Early on a cool spring morning, in far Downeast Maine, Severine von Tscharner Welcome and her husband, Terran, scrambled along a point jutting into Cobscook Bay. The Passamaquoddy people named the bay Kapskuk after the immense tides and wild currents that make the water seem to boil. These turbulent waters support a rich array of life, including Atlantic salmon, sturgeon, and alewives, as well as many edible species of seaweed.

Harvesting in a spot that’s accessible fewer than 20 days per year, during negative tides, Welcome pulled a long strand of alaria, a golden ruffled kelp, from the riffles. She piled vivid green sea lettuce that tastes surprisingly like its earthbound namesake in a plastic fish basket, and handed me a hank of fine red strands of gracilaria algae to taste. It was slippery, al dente, and tasted a little, but not unpleasantly, like blood. 

In a more sheltered part of the bay, the Welcomes farm sugar kelp and oysters. They sell the wild and cultivated seaweed dried, and use the less delicious, more abundant kinds to fertilize the saltwater farm they’re reviving nearby. For years, much of the seaweed harvest in North America looked a lot like this: Smalltimers scrabbling on rocky shores or drifting in little boats — often on both the economic and geographic margins.

But the industry is at an inflection point. While the North American seaweed industry remains tiny by global standards — 95 percent of farmed seaweed comes from Asia — it is the fastest-growing type of aquaculture in the United States. Fueling this rise is a surging appetite for seaweed as a food and an ingredient, and the hope that it could play an important role in mitigating climate change. 

In recent years, seaweed has been put forth as what one writer called a Swiss Army knife solution to climate change: able to absorb atmospheric carbon, reduce cattle’s methane emissions, provide feedstock for biofuels, and feed the world — no fertilizers, fresh water, or even land required.

Now, a new crop of seaweed startups, many funded by venture capital and tech industry players, is pouring millions into projects using seaweed to mitigate climate change. They’re driven by a slew of corporate net zero pledges — which are increasing demand for carbon offsets — and the growing realization that reducing emissions won’t be enough to keep global warming at or below 1.5 degrees Celsius: carbon will also have to be removed from the atmosphere. 

To play a significant role in stabilizing the climate, though, seaweed farming would need to scale to a level larger than anything North America has ever seen. Much of that expansion would likely be offshore, since the amount of suitable coastline is limited, and potential conflicts with inshore industries like fishing and shipping abound. This nascent transformation — from small-scale, coastal operations to a large, increasingly offshore industry — is raising a host of scientific and ethical questions as well as worries about the environmental and social effects of growing vast amounts of seaweed.

As climate change intensifies, people are “panicking,” said Kristen Davis, a professor of civil and environmental engineering and earth system science at University of California, Irvine. Added to this sense of urgency is a culture clash between scientists and environmentalists who favor a more precautionary approach and the tech industry’s “fake it till you make it” ethos, she said. Davis is part of a growing number of scientists, small-scale harvesters, and environmental groups that caution that some of these new ventures are rushing ahead before fundamental questions about how much kelp can responsibly be farmed – and how much carbon it can actually sequester — are answered.

“The clock is ticking,” Davis said, “and people just want to move really fast.” 

Many of these new companies imply that seaweed is going to save the world, Welcome said. And they have a point — the ocean itself is by far the Earth’s largest carbon sink.  Since the Industrial Revolution, it has absorbed at least a quarter of humanity’s CO2 emissions. And wild seaweed forests already sequester huge amounts of carbon. But if the world’s seaweed stocks aren’t expanded carefully, critical ecosystems could be harmed or attention diverted from known solutions like quitting fossil fuels and protecting and restoring the oceans and forests.

“Seaweed is already saving the world,” Welcome said.

One of the bolder experiments to use seaweed as a climate change solution is underway in Iceland, where millions of basketball-sized buoys, made with wood byproducts and limestone and seeded with seaweed tissue, will be dropped into the ocean in the coming months. 

