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Carbon storage in wetlands has declined due to anthropogenic land use and land cover change, primarily from conversion to cropland, forestry, urban area, and peat extraction over the past millennia and centuries (Joosten and Clarke, 2002; Asselen et al., 2013). Artificial soil drainage also exposes soil organic carbon, accumulated over millennia, to aerobic oxidation, leading to large carbon fluxes to the atmosphere (Erb et al., 2017; Armentano, 1980). The global area of drained wetlands is estimated to be as high as 71% since 1700 (Davidson, 2014), and 35% since 1970 according to recent meta‐analyses (Dixon et al., 2016; Darrah et al., 2019), while mapping approaches estimate cumulative wetland losses to be 33% (Hu et al., 2017). The uncertainty in wetland area loss presents a challenge to estimating losses in soil carbon storage.

Global inventories of land‐use related emissions have not considered the impact of wetland drainage outside of recent drainage in Southeast Asia (Pongratz et al., 2018). Drainage of peat swamps in Indonesia alone are estimated to have emitted 6 PgC from 1850–2015 (Houghton and Nassikas, 2017). Peat drainage in this region still occurs at a rapid pace, i.e., 14,500 km² of peat swamp forest have been converted to oil palm and pulpwood plantations between 2000–2010 (Page and Hooijer, 2016). Separate accounting efforts using geospatial data and emission factors have estimated that >250,000 Mkm² of organic soiled wetlands were drained for agriculture globally, leading to a CO₂ release of 0.078 Pg/yr, more than one‐fourth of all land‐use CO₂ emissions (Tubiello et al., 2016). Nearly 13% of these emissions have occurred since 1990 (Conchedda and Tubiello, 2020). A separate bookkeeping approach estimates peatland degradation and losses to 510,000 Mkm² and a cumulative release of 80.8 PgC (Leifeld and Menichetti, 2018). Following drainage, carbon is likely also transported to the river network then to the ocean as dissolved carbon, though this pathway and emissions of carbon to the atmosphere is uncertain (Cole et al., 2007). The decline in global wetland area since 1850 is estimated to have reduced methane emissions by 56 Tg CH₄/yr with most of the decline from the northern temperate zone (Paudel et al., 2016).