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The largest extent of peatlands and wetlands globally are in Boreal and Arctic biomes (here defined as >50°N latitude) where waterlogging and low soil temperatures promotes the accumulation of organic matter (Gorham, 1991). Peatlands >50°N latitude cover ca. 3.3 M km², which is >70% of the global peatland area (Hugelius et al., 2020) These northern peatlands have been expanding throughout the Holocene, sequestering C into peat deposits and acting as a sink for atmospheric carbon (Smith, 2004; MacDonald et al., 2006; Brovkin et al., 2016). Peatlands are a type of wetland defined by an accumulation of peat (organic soil) at the surface, usually to a thickness of at least 30 or 40 cm (Joosten & Clarke, 2002). Gorham (1991) provided an extensive review of northern peatlands and made a benchmark carbon stock estimate of 455 PgC in the northern high‐latitude region of the world. Over broad spatial scales, peatland initiation is favored by relatively low temperature and high precipitation (Kivinen & Pakarinen, 1981). At local scales, topography and substrate permeability are the main controls on soil water retention (Loisel et al., 2017).

The two main processes of peatland formation (and expansion) are paludification or terrestrialization. Paludification is the most common form of northern peatland formation and involves initiation and expansion of peatlands into other established terrestrial ecosystems (such as poorly drained forests) and terrestrialization is peatland formation due to gradual in‐filling of water bodies (Kuhry & Turunen, 2006). While peatland initiation is favored by wet and cold conditions, the vertical growth and carbon accumulation rate of peat in established peatlands is favored by long growing seasons and high incoming photosynthetically active radiation, so that ecosystem productivity seems more important than decomposition rates for the rate of long‐term carbon accumulation (Charman et al., 2013). Over the past seven millennia, the mean net carbon accumulation rate in northern peatlands has been slightly above 10 g C/m²/yr, with present day flux measurements indicating a mean sink strength of around 30 g C/m²/yr (Yu et al., 2012). Northern peatlands and wetlands are a sink for atmospheric carbon, and this sink is expected to remain or grow in coming decades (Frolking et al., 2011; Chaudhary et al., 2020). However, it has been shown that the sink strength may saturate (Gallego‐Sala et al., 2018) or that peat growth may partly be offset by disturbances, including higher fire frequency (Turetsky et al., 2015), drought (Fenner and Freeman, 2011), and thawing of permafrost (Schuur et al., 2015).

A recent study presented an estimate of northern peatland stocks of 415±150 (min/max) Pg C, of which 185±66 PgC is in permafrost peatlands (calculated for >50°N latitude from Hugelius et al., 2020). This estimate is in line with most earlier studies and is based on harmonized soil maps and >7000 field observations. A fully consistent estimate of northern peatland extent and carbon stocks has long been lacking (Loisel et al., 2017; Yu et al., 2014). The large spread in estimates is due to a lack of unified methods or maps, as well as different definitions used. Studies in recent decades have quantified northern peat C stocks ranging from 270 to 604 PgC, but numbers of 400 to 500 Pg are most commonly used and cited (Yu et al., 2014; Loisel et al., 2017). The estimates are similar in that they all rely on combining data from peat cores (either peat depth or historical peat accumulation) with areal estimates of present day peatland coverage. In most cases however, these studies have not defined a specific lower latitude for the spatial extent, and thus include peat C stored south of >50°N latitude. Most recent studies have used areal estimates for northern peatland extent from Gorham (1991), 3.42 × 10⁶ km², or from Joostens and Clark (2002), 3.46 × 10⁶ km². These areal estimates are primarily derived from combining various national inventories of peat resources, which are not available as maps. Global soil or peatland map products that have been used to support broad‐scale modeling, such as WISE30sec or PEATMAP (Batjes, 2016; Xu et al., 2018) show significantly lower coverage of northern peatlands (< 3 ×10⁶ km²). Within the northern permafrost region, more detailed regional soil maps have been used to estimate peatland carbon stocks of ca 300±50 PgC, of which ca 150±30 Pg is in permafrost (Hugelius et al., 2014).

Globally, mineral wetland soils cover an estimated 2.3 × 10⁶ km² and store ca. 50 PgC (Bridgham et al., 2006). There is no specific estimate for Boreal and Arctic biomes, but spatial analyses of the WISE30sec database (Batjes, 2016) suggest that ca. half of this area and stock is located >50°N. Vegetation C stocks in northern wetlands is usually a small fraction of total ecosystem carbon. In forested northern wetlands, vegetation C is usually 5–10 kg C m^–2, but in open wetland systems it rarely exceeds 1–2 kg C/m² (Alexeyev & Birdsey, 1998). With a total Boreal and Arctic wetland area of 4.5 × 10⁶ km², of which a quarter is forested, the total vegetation carbon stock is estimated to ca. 10–15 PgC. We estimate that total soil organic carbon stocks in wetlands north of 50° latitude ranges between 400–500 PgC. Defining high‐latitude wetlands to include all permafrost zones (continuous, discontinuous, sporadic) would give an estimate of up to 1672 PgC, which 1466 PgC is in perennially frozen soils and deposits, i.e., Yedoma (Tarnocai et al., 2009).