The buoys look like manikin heads with monk haircuts: bald on top, flowing algal locks below. They are designed to sink to the deep ocean floor where the carbon they hold will remain sequestered for 800 years or more, according to Running Tide, the Maine-based company behind the project. Of course, it’s hard to pin down this timeframe; nothing like this has ever been done before. 

Still, Running Tide’s carbon removal plans got a big boost in March, when the company announced a contract with Microsoft to take 12,000 tons of carbon dioxide equivalent out of the atmosphere over the next two years. 

Running Tide is one of several ventures looking to use seaweed to remove carbon from the atmosphere. Others want to use kelp to reduce emissions by replacing carbon-intensive materials like soy, fertilizers, plastic, and petroleum with seaweed-derived versions. 

This cohort includes the Climate Foundation, which plans to grow massive floating kelp farms offshore in the Philippines and California and sink the kelp to the seafloor to sequester carbon. The project won a million-dollar prize from the Musk Foundation last year. California-based Umaro Foods, funded by the Department of Energy’s ARPA-E program, is working on seaweed bacon and also to develop seaweed farming at “terrestrial farming scales in the deep ocean.” Another, Marine Bioenergy Inc., also ARPA-E funded, plans to grow massive lines of seaweed off the Pacific coast and use drones to move them between the ocean surface, where light abounds, to depths where there are more nutrients. 

Running Tide, which operates on Portland’s fish pier, was founded by Marty Odlin, an engineer and fourth-generation commercial fisherman. The Gulf of Maine is warming at a faster rate than nearly any other part of the ocean, and Odlin has seen the changes firsthand — fish moving north to colder ocean, clam shells dissolving in acidifying waters. 

About 15 years ago, Odlin heard a talk from Klaus Lackner — the physicist, now at Arizona State University, who popularized the idea of removing carbon from the atmosphere. It clicked. “It was like, ‘Oh, this is right because there’s no way we’re going to get off fossil fuels in the next 50 years,’” Odlin said in an interview. “It’s just not going to happen. We’re going to have to pull it down.”

A recent assessment from the Intergovernmental Panel on Climate Change echoes this prescription. In addition to swiftly cutting emissions — which is mandatory if we are to limit climate change — the panel also estimates that we’ll need to remove and sequester about 10 gigatons of CO2 equivalent from the atmosphere annually by 2050, and double that amount, annually, by the end of the century. Right now, there are about 2,000 square kilometers’ worth of seaweed farms in the world; to sequester a tenth of a gigaton of carbon annually would require 73,000 square kilometers — equivalent to planting a nearly half-kilometer-wide belt of seaweed farms along the entire U.S. coast, according to a report from the National Academies of Sciences, Engineering, and Medicine. 

Running Tide plans to sell credits for the carbon it removes from the atmosphere. While the company won’t disclose how much Microsoft is paying, it is just one of several tech companies, including Shopify and Stripe, that have paid for future carbon removal services in order to help kick-start the company. The credits sell for roughly $250 per ton, said Running Tide. 

By way of comparison, Climeworks, which uses mechanical processes to remove carbon dioxide from the atmosphere and store it in rock, reportedly charges between $500 and $1,000 per ton of carbon dioxide equivalent removed. Climeworks is currently capable of removing 4,000 tons of CO2 equivalent per year, though it plans to scale up operations and remove millions of tons annually by 2030.

There’s a logic to the idea of using kelp to draw down carbon, and it would be an elegant, nature-based solution if it works. Some scientists call the Earth’s seaweed forests, which cover an estimated 2 million square kilometers, a “Marine Amazonia” because they absorb as much carbon as the planet’s largest rainforest. But much of that sequestration is short-lived. When the seaweed is harvested, eaten by animals or washes up on shore, its stored carbon is released … back into the atmosphere.

