Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.5.2.1 Observed and Reconstructed Changes ====

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The current extent of the global permafrost region is about 22 ± 3×10 <sup>6</sup> km <sup>2</sup> ( [[#Gruber--2012|Gruber, 2012]] ). Permafrost underlies about 15% of Northern Hemisphere land and more than 50% of the unglacierized land north of 60°N ( [[#Zhang--1999|Zhang et al., 1999]] ; [[#Gruber--2012|Gruber, 2012]] ; [[#Obu--2019|Obu et al., 2019]] ). It is also found in high-altitude areas of mountain ranges in both hemispheres – estimated in SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) as representing about 27–29% of the global permafrost area ( ''medium confidence'' ) and most unglacierized areas in Antarctica ( [[#Vieira--2010|Vieira et al., 2010]] ; [[#Obu--2020|Obu et al., 2020]] ). Ground ice volume in permafrost is variable, reaching up to 90% in syngenetic permafrost deposits ( [[#Kanevskiy--2013|Kanevskiy et al., 2013]] ; [[#Gilbert--2016|Gilbert et al., 2016]] ). The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) reported ''medium confidence'' in the estimation that Earth’s total perennial ground ice volume is equivalent to 2–10 cm of global sea level ( [[#Zhang--2000|Zhang et al., 2000]] ). There is no evidence suggesting that a large part of this volume, if melted, would run off and contribute to global sea level. Therefore, and because of the modest total volume of mobilizable water, the contribution of permafrost thaw to past and future sea level budgets is usually neglected (see [[#9.6.3.2|Section 9.6.3.2]] ).

Permafrost changes mostly refer to changes in extent, temperature and active layer thickness (ALT). The SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ; [[#Meredith--2019|Meredith et al., 2019]] ) reported with ''very high confidence'' that record high permafrost temperatures at the depth of the zero annual amplitude (the depth about 10–20 m below the surface where the seasonal soil temperature cycle vanishes) were attained in recent decades in the Northern circumpolar permafrost region, ''high confidence'' that permafrost has warmed over recent decades in many mountain ranges, and overall ''very high confidence'' that global warming over the last decades has led to widespread permafrost warming. As reported in SROCC, the global (polar and mountain) permafrost temperature has increased at 0.29°C ± 0.12°C near the depth of zero annual amplitude between 2007 and 2016 ( [[#Biskaborn--2019|Biskaborn et al., 2019]] ). Stronger warming has been observed in the continuous permafrost zone (0.39°C ± 0.15°C) compared to the discontinuous zone (0.20°C ± 0.10°C), consistent with the fact that, near the melting point, a large amount of energy is required for melting the ice (Figure 9.22), and because of the reduced effect of Arctic amplification in more southerly locations ( [[#Romanovsky--2017|Romanovsky et al., 2017]] ). This is consistent with longer-term Arctic trends from deep boreholes shown in Figure 2.22. Mountain permafrost temperature trends are heterogeneous, reflecting variations in local conditions such as topography, surface type, soil texture and snow cover, but again, generally weaker warming rates are observed in warmer permafrost at temperatures close to 0°C, particularly when ice content is high (e.g., [[#Mollaret--2019|Mollaret et al., 2019]] ; [[#Noetzli--2019|Noetzli et al., 2019]] ; [[#PERMOS--2019|PERMOS, 2019]] ). In summary, strong variability in recent permafrost temperature trends is linked to local conditions, regionally varying temperature trends, and the thermal state of permafrost itself. However, as discussed in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] , there is overall ''high confidence'' in the observed increases in permafrost temperature over the past three to four decades throughout the permafrost regions.

Closer to the surface, the active layer undergoes annual cycles of freeze and thaw. The SROCC reported ''medium confidence'' in ALT increase as a pan-Arctic phenomenon. Recent evidence presented in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] shows pervasive ALT increase in the European and Russian Arctic in the 21st century, and in high elevation areas in Europe and Asia since the mid-1990s. Emergence of a clearer global picture is hampered by: (i) uneven distribution of observing sites; (ii) substantial variability among the existing sites, strongly influenced by local conditions (soil constituents and moisture, snow cover, vegetation); (iii) interannual variability; and (iv) thaw settlement in ice-rich terrain ( [[#Streletskiy--2017|Streletskiy et al., 2017]] ; [[#O’Neill--2019|O’Neill et al., 2019]] ). In summary, in agreement with SROCC and recent evidence presented in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] , there is ''medium confidence'' that ALT increase is a pan-Arctic phenomenon.

