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

==== 2.5.3.2 Feedbacks to climate from high-latitude land-surface changes ====

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In high latitudes, snow albedo and permafrost carbon feedbacks are the most well-known and most important surface-related climate feedbacks because of their large-scale impacts.

In response to ongoing and projected decrease in seasonal snow cover (Derksen and Brown 2012 <sup>[[#fn:r1266|1266]]</sup> ; Brutel-Vuilmet et al. 2013 <sup>[[#fn:r1267|1267]]</sup> ) warming is, and will continue to be, enhanced in boreal regions ( ''high confidence'' ) (Brutel-Vuilmet et al. 2013 <sup>[[#fn:r1268|1268]]</sup> ; Perket et al. 2014 <sup>[[#fn:r1269|1269]]</sup> ; Thackeray and Fletcher 2015 <sup>[[#fn:r1270|1270]]</sup> ; Mudryk et al. 2017 <sup>[[#fn:r1271|1271]]</sup> ). One reason for this is the large reflectivity (albedo) the snow exerts on shortwave radiative forcing: the all- sky global land snow shortwave radiative effect is evaluated to be around –2.5 ± 0.5 W m <sup>–2</sup> (Flanner et al. 2011 <sup>[[#fn:r1272|1272]]</sup> ; Singh et al. 2015 <sup>[[#fn:r1273|1273]]</sup> ). In the southern hemisphere, perennial snow on the Antarctic is the dominant contribution, while in the northern hemisphere, this is essentially attributable to seasonal snow, with a smaller contribution  from snow on glaciated areas. Another reason is the sensitivity of snow cover to temperature: Mudryk et al. (2017) <sup>[[#fn:r1274|1274]]</sup> recently showed that, in the high latitudes, climate models tend to correctly represent this sensitivity, while in mid-latitude and alpine regions, the simulated snow cover sensitivity to temperature variations tends to be biased low. In total, the global snow albedo feedback is about 0.1 W m <sup>–2</sup> K <sup>–1</sup> , which amounts to about 7% of the strength of the globally dominant water vapour feedback (e.g., Thackeray and Fletcher (2015) <sup>[[#fn:r1275|1275]]</sup> . While climate models do represent this feedback, a persistent spread in the modelled feedback strength has been noticed (Qu and Hall 2014 <sup>[[#fn:r1276|1276]]</sup> ) and, on average, the simulated snow albedo feedback strength tends to be somewhat weaker than in reality ( ''medium confidence'' ) (Flanner et al. 2011 <sup>[[#fn:r1277|1277]]</sup> ; Thackeray and Fletcher 2015 <sup>[[#fn:r1278|1278]]</sup> ). Various reasons for the spread and biases of the simulated snow albedo feedback have been identified, notably inadequate representations of vegetation masking snow in forested areas (Loranty et al. 2014 <sup>[[#fn:r1279|1279]]</sup> ; Wang et al. 2016c <sup>[[#fn:r1280|1280]]</sup> ; Thackeray and Fletcher 2015 <sup>[[#fn:r1281|1281]]</sup> ).

The second most important potential feedback from land to climate relates to permafrost decay. There is ''high confidence'' that, following permafrost decay from a warming climate, the resulting emissions of CO <sub>2</sub> and/or CH <sub>4</sub> (caused by the decomposition of organic matter in previously frozen soil) will produce additional GHG- induced warming. There is, however, substantial uncertainty on the magnitude of this feedback, although recent years have seen large progress in its quantification. Lack of agreement results from several critical factors that carry large uncertainties. The most important are (i) the size of the permafrost carbon pool, (ii) its decomposability, (iii) the magnitude, timing and pathway of future high-latitude climate change, and (iv) the correct identification and model representation of the processes at play (Schuur et al. 2015 <sup>[[#fn:r1282|1282]]</sup> ). The most recent comprehensive estimates establish a total soil organic carbon storage in permafrost of about 1500 ± 200 PgC (Hugelius et al. 2014 <sup>[[#fn:r1283|1283]]</sup> , 2013 <sup>[[#fn:r1284|1284]]</sup> ; Olefeldt et al. 2016 <sup>[[#fn:r1285|1285]]</sup> ), which is about 300 Pg C lower than previous estimates ( ''low confidence'' ). Important progress has been made in recent years at incorporating permafrost-related processes in complex ESMs (e.g., McGuire et al. (2018) <sup>[[#fn:r1286|1286]]</sup> ), but representations of some critical processes such as thermokarst formation are still in their infancy (Schuur et al. 2015) <sup>[[#fn:r1287|1287]]</sup> . Recent model-based estimates of future permafrost carbon release (Koven et al. 2015 <sup>[[#fn:r1288|1288]]</sup> ; McGuire et al. 2018 <sup>[[#fn:r1289|1289]]</sup> ) have converged on an important insight. Their results suggest that substantial net carbon release of the coupled vegetation-permafrost system will probably not occur before about 2100 because carbon uptake by increased vegetation growth will initially compensate for GHG releases from permafrost ( ''limited evidence, high agreement'' ).

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