Editing IPCC:AR6/WGII/Chapter-4 (section)

== 4.1 Centrality of Water Security in Climate Change and Climate Resilient Development ==

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Water security is defined as ‘ ''the capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development, for ensuring protection against water-borne pollution and water-related disasters, and for preserving ecosystems in a climate of peace and political stability'' ’ ( [[#Grey--2007|Grey and Sadoff, 2007]] ). Risks emanating from various aspects of water insecurity have emerged as a significant global challenge. The Global Risks Report by the World Economic Forum lists water crisis as one of the top five risks in all its reports since 2015 ( [[#WEF--2015|WEF, 2015]] ; [[#WEF--2016|WEF, 2016]] ; [[#WEF--2017|WEF, 2017]] ; [[#WEF--2018|WEF, 2018]] ; [[#WEF--2019|WEF, 2019]] ; [[#WEF--2020|WEF, 2020]] ). Water also features prominently in the SDGs ( [[#4.8|Section 4.8]] ) and plays a central role in various systems transitions needed for climate resilient development. Most SDGs cannot be met without access to adequate and safe water ( [[#Ait-Kadi--2016|Ait-Kadi, 2016]] ; Mugagga, 2016). In addition, without adequate adaptation, future water-related impacts of climate change on various sectors of the economy are projected to lower the global GDP by mid-century, with higher projected losses expected in low- and middle-income countries ( [[#World%20Bank--2017|World Bank, 2017]] ; [[#GCA--2019|GCA, 2019]] ).

There are at least four reasons for the centrality of water security in adapting to, and mitigating climate change.

First, approximately half the world’s population (~4 billion out of ~8 billion people) are assessed as being currently subject to severe water scarcity for at least some part fo the year ( ''medium confidence'' ) due to climatic and non-climate factors (Box 4.1). Water insecurity arises from many factors, both environmental and societal. Environmental factors include too little freshwater due to drought or pollution, and too much water, due to extreme precipitation and flooding, and are being affected by climate change. Societal factors include economic and governance-related barriers to water access or protection from water-related damages. Currently, many people are experiencing climate change on a day-to-day basis through water-related impacts such as the increased frequency and intensity of heavy precipitation ( ''high confidence'' ) ( [[#4.2.1.1|Section 4.2.1.1]] , [[#Seneviratne--2021|Seneviratne et al., 2021]] ); accelerated melting of glaciers ( ''high confidence'' ) ( [[#4.2.2|Section 4.2.2]] , [[#Douville--2021|Douville et al., 2021]] ); changes in frequency, magnitude and timing of floods ( ''high confidence'' ) ( [[#4.2.4|Section 4.2.4]] , [[#Seneviratne--2021|Seneviratne et al., 2021]] ); more frequent and severe droughts in some places ( ''high confidence'' ) ( [[#4.2.5|Section 4.2.5]] , [[#Seneviratne--2021|Seneviratne et al., 2021]] ); decline in groundwater storage and reduction in recharge ( ''medium confidence'' ) ( [[#4.2.6|Section 4.2.6]] , [[#Douville--2021|Douville et al., 2021]] ) and water quality deterioration due to extreme events ( ''medium confidence'' ) ( [[#4.2.7|Section 4.2.7]] ). For example, since the 1970s, 44% of all disaster events have been flood-related ( [[#WMO--2020|WMO, 2020]] ). With the added stressor of climate change, globally, a larger fraction of land and population are projected to face increased water scarcity due to climate change. For example, at an approximately 2°C GWL, between 0.9 and 3.9 billion people are projected to be at increased exposure to water stress, depending on regional patterns of climate change and the socioeconomic scenarios considered ( [[#Koutroulis--2019|Koutroulis et al., 2019]] ).

