Takemura

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On the Black Sea, three people on a green ship deck probe some machinery that has collected sediment samples
In partnership with carbon-removal startup Rewind, researchers in Turkey remove sediment samples taken from the bottom of the Black Sea in February. (Rewind)

Here’s an inconvenient truth: We’ve polluted the atmosphere with so many gigatons of greenhouse gases that just slashing emissions — which we still absolutely have to do — will no longer be enough to avert the worst impacts of climate change. We’ll also need to actively remove greenhouse gases from the atmosphere. The United Nations’ Intergovernmental Panel on Climate Change estimates that by 2050, even assuming steep emissions cuts in the next few decades (and emissions have been rising, not falling), we’ll need to take out something like 5 to 10 gigatons of CO2-equivalent each year.

The problem is, no one knows yet how to remove carbon from the atmosphere at anywhere close to that scale.

But the race is on to figure it out: Hundreds of companies are pursuing a broad range of approaches, from direct air capture (deploying large machines to pull CO2 out of the air) to carbon-removal strategies that purport to harness natural systems to sequester carbon.

In this last category, two Israeli companies, Rewind and BlueGreen Water Technologies, have carved out a niche for themselves in the nascent field of aquatic carbon removal. Both companies are betting they can lock away more CO2 per year more quickly and with less capital investment by taking readily available biomass and sinking it to the bottom of large bodies of water. They aim to be removing carbon by the gigaton annually by 2030.

But outside experts say the companies have more work to do to prove they can effectively sequester carbon without doing more harm than good.

For one, organic carbon sequestration is notoriously difficult to measure, so it will be hard to independently verify the claims Rewind and BlueGreen make. And both startups employ methods that could be stymied by hard-to-predict interactions with microbes.

These issues speak to the larger uncertainties surrounding the field of aquatic carbon removal (as well as the related category of ocean-based carbon removal).

People are still trying to learn about and understand…the efficacy of these [approaches] and their impacts on ecosystems,” said Sifang Chen, an expert in ocean-based carbon removal at nonprofit Carbon180. This is a very new field.”

Burying carbon in the Black Sea

Compared to other companies’ carbon-removal strategies, Rewind has a very simple, almost laughable solution,” CEO Ram Amar said. We want to collect plants and transport them into the bottom of the Black Sea.”

The company is a pursuit of passion for Amar. In 2019, the entrepreneur sold a cloud-data startup to Google for an undisclosed amount. He then set his sights on addressing climate change, around the time his first daughter was born.

Unchecked, climate change will disrupt food systems, intensifying tensions in the volatile Middle East, where Amar and his family live, he said. I wanted to work on a problem with more meaning to my life or to my children’s life.”

Amar’s research into potential climate solutions eventually led him to the Black Sea. Northeast of the Mediterranean, the vast body of water could, in theory, function as a carbon-storage vault. It has an anoxic zone below a depth of about 150 meters that is devoid of oxygen and high in hydrogen sulfide concentrations. These conditions inhibit microbes that depend on oxygen to decompose biomass into CO2, thus creating an environment that preserves organic matter very well,” Amar said.

Map of the Black Sea surrounded by other countries, including Ukraine, Romania, Georgia, Bulgaria, Turkey.
The Black Sea (Google Maps)

Don’t take his word for it, though; just look at the shipwrecks, he said. Sixty astonishingly well-preserved sunk ships lie at the bottom of the Black Sea, their wooden hulls, masts and rudders virtually unravaged by time and decomposing microbes. One ancient Greek ship has lain undisturbed for the last 2,400 years.

Rewind plans to collect huge quantities of plant material and sink it to the bottom of the Black Sea, where, like the ancient ships, it could remain intact for centuries or even millennia, locking away all the carbon that would be released into the atmosphere if that biomass decomposed on land.

