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

==== 3.2.3.1 Ocean Acidification ====

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The ocean’s uptake of anthropogenic carbon affects its chemistry in a process referred to as ocean acidification, which increases the concentrations of aqueous CO 2 , bicarbonate and hydrogen ions, and decreases pH, carbonate ion concentrations and calcium carbonate mineral saturation states ( [[#Doney--2009|Doney et al., 2009]] ). Ocean acidification affects a variety of biological processes with, for example, lower calcium carbonate saturation states reducing net calcification rates for some shell-forming organisms and higher CO 2 concentrations increasing photosynthesis for some phytoplankton and macroalgal species ( [[#3.3.2|Section 3.3.2]] ).

Direct measurements of ocean acidity from ocean time series, as well as pH changes determined from other shipboard studies, show consistent decreases in ocean surface pH over the past few decades ( ''virtually certain'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3.2|Section 5.3.2.2]] ; [[#Takahashi--2014|Takahashi et al., 2014]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Sutton--2019|Sutton et al., 2019]] ; [[#Canadell--2021|Canadell et al., 2021]] ).

Since the 1980s, surface ocean pH has declined by a ''very likely'' rate of 0.016–0.020 per decade in the subtropics and 0.002–0.026 per decade in the subpolar and polar zones (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3.2|Section 5.3.2.2]] ; [[#Canadell--2021|Canadell et al., 2021]] ). Typically, the pH of global surface waters has decreased from 8.2 to 8.1 since the pre-industrial era (1750 CE), a trend attributable to rising atmospheric CO 2 ( ''virtually certain'' ) ( [[#Orr--2005|Orr et al., 2005]] ; [[#Jiang--2019|Jiang et al., 2019]] ).

Ocean acidification is also developing in the ocean interior ( ''very high confidence'' ) due to the transport of anthropogenic CO 2 to depth by ocean currents and mixing (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.3.1]] ; [[#Canadell--2021|Canadell et al., 2021]] ). There, it leads to the shoaling of saturation horizons of aragonite and calcite ( ''high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.3.1]] ; [[#Canadell--2021|Canadell et al., 2021]] ), below which dissolution of these calcium carbonate minerals is thermodynamically favoured. The calcite or aragonite saturation horizons have migrated upwards in the North Pacific (1–2 m yr –1 over 1991–2006) ( [[#Feely--2012|Feely et al., 2012]] ) and in the Irminger Sea (10–15 m yr –1 for the aragonite saturation horizon over 1991–2016) ( [[#Perez--2018|Perez et al., 2018]] ). In some locations of the western Atlantic Ocean, calcite saturation depth has risen by ~300 m since the pre-industrial era due to increasing concentrations of deep-ocean dissolved inorganic carbon ( [[#Sulpis--2018|Sulpis et al., 2018]] ). In the Arctic, where some coastal surface waters are already undersaturated with respect to aragonite due to the degradation of terrestrial organic matter ( [[#Mathis--2015|Mathis et al., 2015]] ; [[#Semiletov--2016|Semiletov et al., 2016]] ), the deep aragonite saturation horizon shoaled on average 270 ± 60 m during 1765–2005 ( [[#Terhaar--2020|Terhaar et al., 2020]] ).

Detection and attribution of ocean acidification in coastal environments are more difficult than in the open ocean due to larger spatio-temporal variability of carbonate chemistry ( [[#Duarte--2013|Duarte et al., 2013]] ; [[#Laruelle--2017|Laruelle et al., 2017]] ; [[#Torres--2021|Torres et al., 2021]] ) and to the influence of other natural acidification drivers such as freshwater and high-nutrient riverine inputs ( [[#Cai--2011|Cai et al., 2011]] ; [[#Laurent--2017|Laurent et al., 2017]] ; [[#Fennel--2019|Fennel et al., 2019]] ; [[#Cai--2020|Cai et al., 2020]] ) or anthropogenic acidification drivers ( [[#3.1|Section 3.1]] ) like atmospherically deposited nitrogen and sulphur ( [[#Doney--2007|Doney et al., 2007]] ; [[#Hagens--2014|Hagens et al., 2014]] ). Since AR5, the observing network in coastal oceans has expanded substantially, improving understanding of both the drivers and amplitude of observed variability ( [[#Sutton--2016|Sutton et al., 2016]] ). Recent studies indicate that two more decades of observations may be required before anthropogenic ocean acidification emerges over natural variability in some coastal sites and regions (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.5.2]] ; [[#Sutton--2019|Sutton et al., 2019]] ; [[#Turk--2019|Turk et al., 2019]] ; [[#Canadell--2021|Canadell et al., 2021]] ).