Second, while climate change directly affects freshwater availability across space and time, it also affects water requirements for different uses, such as irrigation, potentially adding to existing societal challenges ( [[#Bijl--2018|Bijl et al., 2018]] ). Vulnerability to water-related impacts of climate change and extreme weather are already felt in all major sectors and are projected to intensify in the future, for example, in agriculture ( ''high confidence'' ) (Sections 4.3.1, 4.5.1); energy and industry ( ''high confidence'' for observed drought impacts and projected impacts) (Sections 4.3.2, 4.5.2); water for health and sanitation ( ''high confidence'' about links to precipitation extremes and disease outbreaks) (Sections 4.3.3, 4.5.3); water for urban, peri-urban and municipal sectors ( ''medium confidence'' ) (Sections 4.3.4, 4.5.4) and freshwater ecosystems ( ''high confidence'' in climate change as a driver in degradation of freshwater ecosystems) (Sections 4.3.5, 4.5.5). Agriculture and irrigation account for the most significant proportion of consumptive water use and account for 60–70% of total water withdrawals ( [[#Hanasaki--2018|Hanasaki et al., 2018]] ; [[#Burke--2020|Burke et al., 2020]] ; [[#Müller%20Schmied--2021|Müller Schmied et al., 2021]] ). Globally, 10% of the most water-stressed basins account for 35% of global irrigated calorie production ( [[#Qin--2019|Qin et al., 2019]] ), and food production is at risk in those basins and worldwide due to changes in hydrological components of climate change. Lack of access to clean water and sanitation has been one of the leading causes of water-borne diseases. In 2017, approximately 2.2 billion people lacked access to safe drinking water, and roughly 4.2 billion people could not access safe sanitation ( [[#WHO%20and%20UNICEF--2019|WHO and UNICEF, 2019]] ). Inequities in access to safe water are being amplified during the current COVID-19 pandemic (Box 4. 5 and Cross-Chapter Box COVID in Chapter 7). The same 10% of most water-stressed basins also account for 19% of global thermal electricity generation ( [[#Qin--2019|Qin et al., 2019]] ), and globally, both production of hydropower and thermal power has been negatively affected by droughts and other extreme events. Globally, between 16% and 39% of cities experienced surface-water deficits between 1971 and 2000. If environmental flow requirements (EFRs) are accounted for, these numbers increase to 36% and 63%, respectively. Even under a scenario where urban water gets the highest priority, more than 440.5 million people in cities globally are projected to face a water deficit by 2050 ( [[#Flörke--2018|Flörke et al., 2018]] ). The situation is particularly precarious in the Global South, where most of the population lacks access to piped water ( [[#WRI--2019|WRI, 2019]] ).

Third, a large majority (~60%) of all adaptation responses documented since 2014 are about adapting to water-related hazards like droughts, floods and rainfall variability ( [[#Berrang-Ford--2021b|Berrang-Ford et al., 2021b]] ) ( ''high confidence'' ). Water-related adaptation action features prominently in NDC pledges by a large majority of countries in both Global North and Global South ( [[#GWP--2018|GWP, 2018]] ). These adaptation responses and their current benefits and effectiveness in reducing water-related risks in the future are systematically assessed in this chapter (Sections 4.6, 4.7.1, 4.7.2 and 4.7.3). These adaptation measures aim to reduce impacts of water-related hazards through responses such as irrigation, water and soil moisture conservation, rainwater harvesting, changes in crops and cultivars, improved agronomic practices, among others (Sections 4.6.2; 4.7.1). Only ~20% of all documented case studies on observed water-related adaptations measure outcomes (positive or negative), but the link between positive outcomes and climate risk reduction is unclear and remains challenging to assess ( [[#4.7.1|Section 4.7.1]] ) ( ''medium confidence'' ). On the other hand, most of the future projected water-related adaptations are more effective at lower GWLs (1.5°C) than at higher GWLs, showing the importance of mitigation for future adaptations to remain effective ( ''high confidence'' ).

Finally, while limiting global warming to 1.5°C would minimise the increase in risks in the various water use sectors and keep adaptation effective, many mitigation measures can potentially impact future water security. For example, bioenergy with carbon capture and storage (BECCS) and afforestation and reforestation can have a considerable water footprint if done at inappropriate locations ( [[#4.7.6|Section 4.7.6]] , see also [[#Canadell--2021|Canadell et al., 2021]] ). Therefore, minimising the risks to water security from climate change will require a full-systems view that considers the direct impacts of mitigation measures on water resources and their indirect effect via limiting climate change ( ''high confidence'' ).

