Key Finding 1
The assessment of terrestrial wetland carbon stocks has improved greatly since the First State of the Carbon Cycle Report (CCSP 2007) because of recent national inventories and the development of a U.S. soils database. Terrestrial wetlands in North America encompass an estimated 2.2 million km2, which constitutes about 37% of the global wetland area, with a soil and vegetation carbon pool of about 161 petagrams of carbon that represents approximately 36% of global wetland carbon stock. Forested wetlands compose 55% of the total terrestrial wetland area, with the vast majority occurring in Canada. Organic soil wetlands or peatlands contain 58% of the total terrestrial wetland area and 80% of the carbon (high confidence, likely).
Description of evidence base
Key Finding 1 is supported by an extensive analysis of the most current wetland soil and vegetation information available across the conterminous United States (CONUS), Alaska, Hawai’i, Puerto Rico, Canada, and Mexico, updating previous estimates made in SOCCR1 (see SOCCR2 Appendices 13A and 13B).
Major uncertainties
Uncertainties are high where wetlands are present but not extensively mapped, such as in Alaska.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Over much of the area under consideration, confidence is high that this assessment has accurately mapped carbon pools in mineral soil wetlands and peatlands.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Understanding current carbon pools is critical in predicting how changes in, for example, climate, land use, and restoration will affect the carbon stored in terrestrial wetlands.
Summary sentence or paragraph that integrates the above information
Terrestrial wetlands are the largest reservoir of carbon in North America. Understanding the processes that lead to carbon storage and fluxes is important to predict how future changes will influence this large carbon pool and subsequent feedbacks to the atmosphere.
Key Finding 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 as CH4 (CH4 -C) per year (medium confidence, likely).
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).
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 and agreement, including short description of nature of evidence and level of agreement
Confidence is medium, given both the incompleteness in mapping and variability in flux rates.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Greenhouse gas fluxes from terrestrial wetlands in North America contribute to the global CO2 and CH4 cycles and associated climate forcing.
Summary sentence or paragraph that integrates the above information
Understanding both terrestrial wetland carbon pools (Key Finding 1) and net fluxes to the atmosphere (Key Finding 2) is critical because these wetlands are stable long-term carbon sinks and also an important source of CH4.
Key Finding 3
The current rate of terrestrial wetland loss is much less than historical rates (about 0.06% of the wetland area from 2004 to 2009) with restoration and creation nearly offsetting losses of natural wetlands. Although area losses are nearly offset, there is considerable uncertainty about the functional equivalence of disturbed, created, and restored wetlands when comparing them to undisturbed natural wetlands. Correspondingly, there remains considerable uncertainty about the effects of disturbance regimes on carbon stocks and greenhouse gas (GHG) fluxes. For this reason, studies and monitoring systems are needed that compare carbon pools, rates of carbon accumulation, and GHG fluxes across disturbance gradients, including restored and created wetlands. Those studies will produce data that are needed for model applications (high confidence, likely).
Description of evidence base
The evidence for Key Finding 3 is from updated published literature for the United States and Mexico (Casasola 2008; Landgrave and Moreno-Casasola 2012; USFWS 2011) and the same data reported in SOCCR1 (CCSP 2007) for Canada. The amount of wetlands being restored is also a function of recent literature estimates (e.g., Dahl 2011). Disturbance also needs to be considered in the context of changes to carbon cycling processes.
Major uncertainties
Where wetlands are mapped well, the area of wetland loss is very certain. Some areas not mapped well, such as remote locations in Alaska, generally are not under threat from development, but changes in climatic conditions threatened the boreal region more than temperate and tropical regions. However, the opposite is true for areas under development in Mexico. The amount of area being restored is also not tracked very well, especially when restoration fails. Crossing the gradient from disturbed to restored and/or created wetlands, there exists considerable uncertainty about the level of functions that those wetlands provide.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is high confidence that systems for reporting wetland losses and gains are accurate in the United States, but periodic inventories in other countries are lacking. Also, tracking the amount of wetlands that have been disturbed in some way is very difficult.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Although the area of restored or created wetlands is small relative to the total wetland area of North America, the impact is likely important because understanding even small changes in wetland area is critical to scaling up carbon pools and fluxes.
Summary sentence or paragraph that integrates the above information
Although there are very reliable data that track wetland change across CONUS, no such data are available for Canada because regular wetland assessments for that country are lacking. In addition, field-based wetland mapping is generally poor in Alaska and Mexico, and restored and disturbed wetland areas also are difficult to track.
