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

=== 9.5.2 Permafrost ===

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This section focuses on the physical aspects of permafrost (perennially frozen ground) as an element of the climate system, drawing on the assessment of observed global permafrost changes provided in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] , and more specifically model evaluation and projections. The permafrost carbon feedback is assessed in Box 5.1. [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] of this Report provides permafrost information relevant to impacts and risk on regional scales.

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==== 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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==== 9.5.2.2 Evaluation of Permafrost in Climate Models ====

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As stated in AR5 ( [[#Flato--2013|Flato et al., 2013]] ), coupled models contributing to CMIP5 showed large inter-model variability of permafrost extent due to deficiencies in reproducing surface characteristics and processes ( [[#Koven--2013|Koven et al., 2013]] ), particularly thermal properties of the ground and snow. These deficiencies led SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) to express only ''medium confidence'' in the models’ capacity to correctly project the magnitude of future permafrost changes, in spite of ''high confidence'' in the models’ projection of a general thaw depth increase and a substantial loss of shallow permafrost. The SROCC further noted that several types of physical ‘pulse’ disturbances, in particular fire and thermokarst formation, are usually not represented in coupled climate models. This has been discussed in detail in SROCC, which assessed that there is ''high confidence'' that permafrost degradation through fire ( [[#Jones--2015|Jones et al., 2015]] ; [[#Gibson--2018|Gibson et al., 2018]] ) is currently occurring faster in some well-studied regions than during the first half of the 20th century, and ''medium confidence'' that thermokarst formation, to which about 20% of the northern permafrost region is vulnerable ( [[#Olefeldt--2016|Olefeldt et al., 2016]] ), can lead to faster large-scale permafrost degradation in response to climate change.

Since SROCC, dedicated modelling of the evolution of ice- and organic-rich permafrost in the north-east Siberian lowlands ( [[#Nitzbon--2020|Nitzbon et al., 2020]] ) has shown that not representing thermokarst-inducing processes in ice-rich terrain leads to a systematic underestimation of the rapidity and magnitude of permafrost thaw. Simplified inventory-based modelling ( [[#Turetsky--2020|Turetsky et al., 2020]] ) points towards similar conclusions. Although these pulse disturbances still need to be represented in CMIP-type models, there have been many new developments to that type of model since CMIP5 and AR5. Soil freezing and its thermal and hydrological effects are now included in a large number of land-surface modules that are part of the CMIP6 ensemble (S. [[#Chadburn--2015|]] [[#Chadburn--2015|Chadburn et al., 2015]] ; [[#Hagemann--2016|Hagemann et al., 2016]] ; [[#Cuntz--2018|Cuntz and Haverd, 2018]] ; [[#Guimberteau--2018|Guimberteau et al., 2018]] ; [[#Yokohata--2020|Yokohata et al., 2020]] ), sometimes allowing for the effects of excess ice ( [[#Lee--2014|Lee et al., 2014]] ). Improved representation of snow insulation in models has led to more realistic simulated permafrost extents (e.g., [[#Paquin--2015|Paquin and Sushama, 2015]] ). In a post-CMIP5 ensemble of land-surface models driven by observed meteorological conditions ( [[#McGuire--2016|McGuire et al., 2016]] ), inter-model spread was substantially reduced when the ensemble was restricted to models that appropriately represented the effect of snow insulation on the underlying soil (W. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ). More detailed descriptions of high-latitude vegetation characteristics, vegetation dynamics, and snow-vegetation interactions have been included in several models since AR5 (S.E. [[#Chadburn--2015|]] [[#Chadburn--2015|Chadburn et al., 2015]] ; [[#Porada--2016|Porada et al., 2016]] ; [[#Druel--2017|Druel et al., 2017]] ).

A total soil column depth of at least about 10 m is required to adequately represent the dampening effect of seasonal-scale heat exchanges between shallow and deeper ground, and thus to correctly simulate ALT ( [[#Lawrence--2008|Lawrence et al., 2008]] ; [[#Ekici--2015|Ekici et al., 2015]] ). However, many CMIP6 models still have shallower total soil columns ( [[#Burke--2020|Burke et al., 2020]] ) and the proportion of models with deeper total soil columns has not increased since CMIP5 ( [[#Koven--2013|Koven et al., 2013]] ). Another recently identified process, usually not represented in the current (CMIP6) generation of climate models ( [[#Zhu--2019|Zhu et al., 2019]] ), is warming-driven decomposition and burning of organic material that provides strong thermal insulation of underlying ground. Decay of the insulating organic material can lead to increased permafrost thaw, creating a positive feedback loop.

In spite of the aforementioned structural improvements to many models, the simulated current permafrost extent from available CMIP6 models shows no substantial improvement with respect to CMIP5 (see Figure 9.22a). The extent of the region where permafrost is simulated within the top 15 m in the Northern Hemisphere for the 1979–1998 period is characterized by very large scatter in the coupled CMIP5 and CMIP6 historical simulations compared to estimates of the present permafrost extent based on multiple observational lines of evidence ( [[#Zhang--1999|Zhang et al., 1999]] ) and models based on satellite observations and reanalyses ( [[#Gruber--2012|Gruber, 2012]] ; [[#Obu--2019|Obu et al., 2019]] ). Outliers with very low simulated permafrost extent are models that have only a very shallow soil column (leading to an underestimate of thermal inertia at depth) and do not take into account soil water phase changes. These inadequacies lead to an overestimate of seasonal thaw depth, exceeding the total thickness of the models’ soil columns ( [[#Burke--2020|Burke et al., 2020]] ). Excessive simulated permafrost extent can in several cases be traced to insufficient thermal insulation by the winter snow cover ( [[#Burke--2020|Burke et al., 2020]] ).

