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

==== 5.3.3.1 Ocean Memory: Acidification in the Ocean Interior ====

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Advances in observations and modelling for ocean physics and biogeochemistry and established knowledge of ocean carbonate chemistry show with ''very high confidence'' that anthropogenic CO <sub>2</sub> taken up into the ocean surface layer is further spreading into the ocean interior through ventilation processes, including vertical mixing, diffusion, subduction and meridional overturning circulations (Sections 2.3.3.5, 5.2.1.3 and 9.2.2.3; [[#Sallée--2012|Sallée et al., 2012]] ; [[#Bopp--2015|Bopp et al., 2015]] ; [[#Nakano--2015|Nakano et al., 2015]] ; [[#Iudicone--2016|Iudicone et al., 2016]] ; [[#Toyama--2017|Toyama et al., 2017]] ; [[#Pérez--2018|Pérez et al., 2018]] ; [[#Gruber--2019b|Gruber et al., 2019b]] ) and is causing acidification in the ocean interior. The net change in oxygen consumption by aerobic respiration of marine organisms further influences acidification by releasing CO <sub>2</sub> ( [[#5.3.3.2|Section 5.3.3.2]] ; [[#Chen--2017|Chen et al., 2017]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ; [[#Robinson--2019|Robinson, 2019]] ).

Observations over past decades of basin-wide and global syntheses of ocean interior carbon show that the extent of acidification due to anthropogenic CO <sub>2</sub> invasion tends to diminish with depth ( ''very high confidence'' ) ( [[#5.2.1.3.3|Section 5.2.1.3.3]] and Figure 5.21; [[#Woosley--2016|Woosley et al., 2016]] ; [[#Carter--2017|Carter et al., 2017]] ; [[#Lauvset--2020|Lauvset et al., 2020]] ). The regions of deep convection such as subpolar North Atlantic and Southern Ocean present the deepest acidification detections below 2000 m ( ''medium confidence'' ). Mid-latitudinal zones within the subtropical cells and tropical regions present a relatively deep and shallow detection, respectively. A pH decrease has also been observed on the Antarctic continental shelf ( [[#Hauck--2010|Hauck et al., 2010]] ; [[#Williams--2015|Williams et al., 2015]] ). Acidification is also underway in the subsurface to intermediate layers of the Arctic Ocean due to the inflow of ventilated waters from the North Atlantic and the North Pacific ( [[#Qi--2017|Qi et al., 2017]] ; [[#Ulfsbo--2018|Ulfsbo et al., 2018]] ).

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[[File:003c6023a9968316104dcfa15c261c17 IPCC_AR6_WGI_Figure_5_21.png]]

'''Figure 5.21 |''' '''Spread of ocean acidification from the surface into the interior of ocean since pre-industrial times''' . '''(a)''' Map showing the three transects used to create the cross sections shown in (b) and (c); vertical sections of the changes in '''(b)''' pH and '''(c)''' saturation state of aragonite (Ω <sub>arag</sub> ) between 1800–2002 due to anthropogenic CO <sub>2</sub> invasion (colour). Contour lines are their contemporary values in 2002. The red transect begins in the Nordic Seas and then follows the GO-SHIP lines A16 southward in the Atlantic Ocean, SR04 and S04P westward in the Southern Ocean, and P16 northward in the Pacific Ocean. The purple line follows the GO-SHIP line I09 southward in the Indian Ocean. The green line on the smaller inset crosses the Arctic Ocean from the Bering Strait to North Pole along 175°W and from the North Pole to the Fram Strait along 5°E (Lauvset et al., 2020). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

A significant increase in acidification resulting from net metabolic CO <sub>2</sub> release coupled with ocean circulation changes has been shown with ''high confidence'' in large swathes of intermediate waters in the Pacific and Atlantic oceans ( [[#Dore--2009|Dore et al., 2009]] ; [[#Byrne--2010|Byrne et al., 2010]] ; [[#Ríos--2015|Ríos et al., 2015]] ; [[#Chu--2016|Chu et al., 2016]] ; [[#Carter--2017|Carter et al., 2017]] ; [[#Lauvset--2020|Lauvset et al., 2020]] ). For example, ocean circulation contributes a pH change of –0.013 ± 0.013 to the overall observed change of –0.029 ± 0.014 for 1993–2013 at depths around 1000 m at 30°S–40°S in the South Atlantic ocean ( [[#Ríos--2015|Ríos et al., 2015]] ). Long-term repeated observations in the North Pacific show a decline in dissolved oxygen (–4.0 μmol kg <sup>−1</sup> per decade at maximum) being sustained in the intermediate water since the 1980s (Takatani et al., 2012; [[#Sasano--2015|Sasano et al., 2015]] ). The amplification of acidification associated with the weakening ventilation is thought to have been occurring persistently. In contrast, for the North Pacific subtropical mode water, large decadal variability in pH and aragonite saturation state with amplitudes of about 0.02 and about 0.1, respectively, are superimposed on secular declining trends due to anthropogenic CO <sub>2</sub> invasion ( [[#Oka--2019|Oka et al., 2019]] ). This is associated with the variability in ventilation due to the approximately 50% variation in the formation volume of the mode water that is forced remotely by the Pacific Decadal Oscillation ( [[#Qiu--2013|Qiu et al., 2013]] ; [[#Oka--2015|Oka et al., 2015]] ).

These trends of acidification in the ocean interior lead to ''high confidence'' in shoaling of the saturation horizons of calcium carbonate minerals where Ω = 1. In the Pacific Ocean where the aragonite saturation horizon is shallower (a few hundred metres to 1200 m; Figure 5.21c), the rate of its shoaling is in the order of 1–2 m yr <sup>–1</sup> ( [[#Feely--2012|Feely et al., 2012]] ; [[#Ross--2020|Ross et al., 2020]] ). In contrast, shoaling rates of 4 m yr <sup>–1</sup> to 1710 m for 1984–2008 and of 10–15 m yr <sup>–1</sup> to 2250 m for 1991–2016 have been observed in the Iceland sea and the Irminger sea, respectively ( [[#Olafsson--2009|Olafsson et al., 2009]] ; [[#Pérez--2018|Pérez et al., 2018]] ).

In summary, ocean acidification is spreading into the ocean interior. Its rates at depths are controlled by the ventilation of the ocean interior as well as anthropogenic CO <sub>2</sub> uptake at the surface, thereby diminishing with depth ( ''very high confidence'' ) (Figure 5.21). Variability in ocean circulation modulates the trend of ocean acidification at depths through the changes in ventilation and their impacts on metabolic CO <sub>2</sub> content. However, the large knowledge gap around ventilation changes leads to ''low confidence'' in their impacts in many ocean regions (Sections 5.3.3.2; 9.2.2.3 and 9.3.2).

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