Collecting biomass to toss into the sea

Where will Rewind find the billion tons of unwanted organic material that it needs to put its plan into action? The region around the Black Sea is the heart of Europe’s breadbasket, annually producing gigatons of combined agricultural, orchard and forestry residues — in the form of wheat stalks, leaves and trimmed tree branches, according to Amar. Much of this waste is piled up and left to rot, or worse, burned, Amar said.

Rows of trees without leaves, with person on ladder in the distance, using clippers to prune treetops.
An Israeli farmer pruning apple trees in the Golan Heights in January. Rewind used the trimmings for experiments. (Rewind)

Rewind intends to work with countries’ agriculture and forestry industries to collect and transport biomass to the Black Sea. The company calculates the cost in emissions of doing this would be just 3 percent of what the company can sequester. And far from filling up the Black Sea, all that organic matter, Rewind estimates, would take up less than 0.1 percent of the sea’s volume.

The company is striving to sequester a gigaton per year by 2030, initially at $200 per ton of CO2, then decreasing to $80 per ton of CO2 once Rewind has optimized its supply chain, he said.

So I’m sure you’re thinking, Why is this guy looking to throw trash in the Black Sea?’ I’m not,” Amar said. All rivers in the world carry branches and leaves out to the seas where they flow, and they just dump it on the bottom. This is a natural process, and we are accelerating it.”

Concerns about Black Sea ecosystem, greenhouse gases

For the moment, Rewind is conducting research, carrying out degradation experiments with plant matter and analyzing ancient shipwreck wood in order to calibrate models of how the Black Sea’s anoxic conditions preserve carbon.

One crucial question that has yet to be resolved: Will that carbon really stay put?

While the bottom of the Black Sea is devoid of oxygen, that doesn’t mean it’s devoid of life. Microbes called methanogens inhabit its depths; they can devour organic matter in anoxic environments. When that happens, they release carbon-containing methane (CH4), a greenhouse gas that is shorter-lived than CO2 but 84 times more potent over a 20-year timeframe.

Is there going to be any kind of interaction between those microbes and the biomass? […] Is there going to be any leakage of other types of greenhouse gases?” Chen said.

Methane was a big concern for us when we started more than a year ago,” Amar acknowledged. But talks with scientists have led the Rewind team to believe methane won’t be a problem, notably because the Black Sea essentially doesn’t mix vertically — so even if methane is produced in the anoxic zone, Amar said it’s unlikely to reach surface waters and escape to the atmosphere.

But the company isn’t going to rely on theory, he added. They plan to monitor their sunk biomass, using cameras as well as sensors to track the water chemistry: traces of methane, carbon dioxide, pH, oxygen and more.

Scientist sitting with laptop connected to wires leading to metal crate containing white mesh bag of straw
Kobi Kaminitz, Rewind's CTO, prepares an underwater camera for a field test to monitor the degradation of a bag of straw in the Mediterranean Sea in January 2023. (Rewind)
Rock covered with green and purple sea life, including anemones, surrounded by white sand
The first test of Rewind’s underwater camera on the floor of the Mediterranean Sea in October 2022 (Rewind)

We’re not just counting on the fact that [the chemistry] is not going to change,” he said. We’re going to measure and monitor — and make sure that whatever we expect really does happen.”

The company will document its results on its website as they’re available, as well as publish in peer-reviewed scientific journals, Amar said.

A dark red-brown lump of ancient wood sits on a yellow tray with an electronic, flat-panelled instrument in the background
Ancient wood from 600–700 CE being prepared for density measurements at the University of Haifa, Israel in February. The data will be used to calibrate Rewind’s models of anoxic carbon storage in the Black Sea. (Rewind)

Matt Long is an oceanographer at the National Center for Atmospheric Research and is not affiliated with Rewind. He provided early feedback on a scientific literature review the company plans to publish later this year. I’ve been pleased with Rewind’s willingness to expose their thinking and documentation,” he said.

It’s just really critical for early companies to set a paradigm that embraces full transparency, so that the industry can be built on credible science,” Long said. In particular, as Rewind and other companies prepare to sell carbon removal credits, he said those credits need to come with robust verification standards — unlike the relatively lax standards for many emissions-avoidance credits available today on the voluntary carbon market. A lot of the carbon credits that are traded are specious,” he said, and are not significantly helping address the climate crisis.

