Scientists Are Trying to Coax the Ocean to Absorb More CO2

Last May in Grundartangi, a small port in western Iceland, a barge piled high with wood chips began making regular trips to a patch of ocean 190 miles from the coast. By September, almost 20,000 tons — about 1,400 dump trucks’ worth — of “wood waste” had been pushed overboard. This was no attempt to clandestinely offload trash into the sea. Instead, it was one of the latest efforts in the race to rid the atmosphere of excess carbon dioxide.

Most people have heard of land-based carbon dioxide removal (CDR) schemes, most notably “direct air capture.” Iceland is a pioneer in this realm — the Climeworks Orca plant, located not far from the country’s capital, Reykjavik, is vacuuming 4,000 metric tons of CO2 from the atmosphere and injecting it deep into the earth each year. But that is far from the amount experts say is necessary. In its most recent assessment report, the Intergovernmental Panel on Climate Change (IPCC) noted that carbon dioxide removal “is required to achieve global and national targets” of limiting warming to between 1.5 and 2 degrees C to avoid “major, irreversible ecological and social impacts.”

To be exact, the IPCC said that removing as much as 15 gigatons of carbon per year may be necessary to stay below the 1.5-degree threshold. That means capturing the equivalent of the annual exhaust of 3.3 billion gasoline-powered cars for 80 years. “This isn’t a one solution problem,” says Nicholas Ward, an earth scientist at the Pacific Northwest National Laboratory (PNNL). “We don’t necessarily need that 15-gigaton solution that’s going to solve all of our emissions problems — we need a tool bag of a bunch of five-percent solutions.”

Little is known about how effective these techniques are in combatting global warming — or about the range of unintended consequences

Running Tide, the seven-year-old U.S.-based start-up that’s experimenting with woodchips in Iceland, is betting that the ocean is just the place to perfect these small solutions, in the form of marine carbon dioxide removal, or mCDR. For billions of years, the ocean has been absorbing CO2 from the atmosphere, reducing it to different forms of carbon that circulate via currents or settle to the seafloor. Today, the ocean soaks up 30 percent of anthropogenic CO2 emissions, a capacity that is 42 times greater than the atmosphere’s. Running Tide wants to take advantage of this system by “deploying” timber industry “wood waste,” which is often sold as feedstock to be burned for bioenergy or simply left to rot — processes that release the wood’s stored carbon back into the air.

“We want the simplest possible way of doing what we need to do,” says Kristinn Hróbjartsson, the general manager of Running Tide in Iceland. “If we can do that with a golf-ball-sized piece of wood, we would rather do that than through some giant process.”

Other mCDR companies are experimenting with growing CO2-consuming aquatic plants, like kelp and algae, then sinking them into the deep ocean or burying them in the earth, much like Iceland’s Climeworks Orca facility buries the carbon it captures from the air. Still others are pumping seawater through electrodialysis filtering systems that both increase the water’s ability to sequester carbon and remove excess acid, a byproduct of CO2 that can devastate marine species.

The Running Tide facility gathers wood chips that will be dumped into the ocean.

The Running Tide facility gathers wood chips that will be dumped into the ocean.
Running Tide

No matter the method, and despite the rapidly expanding ecosystem of researchers, private companies, venture capitalists, and governments focusing their attention and money on mCDR, the reality is that little is known about how effective these techniques are in combatting global warming — or about the range of unintended consequences they may have on the marine environment. “It’s fair to say that regulations are lagging behind the pace of both research and industry investment,” says Ward, whose lab is part of the U.S. Department of Energy. “Research dollars aren’t unlimited, and in our case they’re taxpayer money, so we should be thinking about the solutions with the most potential.”

