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The Great Carbon Sink in the Sea: Geoengineering, Seaweed, and the Race to Beat Climate Change

Each month we delve into a new story inspired by our scent of the month. This month, in honor of Waves we celebrate seaweed’s role in fighting climate change through natural growth and geoengineering.

Under a bright flood of warm sunlight, the gentle surf rhythmically crashes along a clean, unbroken stretch of soft sand. A cool, salt-savory breeze floats inland while, in the distance, two elegant terns glide above the spray.

Conspicuously absent from this picture-perfect beach scene: slick bundles of seaweed drifting with the tide, lying in wait to brush against an unguarded ankle like a kraken’s tentacle rising from the abyss.

At best, seaweed’s an unremarkable fixture of the marine landscape. At worst, it evokes genuine terror (solidarity with all our fykiaphobic friends out there!).1 Certainly, no one rushes to the coastline for seaweed’s sake.

Seaweed in the surf looks an awful lot like a seamonster; Source: Phobia Wiki


Unless, that is, you were a woman living in Victorian England, in which case you may have been an eager algologist, or seaweed enthusiast. Seaweed sparked a craze among British women in the 19th century. At a time when their everyday agency was severely restricted, women found in algology a chance to strike out on their own, unbound by the rules of “civil society.” They traded petticoats for men’s clothing and combed beaches for specimens to identify, catalogue, and preserve. One prominent algologist, Margaret Gatty, wrote a comprehensive two-volume study on the subject. While her work was respected by many of her male peers, her gender barred her from membership in the Royal Society, England’s preeminent scientific academy.2

“Seaweed sparked a craze among British women in the 19th century.”


Of course, Victorian women were not the first to fall in love with seaweed. People have long been cooking with the stuff, and while many Western readers may associate it most heavily with Japanese, Chinese, and Korean cuisine, citizens of Western Scotland have chowed down on a variety called “dulse” since at least the sixth century A.D. Writer Annalena Mcafee describes its taste as a “mysterious sweet-bacon tang.”3

That might raise some eyebrows, but that’s because our preconceived notions of seaweed as nothing more than soggy, stinky sea wrack are misguided. Heck, it even smells pretty good, in the right contexts. Perfumers prize its grassy, marine aroma, and when the EU banned oak moss as part of an anti-allergy crackdown in 2015, many scent makers turned to seaweed for salvation. Marc-Antoine Corticchiato of Parfum d’Empire told Reuters that seaweed’s “wet, iodized smell” could easily mimic the woody earthiness of oak moss.4

Seaweed has also attracted its fair share of literary admirers. Middlemarch author George Eliot was “quite in love with sea-weeds” during the heyday of algology,5 and in 2006, the American poet D.A. Powell saw it fit to compare love — of all things! — to algae: “who would have guessed love’s a palpable thing: a dark splotch of kelp in the shoals.”6

From Coral Reefs to Kelp Forests, the Ocean Runs on Algae


You may still not be sold on seaweed’s beauty. Perhaps a coral reef is more to your taste — and who could blame you? These brilliant formations are the largest living structures on the planet, with hundreds or thousands of miniscule polyps working together to create a single reef.7 While coral reefs only occupy one percent of the ocean floor, they are home to 25 percent of all ocean life.8 Reefs also defend coastal environments from destruction, acting as shields to dampen the power of storm-produced waves. Experts estimate more than half a billion people depend on coral reefs in some way for food and life.9

Anatomy of a coral polyp; Source: ICRI Forum


And the reefs, in turn, all depend on algae. Coral polyps allow algae to grow in their body tissues, and the algae shares the food it photosynthesizes with the polyps. Think of it as a form of rent: in exchange for a place to live, the algae keeps its landlord fed.10

These microscopic coral-dwellers are but seaweed on a smaller scale. Many of us consider seaweed an oceanic plant, but in truth it is a large, multicellular type of algae known as “macroalgae.”11 To love a coral reef, then, is to appreciate the necessity of our algal pals in this beautiful ecosystem.

A seal in a kelp forest; Source: Smithsonian



While microscopic algae keep coral reefs healthy and vibrant, larger varieties of seaweed provide marine life with another integral underwater ecosystem: kelp forests. Found in oceans around the world, kelp forests are dense, jungle-like copses of kelp, some species of which can grow up to 100 feet long. Hundreds of species of fish, shellfish, and aquatic mammals make their homes in kelp forests, including the indomitably adorable sea otter.12

“Otters are a keystone species for kelp forests, meaning they are critical to the ecosystem’s continued existence.”