Treating algal blooms to store carbon

BlueGreen Water Technologies takes a different approach to aquatic carbon removal. Instead of collecting material grown on land and sinking it underwater, the company is sinking biomass already teeming in aquatic environments: harmful algal blooms made up of cyanobacteria, also known as blue-green algae.

Cyanobacterial blooms wreak havoc on ecosystems. Caused by warming waters and excess nutrients, they can choke aquatic environments, creating dead zones with oxygen levels too low to sustain fish and other aquatic life. Cyanobacterial blooms also produce toxins that can harm people and animals.

Gloved hand holding cup that has scooped up some deep green slurry - a harmful algal bloom
Harmful algal bloom, not a green smoothie vat (BlueGreen Water Technologies)

The good news is…they also suck a lot of carbon out of the atmosphere,” said CEO Eyal Harel.

The company was founded in 2014 with the goal of cleaning up water pollution, but it has since evolved into a climate-solutions company. 

In fact, Harel said, BlueGreen used to have trouble raising money because investors didn’t think there was enough of a market for treating water.

One of the best things that happened to us is actually the rise of the climatetech industry,” Harel said. We’re no longer a company that remediates harmful algal blooms and, by the way, removes fantastic amounts of carbon. Today, we are a company that removes fantastic amounts of carbon, and by the way, cleans up lakes.”

The company claims it can sequester, on average, 8,000 tons of CO2 per square treated kilometer, and that across thousands of commercial applications, it has already locked away 1.5 million to 3 million tons of CO2 to date.

How? The company administers a buoyant powder containing hydrogen peroxide, which triggers programmed cell death or cyanobacterial suicide. The treatment clears afflicted waters in just a day or two. While some of the algal bloom decomposes in the water and returns to the atmosphere as CO2, some remnants fall to the sediment and stay there. According to BlueGreen, their products leave no trace, and they’re approved by the U.S. Environmental Protection Agency and the National Sanitation Foundation (NSF) for use in drinking water.

Person wearing gloves holds white bucket over water body, drizzling a sky blue powder into it as they walk along the shore
BlueGreen’s hydrogen-peroxide-based treatment (BlueGreen Water Technologies)

Many algal blooms already have a boom-bust cycle that can naturally sequester carbon. Some blooms take off during warm summer months before collapsing in the winter, with a fraction of the algae sinking to the bottom, and a fraction of that fraction getting permanently stored. (Other blooms, though, can persist through cold temperatures, which means less carbon is naturally sequestered.) So for any candidate body of water, BlueGreen has to assess how much more carbon the company can sequester through treatment than would have occurred anyway.

BlueGreen looks at sediment cores and three years of satellite data to establish a baseline for the amount of carbon from cyanobacterial blooms that has historically been permanently buried in the sediments.

The company has a detailed methodology for this carbon accounting that they produced in collaboration with Social Carbon, a carbon accreditation standard like Verra and The Gold Standard. Now, BlueGreen can use the methodology to quantify, and therefore sell, carbon removal credits, which it’s currently marketing at $100 per ton.

Mike Davies, CEO of Social Carbon, said the methodology will open up finance for globally critical problems — cleaning up freshwater sources, protecting biodiversity and fighting the climate crisis — all at the same time.

As to rigor, the methodology has been independently checked by our team but also experts in the field,” Davies told Canary Media. We’re putting our reputation on the line, so we want to make sure it’s airtight. […] Reputation takes a long time to build, and you can lose it extremely quickly.”

Other approaches to aquatic carbon removal — such as sinking macroalgae, a method employed by Maine-based Running Tide and U.K.-based startup Seafieldshaven’t gone through that rigorous process to get accreditation,” he said. (Companies are pursuing it, though. Rewind, for example, is working with on a carbon-accounting method.)

