Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.3.3.2 Ocean Deoxygenation and its Implications for Greenhouse Gases ====

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As summarized in SROCC ( [[#5.2.2.4|Section 5.2.2.4]] ), there is a growing consensus that between 1970 and 2010 the open ocean has ''very likely'' lost 0.5–3.3% of its dissolved oxygen in the upper 1000 m depth ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.6|Section 2.3.3.6]] ; [[#Helm--2011|Helm et al., 2011]] ; [[#Ito--2017|Ito et al., 2017]] ; [[#Schmidtko--2017|Schmidtko et al., 2017]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ). Regionally, the equatorial and North Pacific, the Southern Ocean and the South Atlantic have shown the greatest oxygen loss of up to 30 mol m <sup>–2</sup> per decade ( [[#Schmidtko--2017|Schmidtko et al., 2017]] ). Warming – via solubility reduction and circulation changes – mixing and respiration are considered the major drivers, with 50% of the oxygen loss for the upper 1000 m of the global oceans attributable to the solubility reduction ( [[#Schmidtko--2017|Schmidtko et al., 2017]] ). Climate variability also modifies the oxygen loss on interannual and decadal time scales especially for the tropical ocean OMZs ( [[#Deutsch--2011|Deutsch et al., 2011]] , 2014; [[#Llanillo--2013|Llanillo et al., 2013]] ) and the North Pacific subarctic zone ( [[#Whitney--2007|Whitney et al., 2007]] ; [[#Sasano--2018|Sasano et al., 2018]] ; [[#Cummins--2020|Cummins and Ross, 2020]] ). However, quantifying the oxygen decline and variability and attributing them to processes in different regions remains challenging (Levin, 2018; [[#Oschlies--2018|Oschlies et al., 2018]] ). Earth system models (ESMs) in CMIP5 and CMIP6 corroborate the decline in ocean oxygen, and project a continuing and accelerating decline with a strong impact of natural climate variability under high-emissions scenarios (Bopp et al., 2013; [[#Long--2016|Long et al., 2016]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). However, CMIP5 models did not reproduce observed patterns for oxygen changes in the tropical thermocline, and generally simulated only about half the oxygen loss inferred from observations ( [[#Oschlies--2018|Oschlies et al., 2018]] ). CMIP6 models have a more realistic simulated mean state of ocean biogeochemistry than CMIP5 models due to improved ocean physical processes and better representation of biogeochemical processes ( [[#Séférian--2020|Séférian et al., 2020]] ). Theyalso exhibit enhanced ocean warming as a result of an increase in the equilibrium climate sensitivity (ECS) of CMIP6 relative to CMIP5 models, which contributes to increased stratification and reduced subsurface ventilation (Sections 4.3.1, 4.3.4, 5.3.3.2, 7.4.2, 7.5.6, 9.2.1, and TS2.4). Consequently, CMIP6 model ensembles reproduce the ocean deoxygenation trend of −0.30 to −1.52 mmol m <sup>−3</sup> per decade between 1970–2010 reported in SROCC ( [[#5.2.2.4|Section 5.2.2.4]] ) with a very ''likely'' range, and also project 32–71% greater subsurface (100–600 m) oxygen decline relative to their Representative Concentration Pathway (RCP) analogues in CMIP5, reaching to the ''likely'' range of decline of 6.4 ± 2.9 mmol m <sup>–3</sup> under SSP1–2.6 and 13.3 ± 5.3 mmol m <sup>–3</sup> under SSP5–8.5, from 1870–1899 to 2080–2099 ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). It is concluded that the oxygen content of subsurface ocean is projected to transition to historically unprecedented condition with decline over the 21st century ( ''medi'' ''um confidence'' ).

