A comprehensive field and modeling study indicates vulnerability of tidal wetlands to sea-level rise on the U.S. Pacific coast.

Tidal wetlands produce long-term soil organic carbon (C) stocks. Thus for carbon accounting purposes, we need accurate and precise information on the magnitude and spatial distribution of those stocks. We assembled and analyzed an unprecedented soil core dataset, and tested three strategies for mapping carbon stocks: applying the average value from the synthesis to mapped tidal wetlands, applying models fit using empirical data and applied using soil, vegetation and salinity maps, and relying on independently generated soil carbon maps. Soil carbon stocks were far lower on average and varied less spatially and with depth than stocks calculated from available soils maps. Further, variation in carbon density was not well-predicted based on climate, salinity, vegetation, or soil classes. Instead, the assembled dataset showed that carbon density across the conterminous united states (CONUS) was normally distributed, with a predictable range of observations. We identified the simplest strategy, applying mean carbon density (27.0 kg C m−3), as the best performing strategy, and conservatively estimated that the top meter of CONUS tidal wetland soil contains 0.72 petagrams C. This strategy could provide standardization in CONUS tidal carbon accounting until such a time as modeling and mapping advancements can quantitatively improve accuracy and precision.

 We quantified capacity of tidal wetlands to accrete relative to sea-level rise or migrate inland, using localized sea-level rise scenarios.• More northerly wetlands have more capacity for relative vertical accretion, and more southerly wetlands have more lateral migration space.• Between 43% and 48% of current coastal wetland area is vulnerable to vertical accretion deficit and lack of capacity for inland migration.

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