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

== 4.6 Key Risks and Adaptation Responses in Various Water Use Sectors ==

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Anthropogenic climate change has impacted every aspect of the water cycle ( [[#4.2|Section 4.2]] ), and risks are projected to intensify with every degree of global warming ( [[#4.4|Section 4.4]] ), with impacts already visible in all sectors of the economy and ecosystems ( [[#4.3|Section 4.3]] ) and projected to intensify further ( [[#4.5|Section 4.5]] ). In response to climate- and non-climate-induced water insecurity, people and governments worldwide are undertaking various adaptation responses across all sectors. In addition, there are several projected studies for future adaptation responses. We draw upon a list of 359 case studies of observed adaptation and 45 articles on projected future adaptation. Further information on selection and inclusion criteria is available in SM4.2. In this section, we document those adaptation responses (current and future) in different water use sectors. In the next (Sections 4.7.1, 4.7.2, 4.7.3), benefits of current adaptation and effectiveness of future adaptation are discussed.

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=== 4.6.1 Key Risks Related to Water ===

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The preceding sections have outlined the various pathways along which climate affects water resources and water-using sectors. In synthesis, fundamental changes in observed climate are already visible in water-related outcomes ( ''high confidence'' ), including ~500 million people experiencing historically unfamiliar precipitation regimes ( [[#4.2.1.1|Section 4.2.1.1]] ); cryospheric changes impacting various societal and ecosystem components ( [[#4.2.2|Section 4.2.2]] ); increasing vulnerability to flood impacts, driven by both by climate and socioeconomic factors ( [[#4.2.4|Section 4.2.4]] ); and as climate change-driven increases in drought impacts ( [[#4.2.5|Section 4.2.5]] ).

Further increases in risks are projected to manifest at different levels of warming. Climate change is impacting all components of the hydrological cycle, but the water use sectors are also facing the consequences of climate change, given the central role of water for all aspects of human and environmental systems ( [[#4.1|Section 4.1]] , Box 4.1). Therefore, risks to water security are also identified as a representative key risk (RKR) (WGII, Chapter 16, [[IPCC:Wg2:Chapter:Chapter-16#16.5.2.3.7|Section 16.5.2.3.7]] ).

Approximately 4 billion people globally face physical water scarcity for at least one month yr –1 which is driven by both climatic and non-climatic factors ( [[#Mekonnen--2016|Mekonnen and Hoekstra, 2016]] ). Increases in physical water scarcity are projected, with estimates between 800 million and 3 billion for 2°C global warming and up to approximately 4 billion for 4°C global warming ( [[#Gosling--2016|Gosling and Arnell, 2016]] ). Projected increases in hydrological extremes pose increasing risks to societal systems globally ( ''high confidence'' ), with a potential doubling of flood risk between 1.5°C and 3°C of warming ( [[#Dottori--2018|Dottori et al., 2018]] ) and an estimated 120–400% increase in population at risk of river flooding at 2°C and 4°C, respectively ( [[#Alfieri--2017|Alfieri et al., 2017]] ). Also projected are increasing risks of fatalities and socioeconomic impacts ( [[#4.4.4|Section 4.4.4]] ). Similarly, a near doubling of drought duration ( [[#Naumann--2018|Naumann et al., 2018]] ) and an increasing share of the population affected by various types, durations and severity levels of drought are projected ( ''high confidence'' ) ( [[#4.4.5|Section 4.4.5]] ). Increasing return periods of high-end hydrological extremes pose significant challenges to adaptation, requiring integrated approaches to risk management, which take the various economic and non-economic, as well as direct and indirect losses and damages into account ( [[#Jongman--2018|Jongman, 2018]] ).

Increasing sectoral risks are reported across regions and sectors with rising temperatures and associated hydrometeorological changes (Cross-Chapter Box INTEREG in Chapter 16). Risks to agricultural yields due to combined effects of water and temperature changes, for example, could be three times higher at 3°C compared to 2°C ( [[#Ren--2018b|Ren et al., 2018b]] ), with additional risks as a consequence of increasing climate extremes ( [[#Leng--2019|Leng and Hall, 2019]] ). In addition, climate-driven water scarcity and increasing crop water demands, including for irrigation, pose additional challenges for agricultural production in many regions ( ''high confidence'' ). Regional water-related risks to agricultural production are diverse and vary strongly across regions and crops ( [[#4.5.1|Section 4.5.1]] ). As there are limitations to how well global agricultural models can represent available water resources ( [[#Elliott--2014|Elliott et al., 2014]] ; [[#Jägermeyr--2017|Jägermeyr et al., 2017]] ), water limitations to agricultural production may well be underestimated. For example, the potential for irrigation, commonly assumed to play an important role in ensuring food security, could be more limited than models assume (Box 4.3).

With higher levels of warming, risks to water-dependent energy production increase substantially across regions ( [[#van%20Vliet--2017|van Vliet et al., 2017]] ). While there are increasing potentials of ~2–6% for hydropower production by 2080 ( ''medium confidence'' ), risks to thermoelectric power production increase for most regions ( ''high confidence'' ), for example, with potentially near doubling of the risk to European electricity production from 1.5°C to 3°C ( [[#Tobin--2018|Tobin et al., 2018]] ). Shifting to a higher share of renewable sources less dependent on water resources for energy production could substantially reduce the vulnerability of this sector ( [[#4.5.2|Section 4.5.2]] ).

Increasing hydrological extremes also have consequences for the maintenance and further improvement of the provision of WaSH services ( ''medium confidence'' ). Risks related to the lack or failure of WaSH services under climate change include increased incidence and outbreaks of water-related diseases, physical injuries, stress, exacerbation of the underlying disease, and risk of violence, which is often gendered ( [[#4.5.3|Section 4.5.3]] ). Although globally, the regional potential infestation areas for disease-carrying vectors could be five times higher at 4°C than at 2°C ( [[#Liu-Helmersson--2019|Liu-Helmersson et al., 2019]] ), climate projections suggest up to 2.2 million more cases of ''E. coli'' by 2100 (2.1°C increase) in Bangladesh ( [[#Philipsborn--2016|Philipsborn et al., 2016]] ), up to an 11-fold and 25-fold increase by 2050 and 2080, respectively (2°C–4°C increase), in disability-adjusted life years associated with cryptosporidiosis and giardiasis in Canada ( [[#Smith--2015|Smith et al., 2015]] ), and an additional 48,000 deaths of children under 15 years of age globally from diarrhoea by 2030 ( [[#WHO--2014|WHO, 2014]] ).

Increasing water demand in conjunctions with changing precipitation patterns will pose risks to urban water security by mid-century, with water demand in nearly a third of the world’s largest cities potentially exceeding surface water availability by 2050 (RCP6.0) ( [[#Flörke--2018|Flörke et al., 2018]] ) and the global volume of domestic water withdrawal projected to increase by 50–250% ( [[#Wada--2016|Wada et al., 2016]] ) ( [[#4.5.4|Section 4.5.4]] ). Globally, climate change will exacerbate existing challenges for urban water services, driven by further population growth, the rapid pace of urbanisation and inadequate investment, particularly in less developed economies with limited governance capacity ( ''high confidence'' ).

Risks to freshwater ecosystems increase with progressing climate change, with freshwater biodiversity decreasing proportionally with increasing warming if 1.5°C is exceeded ( ''medium evidence, high agreement'' ). Risks include range shift, a decline in species population, extirpation and extinction ( [[#4.5.5|Section 4.5.5]] ).

The potential for climate change to influence conflict is highly contextual and depends on various socioeconomic and political factors. However, water-specific conflicts between sectors and users may be exacerbated for some regions of the world ( ''high confidence'' ) ( [[#4.5.7|Section 4.5.7]] ).

Human migration takes many forms and can be considered a consequence and impact of climate change and an adaptation response ( [[#4.5.8|Section 4.5.8]] ). Projections indicate a potentially substantial increase in internal and international displacement due to water-related climate risks ( [[#Missirian--2017|Missirian and Schlenker, 2017]] ; [[#Rigaud--2018|Rigaud et al., 2018]] ). In the context of water-related adaptation, short-term migration as an income diversification approach is commonly documented. However, permanent relocation and fundamental changes to livelihoods are more transformational and yet can be associated with tangible and intangible losses ( [[#Mechler--2019|Mechler et al., 2019]] ). In the context of climate-induced hydrological change, increased vulnerability among migrants and the risk of trapped populations poses significant additional risks. However, quantifications that disentangle different climate drivers and show specific risks emanating from hydrological change are unavailable ( [[#Rigaud--2018|Rigaud et al., 2018]] ).

Hydrological change, especially increasing extreme events, pose risks to the cultural uses of water of Indigenous Peoples, local communities and traditional peoples ( ''high confidence'' ), with implications for the physical well-being of these groups ( ''high confidence'' ). Increasing risks are documented across groups and regions; however, partly due to the unquantifiable nature of these risks, the lack of research funding for the social dimensions of climate change, particularly in the Global South, and the systemic underrepresentation of marginalised groups in scientific research, quantitative projections are limited ( [[#4.5.8|Section 4.5.8]] ).

Adaptation is already playing an integral part in reducing climate impacts and preparing for increasing climate risk, and it will grow in importance evermore with increasing risks at higher levels of warming. The remaining subsections describe these adaptation responses.

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=== 4.6.2 Adaptation in the Agricultural Sector ===

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AR5 reported a range of available hard and soft adaptation options for water-related adaptation in the agricultural sector. However, the evidence on the effectiveness of these adaptation responses, now and in the future, was not assessed ( [[#Noble--2014|Noble et al., 2014]] ; [[#Porter--2014|Porter et al., 2014]] ). Assessing the feasibility of different irrigation measures as adaptation, SR1.5 ( [[#de%20Coninck--2018|de Coninck et al., 2018]] ) found mixed evidence, depending on the applied methodology.

There is ''high confidence'' that water-related adaptation is occurring in the agricultural sector ( [[#Acevedo--2020|Acevedo et al., 2020]] ; [[#Ricciardi--2020|Ricciardi et al., 2020]] ), and water-related adaptation in the agricultural sector makes up the majority of documented local, regional and global evidence of implemented adaptation ( ''high confidence'' ) ( [[#4.7.1|Section 4.7.1]] , Figure 4.23 and Figure 4.24, Table 4.8). However, while there is increasing evidence of adaptation and its benefits across multiple dimensions, the link between adaptation benefits and climate risk reduction is unclear due to methodological challenges ( ''medium confidence'' ) ( [[#4.7.1|Section 4.7.1]] ). On the other hand, while it is methodologically possible to measure the effectiveness of future adaptation in reducing climate risks, the main limitation here is that not all possible ranges of future adaptations can be modelled given the limitations of climate and impact models ( ''high confidence'' ) ( [[#4.7.2|Section 4.7.2]] ). Furthermore, findings on current adaptation are constrained by what is documented in peer-reviewed articles. At the same time, there may be a range of options implemented on the ground by local governments or as a part of corporate social responsibility that is not published in peer-reviewed publications.