This chapter draws on previous IPCC reports and new methodologies ( [[#4.1.1|Section 4.1.1]] and SM4.1, SM4.2) and assesses the impacts of climate change on natural and human dimensions of the water cycle with a particular focus on water-related vulnerabilities and adaptation responses (Figure 4.1). [[#4.2|Section 4.2]] assesses observed changes in the hydrological cycle, and [[#4.3|Section 4.3]] focuses on their societal impacts and detects which parts of these changes are directly attributable to climate change. [[#4.4|Section 4.4]] assesses projected risks of changes in the hydrological cycle on various components of the hydrological cycle, and [[#4.5|Section 4.5]] assesses the same for sectoral risks. Projections and risks assessments for future impacts are framed in terms of GWLs and time horizons, as these are useful for informing mitigation policy under the Paris Agreement and informing adaptation planning. Sections 4.6 and 4.7 assesses current and future water-related adaptation responses in reducing climate and associated impacts and risks and looks at limits to adaptations, especially in a future warmer world. Finally, [[#4.8|Section 4.8]] outlines the enabling principles for meeting water security, SDGs and climate resilient development.

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'''Figure 4.1 |''' '''Chapter structure.'''

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=== 4.1.1 Points of Departure and Advancements since AR5 ===

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The Fifth Assessment Report (AR5, [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ) concluded that for each degree of global warming, approximately 7% of the global population, under a scenario of moderate population growth, was projected to be exposed to a decrease of renewable water resources of at least 20%. In addition, AR5 reported negative impacts on streamflow volumes, its seasonality (specifically in cryospheric zones), a decline in raw water quality ( ''medium evidence, high agreement'' ) and projected reduction in renewable surface water and groundwater in most dry tropical regions. AR5 projected an increase in meteorological, agricultural and hydrological droughts in dry regions ( ''medium confidence'' ) ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ).

The Special Report on Global Warming of 1.5°C (SR1.5) assessed that limiting global warming to 1.5°C is expected to substantially reduce the probability of extreme droughts, precipitation deficits and risks associated with water availability in some regions ( ''medium confidence'' ). On the other hand, higher risks to natural and human systems in a 2.0°C world would mean increased vulnerability for the poor, showing that socioeconomic drivers are expected to have a more significant influence on water-related risks and vulnerabilities than changes in climate alone ( ''medium confidence'' ) ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ).

The Special Report on Oceans and Cryosphere in a Changing Climate (SROCC) confirmed findings from AR5, with ''robust evidence'' of declines in snow cover and negative mass balance in most glaciers globally. Glacier melting seriously threatens water supply to mountain communities and millions living downstream through water shortages, jeopardising hydropower generation, irrigation and urban water uses ( [[#Hock--2019b|Hock et al., 2019b]] ). Additionally, Arctic hydrology will be affected by permafrost changes, negatively impacting Arctic communities’ health and cultural identity ( [[#Meredith--2019|Meredith et al., 2019]] ).

The Special Report on Climate Change and Land (SRCCL) stated that groundwater over-extraction for irrigation is causing depletion of groundwater storage ( ''high confidence'' ). The report also noted that precipitation changes, coupled with human drivers, will have a role in causing desertification, and water-driven soil erosion is projected to increase due to climate change ( ''medium confidence'' ). The population vulnerable to impacts related to water is projected to increase progressively at 1.5°C, 2°C and 3°C of global warming, with half of those impacted residing in South Asia, followed by Central Asia, West Africa and East Asia. SRCCL stated that improved irrigation techniques (e.g., drip irrigation) and moisture conservation (e.g., rainwater harvesting using indigenous and local practices) could increase farmers’ adaptive capacity ( ''high confidence'' ) ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ).

The Sixth Assessment Report (AR6) Working Group I (WGI) ( [[#Douville--2021|Douville et al., 2021]] ) concluded that anthropogenic climate change has increased atmospheric moisture and precipitation intensity ( ''very likely'' by 2–3% per 1°C) ( ''high confidence'' ), increased terrestrial ET ( ''medium confidence'' ) and contributed to drying in dry summer climates including in the Mediterranean, southwestern Australia, southwestern South America, South Africa and western North America ( ''medium to high confidenc'' e), and has caused earlier onset of snowmelt and increased melting of glaciers ( ''high confidence'' ) since the mid-20 th century. The report also stated with ''high confidence'' that the water cycle variability and extremes are projected to intensify, regardless of the mitigation policy. The share of the global population affected by water-related hazards and water availability issues is projected to increase with the intensification of water cycle variability and extremes. They concluded with ''high confidence'' that strong and rapid mitigation initiatives are needed to avert the manifestation of climate change in all components of the global water cycle.

Building on these previous reports, this chapter advances understanding climate change-induced hydrological changes and their societal impacts and risk in several key ways.

