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

== Box 2.2 Local, Regional and Global Climate Feedbacks Involving the Mountain Cryosphere ==

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The cryosphere interacts with the environment and contributes to several climate feedbacks, most notably ones involving the snow cover, referred to as the snow albedo feedback. The presence or absence of snow on the ground drives profound changes in the energy budget of land surfaces, hence influencing the physical state of the overlying atmosphere (Armstrong and Brun, 2008 <sup>[[#fn:r221|221]]</sup> ). The reduction of snow on the ground, potentially amplified by aerosol deposition and modulated by interactions with the vegetation, increases the absorption of incoming solar radiation and leads to atmospheric warming. In mountain regions, this positive feedback loop mostly operates at the local scale and is seasonally variable, with most visible effects at the beginning and end of the snow season (Scherrer et al., 2012 <sup>[[#fn:r222|222]]</sup> ). Examples of other mechanisms contributing to local feedbacks are introduced in Box 2.1. At the regional scale, feedbacks associated with deposition of light absorbing particles and enhanced snow albedo feedback were shown to induce surface air warming (locally up to 2oC) (Ménégoz et al., 2014 <sup>[[#fn:r223|223]]</sup> ) with accelerated snow cover reduction (Ji, 2016; Xu et al., 2016 <sup>[[#fn:r224|224]]</sup> ), and may also influence the Asian monsoon system (Yasunari et al., 2015). However, many of these studies have considered so-called rapid adjustments, without changes in large-scale atmospheric circulation patterns, because they used regional or global models constrained by large-scale synoptic fields. In summary, regional climate feedbacks involving the high mountain cryosphere, particularly the snow albedo feedback, have only been detected in large mountain regions such as the Himalaya, using global and regional climate models (medium confidence).

Global-scale climate feedbacks from the cryosphere remain largely unexplored with respect to the proportion originating from high mountains. Although mountain topography affects global climate (e.g., Naiman et al., 2017), there is little evidence for mountain-cryosphere specific feedbacks, largely because of the limited spatial extent of the mountain cryosphere. The most relevant feedback probably relates to permafrost in mountains, which contain about 28% of the global permafrost area (Section 2.2.4). Organic carbon stored in permafrost can be decayed following thaw and transferred to the atmosphere as carbon dioxide or methane (Schuur et al., 2015 <sup>[[#fn:r226|226]]</sup> ). This self-reinforcing effect accelerates the pace of climate change and operates in polar (Section 3.4.1.2.3) and mountain areas alike (Mu et al., 2017 <sup>[[#fn:r227|227]]</sup> ; Sun et al., 2018a <sup>[[#fn:r228|228]]</sup> ). In contrast to polar areas, however, there is ''limited evidence'' and ''low agreement'' on the total amount of permafrost carbon in mountains because of differences in upscaling and difficulties to distinguish permafrost and seasonally frozen soils due to the lack of data. For example, on the Tibetan Plateau, the top 3 m of permafrost are estimated to contain about 15 petagrams (Ding et al., 2016 <sup>[[#fn:r229|229]]</sup> ) and mountain soils with permafrost globally are estimated to contain approximately 66 petagrams of organic carbon (Bockheim and Munroe, 2014 <sup>[[#fn:r230|230]]</sup> ). At the same time, there is ''limited evidence'' and ''high agreement'' that the average density (kg C m -2 ) of permafrost carbon in mountains is lower than in other areas. For example, densities of soil organic carbon are low in the sub-arctic Ural (Dymov et al., 2015 <sup>[[#fn:r231|231]]</sup> ) and 1–2 orders of magnitude lower in subarctic Sweden (Fuchs et al., 2015 <sup>[[#fn:r232|232]]</sup> ) in comparison to lowland permafrost, and 50% lower in mountains than in steppe-tundra in Siberia and Alaska (Zimov et al., 2006 <sup>[[#fn:r233|233]]</sup> ). Some mechanisms of soil carbon decay and transfer to the atmosphere in mountains are similar to those in lowland areas, for example collapse following thaw in peatlands (Mu et al., 2016 <sup>[[#fn:r234|234]]</sup> ; Mamet et al., 2017 <sup>[[#fn:r235|235]]</sup> ), and some are specific to areas with steep slopes, for example drainage of water from thawing permafrost leading to soil aeration (Dymov et al., 2015 <sup>[[#fn:r236|236]]</sup> ). There is no global-scale analysis of the climate feedback from permafrost in mountains. Given that projections indicate increasing thaw and degradation of permafrost in mountains during the 21st century ( ''very'' ''high confidence'' ) (Section 2.2.4), a corresponding increase in greenhouse gas emissions can be anticipated but is not quantified.

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