Water and soil conservation measures (e.g., reduced tillage, contour ridges or mulching) are frequently documented as adaptation responses to reduce water-related climate impacts ( [[#Kimaro--2016|Kimaro et al., 2016]] ; [[#Traore--2017|Traore et al., 2017]] ). This measure features in all continents’ top four adaptation responses except Australasia (Figure 4.27). Especially for rain-fed farming, which currently is the norm in most of Africa, large parts of Central and South America and Europe, water and soil conservation measures and various components of conservation agriculture are some of the most frequently used adaptation responses ( [[#Jat--2019|Jat et al., 2019]] ). This measure is deemed to have economic benefits and benefits for vulnerable communities who adopt this measure ( ''high confidence'' ) and benefits in terms of water saving and positive ecological and sociocultural benefits ( ''medium confidence'' ). However, this measure can be sometimes maladaptive ( ''low evidence, medium agreement'' ) and can have mitigation co-benefits ( ''low evidence, high agreement'' ) (Figure 4.29). Furthermore, water- and soil management-related measures show high potential efficacy in reducing impacts in a 1.5°C world, with declining effectiveness at higher levels of warming (Figure 4.28 and Figure 4.29)

Changes in cropping patterns, the timing of sowing and harvesting, crop diversification towards cash crops and the adoption of improved crop cultivars that can better withstand hazards like floods and drought are among the most used adaptation responses by farmers. This is among the top two measures in Asia and Africa (Figure 4.27). Extra income allows households to re-invest in improved agricultural techniques and improved cultivars ( [[#Taboada--2017|Taboada et al., 2017]] ; [[#Khanal--2018b|Khanal et al., 2018b]] ). Beneficial outcomes are documented in terms of increases in incomes and yields and water-related outcomes ( ''medium confidence'' , from ''robust evidence, but medium agreement'' ), but benefits to vulnerable communities are not always apparent on the whole (Figure FAQ4.4.1). Changes in cropping patterns and systems are also among those adaptation options assessed for their potential to reduce future climate impacts, though effectiveness is shown to be limited ( [[#Brouziyne--2018|Brouziyne et al., 2018]] ; [[#Paymard--2018|Paymard et al., 2018]] ). Assessments of the future effectiveness of crop rotation systems for adaptation show a continued reduction in required irrigation water use, though studies of effectiveness beyond 2°C global mean temperature increase are not available ( [[#Kothari--2019|Kothari et al., 2019]] ; [[#Yang--2019b|Yang et al., 2019b]] ) (Figure 4.28 and Figure 4.29).

Conservation agriculture and climate-smart agriculture (includes improved cultivars and agronomic practices) have proven to increase soil carbon, yields and technical efficiency ( [[#Penot--2018|Penot et al., 2018]] ; [[#Salat--2018|Salat and Swallow, 2018]] ; [[#Ho--2019|Ho and Shimada, 2019]] ; [[#Makate--2019|Makate and Makate, 2019]] ; [[#Okunlola--2019|Okunlola et al., 2019]] ). Some water-related measures in conservation agriculture include allowing for shading and soil moisture retention, with the co-benefit of reducing pest attacks ( [[#Thierfelder--2015|Thierfelder et al., 2015]] ; [[#Raghavendra--2018|Raghavendra and Suresh, 2018]] ; [[#Islam--2019a|Islam et al., 2019a]] ). Especially for traditional food grains in small-holder agriculture, improved practices such as modern varieties or integrated nutrient management can play an important role in making production more resilient to climate stress ( [[#Handschuch--2016|Handschuch and Wollni, 2016]] ). This measure is also among the top four most frequent adaptation measures in all continents except Australia and North America (Figure 4.27). In addition, this measure is shown to have positive economic benefits ( ''high confidence'' ) and also benefits on other parameters ( ''medium confidence'' ) (Figure FAQ4.4.1). Such approaches are also among those most frequently assessed for their effectiveness in addressing future climate change, but show limited effectiveness across warming levels (Figure 4.28 and Figure 4.29).

The use of non-conventional water sources, that is, desalinated and treated waste water, is emerging as an important component of increasing water availability for agriculture ( [[#DeNicola--2015|DeNicola et al., 2015]] ; [[#Martínez-Alvarez--2018b|Martínez-Alvarez et al., 2018b]] ; [[#Morote--2019|Morote et al., 2019]] ). While desalination has a high potential in alleviating agricultural water stress in arid coastal regions, proper management and water quality standards for desalinated irrigation water are essential to ensure continued or increased crop productivity. In addition to the energy intensity ( [[#4.7.6|Section 4.7.6]] ), risks of desalinated water include lower mineral content, higher salinity, crop toxicity and soil sodicity ( [[#Martínez-Alvarez--2018b|Martínez-Alvarez et al., 2018b]] ). Similarly, waste-water reuse can be an important contribution to buffer against the increasing variability of water resources. However, waste-water guidelines that ensure the adequate treatment to reduce adverse health and environmental outcomes due to pathogens or other chemical and organic contaminants will be essential ( [[#Angelakis--2015|Angelakis and Snyder, 2015]] ; [[#Dickin--2016|Dickin et al., 2016]] ) (Box 4.5; 4.6.4).

IKLK are crucial determinants of adaptation in agriculture for many communities globally. Indigenous Peoples have intimate knowledge about their surrounding environment and are attentive observers of climate changes. As a result, they are often best placed to enact successful adaptation measures, including shifting to different crops, changing cropping times or returning to traditional varieties ( [[#Mugambiwa--2018|Mugambiwa, 2018]] ; [[#Kamara--2019|Kamara et al., 2019]] ; [[#Nelson--2019|Nelson et al., 2019]] ) ( [[#4.8.4|Section 4.8.4]] ).

Migration and livelihood diversification is often an adaptation response to water-related hazards and involves securing income sources away from agriculture, including off-farm employment and temporary or permanent migration, and these are particularly important in Asia and Africa (Figure 4.27). Income and remittances are sometimes re-invested, for instance, for crop diversification ( [[#Rodriguez-Solorzano--2014|Rodriguez-Solorzano, 2014]] ; [[#Musah-Surugu--2018|Musah-Surugu et al., 2018]] ; [[#Mashizha--2019|Mashizha, 2019]] ). While there is extensive documentation on the benefits of migration, the quality of studies is such that links between migration and subsequent benefits are not clear, making our conclusion of benefits from this measure as having ''medium confidence'' . On the other hand, there is more rigorous evidence on the maladaptive nature of migration as an adaptation measure (Figure FAQ4.4.1). However, adverse climatic conditions, especially droughts, have been found to reduce international migration, as resources are unavailable to consider this option ( [[#Nawrotzki--2017|Nawrotzki and Bakhtsiyarava, 2017]] ), resulting in limits to adaptation ( [[#Ayeb-Karlsson--2016|Ayeb-Karlsson et al., 2016]] ; [[#Brottem--2018|Brottem and Brooks, 2018]] ; [[#Ferdous--2019|Ferdous et al., 2019]] ). In addition, it is difficult to model this option in future climate adaptation models.

Policies, institutions and capacity building are important adaptation measures in agriculture and often have beneficial outcomes, but the quality of studies precludes a high degree of certainty about those impacts (Figure FAQ4.4.1). Access to credits, subsidies or insurance builds an important portfolio of reducing reliance on agricultural income alone ( [[#Rahut--2017|Rahut and Ali, 2017]] ; [[#Wossen--2018|Wossen et al., 2018]] ). Training and capacity building are essential tools to ensure effective adaptation in agriculture, increasing food security ( [[#Chesterman--2019|Chesterman et al., 2019]] ; [[#Makate--2019|Makate and Makate, 2019]] ). Through better understanding, the implementation of available responses reduces exposure to climate impacts. In addition, public regulations, including water policies and allocations and incentive instruments, and availability of appropriate finance play an essential role in shaping and enabling (Sections 4.8.5, 4.8.6, 4.8.7), but also limiting ( [[#4.8.2|Section 4.8.2]] ), water-related adaptation for agriculture (see also Chapter 17).

Water-stressed regions already rely on importing agricultural resources, thus importing water embedded in these commodities ( [[#D’Odorico--2014|D’Odorico et al., 2014]] ). Virtual water trade will continue to play a role in reducing water-related food insecurity (Cross-Chapter Box INTERREG in Chapter 16) ( [[#Pastor--2014|Pastor et al., 2014]] ; [[#Graham--2020b|Graham et al., 2020b]] ).

While an increasing body of literature documents water-related adaptation in the agricultural sector, both in reducing current climate impacts and addressing future climate risk, knowledge gaps remain about assessing the effectiveness of such measures to reduce impacts and risks. Additional considerations on co-benefits of trade-offs for overall sustainable development are not always sufficiently considered in the available literature.

In sum, water-related adaptation in the agricultural sector is widely documented, with irrigation, agricultural water management, crop diversification and improved agronomic practices among the most common adaptation measures adopted ( ''high confidence'' ). However, the projected future effectiveness of available water-related adaptation for agriculture decreases with increasing warming ( ''medium evidence, high agreement'' ).

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'''Box 4.2 | Observed Risks, Projected Impacts and Adaptation Responses to Water Security in Small Island States'''
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AR5 and SR1.5 recognised the exceptional vulnerability of islands, especially concerning water security and potential limits to adaptation that may be reached due to freshwater resources ( [[#Klein--2014|Klein et al., 2014]] ; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#Roy--2018|Roy et al., 2018]] ).

Small islands are already regularly experiencing droughts and freshwater shortages ( ''high confidence'' ) ( [[#Holding--2016|Holding et al., 2016]] ; [[#Pearce--2018|Pearce et al., 2018]] ; [[#Gheuens--2019|Gheuens et al., 2019]] ; [[#MacDonald--2020|MacDonald et al., 2020]] ). Freshwater supply systems vary from household or small community systems such as rainwater harvesting systems and private wells to large public water supply systems using surface, groundwater and, in some cases, desalinated water ( [[#Alsumaiei--2018b|Alsumaiei and Bailey, 2018b]] ; [[#Falkland--2020|Falkland and White, 2020]] ). In many cases, communities rely on more than one water source, including a strong reliance on rainwater and groundwater ( [[#Elliott--2017|Elliott et al., 2017]] ; [[#MacDonald--2020|MacDonald et al., 2020]] ). Groundwater resources in freshwater lenses (FWLs) are essential in providing access to freshwater resources, especially during droughts when the collected rainwater is insufficient ( [[#Barkey--2017|Barkey and Bailey, 2017]] ; [[#Bailey--2018|Bailey et al., 2018]] ), leading to greater risks of water-borne diseases, with significant effects on nutrition ( [[#Elliott--2017|Elliott et al., 2017]] ; [[#Savage--2020|Savage et al., 2020]] ), and improper sanitation poses additional risks to the limited groundwater resources ( [[#MacDonald--2017|MacDonald et al., 2017]] ). Drought events have also severely affected FWL recharge ( [[#Barkey--2017|Barkey and Bailey, 2017]] ), with extraction rates further threatening available groundwater volumes ( [[#Post--2018|Post et al., 2018]] ). In conjunction with sea level rise, this poses serious risks to groundwater salinisation ( [[#Alsumaiei--2018b|Alsumaiei and Bailey, 2018b]] ; [[#Storlazzi--2018|Storlazzi et al., 2018]] ; [[#Deng--2019|Deng and Bailey, 2019]] ). In addition, FWLs are threatened by climate change due to changes in rainfall patterns, extended droughts and wash-over events caused by storm surges and sea level rise ( ''high confidence'' ) (see Chapter 15) ( [[#Chui--2015|Chui and Terry, 2015]] ; [[#Alsumaiei--2018a|Alsumaiei and Bailey, 2018a]] ; [[#Alsumaiei--2018b|Alsumaiei and Bailey, 2018b]] ; [[#Post--2018|Post et al., 2018]] ; [[#Storlazzi--2018|Storlazzi et al., 2018]] ; [[#Deng--2019|Deng and Bailey, 2019]] ). After small-scale wash over events, the FWLs have been shown to recover to pre-wash over salinity levels within a month ( [[#Oberle--2017|Oberle et al., 2017]] ).

Due to wash-over events exacerbated by sea level rise and lens thinning due to pumping, recovery time for FWLs is projected to take substantially longer ( [[#Oberle--2017|Oberle et al., 2017]] ; [[#Alsumaiei--2018a|Alsumaiei and Bailey, 2018a]] ; [[#Storlazzi--2018|Storlazzi et al., 2018]] ). Projections indicate that atolls may be unable to provide domestic freshwater resources due to the lack of potable groundwater by 2030 (RCP8.5+ ice-sheet collapse), 2040 (RCP8.5) or the 2060s (RCP4.5) ( [[#Storlazzi--2018|Storlazzi et al., 2018]] ). Projections of future freshwater availability in small islands further underline these substantial risks to island water security ( [[#Karnauskas--2016|Karnauskas et al., 2016]] ; [[#Karnauskas--2018|Karnauskas et al., 2018]] ). Population growth, changes in rainfall patterns and agricultural demand are projected to increase water stress in small islands ( [[#Gohar--2019|Gohar et al., 2019]] ; [[#Townsend--2020|Townsend et al., 2020]] ). While some islands are projected to experience an increase in rainfall patterns, this may refer to shorter intense rainfall events, thereby increasing the risk of flooding during the wet season, while not decreasing their risk of droughts during dry periods ( [[#Aladenola--2016|Aladenola et al., 2016]] ; [[#Gheuens--2019|Gheuens et al., 2019]] ). In addition, projected shifts in the timing of the rainfall season might pose an additional risk for water supply systems ( [[#Townsend--2020|Townsend et al., 2020]] ).

Observed adaptation during drought events includes community water sharing ( [[#Bailey--2018|Bailey et al., 2018]] ; [[#Pearce--2018|Pearce et al., 2018]] ) as well as using alternative water resources such as water purchased from private companies ( [[#Aladenola--2016|Aladenola et al., 2016]] ), desalination units ( [[#Cashman--2019|Cashman and Yawson, 2019]] ; [[#MacDonald--2020|MacDonald et al., 2020]] ) or accessing deeper or new groundwater resources ( [[#Pearce--2018|Pearce et al., 2018]] ). Rainwater harvesting to adapt the water supply system in the Kingston Basin in Jamaica was able to significantly alleviate water stress, for example. Still, it would not fill the total supply gap caused by climate change ( [[#Townsend--2020|Townsend et al., 2020]] ). Likewise, groundwater sustainability with increasing climate change in Barbados cannot be ensured without aquifer protection, leading to higher optimised food prices if no additional adaptation measures are implemented ( [[#Gohar--2019|Gohar et al., 2019]] ). The potential of using multiple water sources is rarely assessed in future water supply projections in small islands ( [[#Elliott--2017|Elliott et al., 2017]] ). In the Republic of Marshall Islands, more than half of all interviewed households have already had to migrate once due to a water shortage ( [[#MacDonald--2020|MacDonald et al., 2020]] ). In Cariacou, Grenada, increases in migration rates have been observed following drought events ( [[#Cashman--2019|Cashman and Yawson, 2019]] ). with long-term cross border and internal migration shown to be having significant impacts on well-being, community-cohesion, livelihoods and people-land relationships ( [[#Yates--2021|Yates et al., 2021]] ).

In sum, small islands are already regularly experiencing droughts and freshwater shortages ( ''high confidence'' ). For atoll islands, freshwater availability may be severely limited as early as 2030 ( ''low confidence'' ). The effects of temperature increase, changing rainfall patterns, sea level rise and population pressure combined with limited options available for water-related adaptation leave small islands partially water-insecure currently, with increasing risks in the near-term and at warming above 1.5°C ( ''high confidence'' ).

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=== 4.6.3 Adaptation in Energy and Industrial Sectors ===

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While AR5 ( [[#Arent--2014|Arent et al., 2014]] ) had looked at demand and supply changes in the energy sector due to climate change, none of the AR5 chapters had assessed adaptations in the energy sector per se. A modelling study by [[#van%20Vliet--2016b|van Vliet et al. (2016b)]] demonstrated that increasing the efficiency of hydropower plants by up to 10% could offset the impacts of decreased water availability in most regions by mid-century, under both RCP2.6 and RCP8.5 scenarios ( ''medium confidence'' ). Changing hydropower operation protocol and plant design can be effective adaptation measures, yet may be insufficient to mitigate all future risks related to increased floods and sediment loads ( [[#Lee--2016|Lee et al., 2016]] ).

[[#van%20Vliet--2016b|van Vliet et al. (2016b)]] projected that even a 20% increase in efficiency of thermoelectric power plants might not be enough to offset the risks of water stress by mid-century ( ''medium confidence'' ). Therefore, thermoelectric power plants will need additional adaptation measures such as changes in cooling water sources and alternative cooling technologies ( [[#van%20Vliet--2016c|van Vliet et al., 2016c]] ). In China, many CFPPs in water-scarce North China have adopted air cooling technologies ( [[#Zhang--2016a|]] [[#Zhang--2016|Zhang et al., 2016]] a ). In Europe, wet/dry cooling towers ( [[#Byers--2016|Byers et al., 2016]] ) and seawater cooling ( [[#Behrens--2017|Behrens et al., 2017]] ) have been the preferred options. Overall, freshwater withdrawals for adapted cooling systems under all scenarios are projected to decline by −3% to −63% by 2100 compared to the base year of 2000 ( [[#Fricko--2016|Fricko et al., 2016]] ) ( ''medium confidence'' ).

Diversifying energy portfolios to reduce water-related impacts on the energy sector is another effective adaptation strategy with high mitigation co-benefits. A modelling study from Europe shows that for a 3°C scenario, an energy mix with an 80% share of renewable energy can potentially reduce the overall negative impacts on the energy sector by a factor of 1.5 times or more ( [[#Tobin--2018|Tobin et al., 2018]] ). In addition, hydropower can also play a role in compensating for the intermittency of other renewable energies ( [[#François--2014|François et al., 2014]] ). For example, integrating hydro, solar and wind power in energy generation strategies in the Grand Ethiopian Renaissance Dam can potentially deliver multiple benefits, including decarbonisation, compliance with environmental flow norms and reduce potential conflicts among Nile riparian countries ( [[#Sterl--2021|Sterl et al., 2021]] ). Furthermore, reducing the share of thermoelectric power with solar and wind energy ( [[#Tobin--2018|Tobin et al., 2018]] ; [[#Arango-Aramburo--2019|Arango-Aramburo et al., 2019]] ; [[#Emodi--2019|Emodi et al., 2019]] ) can be synergistic from both climate and water perspectives, as solar and wind energy have lower water footprints ( ''high confidence'' ).

Indigenous Peoples, mountain communities and marginalised minorities often bear the brunt of environmental and social disruptions due to hydropower. As a consequence, hydropower operators face resistance prior to and during construction. Benefit-sharing mechanisms help redistribute some of the gains from hydropower generation to the communities in the immediate vicinity of the project. For instance, sharing of hydropower revenues and profits to fund local infrastructure and pay dividends to local people has been practiced in Nepal and in some countries of the Mekong basin to enhance the social acceptability of hydropower projects ( [[#Balasubramanya--2014|Balasubramanya et al., 2014]] ; [[#Shrestha--2016|Shrestha et al., 2016]] ) ( ''low confidence'' ).

Most water-intensive industries are increasingly facing water stress, making the reuse of water an attractive adaptation strategy (see Box 4.5). For example, Singapore, where the share of industrial water use is projected to grow from 55% in 2016 to 70% in 2060, is increasing its NEWater (highly treated wastewater) supply share from 30% to 55% to meet the growing demand of industrial and cooling activities ( [[#PUB--2016|PUB, 2016]] ). In addition, the mining industry has also adopted water adaptations measures, such as water recycling and reuse; using brackish or saline sources; and working with regional water utilities to reduce water extraction and improve water use efficiency ( [[#Northey--2017|Northey et al., 2017]] ; [[#Odell--2018|Odell et al., 2018]] ).

In summary, energy and industrial sector companies have undertaken several adaptation measures to reduce water stress, with varying effectiveness levels. However, residual risks will remain, especially at higher levels of warming ( ''medium confidence'' ).

<div id="box-4.3" class="h2-container box-container"></div>

'''Box 4.3 | Irrigation as an Adaptation Response'''
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Irrigation has consistently been used as a crop protection and yield enhancement strategy and has become even more critical in a warming world ( [[#Siebert--2014|Siebert et al., 2014]] ). Approximately 40% of global yields come from irrigated agriculture, with a doubling of irrigated areas over the last 50 years and now constituting around 20% of the total harvested area ( [[#FAO--2018b|FAO, 2018b]] ; [[#Meier--2018|Meier et al., 2018]] ; [[#Rosa--2020b|Rosa et al., 2020b]] ). Thus, irrigation is one of the most frequently applied adaptation responses in agriculture and features centrally in projections of adaptation at all scales. Expansions of irrigated areas over the coming century are projected, leading to shifts from rain-fed to irrigated agriculture in response to climate change ( [[#Malek--2018|Malek and Verburg, 2018]] ; [[#Huang--2019|Huang et al., 2019]] ; [[#Nechifor--2019|Nechifor and Winning, 2019]] ). However, there are regional limitations to this expansion due to renewable water resource limitations, including water quality issues ( [[#Zaveri--2016|Zaveri et al., 2016]] ; [[#Turner--2019|Turner et al., 2019]] ). Depending on the specific spatial, temporal and technological characteristics of irrigation expansion, up to 35% of current rain-fed production could sustainably shift to irrigation with limited negative environmental effects ( [[#Rosa--2020b|Rosa et al., 2020b]] ).

Irrigation increases resilience and productivity relative to rain-fed production by reducing drought and heat stress on crop yields and by lowering ET demand by cooling canopy temperatures ( [[#Siebert--2014|Siebert et al., 2014]] ; [[#Tack--2017|Tack et al., 2017]] ; [[#Li--2018|Li and Troy, 2018]] ; Zaveri and B. Lobell, 2019; [[#Agnolucci--2020|Agnolucci et al., 2020]] ; [[#Rosa--2020b|Rosa et al., 2020b]] ). Large-scale irrigation also affects local and regional climates ( [[#Cook--2020b|Cook et al., 2020b]] ). While cooling effects, including reduction of the extreme heat due to irrigation, have been observed ( [[#Qian--2020|Qian et al., 2020]] ; [[#Thiery--2020|Thiery et al., 2020]] ), increases in humid heat extremes because of irrigation with potentially detrimental health outcomes have also been reported ( [[#Krakauer--2020|Krakauer et al., 2020]] ; [[#Mishra--2020|Mishra et al., 2020]] ). For the heavily irrigated North China Plain, a night-time temperature increase overcompensated daytime cooling effects, leading to an overall warming effect ( [[#Chen--2018|Chen and Jeong, 2018]] ). In addition, modification of rainfall patterns has been linked to irrigation ( [[#Alter--2015|Alter et al., 2015]] ; [[#Kang--2019|Kang and Eltahir, 2019]] ; [[#Mathur--2020|Mathur and AchutaRao, 2020]] ). For example, increases in extreme rainfall in central India in recent decades has been linked to the intensification of irrigated paddy cultivation in northwest India ( [[#Devanand--2019|Devanand et al., 2019]] ).

Different irrigation techniques are associated with significant differences in irrigation water productivity ( [[#Deligios--2019|Deligios et al., 2019]] ), and replacing inefficient systems can reduce average non-beneficial water consumption by up to 76% while maintaining stable crop yields ( [[#Jägermeyr--2015|Jägermeyr et al., 2015]] ). Several adjustments can improve water use efficiency, including extending irrigation intervals, shortening the time of watering crops or reducing the size of the plot being irrigated ( [[#Caretta--2015|Caretta and Börjeson, 2015]] ; [[#da%20Cunha--2015|da Cunha et al., 2015]] ; [[#Dumenu--2016|Dumenu and Obeng, 2016]] ). Deficit irrigation is an important mechanism for improving water productivity ( [[#Zheng--2018|Zheng et al., 2018]] ) and increasing regional crop production under drying conditions ( [[#Malek--2018|Malek and Verburg, 2018]] ). Access to irrigation can also play a role in alleviating poverty, contributing to reducing vulnerability and risks ( [[#Balasubramanya--2020|Balasubramanya and Stifel, 2020]] ). However, the diversity of irrigation-related techniques and the consequent differences in effect and water-use intensity is often underreported ( [[#Vanschoenwinkel--2018|Vanschoenwinkel and Van Passel, 2018]] ).

The use of water-saving technologies like laser levelling, micro-irrigation, efficient pumps and water distribution systems ( [[#Kumar--2016|Kumar et al., 2016]] ); increasing irrigation efficiency ( [[#Wang--2019a|Wang et al., 2019a]] ) through improved agronomic practices ( [[#Kakumanu--2018|Kakumanu et al., 2018]] ) and economic instruments like water trading in developed countries like Australia ( [[#Kirby--2014|Kirby et al., 2014]] ) are known to reduce water application rates and increase yields, and ‘save’ water at the plot level, but may exacerbate basin-scale water scarcity ( [[#van%20der%20Kooij--2013|van der Kooij et al., 2013]] ; [[#Zhou--2021|Zhou et al., 2021]] ).

Asia accounts for 69–73% of the world’s irrigated area. However, irrigation currently plays a relatively minor role in most of Africa, except in the contiguous irrigated area along the Nile basin and North Africa and South Africa ( [[#Meier--2018|Meier et al., 2018]] ). In India, long-term data (1956–1999) on the irrigated area shows that farmers adjust their irrigation investments and crop choices in response to medium-run rainfall variability ( [[#Taraz--2017|Taraz, 2017]] ). da Cunha et al. (2015) report that farmers’ income tends to be higher on irrigated lands in Brazil. In Bangladesh, farmers invest a part of their increased incomes in improving irrigation access ( [[#Delaporte--2018|Delaporte and Maurel, 2018]] ). The severity of drought increases the likelihood of farmers adopting supplementary irrigation in Bangladesh ( [[#Alauddin--2014|Alauddin and Sarker, 2014]] ). In Vietnam, irrigation improvement had the highest positive impact on crop yield among all farm-level adaptive practices ( [[#Ho--2019|Ho and Shimada, 2019]] ). In South Africa, access to irrigation was one of the most important predictors of whether or not farmers would adopt a whole suite of other adaptation responses ( [[#Samuel--2019|Samuel and Sylvia, 2019]] ).

Irrigation is also associated with adverse environmental and socioeconomic outcomes, including groundwater over-abstraction, aquifer salinisation ( [[#Foster--2018|Foster et al., 2018]] ; [[#Pulido-Bosch--2018|Pulido-Bosch et al., 2018]] ; [[#Quan--2019|Quan et al., 2019]] ; [[#Blakeslee--2020|Blakeslee et al., 2020]] ) and land degradation ( [[#Singh--2018|Singh et al., 2018]] ). Further, while irrigation expansion is one of the most commonly proposed adaptation responses, there are limitations to further increases in water use, as many regions are already facing water limitations under current climatic conditions ''(high confidence)'' ( [[#Rockström--2014|Rockström et al., 2014]] ; [[#Steffen--2015|Steffen et al., 2015]] ; [[#Kummu--2016|Kummu et al., 2016]] ).

Projections of the future effectiveness of irrigation indicate a varying degree of effectiveness depending on the region and specific type and combination of approaches used. At the same time, overall residual impacts increase at higher levels of warming ( [[#4.7.1.2|Section 4.7.1.2]] ). Uncertainties in regional climate projections and limitations in the ability of agricultural models to fully represent water resources are important limitations in our understanding of the potential of further irrigation expansion ( [[#4.5.1|Section 4.5.1]] ) ( [[#Greve--2018|Greve et al., 2018]] ).

In light of the volume of irrigated agriculture globally, and the projected increase in water requirements for food production, increasing water productivity and thus improving the ratio of water used per unit of agricultural output is necessary globally to meet agricultural water demand ( [[#4.5.1|Section 4.5.1]] ) ( [[#Jägermeyr--2015|Jägermeyr et al., 2015]] ; [[#Jägermeyr--2017|Jägermeyr et al., 2017]] ). For example, assuming a doubling of global maize production by 2050 increased water productivity could reduce total water consumption compared to the baseline productivity by 20–60% ( [[#Zheng--2018|Zheng et al., 2018]] ). Under economic optimisation assumptions, shifts towards less water-intensive and less climate-sensitive crops would be optimal in terms of water use efficiency and absolute yield increases; however, this could pose risks to food security as production shifts away from main staple crops ( [[#Nechifor--2019|Nechifor and Winning, 2019]] ). Shifting currently rain-fed production areas to irrigation will be an important element in ensuring food security with increasing temperatures, though investment in storage capacities to buffer seasonal water shortage will be essential to ensure negative environmental impacts are minimised ( [[#Rosa--2020b|Rosa et al., 2020b]] ).

<div id="_idContainer075" class="Box_Header-continued"></div>

Box 4.3

<div id="4.6.4" class="h2-container"></div>

<span id="adaptation-in-the-water-sanitation-and-hygiene-sector"></span>
=== 4.6.4 Adaptation in the Water, Sanitation and Hygiene Sector ===

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AR5 pointed to adaptive water management techniques ( ''limited evidence, high agreement'' ) ( [[#Field--2014b|Field et al., 2014b]] ), while SR1.5 documented the need for reducing vulnerabilities and promoting sustainable development and disaster risk reduction synergies ( ''high confidence'' ) ( [[#IPCC--2018a|IPCC, 2018a]] ). WaSH has also been identified as a low-regrets adaptation measure ( [[#Cutter--2012|Cutter et al., 2012]] ).

Access to appropriate, reliable WaSH protects against water-related diseases, particularly after climate hazards such as heavy rainfalls and floods ( [[#Carlton--2014|Carlton et al., 2014]] ; [[#Jones--2020|Jones et al., 2020]] ). WaSH interventions have been demonstrated to reduce diarrhoea risk by 25–75% depending on the specific intervention ( [[#Wolf--2018|Wolf et al., 2018]] ) ( ''high confidence'' ). Conversely, inadequate WaSH is associated with an estimated annual loss of 50 million daily adjusted life years ( [[#Prüss-Ustün--2019|Prüss-Ustün et al., 2019]] ), of which 89% of deaths are due to diarrhoea, and 8% of deaths from acute respiratory infections ( [[IPCC:Wg2:Chapter:Chapter-7|Chapter 7]] WGII 7.3.2), making universal access to WaSH (i.e., achievement of SDG 6.1, 6.2) a critical adaptation strategy ( ''high confidence'' ). However, not all WaSH solutions are suited to all climate conditions ( [[#Sherpa--2014|Sherpa et al., 2014]] ; [[#Howard--2016|Howard et al., 2016]] ), so health outcome improvements are not always sustained under changing climate impacts ( [[#Dey--2019|Dey et al., 2019]] ) ( ''medium evidence, high agreement'' ). As such, WaSH infrastructure also needs to be climate-resilient ( [[#Smith--2015|Smith et al., 2015]] ; [[#Shah--2020|Shah et al., 2020]] ). In addition to new WaSH infrastructure design and implementation, expansion and replacement of existing infrastructure offer opportunities to implement climate-resilient designs and reduce greenhouse emissions ( [[#Boholm--2017|Boholm and Prutzer, 2017]] ; [[#Dickin--2020|Dickin et al., 2020]] ) ( ''medium evidence, high agreement'' ).

Effective adaptation strategies include protecting source water and managing both water supply and demand. Source water protection ( [[#Shaffril--2020|Shaffril et al., 2020]] ) has proven effective in reducing contamination. Improved integrated (urban) water resources management ( [[#Kirshen--2018|Kirshen et al., 2018]] ; [[#Tosun--2019|Tosun and Leopold, 2019]] ) and governance ( [[#Chu--2017|Chu, 2017]] ; [[#Miller--2020|Miller et al., 2020]] ) and enhanced ecosystem management ( [[#Adhikari--2018b|Adhikari et al., 2018b]] ) lead to policies and regulations that reduce water insecurity and, when developed appropriately, reduce inequities ( ''medium confidence'' ). Supply (source) augmentation, including dams, storage and rainwater/fog harvesting, can increase the supply or reliability of water for drinking, sanitation and hygiene ( [[#DeNicola--2015|DeNicola et al., 2015]] ; [[#Pearson--2015|Pearson et al., 2015]] ; [[#Majuru--2016|Majuru et al., 2016]] ; [[#Poudel--2017|Poudel and Duex, 2017]] ; [[#Lucier--2018|Lucier and Qadir, 2018]] ; [[#Goodrich--2019|Goodrich et al., 2019]] ) ( ''high confidence'' ). For example, rainwater harvesting in an Inuit community increased water for hygiene by 17%, reduced water retrieval efforts by 40% and improved psychological and financial health (Mercer and [[#Hanrahan--2017|Hanrahan, 2017]] ). However, climate change impacts will affect amounts of rainwater available. A recent study concluded that domestic water demand met through rainwater harvesting generally improves under climate change scenarios for select communities in Canada and Uganda, with the exception of drier summers in some areas of Canada ( [[#Schuster-Wallace--2021|Schuster-Wallace et al., 2021]] ). Further, it is important to recognise that many of these interventions require financial investments that make them inaccessible to the poorest ( [[#Eakin--2016|Eakin et al., 2016]] ). Demand for water can be decreased through reductions in water loss from the system (e.g., pipe leakage) ( [[#Orlove--2019|Orlove et al., 2019]] ) and water conservation measures ( [[#Duran-Encalada--2017|Duran-Encalada et al., 2017]] ) ( ''medium confidence'' ).