First, since AR5, the methodology of climate change impact studies has advanced and these methodological advances are described in SM4.1. AR6 uses new projections (CMIP6) based on the SSPs and other scenarios, and we assess those results in this chapter alongside those using other projections and scenarios.

Second, this chapter follows the developments set in motion by SR1.5, SRCCL and SROCC to incorporate Indigenous knowledge (IK), traditional knowledge (TK) and local knowledge (LK). SR1.5 stated that disadvantaged and vulnerable populations, including Indigenous Peoples and certain local communities, are at disproportionately higher risk of suffering adverse consequences due to global warming of 1.5°C or more ( [[#Roy--2018|Roy et al., 2018]] ). SRCCL highlighted the enhanced efficacy of decision-making and governance with the involvement of local stakeholders, particularly those most vulnerable to climate change, such as Indigenous Peoples ( [[#Arneth--2019|Arneth et al., 2019]] ). SROCC found adaptation efforts have benefited from the inclusion of IK and LK (IKLK) ( [[#Abram--2019|Abram et al., 2019]] ). In this chapter, we engage directly with Indigenous contributing authors and use multiple evidence-based approaches, as undertaken by the IPBES ( [[#Tengö--2014|Tengö et al., 2014]] ; [[#Tengö--2017|Tengö et al., 2017]] ). This approach is guided by the understanding that the co-production of knowledge (between scholars and local communities) about water and climate change vulnerability, impacts and adaptation has the potential to lead to new water knowledge and context-specific governance strategies ( [[#Arsenault--2019|Arsenault et al., 2019]] ; [[#Chakraborty--2021|Chakraborty and Sherpa, 2021]] ). Additionally, shifting beyond the exclusive use of technical knowledge and Western viewpoints redresses the shortcomings of resource- and security-oriented understandings to water and acknowledges the more holistic and relational approaches common to IKLK ( [[#4.8.4|Section 4.8.4]] ) ( [[#Stefanelli--2017|Stefanelli et al., 2017]] ; [[#Wilson--2019|Wilson, 2019]] ; [[#Chakraborty--2021|Chakraborty and Sherpa, 2021]] ).

Finally, grounded in the AR6 goal to expand the solution space, this chapter advances the understanding of adaptation in the water sector since AR5 by deploying a meta-analysis of adaptation measures. The meta-analysis focuses on both current adaptation responses ( [[#4.7.1|Section 4.7.1]] ) and future projected adaptation responses, which have been modelled ( [[#4.7.2|Section 4.7.2]] ). The meta-review assesses the outcomes of current adaptation responses and effectiveness of future projected adaptations in reducing climate and associated risks. Studies derived from the Global Adaptation Mapping Initiative (GAMI) database ( [[#Berrang-Ford--2021a|Berrang-Ford et al., 2021a]] ) (see Chapter 16) were coded systematically following a meta-review protocol developed specifically for this assessment ( [[#Mukherji--2021|Mukherji et al., 2021]] ; SM4.2). A similar meta-review protocol was also developed to assess effectiveness of adaptations to reduce projected climate risks ( [[#4.7.2|Section 4.7.2]] ; SM4.2).

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'''Box 4.1 | Implications of Climate Change for Water Scarcity and Water Insecurity'''
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Water scarcity and water insecurity are related concepts but not identical, and each has a range of interpretations leading to some overlap. Water scarcity can be broadly described as a mismatch between the demand for fresh water and its availability, quantified in physical terms. Water security/insecurity is a broader concept with definitions beyond physical water scarcity, encompassing access to water services, safety from poor water quality and flooding, and appropriate water governance that ensures access to safe water ( [[#Sadoff--2020|Sadoff et al., 2020]] ). Metrics of water security include both physical and socioeconomic components and are a tool for comparison between different locations and countries regarding relative levels of water security in the context of water-related risks. Some definitions of water scarcity also incorporate these broader issues. For example, ‘economic water scarcity’ has been defined as a situation where ‘human, institutional, and financial capital limit access to water, even though water in nature is available locally to meet human demands’ ( [[#Comprehensive%20Assessment%20of%20Water%20Management%20in%20Agriculture--2007|Comprehensive Assessment of Water Management in Agriculture, 2007]] ). Economic water scarcity can also occur where infrastructure exists, but water distribution is inequitable ( [[#Jaeger--2017|Jaeger et al., 2017]] ). Much of the literature exploring the impacts of climate change on water security, however, focuses on quantifying physical water scarcity. Discussions in this box consider physical water scarcity as a quantifiable measure of water availability compared to its demand and consider the societal elements of economic water scarcity to be part of the more comprehensive concept of water insecurity.

