Editing IPCC:AR6/WGII/Chapter-3 (section)

==== 3.5.5.3 Provision of Freshwater, Maintenance of Water Quality and Regulation of Pathogens ====

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The salinities of many estuaries, deltas, coastal freshwater aquifers and soils around the world are increasing, and this decrease in water quality is endangering human health and agricultural yields ( ''very high confidence'' ) ( [[#3.4.2.4|Section 3.4.2.4]] ; Table 3.26; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Bouderbala--2019|Bouderbala, 2019]] ; [[#Rahman--2019|Rahman et al., 2019]] ; [[#Naser--2020|Naser et al., 2020]] ; [[#Rakib--2020|Rakib et al., 2020]] ; [[#Mastrocicco--2021|Mastrocicco and Colombani, 2021]] ). Coastal salinisation is attributed to regionally varying combinations of climate-induced drivers, like SLR and storm-related flooding by seawater, and non-climate drivers, like water withdrawal and land-use changes ( ''very high confidence'' ) ( [[#Islam--2019|Islam et al., 2019]] ; [[#Rahman--2019|Rahman et al., 2019]] ; [[#Paldor--2021|Paldor and Michael, 2021]] ). Monitoring-related adaptations ( [[#3.6.2.2|Section 3.6.2.2.2]] ), including advances in modelling and monitoring, are providing decision-relevant, regional-scale information ( [[#Colombani--2016|Colombani et al., 2016]] ; [[#Mukhopadhyay--2019|Mukhopadhyay et al., 2019]] ; [[#Slama--2020|Slama et al., 2020]] ; [[#Corwin--2021|Corwin, 2021]] ). For example, new projections indicate which drinking-water intake stations on China’s Pearl River Estuary will be unable to meet demands by 2100 due to SLR and drought ( [[#Wang--2021|Wang and Hong, 2021]] ), while others show that SLR effects on seawater intrusion into the coastal aquifer in Kerala, India, under both RCP4.5 and RCP8.5 scenarios are negligible ( [[#Sithara--2020|Sithara et al., 2020]] ). Salinisation-associated changes may disproportionately burden women responsible for securing drinking water and fuel, such as in the Indian Sundarbans ( [[#Mukhopadhyay--2019|Mukhopadhyay et al., 2019]] ). Salinisation will continue to endanger coastal water and soil quality in the future ( ''high confidence'' ) ( [[#Islam--2019|Islam et al., 2019]] ; [[#Paldor--2021|Paldor and Michael, 2021]] ), but the evidence assessed above shows that subsequent impacts to human health and agriculture will depend heavily on regional variations in environment and human behaviour ( ''medium confidence'' ).