During periods of water insecurity, people often implement maladaptive strategies ( [[#Magnan--2016|Magnan et al., 2016]] ), that is, strategies that can increase the risk of adverse health impacts, increase exposure to violence or cause malnutrition ( [[#Kher--2015|Kher et al., 2015]] ; [[#Pommells--2018|Pommells et al., 2018]] ; [[#Collins--2019a|Collins et al., 2019a]] ; [[#Schuster--2020|Schuster et al., 2020]] ) ( ''medium evidence, high agreement'' ). Examples include walking further, using less safe water sources, prioritising drinking and cooking over personal/household hygiene, or reducing food/water intake. Conversely, some rebalancing of gender roles can occur when women and girls cannot source sufficient water, with men building additional water supply or storage infrastructure or fetching water ( [[#Singh--2015|Singh and Singh, 2015]] ; [[#Magesa--2016|Magesa and Pauline, 2016]] ; [[#Shrestha--2019b|Shrestha et al., 2019b]] ). Some adaptation strategies create unintended health threats such as increased odds (1.55) of mosquito larvae in water storage pots ( [[#Ferdousi--2015|Ferdousi et al., 2015]] ), which could have even more significant impacts in the future given projected range expansion for vectors as a result of climate change ( [[#Liu-Helmersson--2019|Liu-Helmersson et al., 2019]] ). Other unintended consequences include pathogen contamination ( [[#Gwenzi--2015|Gwenzi et al., 2015]] ) and time or financial trade-offs ( [[#Schuster--2020|Schuster et al., 2020]] ) ( ''medium evidence, high agreement'' ). Wastewater reuse for irrigation may have adverse health impacts if wastewater is not treated ( [[#Dickin--2016|Dickin et al., 2016]] ). Conversely, especially where women are responsible for domestic and productive water management, adaptive agricultural water strategies, such as water-efficient irrigation or low-water crops, mean that less water from finite water supplies are used for agriculture, leaving more water locally available for domestic purposes (see section 4.6.2). These co-benefits across sectors become important community water stress adaptations ( [[#Chinwendu--2017|Chinwendu et al., 2017]] ), with water savings from one use leading to more water available for other uses. This can reduce domestic water burdens and, therefore, gender inequities ( [[#4.8.3|Section 4.8.3]] ) ( ''limited evidence, high agreement'' ). Further analyses of co-benefits, particularly employing a gender lens, are required to improve adaptation strategies ( [[#McIver--2016|McIver et al., 2016]] ).

In summary, ensuring access to climate-resilient WaSH infrastructure and practices represents a key adaptation strategy that can protect beneficiaries against water-related diseases induced by climate change ( ''high confidence'' ). Better management of water resources, supply augmentation and demand management are important adaptation strategies ( ''high confidence'' ). Reliable, safe drinking water reduces adverse physical and psychological impacts of climate-related water stress and extreme events ( ''robust evidence, medium agreement'' ). WaSH infrastructure expansion and replacement provide opportunities to redesign and increase resilience in rural and urban contexts ( ''limited evidence, high agreement'' ).

<div id="box-4.4" class="h2-container box-container"></div>

'''Box 4.4 | COVID-19 Amplifies Challenges for WaSH Adaptation'''
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While COVID-19 is an airborne disease (see Cross-Chapter Box COVID in Chapter 7), public health responses to the COVID-19 pandemic and the associated socioeconomic and environmental impacts of these measures intersect with WaSH ( [[#Armitage--2020a|Armitage and Nellums, 2020a]] ). Notably, COVID-19 and climate change act as compound risks in the context of water-induced disasters, exacerbating existing threats to sustainable development ( [[#Neal--2020|Neal, 2020]] ; Pelling et al. 2021).

The principal WaSH response to COVID-19 relates to hand hygiene, an infection control intervention that requires access to sufficient, clean and affordable water beyond cooking, hydration and general sanitation needs, as outlined in SDG6 ( [[#Armitage--2020a|Armitage and Nellums, 2020a]] ). However, despite significant progress, more than 800 million people in central and southern Asia, and 760 million in sub-Saharan Africa, lack basic hand-washing facilities in the home (UNICEF, 2020). Notably, one in four healthcare facilities in select low- and middle-income countries lacks basic water access, and one in six lacks hand-washing facilities ( [[#WHO--2019|WHO, 2019]] ) ( [[#4.3.3|Section 4.3.3]] ). Moreover, household water insecurity also impacts marginalised and minority groups in the Global North ( [[#Deitz--2019|Deitz and Meehan, 2019]] ; [[#Rodriguez-Lonebear--2020|Rodriguez-Lonebear et al., 2020]] ; [[#Stoler--2021|Stoler et al., 2021]] ).

Compound disasters have arisen due to either the co-occurrence of drought, storms or floods and COVID-19. COVID-19 acts as a stress multiplier for women and girls in charge of water collection and minorities and disabled people who are not engaged in water management ( [[#Phillips--2020|Phillips et al., 2020]] ; [[#Rodriguez-Lonebear--2020|Rodriguez-Lonebear et al., 2020]] ). Across the world, existing inequalities deepened due to lockdowns, which further limited access to clean water and education for women and girls, and reinstated gendered responsibilities of child, elderly and sick care, which had been previously externalised ( [[#Cousins--2020|Cousins, 2020]] ; [[#Neal--2020|Neal, 2020]] ; [[#Zavaleta-Cortijo--2020|Zavaleta-Cortijo et al., 2020]] ). Accordingly, COVID-19 has further steepened the path to reach SDGs 2, 3, 4, 5 and 11 ( [[#Lambert--2020|Lambert et al., 2020]] ; [[#Mukherjee--2020|Mukherjee et al., 2020]] ; [[#Neal--2020|Neal, 2020]] ; [[#Pramanik--2021|Pramanik et al., 2021]] ). In addition, the pandemic exacerbated food insecurity in drought-affected eastern and southern Africa ( [[#Phillips--2020|Phillips et al., 2020]] ; [[#Mishra--2021|Mishra et al., 2021]] ). As the twin risk of COVID-19 and hurricanes on the US Gulf Coast ( [[#Pei--2020|Pei et al., 2020]] ; [[#Shultz--2020|Shultz et al., 2020]] ) and cyclone Amphan in Bangladesh ( [[#Pramanik--2021|Pramanik et al., 2021]] ) showed, increased hand washing, additional WaSH and evacuation and shelter infrastructures proved essential for preventing further spread of COVID-19 ( [[#Baidya--2020|Baidya et al., 2020]] ; [[#Ebrahim--2020|Ebrahim et al., 2020]] ; [[#Guo--2020|Guo et al., 2020]] ; [[#Mukherjee--2020|Mukherjee et al., 2020]] ; [[#Pei--2020|Pei et al., 2020]] ; [[#Shultz--2020|Shultz et al., 2020]] ; [[#Pramanik--2021|Pramanik et al., 2021]] ). Moreover, while immediate steps can be taken during disaster response to minimise climate-attributable loss of life, climate adaptation requires long-term strategies that intersect with pandemic preparedness ( [[#Phillips--2020|Phillips et al., 2020]] ).

Public health responses to COVID-19 geared towards infection control and caring for the sick can trigger increased water demand where population numbers and density are high ( [[#Mukherjee--2020|Mukherjee et al., 2020]] ; [[#Sivakumar--2021|Sivakumar, 2021]] ). As COVID-19 has highlighted the importance of WaSH ( [[#4.3.3|Section 4.3.3]] ), this pandemic could also result in long-term positive outcomes in community resilience, improved infection control and health protection while addressing longer-term environmental challenges of climate change ( [[#Phillips--2020|Phillips et al., 2020]] ).

<div id="4.6.5" class="h2-container"></div>

<span id="adaptation-in-urban-and-peri-urban-sectors"></span>
=== 4.6.5 Adaptation in Urban and Peri-Urban Sectors ===

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AR5 reported that although case studies of the potential effectiveness of adaptation measures in cities are growing, not all considered how adaptation would be implemented in practice ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). Furthermore, AR5 concluded that more attention had been given to adaptations that help ensure sufficient water supplies than to increasing the capacity of sewage and drainage systems to adapt to heavier rainfall or sea level rise ( [[#Revi--2014|Revi et al., 2014]] ).

Since AR5 knowledge on urban adaptation has advanced, even though there is still limited documentation of water adaptation in urban contexts as compared to other adaptation responses (Figure 4.23.) The majority of case studies on urban adaptation are also from developed countries, most commonly in Europe and Australasia (Figure 4.24). Water-related urban and peri-urban climate change adaptation can involve ‘hard’ engineering structures (grey), managed or restored biophysical systems (green and blue) or hybrid approaches that combine these strategies ( [[#Ngoran--2015|Ngoran and Xue, 2015]] ; [[#Palmer--2015|Palmer et al., 2015]] ) (Figure 4.21, also see Figure 4.22 for types of urban adaptation options).

<div id="_idContainer085" class="Figure"></div>

[[File:874b9c2ec605731213e4dc9930214023 IPCC_AR6_WGII_Figure_4_022.png]]

'''Figure 4.22 |''' '''Decision tree, documenting the classification of water-related adaptation responses across 48 subcategories into 16 intermediate and 8 larger categories.''' We use the 16 intermediate categories of adaptation responses for further analysis in this section.

<div id="_idContainer079" class="Figure"></div>

[[File:6c6c877225b16479b251c17fedec9d90 IPCC_AR6_WGII_Figure_4_021.png]]

'''Figure 4.21 |''' '''Strategies for urban water adaptation.'''

'''(a)''' Green and blue strategies of urban water adaptation prioritise ecosystem restoration, such as wetlands restoration.

'''(b)''' Grey water strategies are hard engineering approaches to urban water adaptation, including infrastructure such as pipes and canals, with extensive areas of impervious surfaces.

'''(c)''' Hybrid approaches combine green, blue and grey adaptation strategies, such that ecosystem functions are complemented by engineered infrastructure, such as constructed wetlands, green roofs and riparian buffers. Green and blue and hybrid approaches are variously classified in terms of a circular economy, water sensitive urban design, nature-based solutions (NbS), integrated urban water management, and ecological infrastructure. Adapted from [[#Depietri--2017|Depietri and McPhearson (2017)]] .

In most regions, hybrid adaptation approaches are underway. For example, sustainable urban drainage systems (SUDS) are a common adaptation measure that can reduce flooding and improve stormwater quality while reducing the urban heat island effect (e.g., [[#Chan--2019|Chan et al., 2019]] ; [[#Loiola--2019|Loiola et al., 2019]] ; [[#Song--2019|Song et al., 2019]] ; [[#Huang--2020|Huang et al., 2020]] ; [[#Lin--2020|Lin et al., 2020]] ) (Box 4.6; 12.5.5.3.2; 12.7.1). Municipal, catchment and local community plans to minimise water-related climate risks are another form of adaptation ( [[#Stults--2018|Stults and Larsen, 2018]] ). Plans involve supply augmentation ( [[#Chu--2017|Chu, 2017]] ; [[#Bekele--2018|Bekele et al., 2018]] ), as well as floodplain management, land use planning, stakeholder coordination and water demand management ( [[#Andrew--2017|Andrew and Sauquet, 2017]] ; [[#Flyen--2018|Flyen et al., 2018]] ; [[#Robb--2019|Robb et al., 2019]] ; [[#Tosun--2019|Tosun and Leopold, 2019]] ), with some US cities including strategies to address social inequalities that climate change may exacerbate ( [[#Chu--2021|Chu and Cannon, 2021]] ).

Such adaptation measures are concentrated in more developed countries ( [[#Olazabal--2019|Olazabal et al., 2019]] ). For example, about 80% of European cities with more than 500,000 inhabitants have either mitigation and/or adaptation plans ( [[#Reckien--2018|Reckien et al., 2018]] ). In contrast, a survey of cities with more than one million inhabitants found 92% of Asian cities, 89% of African cities and 87% of Latin American cities did not report adaptation initiatives ( [[#Araos--2016|Araos et al., 2016]] ) (12.5.8.1). Autonomous adaptation measures (e.g., elevating housing and drainage maintenance) are pursued to reduce flood risk in urban Senegal ( [[#Schaer--2015|Schaer, 2015]] ), Kenya ( [[#Thorn--2015|Thorn et al., 2015]] ), Brazil ( [[#Mansur--2018|Mansur et al., 2018]] ) and Guyana ( [[#Mycoo--2014|Mycoo, 2014]] ) (Box 4.7; 9.8.5.1; 12.5.5.3; FAQ12.2).