Physical water scarcity

Definitions of water scarcity have evolved to take account of a broader set of factors. For example, physical water scarcity indicates that an insufficient quantity of water is available to meet requirements. A commonly used measure of physical water scarcity is the Falkenmark index which measures the amount of renewable freshwater available per capita ( [[#Falkenmark--1989|Falkenmark et al., 1989]] ; [[#White--2014|White et al., 2014]] ). However, the Falkenmark index is now regarded as an incomplete measure, as it does not account for water needed for non-human needs (as quantified with EFRs). Therefore, EFRs have begun to be incorporated in recent water scarcity assessments ( [[#Liu--2016|Liu et al., 2016]] ; [[#Liu--2017b|Liu et al., 2017b]] ). Quality-induced water scarcity is an additional factor beginning to be considered ( [[#Liu--2020|Liu and Zhao, 2020]] ) .

Using a Water Scarcity Index (WSI) defined as the ratio of demand and availability, accounting for EFRs, it is estimated that 4 billion people live under conditions of severe water scarcity for at least one month per year (Figure Box 4.1.1a; [[#Mekonnen--2016|Mekonnen and Hoekstra, 2016]] ). Nearly half of these people live in India and China. Although regions with high water scarcity are already naturally dry ( ''virtually certain'' 2 ), [[#footnote-000|1]] human influence on climate is leading to reduced water availability in many regions. It is ''very likely'' that global patterns of soil moisture change are being driven by human influence on climate, and an overall global decline in soil moisture is attributable to greenhouse forcing [4.2.1.3]. Climate change patterns of streamflow change include declines in western North America, northeast South America, the Mediterranean and South Asia ( ''medium confidence'' ) [4.2.3]. However, quantification of the contribution of anthropogenic climate change to current levels of water scarcity is not yet available.

Water demand is projected to change as a direct result of socioeconomic changes. For example, the global water demand for domestic, industrial and agricultural uses, at present about 4600 km³ yr –1 , is projected to increase by 20–30% by 2050 ( [[#Greve--2018|Greve et al., 2018]] ), depending on the socioeconomic scenario. Changes in water availability and demand have been projected in several studies using climate models and socioeconomic scenarios (e.g., [[#Arnell--2014|Arnell and Lloyd-Hughes, 2014]] ; [[#Gosling--2016|Gosling and Arnell, 2016]] ; [[#Greve--2018|Greve et al., 2018]] ; [[#Koutroulis--2019|Koutroulis et al., 2019]] ). In such studies, the projected changes in water availability arise from differences in precipitation and evapotranspiration (ET). However, both precipitation and evapotranspiration are also subject to very high uncertainty in key processes such as regional climate change patterns ( [[#Uhe--2021|Uhe et al., 2021]] ) and the influence of vegetation responses to elevated CO 2 on transpiration ( [[#Betts--2015|Betts et al., 2015]] ).

Human factors are projected to be the dominant driver of future water scarcity on a global scale ( [[#Graham--2020a|Graham et al., 2020a]] ). However, at regional scales, high uncertainty in climate changes means that reduced water availability is ''more likely than not'' in many major river basins and remains a risk in most basins even where the central estimate is for increased water availability due to climate change (Figure 4.16). Such substantial uncertainties in projected water scarcity are crucial factors causing water management policies and planning challenges in the future. Therefore, locations projected to see significant increases in water scarcity with large uncertainty can be considered to be subject to the highest challenges for water management policy (Figure Box 4.1.1b; [[#Greve--2018|Greve et al., 2018]] ).

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'''Figure Box 4.1.1 |''' '''Geographical distributions of current water scarcity and levels of challenge for policies addressing future change.'''

'''(a)''' The number of months per year with severe water scarcity (ratio of water demand to availability &gt; 1.0). Reproduced from [[#Mekonnen--2016|Mekonnen and Hoekstra (2016)]] .

