Startups Begin Geoengineering the Sea

If you search up Hawaii’s Keāhole Point on Google Maps, center it on your screen, and then zoom out until you can see the edges of the globe, one thing will become abundantly clear: The Pacific Ocean is very, very big.

In a few months, on this volcanic headland on Hawaii’s Big Island, marine-tech startup Captura will begin pumping as much of the mighty Pacific through its pipes and tanks as it can. The company’s plan is to electrochemically strip carbon dioxide out of the ocean, store or use the CO₂, and then return the water to the sea, where it will naturally absorb more CO₂ from the air.

This article is part of our special report Top Tech 2025.

Captura is one of a cadre of startups eyeing Earth’s oceans as a carbon sink ready to be harnessed. The bioengineering strategies it’s deploying aim to accelerate what the oceans already do: absorb carbon emissions on a massive scale. This natural process has helped keep atmospheric CO₂ levels in check for millions of years, but it can’t keep up with present-day industrial emissions. Dozens of field trials and pilot projects have begun, and in 2025, Captura and several other companies will begin scaling up their facilities.

Their approaches are as diverse as they are bold. Some groups are growing kelp forests or microalgae in the sea. Others propose pumping seawater between shallow and deep layers to move carbon around. Two strategies caught IEEE Spectrum’s gaze—Captura’s ocean carbon dioxide removal approach, which sucks carbon out of the sea, and ocean alkalinity enhancement, which stores carbon in the sea. Both have inspired the engineering of novel, highly efficient electrochemical systems to treat copious amounts of seawater.

Big funding entities support these ideas. The finalists for both the US $100 million XPrize for Carbon Removal and the $35 million Carbon Dioxide Removal Purchase Pilot Prize from the U.S. Department of Energy include marine-based strategies, alongside atmospheric ones.

But the challenges facing marine carbon companies feel as grand as the companies’ plans. Most of their business models ultimately depend on selling carbon credits on voluntary markets. And to sell carbon credits, they must quantify how much CO₂ they’re causing the oceans to draw down from the air. This can’t be done with physical measurements alone; instead, they must rely on numerical models that come with considerable uncertainty.

On top of that, a lot of environmental monitoring needs to be done to prove that marine carbon-removal strategies aren’t harming aquatic life. And then there’s the issue of scale. To make a dent in the more than 1,000 gigatonnes of excess CO₂ lingering in Earth’s atmosphere, and the few dozen gigatonnes continuing to be emitted each year from human activities, companies would have to process ocean water in biblical proportions.

“If you want to strip out 1 gigatonne of CO₂ from the ocean, you probably have to put the upper few meters of the Atlantic through your machines every year,” says Andreas Oschlies, head of biogeochemical modeling at the Geomar Helmholtz Centre for Ocean Research, in Kiel, Germany. “That’s a huge amount of water. But it’s not impossible.”

Hanging on to “not impossible,” companies around the world will be giving it a go in 2025.

How Captura Removes Carbon from the Ocean

To maintain equilibrium, Earth’s oceans and atmosphere constantly exchange CO₂ gas. Oceans take in more during periods of higher atmospheric CO₂ levels, including the post–Industrial Revolution age. Currently, oceans absorb about a quarter of carbon emissions, land takes up another 30 percent, and the rest lingers in the atmosphere, warming the planet.

Many groups have embarked on a mission to suck CO₂ out of the air using direct air capture (DAC) systems. This energy-intensive approach involves passing ambient air through chemical solvents or filters, and then storing or reusing the captured carbon.

The Hawaii Ocean Science and Technology Park at Keāhole Point will host Captura’s next plant, which will remove about 10,000 tonnes of CO₂ from the Pacific Ocean.Tetrachrome

But CO2 in the ocean is 150 times as concentrated as in the air. “The advantage of using the ocean is that it’s already there doing this job, at a massive scale,” says Steve Oldham, CEO at Captura, in Pasadena, Calif., who previously worked in the DAC industry.

For the last year, Captura has been testing its strategy at a pilot plant built on the basketball-court-size deck of a retired U.S. Navy barge. The barge floats in an old section of the Port of Los Angeles, tethered to an out-of-use terminal where sea lions sometimes visit.

