by Sylvia Hurlimann
figures by Hannah Zucker

When we think of kelp, we conjure up images of magical underwater forests. Recent research, however, suggests that in addition to creating beautiful habitats, macroalgae such as kelp play a large role reducing the effects of global warming. Kelp has an incredibly fast growth rate (up to two feet per day) and exports a large portion of its biomass out into the deep sea, allowing kelp to permanently remove carbon dioxide from the atmosphere. Removing carbon dioxide from the atmosphere will play a necessary role in preventing rising temperatures and future climate catastrophe.

Sequestering greenhouse gases

As the concentration of greenhouse gases such as carbon dioxide rise at unprecedented rates, people are focused on decreasing the amount of carbon dioxide we put into the air. While the most effective way of doing this is by reducing carbon emissions, experts increasingly think that this will not be enough. According to the Intergovernmental Panel on Climate Change, the leading international body on climate change, we need to actively remove or sequester away carbon dioxide from the atmosphere to achieve negative carbon emissions and  prevent climate catastrophe. By 2050, we should plan to have net zero emissions, meaning that all carbon emissions need to be balanced by carbon removal.

One way to sequester carbon dioxide is using biology. When plants such as trees photosynthesize and grow, carbon in the form of carbon dioxide is removed from the atmosphere and converted into biomass, such as a branch or leaf on a growing tree. Although trees store carbon, this storage is vulnerable since deforestation or forest degradation release this carbon back into the atmosphere, undoing the benefits. When thinking about carbon sequestration, we need to focus on permanent solutions.

Coastal ecosystems sequester away surprisingly large amounts of carbon – they can sequester up to 20 times more carbon per acre than land forests. Marine plants that contribute to this carbon sequestration, such as mangroves and seagrass, live in rich soil. When these plants die, some of the leaves, branches, roots, and stems get buried underwater in the soil – and because of low oxygen concentrations underwater, the plant material can stay buried for decades or longer before breaking down and releasing carbon dioxide. Unfortunately, because the carbon is stored close to the shore, it can be easily disturbed by runoff, human activity, or storms and released back into the atmosphere sooner than it otherwise might have.

What makes macroalgae so special?

Unlike mangroves and seagrass, macroalgae such as kelp usually grow near the shore in rocky and eroding conditions where plant materials cannot get buried. Instead, bits of macroalgae get exported to the deep sea, where the carbon can be sequestered. Because the carbon from macroalgae is stored far away from the shore, it is less likely to be disturbed and returned to the atmosphere.

In addition to leaf-like structures and roots that we are generally familiar with, macroalgae have gas-filled bladders that help them float towards the surface where they receives more sunlight for photosynthesis (Figure 1). These gas-filled bladders allow bits of macroalgae to float for long distances and be carried far away from where the macroalgae is grown. Because they contain unpalatable compounds, macroalgae remain mostly uneaten as they travels across the ocean. Eventually, the air bladders burst and the macroalgae sink down towards the deep-sea floor, where the carbon is thought to be sequestered away from the atmosphere for centuries (and potentially up to millions of years).

Figure 1: Pathways for sequestration of macroalgae carbon into the deep sea. As macroalgae grow, they removes carbon dioxide from the atmosphere. Most of the carbon sequestered by macroalgae is sent to the deep sea either in the form of dissolved carbon or in the form of plant detritus which easily floats out to sea thanks to gas-filled bladders. This figure was adapted from Krause-Jensen and Duarte, 2016.

From macroalgae found in the guts of deep-sea crustaceans, we have inferred for decades that macroalgae travel far from where they are grown and make their way to depths of over 6000 meters under water. The importance of macroalgae in sequestering away carbon has been overlooked until recently, however, because it is difficult to precisely measure how much carbon is sequestered and exported to the deep sea.

Research estimates of carbon sequestration by macroalgae

A paper published in 2016 in Nature Geosciences compiled data from previous studies in order to provide an estimate of how much atmospheric carbon is being removed by macroalgae. Their rough estimate suggests that around 200 million tons of carbon dioxide are being sequestered by macroalgae every year – about as much as the annual emissions of the state of New York.

These estimations, however, rely on indirect calculations. To improve the numbers on how much carbon is being sequestered by macroalgae, we need to be able to measure how much macroalgae ends up in the deep-sea. As macroalgae slowly degrade, they expel bits of DNA into the environment. Research groups are planning on experimentally measuring how much macroalgae gets buried each year by taking samples from the deep-sea and measuring the amount of macroalgal DNA.

Although carbon sequestration is necessary to slow climate change, carbon sequestration alone cannot prevent climate catastrophe unless we reduce our use of fossil fuels.  Studies like this, however, highlight the importance of protecting valuable marine ecosystems such as kelp forests from environmental damage. As we decrease our use of fossil fuels, carbon sinks such as kelp forests will play a key role in getting us to net zero emissions.

Sylvia Hurlimann is a third-year graduate student in the Department of Molecular and Cellular Biology at Harvard University.