Marine carbon dioxide removal is a form of geoengineering, perhaps the most polarizing word in climate science today. Although scientists and governments seem more alarmed by atmospheric geoengineering efforts, like altering the reflective capacity of the sky with injections of sulfur dioxide, they have also expressed concerns about mCDR, if only because it is so little understood. Iceland’s small size, and its acknowledgement of the urgency of the climate crisis, have made it an ideal incubator for mCDR science. But the IPCC’s and other groups’ increasingly dire warnings about the need to act now, and to act in a multitude of ways that include mCDR, has been loud enough to get the attention of much larger governments, including the Biden administration.

Instead of using electricity to shock the acid from seawater, some companies are experimenting with alkaline rocks.

Last year, the Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) created a program specifically aimed at supporting the development of technology to improve scientists’ ability to measure and monitor carbon in the ocean. In October, the program — called Sensing Exports of Anthropogenic Carbon Through Ocean Observation (SEA-CO2) — announced its first tranche of funding: $36 million to be distributed among 11 labs, academic institutions, and private companies in nine states.

Ward and his colleagues at PNNL’s Sequim, Washington, campus have been awarded just over $2 million to develop models and lab experiments to better understand the effectiveness and impact of one of the better-understood mCDR techniques — ocean alkalinity enhancement (OAE). Reducing the acidity of seawater increases its alkalinity; the higher its alkalinity, the more atmospheric CO2 it can absorb and transform into stabler, inorganic carbon, which is less likely to seep back into the air. Since last year, the company Ebb Carbon has been operating its OAE system at the Sequin campus, filtering seawater through a series of membranes that, when pulsed with electricity, extract its acid. With its federal grant, Ward says, the lab will be testing OAE methods from a handful of other companies.

An EBB Carbon system that can make seawater less acidic, allowing it to absorb more carbon dioxide.

An EBB Carbon system that can make seawater less acidic, allowing it to absorb more carbon dioxide.

Matthew Eisaman, Ebb Carbon’s cofounder and an associate professor in Yale University’s Department of Earth and Planetary Sciences, says that electrodialysis can also be used to alkalinize salt brine from desalination plants. Such facilities, according to Eisaman, yield around 40 billion gallons of brine a day, globally, most of which is discharged into the sea. “If you convert all that salt to alkalinity with the process we’re pursuing,” he says, more than 1 billion tons of CO2 are pulled from the air and stored in the ocean per year. To reduce acidification at oceanic scales would be impossible, but Eisaman says that isn’t Ebb Carbon’s goal. He pointed to the oyster farms in the bays and estuaries around Sequin that have been suffering from acidification, which thins the shells of bivalves. “You could use this to kind of keep that relatively small local body of water similar to a preindustrial equilibrium that is ideal for shellfish.”

Instead of using electricity to shock the acid from seawater, other companies are experimenting with alkaline rocks, like basalt or olivine, which over thousands of years break down and make their way into the oceans. To speed up that natural process, often called “enhanced rock weathering,” the material can be mined, pulverized, and mixed into the ocean. Eisaman argues that “open systems” like this, which use the Earth’s own carbon cycle instead of “closed systems” like direct air capture, are “approaches that will have the potential to scale gigatons and at low cost.” (Running Tide also coats its wood chips with an alkaline material, with the goal of simultaneously transferring CO2 to the ocean and combating acidification.)

“We’re intervening in the natural world, which means we need to be very careful about what we do,” says a startup company manager.

At Woods Hole Oceanographic Institution in Massachusetts, scientists will be using their portion of the SEA-CO2 funding to develop sensors that can be attached to scientific moorings and autonomous underwater vehicles to continuously measure carbon concentrations in seawater. Such sensors would benefit companies that aim to grow macroalgae — seaweed and kelp — that draw large amounts of CO2 from the air, then sink them into the deep ocean. But some scientists say such efforts are risky. As it breaks down in the water column, this biomass produces organic carbon that “can be eaten by bacteria, which causes all sorts of transformations in the environment,” says Ward. “For example, on the seafloor, you have organic carbon that can transform into different greenhouse gases, like nitrous oxide and methane, depending on the conditions. If we’re not monitoring for production of these other greenhouse gases, we can’t tell you exactly how much carbon was truly sequestered.”