Together, climate-induced and non-climate drivers can mobilise toxins and contaminants in ways that harm human and marine species health ( ''very high confidence'' ) (see Box 3.2), and climate change is altering these relationships ( ''high confidence'' ) (Table 3.26; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). Under warming or ocean acidification, marine molluscs exposed to pharmaceuticals via wastewater experience more detrimental biological consequences or greater bioaccumulation ( ''limited evidence, high agreement'' ) ( [[#Costa--2020a|Costa et al., 2020a]] ; [[#Costa--2020b|Costa et al., 2020b]] ; [[#Dionísio--2020|Dionísio et al., 2020]] ; [[#Freitas--2020|Freitas et al., 2020]] ; [[#Kibria--2021|Kibria et al., 2021]] ). Physical circulation, temperature and biogeochemical characteristics ( [[#Bowman--2020|Bowman et al., 2020]] ; [[#Liu--2020a|Liu et al., 2020a]] ; [[#Liu--2020b|Liu et al., 2020b]] ; [[#Sun--2020|Sun et al., 2020]] ; [[#Zhang--2020b|Zhang et al., 2020b]] ) control the ubiquitous oceanic distribution of methylmercury, and ocean acidification- and warming-driven changes in planktonic speciation and interactions can promote additional food-web bioaccumulation of methylmercury ( [[#Tada--2020|Tada and Marumoto, 2020]] ; [[#Wu--2020b|Wu et al., 2020b]] ; [[#Zhang--2020b|Zhang et al., 2020b]] ; [[#Zhang--2021a|Zhang et al., 2021a]] ). Interactions among drivers also matter: temperature plus overfishing increased tissue methylmercury concentrations in Atlantic bluefin tuna from the 1970s to the 2000s more than the decreases in the late 1990s and 2000s from lower environmental mercury levels ( [[#Schartup--2019|Schartup et al., 2019]] ). This appears true for persistent organic pollutants as well, but their bioaccumulation is related more to temperature effects on animal behaviour than on pollutant dynamics ( [[#Houde--2019|Houde et al., 2019]] ; [[#Wagner--2019|Wagner et al., 2019]] ; [[#Kalia--2021|Kalia et al., 2021]] ). By 2100 under RCP8.5, productivity changes and community structure shifts are expected to increase methylmercury concentrations in polar oceans and high-latitude phytoplankton and decrease it in low latitudes ( [[#Zhang--2021a|Zhang et al., 2021a]] ). The estimated average global cost of mercury-related health effects by 2050, mainly from seafood consumption during 2010–2050, will be 19 trillion USD (2020), assuming a 3% discount rate, if methylmercury emissions are not reduced ( [[#Zhang--2021b|Zhang et al., 2021b]] ).

Since previous assessments, evidence has increased that climate impacts, such as warming, extreme weather and SLR, are increasing the geographic spread and risk of marine-borne human pathogen outbreaks, including ''Vibrio'' spp. ( ''very high confidence'' ) (Table 3.26; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Logar-Henderson--2019|Logar-Henderson et al., 2019]] ; [[#Froelich--2020|Froelich and Daines, 2020]] ; [[#Montánchez--2020|Montánchez and Kaberdin, 2020]] ; [[#Semenza--2020|Semenza, 2020]] ; [[#Ferchichi--2021|Ferchichi et al., 2021]] ). Climate change affects at least 30 human pathogens with aquatic-system infection routes (e.g., ingestion of contaminated water or seafood, or contact with wounds; Table 3.SM.2; Cross-Chapter Box ILLNESS in Chapter 2; [[#Nichols--2018|Nichols et al., 2018]] ). Conditions favourable for ''Vibrio cholerae'' are increasing globally, which raises the risk to humans (Cross-Chapter Box ILLNESS in Chapter 2). Increased storm-related flooding and SLR further increase human encounters with ''Vibrio'' spp. ( [[#Froelich--2020|Froelich and Daines, 2020]] ). Aquatic diseases, particularly ''Vibrio'' spp., have caused large economic losses in aquaculture by decreasing the quality or survival of cultured species ( [[#Lafferty--2015|Lafferty et al., 2015]] ; [[#Novriadi--2016|Novriadi, 2016]] ). Temperature-based model projections show that all Canadian shellfish beds will experience conditions that promote high risk of ''Vibrio'' spp. growth by 2100 for both RCP4.5 and RCP8.5 scenarios ( [[#Ferchichi--2021|Ferchichi et al., 2021]] ). Climate-induced drivers may increase ''Vibrio'' spp. loads in seafood species: laboratory-simulated heatwaves increase ''Vibrio'' spp. abundance in Pacific oyster ( ''Crassostrea gigas'' ) ( [[#Green--2019|Green et al., 2019]] ) and simulated ocean acidification increases hard clam ( ''Mercenaria mercenaria'' ) susceptibility to ''Vibrio'' spp. infection ( [[#Schwaner--2020|Schwaner et al., 2020]] ). Projected increases in temperature, extreme and variable rainfall conditions, coastal flooding and SLR ( [[#3.2|Section 3.2]] ; Cross-Chapter Box SLR in Chapter 3) strongly increase the risk of frequent and severe aquatic human pathogen outbreaks in ocean and coastal areas that will continue to harm human health and cause economic losses ( ''high confidence'' ) (Cross-Chapter Box ILLNESS in Chapter 2; [[#Froelich--2020|Froelich and Daines, 2020]] ; [[#Semenza--2020|Semenza, 2020]] ; [[#Ferchichi--2021|Ferchichi et al., 2021]] ). [[#3.6.3.1.5|Section 3.6.3.1.5]] assesses human adaptations to increasing risk of marine-borne pathogens.

