Editing IPCC:AR6/SROCC/Chapter-2 (section)

=== 2.2.4 Permafrost ===

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This section assesses permafrost, but not seasonally frozen ground, in high mountain areas. As mountains also exist in polar areas, some overlap exists between this section and Chapter 3. Observations of permafrost are scarce (Tables 2.1 and 2.2, PERMOS, 2016; Bolch et al., 2018 <sup>[[#fn:r146|146]]</sup> ) and unevenly distributed among and within mountain regions. Unlike glaciers and snow, permafrost is a subsurface phenomenon that cannot easily be observed remotely. As a consequence, its distribution and change are less understood than for glaciers or snow, and in many mountain regions it can only be inferred (Gruber et al., 2017 <sup>[[#fn:r147|147]]</sup> ). Permafrost thaw and degradation impact people via runoff and water quality (Section 2.3.1), hazards and infrastructure (Section 2.3.2) and greenhouse gas emissions (Box 2.2).

AR5 and IPCC’s Special Report on ‘Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation’ (SREX) assessed permafrost change globally, but not separately for mountains. AR5 concluded that permafrost temperatures had increased in most regions since the early 1980s ( ''high confidence'' ), although warming rates varied regionally, and attributed this warming to increased air temperature and changes in snow cover ( ''high confidence'' ). The temperature increase for colder permafrost was generally greater than for warmer permafrost ( ''high confidence'' ). SREX found a ''likely'' warming of permafrost in recent decades and expressed ''high confidence'' that its temperatures will continue to increase. AR5 found decreases of northern high-latitude near surface permafrost for 2016–2035 to be ''very likely'' and a general retreat of permafrost extent for the end of the 21st century and beyond to be ''virtually certain'' . While some permafrost phenomena, methods of observation and scale issues in scenario simulations are specific to mountainous terrain, the basic mechanisms connecting climate and permafrost are the same in mountains and polar regions.

Between 3.6–5.2 million km 2 are underlain by permafrost in the eleven high mountain regions outlined in Figure 2.1 ( ''medium confidence'' ) based on data from two modelling studies (Gruber, 2012 <sup>[[#fn:r148|148]]</sup> ; Obu et al., 2019 <sup>[[#fn:r149|149]]</sup> ). For comparison, this is 14–21 times the area of glaciers (Section 2.2.3) in these regions (Figure 2.1) or 27–29% of the global permafrost area. The distribution of permafrost in mountains is spatially highly heterogeneous, as shown in detailed regional modelling studies (Boeckli et al., 2012 <sup>[[#fn:r150|150]]</sup> ; Bonnaventure et al., 2012 <sup>[[#fn:r151|151]]</sup> ; Westermann et al., 2015 <sup>[[#fn:r152|152]]</sup> ; Azócar et al., 2017 <sup>[[#fn:r153|153]]</sup> ; Zou et al., 2017 <sup>[[#fn:r154|154]]</sup> ).

Permafrost in the European Alps, Scandinavia, Canada, Mongolia, the Tien Shan and the Tibetan Plateau has warmed during recent decades and some observations reveal ground-ice loss and permafrost degradation ( ''high confidence'' ). The heterogeneity of mountain environments and scarcity of long-term observations challenge the quantification of representative regional or global warming rates. A recent analysis finds that permafrost at 28 mountain locations in the European Alps, Scandinavia, Canada as well as High Mountain Asia and North Asia, warmed on average by 0.19 ± 0.05 °C per decade between 2007–2016 (Biskaborn et al., 2019 <sup>[[#fn:r155|155]]</sup> ). Over longer periods, observations in the European Alps, Scandinavia, Mongolia, the Tien Shan and the Tibetan Plateau (see also Cao et al., 2018) show general warming (Table 2.1, Figure 2.5) and degradation of permafrost at individual sites (e.g., Phillips et al., 2009). Permafrost close to 0ºC warms at a lower rate than colder permafrost because ground-ice melt slows warming. Similarly, bedrock warms faster than debris or soil because of low ice content. For example, several European bedrock sites (Table 2.1) have warmed rapidly, by up to 1ºC per decade, during the past two decades. By contrast, total warming of 0.5ºC–0.8ºC has been inferred for the second half of the 20th century based on thermal gradients at depth in an ensemble of European bedrock sites (Isaksen et al., 2001 <sup>[[#fn:r156|156]]</sup> ; Harris et al., 2003 <sup>[[#fn:r157|157]]</sup> ). Warming has been shown to accelerate at sites in Scandinavia (Isaksen et al., 2007 <sup>[[#fn:r158|158]]</sup> ) and in mountains globally within the past decade (Biskaborn et al., 2019 <sup>[[#fn:r159|159]]</sup> ). During recent decades, rates of permafrost warming in the European Alps and Scandinavia exceeded values of the late 20th century ( ''limited evidence, high agreement'' ).

