Potential of many marine CDR methods is overstated
Seaweeds seem to be the answer to everything climate these days. Food security, alternate proteins, carbon removal- the answer inevitably lies in growing tons of seaweed. They are great, in general, for the ocean ecosystem but like planting trees, their carbon removal impact (and many other ocean restoration methods) is overstated.
Blue carbon refers to carbon stored in marine and coastal ecosystems such as mangroves, seagrass meadows and salt marshes. While they store huge amounts of carbon and keeping them safe from harm is essential, they are not a viable set of carbon removal solutions.
Biodiversity growth and ecosystem preservation is often conflated with carbon dioxide removal. These two are correlated but they are not the same thing. A successful project for either one should have a clear goal in mind and should not mistake increased whale population, for instance, for tons stored. Climate action, like other things, should be a quest for truth and a well defined goal means we are not deluding ourselves.
Solutions such as mangrove restoration and sea weed farming are hard to scale to make their CDR meaningful. Carbon Plan has a good primer highlighting these issues.1 Carbon storage of blue carbon ecosystems (which is 20-30 times than terrestrial forests), is confused with their CDR potential. CDR potential of mangroves, seagrass meadows and salt marshes is limited and scaling up would come in direct conflict with human settlements and aquaculture.
Carbon accounting for blue carbon solutions is uncertain and challenging. The carbon sequestration estimates of seagrasses are biased high by up to three levels of magnitude.2 Other uncertainties include methane and nitrous oxide emissions and carbonate precipitation.3
Dollars spent on blue carbon restoration methods present an opportunity cost that could be spent on methods that are scalable and durable.4 The carbon offset market based on blue carbon solutions can lead to interventions that falsely claim to be ‘net zero’ (aka. greenwashing). Nevertheless, it is imperative to understand the benefits of blue carbon ecosystems, such as coastal protection and improved biodiversity, which are important goals in of themselves.
Sophia C Johannessen and Robie W Macdonald 2016 Environ. Res. Lett. 11 113001
Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666
P W Boyd et al 2024 Environ. Res. Lett. 19 061002