Editing IPCC:AR6/WGII/Cross-Chapter-Paper-5 (section)

==== CCP5.2.2.1 Water ====

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Water is a fundamental source of life in mountain regions; it is also a central element and ‘connector’ in coupled natural–human systems and carries diverse meanings in different sociocultural contexts, including in indigenous ontologies ( [[#Boelens--2014|Boelens, 2014]] ). In addition, water is a key component connecting upstream mountains and downstream lowlands ( [[#Salzmann--2016|Salzmann et al., 2016]] ; [[#Di%20Baldassarre--2018|Di Baldassarre et al., 2018]] ; [[#Encalada--2019|Encalada et al., 2019]] ). Mountains are of paramount importance as water towers for people living there and for around two billion people living in connected lowland areas ( [[#Immerzeel--2020|Immerzeel et al., 2020]] ; [[#Viviroli--2020|Viviroli et al., 2020]] ).

Mountain river systems are especially sensitive to and affected by climate change and continuing anthropogenic disturbance, including water pollution, hydropower development, water withdrawals for agriculture and human consumption and biodiversity loss and ecosystem changes ( ''high confidence'' ) ( [[#Honda--2016|Honda and Durigan, 2016]] ; [[#Encalada--2019|Encalada et al., 2019]] ; [[#Bissenbayeva--2021|Bissenbayeva et al., 2021]] ; [[#Chen--2021|Chen et al., 2021]] ). The effects of climate and cryosphere change in mountains on downstream water and river systems have been studied and quantified for many regions worldwide ( [[#Barnett--2005|Barnett et al., 2005]] ; [[#Huss--2011|Huss, 2011]] ; [[#Lutz--2014|Lutz et al., 2014]] ; [[#O’Neel--2015|O’Neel et al., 2015]] ; [[#Huss--2018|Huss and Hock, 2018]] ). Comprehensive approaches focusing on both water demand and supply aspects provide regionally or locally specified information on water availability, scarcity and security ( [[#Buytaert--2014|Buytaert et al., 2014]] ; [[#Drenkhan--2015|Drenkhan et al., 2015]] ; [[#Brunner--2019|Brunner et al., 2019]] ) (Chapter 4). Present and potential future hotspot regions of water scarcity that rely heavily on mountainous water sources include Central Asia, South Asia, tropical and subtropical western South America and southwestern North America ( ''robust evidence, medium agreement'' ) ( [[#Kummu--2016|Kummu et al., 2016]] ; [[#Biemans--2019|Biemans et al., 2019]] ; [[#Immerzeel--2020|Immerzeel et al., 2020]] ; [[#Viviroli--2020|Viviroli et al., 2020]] ).

Figure CCP5.2 represents different levels of dependences of lowland areas on mountain water. At a global scale, 68% of irrigated agricultural areas in lowlands depend on essential runoff contributions from the mountains. The dependence of lowland populations on essential mountain runoff contributions increased by a factor of more than three from the 1960s to the 2000s, with increases of up to ten-fold in some major river catchments ( [[#Viviroli--2020|Viviroli et al., 2020]] ).

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[[File:eb0e8abf4a6dfae6551c0ce1bf09c8ee IPCC_AR6_WGII_Figure_CCP5_002.png]]

'''Figure CCP5.2 |''' '''Dependence of land surface areas and population on mountain water resources, 1961–2050.''' Results are shown as decadal averages for lowland populations in each category of dependence on mountain water from no surplus and negligible to essential;

'''(a)''' map of global mountain regions and their differentiated importance for lowland water resources;

'''(b)''' map of lowland populations and their differentiated dependence on mountain water resources, both for the scenario combination SSP2-RCP6.0 and for the time period 2041–2050;

'''(c)''' number of lowland populations and their differentiated dependence on mountain water resources from 1960s to 2040s for three different scenario combinations (based on [[#Viviroli--2020|Viviroli et al., 2020]] ).

