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

== 2.1 Introduction ==

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High mountain regions share common features, including rugged terrain, a low-temperature climate regime, steep slopes and institutional and spatial remoteness. These features are often linked to physical and social-ecological processes that, although not unique to mountain regions, typify many of the special aspects of these regions. Due to their higher elevation compared with the surrounding landscape, mountains often feature cryosphere components, such as glaciers, snow cover and permafrost, with a significant influence on surrounding lowland areas even far from the mountains (Huggel et al., 2015a <sup>[[#fn:r1|1]]</sup> ). Hence the mountain cryosphere plays a major role in large parts of the world. Considering the close relationship between mountains and the cryosphere, high mountain areas are addressed in a dedicated chapter within this special report. Almost 10% (671 million people) of the global population lived in high mountain regions in 2010, based on gridded population data (Jones and O’Neill, 2016 <sup>[[#fn:r2|2]]</sup> ) and a distance of less than 100 km from glaciers or permafrost located in mountains areas as defined in Figure 2.1. This population is expected to grow to 736–844 million across the shared socioeconomic pathways by 2050. Many people living outside of mountain areas and not included in these numbers are also affected by changes in the mountain cryosphere.

This chapter assesses recent and projected changes in glaciers, snow cover, permafrost and lake and river ice in high mountain areas, their drivers, as well as their impact on the different services provided by the cryosphere and related adaptation, with a focus on literature published after AR5. The assessment of cryospheric change is focused on recent decades rather than a perspective over a longer period, and future changes spanning the 21st century. A palaeo-perspective is covered in IPCC Sixth Assessment Report (AR6) Working Group I contribution on ‘The Physical Science Basis’. High mountain areas, as discussed here, include all mountain regions where glaciers, snow or permafrost are prominent features of the landscape, without a strict and quantitative demarcation, but with a focus on distinct regions (Figure 2.1). Mountain regions located in the polar regions are considered in Chapter 3 except those in Iceland, Scandinavia and Alaska and parts of adjacent Yukon Territory and British Columbia, which are included in this chapter. Many changes in the mountain environment are not solely or directly related to climate change induced changes in the cryosphere, but to other direct or indirect effects of climate change, or to other consequences of socioeconomic development. Consistent with the scope of this report with a focus on the ocean and the cryosphere, this section deals primarily with the impacts that can at least partially be attributed to cryosphere changes. Even though other drivers may be the dominant driver of change in many cases, they are not considered explicitly in this chapter, although unambiguous attribution to cryosphere changes is often difficult.

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'''Figure 2.1 | Distribution of mountain areas (orange shading) and glaciers (blue) as well as regional summary statistics for glaciers and permafrost in mountains. Mountains are distinguished based on a ruggedness index (&gt;3.5), a logarithmically scaled measure of relative relief (Gruber, 2012). Eleven distinct regions with glaciers, generally corresponding to the primary regions in the […]'''
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Figure 2.1 | Distribution of mountain areas (orange shading) and glaciers (blue) as well as regional summary statistics for glaciers and permafrost in mountains. Mountains are distinguished based on a ruggedness index (&gt;3.5), a logarithmically scaled measure of relative relief (Gruber, 2012 <sup>[[#fn:r3|3]]</sup> ). Eleven distinct regions with glaciers, generally corresponding to the primary regions in the Randolph Glacier Inventory, RGI v6.0 (RGI Consortium, 2017) are outlined, although some cryosphere related impacts presented in this chapter may go beyond these regions. Region names correspond to those in the RGI. Diamonds represent regional glacier area (RGI 6.0) and circles the permafrost area in all mountains within each region boundary (Obu et al., 2019 <sup>[[#fn:r4|4]]</sup> ). Histograms for each region show glacier and permafrost area in 200 m elevation bins as a percentage of total regional glacier and permafrost area, respectively. Also shown is the median elevation of the annual mean 0oC free-atmosphere isotherm calculated from the ERA-5 re-analysis of the European Centre for Medium Range Weather Forecasts over each region’s mountain area for the period 2006–2015, with 25–75% quantiles in grey. The annual 0oC isotherm elevation roughly separates the areas where precipitation predominantly falls as snow and rain. Areas above and below this elevation are loosely referred to as high and low elevations, respectively, in this chapter.

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