Kelp and carbon sequestration: bringing terrestrial carbon accounting to the deep sea

Permanence and sequestered carbon

“Permanence” is a favorite topic of the forest and climate change community.


When you reduce or avoid emissions – by switching from coal to wind energy, or by preventing deforestation – these are emissions that would have happened that year, but that did not happen because of an intervention. As long as you have the proper baseline (no easy task), you can credibly state that a business-as-usual ton of carbon dioxide or other greenhouse gas was not emitted that year. The non-existence of this emission is permanent.


When reforestation or afforestation occurs, and trees grow by performing photosynthesis, the carbon dioxide removed from the atmosphere is stored (“sequestered”) in biomass. Unlike reduced emissions which “no longer exist,” the carbon in this biomass is physically vulnerable.


Deforestation and forest degradation release this stored carbon back into the atmosphere. When this occurs the beneficial effects of the carbon sequestration have been reversed. Concerns about “reversals” have challenged reforestation and afforestation as a practical approach to climate change mitigation. Carbon offset markets (e.g., for California’s cap-and-trade system) employ technical fixes like “buffer accounts” to reduce the risk of over-crediting, given the statistical risks of reversal. However, concerns around permanence continue and may limit both financial and political investment into restoring ecosystems for climate benefits.


Concerns around permanence apply to all “enhancements” of biological carbon stocks, because of their physical vulnerability. All actions to improve biological carbon stocks carry real risks of reversal through human disturbance, whether through reforestation, afforestation, in soils, in terrestrial non-forest ecosystems, or with coastal blue carbon (e.g., planting mangroves).



The macroalgal carbon cycle

One recent study suggests that kelp forests may work differently. Carbon sequestration by kelp forests may be unusually “permanent,” due to their specific carbon cycle and the “wild” nature of the deep oceans.


In their Nature Geoscience paper, “Substantial role of macroalgae in marine carbon sequestration,” Professors Dorte Krause-Jensen and Carlos M. Duarte synthesize existing data on carbon sequestration by kelp and other macroalgae to develop a “first-order” estimate of kelp sequestration processes, destinations, and quantities. Their initial estimate suggests macroalgae sequesters about 634 million tonnes of carbon dioxide per year (173 TgC), greater than the emissions of Australia, the world’s 13th largest emitter. While 10% of this is buried in coastal sediments with a risk of anthropogenic reversals, about 90% is exported to the deep sea.


Despite the name, kelp forests have more in common with grasslands than with terrestrial forests in terms of their carbon cycle. Like terrestrial grasslands, kelp forests can be perennial (in warmer seas) or annual (in colder ocean regions). Whereas most forest types concentrate carbon in living biomass, grasslands and kelp forests have relatively short life cycles and act as “carbon donors” to rapidly enhance other carbon pools.


Most immediately, carbon moves from seaweed biomass to the Dissolved Organic Carbon the Particulate Organic Carbon pools. Seaweeds and other marine photosynthesizers continually release organic material that readily dissolves in water. This material can settle in place, travel to other parts of the ocean, or be rapidly consumed by microbes. This material is also known as Dissolved Organic Carbon, which refers broadly to organic molecules dissolved in water.


Kelp continually sheds old fronds. These drifting pieces of biomass are placed in a carbon pool called Particulate Organic Carbon. This may float great distances, due to the air bladders and enabled by chemical compounds that render kelp unpalatable to most animal species. This process of drift occurs with other macroalgae such as Sargassum and Desmarestiales as well. Combined with ocean circulation patterns, the carbon sequestered by macroalgae arrives in the deep sea, especially submarine canyons. Storms, in particular, rip fronds and whole plants from the ground to accelerate the export of material and sequestration of carbon into the deep sea.



Moving carbon from the atmosphere to the deep sea

For those interested in the overall numbers, the authors of the Nature Geoscience paper estimate the following average carbon fluxes (flows), synthesizing existing data sources and studies:

  • Sequestration of carbon dioxide from the atmosphere begins with Net Primary Production – algal growth, driven by photosynthesis.
  • On average, an estimated 43% of this Net Primary Production (NPP) is exported from the algal bed. Most of the remainder is grazed by marine animals or remineralized into its simplest elements by microbes, recycling it through the ecosystem. Only about 4% is directly buried in the algal bed.
  • This carbon exported from the algal bed travels in two carbon pools: about half moves as Dissolved Organic Carbon (explanation) and about half as Particulate Organic Carbon (explanation).
  • About one-third of the Dissolved Organic Carbon is exported below the mixed layer, of which some fraction enters the deep sea (this study assumes 100%). The rest is remineralized in the shelf.
  • About 11% of the Particulate Organic Carbon is exported to the deep sea and 4% is buried in the shelf. The remainder is remineralized, grazed, or cast onto beaches.
  • Together, long-term carbon sequestration by macroalgae occurs through 4 processes: carbon directly buried in the algal bed, Dissolved Organic Carbon exported below the mixed layer, Particulate Organic Carbon exported to the deep sea, and Particulate Organic Carbon buried in the shelf. Ultimately, about 11% of the carbon involved in biomass growth (Net Primary Production) is sequestered into other carbon pools longer-term, and the remainder is quickly recycled through the ecosystem.


