Seaweed afforestation at large-scales exclusively for carbon sequestration: Critical assessment of risks, viability and the state of knowledge

Abstract

There have been discussions of scaling up offshore seaweed cultivation and sinking it exclusively for carbon sequestration (‘ocean afforestation’) and thereby help mitigate climate change, but is this concept feasible? Here we investigate the feasibility of ocean afforestation across five perspectives: 1) Ecological feasibility; 2) Technical feasibility; 3) Economic feasibility; 4) Co-benefits and risks; and 5) Governance and social considerations. Optimising ecological factors such as species selection and use of currents, alongside the use of low-cost biodegradable rafts in theory could see this concept scaled globally. An area of 400,000km2 or 16.4 billion biodegradable rafts would be needed for 1 gigatonne of CO2 fixation given roughly 16 rafts of 25m2 each would be needed per tonne of CO2 fixation. However, CO2 fixation (calculated from net primary productivity) and carbon sequestration (carbon permanently removed from the atmosphere) are fundamentally different processes, yet this distinction is often overlooked. Quantifying carbon sequestration from ocean afforestation remains elusive given several outstanding oceanic biogeochemical considerations. For example, the displacement of phytoplankton communities and their associated carbon sequestration via nutrient reallocation is a critical knowledge gap in understanding the climate change mitigation potential of ocean afforestation. Ocean afforestation also carries complex risks to marine ecosystems, for example, the impact on benthic communities of seaweed deposition. Additionally, governance and social challenges exist such as the legality of operation in relation to ocean treaties. The concept of ocean afforestation is still in its infancy, and while there are large research gaps, further investment into research should be given before the concept can be adequately compared against the suite of potential ocean-based climate change mitigation strategies.

The Sea's best kept secret: the use of seaweed as a source of biohydrogen for clean and renewable energy

“…33 Seaweed is able to act as a carbon-neutral material by absorbing CO 2 from the atmosphere during photosynthesis, whereby CO 2 produced as emissions can be mitigated. 34 A range of methods have been explored to dewater and ultimately dry the seaweed feedstock: osmotic media, organic/mineral acids, screw-pressing, and conventional drying. 35 As seen in other research studies using seaweed towards biorefinery applications, these processes are species-dependent.…”

Carbon sequestration and climate change mitigation using macroalgae: a state of knowledge review

“…We focus on wild macroalgal ecosystems and intentionally refrain from contextualising these topics in relation to macroalgae farming. This focus arises because interventions around wild ecosystems presently have substantially higher potential abatement (NASEM, 2021), present more co-benefits (Forbes et al, 2022), and the CO 2removal capacity of macroalgae farming has been explored elsewhere (Arzeno-Soltero et al, 2023;Ross et al, 2022;Wu, Keller & Oschlies, 2023). Specifically, our review is largely centered on macroalgal forests formed by large brown algae (sensu Wernberg & Filbee-Dexter, 2019), which draw the greatest atmospheric CO 2 flux of any macroalgae habitat (Duarte et al, 2022;Pessarrodona et al, 2022) and whose contribution to sequestration is presumably the largest (Krause-Jensen & .…”

“…The need to ascertain new ocean‐based pathways to mitigate the effects of climate change has recently prompted revisiting of the role of macroalgae as an actionable blue carbon ecosystem (Macreadie et al ., 2019, 2021), including farmed macroalgae (Duarte et al ., 2017; Sondak et al ., 2017; Froehlich et al ., 2019). This in turn has attracted interest from a wide range of actors in government and private industry sectors (Coleman et al ., 2022; Friess et al ., 2022; Kuwae et al ., 2022 a ; Ross, Tarbuck & Macreadie, 2022), catalysing research and debate about their use as a climate mitigation tool.…”

Nitrogen availability regulates the effects of a simulated marine heatwave on carbon sequestration and phycosphere bacteria of a marine crop

“…Seaweeds play an essential role in coastal ecosystems, such as the provision of food, a spatial refuge against predators and the cycling of nutrients (Teagle et al 2017; Gao et al 2021 a ). In addition, the huge potential of seaweeds to sequester carbon is attracting increasing global attention as the need for climate mitigation techniques grows more acute (Gao et al 2022; Ross et al 2022). Climate change is expected to impact seaweed carbon fixation and export (Wada et al 2021).…”

“…In addition, the huge potential of seaweeds to sequester carbon is attracting increasing global attention as the need for climate mitigation techniques grows more acute (Gao et al 2022;Ross et al 2022). Climate change is expected to impact seaweed carbon fixation and export (Wada et al 2021).…”