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

==== 3.2.3.2 Ocean Deoxygenation ====

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Ocean deoxygenation, the loss of oxygen in the ocean, results from ocean warming, through a reduction in oxygen saturation, increased oxygen consumption, increased ocean stratification and ventilation changes ( [[#Keeling--2010|Keeling et al., 2010]] ; [[#IPCC--2019a|IPCC, 2019a]] ). In recent decades, anthropogenic inputs of nutrients and organic matter ( [[#3.1|Section 3.1]] ) have increased the extent, duration and intensity of coastal hypoxia events worldwide ( [[#Diaz--2008|Diaz and Rosenberg, 2008]] ; [[#Rabalais--2010|Rabalais et al., 2010]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ), while pollution-induced atmospheric deposition of soluble iron over the ocean has accelerated open-ocean deoxygenation ( [[#Ito--2016|Ito et al., 2016]] ). Deoxygenation and acidification often coincide because biological consumption of oxygen produces CO 2 . Deoxygenation can have a range of detrimental effects on marine organisms and reduce the extent of marine habitats (Sections 3.3.2, 3.4.3.1; [[#Vaquer-Sunyer--2008|Vaquer-Sunyer and Duarte, 2008]] ; [[#Chu--2015|Chu and Tunnicliffe, 2015]] ).

Changes in ocean oxygen concentrations have been analysed from compilations of ''in situ'' data dating back to the 1960s ( [[#Helm--2011|Helm et al., 2011]] ; [[#Ito--2017|Ito et al., 2017]] ; [[#Schmidtko--2017|Schmidtko et al., 2017]] ). SROCC concluded that a loss of oxygen had occurred in the upper 1000 m of the ocean ( ''medium confidence'' ), with a global mean decrease of 0.5–3.3% ( ''very likely range'' ) over 1970–2010 ( [[#Bindoff--2019a|Bindoff et al., 2019a]] ). Based on new regional assessments ( [[#Queste--2018|Queste et al., 2018]] ; [[#Bronselaer--2020|Bronselaer et al., 2020]] ; [[#Cummins--2020|Cummins and Ross, 2020]] ; [[#Stramma--2020|Stramma et al., 2020]] ), WGI AR6 assesses that ocean deoxygenation has occurred in most regions of the open ocean since the mid-20th century ( ''high confidence'' ), but it is modified by climate variability on interannual and inter-decadal time scales ( ''medium confidence'' ) (WGI AR6 Sections 2.3.3.6, 5.3.3.2; [[#Canadell--2021|Canadell et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ). New findings since SROCC also confirm that the volume of oxygen minimum zones (OMZs) are expanding at many locations ( ''high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.3.2]] ; [[#Canadell--2021|Canadell et al., 2021]] ).

The most recent estimates of future oxygen loss in the subsurface ocean (100–600 m), using CMIP6 models, amount to −4.1 ± 4.2 ( ''very likely range'' ), −6.6 ± 5.7, −10.1 ± 6.7 and −11.2 ± 7.7% in 2081–2100 relative to 1995–2014 for SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5, respectively (Figure 3.5; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). Based on these CMIP6 projections, WGI AR6 concludes that the oxygen content of the subsurface ocean is projected to decline to historically unprecedented conditions over the 21st century ( ''medium confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.3.2]] ; [[#Canadell--2021|Canadell et al., 2021]] ). These declines are greater (by 31–72%) than simulated by the CMIP5 models in their Representative Concentration Pathway (RCP) analogues, a ''likely'' consequence of enhanced surface warming and stratification in CMIP6 models (Figure 3.5; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). At the regional scale and for subsurface waters, projected changes are not spatially uniform, and there is ''lower agreement'' among models than they show for the global mean trend ( [[#Bopp--2013|Bopp et al., 2013]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). In particular, large uncertainties remain for these future projections of ocean deoxygenation in the subsurface tropical oceans, where the major OMZs are located ( [[#Cabré--2015|Cabré et al., 2015]] ; [[#Bopp--2017|Bopp et al., 2017]] ).

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