Climate-driven changes in temperature, salinity (from ice melt and precipitation changes), deoxygenation and ocean acidification can alter dynamics of infectious diseases that target ocean and coastal species by increasing hosts’ susceptibility or pathogens’ abundance or virulence ( ''high confidence'' ) ( [[#Burge--2020|Burge and Hershberger, 2020]] ; [[#Byers--2021|Byers, 2021]] ). Coral and urchin diseases have increased over time driven by warming-related declines in organism recovery and survival or immunity ( ''medium confidence'' ) ( [[#Cohen--2018|Cohen et al., 2018]] ; [[#Tracy--2019|Tracy et al., 2019]] ). Seagrass and sea star wasting disease outbreaks have occurred under combinations of ocean warming or MHWs and non-climate drivers (e.g., eutrophication, bottom trawling), but attribution of these outbreaks to specific drivers is still not resolved ( [[#Harvell--2019|Harvell et al., 2019]] ; [[#Jakobsson-Thor--2020|Jakobsson-Thor et al., 2020]] ; [[#Krause-Jensen--2021|Krause-Jensen et al., 2021]] ). Disease outbreaks threaten marine biodiversity, species that create habitat or dampen wave action, and keystone species ( [[#Harvell--2020|Harvell and Lamb, 2020]] ). Attributing observed changes in marine disease patterns to climate remains extremely difficult owing to interacting climate and non-climate drivers ( [[#Burge--2020|Burge and Hershberger, 2020]] ) and lack of baseline data ( [[#Tracy--2019|Tracy et al., 2019]] ). Projected increases in the frequency, duration and intensity of warming events would reduce survival and recovery of some species from hot events, reduce immunity of other species to pathogens, extend poleward ranges of some pathogens and increase infection risk when host species congregate in scarce habitat ( [[#Cohen--2018|Cohen et al., 2018]] ). Pathogens that target ocean and coastal organisms may themselves be sensitive to future climate conditions or subsequent ecosystem changes, which challenges development of projections ( [[#Cohen--2018|Cohen et al., 2018]] ; [[#Burge--2020|Burge and Hershberger, 2020]] ).

New examples have illustrated how toxic HABs interfere with regulating, provisioning ( [[#3.5.3|Section 3.5.3]] ) and cultural ecosystem services ( [[#3.5.6|Section 3.5.6]] ) in interconnected ways ( ''limited evidence, high agreement'' ). A massive toxic ''Pseudo-nitzschia'' spp. bloom in 2013–2016 along the USA West Coast triggered Dungeness crab, rock crab and razor clam fishery closures to protect human consumers (Sections 3.6.2, 3.6.3.1.5; [[#McCabe--2016|McCabe et al., 2016]] ), and this disproportionately harmed fishers, especially small-vessel owners, and fishing-support service industries, primarily through lost revenue ( [[#Ritzman--2018|Ritzman et al., 2018]] ; [[#Moore--2019|Moore et al., 2019]] ; [[#Trainer--2019|Trainer et al., 2019]] ; [[#Jardine--2020|Jardine et al., 2020]] ; [[#Moore--2020a|Moore et al., 2020a]] ). Toxic ''Alexandrium'' spp. blooms promoted by climate-driven coastal extremes (e.g., MHWs, stratification, runoff) in Tasmania, Australia, in 2012 and Chile in 2016 caused fish kills, shellfish product recalls, substantial economic losses, and human sickness and death ( [[#Trainer--2019|Trainer et al., 2019]] ). The Chile event caused an estimated loss of 800 million USD in the farmed salmon industry ( [[#Díaz--2019|Díaz et al., 2019]] ) and resulted in a series of large, long-lasting regional protests calling for national aid ( [[#Delgado--2019|Delgado et al., 2019]] ). New evidence, however, suggests that the perceived global increase in harmful algal blooms results from better monitoring and more detrimental bloom impacts, rather than a climate-linked mechanism ( [[#Hallegraeff--2021|Hallegraeff et al., 2021]] ).