You have more than likely seen the cute photos of otters using giant kelp as an anchor, wrapping themselves in it so they can sleep safely without floating out to sea, but there is more to the relationship. Otters are a keystone species for kelp forests, meaning they are critical to the ecosystem’s continued existence. As predators, they hunt urchins and other animals that eat the kelp. This keeps herbivore populations in check and allows the kelp forest to flourish.13

A sea otter wrapped in kelp; Source: SeaOtters.com



Our endearing otters are unwitting actors in the fight against climate change: the more robust a kelp forest is, the more it can help keep climate change at bay. Like terrestrial forests, kelp forests absorb a lot of carbon dioxide — and they can absorb even more of it when the otters are around. According to one study, a kelp forest without otters can capture 4.4 megatons of carbon dioxide, whereas a kelp forest with otter protection can capture 8.7 megatons.14

But otters cannot fend off every threat that comes a kelp forest’s way. As climate change warms the ocean, kelp forests wilt. Warm water also allows urchin populations to explode beyond natural management. A hardy, voracious species, urchins won’t hesitate to gobble up all the kelp in sight. When urchin populations reach critical levels, they can turn kelp forests into “urchin barrens”: vast expanses of ocean floor totally devoid of vegetation — or, really, of any life beyond the urchins.

Sea urchins march through a decimated kelp forest; Source: Noyo Center

Once an area becomes an urchin barren, it is almost impossible to reverse the process. In times of hunger, urchins’ jaws and teeth actually enlarge, allowing them to consume more kinds of food — including borderline inedible fare like barnacles and calcified algae. As food options diminish, the urchins become more and more aggressive. As marine biologist Mark Carr puts it, “They form these fronts, and they graze along the bottom and eat everything.”15

Reefs, too, suffer under climate change. Warming waters cause corals to reject their algae tenants. This, in fact, is what coral bleaching is: the process of coral polyps expelling the algae from their tissues. As a result, the reef starves and collapses — and so does the massive ecosystem it supported.16

Bleached coral; The NOAA hosts a helpful infographic charting the mechanism of bleaching; Source: SquareSpace


Save the Planet — Salute Your Seaweed


Reducing emissions is a necessary but insufficient step toward fighting climate change. In addition, scientists argue, we need to capture more of the carbon that enters the atmosphere. Reforestation — the act of planting massive numbers of trees in areas that used to be forests — is one key strategy for sequestering carbon emissions, but eventually, you run out of dry land.17

Enter ocean afforestation, the process of “reforesting” the ocean with seaweed, effectively turning the world’s waters into a giant carbon sink. Ocean afforestation also includes the strategic harvesting of seaweed, which would then be processed into biofuel. Antoine de Ramon N’Yeurt, one of the researchers who first developed the concept, estimates that if just 9 percent of the world’s ocean surface were afforested, we could replace fossil fuel energy entirely while removing 53 billion tons of carbon dioxide from the atmosphere each year. Currently, humans emit 40 billion tons of carbon dioxide every year, which means ocean afforestation could not only stop further warming, but actually lower overall carbon dioxide levels in the long run.

“Meaningful interventions against environmental catastrophe require a delicate balance between respecting the rhythms of the natural world and altering the operations of the global ecosystem.”



Of course, it is possible to have too much of a good thing, and seaweed is no different. Citizens of Mexico’s Caribbean coast can attest to that. Since 2018, they have been battling a brown-red species of seaweed named Sargassum. While Sargassum is a normal part of ocean life, the warmer waters produced by climate change allow it to proliferate at an explosive rate. Appearing in such vast quantities, Sargassum quickly turns dangerous. Aggregations of Sargassum can block the sunlight from coral, cutting the algae off from its food supply and killing the coral polyps. When the seaweed eventually dies, it unleashes massive amounts of nitrogen and phosphorus, contaminating fresh water supplies. All the carbon dioxide the Sargassum had previously sucked out of the atmosphere also gets released back into the air.18

Mexican researchers are experimenting with technologies to skim this excess seaweed and convert it into biofuel, but it would be a mistake to assume we can geoengineer our way out of any and every ecological crisis. Meaningful interventions against environmental catastrophe require a delicate balance between respecting the rhythms of the natural world and altering the operations of the global ecosystem. To go too far in either direction may only bring new problems to bear.19

A giant Sargassum bloom; Source: Weather.com


Seaweed’s Not So Simple After All


Let’s head back to our idyllic beach scene again: the soft sand, the rhythmic surf, those elegant terns — and yes, now we see the gemlike clumps of seaweed speckling the shore.

It’s easy to overlook algae, but consider all it has done and will do for us, from proto-feminist cause to coral guardian and even a potential replacement for fossil fuels.

A seaweed-spangled tide pool; Source: Pampering Campers

There’s a lesson here, and it goes far beyond seaweed. The earth’s ecosystem truly is a system, and the continuation of the whole depends on each and every one of its parts — from the tiny to the large, the scary to the cute. Humans would do well to remember that without the humblest forms of life at the bottom of the food chain, we could lose it all.

Seaweed certainly doesn’t have to be your favorite part of a day at the beach, but the next time you find it clinging to your forearm, maybe hold it close like a sea otter, and say a little “thank you” before you fling it back into the waves.

— The Keap Team



The Ignite Series

We select a scent of the month to send to our seasonal candle subscribers. We use the opportunity to uncover a facet of that scent through the written word with a monthly article. For our subscribers, this is complemented by a limited edition art label and art card in their monthly package. Learn more about the Keap candle subscription.