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

=== 4.2.7 Observed Changes in Water Quality ===

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AR5 ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ) concluded with ''medium evidence'' and ''high agreement'' that climate change affected water quality, posing additional risks to drinking water quality and human health ( [[#Field--2014b|Field et al., 2014b]] ), particularly due to increased eutrophication at higher temperatures or release of contaminants due to extreme floods ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). In addition, SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ; [[#Meredith--2019|Meredith et al., 2019]] ) assessed that glacier decline and permafrost degradation impacts water quality through increases in legacy contaminants ( ''medium evidence, high agreement'' ).

Warming temperatures and extreme weather events can potentially impact water quality ( [[#Khan--2015|Khan et al., 2015]] ). Water quality can be compromised through algal blooms that affect the taste and odour of recreational and drinking water and can harbour toxins and pathogens ( [[#Khan--2015|Khan et al., 2015]] ). Warming directly affects thermal water regimes, promoting harmful algal blooms ( [[#Li--2018|Li et al., 2018]] ; [[#Noori--2018|Noori et al., 2018]] ) ( [[#4.3.5|Section 4.3.5]] ). Additionally, permafrost degradation leads to an increased flux of contaminants ( [[#MacMillan--2015|MacMillan et al., 2015]] ; [[#Roberts--2017|Roberts et al., 2017]] ; [[#Mu--2019|Mu et al., 2019]] ). The increased meltwater from glaciers ( [[#Zhang--2019|Zhang et al., 2019]] ) releases deposited contaminants and reduces water quality downstream ( [[#Zhang--2017|Zhang et al., 2017]] ; [[#Hock--2019b|Hock et al., 2019b]] ).

Floods intensify the mixing of floodwater with wastewater and the redistribution of pollutants ( [[#Andrade--2018|Andrade et al., 2018]] ). In addition, contaminated floodwaters pose an immediate health risk through waterborne diseases ( [[#Huang--2016b|Huang et al., 2016b]] ; [[#Paterson--2018|Paterson et al., 2018]] ; [[#Setty--2018|Setty et al., 2018]] ). Wildfires, along with heavy rainfalls and floods, can also affect turbidity, which increases drinking water treatment challenges and has been linked to increases in gastrointestinal illness ( [[#de%20Roos--2017|de Roos et al., 2017]] ). Droughts reduce river dilution capacities and groundwater levels (Wen et al., 2017) increasing the risk of groundwater contamination ( [[#Kløve--2014|Kløve et al., 2014]] ). More generally, contaminated water diminishes its aesthetic value, compromising recreational activities, reducing tourism and property values and creating challenges for management and drinking water treatment ( [[#Eves--2014|Eves and Wilkinson, 2014]] ; [[#Khan--2015|Khan et al., 2015]] ; [[#Walters--2015|Walters et al., 2015]] ).

Between 2000 and 2010, ~10% of the global population faced adverse water quality issues ( [[#van%20Vliet--2021|van Vliet et al., 2021]] ). Adverse drinking water quality has been associated with extreme weather events in countries located in Asia, Africa and South and North America ( [[#Jagai--2015|Jagai et al., 2015]] ; [[#Levy--2016|Levy et al., 2016]] ; [[#Huynh--2018|Huynh and Stringer, 2018]] ; [[#Leal%20Filho--2018|Leal Filho et al., 2018]] ; [[#Abedin--2019|Abedin et al., 2019]] ) ( ''medium evidence, high agreement'' ). Dilution factors in 635 of 1049 US streams fell extremely low during drought conditions. Additionally, the safety threshold for endocrine-disrupting compound concentration exceeded in roughly a third of streams studied ( [[#Rice--2017|Rice and Westerhoff, 2017]] ). Natural acid rock drainage, which can potentially release toxic substances, has experienced intensification in an alpine catchment of the Central Pyrenees due to climate change and severe droughts in the last decade. River length affected by natural acid drainage increased from 5 km in 1945 to 35 km in 2018 ( [[#Zarroca--2021|Zarroca et al., 2021]] ). Threefold increases in contaminants and fivefold increases in nutrients have been observed in water sources after wildfires ( [[#Khan--2015|Khan et al., 2015]] ). Due to permafrost thawing, the concentration of major ions, especially SO 4 2− in two high Arctic lakes, has rapidly increased up to 500% and 340% during 2006–2016 and 2008–2016, respectively ( [[#Roberts--2017|Roberts et al., 2017]] ). The exports of dissolved organic carbon (DOC), particulate organic carbon and mercury in six Arctic rivers were reported to increase with significant deepening of active layers caused by climate warming during 1999–2015 ( [[#Mu--2019|Mu et al., 2019]] ). Sustained warming in Lake Tanganyika in Zambia during the last ∼ 150 years reduced lake mixing, which has depressed algal production, shrunk the oxygenated benthic habitat by 38% and further reduced fish and mollusc yield ( [[#Cohen--2016|Cohen et al., 2016]] ). From 1994 to 2010, coastal benthos at King George Island in Antarctica have observed a remarkable shift primarily linked to ongoing climate warming and the increased sediment runoff triggered by glacier retreats ( [[#Sahade--2015|Sahade et al., 2015]] ). The recovery time of macroinvertebrates from floods was found longer in cases of pre-existing pollution problems ( [[#Smith--2019a|Smith et al., 2019a]] ).

In summary, although climate-induced water quality degradation due to increases in water and surface temperatures or melting of the cryosphere has been observed ( ''medium confidence'' ), evidence of global-scale changes in water quality is ''limited'' because many studies are isolated and have limited regional coverage.

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