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

===== 3.4.1.2.4 Drivers =====

Changes in temperature and precipitation act as gradual ‘press’ (i.e., continuous) disturbances that directly affect permafrost by modifying the ground thermal regime, as discussed in Section 3.4.1.2.1. Climate change can also modify the occurrence and magnitude of abrupt physical disturbances such as fire, and soil subsidence and erosion resulting from ice rich permafrost thaw (thermokarst). These ‘pulse’ (i.e., discrete) disturbances (Smith et al., 2009 <sup>[[#fn:r1429|1429]]</sup> ) often are part of the ongoing disturbance and successional cycle in Arctic and boreal ecosystems (Grosse et al., 2011 <sup>[[#fn:r1430|1430]]</sup> ), but changing rates of occurrence alter the landscape distribution of successional ecosystem states, with permafrost characteristics defined by the ecosystem and climate state (Kanevskiy et al., 2013 <sup>[[#fn:r1431|1431]]</sup> ).

Pulse disturbances often rapidly remove the insulating soil organic layer, leading to permafrost degradation (Gibson et al., 2018 <sup>[[#fn:r1423|1423]]</sup> ). Of all pulse disturbance types, wildfire affects the most high-latitude land area annually at the continental scale. In some well-studied regions, there is ''high confidence'' that area burned, fire frequency and extreme fire years are higher now than the first half of the last century, or even the last 10,000 years (Kasischke and Turetsky, 2006 <sup>[[#fn:r1433|1433]]</sup> ; Flannigan et al., 2009 <sup>[[#fn:r1434|1434]]</sup> ; Kelly et al., 2013 <sup>[[#fn:r1435|1435]]</sup> ; Hanes et al., 2019 <sup>[[#fn:r1436|1436]]</sup> ) ''.'' Recent climate warming has been linked to increased wildfire activity in the boreal forest regions in Alaska and western Canada where this has been studied (Gillett, 2004 <sup>[[#fn:r1437|1437]]</sup> ; Veraverbeke et al., 2017 <sup>[[#fn:r1438|1438]]</sup> ). Based on satellite imagery, an estimated 80,000 km 2 of boreal area was burned globally per year from 1997 to 2011 (van der Werf et al., 2010 <sup>[[#fn:r1439|1439]]</sup> ; Giglio et al., 2013 <sup>[[#fn:r1440|1440]]</sup> ). Extreme fire years in northwest Canada during 2014 and Alaska during 2015 doubled the long-term (1997–2011) average area burned annually in this region (Canadian Forest Service, 2017), surpassing Eurasia to contribute 60% of the global boreal area burned (van der Werf et al., 2010 <sup>[[#fn:r1441|1441]]</sup> ; Randerson et al., 2012 <sup>[[#fn:r1442|1442]]</sup> ; Giglio et al., 2013 <sup>[[#fn:r1443|1443]]</sup> ). These extreme North American fire years were balanced by lower-than-average area burned in Eurasian forests, resulting in a 5% overall increase in global boreal area burned. The annual area burned in Arctic tundra is generally small compared to the forested boreal biome. In Alaska—the only region where estimates of burned area exist for both boreal forest and tundra vegetation types—tundra burning averaged approximately 270 km 2 yr -1 during the last half century (French et al., 2015 <sup>[[#fn:r1445|1445]]</sup> ), accounting for 7% of the average annual area burned throughout the state (Pastick et al., 2017 <sup>[[#fn:r1446|1446]]</sup> ). There is ''high confidence'' that changes in the fire regime are degrading permafrost faster than had occurred over the historic successional cycle (Turetsky et al., 2011 <sup>[[#fn:r1447|1447]]</sup> ; Rupp et al., 2016 <sup>[[#fn:r1448|1448]]</sup> ; Pastick et al., 2017 <sup>[[#fn:r1449|1449]]</sup> ), and that the effect of this driver of permafrost change is under-represented in the permafrost temperature observation network.

Abrupt permafrost thaw occurs when changing environmental and ecological conditions interact with geomorphological processes. Melting ground ice causes the ground surface to subside. Pooling or flowing water causes localised permafrost thaw and sometimes mass erosion. Together, these localised feedbacks can thaw through meters of permafrost within a short time, much more rapidly than would be caused by increasing air temperature alone. This process is a pulse disturbance to permafrost that can occur in response to climate, such as an extreme precipitation event (Balser et al., 2014 <sup>[[#fn:r1450|1450]]</sup> ; Kokelj et al., 2015 <sup>[[#fn:r1451|1451]]</sup> ), or coupled with other disturbances such as wildfire that affects the ground thermal regime (Jones et al., 2015a <sup>[[#fn:r1452|1452]]</sup> ). There is ''medium confidence'' in the importance of abrupt thaw for driving change in permafrost at the circumpolar scale because it occurs at point locations rather than continuously across the landscape, but the risk for widespread change from this mechanism remains high because of the rapidity of change in these locations (Kokelj et al., 2017 <sup>[[#fn:r1453|1453]]</sup> ; Nitze et al., 2018 <sup>[[#fn:r1454|1454]]</sup> ). New research at the global scale has revealed that 3.6 x 10 6 km 2 , about 20% of the northern permafrost region, appears to be vulnerable to abrupt thaw (Olefeldt et al., 2016 <sup>[[#fn:r1455|1455]]</sup> ).

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