Sinking seaweed could sequester a lot of carbon, but researchers are still grappling with basic questions about reliability, scalability and risks.
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In late January, Elon Musk tweeted that he planned to give $100 million to promising carbon removal technologies, stirring the hopes of researchers and entrepreneurs.
A few weeks later, Arin Crumley, a filmmaker who went on to develop electric skateboards, announced that a team was forming on Clubhouse, the audio app popular in Silicon Valley, to compete for a share of the Musk-funded XPrize.
A group of artists, designers, and engineers assembled there and discussed a variety of possible natural and technical means of sucking carbon dioxide out of the atmosphere. As the conversations continued and a core team coalesced, they formed a company, Pull To Refresh, and eventually settled on growing giant bladder kelp in the ocean.
So far, the venture’s main efforts include growing the seaweed in a tank and testing their control systems on a small fishing boat on a Northern California lake. But it’s already encouraging companies to “get in touch” if they’re interested in purchasing tons of sequestered CO2, as a way to balance out their greenhouse-gas emissions.
Crumley says that huge fleets of semi-autonomous vessels growing kelp could suck up around a trillion tons of carbon dioxide and store it away in the depths of the sea, effectively reversing climate change. “With a small amount of open ocean,” he says, “we can get back to preindustrial levels” of atmospheric carbon dioxide.
'No one knows'
Numerous studies show the world may need to remove billions of tons of carbon dioxide a year from the atmosphere by midcentury to prevent dangerous levels of warming or bring the planet back from them. In addition, more and more corporations are scouring the market for carbon credits that allow them to offset their emissions and claim progress toward the goal of carbon neutrality.
All of that has spurred a growing number of companies, investors, and research groups to explore carbon removal approaches that range from planting trees to grinding up minerals to building giant C02-sucking factories.
Kelp has become an especially active area of inquiry and investment because there’s already an industry that cultivates it on a large scale—and the theoretical carbon removal potential is significant. An expert panel assembled by the Energy Futures Initiative estimated that kelp has the capacity to pull down about 1 billion to 10 billion tons of carbon dioxide per year.
But scientists are still grappling with fundamental questions about this approach. How much kelp can we grow? What will it take to ensure that most of the seaweed sinks to the bottom of the ocean? And how much of the carbon will stay there long enough to really help the climate?
In addition, no one knows what the ecological impact of depositing billions of tons of dead biomass on sea floor would be.
“We just have zero experience with perturbing the bottom of the ocean with that amount of carbon,” says Steven Davis, an associate professor at the University of California, Irvine, who is analyzing the economics of various uses of kelp. “I don’t think anybody has a great idea what it will mean to actively intervene in the system at that scale.”
The scientific unknowns, however, haven’t prevented some ventures from rushing ahead, making bold promises and aiming to sell carbon credits. If the practice doesn’t sequester as much carbon as claimed it could slow or overstate progress on climate change, as the companies buying those credits carry on emitting on the false promise that the oceans are balancing out that pollution, ton for ton.
“For the field as a whole, I think, having this research done by universities in partnership with government scientists and national labs would go a long way toward establishing a basic level of trust before we’re commercializing some of this stuff,” says Holly Buck, an assistant professor at the University at Buffalo, who is studying the social implications of ocean-based carbon removal.
The lure of the ocean
Swaying columns of giant kelp line the rocky shores of California’s Monterey Bay, providing habitat and hunting grounds for rockfish, sea otters, and urchins. The brown macroalgae draws on sunlight, carbon dioxide, and nutrients in the cool coastal waters to grow up to two feet a day. The forests continually shed their blades and fronds, and the seaweed can be knocked loose entirely by waves and storms.
In the late 1980s, researchers at the Monterey Bay Aquarium began a series of experiments to determine where all that seaweed ends up. They attached radio transmitters to large floating rafts of kelp and scanned the ocean depths with remote-operated submarines.
The scientists estimated that the forests released more than 130,000 tons of kelp each year. Most of the rafts of kelp washed up on shore within the bay in a matter of days. But in the underwater observations, they found bundles of seaweed lining the walls and floor of an adjacent underwater gully known as the Carmel Submarine Canyon, hundreds of meters below the surface.
