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

== 3.4.4.6 Deoxygenation ==

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Oxygen levels in the ocean are maintained by a series of processes including ocean mixing, photosynthesis, respiration and solubility (Boyd et al., 2014, 2015; Pörtner et al., 2014; Breitburg et al., 2018) <sup>[[#fn:r610|610]]</sup> . Concentrations of oxygen in the ocean are declining ( ''high confidence'' ) owing to three main factors related to climate change: (i) heat-related stratification of the water column (less ventilation and mixing), (ii) reduced oxygen solubility as ocean temperature increases, and (iii) impacts of warming on biological processes that produce or consume oxygen such as photosynthesis and respiration ( ''high confidence'' ) (Bopp et al., 2013; Pörtner et al., 2014; Altieri and Gedan, 2015; Deutsch et al., 2015; Schmidtko et al., 2017; Shepherd et al., 2017; Breitburg et al., 2018) <sup>[[#fn:r611|611]]</sup> . Further, a range of processes (Section 3.4.11) are acting synergistically, including factors not related to climate change, such as runoff and coastal eutrophication (e.g., from coastal farming and intensive aquaculture). These changes can lead to increased phytoplankton productivity as a result of the increased concentration of dissolved nutrients. Increased supply of organic carbon molecules from coastal run-off can also increase the metabolic activity of coastal microbial communities (Altieri and Gedan, 2015; Bakun et al., 2015; Boyd, 2015) <sup>[[#fn:r612|612]]</sup> . Deep sea areas are ''likely'' to experience some of the greatest challenges, as abyssal seafloor habitats in areas of deep-water formation are projected to experience decreased water column oxygen concentrations by as much as 0.03 mL L <sup>–1</sup> by 2100 (Levin and Le Bris, 2015; Sweetman et al., 2017) <sup>[[#fn:r613|613]]</sup> .

The number of ‘dead zones’ (areas where oxygenated waters have been replaced by hypoxic conditions) has been growing strongly since the 1990s (Diaz and Rosenberg, 2008; Altieri and Gedan, 2015; Schmidtko et al., 2017) <sup>[[#fn:r614|614]]</sup> . While attribution can be difficult because of the complexity of the processes involved, both related and unrelated to climate change, some impacts associated to deoxygenation ( ''low-medium confidence'' ) include the expansion of oxygen minimum zones (OMZ) (Turner et al., 2008; Carstensen et al., 2014; Acharya and Panigrahi, 2016; Lachkar et al., 2018) <sup>[[#fn:r615|615]]</sup> , physiological impacts (Pörtner et al., 2014) <sup>[[#fn:r616|616]]</sup> , and mortality and/or displacement of oxygen dependent organisms such as fish (Hamukuaya et al., 1998; Thronson and Quigg, 2008; Jacinto, 2011) <sup>[[#fn:r617|617]]</sup> and invertebrates (Hobbs and Mcdonald, 2010; Bednaršek et al., 2016; Seibel, 2016; Altieri et al., 2017) <sup>[[#fn:r618|618]]</sup> . In addition, deoxygenation interacts with ocean acidification to present substantial separate and combined challenges for fisheries and aquaculture ( ''medium confidence'' ) (Hamukuaya et al., 1998; Bakun et al., 2015; Rodrigues et al., 2015; Feely et al., 2016; S. Li et al., 2016; Asiedu et al., 2017a; Clements and Chopin, 2017; Clements et al., 2017; Breitburg et al., 2018) <sup>[[#fn:r619|619]]</sup> . Deoxygenation is expected to have greater impacts as ocean warming and acidification increase ( ''high confidence'' ), with impacts being larger and more numerous than today (e.g., greater challenges for aquaculture and fisheries from hypoxia), and as the number of hypoxic areas continues to increase. Risks from deoxygenation are ''virtually certain'' to increase as warming continues, although our understanding of risks at 1.5°C versus 2°C is incomplete ( ''medium confidence'' ). Reducing coastal pollution, and consequently the penetration of organic carbon into deep benthic habitats, is expected to reduce the loss of oxygen in coastal waters and hypoxic areas in general ( ''high confidence'' ) (Breitburg et al., 2018) <sup>[[#fn:r620|620]]</sup> .

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