Running Tide’s model, in theory at least, would take that sequestered carbon and sink it to the ocean floor where, in cold and dark conditions, it would remain for centuries — moving the carbon from what is sometimes called the fast carbon cycle to the slow, geologic cycle. But coming up with a credible accounting system for this model is an enormous challenge. (Even much more straightforward carbon sequestration projects, like forest offsets, are hard to measure and verify and have been shown, in some cases, to amount to little more than corporate greenwashing.) 

In the last couple of years, Running Tide and other kelp-based CO2-removal ventures have gotten heat for racing to sell credits before the science of deep sequestration has proved out. The MIT Technology Review published a pair of articles critical of the general idea and of Running Tide specifically. In editorials and articles, scientists also have raised questions about the wisdom, feasibility, and ethics of sinking seaweed to the ocean floor.

“There is no need for another yet-to-be proven technology-driven approach to climate change mitigation that is not based in sound science and marketability and distracts from other, more effective actions, like reducing reliance on fossil fuels,” a group of scientists wrote in the journal Reviews in Fisheries Science & Aquaculture last year. 

Another editorial, coauthored by some of the field’s most prominent scientists, argued that seaweed-sinking ventures were “surging past even perfunctory evaluation of the environmental impacts and social benefits.”

These concerns aren’t hypothetical. Costly attempts to develop algae-based biofuels haven’t panned out. And there have been disasters. In the 1970s, entrepreneurs introduced a species of seaweed off Hawaii, intending to manufacture a food thickener. The business failed, but the species thrived to the point of becoming invasive; it has taken over coral reefs and now dominates entire bays. And disease outbreaks have devastated large seaweed farms in Zanzibar, Indonesia, and elsewhere.

If the industry doesn’t get things right this time around — if it grows too fast, brings unintended consequences, or just fails to deliver on its promises — it could undercut the small-scale, sustainability-focused seaweed economy that people like Welcome have been nurturing.

Welcome got her start building a movement of young farmers before becoming enraptured by seaweed, “beings that haven’t been corralled, that haven’t been weeded or bred or contained or plowed up,” she said. 

Welcome is excited about seaweed’s potential. In addition to harvesting and farming it, she is developing a mycobuoy project with mycologist Sue Van Hook that would help reduce the amount of plastic used in aquaculture, and working with organic groups to address the overharvesting of wild seaweed to make non-chemical fertilizers.

But she worries that the current rush to farm the ocean at larger scale risks replicating many of the same problems of terrestrial agriculture — consolidation, monoculture, a lack of local control, environmental harm.

The point was echoed by Bren Smith, a commercial fisherman turned ocean farmer in Long Island Sound who has helped seed dozens of oyster and seaweed farms over the last decade through his nonprofit GreenWave. When he started out, the sector was a “little experimental petri-dish,” something easily ignored. Now, the industry’s expanded so fast that it’s unrecognizable, he said. And while that’s exciting in some ways, it’s also alarming. “Is this going to be some vertically integrated industry with thousand-acre farms owned by three guys who are getting all the benefits?” he asked. “Is this going to be the Monsanto of the sea?”

Welcome is part of the Seaweed Commons, an affiliation of small-scale harvesters and growers, scientists and environmentalists who have banded together in response to the rapid expansion and transition in the seaweed industry. 

In a recent position paper, the group argued for a precautionary approach, warning that the industry is evolving before adequate regulations are in place and before the potential environmental and social impacts of mass-scale seaweed farming are fully understood. Without adequate safeguards, they wrote, “seaweed could become the next boom and bust crop that was supposed to ‘save the world.’”

Perhaps the largest scientific unknown in this field is exactly how much carbon seaweed can actually remove from the atmosphere — the amount can vary depending on location and weather. It’s also not clear how much farmed and wild seaweed winds up on the deep ocean floor, as opposed to drifting elsewhere. And there are critical questions about how growing large amounts of seaweed or sinking it to the seafloor could affect marine ecosystems. Could large kelp monocultures spread disease to wild seaweed populations? Could an expansion of seaweed farming disrupt phytoplankton — and the marine food web that depends on it — by outcompeting it for nutrients and sunlight?