The observed thickness of the active layer (see Annex I: Glossary), the layer of ground above permafrost subject to annual thawing and freezing, increased in the European Alps, Scandinavia (Christiansen et al., 2010 <sup>[[#fn:r160|160]]</sup> ), and on the Tibetan Plateau during the past few decades (Table 2.2), indicating permafrost degradation. Geophysical monitoring in the European Alps during approximately the past 15 years revealed increasing subsurface liquid water content (Hilbich et al., 2008 <sup>[[#fn:r161|161]]</sup> ; Bodin et al., 2009 <sup>[[#fn:r162|162]]</sup> ; PERMOS, 2016 <sup>[[#fn:r163|163]]</sup> ), indicating gradual ground-ice loss.

During recent decades, the velocity of rock glaciers in the European Alps exceeded values of the late 20th century ( ''limited evidence, high agreement'' ). Some rock glaciers, that is, masses of ice-rich debris that show evidence of past or present movement, show increasing velocity as a transient response to warming and water input, although continued permafrost degradation would eventually inactivate them (Ikeda and Matsuoka, 2002 <sup>[[#fn:r164|164]]</sup> ). Rock glacier velocities observed in the European Alps in the 1990s were on the order of a few decimetres per year and during approximately the past 15 years they often were about 2–10 times higher (Bodin et al., 2009 <sup>[[#fn:r165|165]]</sup> ; Lugon and Stoffel, 2010 <sup>[[#fn:r166|166]]</sup> ; PERMOS, 2016 <sup>[[#fn:r167|167]]</sup> ). Destabilisation, including collapse and rapid acceleration, has been documented (Delaloye et al., 2010 <sup>[[#fn:r168|168]]</sup> ; Buchli et al., 2013 <sup>[[#fn:r169|169]]</sup> ; Bodin et al., 2016 <sup>[[#fn:r170|170]]</sup> ). One particularly long time series shows velocities around 1960 just slightly lower than during recent years (Hartl et al., 2016 <sup>[[#fn:r171|171]]</sup> ). In contrast to nearby glaciers, no clear change in rock glacier velocity or elevation was detected at a site in the Andes between 1955–1996 (Bodin et al., 2010 <sup>[[#fn:r172|172]]</sup> ). The majority of similar landforms investigated in the Alaska Brooks Range increased their velocity since the 1950s, while few others slowed down (Darrow et al., 2016 <sup>[[#fn:r173|173]]</sup> ).

Decadal-scale permafrost warming and degradation are driven by air temperature increase and additionally affected by changes in snow cover, vegetation and soil moisture. Bedrock locations, especially when steep and free of snow, produce the most direct signal of climate change on the ground thermal regime (Smith and Riseborough, 1996 <sup>[[#fn:r174|174]]</sup> ), increasing the confidence in attribution. Periods of cooling, one or few years long, have been observed and attributed to extraordinary low-snow conditions (PERMOS, 2016 <sup>[[#fn:r175|175]]</sup> ). Extreme increases of active-layer thickness often correspond with summer heat waves (PERMOS, 2016 <sup>[[#fn:r176|176]]</sup> ) and permafrost degradation can be accelerated by water percolation (Luethi et al., 2017 <sup>[[#fn:r177|177]]</sup> ). Similarity and synchronicity of interannual to decadal velocity changes of rock glaciers within the European Alps (Bodin et al., 2009 <sup>[[#fn:r178|178]]</sup> ; Delaloye et al., 2010 <sup>[[#fn:r179|179]]</sup> ) and the Tien Shan (Sorg et al., 2015 <sup>[[#fn:r180|180]]</sup> ), suggest common regional forcing such as summer air temperature or snow cover.

