September 1, 2021
Coastal and marine ecosystems play a significant role in the global carbon cycle, sequestering and storing carbon over long timescales. These "blue carbon" ecosystems help mitigate climate change and its impacts by facilitating the uptake of atmospheric carbon dioxide (CO2) into the ocean and transporting carbon into sediments or deep waters where it can remain indefinitely if undisturbed. Inclusion of these coastal and ocean processes as part of the solution to global climate change is essential in achieving global carbon mitigation and emission reduction goals; however, blue carbon is often overlooked in climate mitigation policies. Further, resource managers of the largest network of U.S. marine protected areas (MPAs), the Office of National Marine Sanctuaries, have not incorporated assessments of blue carbon extent and functionality into their management plans, policies, or decisions, which can result in unintentional carbon emissions and lost opportunities to further protect and enhance carbon sequestration in MPAs. Though blue carbon is a rapidly growing area of research, guidance for how to apply blue carbon information in MPA management is lacking, and for some sequestration processes, completely absent. As requested by Greater Farallones National Marine Sanctuary (GFNMS) in response to Part 1 of this series, the Greater Farallones Association conducted a blue carbon assessment for the sanctuary. This is the first assessment of multiple blue carbon sequestration processes in a U.S. federal MPA, with the primary purpose of informing one of the nation's largest MPAs in its management decision-making. The carbon storage and annual sequestration for two coastal blue carbon habitats, seagrass and salt marsh, and two oceanic carbon sequestration processes, kelp export and dead whale falls, were assessed within the boundaries of the sanctuary using regional and site-specific data. These processes have the potential to sequester 4,950 megagrams of carbon (MgC) each year (or 18,150 metric tons CO2 equivalent), which is valued at $925,650 in societal benefit annually and is 140 times the amount of CO2 that is emitted from annual site operations. Whale falls account for roughly 60% of this annual sequestration; salt marsh, seagrass, and kelp account for roughly equal parts of the remaining 40%, though annual sequestration by the region's kelp forests have declined by 99.7% from 2008 to 2019. Sanctuary coastal blue carbon habitats currently hold approximately 175,000 MgC in their sediments, which, if destroyed, could release approximately 643,000 metric tons of CO2, or the equivalent of adding 140,000 vehicles to the road for one year. Understanding carbon sequestration within national marine sanctuaries is key for managing changes to stored carbon, which has national and global climate relevance. While these estimates are an incomplete characterization of carbon services provided by GFNMS, this report nonetheless serves as a preliminary step in guiding sanctuary management to protect and enhance the critical climate mitigation services of its coast and ocean resources.