Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.10.4 Mountains ====

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Mountains cover about 30% of the land areas on Earth (not counting Antarctica) and deliver a number of vital services to humanity (WGII Cross-Chapter Paper 5; [[#IPCC--2019b|IPCC, 2019b]] ). Climate change in high mountains was addressed in SROCC, which emphasized changes in several climatic impact-drivers. These included an observed general decline in low-elevation snow cover, glaciers and permafrost ( ''high confidence'' ), which induced changes in natural hazards such as decrease in slope stability ( ''high confidence'' ), changes to the frequency of glacial lake outbursts ( ''limited evidence'' ), and climate effects on other climatic impact-drivers (avalanche, rain-on-snow floods) with various degrees of confidence ( [[#Hock--2019|Hock et al., 2019]] ).

There is a growing body of literature indicating elevation-dependent warming (EDW; different rates of warming by altitude although not necessarily increasing with altitude) in several mountain regions but not globally ( [[#Hock--2019|Hock et al., 2019]] ; [[#Pepin--2019|Pepin et al., 2019]] ; [[#Ahmed--2020|Ahmed et al., 2020]] ; [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|B. Li et al., 2020]] ; [[#Williamson--2020|Williamson et al., 2020]] ; [[#You--2020|You et al., 2020]] ; [[#Micu--2021|Micu et al., 2021]] ). Statistically significant elevational enhancement to long-term trends in maximum near-surface air temperatures and diurnal temperature range were observed in southern central Himalaya and in the Swiss Alps ( [[#Rottler--2019|Rottler et al., 2019]] ; [[#Thakuri--2019|Thakuri et al., 2019]] ). [[#Aguilar-Lome--2019|Aguilar-Lome et al. (2019)]] reported that winter daytime land surface temperatures in the Andean region between 7°S and 20°S show the strongest trends at higher elevations: +1.7°C per decade above 5000 m above sea level. [[#Palazzi--2019|Palazzi et al. (2019)]] identified changes in albedo and downward thermal radiation as key drivers of EDW according to the simulation outputs of a high-spatial-resolution model in three important mountainous areas: the Colorado Rocky Mountains, the Greater Alpine Region and the Himalayas–Tibetan Plateau, but mechanisms for EDW remain complex ( [[#Hock--2019|Hock et al., 2019]] ). Warming is also affecting mountain lake surface temperatures, increasing probabilities of ice-free winters and the frequency and duration of ‘lake heatwaves’ ( ''high confidence'' ) ( [[#O’Reilly--2015|O’Reilly et al., 2015]] ; [[#Woolway--2020|Woolway et al., 2020]] , 2021) with a high variability from lake to lake.

Elevation-dependent warming could speed up the observed, rapid upward shifts of the freezing level height (FLH) in several mountainous regions of the world and lead to faster changes in the snowline, the glacier equilibrium-line altitude and the snow/rain transition height ( ''high confidence'' ). In the Indus, Ganges and Brahmaputra basins in Asia, the FLH is projected to rise at a rate of 4.4 to 10.0 m yr <sup>–1</sup> under RCP8.5 ( [[#Viste--2015|Viste and Sorteberg, 2015]] ). In the Argentinian Andes, FLH is projected under RCP8.5 to move up more than twice as much by 2070 as during the entire Holocene under the worst case scenario ( [[#Drewes--2018|Drewes et al., 2018]] ). On the western slope of the subtropical Andes (30°S–38°S) in central Chile, the mean value of the free tropospheric height of the 0°C isotherm under wet conditions is projected to be close to or higher than the upper quartile of the distribution in the current climate, towards the end of the century and under RCP8.5 ( [[#Mardones--2020|Mardones and Garreaud, 2020]] ). In the Alps and the Pyrenees, [[#Spandre--2019|Spandre et al. (2019)]] projected a rise in the natural snow elevation of between 200–300 m and 400–600 m by mid-century under RCP2.6 and RCP8.5, respectively. In the same region, the environmental equilibrium-line altitude is projected to exceed the maximum elevation of 69%, 81% and 92% of the glaciers by the end of the century under RCPs 2.6, 4.5 and 8.5, respectively ( [[#Žebre--2021|Žebre et al., 2021]] ).

