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150,000 km² of freshwater peatlands are located less than 5 m in elevation (Henman & Poulter, 2008), highlighting the need to understand the impacts on carbon pools and fluxes. In a coastal freshwater peatland in North Carolina, USA, total existing carbon storage (peat and vegetation) ranged between 155.5 TgC and 201.0 TgC, with potential losses between 99.4–128.0 TgC by the end of the century due to inundation from sea level rise (Henman & Poulter, 2008). In low‐lying coastal ecosystems, restoring peatland structure and function can improve climate resiliency as peatlands play a critical role in ecosystem adaptation to sea level rise, by preventing soil loss through oxidation while allowing soil accretion to resume. Restoring peatlands through reintroduction of wetland hydrology (rewetting) can prevent soil carbon loss and ensure these peatlands continue to serve as carbon sinks (Limpens et al., 2008; Lamers et al., 2015; Chimner et al., 2017). Rewetting also has the benefit of reducing the likelihood of catastrophic wildfires (Wurster et al., 2016).

      There is a significant amount of uncertainty in temperate wetland carbon stock estimates. Most studies reporting carbon stock estimates are generally focused on either boreal or tropical peatlands, leaving temperate freshwater peatlands largely understudied. Difficulty in differentiating between mineral‐soil and organic‐soil wetlands at landscape‐scales also increases the uncertainty in the areal extents and estimates of carbon stores for temperate wetlands. Drainage and conversion have fragmented and decreased temperate wetland areal extents, making measurements even more difficult. However, with the increasing availability of aerial‐ and satellite‐remote sensing data from both active and passive sensors, opportunities exist to address this knowledge gap. Studies that combine remote‐sensing data, repeat field measurements, and intensive inventories would improve our understanding of these systems and provide a critical baseline for monitoring and forecasting future changes related to climate and anthropogenic pressures in these ecosystems.

Table 1.2 Summary of global wetland carbon stocks. The temperate wetland stock estimate includes the conterminous United States and China

Wetland Type	Aboveground stocks (PgC)	Belowground stocks (PgC)	Total Ecosystem Stocks (PgC)
Mangroves	1.52–1.75	1.93–6.4	3.45–8.15
Salt marshes	–	0.4–6.5	0.4–6.5
Seagrass	–	–	4.2–8.4
Tropical peatlands	8.5–9.6	69–129	77.5–138.6
Without permafrost, Boreal wetlands (mineral and organic combined)	10.0–15.0	400–500	410–515
With permafrost, Boreal wetlands	10.0–15.0	1672	1682–1687
Temperate wetlands (for China, US (lower 48) mineral and organic combined)	1.2–3.2	27.3–38.1	29.5–41.3
Global total	21.2–29.6	498.6–680	519.8–709.6
Global total w. permafrost (1672 PgC)	21.2–29.6	1770.6–1852.0	1791.8–1881.6

1.5. GLOBAL SUMMARY OF WETLAND CARBON STOCKS

Combining the wetland carbon stock estimates we calculate a global total above and below ground storage of 520–710 PgC, and 1792–1882 PgC with permafrost included (Table 1.2). Boreal wetlands contribute 88% to the global total due to the high amount of carbon stored in soils, ~500 PgC in non‐permafrost soil and ~800 PgC in perennially frozen soils (Hugelius et al., 2014). Compared with the IPCC 5^th Assessment Report, our estimate is lower by 600–800 PgC, primarily because of the lower estimate in permafrost carbon provided by Hugelius et al. (2014) where the updated carbon density for Gelisol soil order was significantly lower than in the 5^th Assessment Report. Tropical wetlands, mainly peatlands, contain ~124 PgC, with recent estimates from the Cuvette Centrale in the Congo Basin shifting focus from the better‐known deposits found in Southeast Asia (Page et al., 2011). Dargie et al. (2017) found that the extent of Cuvette Centrale peatlands to be five times larger than earlier African estimates, expanding the global extent of tropical peatlands by 29%.

      In this chapter we also highlight recent work on “blue carbon” estimates for mangroves and salt marshes, with an estimate also provided for seagrass. Combined, blue carbon stocks range from 8.3–23.1 PgC, which increases the earlier estimates of Chmura et al. (2003) of >10 PgC for coastal‐wetland sediments. The expanding number of coastal field studies, meta analyses, and applications of high‐resolution remote sensing data to distinguish more precisely mangrove habitat have contributed to refining the range of coastal carbon stocks. In addition, the numerous ecosystem benefits provided by coastal ecosystems, such as fish habitat, buffering of storms and tropical cyclones, biodiversity habitat for birds, and fuel and fiber for people, have increased interest in protecting these ecosystems given the numerous co‐benefits they provide. Corals and kelp forests have locally important roles in marine ecosystems but are not significant in terms of the carbon stocks they contain (Howard et al., 2017).

      Surprisingly, temperate wetland carbon stocks remain uncertain in our budget (29–41 PgC), and our estimate is likely biased toward low total carbon stocks because of missing data for Europe, India, Japan, and other countries. Part of the challenge is related to the impact of land use activities on freshwater wetlands, where drainage and conversion to agriculture have decreased the areal extent of these wetlands. Another challenge is that the size of these wetlands is smaller and more fragmented than the wetlands found in the boreal or tropical regions. Our budget does not consider carbon stocks for managed wetlands, like areas in cultivation for rice agriculture, or other food crops.

1.6. FUTURE CHANGES IN WETLAND CARBON STOCKS

      Future changes to the carbon stock dynamics of wetlands will be driven by changes to either the plant productivity that serves as carbon inputs to these ecosystems, or by the rates of carbon loss through decomposition and combustion. The first of these is likely to be most strongly driven by CO₂ fertilization, which has increased global primary productivity substantially over the historical period (Campbell et al., 2017). The second of these is strongly driven by changes to wetland hydrology and associated anoxia, which in turn may be heavily influenced by changes in temperature, particularly in permafrost‐affected soils, and by anthropogenic disturbance, particularly in tropical peatlands.

      The dynamics of wetland carbon stocks are not well captured in existing global models, and thus not well integrated into assessments of climate feedbacks arising from these ecosystems.

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