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

=== 3.3.10 Ocean Chemistry ===

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Ocean chemistry includes pH, salinity, oxygen, CO <sub>2</sub> , and a range of other ions and gases, which are in turn affected by precipitation, evaporation, storms, river runoff, coastal erosion, up-welling, ice formation, and the activities of organisms and ecosystems (Stocker et al., 2013) <sup>[[#fn:r345|345]]</sup> . Ocean chemistry is changing alongside increasing global temperature, with impacts projected at 1.5°C and, more so, at 2°C of global warming (Doney et al., 2014) <sup>[[#fn:r346|346]]</sup> ( ''medium to high confidence'' ). Projected changes in the upper layers of the ocean include altered pH, oxygen content and sea level. Despite its many component processes, ocean chemistry has been relatively stable for long periods of time prior to the industrial period (Hönisch et al., 2012) <sup>[[#fn:r347|347]]</sup> . Ocean chemistry is changing under the influence of human activities and rising greenhouse gases ( ''virtually certain'' ; Rhein et al., 2013; Stocker et al., 2013) <sup>[[#fn:r348|348]]</sup> . About 30% of CO <sub>2</sub> emitted by human activities, for example, has been absorbed by the upper layers of the ocean, where it has combined with water to produce a dilute acid that dissociates and drives ocean acidification ( ''high confidence'' ) (Cao et al., 2007; Stocker et al., 2013) <sup>[[#fn:r349|349]]</sup> . Ocean pH has decreased by 0.1 pH units since the pre-industrial period, a shift that is unprecedented in the last 65 Ma ( ''high confidence'' ) (Ridgwell and Schmidt, 2010) <sup>[[#fn:r350|350]]</sup> or even 300 Ma of Earth’s history ( ''medium confidence'' ) (Hönisch et al., 2012) <sup>[[#fn:r351|351]]</sup> .

Ocean acidification is a result of increasing CO <sub>2</sub> in the atmosphere ( ''very high confidence'' ) and is most pronounced where temperatures are lowest (e.g., polar regions) or where CO <sub>2</sub> -rich water is brought to the ocean surface by upwelling (Feely et al., 2008) <sup>[[#fn:r352|352]]</sup> . Acidification can also be influenced by effluents from natural or disturbed coastal land use (Salisbury et al., 2008) <sup>[[#fn:r353|353]]</sup> , plankton blooms (Cai et al., 2011) <sup>[[#fn:r354|354]]</sup> , and the atmospheric deposition of acidic materials (Omstedt et al., 2015) <sup>[[#fn:r355|355]]</sup> . These sources may not be directly attributable to climate change, but they may amplify the impacts of ocean acidification (Bates and Peters, 2007; Duarte et al., 2013) <sup>[[#fn:r356|356]]</sup> . Ocean acidification also influences the ionic composition of seawater by changing the organic and inorganic speciation of trace metals (e.g., 20-fold increases in free ion concentrations of metals such as aluminium) – with changes expected to have impacts although they are currently poorly documented and understood ( ''low confidence'' ) (Stockdale et al., 2016) <sup>[[#fn:r357|357]]</sup> .

Oxygen varies regionally and with depth; it is highest in polar regions and lowest in the eastern basins of the Atlantic and Pacific Oceans and in the northern Indian Ocean (Doney et al., 2014; Karstensen et al., 2015; Schmidtko et al., 2017) <sup>[[#fn:r358|358]]</sup> . Increasing surface water temperatures have reduced oxygen in the ocean by 2% since 1960, with other variables such as ocean acidification, sea level rise, precipitation, wind and storm patterns playing roles (Schmidtko et al., 2017) <sup>[[#fn:r359|359]]</sup> . Changes to ocean mixing and metabolic rates, due to increased temperature and greater supply of organic carbon to deep areas, has increased the frequency of ‘dead zones’, areas where oxygen levels are so low that they no longer support oxygen dependent life (Diaz and Rosenberg, 2008) <sup>[[#fn:r360|360]]</sup> . The changes are complex and include both climate change and other variables (Altieri and Gedan, 2015) <sup>[[#fn:r361|361]]</sup> , and are increasing in tropical as well as temperate regions (Altieri et al., 2017) <sup>[[#fn:r362|362]]</sup> .

Ocean salinity is changing in directions that are consistent with surface temperatures and the global water cycle (i.e., precipitation versus evaporation). Some regions, such as northern oceans and the Arctic, have decreased in salinity, owing to melting glaciers and ice sheets, while others have increased in salinity, owing to higher sea surface temperatures and evaporation (Durack et al., 2012) <sup>[[#fn:r363|363]]</sup> . These changes in salinity (i.e., density) are also potentially contributing to large-scale changes in water movement (Section 3.3.8).

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