Introduction

It is becoming increasingly clear that achievement of the temperature objectives of the Paris Climate Agreement will require the global community to both engage in extremely rapid and aggressive decarbonization of the world economy and deployment of atmospheric carbon dioxide removal (CDR) approaches at a substantial scale. Of the integrated assessment model scenarios in the IPCC’s Fifth Assessment Report that achieve the Paris Agreement’s upper-level temperature targets, 87% contemplated the need for extensive deployment of CDR options in the second half of the century.

The IPCC’s Sixth Assessment Report, released in 2021/2022, affirms this conclusion, stating that “deployment of carbon dioxide removal (CDR) to counterbalance hard-to-abate residual emissions is unavoidable if net zero CO2 or GHG emissions are to be achieved.” This may ultimately translate into the need to remove hundreds of gigatons of carbon dioxide from the atmosphere to meet the Paris Agreement’s temperature objectives.

The emphasis in recent IPCC reports has been on terrestrial carbon removal options, including afforestation/reforestation, bioenergy with carbon capture and storage, and direct air capture. However, there is increasing recognition that ocean-based approaches may also have to play a substantial role in building a viable portfolio of carbon dioxide removal options.

One of the most widely discussed of these ocean-based approaches is termed “ocean afforestation,” or “seaweed farming.” Wild seaweed incorporates dissolved inorganic carbon from the upper layer of the ocean into tissue through the photosynthetic process. The conversion of dissolved carbon dioxide into biomass, in turn, results in drawdown of atmospheric carbon dioxide by air-sea fluxes into ocean surfaces. Some species, such as kelp, float near the surface because of gas-filled bladders in their leaves. When these bladders burst, kelp can sink to the deep oceans, or get locked away in sediments, potentially resulting in the storage of carbon for hundreds to millions of years. One study concluded that natural seaweed ecosystems sequester approximately 173 million metric ton of carbon annually.

Ocean afforestation entails large-scale cultivation of species such as kelp to try to supercharge carbon dioxide removal. It’s contemplated that long-term storage of carbon dioxide in kelp tissues can be effectuated by either the sinking of seaweed biomass or pumping it to depth.

In recent years, a number of start-up companies have begun exploring the potential for large-scale seaweed farming for carbon removal purposes. For example, Maine-based Running Tide, is growing kelp seeds in hatcheries, and then placing them on biodegradable buoys. The idea is that the kelp can capture carbon dioxide as the buoys float along on the surface of the ocean, with the buoys sinking deep into the ocean when the kelp reaches a certain weight, likely over a 6-8 month growth season. California-based Pull to Refresh contemplates developing a fleet of semi-autonomous vessels that would tow vases in which giant bladder kelp can be grown, with the kelp eventually dying and sinking to the bottom of the ocean.

A number of recent articles in the popular press have, without much scientific analysis, touted this approach. This includes a recent Atlantic piece proclaiming that “[k]elp is weirdly great at sucking carbon out of the sky,” and an NPR piece echoing the claims of both Running Tide and Pull to Refresh that kelp farming could potentially store prodigious amounts of carbon dioxide. Indeed, Pull to Refresh contends that kelp cultivation could result in the storage of up to a trillion metric tons of carbon dioxide.

I would like to take a more precautionary position in this article, both in terms of the actual carbon sequestration potential of ocean afforestation, as well as the specter of negative impacts of deploying this approach at large scales.

Sequestration potential of kelp-farming

Kelp assuredly has remarkable carbon sequestration properties, taking up roughly twenty times more carbon than terrestrial plants of commensurate volumes. However, several recent studies have concluded that the contribution of kelp farming in a carbon removal portfolio would likely be very modest. For example, a report on ocean-based CDR approaches released late last year by the U.S. National Academies of Science, Engineering, and Medicine (NASEM), concluded that a “reasonably successful goal” for a kelp farming program would be to grow and sequester enough macrophyte biomass to sequester a tenth of a gigaton of carbon dioxide annually. This is equivalent to 0.2% of annual global emissions. To put this in perspective, very large-scale kelp farming might be able to sequester enough carbon to offset emissions associated with the aquaculture industry, but this would pale into insignificance compared to the hundreds of billions of tons that we need from carbon dioxide removal approaches. It may thus be time to put aside the hyperbolic claims of some supporters, and view seaweed cultivation as a niche form of carbon removal.

Moreover, the study emphasized the titanic logistics of such an enterprise. Even sequestering 0.1 gigaton of carbon dioxide annually would require an area equivalent to the land mass of Ireland, or if sited in coastal regions, a 100-meter-wide continuous belt encompassing 63% of the global coastline. This seems wholly unrealistic, especially given the particularly imposing challenges of constructing and operating farming systems in difficult offshore environments, where much of this activity would likely have to take place.

Recent research also calls into question even these modest estimates of potential carbon sequestration. A new study by Gallagher et al. concluded that seaweed farming could constitute a net source of carbon dioxide to the atmosphere. The reason is that as coastal waters flow through seaweed canopies they introduce large quantities of plankton and other organic material, which in turn can be fed upon by species that live among seaweed, such as sea squirts and bryozoan animals. These creatures breathe out large amounts of carbon dioxide as they consume these new sources of food supply. The authors conclude that rather than sequestering an estimated 50 tons of carbon dioxide per square kilometer, seaweed may emit as much as 150 tons per square kilometer annually to the atmosphere.

