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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.