The carbon buried in the algal bed and buried in the shelf is vulnerable to reversal due to human influence on coastal sediments. This sequestered carbon is conceptually similar to the carbon sequestered in other blue carbon ecosystems.


The carbon from the export of Dissolved Organic Carbon and Particulate Organic Carbon, however, largely ends up in the deep sea, where it may have minimal risk of reversal.


This “deep sea sequestration” represents an estimated 88% of the carbon sequestration by macroalgae, or about 10% of biomass growth.[1] This suggests this carbon may skirt the permanence issues that complicate sequestration for forests, other terrestrial ecosystems, and other blue carbon ecosystems. Because the sediments of the deep sea have so little human contact, compared to nearly all other ecosystems, this carbon may be “safer” from anthropogenic disturbances.



Next steps and policy mechanisms

Until now, kelp forests and other seaweeds have received almost no attention in terms of climate change mitigation policy discussions. Even other blue carbon ecosystems (seagrass meadows, tidal marshes, and mangroves) are a relatively new field. Why is this research into seaweed carbon sequestration important?


First, this paper is exiting in that it raises the profile of carbon sequestration by seaweeds. Better understanding the role of seaweeds in the global carbon cycle is inherently important as a scientific pursuit. This paper is simply a “first order estimate,” and as such there is tremendous uncertainty in the numbers: the authors state that their estimate varies by an order of magnitude. Much more scientific muscle will be required to increase the resolution and tighten these estimates. For example, future studies will need to confirm that there are no other natural or anthropogenic processes that causes this carbon to cycle away from the deep sea and back into the atmosphere.


Second, increased focus on seaweed sequestration highlights the potential for kelp forest “afforestation” as a novel climate change mitigation strategy. Policy interventions can encourage kelp production and the carbon sequestration it brings.

  1. Governments could directly implement kelp reforestation and protection programs, which have substantial benefits for marine ecosystems can spur dive tourism.
  2. Carbon offset standards can develop kelp offset methodologies, inside or outside a government context (compliance or voluntary offset markets, respectively). Where kelp is allowed to fully grow and is not used commercially, kelp cultivation efforts should hypothetically be additional, and therefore a credible source of offsets. Managing natural kelp beds, which face numerous existential threats, could also be a credible offset type.
  3. Another option is for governments to promote kelp cultivation through efficient economic incentives for the private sector. Kelp has various commercial uses: as a gel (hypercolloid industry), as an energy source, in pharmaceuticals, in fertilizers, and for invertebrate aquaculture (e.g., abalone, shrimp, sea urchins).[2] Tax breaks for kelp cultivation could provide a “discount” to match the unmonetized social good that cultivation brings through carbon sequestration.[3]


Third, if the implications of the Nature Geoscience article are validated, the reduced risk of reversals from seaweed carbon sequestration could lead to creative possibilities.


Unlike terrestrial ecosystems and coastal marine sediments, the deep sea has minimal direct land-uses, such as farms, urbanization, industry, or coastal resorts. As a result, carbon that enters the deep sea is theoretically insulated against the risks of reversals that affect other biological carbon.


Pending further research, this could lead to benefits for future kelp reforestation methodologies in the offset markets. Under California’s cap-and-trade system, to address permanence concerns, forestry projects sign 100-year contracts and place a share of their credits into a buffer account to protect against the risk of unintentional reversals (e.g., wildfires). Kelp projects could potentially skirt these requirements, making this more appealing to project developers.


Additionally, if reduced reversal risk is confirmed, kelp-based offsets may command a premium price in the market.  Although the carbon offset is theoretically a fungible commodity, the characteristics of offset projects do affect offset price in practice in the voluntary market.[4] Factors such as project charisma, co-benefits, and additionality can lead to higher prices. The “additional permanence” of kelp sequestration, if supported by further research, could lead kelp credits to command a higher price in the voluntary market.


For now, carbon sequestration by kelp is still in the initial stages of research science. At present, the Nature Geoscience paper is an interesting publication, that provides an initial estimate of carbon fluxes by macroalgae. As research expands over the next several years, the seaweed aquaculture sector may provide new, creative options for conservation and climate change mitigation that can be amplified by incentivizing policies and the offset markets.


Article by Patrick Cage, Program Officer


[1] For reference, this is about 70% through the Dissolved Organic Carbon pool and about 30% through the Particulate Organic Carbon pool.


[3] To be economically efficient, research would need to quantify how cultivation methods and harvest timing affect carbon release compared to a natural growth baseline. Typically, we would expect reduced carbon released in cultivated systems, since commercial users want to maximize their take of kelp biomass. This may be partially or entirely offset by the increased productivity of complex aquaculture techniques.

[4] State of the Voluntary Carbon Market report 2017