A hundred years ago, this narrow strip of reclaimed land served as a loading dock for crates of fruit and cotton. Now it houses a long row of mostly dilapidated warehouses, and a single recently remodeled one belonging to Captura’s host, AltaSea. Across the water, the newer terminals of North America’s busiest port host massive international cargo ships that dock and unload shipping containers into hulking piles.

Captura improved upon commercial electrodialysis designs by developing more efficient membranes, and by changing the geometry of the membrane stack.Captura

Dressed in matching green polo shirts embroidered with the company logo, Captura oceanographer Sophie Chu and mechanical engineer Eric Marks gave Spectrum a tour of their plant. We weaved through the pipes and tanks crammed onto the barge, and paused at one of the system’s key features: a custom electrodialysis machine.

After ocean water is pumped on board, this machine applies a voltage to a portion of it as it moves through a series of ion-selective membranes. This chemically rearranges seawater molecules (hydrogen, oxygen, and sodium chloride) based on their constituent ions’ charge, resulting in the production of an acid (hydrochloric acid) and a base (sodium hydroxide). The acid reacts with dissolved inorganic carbon in the seawater, converting it to dissolved CO₂, which gets separated and captured using a vacuum that pulls it across gas-liquid membrane contactors. The base is then added to restore the water’s alkalinity before it is returned to the sea, where it will naturally draw down more CO₂ as it equilibrates with the atmosphere [see diagram, “How to Strip CO₂ Out of Seawater”].

How to Strip CO2 Out of Seawater

Captura is stripping CO₂ out of the Pacific Ocean so that the seawater will naturally draw down more CO₂ from the atmosphere. Here’s how:

1. A stream of screen-filtered seawater is drawn into the facility.
2. A small fraction of the seawater (about 0.5 percent) is diverted and pretreated to produce softened saltwater.
3. The softened water passes through the electrodialysis unit, which applies a voltage. Ion-selective membranes separate the salt and water into their constituent ions based on their charge, forming acid (hydrochloric acid) and base (sodium hydroxide) streams.
4. The acid stream is added to the original 99.5 percent seawater flow where it reacts with the dissolved inorganic carbon in it, converting it to dissolved CO₂.
5. To extract the dissolved CO₂, a vacuum pulls the seawater through a gas-liquid membrane contactor.
6. The base stream generated in the electrodialysis unit is added to the acidified, CO₂-depleted seawater to neutralize the acid.
7. The CO₂-depleted seawater is released back into the ocean where it can absorb atmospheric CO₂.

Desalination plants commonly use electrodialysis systems. To make one feasible for carbon capture, Captura engineers improved upon commercial designs by developing high-performing membranes that reduce costs and energy use, and by changing the geometry of the membrane stack. To further reduce energy consumption and enable the system to run on intermittent renewable energy sources, Captura engineered the electrodialysis to run for shorter periods of time, such as during periods of the day when electricity is cheap, or when the sun is shining.

Captura’s Port of Los Angeles pilot can remove about 100 tonnes of CO₂ per year from seawater. The company’s new plant under construction in Hawaii will capture 10 times that amount—a measurement the company can definitively quantify.

Problems and Solutions in Marine Carbon Capture

What’s not easy to quantify is what happens after the CO₂-depleted effluent is returned to the sea. Theoretically, if 1,000 tonnes of CO₂ is artificially pulled out of the ocean’s upper layer, the ocean will eventually draw down 1,000 tonnes of CO₂ from the air. How fast that happens depends on ocean currents, temperatures, and wind.

It takes about a year, on average, for CO₂ to equilibrate at the ocean’s surface through natural air-sea gas exchange, when the difference in air-sea CO₂ concentration is small, Chu says. But artificial CO₂ removal will create larger differences in concentration, so the equilibration process will likely take longer, she says.

