With half of global CO2 fixation occurring in the oceans (Nellemann et al. 2009), there’s been a tremendous recent upsurge in interest from governments and the private sector in the potential for carbon sequestered in the ocean or Blue Carbon. Blue carbon ecosystems include coastal ecosystems, seagrasses, salt marshes and mangroves. There have been discussions about whether seaweed should also be considered as blue carbon given that seaweed habitats are larger in area than all other coastal blue carbon habitats combined. The area of seaweed that grows on soft-sediments, where carbon can accumulate, is estimated at 1.5 million km2 – relative to the 7.2 million km2 of total global wild seaweed area (Duarte et al. 2022). However, the science of seaweed carbon sequestration has been largely outpaced by policy and practice, and the scale of opportunity for seaweed carbon sequestration is unclear.
There are four key pathways through which seaweed can help sequester carbon.
1. Protecting and restoring wild seaweed forests
Protecting and restoring wild seaweed forests are absolutely critical for biodiversity, fisheries, nutrient alleviation and tourism. However, it has proven to be challenging to track the carbon that is exported from these forests to the deep sea as there is no guarantee that the carbon has been sequestered until it reaches a permanent sink site in the deep ocean.
Seaweed grows very fast, but also breaks down really fast with the carbon returning to the water column. While it is estimated that 173 Tg year–1 (with a range of 61–268 Tg C year–1) is sequestered by wild seaweed forests globally (Krause-Jensen and Duarte 2016), seaweed forests have degraded at a rate of 1.8% each year (Krumhansl et al. 2016). An example of this is in Tasmania where up to 90% of Giant Kelp forests have been lost.
Carbon sequestration aside, restoration of seaweed forests is a critical part of enabling a nature-positive future. You can read more about global seaweed restoration efforts here.
2. Expanding sustainable nearshore seaweed aquaculture
Seaweed farms can sequester carbon by depositing it in sediments below the farms or by exporting the carbon from the farms. It is critical that we scale up seaweed aquaculture around the world for the many co-benefits it provides such as lowering ocean acidification, enhancing fisheries and supporting community development (Duarte et al. 2021). In 2019, seaweed aquaculture production had grown to 35.8 million tonnes of fresh weight (Cai et al. 2021) which meant a maximum carbon sequestration of 3,254,690 tonnes of CO2 per year. Currently, seaweed is largely a low-value commodity crop which limits its scalability and potential to support developing communities and have a positive impact on the environment. This is partly because there is currently little to no traceability in the seaweed industry. But with companies like Sea Green, technology can now be used to enable farmers to monitor and manage input their environmental and social data via a digital platform, empowering them to enhance seaweed production operations. Sea Green’s platform helps farmers demonstrate the positive environmental, social, and governance (ESG) impacts they are achieving along the supply chain, with a focus on sustainability. If improved seaweed traceability is able to occur concurrently with financial recognition of the co-benefits of seaweed production, for example carbon credits, seaweed farmers will be able to unlock an additional income source.
3. Using low emission seaweed products instead of high emission alternatives
This is likley to be the biggest quantifiable way for seaweed to help lower emissions. There are many new exciting 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 alternatives. However, currently, 90% of cultivated seaweeds are consumed as food or additives (Duarte et al. 2021). Developing new markets for novel seaweed industrial products is critical to helping scale up seaweed farming and making a positive climate impact.
4. Directly sinking seaweed into the deep sea to sequester CO2
This is a ‘moonshot’ seaweed concept that has received much scrutiny and which is discussed further in a paper here. The science presents numerous challenges, and many difficult questions still surround this concept. This is a controversial topic that is analysed in more detail in the mentioned paper.
Seaweed is the fastest-growing crop on the planet , requiring no nutrient or fertiliser inputs. There is massive potential for seaweed aquaculture to contribute to a sustainable, low-carbon future. Similarly, restoring and protecting wild seaweed forests has massive economic benefits alongside the co-benefit of sequestering carbon. Enabling the traceability of seaweed products and accurately verifying their environmental benefits are crucial steps for the industry to advance and broaden the contribution of seaweed towards a more sustainable and equitable future.
References
Cai, J., A. Lovatelli, A. Stankus, and X. Zhou. 2021. Seaweed Revolution: Where is the Next Milestone? FAO Aquaculture Newsletter:13-16.
Duarte, C. M., A. Bruhn, and D. Krause-Jensen. 2021. A seaweed aquaculture imperative to meet global sustainability targets. Nature Sustainability:1-9.
Duarte, C. M., J. P. Gattuso, K. Hancke, H. Gundersen, K. Filbee‐Dexter, M. F. Pedersen, J. J. Middelburg, M. T. Burrows, K. A. Krumhansl, and T. Wernberg. 2022. Global estimates of the extent and production of macroalgal forests. Global Ecology and Biogeography.
Krause-Jensen, D., and C. M. Duarte. 2016. Substantial role of macroalgae in marine carbon sequestration. Nature Geoscience 9:737-742.
Nellemann, C., E. Corcoran, C. M. Duarte, C. De Young, L. E. Fonseca, and G. Grimsdith. 2009. Blue carbon: the role of healthy oceans in binding carbon.
