Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.3.6 Ocean Deoxygenation ====

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SROCC concluded that there has ''very likely'' been a net loss of oxygen over all ocean depths since the 1960s linked to global ocean deoxygenation at a range of 0.3–2.0%, and that the oxygen levels in the global upper 1000 m of the ocean had decreased by 0.5–3.3% during 1970–2010 ( ''medium confidence'' ), alongside an expansion of oxygen minimum zones (OMZ) by 3–8%. For the surface ocean (0–100 m) and the thermocline at 100–600 m, the ''very likely'' range of oxygen decline was assessed to be 0.2–2.1% and 0.7–3.5%, respectively. Multidecadal rates of deoxygenation showed variability throughout the water column and across ocean basins ( ''high confidence'' ).

Since AR5 evidence for changes in oxygen content based on new proxy reconstructions has increased ( [[#Hoogakker--2015|Hoogakker et al., 2015]] , 2018; [[#Gottschalk--2016|Gottschalk et al., 2016]] , 2020; [[#Anderson--2019|Anderson et al., 2019]] ). Paleo records point to past periods of reduced oceanic oxygen levels during the late Permian (about 250 Ma), the Jurassic and the Cretaceous (180–100 Ma), alongside associated global scale disturbances of the global carbon cycle. Emerging studies for the Cenozoic Era suggest more stable ocean oxygenation conditions throughout the interval on million-year time scales (X. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ). Sedimentary proxy data indicate, however, that the seemingly stable Cenozoic was punctuated by transient, widespread deoxygenation during the PETM ( [[#Dickson--2012|Dickson et al., 2012]] ; [[#Winguth--2012|Winguth et al., 2012]] ; [[#Remmelzwaal--2019|Remmelzwaal et al., 2019]] ), with parts of the ocean reaching anoxic levels (Section 5.3.1.1; [[#Yao--2018|Yao et al., 2018]] ). Since the LGM, there was an overall emergence and expansion of low-oxygen waters into the ocean intermediate depths as a result of rapid warming and a reorganization of the global overturning circulation ( [[#Galbraith--2015|Galbraith and Jaccard, 2015]] ). The maximum expansion of oxygen-depleted waters during the LDT occurred coincidently with rapid warming in the NH at 14.7–12.9 ka ( [[#Jaccard--2012|Jaccard and Galbraith, 2012]] ; [[#Moffitt--2015|Moffitt et al., 2015]] ; [[#Hoogakker--2018|Hoogakker et al., 2018]] ). Deep (&gt;1500 m) ocean oxygen levels increased by 100–150 µmol kg <sup>–1</sup> since the LGM, reaching modern oxygen levels at about 10 ka (Section 5.3.1.2; [[#Hoogakker--2015|Hoogakker et al., 2015]] ; [[#Gottschalk--2016|Gottschalk et al., 2016]] ; [[#Anderson--2019|Anderson et al., 2019]] ).

New findings for ocean oxygen content since SROCC are limited to regional scale assessments. The magnitude of change is difficult to compare across regions to arrive at a global assessment due to differences in depth range, time period, baseline climatology, methodology, and particularly the use of different units. To facilitate comparisons, data are presented as change per decade, and conversions of the SROCC global mean percentage of oxygen decline estimates are provided as a loss of 3.2 μmol kg <sup>−1</sup> in the upper 1000 m of the global ocean (1.93%), 2.0 μmol kg <sup>−1</sup> (0.8 μmol kg <sup>−1</sup> per decade) in the upper 1000 m of the global ocean (1.93%), and 2.0 μmol kg <sup>−1</sup> (0.5 μmol kg <sup>−1</sup> per decade) in the entire water column (1.15%) between 1970 and 2010 ( [[#Bindoff--2019|Bindoff et al., 2019]] ). Oxygen change also shows decadal variability ( [[#Ito--2016|Ito et al., 2016]] ; [[#Stramma--2020|Stramma et al., 2020]] ) that can influence estimates of trends.

Subsurface (100–400 m) oxygen in the California Current system is estimated to have declined by 24 ± 2 μmol kg <sup>−1</sup> (1.0 μmol kg <sup>−1</sup> per decade) between 1993 and 2018, a rate similar to the global upper 1000 m average ( [[#Bindoff--2019|Bindoff et al., 2019]] ). In some locations, however, the magnitude of oxygen loss substantially exceeds global averages ( [[#Queste--2018|Queste et al., 2018]] ; [[#Bronselaer--2020|Bronselaer et al., 2020]] ; [[#Cummins--2020|Cummins and Ross, 2020]] ; [[#Stramma--2020|Stramma et al., 2020]] ). For example, a decline in oxygen content of 11.7 ± 3.5% in the upper 4000 m, including a decline of 20.4 ± 7.2% in the upper 1550 m, is reported in the North Pacific over the period 1958–2018 ( [[#Cummins--2020|Cummins and Ross, 2020]] ) (equivalent to 2.3 μmol kg <sup>−1</sup> per decade in the upper 1550 m and 2.0 μmol kg <sup>−1</sup> decade <sup>− 1</sup> throughout the 4000 m water column). In some regions of the Southern Ocean south of 65°S oxygen in the upper 2000 m has declined by 60 μmol kg <sup>−1</sup> (~52 μmol kg <sup>−1</sup> per decade) based on comparisons of 2014–2019 and 1985–2005 observations ( [[#Bronselaer--2020|Bronselaer et al., 2020]] ). Within some OMZs regions of the Indian ocean, oxygen has declined from 6–12 to &lt;2 μmol kg <sup>−1</sup> between the 1960s and 2015–2016 ( [[#Bristow--2017|Bristow et al., 2017]] ; [[#Al-Said--2018|Al-Said et al., 2018]] ; [[#Queste--2018|Queste et al., 2018]] ).

Findings since SROCC provide further support that the volume of severely oxygen-depleted water has expanded in some locations of the global ocean (Section 5.3.3.2). For example, vertical expansion of low oxygen zones is reported in the North Pacific at a rate of 3.1 ± 0.5 m yr <sup>–1</sup> ( [[#Ross--2020|Ross et al., 2020]] ), and suboxic waters have increased by 20% at a rate of about 19 m per decade from 1982–2010 in the Arabian Sea ( [[#Al-Said--2018|Al-Said et al., 2018]] ; [[#Lachkar--2019|Lachkar et al., 2019]] ), and expanded off the coast of Mexico ( [[#Sánchez-Velasco--2019|Sánchez-Velasco et al., 2019]] ).

In summary, episodes of widespread and long-lasting (100 ka scales) open-ocean deoxygenation were related to warm climate intervals of the Permian-Cretaceous, with conditions becoming generally better oxygenated as the climate cooled over the course of the Cenozoic ( ''high confidence'' ). The largest expansions of oxygen depleted waters over the past 25 ka were strongly linked to rapid warming rates ( ''medium confidence'' ). Open-ocean deoxygenation has occurred in most regions of the open ocean during the mid-20th to early 21st centuries ( ''high confidence'' ), and shows decadal variability ( ''medium confidence'' ). Evidence further confirms SROCC that OMZs are expanding at many locations ( ''high confidence'' ).

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