Proving any of that with enough physical measurements, however, is nearly impossible because it requires laboriously taking water samples across huge swaths of ocean and bringing them back to labs on land. Automation would help. “Ideally we want to miniaturize sensors that we can put on autonomous platforms in the ocean and get a lot of data over space and time,” says Katja Fennel, an oceanographer at Dalhousie University in Halifax, Canada.

These sensors would measure key metrics such as alkalinity, dissolved inorganic carbon, pH, and partial pressure of CO₂ (pCO₂), which is the pressure exerted by carbon dioxide in seawater, indicating how much CO₂ is present. Sensors for pH and pCO₂ are available, and companies are developing them for the other properties, Fennel says.

In the meantime, marine carbon companies must rely on numerical models. ROMS (Regional Ocean Modeling System), developed by university researchers, and MARBL (Marine Biogeochemistry Library), from the National Center for Atmospheric Research, have been used for decades for the general study of climate and oceans and can be refined to quantify marine CO₂ removal, says Alicia Karspeck, chief technology officer at [C]Worthy. To that end, Karspeck’s organization is building software infrastructure to help standardize and deploy these models and hopes to launch its first version in 2025. Data from physical measurements will also help improve modeling.

Whether investors or buyers of carbon credits will accept modeling projections over physical measurements remains to be seen. Then there’s the question of what to do with all the captured CO₂. It can be used to make plastics or synthetic fuels, which could send it back into the atmosphere, or it can be permanently sequestered underground, which is expensive. Oldham envisions building Captura plants on retired oil and gas platforms, using the existing pipes to send the captured CO₂ under the seafloor.

There’s no carbon-sequestration facility near Captura’s Hawaii plant, which sits on a volcanic rock beach at the Hawaii Ocean Science and Technology Park. So the company hopes to partner with another tenant at the park that will utilize the captured CO₂.

Oceans Store CO₂ with Alkalinity Enhancement

To eliminate the challenge of transporting and sequestering captured CO₂, some research groups are looking to the ocean itself as a place of permanent storage. Rather than sucking out CO₂, this approach, called ocean alkalinity enhancement, essentially speeds up the acid-base balancing act that has been regulating the ocean’s pH for billions of years.

In that geologic process, called weathering, CO₂ in the atmosphere reacts with alkaline rocks on land to form bicarbonate and carbonate ions. Rain washes these ions and others from the rocks into rivers and eventually oceans. This increases the alkalinity and pH of the ocean, reduces acidification from fossil fuel emissions, and shifts the balance of dissolved inorganic carbon in the sea toward bicarbonate and carbonate ions. In these forms, carbon stays locked away for thousands of years, ocean biochemists estimate.

Ocean alkalinity enhancement bypasses the weathering process by boosting ocean alkalinity directly. This approach boosts the pH of the water, shifting dissolved CO₂ into the more stable bicarbonate and carbonate ions. This can be done by adding alkaline material to oceans or beaches. In July, Vesta announced it had added 8,200 tonnes of olivine sand off the coast of Duck, N.C. And Nova Scotia–based Planetary Technologies adds magnesium hydroxide to seawater. The company announced in November that it had removed 138 tonnes of CO₂, and sold the carbon credits to Shopify and Stripe.

Ocean alkalinity enhancement can also be done electrochemically. Ebb Carbon has been testing this strategy for over a year at a pilot plant the size of a shipping container at the Pacific Northwest National Lab in Sequim, Wash. The plant draws in saltwater from Sequim Bay and sends it through an electrodialysis machine. A voltage is applied as the water passes through up to 200 stacks of ion-selective membranes laced with a catalyst. This selectively rearranges the ions in the water, creating an acid stream (hydrochloric acid) and a base stream (sodium hydroxide).

At its headquarters in South San Francisco, Ebb Carbon is constructing electrodialysis stacks, which will be used to boost the alkalinity of seawater. The system will be deployed to the company’s new pilot plant, called Project Macoma, in Port Angeles, Wash.Ebb Carbon

The base, or alkaline, stream is returned to the ocean, where it mixes with the seawater. There it converts dissolved CO₂ into carbonate and bicarbonate ions, making room for additional CO₂ from the air to enter. The acid stream, however, becomes a waste by-product that will need to be neutralized.