'''(b)''' Local levels of policy challenges for addressing water scarcity by 2050, considering both the central estimate (median) and the change uncertainty in projections of a Water Scarcity Index (WSI) from the present day to 2050 ( [[#Greve--2018|Greve et al., 2018]] ). Projections used five CMIP5 climate models, three global hydrological models from ISIMIP and three Shared Socioeconomic Pathways (SSPs). Levels of policy challenges refer to the scale and nature of policies to address water scarcity and range from monitoring and reviewing risks (‘low’) through transitional changes in water systems (‘medium’) to transformational changes (‘high’). Low policy challenges arise when the projected water scarcity in 2050 is lower (&lt;0.4), and the level of uncertainty remains relatively stable in future projections. Medium policy challenge arises when either the central estimate of water scarcity remains low but uncertainty increases, or the uncertainty is stable but the central estimate of water scarcity for 2050 is higher (&gt;0.4). High policy challenges arise when the central estimate of water scarcity is higher and the uncertainty increases. White areas show grid points defined as non-water-scarce (75th quantile of the WSI &lt; 0.1 at all times) or very low average water demand. Reproduced from [[#Greve--2018|Greve et al. (2018)]] .

Water security and insecurity

Unlike physical water scarcity, water security or insecurity cannot be quantified in absolute terms. However, relative levels of water security in different places can be compared using metrics representing critical aspects of security (Gain et al., 2016; [[#Young--2019|Young et al., 2019]] ), ideally with thresholds for secure/insecure compared with local experience to assess validity ( [[#Young--2019|Young et al., 2019]] ).

Gain et al. (2016) define a Global Water Security Index (GWSI) metric on a scale of 0 to 1 combining indicators of relative levels of availability of freshwater, accessibility to water services, water management and water quality and safety (including flood risk, which can affect water quality as well as being a direct physical hazard). Global application of this index indicates large worldwide differences in water security arising from different combinations of reasons (Figure Box 4.1.2a). In North Africa, the Middle East, large parts of the Indian sub-continent and north China, low water security arises predominantly from low water availability. However, many areas with relatively high water availability have relatively low levels of water security due to other factors. In 2015, 29% of the world’s population did not have access to safe drinking water (Ritchie and Roser, 2019). In large parts of South and Southeast Asia, significant contributions to water insecurity came from increased flood risk and deteriorated water quality ( [[#Burgess--2010|Burgess et al., 2010]] ; [[#Ward--2017|Ward et al., 2017]] ; [[#Farinosi--2018|Farinosi et al., 2018]] ). Water availability is relatively high across most of Africa, but water security is relatively low due to low accessibility, management and safety/quality standards. Most people in Africa do not have access to safe drinking water and improved sanitation ( [[#Marson--2015|Marson and Savin, 2015]] ; [[#Naik--2017|Naik, 2017]] ; [[#Armah--2018|Armah et al., 2018]] ).

In contrast, some areas with high physical water scarcity, such as some parts of the USA, Australia and southern Europe, show relatively high water security levels due to good governance, safety/quality and accessibility. Nevertheless, marginalised groups such as Indigenous Peoples experience reduced access to water even within regions in the Global North. For example, in both Canada and the USA, many Indigenous Peoples living on reserves lack access to piped water ( [[#Collins--2017|Collins et al., 2017]] ; [[#Hanrahan--2017|Hanrahan, 2017]] ; [[#Marshall--2018|Marshall et al., 2018]] ) and (or) are on boil water advisories ( [[#Patrick--2019|Patrick et al., 2019]] ). In Australia, 25–40% of Aboriginal people live in remote rural areas with poor access to clean water ( [[#Bowles--2015|Bowles, 2015]] ; [[#NCCARF--2018|NCCARF, 2018]] ).

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'''Figure Box 4.1.2 |''' '''Global Water Security Index (GWSI) and its components for the present day, and factors affecting future change in water security.'''

'''(a)''' Top: a global map of local values of GWSI constructed from the following components with their subjectively weighted contribution to the combined metric indicated in brackets. Middle left: relative effectiveness of water management (15%), comprising a World Governance Index at country scale (itself representing six components: voice and accountability, political stability and absence of violence/terrorism, government effectiveness, regulatory quality, rule of law and control of corruption) and indicators of transboundary legal frameworks and political tensions at a river-basin scale. Middle right: relative accessibility to water services (20%), including drinking water and sanitation. Bottom left: relative water quality and safety (20%), including a Water Quality Index and Flood Frequency Index. Bottom right: relative availability of fresh water (45%), comprising a Water Scarcity Index, Drought Index and the groundwater depletion rate. Data for the components do not apply to the same set of dates but are generally applicable to recent decades up to 2010. White areas indicate no data available for at least one component. For further details, see Gain et al. (2016).