In oxygen-depleted waters, microbial processes (denitrification and anammox, i.e., anaerobic ammonium oxidation; [[#Kuypers--2005|Kuypers et al., 2005]] ; [[#Codispoti--2007|Codispoti, 2007]] ; [[#Gruber--2008|Gruber and Galloway, 2008]] ) remove fixed nitrogen, and when upwelled waters reach the photic zone, primary production becomes nitrogen-limited ( [[#Tyrrell--2002|Tyrrell and Lucas, 2002]] ). However, in other oceanic regions, increased water-column stratification due to warming may reduce the amount of N <sub>2</sub> O reaching the surface and thereby decrease N <sub>2</sub> O flux to the atmosphere. [[#Landolfi--2017|Landolfi et al. (2017)]] suggest that, by 2100, under the RCP8.5 scenario, total N <sub>2</sub> O production in the ocean may decline by 5% and N <sub>2</sub> O emissions be reduced by 24% relative to the pre-industrial era due to decreased organic matter export and anthropogenic-driven changes in ocean circulation and atmospheric N <sub>2</sub> O concentrations. Projected oxygen loss in the ocean is thought to result in an ocean-climate feedback through changes in the natural emissions of GHGs ( ''l'' ''ow confidence'' ).

The areas with relatively rapid oxygen decrease include OMZs in the tropical oceans, where oxygen content has been decreasing at a rate of 0.9–3.4 µmol kg <sup>–1</sup> per decade in the thermocline for the past five decades (Stramma et al., 2008). Low oxygen, low pH and shallow aragonite saturation horizons in the OMZs of the eastern boundary upwelling regions co-occur, affecting ecosystem structure ( [[#Chavez--2008|Chavez et al., 2008]] ) and function in the water column, including the presently unbalanced nitrogen cycle ( [[#Paulmier--2009|Paulmier and Ruiz-Pino, 2009]] ). The coupling between upwelling, productivity, and oxygen depletion feeds back to biological productivity and the role of these regions as sinks or sources of climate active gases. When OMZ waters upwell and impinge on the euphotic zone, they release significant quantities of GHGs, including N <sub>2</sub> O (0.81–1.35 TgN yr <sup>–1</sup> ), CH <sub>4</sub> (0.27–0.38 TgCH <sub>4</sub> yr <sup>–1</sup> ), and CO <sub>2</sub> (yet to be quantified) to the atmosphere, exacerbating global warming ( [[#Paulmier--2008|Paulmier et al., 2008]] ; [[#Naqvi--2010|Naqvi et al., 2010]] ; [[#Kock--2012|Kock et al., 2012]] ; [[#Arévalo-Martínez--2015|Arévalo-Martínez et al., 2015]] ; [[#Babbin--2015|Babbin et al., 2015]] ; [[#Farías--2015|Farías et al., 2015]] ). Modelling projectionssuggest a global decrease of 4–12% in oceanic N <sub>2</sub> O emissions (from 3.71–4.03 TgN yr <sup>–1</sup> <sup></sup> to 3.54–3.56 TgN yr <sup>–1</sup> ) from 2005 to 2100 under RCP8.5, despite a tendency to increased N <sub>2</sub> O production in the OMZs, associated primarily with denitrification ( [[#Martinez-Rey--2015|Martinez-Rey et al., 2015]] ). It is difficult to single out the contribution of nitrification and denitrification, which can occur simultaneously. A rigorous separation of these two processes would require more mechanistic parametrization, which has been hindered by the still large conceptual and parametric uncertainties ( [[#Babbin--2015|Babbin et al., 2015]] ; [[#Trimmer--2016|Trimmer et al., 2016]] ; [[#Landolfi--2017|Landolfi et al., 2017]] ). Furthermore, the correlation between N <sub>2</sub> O and oxygen varies with microorganisms present, nutrient concentrations, and other environmental variables ( [[#Voss--2013|Voss et al., 2013]] ).

In summary, total oceanic N <sub>2</sub> O emissions were projected to decline by 4–12% from 2005–2100 ( [[#Martinez-Rey--2015|Martinez-Rey et al., 2015]] ) and by 24% from the pre-industrial era to 2100 ( [[#Landolfi--2017|Landolfi et al., 2017]] ) under RCP8.5. However, there is ''low confidence'' in the reduction in N <sub>2</sub> O emissions to the atmosphere, because of large conceptual and parametric uncertainties, a limited number of modelling studies that explored this process, and greater oxygen losses simulated in CMIP6 models than in CMIP5 models ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ).

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