GO-BC
Research Highlight:
Additionality in Blue Carbon Ecosystems: Recommendations for a Universally Applicable Accounting Methodology
GO-BC interviewed PhD student Alex Houston who gave us insights into his vision and take home messages from his latest publication which looked at ways in which allochthonous carbon is accounted for in various blue carbon methodologies. Read on below to discover what his vision was for the paper, what difficulties he encountered whilst writing this publication, and what he hopes to do following on from this research milestone and during the closing stages of his PhD.
Tell us about your research
I am a PhD student at the University of St Andrews, researching carbon cycling in blue carbon ecosystems (tidal marshes, seagrasses and mangroves). These ecosystems accumulate organic carbon as plants photosynthesise, drawing down atmospheric CO2 which ‘cools’ the climate. This in-situ process is known as ‘autochthonous’ carbon. Blue carbon ecosystems also accumulate organic carbon associated with sediments deposited during high tide. These sediments are ‘allochthonous’ as they originated from outside the habitat boundary – e.g., marine sediments and terrestrial soils.
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As the organic carbon accumulates in blue carbon ecosystems, a portion gets decomposed by microorganisms living in these soils, releasing greenhouse gases into the atmosphere and contributing to climate warming. Due to their waterlogged, low oxygen soils, rates of decomposition are slow, and some of the soil organic carbon can be stored for 100’s to 1000’s of years.
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When they are healthy, blue carbon ecosystems draw down more carbon than they emit – but when degraded the opposite is the case. My research focuses on understanding which types of organic carbon (autochthonous vs allochthonous) remain stored in blue carbon ecosystem soils long-term and which are decomposed and released more quickly. This research aims to provide insights into the role of these ecosystems in mitigating climate change.
Photograph: Kyle of Tongue saltmarsh, Scotland. this site is degrading at its edges, potentially releasing vast amounts of stored carbon back into the atmosphere.
Why is allochthonous carbon important?
The protection and restoration of blue carbon ecosystems could offset 3% of global emissions by 2030, according to some estimates. Due to the potential for climate mitigation, a growing number of Governments are including blue carbon ecosystems in their Nationally Determined Contributions to the 2016 Paris Agreement – their pathway to net-zero emissions. As well as this, multiple voluntary carbon market mechanisms have been developed to allow the purchasing of carbon credits by individuals and private organizations, to help them reach their emissions reductions targets.
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To claim any emissions reductions, it must be demonstrated that a given management intervention (e.g., restoration of a seagrass meadow) either draws down additional greenhouse gases from the atmosphere or reduces the release of stored carbon to the atmosphere. This is termed ‘additionality’ and it is crucial for ensuring that all claimed climate benefits are genuine.
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In some blue carbon methodologies, allochthonous carbon is discounted from additionality assessments as it was sequestered from the atmosphere elsewhere before being transported to the ecosystem – and therefore may have remained stored in a different environment. Other methodologies do not require the discounting of allochthonous carbon – arguing that due to the slow rates of organic carbon decomposition in blue carbon ecosystems, that these soils are a longer-term store than alternative environments.
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The amount of allochthonous carbon stored in blue carbon ecosystem soils ranges from 0-90% of the total organic carbon and depends on factors such as the environment surrounding the site – those in estuaries with a large tidal range tend to accumulate a greater amount of allochthonous carbon than those with a lesser tidal range. It is therefore crucial to determine whether allochthonous carbon should be included in or discounted from blue carbon methodologies.
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If allochthonous carbon is not additional, its inclusion risks overstating the climate mitigation service provided by blue carbon projects and accounting for non-genuine emissions abatement. If allochthonous carbon is additional, then its exclusion risks unnecessarily reducing the amount of viable blue carbon projects, as the amount of carbon credits awarded is a financial constraint for implementing many projects.
Photograph: Salt ponds in Gujarat, India. Saltmarshes and mangroves in Gujarat have been extensively converted to ponds in which salt is extracted from for use in the oil and chemical industries. The degradation of saltmarshes and mangroves can be cause the loss of vast amounts of stored carbon to the atmosphere, so their protection can help avoid these emissions.
What was the vision of this paper?
The idea for this paper came about during a GO-BC workshop on the state of blue carbon knowledge at Ahmedabad University, India. During a long bus ride to visit a mangrove restoration site in the Gulf of Kachchh, Hilary and I were discussing how allochthonous carbon gets accounted for in different blue carbon methodologies and we realised that there isn’t a consistent treatment of allochthonous carbon across methodologies, risking undermining carbon accounting credibility. When we got back to the UK we decided to write this opinion article together with Bill.
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We reviewed the scientific evidence for the treatment of allochthonous carbon in a number of blue carbon methodologies we were able to access online. We found that due to a lack of empirical data, there is no scientific basis for the blanket inclusion or exclusion of allochthonous carbon from assessments of additionality. However, we did find that certain methodologies were estimating the amount of allochthonous carbon using mathematical approaches based on inappropriate default factors, which if applied globally would result in > 25% of mangroves, > 30% of tidal marshes and > 50% of seagrasses being deemed to contribute no ‘additional’ soil carbon and therefore no climate mitigation benefit – this is unlikely to be true. We argue that observational and experimental approaches should be used to determine the ‘additionality’ of allochthonous carbon on a case-by-case basis. When this is not feasible, we argue that allochthonous carbon should be included in these assessments.
Photograph: GO-BC research team learning about mangrove restoration sites in the Gulf of Kachchh, India. This site was restored about 10 years ago and is managed by local fishermen who benefit from improved fisheries.
What are the three take home messages?
1. Allochthonous carbon is discounted from some blue carbon methodologies but included in others, risking undermining the credibility of these methodologies
2. Blue Carbon ecosystems can provide a stable "home" for allochthonous carbon - their degradation threatens this.
3. Deductions of allochthonous carbon from Blue Carbon projects should be based on an observational and experimental approach.
What's next for your PhD?
I would like to continue working on observational and experimental approaches towards determining the additionality of allochthonous carbon in blue carbon ecosystems. I am interested in different types of disturbances which may destabilise previously stable carbon, and whether blue carbon ecosystems are better stores of carbon than alternative environments (like mudflats), or not. However, I am in my final year of my PhD, so probably back to writing up!
Photograph: Alex finding his way through a restored mangrove in the Gulf of Kachchh, India. Alex is holding the EGM-5 which he uses to measure the CO2 fluxes from these types of sites to help assess the amount of carbon that mangroves are drawing down from, and emitting bck into, the atmosphere.
Where can you find this paper?
The paper is published fully open access in the Global Change Biology Journal - see below: