Can seaweed really help fight global warming? Here’s what we know so far.

Kelp in the ocean

We’re seeing the impacts of climate change in real time. There is an urgent need to reduce greenhouse gases (GHGs) in the atmosphere to avoid more catastrophic climate change impacts. This will require both reductions in GHG emissions and the removal of GHGs from the atmosphere. In recent years, this need has driven interest in seaweed as a natural climate solution, as seaweed can absorb carbon and grow quickly.   

But it’s not that simple. Seaweed mostly absorbs carbon dissolved in seawater – not atmospheric carbon. That means conditions have to be right to result in climate change benefits. Carbon sequestration by seaweed is variable and difficult to measure because of the complexity of ocean ecosystems and seaweed carbon processing. There is still a lot we don’t know.  

Last year, EDF convened experts to identify knowledge gaps that need to be filled so that seaweed carbon sequestration can be accurately measured. These findings are summarized in our recent paper. Now we are filling these knowledge gaps by carrying out research on our demonstration seaweed farm in the Philippines.  

A major uncertainty revolves around the effects of products made from seaweed on GHG levels in the atmosphere. Most seaweed is used to make food and colloids (the gelatinous stuff in seaweed that is used in countless products to make them smooth and creamy). These do not sequester much carbon because they have short life spans, returning carbon to the atmosphere after they are used. But food and colloids are not the only products that can be made from seaweed.   

Our new report, ‘Seaweed Product Analysis: Process Maps for Seaweed Products with the Potential to Mitigate Climate Change’ comprehensively examines the products made from seaweed and assesses their potential to store carbon and avoid GHG emissions. This report describes how these products are made and how they could contribute to climate change mitigation. It will serve as the basis for life cycle assessments aimed at quantifying the net impacts of these products after accounting for GHG emissions associated with producing the seaweed, manufacturing the products, using the products, and disposing of the products. This is part of our strategy aimed at increasing the climate change mitigation benefits of seaweed.

Biostimulants are soil additives mixed with other products and used to stimulate plant growth. Biostimulants made from seaweed have been observed to increase crop yield, which can allow farmers to use less fertilizer. If farmers substitute biostimulants for some fertilizer, this would eliminate the emissions of GHGs that would have been released by making and shipping the fertilizer. But, these climate mitigation benefits are still uncertain as they are dependent upon many variable factors, like climate conditions and farmer behavior. 

Seaweed can be turned into construction materials that store carbon. While this is a promising avenue for GHG reduction, there are uncertainties about the impacts of making, distributing, using and disposing of these products. Storing carbon in such products for decades has an uncertain impact on the climate system. Ideally, carbon would be stored for centuries or millennia.

Certain species of seaweed can reduce methane emissions by cattle when small amounts are included in their feed. As methane is a much more powerful GHG than carbon dioxide, this can have major positive climate change impacts. However, more research is needed to investigate whether feeding cattle seaweed has any health impacts and if it is a feasible long-term strategy for methane suppression. 

Most plastic is made using petroleum feedstocks, so replacing this plastic with seaweed bioplastic could result in GHG reductions. The market for plastics is very large and regulations aimed at reducing plastic waste are creating demand for low-carbon alternatives that do not degrade into microplastics. The most common uses of seaweed bioplastic are for food or clothing packaging, which is an enormous market. These market trends may increase demand for bioplastics made from seaweed. 

Similar to cattle feed, certain seaweed species appear to be capable of suppressing methane emissions from manure. This could be significant, as manure is a major source of methane. But there is lots of uncertainty for this pathway as very few studies exist.  

Many seaweeds can be made into natural gas and biodiesel. These products can result in avoided emissions to the extent they replace fossil fuels and other GHG-intensive sources of biofuels (e.g., corn grown with fertilizers). The market for seaweed-based fuel is in its infancy, and there are concerns over scaling this climate solution given costs relative to fossil fuels. 

Because seaweed can absorb heavy metals and accumulate them, it is possible to extract heavy metals, critical minerals, and rare earth elements from them. Some of these metals are key components of batteries and other aspects of the green economy. The alternative for collecting these metals comes from terrestrial or deep-sea mines, which are dependent on fossil fuels – not to mention other negative impacts like habitat destruction, labor abuses, and pollution. Seaweed heavy metal absorption is carbon neutral or positive, so avoided emissions could be high. The cost-effectiveness of extracting critical metals from seaweeds remains to be seen.

They already are, at least to some extent. Natural seaweed beds and seaweed farms may be sequestering a few hundred million tons of carbon dioxide each year via their natural processes. This natural sequestration process is hard to measure, making carbon financing for these solutions challenging. Capturing the carbon absorbed by seaweed by making the seaweed into products that can store that carbon, replace GHG-intensive products, or suppress GHG emissions can dramatically increase the climate mitigation benefits of natural seaweed beds and farms, and may result in more accurate quantification of these benefits.  

Several concerns and uncertainties must be resolved for this potential to be realized. This includes the effects of heavy metals absorbed by seaweed on the safety and efficacy of seaweed products, the impacts of seaweed-based cattle feeds on cattle health (and on the safety of milk and meat), and high costs of production and other factors that limit the scaling of these seaweed products.

Just as we are seeing the effects of climate change in real time, we are also seeing the rapid development of potential solutions. The seaweed solution is not as simple as it might appear, but we think it’s well worth exploring. 

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