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

==== 3.4.2.2 Permafrost ====

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Circumpolar- or global-scale models represent permafrost degradation in response to warming scenarios as increases in thaw depth only. The CMIP5 models project with ''high confidence'' that thaw depth will increase and areal extent of near-surface permafrost will decrease substantially (Koven et al., 2013 <sup>[[#fn:r1535|1535]]</sup> ; Slater and Lawrence, 2013 <sup>[[#fn:r1536|1536]]</sup> ) (Figure 3.10). However, there is only ''medium confidence'' in the magnitude of these changes due to at least a five-fold range of estimated present day near-surface permafrost area (&lt;5 – &gt;25 x 10 6 km 2 ) by these models. This was caused by a wide range of model sensitivity in permafrost area to air temperature change, resulting in a large range of projected near-surface permafrost loss by 2100: 2–66% for RCP2.6 (24 ± 16%; ''likely'' range), 15–87% under RCP4.5 and 30–99% (69 ± 20%; ''likely'' range) under RCP8.5. A more recent analysis of near-surface permafrost trends from a subset of models that self-identified as structurally representing the permafrost region had a significantly smaller range of estimated present day near-surface permafrost area (13.1–19.3 x 10 6 km 2 ; mean ± SD, 14.1 ± 3.5 x 10 6 km 2 ) (McGuire et al., 2018 <sup>[[#fn:r1537|1537]]</sup> ). This subset of models also showed large reductions of near-surface permafrost area, averaging a 90% loss (12.7 ± 5.1×10 6 km 2 ) of permafrost area by 2300 for RCP8.5 and 29% loss (4.1 ± 0.6×10 6 km 2 ) for RCP4.5, with much of that long-term loss already occurring by 2100.

Pulse disturbances are not included in the permafrost projections described above, and there is ''high confidence'' that fire and abrupt thaw will accelerate change in permafrost relative to climate effects alone, if the rates of these disturbances increase. The observed trend of increasing fire is projected to continue for the rest of the century across most of the tundra and boreal region for many climate scenarios, with the boreal region projected to have the greatest increase in total area burned (Balshi et al., 2009 <sup>[[#fn:r1538|1538]]</sup> ; Kloster et al., 2012 <sup>[[#fn:r1539|1539]]</sup> ; Wotton et al., 2017 <sup>[[#fn:r1540|1540]]</sup> ). Due to vegetation-climate interactions, there is only ''medium confidence'' in projections of future area burned. As fire activity increases, flammable vegetation, such as the black spruce forest that dominates boreal Alaska, is projected to decline as it is replaced by low-flammability deciduous forest (Johnstone et al., 2011 <sup>[[#fn:r1541|1541]]</sup> ; Pastick et al., 2017 <sup>[[#fn:r1542|1542]]</sup> ). In other regions such as western Canada, by contrast, black spruce could be replaced by the even more flammable jack pine, creating regional-scale feedbacks that increase the spread of fire on the landscape (Héon et al., 2014 <sup>[[#fn:r1543|1543]]</sup> ). A regional process-model study of Alaska projected annual median area burned during the 21st century to be 1.3-1.7 times higher compared with the historical average  (Pastick et al., 2017 <sup>[[#fn:r1544|1544]]</sup> ). Fire also appears to be expanding as a novel disturbance into tundra and forest-tundra boundary regions previously protected by a cool, moist climate (Jones et al., 2009 <sup>[[#fn:r1545|1545]]</sup> ; Hu et al., 2010 <sup>[[#fn:r1546|1546]]</sup> ; Hu et al., 2015 <sup>[[#fn:r1547|1547]]</sup> ) ( ''medium confidence'' ). Annual tundra area burned in Alaska is projected to double under RCP6.0 from a historic rate of 270 km 2 yr -1 to 500–610 km 2 yr -1 over the 21st century (Hu et al., 2015 <sup>[[#fn:r1548|1548]]</sup> ). A statistical approach projected a fourfold increase in the 30-year probability of fire occurrence in the forest-tundra boundary by 2100 (Young et al., 2017 <sup>[[#fn:r1549|1549]]</sup> ). In contrast to fire, there has not yet been a comprehensive circumpolar projection of how abrupt thaw rates may change in the future, but one component of abrupt thaw, change in abrupt thaw lake area, has been projected to increase to increase by 53% under RCP8.5 (Walter Anthony et al., 2018 <sup>[[#fn:r1550|1550]]</sup> ) above the 1.4 x 10 6 km 2 of small lakes and ponds that currently exist in the permafrost region (Muster et al., 2017 <sup>[[#fn:r1551|1551]]</sup> ). As a result, there is ''low confidence'' in the ability to assess the magnitude by which abrupt thaw across the entire landscape will affect regional permafrost, even though this mechanism for rapid change appears critically important for projecting future change (Kokelj et al., 2017 <sup>[[#fn:r1552|1552]]</sup> ).

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