Mean open-ocean surface pH is projected to decline by 0.08 ± 0.003 ( ''very likely range'' ), 0.17 ± 0.003, 0.27 ± 0.005 and 0.37 ± 0.007 pH units 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; WGI AR6 [[IPCC:Wg2:Chapter:Chapter-4#4.3.2|Section 4.3.2]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ; [[#Lee--2021|Lee et al., 2021]] ). Projected changes in surface pH are relatively uniform in contrast with those of other surface-ocean variables, but they are largest in the Arctic Ocean (Figure 3.6; WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.4.1]] ; [[#Canadell--2021|Canadell et al., 2021]] ). Similar declines in the concentration of carbonate ions are projected by Earth system models (ESMs; [[#Bopp--2013|Bopp et al., 2013]] ; [[#Gattuso--2015|Gattuso et al., 2015]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). The North Pacific, the Southern Ocean and Arctic Ocean regions will become undersaturated for calcium carbonate minerals first ( [[#Orr--2005|Orr et al., 2005]] ; [[#Pörtner--2014|Pörtner et al., 2014]] ). Concurrent impacts on the seasonal amplitude of carbonate chemistry variables are anticipated (i.e., increased amplitude for ''p'' CO 2 and hydrogen ions, decreased amplitude for carbonate ions; [[#McNeil--2016|McNeil and Sasse, 2016]] ; [[#Kwiatkowski--2018|Kwiatkowski and Orr, 2018]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ).

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[[File:9bdda09a395d9b52ee9d20810614cfc3 IPCC_AR6_WGII_Figure_3_005.png]]

'''Figure 3.5 |''' '''Projected trends in climatic impact-drivers for ocean ecosystems.''' Panels (a,b,c,d) represent Coupled Model Intercomparison Project 5 (CMIP5) Representative Concentration Pathway (RCP) and CMIP6 Shared Socioeconomic Pathway (SSP) end-of-century changes in '''(a)''' global sea level; '''(b)''' average surface pH, '''(c)''' subsurface (100–600 m) dissolved oxygen concentration and '''(d)''' euphotic-zone (0–100 m) nitrate (NO 3 ) concentration against anomalies in sea surface temperature. All anomalies are model-ensemble averages over 2080–2099 relative to the 1870–1899 baseline period (from [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ), except for sea level, which shows model-ensemble median in 2100 relative to 1901 (from AR6 WGI Chapter 9). Error bars represent ''very likely'' ranges, except for SLR where they represent ''likely'' ranges. ''Very likely'' ranges for pH changes are too narrow to appear in the figure (see text). Panels (e,f,g,h) show regions where end-of-century projected CMIP6 surface warming exceeds 2°C, where surface ocean pH decline exceeds 0.3, where subsurface dissolved oxygen decline exceeds 30 mmol m -3 and where euphotic-zone (0–100 m) nitrate decline exceeds 1 mmol m -3 in '''(e)''' SSP1-2.6, '''(f)''' SSP2-4.5, '''(g)''' SSP3-7.0 and '''(h)''' SSP5-8.5. All anomalies are 2080–2099 relative to the 1870–1899 baseline period. (Modified from [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ).

Future declines in subsurface pH (Figure 3.6) will be modulated by changes in ocean overturning and water-mass subduction ( [[#Resplandy--2013|Resplandy et al., 2013]] ), and in organic matter remineralisation ( [[#Chen--2017|Chen et al., 2017]] ). In particular, decreases in pH will be less consistent at the seafloor than at the surface and will be linked to the transport of surface anomalies to depth. For example, &gt;20% of the North Atlantic seafloor deeper than 500 m, including canyons and seamounts designated as marine protected areas (MPAs), will experience pH reductions &gt;0.2 by 2100 under RCP8.5 ( [[#Gehlen--2014|Gehlen et al., 2014]] ). Changes in pH in the abyssal ocean (&gt;3000 m deep) are greatest in the Atlantic and Arctic Oceans, with lesser impacts in the Southern and Pacific Oceans by 2100, mainly due to ventilation time scales ( [[#Sweetman--2017|Sweetman et al., 2017]] ).

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