Seaweed As A Climate Change Solution - A Summary
There are four key pathways through which seaweed can help mitigate climate change. 1) protecting and restoring wild seaweed forests; 2) ocean afforestation by introducing seaweed into the open ocean; 3) offsetting industrial CO2 emissions using seaweed products for emission abatement; and 4) expanding nearshore seaweed aquaculture. In this summary, I summarise the opportunities and challenges surrounding each potential pathway, carbon credit viability for seaweed projects and reflect on the next steps globally for seaweed as a natural climate solution.
1) Protecting and restoring wild seaweed forests
It is estimated that wild seaweed forests globally sequestered 173 Tg year–1 (with a range of 61–268 Tg C year–1) (Krause-Jensen and Duarte 2016). Seaweed forests have been lost at 1.8% each year (Krumhansl et al. 2016). Therefore, it would be assumed that restoring lost lost seaweed forests would restore this natural carbon sequestration. However, tracking the carbon exported from these seaweed forests to the deep sea has proven to be challenging. Seaweed grows very fast, but also degrades quickly with the carbon returning to surface waters. Even if seaweed ecosystems met carbon accounting criteria there is a relatively small contribution of 31 Tg CO₂ year-1 that could be gained from restoring seaweed forests at the same rate they are lost, assuming their decline could be prevented and their restoration achieved at the same rate they have been declining (173x0.018x3.67=31.14). Moreover, the distinction between seaweed primary production as used in the estimate by Krause Jensen and Duarte (2016) and carbon sequestration remains unquantified as re-equilibration and capture of atmospheric CO₂ is an essential step for carbon sequestration, which few studies have quantified (Hurd et al. 2022). However, wild seaweed forests should be restored at scale regardless, given their contribution to ecosystem services such as biodiversity, fisheries, tourism, excess nutrient removal and recreational values (Duarte et al. 2021).
2) Expanding sustainable nearshore seaweed aquaculture
Coastal seaweed aquaculture, an already well-established global industry, can contribute to climate change mitigation by directly depositing carbon in sediments below seaweed farms during the growth cycle (Duarte et al. 2021). For the same reasons as for wild seaweed forest restoration, carbon exported from seaweed farms to oceanic sink sites as dissolved or particulate organic carbon is unlikely to meet current carbon accounting criteria. Krause-Jensen and Duarte (2016) calculated that 11% of the carbon fixed by wild seaweed is sequestered in oceanic sink sites for wild seaweed forests (Duarte et al. 2021). So we can estimate maximum carbon sequestration from seaweed farming by assuming all carbon in seaweed aquaculture products is released back into the atmosphere without abatement in other industries. In 2019, seaweed aquaculture production had grown to 35.8 million tonnes of fresh weight, and hence a maximum theoretical drawdown of 3,254,690 tonnes of CO2 per year (Cai et al. 2021) (35,762,504 x .1 (fresh weight to dry weight) x 0.248 (carbon content of seaweed dry weight) x 3.67 (carbon to CO2) = 3,254,960).
However, only part of the 11% that is being sequestered would have been deposited in sediment below seaweed farms, with the remaining carbon being exported as dissolved or particulate organic carbon. Additionally, seaweed aquaculture can reduce other natural carbon sinks, for example, by shading seagrass (Moreira-Saporiti et al. 2021), and incur operational emissions. Future project-level estimates of carbon sequestration need to include emissions and loss of CO2 sequestration from the loss of these natural carbon sinks (Hurd et al. 2021) and emissions from seaweed farming operations (Pessarrodona et al. 2024).
