It’s Time to Uncover the Mysteries of Blue Carbon

By Rod Fujita and Kristin Kleisner 

To mitigate climate change, sea level rise, and other environmental problems, some experts are looking to nature-based solutions. But which solutions are the most impactful when it comes to sequestering carbon? And what do we still need to learn to improve our scientific understanding of nature-based solutions?  

We co-authored a recent peer-reviewed study that assessed the scientific foundations of blue carbon and other nature-based solution pathways. We found that our scientific understanding of the benefits of restoration and conservation of terrestrial forests is much stronger than our understanding of the benefits of blue carbon, or carbon sequestered by the ocean. Much like the ocean itself, there are still many mysteries left to uncover about blue carbon. 

These findings are not a reason to stop conserving or restoring blue carbon systems like mangrove forests, salt marshes, seagrass meadows, and seaweed beds. Instead, we view this as a call to double down on research that helps address key gaps around blue carbon pathways. 

The study points out that blue carbon efforts are worthy of implementation because of the many social and ecological benefits they provide–even if climate mitigation benefits are less certain at this point. 

Removing Carbon or Avoiding Emissions? 

The study examined the strength of the science of quantifying carbon dioxide (CO2) removal or avoided greenhouse gas (GHG) emissions, looking at each pathway. The results indicate that we can be more confident that mangrove conservation will aid in avoiding large CO2 emissions by storing CO2 in these forests than we can be that mangrove restoration could remove large amounts of CO2 from the atmosphere. However, the study concluded that both pathways still need more research.   

Why is this the case? First, while mangroves sequester carbon rapidly per unit area, mangrove restoration can only take place in nearshore waters where the climate and other factors are conducive to mangrove growth. This limits the geographic scope of efforts to restore mangroves and, therefore, the scale of this pathway.  

Further, under some conditions mangrove systems can release methane, a powerful GHG, which works against the carbon sequestration benefits. Like mangroves, seagrasses and emergent marshes can sequester carbon rapidly and are also constrained by their need for suitable nearshore habitat.  

The durability, or duration that carbon is stored, of all blue carbon sinks is uncertain as climate impacts, such as sea level rise, warming and storms, intensify and become more frequent.  

Experts noted that seaweed farming has the potential to draw down moderate amounts of CO2, but confidence was low due to major uncertainties about how much of the carbon that seaweeds absorb gets sequestered over the long term. This is primarily because, even though seaweeds can absorb carbon very rapidly and farming is less constrained by habitat requirements, marine food webs convert some of that carbon back to CO2. Also, the carbon in most of the products made from seaweed (like foods and colloids) returns to the atmosphere relatively quickly. 

Getting the Science Ready for the Market 

The science behind many blue carbon pathways must be strengthened if they are to be incorporated into carbon markets. Although the uncertainty of the science on GHG mitigation through mangrove conservation and restoration is relatively low, scientists do not yet have the confidence in these pathways that they have for temperate and tropical forest conservation and restoration.  

The research needed to bring more certainty to mangroves’ blue carbon pathways should focus on quantifying emissions of other powerful GHGs, like methane and nitrous oxide, which can counteract carbon sequestration by these systems. In other words, we need to better understand the GHG budget, which can vary in different mangrove systems, depending on other factors like how degraded the mangrove system is by pollution or coastal development. A better understanding of the durability of carbon storage by these systems is also necessary. Environmental Defense Fund is working with partners to do just that for a mangrove restoration project in Ecuador.  

EDF is also reducing uncertainty around carbon sequestration through seaweed farming. Studies suggest that the area of the ocean where seaweed farming could be profitable and feasible far exceeds the current footprint of seaweed farming. Additionally, in contrast to seaweed-based products like food and colloids, there are products that could be developed, like construction materials, that store carbon or replace carbon-intensive products like fossil fuels and plastic. It may also be possible to make seaweed products that suppress GHG emissions, for example, by producing supplements and additives that suppress methane generation by cows or from manure pits.  

However, most of these concepts need more research to understand their potential to sequester or offset GHGs and to understand pathways to scaling and commercial viability, if appropriate. Further development will require a clear understanding of the quantity of GHG emissions associated with farming, processing, and transporting seaweed as well as on the manufacturing, transport, and disposition of the seaweed products. We are collecting data at EDF’s demonstration seaweed farm in the Philippines to start to answer these questions. You can learn more from our blog post covering what we know about seaweed’s role in fighting climate change. 

Blue Carbon Systems Benefit People and Nature 

It’s clear that blue carbon science must be strengthened to understand whether blue carbon pathways are removing as much carbon as we think they are and to develop financing pathways if those estimates indicate that net GHG mitigation occurs. But it’s also clear that blue carbon systems produce multiple, well-documented benefits to people and nature, so the protection of these systems is critical.   

Mangroves and seagrasses should be conserved and restored not only because they may sequester carbon but also because they provide critical habitats for fisheries, provide energy and nutrients for marine food webs, and support marine biodiversity. Sustainable seaweed farming should be supported because it can generate livelihoods and revenue for frontline communities suffering the most from climate change impacts and can also reduce nutrient pollution and alleviate ocean acidification. These many benefits justify increased investment in conserving and restoring blue carbon systems as well as in expanding sustainable seaweed farming.

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