Editing IPCC:AR6/SROCC/Chapter-3 (section)

===== 3.4.1.2.3 Carbon =====

The permafrost region represents a large, climate sensitive reservoir of organic carbon with the potential for some of this pool to be rapidly decayed and transferred to the atmosphere as CO 2 and methane as permafrost thaws in a warming climate, thus accelerating the pace of climate change (Schuur et al., 2015 <sup>[[#fn:r1414|1414]]</sup> ). The current best mean estimate of total (surface plus deep) organic soil carbon (terrestrial) in the northern circumpolar permafrost region (17.8 x 10 6 km 2 area) is 1460 to 1600 petagrams ( ''medium confidence'' ) (Pg; 1 Pg = 1 billion metric tonnes) (Schuur et al., 2018 <sup>[[#fn:r1415|1415]]</sup> ). All permafrost region soils estimated to 3 m in depth (surface) contain 1035 ± 150 Pg C (Tarnocai et al., 2009 <sup>[[#fn:r1416|1416]]</sup> ; Hugelius et al., 2014 <sup>[[#fn:r1417|1417]]</sup> ) ( ''high confidence'' ). Of the carbon in the surface, 800–1000 Pg C is perennially frozen, with the remainder contained in seasonally-thawed soils. The northern circumpolar permafrost region occupies only 15% of the total global soil area, but the 1035 Pg C adds another 50% to the rest of the 3 m soil carbon inventory (2050 Pg C for all global biomes excluding tundra and boreal; Jobbágy and Jackson, 2000 <sup>[[#fn:r1418|1418]]</sup> ; Schuur et al., 2015 <sup>[[#fn:r1419|1419]]</sup> ).

Substantial permafrost carbon exists below 3 m depth ( ''medium confidence'' ). Deep carbon (&gt;3 m) has been best quantified for the Yedoma region of Siberia and Alaska, characterised by wind- and water-moved permafrost sediments tens of meters thick. The Yedoma region covers a 1.4 x 10 6 km 2 area that remained ice-free during the last Ice Age (Strauss et al., 2013 <sup>[[#fn:r1420|1420]]</sup> ) and accounts for 327–466 Pg C in deep sediment accumulations below 3 m (Strauss et al., 2017).

The current inventory has also highlighted additional carbon pools that are likely to be present but are so poorly quantified ( ''low confidence'' ) that they cannot yet be added into the number reported above. There are deep terrestrial soil/sediment deposits outside of the Yedoma region that may contain about 400 Pg C (Schuur et al., 2015 <sup>[[#fn:r1421|1421]]</sup> ). An additional pool is organic carbon remaining in permafrost but that is now submerged on shallow Arctic sea shelves that were formerly exposed as terrestrial ecosystems during the Last Glacial Maximum ~20,000 years ago (Walter et al., 2007 <sup>[[#fn:r1423|1423]]</sup> ). This permafrost is degrading slowly due to seawater intrusion, and it is not clear what amounts of permafrost and organic carbon still remain in the sediment versus what has already been converted to greenhouse gases. A recent synthesis of permafrost extent for the Beaufort Sea shelf showed that most remaining subsea permafrost in that region exists near shore with much reduced area ( ''high confidence'' ) as compared to original subsea permafrost maps that outlined the entire 3 x 10 6 km 2 shelf area (&lt;120 m below sea level depth) that was formerly exposed as land (Ruppel et al., 2016 <sup>[[#fn:r1424|1424]]</sup> ). These observations are supported by similar studies in the Siberian Arctic Seas (Portnov et al., 2013 <sup>[[#fn:r1425|1425]]</sup> ), and by modelling that suggests that subsea permafrost would be thawed many meters below the seabed under current submerged conditions (Anisimov et al., 2012 <sup>[[#fn:r1426|1426]]</sup> ; AMAP, 2017d <sup>[[#fn:r1427|1427]]</sup> ; Angelopoulos et al., 2019 <sup>[[#fn:r1428|1428]]</sup> ).

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