Many mountain regions have one or more cryosphere components (glaciers, permafrost and perennial or seasonal snow), and the mountain cryosphere is among the natural systems most sensitive to climate change worldwide ( ''high confidence'' ). The SROCC assessed a decline in all cryosphere components due to climate change over recent decades, i.e., for low-elevation snow cover ( ''high confidence'' ), permafrost ( ''high confidence'' ) and glaciers ( ''very high confidence'' ) ( [[#Hock--2019|Hock et al., 2019]] ). More recent studies using globally more complete data sets show a considerably higher glacier mass loss (267 ±16 Gt yr -1 ) for 2000–2019 as compared to a ( ''very likely'' [[#footnote-000|2]] ) range of 123 ±24 Gt yr -1 for 2006–2015 in SROCC, with a mass loss acceleration of 48 ±16 Gt yr -1 per decade over 2000–2019 ( [[#Hugonnet--2021|Hugonnet et al., 2021]] ). Assessment conclusions in SROCC found with ''high confidence'' that glacier shrinkage and snow cover changes over the past two decades have led to changes in the amount and timing of runoff in many mountain regions ( [[#Hock--2019|Hock et al., 2019]] ).

The effects of climate and environmental changes in upstream areas on downstream water quantity and quality, including nutrient, pollutant, heavy metals and sediment flux, have been assessed in only a limited number of catchments ( [[#Rakhmatullaev--2009|Rakhmatullaev et al., 2009]] ; [[#Dong--2015|Dong et al., 2015]] ; [[#Milner--2017|Milner et al., 2017]] ; [[#Ilyashuk--2018|Ilyashuk et al., 2018]] ; [[#Lane--2019|Lane et al., 2019]] ; [[#Li--2020|Li et al., 2020]] ; [[#Chen--2021|Chen et al., 2021]] ). Groundwater contributions to streamflow are highly variable in mountains but can be substantial (up to 70 to 80% or more) during low-flow periods ( [[#Frisbee--2011|Frisbee et al., 2011]] ; [[#Baraer--2015|Baraer et al., 2015]] ; [[#Gordon--2015|Gordon et al., 2015]] ; [[#Käser--2016|Käser and Hunkeler, 2016]] ; [[#Somers--2019|Somers et al., 2019]] ). Groundwater may provide some resilience to loss of melt water from glacier and snow decline, but in the longer term groundwater recharge and contribution to streamflow are expected to decrease with ongoing climate change ( ''medium confidence'' ) ( [[#Somers--2020|Somers and McKenzie, 2020]] ). In some mountain regions (e.g., in the Himalaya), springs are a particularly important source of water where large populations depend on them. Observations indicate a reduction of water provision from springs in recent years in the Himalaya, caused by multiple causal factors (human interventions, climatic) ( [[IPCC:Wg2:Chapter:Chapter-10#10.4.4|Section 10.4.4]] .).

Both small-scale interventions (e.g., livestock grazing in sensitive high-elevation wetlands) and high-investment interventions (e.g., hydropower dams and plants) in upstream regions can strongly affect water availability, river connectivity, biodiversity and catchment management ( [[#Anderson--2018|Anderson et al., 2018]] ; [[#Ramsar%20Convention%20on%20Wetlands--2018|Ramsar Convention on Wetlands, 2018]] ; [[#Encalada--2019|Encalada et al., 2019]] ) and are often contested and have led to conflict ( ''medium evidence, high agreement'' ) ( [[#Drenkhan--2015|Drenkhan et al., 2015]] ; [[#French--2015|French et al., 2015]] ). Climate change often exacerbates tensions or conflicts between different users over water at local, national and transboundary or regional scales, and many tensions and social or political conflicts are documented, especially in seasonally dry regions, where large power inequalities exist among users, where clear and established regulations are lacking, and especially in transboundary settings (e.g., Central Asia, Hindu Kush Himalaya [HKH], Andes) ( [[#Carey--2014|Carey et al., 2014]] ; [[#Bocchiola--2017|Bocchiola et al., 2017]] ; [[#Yapiyev--2017|Yapiyev et al., 2017]] ; [[#Hock--2019|Hock et al., 2019]] ; [[#Mukherji--2019|Mukherji et al., 2019]] ).

Water plays a fundamental role in climate change adaptation in mountains. A majority of documented adaptation efforts in mountain regions address water-related aspects (precipitation variability and extremes, including drought, water availability, floods) ( ''high confidence'' ) ( [[#McDowell--2019|McDowell et al., 2019]] , 2020). This is a robust finding across different mountain regions and adaptation project and programme types and is in line with findings for cryosphere-change-related adaptation, as reported in SROCC ( [[#Hock--2019|Hock et al., 2019]] ). Water also plays a role in adaptation in other sectors, such as agriculture, disaster management, and tourism and recreation ( [[#McDowell--2019|McDowell et al., 2019]] ). There is ''high confidence'' that water conservation efforts, including restoration and protection of particularly vulnerable areas (e.g., wetlands) and increase in efficiency in water use, are robust, low-regret adaptation measures.

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