Because air temperature is the major driver of permafrost change, permafrost in high mountain regions is expected to undergo increasing thaw and degradation during the 21st century, with stronger consequences expected for higher greenhouse gas emission scenarios ( ''very high confidence'' ). Scenario simulations for the Tibetan Plateau until 2100 estimate permafrost area to be strongly reduced, for example by 22–64% for RCP2.6 and RCP8.5 and a spatial resolution of 0.5º (Lu et al., 2017 <sup>[[#fn:r181|181]]</sup> ). Such coarse-scale studies (Guo et al., 2012 <sup>[[#fn:r182|182]]</sup> ; Slater and Lawrence, 2013 <sup>[[#fn:r183|183]]</sup> ; Guo and Wang, 2016 <sup>[[#fn:r184|184]]</sup> ), however, are of limited use in quantifying changes and informing impact studies in steep terrain due to inadequate representation of topography (Fiddes and Gruber, 2012 <sup>[[#fn:r185|185]]</sup> ). Fine-scale simulations, on the other hand, are local or regional, limited in areal extent and differ widely in their representation of climate change and permafrost. They reveal regional and elevational differences of warming and degradation (Bonnaventure and Lewkowicz, 2011 <sup>[[#fn:r186|186]]</sup> ; Hipp et al., 2012 <sup>[[#fn:r187|187]]</sup> ; Farbrot et al., 2013 <sup>[[#fn:r188|188]]</sup> ) as well as warming rates that differ between locations (Marmy et al., 2016 <sup>[[#fn:r189|189]]</sup> ) and seasons (Marmy et al., 2013 <sup>[[#fn:r190|190]]</sup> ). While structural differences in simulations preclude a quantitative summary, these studies agree on increasing warming and thaw of permafrost for the 21st century and reveal increased loss of permafrost under stronger atmospheric warming (Chadburn et al., 2017 <sup>[[#fn:r191|191]]</sup> ). Permafrost thaw at depth is slow but can be accelerated by mountain peaks warming from multiple sides (Noetzli and Gruber, 2009 <sup>[[#fn:r192|192]]</sup> ) and deep percolation of water (Hasler et al., 2011 <sup>[[#fn:r193|193]]</sup> ). Near Mont Blanc in the European Alps, narrow peaks below 3,850 m a.s.l. may lose permafrost entirely under RCP8.5 by the end of the 21st century (Magnin et al., 2017 <sup>[[#fn:r194|194]]</sup> ). As ground-ice from permafrost usually melts slower than glacier ice, some mountain regions will transition from having abundant glaciers to having few and small glaciers but large areas of permafrost that is thawing (Haeberli et al., 2017 <sup>[[#fn:r195|195]]</sup> ).

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'''Table 2.1'''

Observed changes in permafrost mean annual ground temperature (MAGT) in mountain regions. Values are based on individual boreholes or ensembles of several boreholes. The MAGT refers to the last year in a period and is taken from a depth of 10–20 m unless the borehole is shallower. Region names refer to Figure 2.1. Numbers in brackets indicate how many sites are summarised for a particular surface type and area; the underscored value is an average. Elevation is metres above sea level (m a.s.l.)

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'''Table 2.2'''

Observed changes of active-layer thickness (ALT) in mountain regions. Numbers in brackets indicate how many sites are summarised for a particular surface type and area. Region names refer to Figure 2.1 . Elevation is metres above sea level (m a.s.l.).

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'''Figure 2.5'''

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'''Figure 2.5 | Mean annual ground temperature from boreholes in debris and bedrock in the European Alps, Scandinavia and High Mountain Asia. Temperatures differ between locations and warming trends can be interspersed by short periods of cooling. One location shows degrading of permafrost. Overall, the number of observed boreholes is small and most records are […]'''
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[[File:dd40b29ee885f4ec7ae85757776387e8 IPCC-SROCC-CH_2_5.jpg]]

Figure 2.5 | Mean annual ground temperature from boreholes in debris and bedrock in the European Alps, Scandinavia and High Mountain Asia. Temperatures differ between locations and warming trends can be interspersed by short periods of cooling. One location shows degrading of permafrost. Overall, the number of observed boreholes is small and most records are short. The depth of measurements is approximately 10 m, and years without sufficient data are omitted (Noetzli et al., 2018).

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