Seaweed to Slow Down Climate Change: Ready? Or Not?

By Rod Fujita

Seaweed is a hot solution for mitigating climate change. Can this carbon-absorbing powerhouse really help?

Seaweed is having a moment. I’ve been working on seaweed for 40 years, and I’ve never seen so many headlines about how seaweed can save the planet. 

I can understand why. The need to save the planet is more pressing than ever. We must now dramatically reduce greenhouse gas emissions and, at the same time, increase the planet’s capacity to remove carbon dioxide (CO2) from the atmosphere to prevent even more catastrophic impacts of global warming. 

Given this urgency, any promising ways to achieve these ambitious goals are going to command a lot of attention. And seaweed looks promising indeed. Some seaweed species can absorb carbon and turn it into biomass very fast. Because seaweed can be made into valuable products, conceivably, this can be done at a profit unlike other approaches for removing carbon, which are costly.  

Seaweed can also remediate ocean acidification, helping many marine organisms grow and thrive, and it can remove excess nutrients from seawater helping to prevent harmful algal blooms and fish die-offs. Natural seaweed stands and farms (collectively called seaweed systems here) also provide food and shelter for myriad types of marine life. 

Seaweed to the rescue? 

Some seaweed systems are probably already helping to mitigate climate change to some extent by sequestering CO2. Seaweed naturally sheds fragments that get swept out to sea and eventually sink into deep sediments for long-term burial or sequestration. However, recent research  summarized in our new paper  suggests that the seaweed story is not that simple.  Many factors affect seaweed’s ability to sequester carbon, making it very difficult to measure its effectiveness as a climate mitigation strategy. For instance, animals eating seaweed convert some of the carbon it absorb back into CO2. And timing is important, too — carbon is only stored by seaweed for a short period of time (a few years) when compared to how long CO2 stays in our atmosphere (over a century). We also convert most of harvested seaweed into food or other products, which reduces the amount of carbon that is sequestered since this also converts the carbon in seaweed back into CO2. Compared to mangrove forests, which sequester large portions of the carbon they absorb in wood, roots and soil, probably a relatively small fraction of the carbon absorbed by seaweed is sequestered in the deep sea.  

Farming and using seaweed is generally low-cost and low-risk.

Can Seaweed Products Help Mitigate Climate Change? 

While the ability of natural seaweed stands to sequester carbon will always be constrained because seaweed stands need shallow water habitats, seaweed farming has some potential for climate change mitigation at scale because we can grow seaweed anywhere in the ocean where there is suitable light and nutrients, and where it makes economic sense (distance to ports, conditions suitable for farm infrastructure, etc.). There is a lot of room for seaweed farming to grow and be profitable, estimated to be an area roughly the size of Egypt and about 150 times the current area of seaweed farming.  

Intentionally sinking seaweed grown in farms has been proposed as a way to both sequester more of the carbon absorbed by seaweed and make it easier to quantify that carbon, thereby making carbon credits more feasible. But the value of seaweed goes beyond its ability to capture carbon, and sinking seaweed means that seaweed can’t be transformed into food or other products, generate profits or provide ecosystem services. Furthermore, multiple risks are involved in transporting lots of organic carbon to the deep sea that quickly — for example, decomposition could further deplete oxygen levels in deep waters that are already quite low in oxygen. Moreover, sinking seaweed could turn out to be rather costly.  

Various types of seaweed products would likely increase the climate change mitigation benefits of seaweed farming, but each comes with constraints on the ability to scale them. Construction materials made from seaweed can store carbon, though scaling is constrained by concerns around functionality and price. Biofuels and bioplastics could reduce greenhouse gas emissions but are constrained by price competitiveness. Feed supplements could reduce emissions of methane (a powerful greenhouse gas), but more research is needed to quantify this and measure health and safety impacts. 

Each of these options is not without its issues, and we can’t know the magnitude of their climate mitigation benefits for certain because making, using and disposing of them will entail some greenhouse gas emissions. But while the full spectrum of environmental impacts associated with these products is uncertain, these issues are under active investigation. 

Next Steps  

Some kinds of seaweed products could take care of a significant amount of carbon; understanding how – and how much – is vital. Although seaweed alone cannot save the planet, it can probably help. At EDF, we think that even though the climate mitigation benefits remain uncertain, expanding well-managed seaweed farming is a low-regrets strategy that would generate ecological and social benefits at a profit. Profitability has proven to be one of the best ways to scale anything, and scale is what we need when it comes to solutions to the climate change crisis.