Seaweed is also much harder to grow in the deep ocean than in coastal environments. Researchers, especially those involved with Department of Energy-funded projects, are trying to figure out how to reliably grow large amounts of seaweed offshore, but they are still a long way off.

But as scale expands, so do potential risks. Kelp farms use rope, and increasing the amount of it raises the risk of entanglement for animals like whales and tortoises. And what happens when a near-shore organism, like kelp, and its associated microbes, flora, and fauna are transported to the deep ocean? Does it create novel, hybrid ecosystems?

“As someone who is looking at the models, we can grow a lot if we paved the ocean,” said Davis, the U.C. Irvine professor. “But we are not going to do that because that would obviously have hugely detrimental effects on ocean health.” The amounts of seaweed that people are talking about aren’t realistic, at least not right now. 

“It has to be a slower ramp,” she said. “And we have to allow time for science to catch up.”

Odlin, Running Tide’s founder, said his team is using modeling and sensors that answer many of these questions; it’s also working with an independent scientific advisory board and has brought in the auditing firm Deloitte to review its carbon accounting process ahead of issuing credits.  

In April, Running Tide  published a lengthy document detailing how it intends to remove carbon from the atmosphere and how it plans to account for how much carbon it is removing. Davis, who reviewed the white paper, said it wasn’t a bad start but that it “needs more detailed explanations to be actionable.” The document doesn’t answer the outstanding questions about the sustainability and viability of using seaweed to sequester carbon, she said, but it does lay out a general methodology for assessing it.

Odlin said he takes the critiques seriously, but rejects the idea that the industry should wait until all scientific questions are answered. The climate emergency is too urgent, he said. “It’s the fourth quarter, we don’t have time to spend 15 to 30 years trying to answer questions that can only really be answered by actually going out and doing these things.”

Odlin favors operating on the best available science. There should be enormous amounts of scientific research going forward, he said, both in collaboration with his company and “in opposition” to it. But given the fact that we know that the planet is warming, that the ocean is acidifying due to the huge amount of carbon it’s absorbing, and that ecosystems are already being disturbed, he said, it is unethical not to act. “There’s a counterpoint to the precautionary principle, and that’s the duty to intervene.”

Amid a flurry of scientific research into the best use of seaweed, Davis was part of a team that recently modeled the costs and potential climate benefits of seaweed — the best emissions mitigation bang for the buck. Sinking seaweed to sequester carbon, the researchers found, was much more expensive than using farmed seaweed to replace foods with high emissions, like soy that’s linked with deforestation.

Still, no matter how seaweed was used, the researchers cited a litany of potential challenges to farming it, including the high cost of seaweed-based carbon removal, potential ecosystem disruption, and the uncertainty about whether large markets for seaweed products exist. “The outlook for a massive scale-up of seaweed climate benefits is thus decidedly murky,” they wrote. 

Nichole Price, a senior research scientist at Maine’s Bigelow Laboratory for Ocean Sciences, who is studying several potential ways to use seaweed to mitigate climate change, said the idea that there are only two options — “sink it or use it”—may be oversimplifying things. It might be possible to produce seaweed products and get credible carbon credits if seaweed farms could gather the parts of the plant they don’t use — the frayed and broken tips of seaweed blades and the tough holdfasts—and deposit them near shore, in areas best suited to sequestering carbon in sediments. This strategy would allow scientists to monitor how much carbon is actually being sequestered and any environmental impacts, which are difficult if not impossible to measure on the ocean floor.

Price said that while there are still many unanswered questions about seaweed and its potential as a climate solution, the science is progressing fast and some of the critical knowledge gaps will be filled within the next five years. 

“The race isn’t really about which pathway is the best at this point,” she said. “It’s which pathway is the fastest and least expensive.” Asked whether that felt appropriate, she said, “I think 20 years ago, we could have gone with the best. Right now, we have to go with what’s the fastest.”