finding 13.2 : key-message-13-2

North American terrestrial wetlands currently are a carbon dioxide sink of about 123 teragrams of carbon (Tg C) per year, with approximately 53% occurring in forested systems. However, North American terrestrial wetlands are a natural source of methane (CH4), with mineral soil wetlands emitting 56% of the estimated total of 45 Tg C as CH4 (CH4 –C) per year (medium confidence, likely).

This finding is from chapter 13 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base: Key Finding 2 and this chapter’s narrative are based on the most recently reported wetland inventories integrated with reported values of soil carbon density (mass per unit area) and gaseous fluxes of carbon dioxide (CO2) and CH4. Accordingly, the projections are dependent on estimates of wetland area and the pool and flux values assigned to the wetland types (see Appendices 13A and 13B).

New information and remaining uncertainties: Major uncertainties Similar to Key Finding 1, one major uncertainty is the mapped area, especially in areas with considerable wetlands that have not been adequately mapped. A second important uncertainty are the flux rates, which are applied globally to wetland types but are highly variable in time and space. Moreover, in many cases, few data exist.

Assessment of confidence based on evidence: Confidence is medium, given both the incompleteness in mapping and variability in flux rates.

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

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