There is ''medium confidence'' that the observed acceleration and destabilization of rock glaciers is related to warming temperatures and increase in water content at the permafrost table in recent decades ( [[#Deline--2015|Deline et al., 2015]] ; [[#Cicoira--2019|Cicoira et al., 2019]] ; [[#Marcer--2019|Marcer et al., 2019]] ; [[#PERMOS--2019|PERMOS, 2019]] ; [[#Kenner--2020|Kenner et al., 2020]] ). There is also ''medium confidence'' that observed increases in size and frequency of rock avalanches are linked to permafrost degradation in rock walls ( [[#Ravanel--2017|Ravanel et al., 2017]] ; [[#Patton--2019|Patton et al., 2019]] ; [[#Tapia%20Baldis--2019|Tapia Baldis and Trombotto Liaudat, 2019]] ). In summary, there is ''medium confidence'' that mountain permafrost degradation at high altitude has increased the instability of mountain slopes in the past decade.

The SROCC assessed with ''high confidence'' that the extent of subsea permafrost, formed before submersion on Arctic continental shelves during the last deglaciation, is much reduced compared to older studies that had estimated the entire formerly exposed Arctic shelf area to be underlain by permafrost. This is supported by observations ( [[#Shakhova--2017|Shakhova et al., 2017]] ) that show rapid thaw of recently submerged permafrost on the East Siberian Shelf. A modelling study ( [[#Overduin--2019|Overduin et al., 2019]] ) estimates that 97% of permafrost under Arctic shelves is currently thinning.

Based on multiple studies, there is ''medium confidence'' that widespread retreat of coastal permafrost is accelerating in the Arctic ( [[#Günther--2015|Günther et al., 2015]] ; [[#Cunliffe--2019|Cunliffe et al., 2019]] ; [[#Isaev--2019|Isaev et al., 2019]] ). There is also consistent evidence of complete permafrost thaw in areas of discontinuous and sporadic permafrost since about 1980, but this evidence is geographically scattered ( [[#Camill--2005|Camill, 2005]] ; [[#Kirpotin--2011|Kirpotin et al., 2011]] ; [[#James--2013|James et al., 2013]] ; B.M. [[#Jones--2016|]] [[#Jones--2016|Jones et al., 2016]] ; [[#Borge--2017|Borge et al., 2017]] ; [[#Chasmer--2017|Chasmer and Hopkinson, 2017]] ; [[#Gibson--2018|Gibson et al., 2018]] ). In spite of increasing evidence of landscape changes from site studies and remote sensing, quantifying permafrost extent change remains challenging because it is a subsurface phenomenon that cannot be observed directly ( [[#Jorgenson--2016|Jorgenson and Grosse, 2016]] ; [[#Trofaier--2017|Trofaier et al., 2017]] ). A modelling study for the Qinghai-Tibet Plateau between the 1960s and the 2000s ( [[#Ran--2018|Ran et al., 2018]] ) suggests transition from permafrost to seasonally frozen ground over an area of more than 400,000 km <sup>2</sup> . In summary, there is ''medium confidence'' that complete permafrost thaw in recent decades is a common phenomenon in discontinuous and sporadic permafrost regions. In addition, paleoclimatic evidence presented in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] confirms a long-term sensitivity of permafrost extent to climatic variations, although an analysis of North American speleothem records over the last two glacial cycles indicates that this apparent high sensitivity could be a consequence of regional-scale variability ( [[#Batchelor--2019|Batchelor et al., 2019]] ).

There is a lack of formal studies attributing observed permafrost changes (thaw depth, thermal state) or associated landscape changes to anthropogenic forcing. However, the observed Arctic warming has been attributed to anthropogenic forcing (e.g., [[#Najafi--2015|Najafi et al., 2015]] ) and an obvious physical link exists between ground temperatures (and thus permafrost) and surface air temperatures. Therefore, physically consistent and convergent lines of evidence lead to ''medium confidence'' in anthropogenic forcing being the dominant cause of the observed pan-Arctic permafrost changes. Added to this, local permafrost change by soil and ecosystem disturbance is induced by increasing human industrial activities in the Arctic (e.g., [[#Raynolds--2014|Raynolds et al., 2014]] ).

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