Adam Reilly , Nathaniel Merrill , Kate Mulvaney, Phil Colarusso, Erin Burman

Salt marsh restoration has the potential to reduce greenhouse gas emissions thereby providing an opportunity for blue carbon crediting, but implementation has been limited to date because of insufficient data and validation. Freshening salt marshes are sources of methane emissions, which present an opportunity for states to address this source of potent greenhouse gas emissions if restored to their more naturally saline state. In this paper, we demonstrate the potential scale of methane emissions that could be avoided if salinity-reducing impairments are mitigated by applying findings from six salt marsh restoration sites in Massachusetts combined with a meta-analysis of the salt marsh salinity-methane relationship. Of the six sites selected, restorations at two sites were successful in improving salinity. Our approach and findings demonstrate the potential benefits in developing consistent accounting methodologies to better track, prioritize, and implement wetlands restoration strategies to mitigate methane emissions and contribute toward state-level emissions reduction targets across at least a percentage of the 475 Massachusetts salt marches with an existing tidal restriction. A significant limitation in estimating benefits, however, is the lack of coordinated, widespread monitoring strategies relevant to methane emissions. While not insurmountable, these challenges will need to be addressed for methane emissions reduction and carbon sequestration through salt marsh restoration to be accepted as an effective strategy. We conclude that while carbon crediting may offer benefits to marsh restoration and state greenhouse gas emissions reduction targets, there remain significant limitations because of a lack of project monitoring and data validation, which could undermine the social benefits that carbon crediting would afford. In the worst case, this could result in the offsetting of actual greenhouse gas emissions with credits that are supported by indirect and less-than-rigorous monitoring data.