'''(b)''' Factors through which climate change or action on mitigation or adaptation could influence water security.

The discrepancy between physical water scarcity and overall water insecurity is a function of socioeconomic vulnerabilities and governance gaps. Therefore, improving societal aspects of water management will be key in adapting to climate change-driven increases in water scarcity in the future ( ''high confidence'' ).

Future water security will depend on the magnitude, rate and regional details of future climate change and non-climatic factors, including agricultural practices, water demand and governance. In many cases, climate change may not be the dominant factor affecting water security. Nevertheless, climate change poses clear risks to water security in many regions through potential impacts on water availability, quality and flooding. The range of possible outcomes is extremely large, and assessing the likelihood of particular outcomes depends on consideration of uncertain regional climate changes and uncertain socioeconomic futures. Uncertainty in future water scarcity projections makes climate change risks to water security and planning for adaptation challenging. Limiting climate change to lower levels of global warming would reduce the risks to water security arising from climate change, partly because uncertainties in regional climate change are smaller at lower levels of warming.

In summary, roughly half of the world’s population are assessed as currently subject to severe water scarcity for at least some part of the year due to climatic and non-climatic factors, and this is projected to be exacerbated at higher levels of warming ( ''medium confidence'' ). General water insecurity issues are seen worldwide, particularly in South Asia, North China, Africa and the Middle East, due to high population densities often coupled with low water availability, accessibility, quality and governance ( ''high confidence'' ). Areas with high water availability can also be water-insecure due to increased flood risk, deteriorated water quality, and poor governance ( ''high confidence'' ). Future water security will depend on the evolution of all these socioeconomic and governance factors and future regional climate change ( ''high confidence'' ). The main climate change contribution to water insecurity is the potential for reduced water availability, with a secondary contribution from increased flooding risk ( ''medium confidence'' ). Future socioeconomic conditions are a crucial driver of water insecurity, implying the need for further adaptation to some level of future climate change ( ''medium confidence'' ). However, policy challenges are high in many regions, with uncertainty in the regional climate outcomes being a key factor ( ''high confidence'' ).

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[[#footnote-000-backlink|1]] 2 In this Report, the following terms have been used to indicate the assessed likelihood of an outcome or a result: Virtually certain 99–100% probability, Very likely 90–100%, Likely 66–100%, About as likely as not 33–66%, Unlikely 0–33%, Very unlikely 0–10%, and Exceptionally unlikely 0–1%. Additional terms (Extremely likely : 95–100%, More likely than not &gt;50–100%, and Extremely unlikely 0–5%) may also be used when appropriate. Assessed likelihood is typeset in italics, e.g., ''very likely'' ). This Report also uses the term ‘ ''likely'' range’ to indicate that the assessed likelihood of an outcome lies within the 17–83% probability range.

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=== 4.1.2 Climatic and Non-Climatic Drivers of Changes in the Water Cycle ===

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The water cycle is affected by both climatic and non-climatic factors ( [[#Douville--2021|Douville et al., 2021]] ). Radiative forcing by changes in greenhouse gas (GHG) concentrations, aerosols and surface albedo drives global and regional changes in evaporation and precipitation ( [[#Douville--2021|Douville et al., 2021]] ). A warmer atmosphere holds more moisture, increasing global and regional mean precipitation, and more extreme precipitation ( [[#Allan--2014|Allan et al., 2014]] ; [[#Giorgi--2019|Giorgi et al., 2019]] ; [[#Allan--2020|Allan et al., 2020]] ). Regional precipitation responses vary according to changes in atmospheric circulation. Geographical variation in aerosols drives changes in atmospheric circulation, affecting precipitation patterns such as the Asian monsoon ( [[#Ganguly--2012|Ganguly et al., 2012]] ; [[#Singh--2019|Singh et al., 2019]] ). ( [[#4.2.1|Section 4.2.1]] )

Warming increases glacier melt and is expected to decrease snowfall globally and lead to shorter snow seasons with earlier but less rapid snowmelt. It can also lead to local increases in snowfall intensity ( [[#Allan--2020|Allan et al., 2020]] ). These changes affect the seasonality of river flows in glacier-fed or snow-dominated basins. ( [[#4.2.2|Section 4.2.2]] )

