Additionality in blue carbon ecosystems: recommendations for a universally applicable accounting methodology

Blue carbon ecosystems (BCEs) remove carbon dioxide from the atmosphere and store significant amounts of organic carbon(OC) in their soils. Consequently, the protection and restoration of BCEs may contribute to net greenhouse gas emissions abate-ment and help address the global challenges of both mitigating and adapting to climate change. An ongoing debate is whetherOC sequestered out with the blue carbon (BC) project and transported to its present location (allochthonous) should be countedas ‘additional’. There are inconsistencies in the treatment of allochthonous carbon between BCE methodologies, potentiallyundermining the credibility of global BC accounting initiatives. To explore these inconsistences, we compare the methodologieswhich we were able to find online, with particular focus on the VERRA, IPCC and BlueCAM methodologies, and review thescience underlying any approach to account for allochthonous OC. Our findings indicate that there are currently no robust sci-entific approaches to define an appropriate apportioning of allochthonous OC for discounting in the calculation of additionality.We therefore advocate for the inclusion of allochthonous OC in BC crediting projects when an observational and experimentalapproach does not support the calculation (and discounting) of the refractory allochthonous carbon contribution.