Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.4.2.4 Water quality ====

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Working Group II of AR5 concluded that most observed changes to water quality from climate change are from isolated studies, mostly of rivers or lakes in high-income countries, using a small number of variables (Jiménez Cisneros et al., 2014) <sup>[[#fn:r427|427]]</sup> . AR5 assessed that climate change is projected to reduce raw water quality, posing risks to drinking water quality with conventional treatment ( ''medium to high confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r428|428]]</sup> .

Since AR5, studies have detected climate change impacts on several indices of water quality in lakes, watersheds and regions (e.g., Patiño et al., 2014; Aguilera et al., 2015; Watts et al., 2015; Marszelewski and Pius, 2016; Capo et al., 2017) <sup>[[#fn:r429|429]]</sup> . The number of studies utilising RCP scenarios at the regional or watershed scale have gradually increased since AR5 (e.g., Boehlert et al., 2015; Teshager et al., 2016; Marcinkowski et al., 2017) <sup>[[#fn:r430|430]]</sup> . Few studies, have explored projected impacts on water quality under 1.5°C versus 2°C of warming, however, the differences are unclear ( ''low confidence'' ) (Bonte and Zwolsman, 2010 <sup>[[#fn:r431|431]]</sup> ; Hosseini et al., 2017) <sup>[[#fn:r432|432]]</sup> . The daily probability of exceeding the chloride standard for drinking water taken from Lake IJsselmeer (Andijk, the Netherlands) is projected to increase by a factor of about five at 2°C relative to the present-day warming level of 1°C since 1990 (Bonte and Zwolsman, 2010) <sup>[[#fn:r433|433]]</sup> . Mean monthly dissolved oxygen concentrations and nutrient concentrations in the upper Qu’Appelle River (Canada) in 2050–2055 are projected to decrease less at about 1.5°C of warming (RCP2.6) compared to concentrations at about 2°C (RCP4.5) (Hosseini et al., 2017) <sup>[[#fn:r434|434]]</sup> . In three river basins in Southeast Asia (Sekong, Sesan and Srepok), about 2°C of warming (corresponding to a 1.05°C increase in the 2030s relative to the baseline period 1981–2008, RCP8.5), impacts posed by land-use change on water quality are projected to be greater than at 1.5°C (corresponding to a 0.89°C increase in the 2030s relative to the baseline period 1981–2008, RCP4.5) (Trang et al., 2017) <sup>[[#fn:r435|435]]</sup> . Under the same warming scenarios, Trang et al. (2017) <sup>[[#fn:r436|436]]</sup> projected changes in the annual nitrogen (N) and phosphorus (P) yields in the 2030s, as well as with combinations of two land-use change scenarios: (i) conversion of forest to grassland, and (ii) conversion of forest to agricultural land. The projected changes in N (P) yield are +7.3% (+5.1%) under a 1.5°C scenario and –6.6% (–3.6%) under 2°C, whereas changes under the combination of land-use scenarios are (i) +5.2% (+12.6%) at 1.5°C and +8.8% (+11.7%) at 2°C, and (ii) +7.5% (+14.9%) at 1.5°C and +3.7% (+8.8%) at 2°C (Trang et al., 2017) <sup>[[#fn:r437|437]]</sup> .

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