Hannah Zucker is a second-year PhD candidate in the Program in Neuroscience at Harvard University. 

For more information:

  • To read about carbon sequestration, check out this article.
  • To learn more about the role of other marine ecosystems in sequestering carbon, read this article.
  • Here is a piece about the role of macroalgae in carbon sequestration.

24 thoughts on “How Kelp Naturally Combats Global Climate Change

  1. I live on the South Coast of England ,up until the 60s our town had a large glass house horticultural Industry ,loose seaweed used to be collected from the beach and placed in jute sacks which was then immersed in large vats of water ,the water eventually turned into dark liqueur which was then poured on the crops.
    Our town was famous for the taste and quality of its tomatoes and other vegetables.

    1. Hello Mike,
      We live and farm on the east coast of Newfoundland in Canada, just across the “pond” from you. We use kelp from beaches in our area to nourish the soil and grow healthy food. The micronutrient rich kelp can be used in the soil or made into a rich , concentrated liquid to apply directly to plants, as you suggest.
      Cheers, Brian
      The Natural Gardener Inc

  2. This article mistakenly reports 200 tons carbon dioxide per year. The original paper reports tons of carbon, which is equivalent to about 600 tons of carbon dioxide

  3. Is it possible to dry kelp without releasing CO2? If so, could the dried kelp be used to enrich soil carbon for agriculture? I am invisaging a giant kelp farm off southern Australia near the desert – hot for drying kelp and good for a solar powered water desalination plant to wash kelp and irrigate carbon rich soil. Would love to know if this is feasible as a carbon positive drawdown and food growing option.

    1. Did you ever get an answer to your questions about if can dry kelp without releasing Co2.
      I am on the East Coast of Aust and interested not in worrying about trying to make money out of seaweed farming, more just in sequestering carbon in seaweed by community groups in coastal areas – if can find a way for seaweed to be used in animal feed, biofuel, skincare products, food great but I just want to get it going without waiting for commercialisation to reduce CO2.
      So your questions interests me too.
      Patricia Rose

    2. I’m currently trying to look into whether large regional kelp farms set up as aquaculture facilities would sequester carbon or whether they need the full ocean environment to do so. I have no substantial evidence as yet!

  4. Incredibly interesting, and stories of ‘hope’ like this help me get over the anxiety I have about the ailing state of our planet:

    The thing is, we have to slow down our polluting actives from CO2 to plastics. We need people to realize this and take actions in their own lives because the rate of pollution is simply not sustainable and I think earth is approaching a tipping point. There’s only so much our planet can take before it reboots itself.

  5. I have a proposal for removing gigatonnes of CO2 from the South Pacific Ocean and in doing so, reducing not only the acidity of the ocean but also reducing atmospheric CO2. To achieve this will require the South Pacific Ocean becoming a marine park.

    The method of removing the CO2 is a vast free-floating kelp forest ecosystem between New Zealand and South America and the key to growing this, is individual bamboo kelp buoys. The kelp buoys carry the iron minerals required by the growing kelp plants and also assist the juvenile kelp plants to stay afloat. ( To save the oceans and ultimate to save the planet it will be necessary within the foreseeable future to include the Southern Ocean and the Arctic Circle in the kelp growing regions.) These additional areas also becoming marine parks. The fishing industries will need to evolve into park management, kelp growing, kelp harvesting, and large scale aquaculture projects. These transitional phases being funded by the United Nations and the World Bank.

    The full proposal can be viewed using the link below,

    Kind regards,

    Geoffrey Peel, Auckland, New Zealand.

  6. Hi,
    What if the kelp is grown for food? Any info on the impact in terms of carbon and climate change in that case?

    1. When dairy cattle are fed Kelp it substantially reduces their greenhouse emissions . It also seems to improve the health of the cattle and the quality of their milk .

  7. Thanks for this post Sylvia and Hannah. I’m not a scientist but have been pondering for a while whether kelp could be grown at scale in the open ocean and then deliberately sunk as a method of carbon capture. I know that research is being done in California on open-ocean kelp farming, using robotic submersibles to raise and lower the kelp beds (for sunlight at daytime, nutrients from deeper water at night), but the purpose is to produce biofuels. See I’m not sure the status of the work or the cost. I also know there are folks who have developed a method of vertical kelp farming in New England to grow both kelp and shellfish for food in shallow waters close to shore. Have you come across any other discussion of ideas of large scale kelp farming, expressly to capture and bury carbon?

    1. Hi I am trying to get Cornwall (Uk) Marine groups to see the benefit of increasing kelp beds, not to farm but to bury as you suggest -I am hitting a brick wall because they say they don’t want to destroy eco systems -I point out that there was kelp there once probably and that isn’t enough to convince them. Any ideas on how to convince them?

      1. Hi Kate,

        Have you gotten any further with this?
        I work for a company that is looking into the many benefits of kelp planting. I would be interested to know how far you’ve gotten with this.
        It seems licensing is a serious issue regarding this particular topic.

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