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

=== 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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