Why ‘Blue Carbon’ Will Not Be the Panacea for Climate Change

Coastal ecosystems can play a pivotal role for climate mitigation, with the term ‘blue carbon’ coined to increase attention to the topic in both scientific and public media. Soon carbon credits markets (a way of compensating for emissions of carbon dioxide) started to expand beyond terrestrial reforestation with methodologies emerging to cover coastal and marine habitats, opening funding opportunities for their conservation and restoration. This resulted in carbon-focussed restoration studies of coastal vegetated ecosystems increasing exponentially in the last decade. The risk with mainstreaming, however, is the misuse of some terms and oversimplifications, that can become an issue if one wants to ensure proper accounting for real climate mitigation and avoid ‘greenwashing’. Very often carbon ‘sequestration’, the only value relevant for climate mitigation and to issue carbon credits, is used interchangeably with ‘storage’, ‘stock’ and ‘sink’, but they have different meanings. Carbon sequestration refers to the long-term (at least 100 years) storage, whereas ‘storage’ can be temporary. ‘Sink’ is the carbon storage process (the opposite of ‘source’ which is the release in the atmosphere), while ‘stock’ refers to the total amount of carbon sequestered and stored in a given space in a specific moment in time.

Currently only vegetated coastal ecosystems (mangroves, saltmarshes and seagrass beds) are fully considered in the carbon markets. The basic reasoning behind this is that during the day plants act as a sink of carbon: through photosynthesis they incorporate part of the carbon in their tissues becoming themselves a temporary storage. Only when the plant dies, and the dead and decaying material becomes incorporated in the soil, some of the carbon will remain locked over a longterm: only this remaining amount should be considered ‘sequestered’. It becomes clear that sequestration is only a small portion of the initial carbon sink. To quantify this, models can become useful but they are not magic and are only usable if information on the surrounding environmental variables are available: temperature for example is needed to quantify decomposition rates of the dead biomass, the speed of current to quantify the entity of dispersal and the soil type to quantify the amount that will remain buried. These are only examples. Every piece of new research shows new processes that may come into play to influence these values. For example; the age of the ecosystems, the species and diversity of vegetated species that are present, the animal species that are associated with the habitat which may influence the photosynthetic activity, consuming the biomass or changing the dechanging the decomposition and burial rates of the dead material. It is important to understand that natural ecosystems can vary largely in time and space and therefore all of the considerations made are very context dependent and site-specific, requiring extensive surveys to enable some accurate accounting beyond initial modelling, before credits can be issued for offsetting.

Some scientists also argue that it is time to broaden what is considered a blue carbon ecosystem (BCE) to allow for more climate mitigation options: kelp forests and macroalgal beds for example are seemingly very close to the above-mentioned vegetated ones, as seaweeds also photosynthesise. However, they miss a large storage component, represented by the roots and rhizomes of the plant and their absence also makes sediment less stable, hindering long-term sequestration as the organic material will be less likely to stay in place. Macroalgae are also eaten at a faster rate than plants by herbivores, have higher rates of breakage and tend to live in areas of faster currents, meaning that the dead material will be dispersed in higher amounts resulting in less long term-sequestration. Many processes may occur over larger scales but most accounting estimates are made at the level of single ecosystem and site. Other suggestions to expand the marine ecosystems that could be considered for climate mitigation include calcifying species, such as shellfish. Oysters, for example, were presented in public medias as ‘unsung heroes in a changing climate’ however scientists are divided on the matter. Some argue that the carbon contained in the shell (made of calcium carbonate, CaCO ) should be considered as carbon stored and when the shell is buried in the sediment (e.g. underneath live shellfish in an oyster reef) is then sequestered longterm. Other contend that shell formation is in itself a carbon releasing reaction that also lowers the capacity of the ocean to buffer atmospheric carbon. The latter idea is now becoming increasingly accepted by a wider scientific community. However, the entity of the carbon release depends on many environmental variables (temperature, salinity, pH…), and using models can help in the choice of optimal sites for shellfish aquaculture which can be the least impactful from a carbon standpoint. Furthermore, for restoration of shellfish reefs, it is important to recognise the other ways, beyond shell formation, of potential carbon sequestration such as the accumulation of carbon-rich sediment within the complex 3D reef, given by the bio-deposition of carbon rich bio-deposits of the shellfish and associated fauna, and the accumulation and trapping of carbon rich detritus and sediments from outside of the reef. In terms of ecosystems that both calcify and photosynthesise (e.g. coral reefs, coralline algae such as maerl…), some carbon recycling may happen where the release from calcification is reused in photosynthesis and a balance between the two processes may result in a sink, but more studies are necessary to understand the implications of their restoration for climate mitigation purposes.

To conclude, it is evident that currently too much remains unknown and understudied to include restoration of BCE and potential BCE in climate mitigation strategies. The multiple assumptions and omissions that still persist in methodologies for blue carbon accounting make some of the current claims such as ‘Blue carbon ecosystems can store up to 10 times more carbon than terrestrial forests’ imprecise and incorrect. More research is needed to enable a mechanistic understanding, improve models and enable the inclusion of these natural habitats in climate mitigation plans.

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As published in Issue 2: United in Discovery. Click any page to view it full size.

First published in Issue 2: United in Discovery, the Innately Science newspaper.