Natural and engineered systems have long been used effectively to manage precipitation and wastewater safely (see Box 4.5), and maintaining and enhancing them is a key nature-based adaptation strategy for coastal communities ( [[#3.6.2.3|Section 3.6.2.3]] ; Cross-Chapter Paper 2). Estimated values of water purification and stormwater management provided by coastal ecosystems are in the hundreds to thousands of USD per hectare [e.g., 272 Euro per 0.01 km 2 yr –1 from the Mediterranean’s sandy coastline ( [[#Hérivaux--2018|Hérivaux et al., 2018]] ); 1100–2800 USD per 0.01 km 2 yr –1 from the state of Maryland, USA ( [[#Campbell--2020b|Campbell et al., 2020b]] ); 600 USD per 0.01 km 2 yr –1 in Zhuzhou City, China ( [[#Zhan--2020|Zhan et al., 2020]] )]. Both wild and cultured organisms also provide filtration services. Seagrasses’ ability to purify water is well recognised by coastal residents and ocean resource users in tropical and temperate locations ( [[#Ambo-Rappe--2019|Ambo-Rappe et al., 2019]] ; [[#Quevedo--2020|Quevedo et al., 2020]] ; [[#Heckwolf--2021|Heckwolf et al., 2021]] ; [[#McKenzie--2021a|McKenzie et al., 2021a]] ). Globally, aquacultured shellfish remove an estimated 49,000 tonnes of nitrogen and 6000 tonnes of phosphorus from coastal waters, worth a potential 1.20 billion USD, and they may help improve existing engineered wastewater treatment systems ( [[#van%20der%20Schatte%20Olivier--2020|van der Schatte Olivier et al., 2020]] ). Climate change, especially episodic extreme rains and RSLR ( [[#Romero-Lankao--2014|Romero-Lankao et al., 2014]] ), is challenging management and design of wastewater and stormwater systems ( ''high confidence'' ) ( [[#Flood--2011|Flood and Cahoon, 2011]] ; [[#Trtanj--2016|Trtanj et al., 2016]] ; [[#Hummel--2018|Hummel et al., 2018]] ; [[#Kirshen--2018|Kirshen et al., 2018]] ; [[#Nazarnia--2020|Nazarnia et al., 2020]] ; [[#Reznik--2020|Reznik et al., 2020]] ; [[#McKenzie--2021b|McKenzie et al., 2021b]] ) and the integrity of coastal landfills ( [[#Beaven--2020|Beaven et al., 2020]] ). Without substantial adaptation that addresses projected wastewater management challenges and community needs ( [[IPCC:Wg2:Chapter:Chapter-4#4.2.6|Section 4.2.6.1]] ; [[#Kirshen--2018|Kirshen et al., 2018]] ; [[#Kirchhoff--2019|Kirchhoff and Watson, 2019]] ; [[#Kool--2020|Kool et al., 2020]] ; [[#Nazarnia--2020|Nazarnia et al., 2020]] ; [[#Hughes--2021|Hughes et al., 2021]] ), coastal water quality in many areas will decrease because of more frequent or severe releases of untreated wastes ( ''high confidence'' ) ( [[#Flood--2011|Flood and Cahoon, 2011]] ; [[#Hummel--2018|Hummel et al., 2018]] ; [[#Hughes--2021|Hughes et al., 2021]] ; [[#McKenzie--2021b|McKenzie et al., 2021b]] ), and this will have harmful consequences for human and coastal ecosystem health ( ''high confidence'' ) ( [[IPCC:Wg2:Chapter:Chapter-4#4.2.6|Section 4.2.6.1]] ; Cross-Chapter Box ILLNESS in Chapter 2; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

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