finding 15.4 : key-message-15-4

In North America, tidal wetlands remove 27 ± 13 Tg C per year from the atmosphere, estuaries outgas 10 ± 10 Tg C per year to the atmosphere, and the net uptake by the combined wetland-estuary system is 17 ± 16 Tg C per year (low confidence, likely).

This finding is from chapter 15 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base: The uptake of atmospheric carbon dioxide (CO2) by tidal wetlands is assessed for Key Finding 4 by net ecosystem exchange (NEE) estimates from eddy covariance measurements. It is similar to an alternative estimate of uptake that assumes uptake as the sum of burial (8 Tg C) and lateral export (16 Tg C). Burial and lateral exports are discussed in the supporting evidence for Key Findings 2 and 3. Estuarine outgassing is based on studies of individual estuary summaries (Chen et al., 2013) and estuarine areas (Laruelle et al., 2013). The flux of the combined system is a simple sum of the fluxes from tidal wetlands and estuaries and compounded error.

New information and remaining uncertainties: The major uncertainties in this Key Finding are the limited spatial and temporal extents of tidal wetland atmospheric flux measurements, burial, lateral flux, and estuarine outgassing measurements. Estuarine outgassing uncertainties also stem from the low spatial resolution of the datasets used to estimate areas.

Assessment of confidence based on evidence: There is low confidence on this calculation at the scale of North America. The low confidence is due to the residual between competing fluxes; on the one hand, there is strong likelihood that tidal wetlands take up CO2 from the atmosphere and estuaries outgas CO2 to the atmosphere and, on the other hand, that there is large uncertainty in the magnitude of each, assessments which stem from the high spatial and temporal variability of these systems and the limited field data. The fate of carbon released from tidal wetland degradation remains unknown.

This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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