Further studies are required to ascertain the effectiveness of adaptation measures implemented since AR5, particularly for the growing populations of informal and peri-urban settlements. For example, in urban Africa, such informal settlements are sites of political contestation as residents resist municipal relocation strategies for flood alleviation ( [[#Douglas--2018|Douglas, 2018]] ). In addition, the growing complexity of challenges facing urban water management, such as climate change, urbanisation and environmental degradation, warrants a transformative shift away from prevailing siloed approaches of water supply, sanitation and drainage to more integrated systems that enhance adaptive capacity ( [[#Ma--2015|Ma et al., 2015]] ; [[#Franco-Torres--2020|Franco-Torres et al., 2020]] ).

In summary, although water-related adaptation is underway in the urban, peri-urban and municipal sectors of some nations, governance, technical and economic barriers remain in implementing locally informed strategies, particularly in developing countries ( ''high confidence'' ).

<div id="4.6.6" class="h2-container"></div>

<span id="adaptation-for-communities-dependent-on-freshwater-ecosystems"></span>
=== 4.6.6 Adaptation for Communities Dependent on Freshwater Ecosystems ===

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AR5 concluded that some adaptation responses in the urban and agricultural sectors could negatively impact freshwater ecosystems ( ''medium confidence'' ) ( [[#Settele--2014|Settele et al., 2014]] ).

Adaptation measures to cope with changes in ecosystems, including freshwater ecosystems, such as ecosystem-based adaptation (EbA) interventions have gained wide recognition at the global policy level ( [[#Reid--2016|Reid, 2016]] ; [[#Barkdull--2019|Barkdull and Harris, 2019]] ; [[#Piggott-McKellar--2019b|Piggott-McKellar et al., 2019b]] ).These have been implemented in many locations around the world, yet, challenges remain, including improving the evidence base of their effectiveness, scaling up of these interventions, mainstreaming across sectors and receiving more adaptation finance ( ''medium confidence'' ).

A systematic review of 132 academic papers and 32 articles from non-peer-reviewed literature ( [[#Doswald--2014|Doswald et al., 2014]] ) provided a comprehensive global overview of EbA, which showed that EbA interventions were used in various ecosystems, including inland wetlands (linked to 30 publications). An investigation of EbA effectiveness by [[#Reid--2019|Reid et al. (2019)]] , where nine case studies covering South Asia, Africa and South America were associated with freshwater systems, concluded that EbA enabled the enhancement of the adaptive capacity or resilience to climate change, particularly for the more vulnerable groups in the community. An assessment of the potential for EbA in three sub-basins of the Murray–Darling Basin, Australia, concluded that EbA can augment catchment management practices but that there were also institutional challenges ( [[#Lukasiewicz--2016|Lukasiewicz et al., 2016]] ). In urban settings, EbA has been associated with ecological structures for reducing risks, including the use of urban wetlands ( [[#Barkdull--2019|Barkdull and Harris, 2019]] ). EbA is a subset of NbS that is rooted in climate change adaptation and covers both mitigation and adaptation ( [[#Pauleit--2017|Pauleit et al., 2017]] ) ( [[#4.6.5|Section 4.6.5]] , Box 4.6). Although adaptation measures for freshwater ecosystems have been implemented in many places ( [[#Shaw--2014|Shaw et al., 2014]] ; [[#Lukasiewicz--2016|Lukasiewicz et al., 2016]] ; [[#Karim--2017|Karim and Thiel, 2017]] ; [[#Milman--2017|Milman and Jagannathan, 2017]] ; [[#FAO--2018a|FAO, 2018a]] ; [[#Piggott-McKellar--2019b|Piggott-McKellar et al., 2019b]] ), the evidence base for the effectiveness of these measures to cope with changes in freshwater ecosystems needs improvement. These measures also require further financial support, mainstreaming across sectors and the scaling up of individual measures ( ''medium confidence'' ).

In summary, adaptation measures to cope with changes in freshwater ecosystems have been implemented in many locations around the world. However, challenges remain, including improving the evidence base of their effectiveness, scaling up these interventions, mainstreaming across sectors and receiving more adaptation finance ( ''medium confidence'' ).

<div id="4.6.7" class="h2-container"></div>

<span id="adaptation-responses-for-water-related-conflicts"></span>
=== 4.6.7 Adaptation Responses for Water-Related Conflicts ===

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AR5 concluded with ''high confidence'' that challenges for adaptation actions (though not water) are particularly high in regions affected by conflicts ( [[#Field--2014a|Field et al., 2014a]] ). Although climate–conflict linkages are disputed ( [[#4.3.6|Section 4.3.6]] ), the potential for synergies between conflict risk reduction and adaptation to climate change exists ( [[#Mach--2019|Mach et al., 2019]] ). For example, discourses around climate–conflict inter-linkages can present opportunities for peace building and cooperation ( [[#Matthew--2014|Matthew, 2014]] ; [[#Abrahams--2020|Abrahams, 2020]] ). Indeed, adaptation efforts are needed in the context of conflict, where the pre-existing vulnerability undermines the capacity to manage climatic stresses. In addition, adaptive capacity depends on contextual factors such as power relations and historical, ethnic tensions ( [[#Petersen-Perlman--2017|Petersen-Perlman et al., 2017]] ; [[#Eriksen--2021|Eriksen et al., 2021]] ), which need to be adequately considered in the design of adaptation strategies.

Some adaptation options, such as water conservation, storage and infrastructure, voluntary migration, planned relocation due to flood risk/sea level rise, and international water treaties, can reduce vulnerability to climate change and conflicts. However, on the other hand, these adaptation options sometimes may have unintended consequences by increasing existing tensions ( [[#Milman--2014|Milman and Arsano, 2014]] ); displacing climate hazards to more vulnerable and marginalised groups ( [[#Milman--2014|Milman and Arsano, 2014]] ; [[#Mach--2019|Mach et al., 2019]] ), for example, pastoralists ( [[#Zografos--2014|Zografos et al., 2014]] ); and favouring some over others, such as industry over agriculture (Iglesias and Garrote, 2015)¸ upstream countries over downstream countries ( [[#Veldkamp--2017|Veldkamp et al., 2017]] ), and men over women ( [[#Chandra--2017|Chandra et al., 2017]] ). Such unintended consequences may happen when adaptation measures intended to reduce vulnerability produce maladaptive outcomes by rebounding or shifting vulnerability to other actors ( [[#Juhola--2016|Juhola et al., 2016]] ). For example, in the Mekong River basin, the construction of dams and water reservoirs contributes to the adaptation efforts of the upstream Southeast Asia countries while increasing current/future vulnerability to floods and droughts in downstream countries and can emerge as a cause of conflict ( [[#Earle--2015|Earle et al., 2015]] ; [[#Ngô--2016|Ngô et al., 2016]] ).

Furthermore, adaptation in the context of water-related conflicts is also constrained by economic, institutional and political factors, competition for development ( [[#Anguelovski--2014|Anguelovski et al., 2014]] ) and gender considerations ( [[#Sultana--2014|Sultana, 2014]] ; [[#Chandra--2017|Chandra et al., 2017]] ), which need to be taken into account when designing adaptation plans/measures.

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'''Box 4.5 | Reduce, Remove, Reuse and Recycle (4Rs): Wastewater Reuse and Desalination as an Adaptation Response'''
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Circular economies can increase the available sustainable adaptation space by moving away from a linear mode of production of ‘extract-produce-use-discard’ to a ‘4Rs’ closed loop to reduce pollution at the source, remove contaminants from wastewater, reuse treated wastewater and recover valuable by-products ( [[#UN%20Water--2017|UN Water, 2017]] ; see WGIII 11.3.3).

It is estimated that 380 billion m 3 of wastewater is produced annually worldwide, which equals about 15% of agricultural water withdrawals. The recovery of nitrogen, phosphorus and potassium from wastewater can offset 13.4% of the global agriculture demand for these nutrients ( [[#Jiménez--2008|Jiménez and Asano, 2008]] ; [[#Fernández-Arévalo--2017|Fernández-Arévalo et al., 2017]] ). Recycling human waste worldwide could satisfy an estimated 22% of the global demand for phosphorus ( [[#UN%20Water--2017|UN Water, 2017]] ). It has been estimated that some 36 million ha worldwide (some 12% of all irrigated land) reuse urban wastewater, mainly for irrigation. However, only around 15% is adequately treated ( [[#Thebo--2017|Thebo et al., 2017]] ), thus the need to invest in sustainable, low-cost wastewater treatment to protect public health. The irrigation potential of this volume of wastewater stands at 42 million ha. Wastewater production is expected to increase globally to 574 billion m 3 by 2050, a 51% increase compared to 2015, mainly due to a growing urban population ( [[#Qadir--2020|Qadir et al., 2020]] ). Water reuse with treated wastewater for potable and non-potable purposes can be practised in a manner that is protective of public health and the environment ( [[#WHO--2006|WHO, 2006]] ; [[#WHO--2017|WHO, 2017]] ). For example, when implemented with sufficient treatment standards, the use of recycled water for the irrigation of crops is protective of public health ( [[#Blaine--2013|Blaine et al., 2013]] ; [[#Paltiel--2016|Paltiel et al., 2016]] ), as was determined by an appointed panel of experts in the state of California ( [[#Cooper--2012|Cooper et al., 2012]] ). However, there are several barriers to the adoption of wastewater reuse; these include technical barriers and public health aspects related to microbiological and pharmaceuticals risks ( [[#Jiménez--2008|Jiménez and Asano, 2008]] ; [[#Jaramillo--2017|Jaramillo and Restrepo, 2017]] ; [[#Saurí--2019|Saurí and Arahuetes, 2019]] ). These are currently being addressed by strengthening regulatory standards, with, for example, 11 out of 22 Arab States adopting legislation permitting the use of treated wastewater ( [[#WHO--2006|WHO, 2006]] ; [[#US%20EPA--2017|US EPA, 2017]] ; [[#WHO--2017|WHO, 2017]] ; [[#EC--2020|EC, 2020]] ). Benefits of wastewater reuse usually outweigh the costs ( [[#Stacklin--2012|Stacklin, 2012]] ; [[#Hernández-Sancho--2015|Hernández-Sancho et al., 2015]] ; [[#UN%20Water--2017|UN Water, 2017]] ).