Assuming the best case above of 3,254,690 tonnes CO2/yr, this is still a relatively small global contribution to CO2 sequestration compared to 841 (621–1,064) Tg CO2 /yr by 2030 from all other blue carbon habitats (Macreadie et al. 2021). Seaweed aquaculture could expand offshore to increase this potential, but is limited in its expansion in areas where shedding of biomass will accumulate in sediment directly below farms to any meaningful extent. Cultivating seaweed on decommissioned oil and gas infrastructure and offshore wind farms could be a key conduit to enable this expansion. These more exposed seaweed aquaculture sites will export a higher portion of difficult-to-measure carbon to the deep sea than deposit the carbon in the sediment below farms.
Consideration should be given to the contribution of seaweed aquaculture to carbon sequestration below seaweed farms for climate mitigation compared to the opportunity cost of impact that could be generated from investment into other climate change solutions that are potentially more scalable. Regardless of carbon sequestration, seaweed aquaculture must be scaled up around the world based solely on the many co-benefits seaweed aquaculture provides such as lowering ocean acidification, enhancing fisheries and supporting community development (Duarte et al. 2021).
3) Offsetting industrial CO2 emissions using seaweed products for emission abatement
Products from seaweed aquaculture can lead to carbon emission abatement if they are meaningful alternatives to terrestrial-based products with higher emissions (Spillas et al. 2023). However, currently 90% of cultivated seaweeds are consumed as food or as additives (Duarte et al. 2021), so the carbon is likely released as methane or CO2 further down in the supply chain. Many novel seaweed products coming to the market, like biofuels, insulation, fertiliser, bioplastics, pharmaceuticals, fabrics, methane reducers for stock and more. There is a significant opportunity for these products to displace emissions from high-emission industrial alternative products. Developing new markets for novel seaweed industrial products like bioplastics, are critical to helping scale up seaweed farming and positively impact the climate. A key component of this will be performing life cycle assessments of seaweed products compared to their terrestrial or industrial alternatives to determine the accuracy of environmental performance claims. Overall, emissions displacement from low-carbon seaweed products appears to be the biggest quantifiable opportunity for climate change mitigation from seaweed which is reinforced by other recent papers such as Bullen et al. (2024) and Spillias et al. (2023).
4) Directly sinking seaweed into the deep sea to sequester CO2, ‘Ocean Afforestation’
Ocean afforestation is the process of growing seaweed offshore and sinking it to the deep sea exclusively as a carbon sink and has been investigated by several start-ups (https://www.runningtide.com, https://southernoceancarbon.com, https://pulltorefresh.team). Ocean afforestation is unlikely to meet carbon accounting criteria (Hurd et al. 2022) and remains very risky (Boyd et al. 2022). It, however, has potential for significant scalability as potentially 48 million km2 are available for seaweed aquaculture (Froehlich et al. 2019). A global collaborative research effort should determine if this methodology is worth pursuing in the next two years, (Hurd et al. 2022). However, because of the many concerns and significant complexities raised by Bach et al. (2021), Hurd et al. (2022), and Boyd et al. (2022) it is unlikely to have a scalable impact in the next decade. Because of these challenges, particularly quantifying the carbon that would have been sequestered by phytoplankton had seaweeds not outcompeted them for nutrients (nutrient reallocation), the potential contribution of ocean afforestation to climate mitigation remains complex, dynamic and unquantified. Overall, while sinking seaweed directly for carbon capture is theoretically possible, it is likely that there are other more viable marine carbon sequestration opportunities to invest in.
Overall, scaling up seaweed aquaculture and wild seaweed forest restoration is important to meet the United Nations Sustainable Development Goals. However, carbon sequestration should primarily be treated as a co-benefit to seaweed aquaculture and wild seaweed restoration as the current scale of seaweed aquaculture and seaweed restoration projects are limited. Sinking seaweed directly for carbon sequestration is theoretically possible but faces many challenges, and it is likely that the costs and complexities will be higher than those of other marine carbon removal strategies. The key opportunity in the future for seaweed to contribute to climate change mitigation is developing seaweed products to displace industrial emissions by providing low-emission product alternatives, which, like the use of seaweed bioplastics, could have a meaningful impact on global emission reduction.