Running Tide, in fact, offers a cautionary tale about sinking macroalgae. In 2021, the company filed a patent for a floating apparatus seeded with kelp and “enhanced with a nutrient payload” of iron oxide, to boost alkalinity. The plan raised enough concerns about unintended consequences that some of the company’s scientists quit. In a 2022 paper published in Nature Ecology & Evolution, the University of Tasmania marine biochemist Phillip Boyd and coauthors modeled the potential impacts of apparatuses like that proposed by Running Tide, concluding that they would likely present a “range of biological threats” for offshore ecosystems, from altered water chemistry to the introduction of invasive microorganisms. Running Tide’s Hróbjartsson says the company has no plans to sink macroalgae, writing in an email, “We would never, and never have, deployed anything into the ocean that isn’t deemed at most a minor or transitory impact on the ocean environment by best available science.”

Chemist Kai Schulz adds rock powder to seawater as part of a study in Kiel, Germany on making the ocean less acidic.

Chemist Kai Schulz adds rock powder to seawater as part of a study in Kiel, Germany on making the ocean less acidic.
Michael Sswat / GEOMAR

Boyd says that electrochemical technology is a safer approach because it’s better understood. “We want to try and incentivize methods that are, number one, safe; number two, are effective for a long duration; and, number three, are readily verifiable.” With biomass deployments, he says, “we’re not sure what the outcome will be.”

Hróbjartsson agrees that more research needs to be done on mCDR, noting that much of the company’s efforts are in the lab, and the wood waste pilot was just a small experiment. “Obviously, we are intervening in the natural world, which means we need to be very careful about what we do. And moving off fossil fuels — it’s not going to be without some impact on the environment, too,” Hróbjartsson says. “But I think everybody agrees that we need to do something, and we need to have a full arsenal of tools available for us to deal with a problem.”

Ultimately, Boyd and his coauthors concluded that there was a great need for just the kind of research that the SEA-CO2 program will produce. “Marine carbon removal is a very challenging field, because it’s not just about the science,” Boyd says. “There’s the technology aspect, the economics, the communicating to the public, the regulatory frameworks, and then there’s the markets. And, as far as I can see, none of those are really fit for purpose at the moment.”

Some scientists say CO2 removal is simply a distraction from the urgency of the climate crisis and an excuse to continue burning fossil fuels.

Despite the fact that scientists are only just beginning to look into the effectiveness of mCDR, private carbon markets are flourishing, and private investors have been betting big on its success. Some of the largest names in tech have already pumped tens of millions of dollars into Ebb Carbon, Running Tide, and others.

Of course, recent years have shown the grave danger of Silicon Valley’s favorite mantra, “move fast and break things.” When it comes to the environment, moving fast may be critical, but breaking things can be catastrophic. Some scientists have made the point that carbon dioxide removal — and the sale of those carbon credits — is simply a distraction from the urgency of the climate crisis and an excuse to continue burning fossil fuels. Friederike Otto, a lead author of the latest IPCC report, recently told The Guardian that the tools to keep warming to 1.5 degrees are already available and completely understood: stop extracting fossil fuels, scale back industrial farming and deforestation, the list goes on. “We should act as if CDR will never be achievable,” Otto said. “We do not have a technology at the moment that works at scale… so we should make our policies as if CDR is not an option.”

Boyd agrees that a CDR technology that is safe, effective at storing carbon for long durations, verifiable, and scalable does not yet exist. But we need the acceleration that private business and carbon markets can bring, he says. “If we are going to build this partnership with businesses, (which) have the tools and the wherewithal and the infrastructure to really drive this, then there has to be a more substantive conversation around what the speed bumps are,” he says. “It’s not that we want to hold (companies) back, but at the same time, the ocean is a complex place.”