Desalination is particularly important in arid and semiarid climates, coastal cities and small island states (Box 4.2). There were 16,000 operational desalination plants globally in 2017, with a daily desalinated water production of 95 million m 3 d -1 ( [[#IDA--2020|IDA, 2020]] ). In 2012, desalinated water was equivalent to 0.6% of the global water supply, and 75.2 TWh of energy per year was used to generate desalinated water; in other words, about 0.4% of the worldwide electricity consumption ( [[#IRENA--2012|IRENA, 2012]] ). Unfortunately, only 1% of total desalinated water uses renewable sources ( [[#IRENA--2012|IRENA, 2012]] ; [[#Amy--2017|Amy et al., 2017]] ; [[#Balaban--2017|Balaban, 2017]] ; [[#Martínez-Alvarez--2018a|Martínez-Alvarez et al., 2018a]] ; [[#Jones--2019|Jones et al., 2019]] ) ( [[#4.7.6|Section 4.7.6]] ). Desalination has already helped to meet urban and peri-urban water supply, particularly during annual or seasonal drought events, with half of the world’s desalination capacity in the Arab region ( [[#UN%20Environment--2019|UN Environment, 2019]] ; [[#UN%20Water--2021|UN Water, 2021]] ). In addition, seawater desalination could help address water scarcity in 146 (50%) large cities (including 12 (63.2%) megacities) ( [[#He--2021|He et al., 2021]] ). Desalination is also being adopted for irrigation. For example, in the island of Gran Canary (Spain), 30% of the agricultural surface area is irrigated with desalinated water to irrigate high-value crops ( [[#Burn--2015|Burn et al., 2015]] ; [[#Martínez-Alvarez--2018a|Martínez-Alvarez et al., 2018a]] ; [[#Monterrey-Viña--2020|Monterrey-Viña et al., 2020]] ). The expected growth of desalination, if not coupled with renewable energy (RE), causes a projected 180% increase in carbon emissions by 2040 ( [[#GCWDA--2015|GCWDA, 2015]] ; [[#Pistocchi--2020|Pistocchi et al., 2020]] ). There have been advances in large-scale and on-farm renewable desalination ( [[#Abdelkareem--2018|Abdelkareem et al., 2018]] ). Using renewable energy to decarbonise desalination has meant that the projected global average levelled cost of water could decrease from €2.4 m –3 (2015) to approximately €1.05 m –3 by 2050, considering unsubsidised fossil fuel costs ( [[#Caldera--2020|Caldera and Breyer, 2020]] ). Desalination will be maladaptive if fossil fuel is used ( [[#Tubi--2021|Tubi and Williams, 2021]] ).

In summary, a resilient circular economy is central to deliver access to water and sanitation, with, wastewater treatment, desalination and water reuse as viable adaptation options compatible with the Paris Agreement, while safeguarding ecological flows according to the SDG6 targets for climate resilient development ( ''medium evidence, high agreement'' ).

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'''Box 4.6 | Nature-based Solutions for Water-Related Adaptation'''
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In the context of climate change-induced water insecurity, nature-based solutions (NbS) are an adaptation response that relies on natural processes to enhance water availability and water quality and mitigate risks associated with water-related disasters while contributing to biodiversity ( [[#IUCN--2020|IUCN, 2020]] ).

Until recently, NbS have been considered mainly for mitigation ( [[#Kapos--2020|Kapos et al., 2020]] ; [[#Seddon--2020|Seddon et al., 2020]] ). Yet, NbS increase the low-cost adaptation options that expand the adaptation space due to their multiple co-benefits (Cross-Chapter Box NATURAL in Chapter 2). Furthermore, a meta-review of 928 NbS measures globally shows that NbS largely addresses water-related hazards like heavy precipitation (37%) and drought (28%) ( [[#Kapos--2020|Kapos et al., 2020]] ).

Natural infrastructure (green and blue) uses natural or semi-natural systems, for example, wetlands, healthy freshwater ecosystems, etc., to supply clean water, regulate flooding, enhance water quality and control erosion (6.3.3.1 to 6.3.3.6.). Grey infrastructure can damage biophysical and hydrological processes, seal soils and bury streams. Compared with grey physical infrastructure, natural infrastructure is often more flexible, cost-effective and can provide multiple societal and environmental benefits simultaneously ( [[#McVittie--2018|McVittie et al., 2018]] ; [[#UN%20Water--2018|UN Water, 2018]] ; [[#IPBES--2019|IPBES, 2019]] ). There is increasing evidence and assessment methods on the role of NbS for climate change adaptation and disaster risk reduction at different scales ( [[#Chausson--2020|Chausson et al., 2020]] ; [[#Seddon--2020|Seddon et al., 2020]] ; [[#Cassin--2021|Cassin and Matthews, 2021]] ) ( [[#4.6.5|Section 4.6.5]] ).

At the landscape scale, there is evidence that impacts from fluvial and coastal floods can be mitigated through water-based NbS like detention/retention basins, river restoration and wetlands ( [[#Thorslund--2017|Thorslund et al., 2017]] ; [[#Debele--2019|Debele et al., 2019]] ; [[#Huang--2020|Huang et al., 2020]] ). Several examples show the effectiveness of floodplain restoration, natural flood management and making room for the river measures (see FAQ2.5, [[#Hartmann--2019|Hartmann et al., 2019]] ; [[#Mansourian--2019|Mansourian et al., 2019]] ; [[#Wilkinson--2019|Wilkinson et al., 2019]] ) ( ''medium evidence, high agreement'' ). Likewise, the use of managed aquifer recharge (MAR) in both urban and rural settings will be crucial for groundwater-related adaptation ( [[#Zhang--2020|Zhang et al., 2020]] a).

At the urban and peri-urban scale, the use and effectiveness of NbS is a crucial feature to build resilience in cities for urban stormwater management and heat mitigation ( [[#Depietri--2017|Depietri and McPhearson, 2017]] ; [[#Carter--2018|Carter et al., 2018]] ; [[#Huang--2020|Huang et al., 2020]] ; [[#Babí%20Almenar--2021|Babí Almenar et al., 2021]] ) ( ''high confidence'' ). NbS have been used for stormwater management by combining water purification and retention functions ( [[#Prudencio--2018|Prudencio and Null, 2018]] ; [[#Oral--2020|Oral et al., 2020]] ). NbS have also been used to mitigate impacts from high-impact extreme precipitation events by integrating large-scale NbS investment plans into urban planning in cities like New York and Copenhagen, highlighting the importance of blended finance and investment (including insurance) to mainstream NbS investments ( [[#Liu--2017|Liu and Jensen, 2017]] ; [[#Rosenzweig--2019|Rosenzweig et al., 2019]] ; [[#Lopez-Gunn--2021|Lopez-Gunn et al., 2021]] ). According to the CDP database, one in three cities use NbS to address climate hazards, and this trend is growing ( [[#Kapos--2020|Kapos et al., 2020]] ).

NbS are cost-effective and can complement or replace grey solutions (Cross-Chapter Box FEASIB in Chapter 18, 3.2.3) ( [[#Chausson--2020|Chausson et al., 2020]] ). Moreover, estimates of NbS are increasingly based on integrated economic valuations that incorporate co-design with stakeholders to incorporate LK ( [[#Pagano--2019|Pagano et al., 2019]] ; [[#Giordano--2020|Giordano et al., 2020]] ; [[#Hérivaux--2021|Hérivaux and Le Coent, 2021]] ; [[#Palomo--2021|Palomo et al., 2021]] ) ( ''medium evidence, high agreement'' ). Yet, the performance of NbS themselves may be limited at higher GWLs ( [[#Calliari--2019|Calliari et al., 2019]] ; [[#Morecroft--2019|Morecroft et al., 2019]] ).

More knowledge is needed on the long-term benefits of NbS, particularly to hydro-meteorological hazards ( [[#Debele--2019|Debele et al., 2019]] ). There is still ''low evidence'' for slow-onset events, including the applicability of NbS to manage highly vulnerable ecosystems and in agriculture (Sonneveld, 2018),

In summary, there is growing evidence on NbS effectiveness as an adaptation measure and its critical role for transformative adaptation to address climate change water-related hazards and water security ( ''medium evidence, high agreement'' ). Moreover, several NbS– —for example, natural (blue and green) and grey infrastructure—can help address water-related hazards such as coastal hazards, heavy precipitation, drought, erosion and low water quality ( ''high confidence'' ).

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=== 4.6.8 Adaptations Through Human Mobility and Migration ===

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AR5 noted that whether migration is adaptive or maladaptive depends on the context and the individuals involved; however, it did not focus specifically on hydrological change-induced migration ( [[#Noble--2014|Noble et al., 2014]] ). Migration is often regarded as a transformational adaptation strategy in response to climate-induced hydrological changes ( [[#Gemenne--2017|Gemenne and Blocher, 2017]] ) but rarely as the primary or only adaptation measure ( [[#Wiederkehr--2018|Wiederkehr et al., 2018]] ; [[#de%20Longueville--2020|de Longueville et al., 2020]] ; Cross-Chapter Box MIGRATE in Chapter 7). Migration is among one of the top five adaptation responses documented in Asia and Africa (Figure 4.27) and confers several benefits to migrants, yet maladaptations are also documented (Figure 4.29). This strategy is not available to everyone. Vulnerable populations exposed to hydrological changes may become trapped due to a lack of economic and social capital required for migration ( [[#Adams--2016|Adams, 2016]] ; [[#Zickgraf--2018|Zickgraf, 2018]] ) ( ''medium confidence'' ).

Spontaneous migration, undertaken without outside assistance, has shown the potential to improve the resilience of migrants and communities ( [[#Call--2017|Call et al., 2017]] ; [[#Jha--2018a|Jha et al., 2018a]] ), but may also lead to increased vulnerability and insecurity in some instances ( [[#Adger--2018|Adger et al., 2018]] ; [[#Linke--2018a|Linke et al., 2018a]] ; [[#Singh--2020|Singh and Basu, 2020]] ). Migration is not a viable strategy for everyone, but age, gender and socioeconomic status play a significant role in encouraging or inhibiting the chances of successful migration ( [[#Maharjan--2020|Maharjan et al., 2020]] ; [[#Bergmann--2021|Bergmann et al., 2021]] ; [[#Erwin--2021|Erwin et al., 2021]] ). Migration has increased vulnerability among women and female-headed households ( [[#Patel--2019|Patel and Giri, 2019]] ), but has also triggered gender-positive processes such as increased female school enrolment ( [[#Gioli--2014|Gioli et al., 2014]] ) ( ''medium confidence'' ). Remittances, that is, transfers of money from migrants to beneficiaries in sending areas, may reduce vulnerability and increase adaptive capacity to climate-induced hydrological changes ( [[#Ng’ang’a--2016|Ng’ang’a et al., 2016]] ; [[#Jha--2018b|Jha et al., 2018b]] ) ( ''medium confidence'' ). Managed retreat refers to the planned and assisted moving of people and assets away from risk areas, such as government- or community-led resettlement ( [[#Hino--2017|Hino et al., 2017]] ; [[#Maldonado--2018|Maldonado and Peterson, 2018]] ; [[#Tadgell--2018|Tadgell et al., 2018]] ; [[#Arnall--2019|Arnall, 2019]] ). Such initiatives may reduce exposure to risk ( [[#Lei--2017|Lei et al., 2017]] ). However, they often fail to include affected populations in the process and may lead to greater impoverishment and increased vulnerability ( [[#Wilmsen--2015|Wilmsen and Webber, 2015]] ) ( ''medium confidence'' ).

More research on how to ensure migration becomes a successful adaptation strategy is needed ( [[#McLeman--2016|McLeman et al., 2016]] ). In addition, impacts on women, youth and marginalised groups ( [[#McLeman--2016|McLeman et al., 2016]] ; Miletto, 2017) and immobility issues need more attention ( [[#Zickgraf--2018|Zickgraf, 2018]] ).

In summary, measures that facilitate successful migration and inclusive resettlement may facilitate adaptation to climate-induced hydrological changes ( ''medium confidence'' ).

