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

==== 3.4.1.2 Permafrost ====

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<span id="temperature-1"></span>
===== 3.4.1.2.1 Temperature =====

Record high temperatures at ~10–20 m depth in the permafrost (near or below the depths affected by intra-annual fluctuation in temperature) have been documented at many long-term monitoring sites in the Northern Hemisphere circumpolar permafrost region (AMAP, 2017d <sup>[[#fn:r1386|1386]]</sup> ) (Figure 3.10) ( ''very high confidence'' ). At some locations, the temperature is 2°C–3°C higher than 30 years ago. During the decade between 2007 and 2016, the rate of increase in permafrost temperatures was 0.39°C ± 0.15°C for colder continuous zone permafrost monitoring sites, 0.20°C ± 0.10°C for warmer discontinuous zone permafrost, giving a global average of 0.29 ± 0.12°C across all polar and mountain permafrost (Biskaborn et al., 2019 <sup>[[#fn:r1387|1387]]</sup> ). Relatively smaller increases in permafrost temperature in warmer sites indicate that permafrost is thawing with heat absorbed by the ice-to-water phase change, and as a result, the active layer may be increasing in thickness. In contrast to temperature, there is only ''medium confidence'' that active layer thickness across the region has increased. This confidence level is because decadal trends vary across regions and sites (Shiklomanov et al., 2012 <sup>[[#fn:r1388|1388]]</sup> ) and because mechanical probing of the active layer can underestimate the degradation of permafrost in some cases because the surface subsides when ground ice melts and drains (Mekonnen et al., 2016 <sup>[[#fn:r1389|1389]]</sup> ; AMAP, 2017d <sup>[[#fn:r1390|1390]]</sup> ; Streletskiy et al., 2017 <sup>[[#fn:r1391|1391]]</sup> ). Permafrost in the Southern Hemisphere polar region occurs in ice-free exposed areas (Bockheim et al., 2013 <sup>[[#fn:r1392|1392]]</sup> ), 0.18% of the total land area of Antarctica (Burton-Johnson et al., 2016). This area is three orders of magnitude smaller than the 13–18 x 10 6 km 2 area underlain by permafrost in the Northern Hemisphere terrestrial permafrost region (Gruber, 2012). Antarctic permafrost temperatures are generally colder (Noetzli et al., 2017 <sup>[[#fn:r1403|1403]]</sup> ) and increased 0.37°C ± 0.10°C between 2007 and 2016 (Biskaborn et al., 2019 <sup>[[#fn:r1404|1404]]</sup> ).

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'''Figure 3.10'''

<span id="schematic-of-important-land-surface-components-influenced-by-the-arctic-terrestrial-cryosphere-permafrost-1-ground-ice-2-river-discharge-3-abrupt-thaw-4-surface-water-5-fire-6-tundra-7-shrubs-8-boreal-forest-9-lake-ice-10-seasonal-snow-11.-time-series-of-snow-cover-extent-anomalies-in-june-relative-to-19812010-climatology-from"></span>
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'''Schematic of important land surface components influenced by the Arctic terrestrial cryosphere: permafrost (1); ground ice (2); river discharge (3); abrupt thaw (4); surface water (5); fire (6); tundra (7); shrubs (8); boreal forest (9); lake ice (10); seasonal snow (11). Time series of snow cover extent anomalies in June (relative to 1981–2010 climatology) from […]'''
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[[File:1f09753851439dd6e693fe4f8198d24b IPCC-SROCC-CH_3_10.jpg]]

Schematic of important land surface components influenced by the Arctic terrestrial cryosphere: permafrost (1); ground ice (2); river discharge (3); abrupt thaw (4); surface water (5); fire (6); tundra (7); shrubs (8); boreal forest (9); lake ice (10); seasonal snow (11). Time series of snow cover extent anomalies in June (relative to 1981–2010 climatology) from 5 products based on the approach of Mudryk et al. (2017) (a); permafrost temperature change normalised to a baseline period (Romanovsky et al., 2017), Region A: Continuous to discontinuous permafrost in Scandanavia, Svalbard, and Russia/Siberia, Region B: Cold continuous permafrost in northern Alaska, Northwest Territories, and NE Siberia, Region C: Cold continuous permafrost in Eastern and High Arctic Canada, Region D: Discontinuous permafrost in Interior Alaska and Northwest Canada (b), and runoff from northern flowing watersheds normalised to a baseline period (1981–2010) (Holmes et al., 2018), multi-station average (± 1 standard deviation) (c). Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model average (± 1 standard deviation) projections for different Representative Concentration Pathway (RCP) scenarios for June snow cover extent change (based on Thackeray et al., 2016) (d), area change of near-surface permafrost (e), and runoff change to the Arctic Ocean (based on McGuire et al., 2018) (f).

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<span id="ground-ice"></span>
===== 3.4.1.2.2 Ground ice =====

Permafrost thaw and loss of ground ice causes the land surface to subside and collapse into the volume previously occupied by ice, resulting in disturbance to overlying ecosystems and human infrastructure (Kanevskiy et al., 2013 <sup>[[#fn:r1405|1405]]</sup> ; Raynolds et al., 2014 <sup>[[#fn:r1406|1406]]</sup> ). Excess ice in permafrost is typical, varying for example from 40% of total volume in some sands up to 80–90% of total volume in fine-grained soil/sediments (Kanevskiy et al., 2013 <sup>[[#fn:r1407|1407]]</sup> ). Ice rich permafrost areas where impacts of thaw could be greatest include the Yedoma deposits in Siberia, Alaska, and the Yukon in Canada, with ice divided between massive wedges interspersed with frozen soil/sediment containing pore ice and smaller ice features (Schirrmeister et al., 2011 <sup>[[#fn:r1408|1408]]</sup> ; Strauss et al., 2017 <sup>[[#fn:r1409|1409]]</sup> ). Other areas including, for example, Northwestern Canada, the Canadian Archipelago, the Yamal and Gydan peninsulas of West Siberia, and smaller portions of Eastern Siberia and Alaska contain buried glacial ice bodies of significant thickness and extent (Lantuit and Pollard, 2008; Leibman et al., 2011; Kokelj et al., 2017; Coulombe et al., 2019). The location and volume of ground ice integrated across the northern permafrost region (5.63–36.55 x 10 3 km 3 , equivalent to 2–10 cm sea level rise) is known with ''medium confidence'' and with no recent updates at the circumpolar scale (Zhang et al., 2008 <sup>[[#fn:r1410|1410]]</sup> ).

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<span id="carbon"></span>
===== 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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<span id="drivers-1"></span>
===== 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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