Orographic effects enhance convection and stratiform heavy precipitation (due to uplift) and make mountains prone to extreme precipitation events. These events are projected to increase in major mountainous regions (Alps, parts of the Andes, British Columbia, North-Western North America, Calabria, Carpathian, Hindu-Kush-Himalaya, Rocky Mountains, Umbria; ''medium'' to ''high confidence'' depending on location), with potential cascading consequences of floods, landslides and lake outbursts in mountainous areas in all scenarios ( ''medium confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-11|Chapter 11]] and Sections 12.4.1–12.4.9; [[#Geertsema--2006|Geertsema et al., 2006]] ; [[#Gaire--2015|Gaire et al., 2015]] ; [[#Kim--2015|Kim et al., 2015]] ; [[#Ciabatta--2016|Ciabatta et al., 2016]] ; [[#Gariano--2016|Gariano and Guzzetti, 2016]] ; [[#Kharuk--2016|Kharuk et al., 2016]] ; [[#Syed--2016|Syed and Al Amin, 2016]] ; [[#Cloutier--2017|Cloutier et al., 2017]] ; [[#Gądek--2017|Gądek et al., 2017]] ; [[#Jurchescu--2017|Jurchescu et al., 2017]] ; [[#Rajczak--2017|Rajczak and Schär, 2017]] ; [[#Alvioli--2018|Alvioli et al., 2018]] ; [[#Coe--2018|Coe et al., 2018]] ; [[#Schlögl--2018|Schlögl and Matulla, 2018]] ; C.-W. [[#Chen--2019|]] [[#Chen--2019|Chen et al., 2019]] ; [[#Handwerger--2019|Handwerger et al., 2019]] ; [[#Hock--2019|Hock et al., 2019]] ; [[#Patton--2019|Patton et al., 2019]] ; [[#Vaidya--2019|Vaidya et al., 2019]] ; [[#Kirschbaum--2020|Kirschbaum et al., 2020]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ).

Declines in low-elevation snow depth and seasonal extent are projected for all SSP-RCPs (see Sections 12.4.1–12.4.6), along with reductions in mountain glacier surface area, increases in permafrost temperature, decreases in permafrost thickness, changes in lake and river ice, changes in the amount and seasonality of streamflows and hydrologic droughts in snow-dominated and glacier-fed river basins (e.g., in Central Asia; [[#Sorg--2014|Sorg et al., 2014]] ; [[#Reyer--2017b|Reyer et al., 2017b]] ) ( ''medium confidence'' ), and decreases in the stability of mountain slopes and snowfields. Glacier recession could lead to the creation of new glacial lakes in places like the Himalaya-Karakoram region ( [[#Linsbauer--2016|Linsbauer et al., 2016]] ) and in Alaska and Canada ( [[#Carrivick--2016|Carrivick and Tweed, 2016]] ; [[#Harrison--2018|Harrison et al., 2018]] ) ( ''medium confidence'' ). With increasing temperature and precipitation these can increase the occurrence of glacier lake outburst floods and landslides over moraine-dammed lakes ( ''high confidence'' ) ( [[#Carey--2012|Carey et al., 2012]] ; [[#Rojas--2014|Rojas et al., 2014]] ; [[#Iribarren%20Anacona--2015|Iribarren Anacona et al., 2015]] ; [[#Cook--2016|Cook et al., 2016]] ; [[#Haeberli--2017|Haeberli et al., 2017]] ; [[#Kapitsa--2017|Kapitsa et al., 2017]] ; [[#Narama--2018|Narama et al., 2018]] ; [[#Wilson--2018|Wilson et al., 2018]] ; [[#Drenkhan--2019|Drenkhan et al., 2019]] ; S. [[#Wang--2020|]] [[#Wang--2020|Wang et al., 2020]] ).

'''In conclusion, mountains face complex challenges from specific climatic impact-drivers drastically influenced by climate change: regional elevation-dependent warming''' ( high confidence '''), low-to-mid-altitude snow cover and sno''' '''w-sea''' '''son decrease even as some high elevations see more snow''' ( high confidence '''), glacier mass reduction and permafrost thawing''' ( high confidence '''), and increases in extreme precipitation and floods in most parts of major mountain ranges''' ( medium confidence ''').'''

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