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

==== 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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