Champions of the kelp industry have made bold claims that farming the fast-growing seaweed will alleviate the climate crisis. But those claims are not substantiated by the science of how kelp removes and sequesters atmospheric carbon. Kelp farming is a Band-Aid solution to mitigating global climate change, pushed to the forefront by decades of insufficiently modifying human behavior to cut carbon emissions.
Instead, the United States has focused its efforts on strategies that are largely centered around carbon capture, which removes carbon dioxide from the atmosphere and from emission point sources such as large industrial facilities and power plants. Most of these carbon capture strategies are costly, complex to implement, and not well studied, whereas kelp farming is cheap and technologically simple, making it an appealing option for those looking to use carbon capture as a climate change mitigation strategy.
Like almost all plants, kelp removes carbon dioxide from the atmosphere via photosynthesis. As one of the fastest growing organisms in the world (some species can grow up to 2 feet per day), kelp gains biomass quickly and thus sequesters carbon quickly. Natural kelp forests capture 5.4 million tons of carbon dioxide annually and are worth an estimated $500 billion, including their value in the food and materials industries. But even more money likely lies in the unexploited coastal ocean suitable for growing kelp, which spans an area of around 18 million square miles. Given the space available for aquaculture and the tremendous value of kelp, it is no surprise that seaweed is one of the fastest growing components of global food production, increasing on average 8 percent per year.
Companies seeking to capitalize on the obvious value of kelp farming are using its supposed carbon sequestration potential as a hook to expand the practice, raise funds, or sell carbon credits. However, scientists have questioned whether kelp farming acts to store carbon long term. Although kelp does absorb carbon dioxide through photosynthesis, the long-term sequestration of that carbon is limited by a process called the slow carbon cycle.
Kelp farming is cheap and technologically simple, making it an appealing option for those looking to use carbon capture as a climate change mitigation strategy.
Global carbon cycling refers to the movement of carbon between the atmosphere, biosphere, ocean, and land. This cycling happens on two timescales: fast and slow. In the fast cycle, carbon is absorbed by plants and sequestered as organic matter. When plants die or are consumed (burned as biofuel, eaten, used as feed, or to make materials, for example), that carbon is usually released back into the atmosphere. This cycle takes years to decades and does not remove carbon from the atmosphere long term.
By contrast, the slow carbon cycle takes 100-200 million years and moves carbon from the atmosphere to rock material and fossil fuels. Carbon moved via the slow carbon cycle is considered truly “removed.” Note that burning fossil fuels (that is, roughly 300-million-year-old carbon) has been adding billions of tons of carbon dioxide annually to natural emissions from the respiration of organisms, disrupting the slow carbon cycle. That is why human perturbations to the carbon cycle have such dire and permanent consequences for the planet.
For kelp to be useful in storing carbon, it needs to be stored as part of the slow carbon cycle, which means kelp needs to be deposited in the deep ocean where it won’t interact with the atmosphere for centuries to millennia. That doesn’t happen if the kelp is being used as food, a biofuel, or in materials production because all those pathways lead to the carbon dioxide being emitted back to the atmosphere via the fast carbon cycle.
To solve this issue, some companies plan or purport to deliberately sink seaweed, thereby enhancing its long-term storage capacity. Then, they profit from this sequestration by selling carbon credits to companies with large carbon emissions. The repercussions of sinking millions of tons of kelp to the seafloor are unknown, but changing the nutrient balance of the deep sea is dangerous because organisms living there can be particularly vulnerable to shifts in things like pH and oxygen availability. What’s more, the technology required to achieve this will likely involve using fossil fuels. In fact, the potential environmental outlook for sinking seaweed is so bad that some scientists have called for a moratorium on the practice, but companies are charging ahead anyway.
For example, Maine-based Running Tide Technologies was the one of the world’s largest kelp-based carbon removal startups but failed to deliver on its promises, going out of business in 2024, only seven years after being founded. Running Tide raised $54 million in funding to grow kelp on flotation devices designed to sink to the deep ocean. When its plan became too costly and its proof of concept failed, the company instead sunk some 21,000 tons of wood chips off the coast of Iceland to fulfill the carbon credit promises it sold to large corporations, including Microsoft and Shopify.
Even if we ignore the many pitfalls and unknowns of farming kelp for carbon sequestration, we would need an enormous amount of space to sequester just a fraction of the carbon dioxide emitted each year. One analysis calculated that 300-foot-wide kelp farms running along 63 percent of all global coastlines would be needed to sequester 0.1 billion tons of carbon dioxide per year — a mere 2 percent of annual U.S. emissions.
Scientists have questioned whether kelp farming acts to store carbon long term.
Some supporters of kelp farming suggest that kelp should not be sunk but rather used to replace fossil-fuel dependent systems. There is potential in using kelp biofuel or kelp-based animal feed to reduce carbon emissions. However, farming kelp on a large scale for such purposes would have negative repercussions on ocean ecosystems and could actually exacerbate climate change. Shading from kelp reduces phytoplankton growth, which could reduce the efficiency of sinking kelp for carbon sequestration by 37 percent. Adding massive amounts of organic carbon to the deep ocean would deplete oxygen, shifting the nutrient balance of the deep ocean and of upwelling regions which supply global ocean fisheries. When kelp grows, it emits halocarbons, which are short-lived but highly potent greenhouse gases. Kelp farming is far from a green solution and puts our oceans at further risk of deterioration.
Ocean governance is notoriously relaxed and difficult to enforce, but the ocean is the primary regulator of the Earth’s climate. It is imperative that we take caution before undertaking global-scale modifications to the ocean’s delicate balance. Ultimately, efforts placed into changing human behavior and developing technologies to disentangle our global systems from their reliance on fossil fuels are far more valuable than working on temporary strategies that allow us to continue business as usual.
Veronique Carignan is an environmental chemist and former academic researcher in chemical oceanography. She now works as an environmental activist and freelance climate journalist.