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=== 4.6.9 Adaptation of the Cultural Water Uses of Indigenous Peoples, Local Communities and Traditional Peoples ===

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AR5 reported that religious and sacred values inform actions taken to adapt to climate change ( [[#Noble--2014|Noble et al., 2014]] ). Neither AR5 nor SR1.5 reviewed adaptation of indigenous, local and traditional uses of water. SROCC highlighted the context-specific adaptation strategies of vulnerable communities in coastal, polar and high-mountain areas, reporting that adaptive capacity and adaptation limits are not only physical, technical, institutional and financial, but also culturally informed ( [[#Hock--2019b|Hock et al., 2019b]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

There is ''high confidence'' that some Indigenous Peoples, local communities and traditional peoples could adapt, and are adapting to climate-driven hydrological changes and their impacts on culturally significant sites, species, ecosystems and practices in polar, high-mountain and coastal areas, where sufficient funding, decision-making power and resourcing exist (e.g., [[#Golden--2015|Golden et al., 2015]] ; [[#Bunce--2016|Bunce et al., 2016]] ; [[#Anderson--2018|Anderson et al., 2018]] ). However, there is also ''high confidence'' that there are significant structural barriers and limits to their adaptation, and that the outcomes of some adaptation strategies can be uneven and maladaptive ( ''medium evidence, high agreement'' ) (Sections 4.7.4; 4.8.3). These barriers include the lack of recognition of Indigenous Peoples’ sovereignty and exclusion of Indigenous Peoples from decision-making institutions ( [[#Ford--2017|Ford et al., 2017]] ; [[#Labbé--2017|Labbé et al., 2017]] ; [[#Eira--2018|Eira et al., 2018]] ; [[#McLeod--2018|McLeod et al., 2018]] ; [[#MacDonald--2020|MacDonald and Birchall, 2020]] ) (14.4.4.2.2; 13.8.1.2). At the same time, the rate and scale of climate change can impede the ability of vulnerable communities to turn their adaptive capacity into effective adaptation responses ( [[#Ford--2015|Ford et al., 2015]] ; [[#Herman-Mercer--2019|Herman-Mercer et al., 2019]] ).

There is ''high confidence'' that local people are adapting to the cultural impacts of climate-driven glacier retreat and decline in snow cover and ice in polar and high-mountain areas. However, there is also ''high confidence'' that such adaptation can be detrimental and disrupt local cultures. For example, in the Peruvian Andes, concerns about water availability for ritual purposes has led to restrictions on pilgrims’ removal of ice and limiting the size of ritual candles to preserve the glacier ( [[#Paerregaard--2013|Paerregaard, 2013]] ; [[#Allison--2015|Allison, 2015]] ). Relatedly, some local people have questioned the cosmological order and have reoriented their spiritual relationships accordingly ( [[#Paerregaard--2013|Paerregaard, 2013]] ; [[#Carey--2017|Carey et al., 2017]] ). In Siberia ( [[#Mustonen--2015|Mustonen, 2015]] ) and northern Finland ( [[#Turunen--2016|Turunen et al., 2016]] ), community-led decisions among herders favour alternative routing, pasture areas and shifts in nomadic cycles in response to changing flood events and permafrost conditions (Box 13.2). However, loss of grazing land and pasture fragmentation pose adaptation limits, and some strategies such as supplementary feeding and new technologies may further affect cultural traditions of herding communities ( [[#Risvoll--2016|Risvoll and Hovelsrud, 2016]] ; [[#Jaakkola--2018|Jaakkola et al., 2018]] ).

There is ''high confidence'' that relocation (managed retreat) is an adaptation response for communities in areas impacted by, or at risk of, inundation and other hydrological changes (15.3.4.7; 15.5.3). However, relocation can be culturally, socially, financially, politically and geographically constrained due to the importance of cultural relationships with traditional, customary or ancestral lands ( ''high confidence'' ) ( [[#Albert--2018|Albert et al., 2018]] ; [[#Narayan--2020|Narayan et al., 2020]] ; [[#Yates--2021|Yates et al., 2021]] ). Among Pacific islands, for example, the prospect of migration raises concerns about the loss of cultural identity and IK and practices, which can impact emotional well-being ( [[#Yates--2021|Yates et al., 2021]] ).

As cultural beliefs influence risk perception, there is ''medium confidence'' that some cultural understandings can foster a false sense of security among Indigenous Peoples, local communities and traditional peoples regarding climate-driven hydrological changes. For example, some members of the Rolwaling Sherpa community in Nepal believe that mountain deities protect them from GLOFs ( [[#Sherry--2017|Sherry and Curtis, 2017]] )( [[#4.2.2|Section 4.2.2]] ). Elsewhere, such as in the islands of Fiji and St. Vincent, cultural beliefs can diminish human agency because change is viewed as inevitable and beyond human intervention ( [[#Smith--2016|Smith and Rhiney, 2016]] ; [[#Currenti--2019|Currenti et al., 2019]] ). Yet such cultural beliefs are not necessarily maladaptive, as they potentially support other resilience factors, such as IKLK ( [[#4.8.5|Section 4.8.5]] ; [[#Ford--2020|Ford et al., 2020]] ), as well as cultural connections and social ties ( [[#Yates--2021|Yates et al., 2021]] ).

In sum, although some Indigenous Peoples, local communities and traditional peoples can adapt, and are adapting to climate-driven hydrological changes, and their impacts on and risks to culturally significant practices and beliefs ( ''medium confidence'' ), these strategies are constrained by structural barriers and adaptation limits ( ''high confidence'' ).

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'''Box 4.7 | Flood-Related Adaptation Responses'''
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Floods, due to their rapid onset and destructive force, require specific adaptation measures. Historically, to address flood damages and risk protection, retreat and accommodation were most common, emphasizing protecting and retreating ( [[#Wong--2014|Wong et al., 2014]] ; [[#Bott--2019|Bott and Braun, 2019]] ). Figure 4.22 identifies five major adaptation strategies from a meta-review of water-related adaptation responses that helps in protecting, retreating and accommodating ( [[#4.7.1|Section 4.7.1]] ).

Globally, structural measures for flood protection through hard infrastructure are the most common measures as they directly manage flood hazards by controlling flow through streams and prevent water overflow ( [[#Andrew--2017|Andrew and Sauquet, 2017]] ; [[#Duží--2017|Duží et al., 2017]] ). These measures include dikes, flood control gates, weirs, dams, storage and proper waste management ( [[#Barua--2017|Barua et al., 2017]] ; [[#Egbinola--2017|Egbinola et al., 2017]] ). Infrastructure measures require high maintenance, such as dredging and clearing channels and overpasses ( [[#Egbinola--2017|Egbinola et al., 2017]] ). A negative aspect of protective infrastructural measures is that, while they eliminate the hazard up to a certain magnitude ( [[#Di%20Baldassarre--2013|Di Baldassarre et al., 2013]] ), they also generate an illusion of no risk by diminishing frequent floods ( [[#Duží--2017|Duží et al., 2017]] ; [[#Logan--2018|Logan et al., 2018]] ). In addition, specific engineering solutions that might be introduced from other localities without proper contextual adjustments may lead to maladaptation ( [[#Mycoo--2014|Mycoo, 2014]] ; [[#Pritchard--2014|Pritchard and Thielemans, 2014]] ). NbS (Box 4.6) have shifted infrastructure measures from purely grey onto mixed engineering and environmental measures. Examples include SUDS, which aid in decreasing flow peaks and are affordable, aesthetically pleasing and socially acceptable while also reducing heat and hence the production of storms ( [[#Chan--2019|Chan et al., 2019]] ) ( [[#4.6.5|Section 4.6.5]] ).

Non-structural or soft measures for flood adaptation include human actions that generate capacities, information and, therefore, awareness of floods ( [[#Du--2020|Du et al., 2020]] ). Soft measures aim to integrate flood resilience within city management and planning ( [[#Wijaya--2015|Wijaya, 2015]] ; [[#Andrew--2017|Andrew and Sauquet, 2017]] ; [[#Abbas--2018|Abbas et al., 2018]] ). Social support between members of a community and economic mechanisms such as loans or remittances are soft measures that promote recovery or resilience to floods ( [[#Barua--2017|Barua et al., 2017]] ; [[#Musah-Surugu--2018|Musah-Surugu et al., 2018]] ; [[#Bott--2019|Bott and Braun, 2019]] ). Communities with heightened awareness and knowledge of floods are probably going to elect political leaders that will affect flood protection and policies that include adaptation ( [[#Abbas--2018|Abbas et al., 2018]] ). Soft measures can be an anchoring factor for policies that promote early warning systems, infrastructure, flood-resilient housing and environmental restoration ( [[#Andrew--2017|Andrew and Sauquet, 2017]] ; [[#Abbas--2018|Abbas et al., 2018]] ). However, soft measures, especially at large scale, may also lead to maladaptation, such as lack of synchronisation between international, national and local levels ( [[#Hedelin--2016|Hedelin, 2016]] ; [[#Lu--2016|Lu, 2016]] ; [[#Jamero--2017|Jamero et al., 2017]] ), and can further be hampered by bureaucracy ( [[#Pinto--2018|Pinto et al., 2018]] ).

Early warning systems (EWS) are defined as integrated systems of hazard monitoring, forecasting and prediction, disaster risk assessment, communication and preparedness activities systems to enable individuals, communities, governments and businesses to take timely action to reduce disaster risks in advance of hazardous events ( [[#UNISDR--2021|UNISDR, 2021]] ). By this definition, EWS are directly dependent on soft and hard infrastructure measures that increase capacity and reduce hazard ( [[#Abbas--2018|Abbas et al., 2018]] ). Aside from the capacity dependent on soft measures and the monitoring infrastructure, communication at all scales, from national weather services to local leaders, needs to be effective for prompt action ( [[#Devkota--2014|Devkota et al., 2014]] ). In many cases, EWS might be the only option to reduce flood casualties ( [[#Kontar--2015|Kontar et al., 2015]] ).

Accommodating floods has gained popularity as the effects of climate change become more apparent and as notable hydroclimatic events exceed the limitations of protective measures ( [[#Pritchard--2014|Pritchard and Thielemans, 2014]] ). NbS measures like wetland restoration can act as modern infrastructure protection with clear mitigation co-benefit and provide opportunities for accommodating floods. For example, initiatives such as ‘Room for the River’ consider flood safety combined with other values such as landscape, environment and cultural values ( [[#Zevenbergen--2015|Zevenbergen et al., 2015]] ). A popular EbA measure has been wetland restoration, which can control flood peaks, serve as storage ponds in addition to restoring the environment ( [[#Pinto--2018|Pinto et al., 2018]] ; [[#Saroar--2018|Saroar, 2018]] ). However, its effectiveness under different conditions is yet to be assessed ( [[#Wamsler--2016|Wamsler et al., 2016]] ). Flood resilient housing is another form of accommodating and living with floods. These comprise mostly of elevated homes or different flood protection measures considering vegetation around the house to make those flood resilient ( [[#Ling--2015|Ling et al., 2015]] ; [[#Abbas--2018|Abbas et al., 2018]] ; [[#Ferdous--2019|Ferdous et al., 2019]] ).

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Box 4.7

Despite different degrees of effectiveness, no flood adaptation measure is uniquely effective to eliminate flood risk. Adaptation to floods needs to be considered at a local level, considering the types of floods, community’s capacities and available livelihoods ( [[#Fenton--2017a|Fenton et al., 2017a]] ). Ideally, flood adaptation strategies need to include short-term actions linked to long-term goals, be flexible, consider multiple strategies and interlink investment agendas of stakeholders ( [[#Zevenbergen--2015|Zevenbergen et al., 2015]] ). Most importantly, flood adaptation and management options have been proven effective to reduce loss of human lives, but not entirely at sustaining livelihoods and reducing infrastructure damages ( [[#Rahman--2016|Rahman and Alam, 2016]] ; [[#Bower--2019|Bower et al., 2019]] ; [[#Ferdous--2019|Ferdous et al., 2019]] ).

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