Cleaning up harmful algal blooms around the world

BlueGreen has grand ambitions. We are working on the biggest-ever-attempted projects in the world — in the United States, China, Israel, South Africa — with the aim to rid the world from harmful algal blooms,” CEO Harel said.

In 2021, BlueGreen treated Florida’s Lake Minneola, and the company is now working through prerequisites to treat Lake Mattamuskeet, the largest natural lake in North Carolina and part of the Mattamuskeet National Wildlife Refuge.

BlueGreen also aims to treat algal blooms in oceans, which have, the company estimates, a carbon-removal potential of 97 gigatons per year, in addition to the 15 gigatons per year in freshwater bodies.

Visually, BlueGreen’s results are striking. For instance, after treating Setumo Dam in South Africa in 2021, the opaque, scummy water cleared in just three days. Moreover, the ecosystem began to recover, Harel said, with the return of nontoxic phytoplankton, zooplankton, crustaceans, fish and flamingoes, which hadn’t been seen for years.”

On left, a 'before' image shows a greener water body; on right, an after image showing deeper blue water.
Setumo Dam in South Africa before and after BlueGreen’s treatment of the water (BlueGreen Water Technologies)

The true impact on remediating these bodies of water…is not just the carbon,” Harel said, but what it means for the local communities that rely upon these water resources, what it means to their prosperity [and] what it means to their health.” 

Community members who rely on Setumo Dam for drinking water and fishing speak about the positive impact of BlueGreen’s remediating the harmful algal bloom. (BlueGreen Water Technologies)

The specter of methane rises again

But as with Rewind’s approach, some scientists have voiced concerns that the climate benefits of BlueGreen’s technique could be undermined — or even canceled out — by the potential to increase methane emissions.

Methane is such a potent greenhouse gas that if a pile of fixed carbon dioxide (i.e., dead algae) lost just 1.2 percent as methane, that would more than offset the climate benefit of the remaining 98.8 percent of carbon that remained sequestered. So an approach that increases methane emissions even just a little immediately turns into a bad deal for the atmosphere,” said Sebastian Sobek, a limnologist at Sweden’s Uppsala University who studies mechanisms that lead to carbon burial in lakes and reservoirs. BlueGreen’s treatment results in a huge amount of organic matter raining down on the sediment, which Sobek said could trigger a microbial feeding frenzy, depleting the oxygen and thus favoring the growth of methane-spewing methanogens.

In its methodology, Social Carbon states that it does take into account methane, nitrous oxide (another powerful greenhouse gas), and carbon dioxide emissions — although it doesn’t require these gases to be explicitly measured. That’s because, after reviewing published studies and consulting with experts, Social Carbon has concluded that these gases’ impact would be negligible for water bodies that met specific eligible conditions required by the methodology,” Davies wrote in an email.

Social Carbon has also stated that methane production in the sediment is ultimately inhibited because the methane produced reacts with halogen ions — including chlorine — resulting in compounds such as the fixative chloroform, putting a stop to pesky methanogens.

But Sobek disagreed. He said that while he doesn’t doubt that this mechanism exists, at the ecosystem scale, it doesn’t seem to be very relevant.” Inland waters produce significant methane emissions, which can in fact increase when organic matter is added.

BlueGreen told Canary Media that although it has not done experiments to measure methane emissions, it plans to do so at some unspecified point in the future.

Sobek is eager to see BlueGreen conduct such a study and subject it to the scrutiny of peer review by a scientific journal. If they can provide evidence that is convincing,” he said, I’m happy to change my opinion.”

While it may be too soon to say whether data will back up the net gigaton carbon-removal expectations of either BlueGreen or Rewind, Sobek admires the growing efforts in the field of aquatic carbon removal. We have a big problem with climate change and all too much CO2 in the atmosphere,” he said. I find it exciting that some people venture to do something about it.”

Alison F. Takemura is staff writer at Canary Media. She reports on home electrification, building decarbonization strategies and the clean energy workforce.