A recent study by Bach, et al., which focused on carbon uptake by floating natural Sargassum seaweed in the North Atlantic, similarly concluded that ocean afforestation could constitute a net source of carbon dioxide, “or, at best, contribute 0.0001–0.001% to the amount of annual CDR required in 2100 under a low emission scenario.” This is due to a range of factors, including releases of carbon dioxide via calcification by species that habitate seaweed, and reduction of the growth of phytoplankton, which also take up carbon dioxide, due to reallocation of nutrient resources. Another recent study, employing new particle tracking models in the ocean, also concluded that the fraction of carbon that remains below the mixed layer for a century or more is far lower than previously estimated. This could further constrain the sequestration potential of kelp farming.

Finally, there are serious risks that a rush to monetize kelp farming could undermine confidence in ocean-based carbon dioxide removal strategies. Some analysts believe that kelp farming companies may start issuing “kelp carbon credits” in voluntary markets as early as this year. Indeed, Pull to Refresh, is already encouraging companies to “get in touch” if they wish to purchase credits. However, this would be extremely injudicious at this point, and may continue to be so for the immediate future.

As the NASEM ocean CDR report of last year emphasized, existing biogeochemical monitoring systems may be inadequate to detect even a one-gigaton per year change in global inventories of carbon dioxide, and to distinguish it from natural fluxes. This may require development of sophisticated numerical modeling, and further development of Earth system models to take into account carbon cycles in higher ocean trophic levels. Moreover, while accounting for the dissolved organic carbon taken up by kelp, and distinguishing it background pools, will be critical to carbon trading, such monitoring is “unfeasible” currently. Finally, measurements of carbon stored in deep ocean regions or sediments currently rely on indirect calculations. The kind of empirical verification that should be required before carbon credits are issued will require development of new, and potentially expensive, methods, such as environmental DNA. This belies the rhetoric of companies, such as Running Tide, which tout the alleged “natural simplicity of this system.”

Verra, a non-profit organization that develops standards for mitigation approaches taken up in the voluntary carbon market, recently launched its Seascape Carbon Initiative. The Initiative seeks to develop carbon crediting methodologies for “seascape carbon activities,” including seaweed/kelp farming. It will be extremely important that it fully engage with the complex science associated with this approach to ensure the integrity of this nascent field, as well as to clearly outline requisite management strategies to minimize potential negative environmental impacts.

Potential risks associated with large-scale seaweed farming

Large-scale seaweed propagation and sinking could impose risks in the upper layers of the ocean where seaweed would be grown, and at depth. In the near-surface ocean, it could reduce ambient nutrient levels and light availability. This could result in reductions in phytoplankton production and trophic exchanges of energy that could adversely impact fisheries and marine mammals. Widespread scale-up of seaweed farming also risks the introduction of invasive species, carried by macroalgae into offshore ocean ecosystems. One recent study, for example, identified 1200 species hitching a ride on macroalgae as it drifted through the ocean. This raises the specter of the spread of disease or crowding out of other species.

Seaweed cultivation at scale would also require a large buildout of infrastructure, including in deeper ocean waters than were most current seaweed farms have been sited. Even a modest goal of sequestering 0.1 gigatons of carbon annually might require 7.3 million hectares of ocean-based farm structures. This could result in entanglement of cetaceans and other vertebrates, hazards to navigation, and displacement of fisheries.

In deeper portions of the ocean, “likely . . . the least understood biome on Earth,” sinking seaweed may reduce oxygen levels, and lead to acidification and eutrophication by increasing carbon dioxide and nutrient loads. Moreover, introduction of large amounts of particulate matter in the deep ocean could imperil visibility and contacts among mesopelagic species, “similar to what might be expected from the improper disposal of tailings from deep-sea mining operations.”

Of course, it also needs to be emphasized that seaweed farming can provide important co-benefits that should be taken into consideration in any policy process. This includes providing critical habitat for many ocean species, combatting ocean acidification and lowering nutrient levels of nitrogen and phosphorus, which contribute to toxic algae blooms.

Is seaweed farming the best use of this resource?

Some researchers question whether seaweed farming for the purposes of deep ocean carbon storage is the optimal use of the resource from an environmental perspective, or whether cultivation for other purposes might make more sense. Recent estimates are that 75% of farmed seaweed could be harvested annually for various purposes that could help address climate change. For example, one study concluded that adding red seaweed to cattle feed can reduce methane emissions by as much as 99 per cent. Another study contends that the optimal use of seaweed is as a replacement for fossil hydrocarbons in fuels, by converting seaweed biomass into a range of bioenergy products, including biogas or liquid or solid biofuels, or chemicals. Seaweed biofuels, for example, “could mitigate about 1500 tons of carbon dioxide per square kilometer of seaweed cultivation.” Seaweed could also help support other terrestrially-based carbon dioxide removal systems. For example, it could also be used as a feedstock for bioenergy with carbon capture and storage systems, helping to reduce potential competition for land with food crops, or for conversion into biochar for long-term deep-burial of carbon. There’s also the potential to substantially reduce carbon dioxide emissions associated with fertilizer production.

Policymakers, in conjunction with the scientific community, need to ascertain how to best use seaweed as an oceanic resource. This should not be primarily driven by the desire of companies wishing to cash in on carbon credits through any given method, but rather by approaches that maximize societal benefits. 

The journey continues.

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