Climate Science and Policy

Blue carbon ecosystems (BCEs) are coastal vegetated habitats with a natural capacity to sequester and store vast amounts of carbon over long timescales. Given rising greenhouse gas concentrations and the subsequent increase in global efforts to reduce and offset emissions, BCEs are increasingly promoted as nature-based climate solutions. Their relevance to climate policy as a tool for climate change mitigation has garnered increasing attention, but heavily depends on ecosystem-specific evidence for carbon storage, sequestration permanence, monitoring feasibility, and governance compatibility. BCEs are also highly valued for providing many ecosystem services, biodiversity, and coastal resilience benefits. This study evaluates two prominent Norwegian BCEs: seagrass (Zostera marina) and macroalgal systems, primarily kelp forests and rockweed beds, as candidates for blue carbon policy and broader marine nature-based solutions. The analysis combines site-to-national carbon and ecosystem-service databases for seagrass and macroalgae, uncertainty analysis, spatial co-location screening from interactive mapping, and policy evaluation. Seagrass occupies a comparatively small national area (modeled extent ≈ 90 km²) but is directly compatible with sediment-based blue carbon accounting. Its sediment carbon density varies strongly among regions — low in the Barents Sea, much higher in Oslofjord and Skagerrak — reflecting the influence of differing environmental controls such as sediment dry bulk density, hydrodynamic exposure, and water column depth. Macroalgae dominate Norway's vegetated coastal area (kelp ≈ 7,417 km², rockweed ≈ 3,090 km²) and hold far larger estimated biomass-carbon pools and annual production fluxes; however, their carbon is living biomass, so any long-term climate benefit depends on export, deposition, and burial outside the production habitat, creating significant attribution and permanence uncertainties for carbon accounting. These differences imply different policy pathways. Norwegian seagrass is conceptually compatible with the Intergovernmental Panel of Climate Change (IPCC) Wetlands Supplement and could progress toward lower-tier inventory treatment as mapping, disturbance, and burial-rate data improve; its strongest near-term policy argument is the avoided degradation of existing sediment carbon. Macroalgae are better treated as an emerging blue carbon frontier and as high-priority biodiversity, fisheries, and coastal-resilience assets, entering policy through nature-based adaptation and co-benefits frameworks while export-driven accounting methods mature. The most defensible near-term pathway is therefore differentiated by habitat type: seagrass can begin IPCC Tier 1 accounting while developing capacity toward Tier 2 and Tier 3 readiness, whereas macroalgae should first be embedded into biodiversity and co-benefits frameworks while macroalgal carbon-fate research advances. Both ecosystems should be protected, monitored, and included into Nationally Determined Contributions immediately, with the differentiated pathway also phased so that each ecosystem is integrated further into formal carbon accounting and blue carbon governance as the underlying science matures.