Scientists have spotted similar remnants of kelp on the deep ocean floors in coastal pockets throughout the world. And it’s clear that some of that carbon in the biomass stays down for millennia, because kelp is a known source of oil deposits.
A 2016 paper published in Nature Geoscience estimated that seaweed may naturally sequester nearly 175 million tons of carbon around the world each year as it sinks into the deep sea or drifts into submarine canyons.
That translates to well below the levels of carbon dioxide that the world will likely need to remove annually by midcentury—let alone the amounts envisioned by Crumley and his team. Which is why Pull To Refresh and other companies are exploring ways to radically scale up the growth of kelp, on offshore vessels or elsewhere.
Reaching the deep seas
But how much of the carbon will remain trapped below the surface and for how long?
Certain species of seaweed, like giant bladder kelp, have tiny gas bladders on their blades, enabling the macroalgae to collect more of the sunlight necessary to drive photosynthesis. The bladders can also keep the remnants or rafts afloat for days or longer depending on the species, helping currents carry dislodged kelp to distant shores.
When the carbon in kelp decomposes on land, or turns into dissolved inorganic carbon dioxide in shallow seawater, it can return to the atmosphere, says David Koweek, science director at Ocean Visions, a research organization that partners with institutions like MIT, Stanford, and the Monterey Bay Aquarium Research Institute. The carbon may also be released if marine creatures digest the kelp in the upper oceans.
But some kelp sinks into the deep ocean as well. Bladders degrade. Storms push the seaweed down so deep that they deflate. Certain species are naturally nonbuoyant. And some amount that breaks free below the surface stays there and may drift down into deeper waters through underwater canyons, like the one off the coast of Monterey.
Ocean circulation models suggest much of the carbon in biomass that reaches great depths of the oceans could remain there for very long times, because the overturning patterns that bring deep waters toward the surface operate so slowly. Below 2,100 meters, for instance, the median sequestration time would exceed 750 years across major parts of the North Pacific, according to a recent paper in Environmental Research Letters.
All of which suggests that deliberately sinking seaweed could store away carbon long enough to ease some of the pressures of climate change. But it will matter a lot where it’s done, and what efforts are taken to ensure that most of the biomatter reaches the deep ocean.
For-profit plans
Pull To Refresh’s plan is to develop semi-autonomous vessels equipped with floats, solar panels, cameras, and satellite antennas, enabling the crafts to adjust their steering and speed to arrive at designated points in the open ocean.
Each of these so-called Canaries will also tow a sort of underwater trellis made of steel wire, known as the Tadpole, tethering together vases in which giant bladder kelp can grow. The vessel will feed the seaweed through tubes from an onboard tank of micronutrients.
Pull To Refresh has tested its control systems on a fishing boat on a lake in Northern California.
Eventually, Crumley says, the kelp will die, fall off, and naturally make its way down to the bottom of the ocean. By putting the vessels far from the coast, the company believes, it can address the risk that the dead seaweed will wash up on shore.
Pull To Refresh has already begun discussions with companies about purchasing “kelp tonnes” from the seaweed it’ll eventually grow.
“We need a business model that works now-ish or as soon as possible,” Crumley says. “The ones we’re talking to are forgiving; they understand that it’s in its infancy. So we will be up-front about anything we don’t know about. But we’ll keep deploying these Canaries until we’ve got enough tonnes to close out your order.”
Crumley said in an email that the company will have two years to get the carbon accounting for its process approved by a third-party accreditor, as part of any transition. He said the company is conducting internal environmental impact efforts, talking to at least one carbon removal registry and that it hopes to receive input from outside researchers working on these issues.
“We are never going to sell a tonne that isn’t third-party verified simply because we don’t want to be a part of anything that could even just sound shady,” he wrote.
‘Scale beyond any other’
Other ventures are taking added steps to ensure that the kelp sinks, and to coordinate with scientific experts in the field.
Running Tide, an aquaculture company based in Portland, Maine, is carrying out field tests in the North Atlantic to determine where and how various types of kelp grow best under a variety of conditions. The company is primarily focused on nonbuoyant species of macroalgae and has also been developing biodegradable floats.
The company isn’t testing sinking yet, but the basic concept is that the floats will break down as the seaweed grows in the ocean. After about six to nine months, the whole thing should readily sink to the bottom of the ocean and stay there.
