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

==== 3.4.4.5 Ocean acidification ====

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Ocean chemistry encompasses a wide range of phenomena and chemical species, many of which are integral to the biology and ecology of the ocean (Section 3.3.10; Gattuso et al., 2014, 2015; Hoegh-Guldberg et al., 2014; Pörtner et al., 2014) <sup>[[#fn:r594|594]]</sup> . While changes to ocean chemistry are ''likely'' to be of central importance, the literature on how climate change might influence ocean chemistry over the short and long term is limited ( ''medium confidence'' ). By contrast, numerous risks from the specific changes associated with ocean acidification have been identified (Dove et al., 2013; Kroeker et al., 2013; Pörtner et al., 2014; Gattuso et al., 2015; Albright et al., 2016) <sup>[[#fn:r595|595]]</sup> , with the consensus that resulting changes to the carbonate chemistry of seawater are having, and are ''likely'' to continue to have, fundamental and substantial impacts on a wide variety of organisms ( ''high confidence'' ). Organisms with shells and skeletons made out of calcium carbonate are particularly at risk, as are the early life history stages of a large number of organisms and processes such as de-calcification, although there are some taxa that have not shown high-sensitivity to changes in CO <sub>2</sub> , pH and carbonate concentrations (Dove et al., 2013; Fang et al., 2013; Kroeker et al., 2013; Pörtner et al., 2014; Gattuso et al., 2015) <sup>[[#fn:r596|596]]</sup> . Risks of these impacts also vary with latitude and depth, with the greatest changes occurring at high latitudes as well as deeper regions. The aragonite saturation horizon (i.e., where concentrations of calcium and carbonate fall below the saturation point for aragonite, a key crystalline form of calcium carbonate) is decreasing with depth as anthropogenic CO <sub>2</sub> penetrates deeper into the ocean over time. Under many models and scenarios, the aragonite saturation is projected to reach the surface by 2030 onwards, with a growing list of impacts and consequences for ocean organisms, ecosystems and people (Orr et al., 2005; Hauri et al., 2016) <sup>[[#fn:r597|597]]</sup> .

Further, it is difficult to reliably separate the impacts of ocean warming and acidification. As ocean waters have increased in sea surface temperature (SST) by approximately 0.9°C they have also decreased by 0.2 pH units since 1870–1899 (‘pre-industrial’; Table 1 in Gattuso et al., 2015; Bopp et al., 2013) <sup>[[#fn:r598|598]]</sup> . As CO <sub>2</sub> concentrations continue to increase along with other GHGs, pH will decrease while sea temperature will increase, reaching 1.7°C and a decrease of 0.2 pH units (by 2100 under RCP4.5) relative to the pre-industrial period. These changes are ''likely'' to continue given the negative correlation of temperature and pH. Experimental manipulation of CO <sub>2</sub> , temperature and consequently acidification indicate that these impacts will continue to increase in size and scale as CO <sub>2</sub> and SST continue to increase in tandem (Dove et al., 2013; Fang et al., 2013; Kroeker et al., 2013) <sup>[[#fn:r599|599]]</sup> .

While many risks have been defined through laboratory and mesocosm experiments, there is a growing list of impacts from the field ( ''medium confidence'' ) that include community-scale impacts on bacterial assemblages and processes (Endres et al., 2014) <sup>[[#fn:r600|600]]</sup> , coccolithophores (K.J.S. Meier et al., 2014) <sup>[[#fn:r601|601]]</sup> , pteropods and polar foodwebs (Bednaršek et al., 2012, 2014) <sup>[[#fn:r602|602]]</sup> , phytoplankton (Moy et al., 2009; Riebesell et al., 2013; Richier et al., 2014) <sup>[[#fn:r603|603]]</sup> , benthic ecosystems (Hall-Spencer et al., 2008; Linares et al., 2015) <sup>[[#fn:r604|604]]</sup> , seagrass (Garrard et al., 2014) <sup>[[#fn:r605|605]]</sup> , and macroalgae (Webster et al., 2013; Ordonez et al., 2014) <sup>[[#fn:r606|606]]</sup> , as well as excavating sponges, endolithic microalgae and reef-building corals (Dove et al., 2013; Reyes-Nivia et al., 2013; Fang et al., 2014) <sup>[[#fn:r607|607]]</sup> , and coral reefs (Box 3.4; Fabricius et al., 2011; Allen et al., 2017) <sup>[[#fn:r608|608]]</sup> . Some ecosystems, such as those from bathyal areas (i.e., 200–3000 m below the surface), are ''likely'' to undergo very large reductions in pH by the year 2100 (0.29 to 0.37 pH units), yet evidence of how deep-water ecosystems will respond is currently limited despite the potential planetary importance of these areas ( ''low to medium confidence'' ) (Hughes and Narayanaswamy, 2013; Sweetman et al., 2017) <sup>[[#fn:r609|609]]</sup> .

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