What is our assessment?

The overall effectiveness of seaweed cultivation for food as a climate solution remains uncertain. It could deliver climate benefits at modest cultivation scales while providing a useful end product. Expansion could also benefit land and food systems by reducing agricultural pressures, but it may introduce environmental trade-offs in the ocean that are not yet well understood. We will “Keep Watching” this solution.

What is it?

This solution involves expanding the cultivation of marine seaweed for human consumption as an alternative to higher-emission, lower-protein foods. These can include grains (e.g., wheat, rye, maize, oats, and rice) and, to a lesser extent, vegetables (e.g., potatoes, cassava, broccoli, and cabbage). By switching part of food production from land to ocean systems, seaweed farming helps avoid some sources of terrestrial agricultural emissions, such as those from fertilizer, irrigation, and soil disturbance. Seaweed cultivation, at modest scales and in suitable locations, does not require additional nutrients or irrigation, which can result in lower emissions. Emissions from physical cultivation activities also differ, with tractor use in land-based agriculture being replaced by emissions from boat operations in seaweed farming. Currently, roughly 80% of cultivated seaweed is consumed by humans in food products. 

Does it work?

The climate impact of cultivating seaweed is understudied, but existing estimates suggest that growing a ton of seaweed generates less than a quarter of the emissions from growing a ton of vegetables, such as broccoli and cabbage. The actual climate impact will depend on which types of foods are displaced in diets. Replacing higher-emission, low-protein foods, such as some grain-based staples (e.g., bread or rice), with seaweed could provide even greater climate benefits. More data are needed to assess full cradle-to-grave emissions for seaweed that include transport, processing, and storage prior to consumption. Actual benefits may be lower once full life cycle emissions are considered, or higher if seaweed replaces more emissions-intensive foods. 

Why are we excited?

Unlike terrestrial crops, seaweed cultivation does not require fresh water for irrigation or pesticides for pest management. It is the fastest-growing sector of global aquaculture, and can produce higher biomass yields per area than some land-based crops. Because it grows in the ocean, seaweed farming reduces land demand, which can therefore support terrestrial biodiversity and conservation efforts. If deployed in the right place, seaweed cultivation can also help reduce nutrient pollution in coastal areas. 

Compared with other seaweed-based climate solutions, farming seaweed for food could achieve a meaningful global climate impact using far less ocean area (1–2 Mha versus 6–7 Mha estimated for solutions like Deploy Ocean Biomass Sinking), though estimates remain highly uncertain. Cultivation might also provide additional carbon removal benefits by selecting for high-productivity cultivars and strategically placing farms in areas where carbon fixation and burial are naturally high. 

Finally, global diets currently overrely on starch-rich grain crops, highlighting a potential opportunity for seaweed, which is a nutritious source of protein, essential fatty acids, and minerals, to replace these foods and diversify diets in many regions. Across commonly consumed species, seaweeds are generally low in fat and calories and can be rich in fiber and micronutrients, including iron, iodine, calcium, and magnesium. 

Why are we concerned?

There are several environmental and feasibility concerns associated with seaweed cultivation, especially if it is expanded to use large areas of ocean habitat. Global estimates of ocean area suitable for seaweed cultivation range substantially, from 10 to 4,800 Mha, but often lack consideration for real-world nutrient limitations or ecological impacts. A more recent analysis that considers nitrogen, phosphorus, and iron limitations suggests that the viable ocean seaweed farming area is closer to 400 Mha. If regions are prioritized based on where cultivation is not nutrient-limited, where it can achieve high carbon removal efficiency, and where there are lower risks of adverse ecological impacts, potential seaweed farming areas could be limited to the western North Pacific and North Atlantic. The costs of such an expansion are also poorly understood, with some cost estimates per ton of CO₂ fairly high.

Similarly, it’s unclear how viable seaweed is as a large-scale substitute for low-protein foods in real-world diets. Using vegetables as an example, achieving a climate impact of at least 0.1 Gt CO₂‑eq/yr could require replacing over 25% of global vegetable production. Assuming productivity typical of subtidal seaweed (6.6 t C/ha/yr), this would translate to an additional ~2.6 Mha of ocean cultivation. An area of 2.6 Mha would equate to a 100-meter-wide continuous belt of seaweed cultivation along 22% of the global coastline. For comparison, seaweed cultivation currently covers less than 400,000 ha. 

At large scales, seaweed cultivation could alter food webs through competition with phytoplankton for nutrients and/or requiring external nutrient inputs, raising serious concerns similar to Deploy Ocean Biomass Sinking. Cultivation can have a range of other negative impacts on coastal ecosystems, too. Seaweed farms established in or near seagrass beds, for instance, can displace existing habitats and species. More research is needed to assess these trade-offs, including the spatial scale required for a globally meaningful climate impact and how seaweed cultivation relates to potential land-use benefits. Further work is also needed to evaluate whether seaweed cultivation could deliver greater climate benefits through other emerging products, rather than as a direct food replacement.