Project Emissions. Emissions in the project scenario are attributed to carbon stock changes in biomass carbon pools, soil processes, or a combination of these factors. In addition, emissions from organic soil burns and fossil fuel use may be quantified where relevant. The long-term average carbon stock must be calculated for the baseline and the project scenario. For projects undertaking even-aged management, the time period n, over which the long-term average GHG benefit is calculated, involves at minimum one full harvest/cutting cycle, including the last harvest/cut in the cycle. For projects under conservation easements with no intention to harvest after the project crediting period (which must be shown in the project description based on verifiable information), or in case of selective cutting, the time period n, over which the long-term average is calculated, is the length of the project crediting period. Measurements of carbon burial rates show high site-specific variability, which are strongly affected by a wide range of environmental factors for mangroves (Adame et al., 2017; Schile et al., 2017), seagrasses (Lavery et al., 2013), and salt marshes (Kelleway et al., 2017b). The reliable determination of sediment accumulation rates is a key consideration, with associated uncertainties only partially reflected in the McLeod et al. (2011) estimates presented above. Geochemical-based studies have indicated that seagrass carbon burial may have been markedly overestimated (Johannessen and Macdonald, 2016). These issues are contentious (Johannessen and Macdonald, 2018a; Johannessen and Macdonald, 2018b; Macreadie et al., 2018; Oreska et al., 2018); their scientific resolution is highly desirable. Lateral transfers are ineffectively quantified. Some of the carbon stored in coastal marine sediments may be recalcitrant carbon from terrestrial or atmospheric sources (and should therefore be excluded) (Chew and Gallagher, 2018), but export of DOC, inorganic carbon, and alkalinity may be considered as additional sequestration (Maher et al., 2018; Santos et al., 2019). Even if well-protected, the permanence of vegetated coastal systems cannot be assumed under future temperature regimes (Ward et al., 2016; Duke et al., 2017; Jennerjahn et al., 2017; Nowicki et al., 2017). Responses to future SLR are also uncertain and complex (Spencer et al., 2016). However, impacts are not necessarily negative: carbon sequestration capacity may increase where totally new habitats are created (Barne...
