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

=== 2.2.1 Atmospheric Drivers of Changes in the Mountain Cryosphere ===

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Past changes of surface air temperature and precipitation in high mountain areas have been documented by ''in situ'' observations and regional reanalyses (Table SM2.2 and Table SM2.4). However, mountain observation networks do not always follow standard measurement procedures (Oyler et al., 2015 <sup>[[#fn:r5|5]]</sup> ; Nitu et al., 2018 <sup>[[#fn:r6|6]]</sup> ) and are often insufficiently dense to capture fine-scale changes (Lawrimore et al., 2011 <sup>[[#fn:r7|7]]</sup> ) and the underlying larger scale patterns. Future changes are projected using General Circulation Models (GCMs) or Regional Climate Models (RCMs) or simplified versions thereof (e.g., Gutmann et al., 2016), used to represent processes at play in a dynamically consistent manner, and to relate mountain changes to larger-scale atmospheric forcing based on physical principles. Existing mountain-specific model studies typically cover individual mountain ranges, and there is currently no initiative found, such as model intercomparisons or coordinated model experiments, which specifically and comprehensively addresses high mountain meteorology and climate globally. This makes it difficult to provide a globally uniform assessment.

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==== 2.2.1.1 Surface Air Temperature ====

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Mountain surface air temperature observations in Western North America, European Alps and High Mountain Asia show warming over recent decades at an average rate of 0.3°C per decade, with a ''likely'' range of ± 0.2°C, thereby outpacing the global warming rate 0.2 ± 0.1 °C per decade, (IPCC, 2018 <sup>[[#fn:r8|8]]</sup> ). Underlying data from global and regional studies are compiled in Table SM2.2, and Figure 2.2 provides a synthesis on mountain warming trends, mostly based on studies using ''in situ'' observations. Local warming rates depend on the season ( ''high confidence'' ). For example, in the European Alps, warming has been found to be more pronounced in summer and spring (Auer et al., 2007 <sup>[[#fn:r9|9]]</sup> ; Ceppi et al., 2012 <sup>[[#fn:r10|10]]</sup> ), while on the Tibetan Plateau warming is stronger in winter (Liu et al., 2009 <sup>[[#fn:r11|11]]</sup> ; You et al., 2010 <sup>[[#fn:r12|12]]</sup> ). Studies comparing observations at lower and higher elevation at the global scale indicate that warming is generally enhanced above 500 m above sea level (a.s.l.) (e.g., Wang et al., 2016a; Qixiang et al., 2018, Table SM2.2). At the local and regional scale, evidence for elevation dependent warming, i.e., that the warming rate is different across elevation bands, is scattered and sometimes contradictory (Box 2.1). On the Tibetan Plateau, evidence based on combining ''in situ'' observations (often scarce at high elevation) with remote sensing and modelling approaches, indicates that warming is amplified around 4,000 m a.s.l., but not above 5,000 m a.s.l. (Qin et al., 2009 <sup>[[#fn:r13|13]]</sup> ; Gao et al., 2018 <sup>[[#fn:r14|14]]</sup> ). Studies in the Italian Alps (Tudoroiu et al., 2016 <sup>[[#fn:r15|15]]</sup> ) and Southern Himalaya (Nepal, 2016 <sup>[[#fn:r16|16]]</sup> ) have shown higher warming at lower elevation. Evidence from Western North and South America is conflicting (Table SM2.2). In other regions, evidence to assess whether warming varies with elevation is insufficient. In summary, there is ''medium evidence'' ( ''medium agreement'' ) that surface warming is different across elevation bands. Observed changes also depend on the type of temperature indicator: changes in daily mean, minimum and maximum temperature can display contrasting patterns depending on region, season and elevation (Table SM2.2).

Attribution studies for changes in surface air temperature specifically in mountain regions are rare. Bonfils et al. (2008) <sup>[[#fn:r28|28]]</sup> and Dileepkumar et al. (2018) <sup>[[#fn:r29|29]]</sup> demonstrated that anthropogenic greenhouse gas emissions are the dominant factor in the recent temperature increases, partially compensated by other anthropogenic factors (land use change and aerosol emissions for Western USA and Western Himalaya, respectively). These findings are consistent with conclusions of AR5 regarding anthropogenic effects (Bindoff et al., 2013 <sup>[[#fn:r30|30]]</sup> ). It is thus ''likely'' that anthropogenic influence is the main contributor to surface temperature increases in high mountain regions since the mid-20th century, amplified by regional feedbacks.

Until the mid-21st century, regardless of the climate scenario (Cross-Chapter Box 1 in Chapter 1), surface air temperature is projected to continue increasing ( ''very high confidence'' ) at an average rate of 0.3°C per decade, with a ''likely'' range of ±0.2°C per decade, locally even more in some regions, generally outpacing global warming rates (0.2 ± 0.1 °C per decade; IPCC, 201 <sup>[[#fn:r31|31]]</sup> 8) ( ''high confidence'' ). Beyond mid-21st century, atmospheric warming in mountains will be stronger under a high greenhouse gas emission scenario (RCP8.5) and will stabilise at mid-21st levels under a low greenhouse gas emission scenario (RCP2.6), similar to global change patterns ( ''very high confidence'' ). The warming rate will result from the combination of regional ( ''high confidence'' ) and elevation-dependent ( ''medium confidence'' ) enhancement factors. Underlying evidence of future projections from global and regional studies is provided in Table SM2.3. Figure 2.3 provides examples of regional climate projections of surface air temperature, as a function of elevation and season (winter and summer) in North America (Rocky Mountains), South America (Subtropical Central Andes), Europe (European Alps) and High Mountain Asia (Hindu Kush, Karakoram, Himalaya), based on global and regional climate projections.

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'''Figure 2.2'''

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'''Figure 2.2 | Synthesis of trends in mean annual surface air temperature in mountain regions, based on 4672 observation stations (partly overlapping) aggregated in 38 datasets reported in 19 studies. Each line refers to a warming rate from one dataset, calculated over the time period indicated by the extent of the line. Colours indicate mountain […]'''
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Figure 2.2 | Synthesis of trends in mean annual surface air temperature in mountain regions, based on 4672 observation stations (partly overlapping) aggregated in 38 datasets reported in 19 studies. Each line refers to a warming rate from one dataset, calculated over the time period indicated by the extent of the line. Colours indicate mountain region (Figure 2.1), and line thickness the number of observation stations used. Detailed references are found in Table SM2.2, which also provides additional information on trends for individual seasons and other temperature indicators (daily minimum or maximum temperature).

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