Marty Odlin, chief executive of Running Tide, stresses that the company is working with scientists to ensure they’re evaluating the carbon removal potential of kelp in rigorous and appropriate ways.
Ocean Visions helped establish a scientific advisory team to guide the company’s field trials, made up of researchers from the Monterey Bay Aquarium Research Institute, UC Santa Barbara, and other institutions. The company is also coordinating with the Centre for Climate Repair at Cambridge on efforts to more precisely determine how much carbon the oceans can take up through these sorts of approaches.
Running Tide plans to carry out tests for at least two and a half years to develop a “robust data set” on the effects of these practices.
“At that point, the conclusion might be we need more data or this doesn’t work or it’s ready to go,” Odlin says.
The company has high hopes for what it might achieve, stating on its website: “Growing kelp and sinking it in the deep ocean is a carbon sequestration solution that can scale beyond any other.”
Running Tide has raised millions of dollars from Venrock, Lowercarbon Capital, and other investors. The tech companies Shopify and Stripe have both provided funds as well, purchasing future carbon dioxide removal at high prices ($250 a ton in Stripe’s case) to help fund research and development efforts.
Several other companies and nonprofits are also exploring ways to sequester carbon dioxide from seaweed. That includes the Climate Foundation, which is selling a $125, blockchain-secured "kelp coin" to support its broader research efforts to increase kelp production for food and other purposes.
The risks
Some carbon removal experts fear that market forces could propel kelp-sinking efforts forward, whatever the research finds about its effectiveness or risks. The companies or nonprofits doing it will have financial incentives to sell credits. Investors will want to earn their money back. Corporate demand for sources of carbon credits is skyrocketing. And offset registries, which earn money by providing a stamp of approval for carbon credit programs, have a clear stake in adding a new category to the carbon marketplace.
One voluntary offset registry, Verra, is already developing a protocol for carbon removal through seagrass cultivation and is “actively watching” the kelp space, according to Yale Environment 360.
We’ve already seen these pressures play out with other approaches to offset credits, says Danny Cullenward, policy director at CarbonPlan, a nonprofit that assesses the scientific integrity of carbon removal efforts.
CarbonPlan and other research groups have highlighted excessive crediting and other problems with programs designed to incentivize, measure, and verify emissions avoided or carbon removal achieved through forest and soil management practices. Yet the carbon credit markets continue to grow as nations and corporations look for ways to offset their ongoing emissions, on paper if not in the atmosphere.
Sinking seaweed to the bottom of the ocean creates especially tricky challenges in verifying that the carbon removal is really happening. After all, it’s far easier to measure trees than it will be to track the flow of carbon dissolved in the deep ocean. That means any carbon accounting system for kelp will rely heavily on models that determine how much carbon should stay under the surface for how long in certain parts of the ocean, under certain circumstances. Getting the assumptions right will be critical to the integrity of any eventual offset program—and any corporate carbon math that relies on them.
Some researchers also worry about the ecological impact of seaweed sinking.
Wil Burns, a visiting professor focused on carbon removal at Northwestern University and a member of Running Tide’s advisory board, notes that growing enough kelp to achieve a billion tons of carbon removal could require millions of buoys in the oceans.
Those floating forests could block the migration paths of marine mammals. Creatures could also hitch aboard the buoys or the vessels delivering them, potentially introducing invasive species into different areas. And the kelp forests themselves could create “gigantic new sushi bars,” Burns says, perhaps tipping food chains in ways that are hard to predict.
An underwater kelp forest off the coast of California.
The addition of that much biomatter and carbon into the deep ocean could alter the biochemistry of the waters, too, and that could have cascading effects on marine life.
“If you’re talking about an approach that could massively alter ocean ecosystems, do you want that in the hands of the private sector?” Burns says.
Running Tide’s Odlin stresses that he has no interest in working on carbon removal methods that don’t work or that harm the oceans. He says the reason he started looking into kelp sinking was that he witnessed firsthand how climate change was affecting marine ecosystems and fish populations.
“I’m trying to fix that problem,” he says. “If this activity doesn’t fix that problem, I’ll go work on something else that will.”
