COP26 in Glasgow will be remembered for many reasons, but none more so than for placing nature-based solutions in the spotlight, specifically blue carbon. Nature-based solutions (NbS) aren’t a new concept, nor are they something (for once) that humans can claim responsibility for. They are simply a way of working with nature to address societal challenges, boost human well-being and restore diversity to our vital ecosystems and planetary life support systems.
An added benefit of NbS is it’s potential to abate carbon emissions through sequestration. Given the stream of net zero commitments (both corporate and government) and a distinct lack of carbon removal projects, it’s easy to see why policymakers and businesses are now turning to NbS. The elevation of blue carbon has led to significant investment in carbon silver bullet solutions, with sights now firmly set on the new star in blue carbon, seaweed, as the next big solution. Presenting seaweed as a global solution to carbon dioxide removal and climate mitigation, however, has come under recent scrutiny from many within the seaweed community.
As the debate rages on, it’s imperative that we follow the science. In this case, there are leading scientists across the world addressing the knowledge gaps and delivering the necessary literature to communicate the research. One such project, Oceans 2050, has taken core samples from coastal sediments below 23 seaweed farms (all farms have been in operation for over 15 years) across the world to better quantify carbon sequestration of seaweed aquaculture. Professor Carlos Duarte, Chief Scientist and leading expert in the field, announced the initial findings from the first few farms in the study during the ‘Seaweed as a Nature-based Solution for Climate Change’ event in the SDG Pavilion at COP26. The preliminary carbon sequestration rate announced by Professor Duarte was an average of 3.5 tonnes CO2 per hectare per year, which of course, will still need to be fully verified over the coming months.
First, it’s vital to note that samples were taken from farms across 12 countries (temperate and tropical climates) where seaweed has been and still is commercially harvested, validating the assumption that sequestration occurs during the growth cycle (see Krause-Jensen & Duarte, 2016). Therefore, the claims that occupying large areas of our ocean with seaweed to sink for carbon dioxide removal could appear somewhat counterproductive, particularly given the increasing demand for seaweed-derived products and the need to source alternatives for fossil fuel-based products. Add to this, uncertainties around the long-term ecological externalities and the socio-economic consideration of sinking valuable biomass amidst increasing global food insecurity, and the case for this activity leaves more questions than answers.
The second element at play here is the concept of scale. Commercial seaweed aquaculture currently occupies less than 2000km2 of our coastlines, so using Oceans 2050 estimates, carbon drawdown from the industry would be just under 700,000 tonnes CO2 per year. This is the equivalent of taking around 150,000 cars off the road a year or offsetting a fifth of the emissions from one coal power plant a year. Whilst this is still impactful and is certain to play a role as the industry continues to scale, we must put the current impact into perspective and maintain realistic expectations of how seaweed aquaculture might contribute to global climate mitigation.
One caveat to these figures is uncertainty around how much carbon is exported to the deep sea, which was not included in the scope of the aforementioned research study. Given the need to exercise rigorous scientific methods to create sound carbon accounting and the countless biogeochemical processes at play in our ocean, it’s difficult to see quantification of deep-sea exports on the immediate horizon for seaweed aquaculture. This is further validated by authors of Chapter 5 ‘Global Carbon and other Biogeochemical Cycles and Feedbacks’ in the recent IPCC AR6, where management and quantification of seaweed as an ocean-based carbon dioxide removal (CDR) mechanism are questioned:
“Wider usage of the term blue carbon occurs in the literature, for example including seaweeds (macroalgae), shelf sea sediments and open ocean carbon exchanges. However, such systems are less amenable to management, with many uncertainties relating to the permanence of their carbon stores”
Markets tend to drive the price of carbon offsets, which can lead to lower quality removal projects, perhaps underscored in terrestrial afforestation projects where pine trees, due to their rapid growth rates and cheaper price points, are planted at large-scale irrespective of the biodiversity implications. This has led to calls for greater transparency, improved market regulation and standards for projects, where carbon is not the only measurement for impact. Ultimately, projects shouldn’t trade off biodiversity gains and social development for carbon sequestration. Instead, impact
assessment needs to be established across a range of universal metrics that consider social, cultural, economic, environmental, and ecological factors.
97% of seaweed production occurs in Asia, and the second largest producer is Indonesia. The reality is that the 250,000 farming households that form the backbone of the seaweed industry in country are living on or below the poverty line. Seaweed farming is simply a secondary livelihood option to other sources of income such as fishing for these households. As fish stocks continue to be fished at biologically unsustainable levels and climate change places entire marine ecosystems on the brink of collapse, more and more fishing communities will be driven towards alternative coastal livelihood options or indeed, away from the coast altogether and into increasingly overpopulated urban areas.
From Sea Green’s perspective, the establishment of market-based solutions that reward these farmers for ecosystem services has the potential to subsidise livelihoods and strengthen financial resilience for coastal communities. Yes, carbon sequestration in seaweed aquaculture is bound to play a part in climate action, but more understanding is needed.
At this stage, the development of any seaweed voluntary carbon market would purely be an enabler for community-based adaptation and resilience, and in ensuring sustainable local economic growth within farming communities. Amidst the hysteria of the seaweed momentum, let’s stay grounded and continue to follow the science.
 Share of seaweed production: China 56% and Indonesia 28%, with majority of Chinese production as wild harvest