Rising atmospheric CO 2 generally decreases plant transpiration, affecting soil moisture, runoff, stream flows, the return of moisture to the atmosphere and surface temperature ( [[#Skinner--2017|Skinner et al., 2017]] ). However, in some regions, these can be offset by increased leaf area (‘global greening’) driven by elevated CO 2 , land use change, nitrogen deposition and effects of climate change itself (Zhu Z. et al., 2016; Zeng Z. et al., 2018). Increased ozone can impact plant functioning, reducing transpiration ( [[#Arnold--2018|Arnold et al., 2018]] ). ( [[#4.2.1|Section 4.2.1]] )

Direct human interventions include abstraction of surface water and groundwater for drinking, irrigation and other freshwater uses, as well as streamflow impoundment behind dams and large-scale inter-basin transfers ( [[#Zhao--2015|Zhao et al., 2015]] ; [[#Donchyts--2016|Donchyts et al., 2016]] ; [[#McMillan--2016|McMillan et al., 2016]] ; [[#Shumilova--2018|Shumilova et al., 2018]] ). The consequences of these interventions are substantial and are discussed below briefly. In addition, these direct human interventions can change due to various societal and economic factors, including changes in land use and urbanisation (Sections 4.3 and 4.5).

Irrigation can reduce river flows and groundwater levels via abstraction and increase local precipitation ( [[#Alter--2015|Alter et al., 2015]] ; [[#Cook--2015|Cook et al., 2015]] ), alter precipitation remotely through moisture advection ( [[#de%20Vrese--2016|de Vrese et al., 2016]] ) and change the timing of monsoons through land–sea temperature contrasts ( [[#Guimberteau--2012|Guimberteau et al., 2012]] ) (Box 4.3). Land cover change affects ET and precipitation ( [[#Li--2015|Li et al., 2015]] ; [[#Douville--2021|Douville et al., 2021]] ), interception of precipitation by vegetation canopies ( [[#de%20Jong--2007|de Jong and Jetten, 2007]] ), infiltration ( [[#Sun--2018a|Sun et al., 2018a]] ) and runoff ( [[#Bosmans--2017|Bosmans et al., 2017]] ). Land cover impacts on the hydrological cycle are of similar magnitude as human water use ( [[#Bosmans--2017|Bosmans et al., 2017]] ).

Urbanisation decreases land surface permeability ( [[#Choi--2016|Choi et al., 2016]] ), which can increase fast runoff and flooding risks and reduce local rainfall by decreasing moisture return to the atmosphere ( [[#Wang--2018|Wang et al., 2018]] ). But urbanisation can also increase the sensible heat flux driving greater or more extreme precipitation ( [[#Kusaka--2014|Kusaka et al., 2014]] ; [[#Niyogi--2017|Niyogi et al., 2017]] ). ( [[#4.3.4|Section 4.3.4]] )

In summary, radiative forcing by GHG and aerosols drives changes in ET and precipitation at global and regional scales, and the associated warming shifts the balance between frozen and liquid water ( ''high confidence'' ). Rising CO 2 concentrations also affect the water cycle via plant physiological responses affecting transpiration, including via reduced stomatal opening and increased leaf area ( ''high confidence'' regarding the individual processes; ''medium confidence'' regarding their net impact). Land cover changes and urbanisation affect both the climate and land hydrology by altering the exchanges of energy and moisture between the atmosphere and surface ( ''high confidence'' ) and changing the permeability of the land surface. Direct human interventions in river systems and groundwater systems are non-climatic drivers with substantial impacts on the water cycle ( ''high confidence'' ) and have the potential to change as part of societal responses to climate change (Figure 4.2).

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[[File:46d780ee64a1bd77a376e0d3ea474b8b IPCC_AR6_WGII_Figure_4_002.png]]

'''Figure 4.2 |''' '''The water cycle, including direct human interventions.''' Water fluxes on land precipitation, land evaporation, river discharge, groundwater recharge and groundwater discharge to the ocean from [[#Douville--2021|Douville et al. (2021)]] . Human water withdrawals for various sectors are shown from [[#Hanasaki--2018|Hanasaki et al. (2018)]] , [[#Sutanudjaja--2018|Sutanudjaja et al. (2018)]] , [[#Burek--2020|Burek et al. (2020)]] , [[#Droppers--2020|Droppers et al. (2020)]] and [[#Müller%20Schmied--2021|Müller Schmied et al. (2021)]] . Green water use ( [[#Abbott--2019|Abbott et al., 2019]] ) refers to the use of soil moisture for agriculture and forestry. Irrigation water use (called blue water) is not included in green water use.

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