Scaling up
Scaling up kelp-based carbon removal from the hundreds of millions of tons estimated to occur naturally to the billions of tons needed will also face some obvious logistical challenges, says John Beardall, an emeritus professor at Monash University in Australia, who has studied the potential and challenges of seaweed cultivation.
For one, only certain parts of the world offer suitable habitat for most kelp. Seaweed largely grows in relatively shallow, cool, nutrient-rich waters along rocky coastlines.
Expanding kelp cultivation near shore will be constrained by existing uses like shipping, fishing, marine protected areas, and indigenous territories, Ocean Visions notes in a “state of technology” assessment. Moving it offshore, with rafts or buoys, will create engineering challenges and add costs.
Moreover, companies may have to overcome legal complications if their primary purpose will be sinking kelp on large, commercial scales. There are complex and evolving sets of rules under treaties like the London Convention and the London Protocol that prevent dumping in the open oceans and regulate “marine geoengineering activities” designed to counteract climate change.
Commercial efforts to move ahead with sinking seaweed in certain areas could be subject to permitting requirements under a resolution of the London Convention, or run afoul of at least the spirit of the rule if they move ahead without environmental assessments, Burns says.
Climate change itself is already devastating kelp forests in certain parts of the world as well, Beardall noted in an email. Warming waters coupled with a population explosion of sea urchins that feed on seaweed have decimated the kelp forests along California’s coastline. The giant kelp forests along Tasmania have also shrunk by about 95% in recent years.
“This is not to say that we shouldn’t look to seaweed harvest and aquaculture as one approach to CO2 sequestration,” Beardall wrote. “But I simply want to make the point that is not going to be a major route.”
Other, better uses
Another question is simply whether sinking seaweed is the best use of it.
It’s a critical food and income source for farmers across significant parts of Asia, and one that’s already under growing strains as climate change accelerates. It’s used in pharmaceuticals, food additives, and animal feed. And it could be employed in other applications that tie up the carbon, like bioplastics or biochar that enriches soils.
“Sustainably farmed seaweed is a valuable product with a very wide range of uses … and a low environmental footprint,” said Dorte Krause-Jensen, a professor at Aarhus University in Denmark who has studied kelp carbon sequestration, in an email. “In my opinion it would be a terrible waste to dump the biomass into the deep sea.”
UC Irvine’s Davis has been conducting a comparative economic analysis of various ways of putting kelp to use, including sinking it, converting it to potentially carbon-neutral biofuels, or using it as animal feed. The preliminary results show that even if every cost was at the lowest end of the ranges, seaweed sinking could run around $200 a ton, which is more than double the long-term, low-end cost estimates for carbon-sucking factories.
Davis says those costs would likely drive kelp cultivators toward uses with higher economic value. “I’m more and more convinced that the biggest climate benefits of farmed kelp won’t involve sinking it,” he says.
‘Get it done’
Pull To Refresh’s Crumley says he and his team hope to begin testing a vessel in the ocean this year. If it works well, they plan to attach baby kelp to the Tadpole and “send it on its voyage,” he says.
He disputed the argument that companies should hold off on selling tons now on the promise of eventual carbon removal. He says that businesses need the resources to develop and scale up these technologies, and that government grants won’t get the field where it needs to be.
“We’ve just decided to get it done,” he says. “If, in the end, we’re wrong, we’ll take responsibility for any mistakes. But we think this is the right move.”
It’s not clear, however, how such a startup could take responsibility for mistakes if the activities harm marine ecosystems. And at least for now, there are no clear mechanisms that would hold companies accountable for overestimating carbon removal through kelp.
At this stage, it’s crucial to carry out controlled field tests to provide more information about the scale, durability, and environmental risks of kelp sinking, Ocean Vision’s Koweek says. Filling in these knowledge gaps will be essential to setting up reliable carbon accounting methods for any voluntary or government-regulated offset programs that eventually allow companies to buy and trade kelp carbon credits.
He does believe that companies can play a helpful role in that, working with scientists and engineers across academia and nonprofits to more quickly deliver the information needed to produce reliable standards and determine best practices. But without addressing any specific company, he also says the science is too premature to start marketing carbon credits from kelp.
“The entire field broadly—the entrepreneurs, startups, investors, philanthropies, scientists, and engineers—we would all benefit by putting time and resources into building out the evidence base together, before we jump the gun and start selling carbon credits,” he says.