In 2025 Ebb plans to build a second plant, called Project Macoma, that can draw down an estimated 500 tonnes of CO₂ per year in Port Angeles, Wash. The company aims to eventually colocate its systems on the back ends of desalination plants and other industrial sites that discharge saltwater into the sea, says Matthew Eisaman, chief scientist and cofounder at Ebb.

Like Captura, Ebb faces the formidable challenge of scale. If Ebb puts a commercial-size version of its system in every desalination plant on the planet, it would draw down about a gigatonne of CO₂ from the atmosphere each year, Eisaman estimates. That’s a lot, but it’s still a fraction of the total CO₂ emitted each year.

Thresholds limiting the concentration of effluent streams could further limit the ability of companies like Ebb to scale. Alkalinity that’s too high can disrupt ecosystems and, if not diluted quickly, can cause chemical reactions that lead to the spontaneous precipitation of limestone, removing alkalinity and outgassing CO₂ into the atmosphere, says Geomar’s Oschlies. “Regulators will have to watch this very carefully,” he says.

Plus, it’s difficult to explain to the public what ocean alkalinity companies do without making it sound like they’re dumping chemicals into the sea—a public relations disaster waiting to happen.

Air Capture Meets Marine Storage

UCLA spin-off Equatic has engineered a solution to sidestep a few of these challenges. Its system combines direct air capture of CO₂ with marine storage, enabling Equatic to precisely measure how much CO₂ is pulled out of the air.

Located on a barge docked just a few meters down from Captura, Equatic’s pilot plant and its engineers give off a more university-meets-beach vibe compared with their polished neighbors. Dressed in a Hawaiian shirt and bucket hat, Aaron Sabin, who works as a lead engineer at UCLA’s Institute for Carbon Management, along with pullover-clad Thomas Traynor, head of engineering at Equatic, gave Spectrum a tour of the plant.

On the next barge down from Captura’s plant at the Port of Los Angeles, Equatic has engineered a system that combines direct air capture of CO₂ with marine storage.Equatic

Equatic’s system draws water out of the sea and runs it through an electrolyzer that separates it into four components: a liquid acid stream, a liquid base stream, hydrogen gas, and oxygen gas. Separately, the system draws in air, which contains CO₂. The air is put into contact with the base stream, turning the CO₂ into bicarbonate ions and solid calcium carbonate.

The acid stream is put into contact with rock to raise the pH, and then combined with the base stream. The water, now nearly chemically similar to what was drawn into the system, is discharged out to sea. As a bonus, the hydrogen produced as a by-product can be sold, giving the company an additional revenue stream beyond carbon credits.

But the trouble with splitting saltwater with an electrolyzer rather than through electrodialysis is that the electrolyzer will also produce toxic chlorine gas. (That’s why electrolyzers for hydrogen production almost always split pure, chloride-free water.) Equatic announced in September that it had developed a method to manufacture oxygen-selective anodes and finely architectured catalysts that don’t react with the salt in seawater. This allows the chloride in the saltwater to remain stable during electrolysis, avoiding the production of chlorine gas, according to the company.

Equatic’s pilot plants in Los Angeles and Singapore remove about 100 kilograms of CO₂ per day from the air. A demonstration plant being built in Singapore will capture about 10,000 kg. In 2025, Equatic plans to begin building a commercial system in Quebec in partnership with Deep Sky, a Canadian startup that specializes in building projects for carbon removal. Using 300 electrolyzers running on hydropower and nuclear energy, the plant will capture over 300 tonnes of CO₂ and produce 8,400 kg of hydrogen per day.

Equatic and other marine carbon companies have captured the attention of large funding authorities. The DOE selected three—Ebb, Equatic, and Vycarb—among the 24 semifinalists in its CO₂ removal prize. And XPrize selected four—Captura, Ebb, Kelp Blue, and Planetary—among its 20 finalists, and plans to choose the winners in April.

Those prizes may make it feel like a competition, but ultimately, preventing the planetary climate crisis will require a combination of many strategies. Says Chu: “There’s enough carbon for everyone.”

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