Seaweed blue carbon: Ready? Or Not?

There is an urgent need to achieve the United Nations Sustainable Development Goals (SDGs). At the same time, greenhouse gasses (GHGs) that contribute to global warming must be reduced to avoid even more severe climate disruption. Macroalgal (seaweed) systems can help the world achieve the SDGs by producing food, other valuable products, livelihoods, and a number of ecological benefits. Seaweed systems may also be drawing down atmospheric carbon dioxide, an important GHG, under some conditions. However, the net impacts of seaweed systems on GHGs ("blue carbon") depends on many context-specific, complex biogeochemical processes and have thus far been difficult to quantify. We engaged experts in a system mapping exercise to support decision-making in the context of the high levels of uncertainty associated with seaweed blue carbon. The conservation and restoration of seaweed stands appears to be a low-regrets intervention that would produce many benefits, including some carbon sequestration under some conditions, with low risk. Increasing the productivity of seaweed farms may have a similar benefit and risk profile. A large expansion of seaweed farming coupled with sinking the seaweed biomass could significantly increase carbon sequestration, but with relatively large social, economic, and ecological risks. Certain products made from seaweed that sequester carbon, replace GHG-intensive products, or suppress GHG emissions could enhance the climate and socioeconomic benefits of seaweed systems while also improving prospects for quantifying and verifying them. More research and interventions will likely be necessary for such products to scale. A portfolio of seaweed systems would probably be necessary to realize the variety of benefits that these systems are capable of generating.