Figure 9.22a also shows that the corresponding land-atmosphere simulations with prescribed observed sea surface temperatures and sea ice concentrations, and the land-only simulations with prescribed reanalysis-based meteorological forcing, do not provide an improved simulation of the current permafrost extent, although, by construction, they can be expected to exhibit lower land surface climate biases. This further points to deficiencies in the land modules as the main reason for biases, consistent with conclusions drawn from the analysis of CMIP5 output ( [[#Koven--2013|Koven et al., 2013]] ), as reported in SROCC and AR5.

In spite of more realistic description of permafrost-related processes in many coupled climate models, the CMIP6 models still produce a very scattered ensemble of estimates of current permafrost extent, and there is ''high confidence'' that this is strongly linked to deficiencies of the representation of soil processes. Furthermore, current-generation climate models tend to neglect several physical disturbances that can lead to faster permafrost thaw. Because of large uncertainties in the future evolution of these drivers (see SROCC), there is ''limited evidence'' that these shortcomings lead to an underestimate of permafrost degradation rates in response to climate change in the CMIP6 ensemble. In summary, there is ''high confidence'' that coupled models correctly simulate the sign of future permafrost changes linked to surface climate changes, but only ''medium confidence'' in the amplitude and timing of the transient response.

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'''Figure 9.22''' '''|''' '''Simulated versus observed permafrost extent and volume change by warming level. (a)''' Diagnosed Northern Hemisphere permafrost extent (area with perennially frozen ground at 15 m depth, or at the deepest model soil level if this is above 15 m) for 1979–1998, for available Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5 and CMIP6) models, from the first ensemble member of the historical coupled run, and for CMIP6 Atmospheric Model Intercomparison Project (AMIP) (atmosphere+land surface, prescribed ocean) and land-hist (land only, prescribed atmospheric forcing) runs. Estimates of current permafrost extents based on physical evidence and reanalyses are indicated as black symbols – triangle: [[#Obu--2018|Obu et al. (2018)]] ; star: [[#Zhang--1999|Zhang et al. (1999)]] ; circle: central value and associated range from [[#Gruber--2012|Gruber (2012)]] . '''(b)''' Simulated global permafrost volume change between the surface and 3 m depth as a function of the simulated global surface air temperature (GSAT) change, from the first ensemble members of a selection of scenarios, for available CMIP6 models. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

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==== 9.5.2.3 Projected Permafrost Changes ====

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The AR5 ( [[#Collins--2013|Collins et al., 2013]] ) and SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) (based on available CMIP5 output) both expressed ''high confidence'' that future pan-Arctic thaw depth will increase and near-surface permafrost extent will decrease under future global warming, and ''medium confidence'' in the magnitude of the simulated changes because of model deficiencies and the large spread of the results.

The equilibrium sensitivity of permafrost extent to stabilized global mean warming has been inferred (by constraining CMIP5 output with diagnosed relationships between the observed present-day spatial distribution of permafrost and air temperature) to be about 4.0×10 <sup>6</sup> km <sup>2</sup> °C <sup>–1</sup> ( [[#Chadburn--2017|Chadburn et al., 2017]] ) for global surface air temperature (GSAT) changes with respect to the present below about +3°C. This equilibrium permafrost sensitivity, relevant for assessing long-term permafrost changes at a stabilized warming level, is about 20% higher than the transient centennial-scale near-surface permafrost extent sensitivity (diagnosed from seasonal thaw down to 3 m depth) suggested by direct analysis of CMIP5 output ( [[#Slater--2013|Slater and Lawrence, 2013]] ). Compared to these and other studies reported in AR5 and SROCC ( [[#Koven--2013|Koven et al., 2013]] ), the recently suggested equilibrium extent sensitivity to GSAT changes of about 1.5×10 <sup>6</sup> km <sup>2</sup> °C <sup>–1</sup> based on idealized ground temperature modelling ( [[#Liu--2021|Liu et al., 2021]] ) appears unrealistically low.

A strong transient temperature sensitivity of the volume of perennially frozen soil in the top 3 m below the surface is consistently suggested by the available CMIP6 models (Figure 9.22b). Relative to the current volume, the transient sensitivity of the modelled permafrost volume in the top 3 m to GSAT changes (with respect to the 1995–2014 average and up to +3°C change, that is, about up to +4°C with respect to pre-industrial levels) is about 25 ± 5 % °C <sup>–1</sup> ( [[#Burke--2020|Burke et al., 2020]] ), but there is only ''medium confidence'' in this value and 1 standard deviation uncertainty range because of the model deficiencies discussed in 9.5.2.2. It is important to note that permafrost loss will not be limited to the top 3 m, with delayed response of deeper permafrost. The simulated transient temperature sensitivity of permafrost volume is slightly stronger in the SSP1-2.6 scenario than in other SSPs because subsurface temperature lag increases with higher atmospheric warming rates, particularly when ground ice melting induces additional delays.

Due to the role of air temperature as a major driver of permafrost change, SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) expressed ''very high confidence'' that 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 emissions scenarios. Recently published studies (e.g., [[#Zhao--2019|Zhao et al., 2019]] ) support this SROCC assessment.

In summary, based on ''high agreement'' across CMIP6 and older model projections, fundamental process understanding, and paleoclimate evidence, it is ''virtually certain'' that permafrost extent and volume will shrink as global climate warms.

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