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

==== 5.3.4.1 Future Projections with Earth System Models (ESMs) ====

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Projections with CMIP5 ESMs, reported in AR5 (Section 6.4.4) and SROCC ( [[#5.2.2.3|Section 5.2.2.3]] ; [[#IPCC--2019b|IPCC, 2019b]] ), showed changes in global mean surface ocean pH from 1870–1899 to 2080–2099 of –0.14 ± 0.001 (inter-model standard deviation) under RCP2.6 and –0.38 ± 0.005 under RCP8.5 with pronounced regional variability ( [[#Bopp--2013|Bopp et al., 2013]] ; [[#Hurd--2018|Hurd et al., 2018]] ). They also projected faster pH declines in mode waters below seasonal mixed layers ( [[#Resplandy--2013|Resplandy et al., 2013]] ; [[#Watanabe--2017|Watanabe and Kawamiya, 2017]] ) as has been observed in the Atlantic ( [[#Salt--2015|Salt et al., 2015]] ) and in the Pacific ( [[#Carter--2019|Carter et al., 2019]] ), because of the net CO <sub>2</sub> release by respiration and lowering CO <sub>2</sub> buffering capacity of seawater. In these CO <sub>2</sub> concentration-driven simulations, the level of acidification in the surface ocean is primarily determined by atmospheric CO <sub>2</sub> concentration and regional seawater carbonate chemistry, thereby providing consistent projections across models. New projections with CMIP6 ESMs show greater surface pH decline of –0.16 ± 0.002 under the SSP1-2.6 and –0.44 ± 0.005 under SSP5-8.5 from 1870–1899 to 2080–2099 ( [[IPCC:Wg1:Chapter:Chapter-4#4.3.2.5|Section 4.3.2.5]] and Cross-Chapter Box 5.3; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). The greater pH declines in CMIP6 are primarily a consequence of higher atmospheric CO <sub>2</sub> concentrations in SSPs than their CMIP5-RCP analogues ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). Ocean acidification is also projected to occur with ''high confidence'' in the Abyssal Bottom Waters in regions such as the northern North Atlantic and the Southern Ocean ( [[#Sulpis--2019|Sulpis et al., 2019]] ), with the rates of global mean pH decline of –0.018 ± 0.001 under SSP1-2.6 and –0.030 ± 0.002 under SSP5-8.5 from 1870–1899 to 2080–2099 in CMIP6 ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ).

In surface ocean, changes in the amplitude of seasonal variations in pH are also projected to occur with ''high confidence.'' ESMs in CMIP6 show +73 ± 12% increase in the amplitude of seasonal variation in hydrogen ion concentration ([H <sup>+</sup> ]) but 10 ± 5% decrease in the seasonal variation in pH (= -log [H <sup>+</sup> ]) from 1995–2014 to 2080–2099 under SSP5-8.5. The simultaneous amplification of [H <sup>+</sup> ] and attenuation of pH seasonal cycles is counterintuitive but is the consequence of a greater increase in the annual mean [H <sup>+</sup> ] due to anthropogenic CO <sub>2</sub> invasion than the corresponding increase in its seasonal amplitude. These changes are consistent with the amplification/attenuation of the seasonal variation of +81 ±16% for [H <sup>+</sup> ] and –16 ± 7% for pH from 1990–1999 to 2090–2099 under RCP8.5 in CMIP5 ( [[#Kwiatkowski--2018|Kwiatkowski and Orr, 2018]] ).

The signal of ocean acidification in surface ocean is large and is projected to emerge beyond the range of natural variability within the time scale of a decade in all ocean basins ( [[#Schlunegger--2019|Schlunegger et al., 2019]] ). There is ''high agreement'' among modelling studies that the largest pH decline and large-scale undersaturation of aragonite in surface seawater start to occur first in polar oceans ( [[#Orr--2005|Orr et al., 2005]] ; [[#Steinacher--2009|Steinacher et al., 2009]] ; [[#Hurd--2018|Hurd et al., 2018]] ; [[#Jiang--2019|Jiang et al., 2019]] ). Under SSP5-8.5, the largest surface pH decline, exceeding 0.45 between 1995–2014 and 2080–2099, occurs in the Arctic Ocean ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). The freshwater input from sea ice melt is an additional factor leading to a faster decline of aragonite saturation level than expected from the anthropogenic CO <sub>2</sub> uptake ( [[#Yamamoto--2012|Yamamoto et al., 2012]] ). The increase in riverine and glacial discharges that provide terrigenous carbon, nutrients and alkalinity as well as freshwater are the other factors modifying the rate of acidification in the Arctic Ocean. However, their impacts have been projected in a limited number of studies with extensive knowledge gaps and model simplifications leading to ''low confidence'' in their impacts ( [[#Terhaar--2019|Terhaar et al., 2019]] ; [[#Hopwood--2020|Hopwood et al., 2020]] ). In the Southern Ocean, the aragonite undersaturation starts in the 2030s in RCP8.5, and the area that experiences aragonite undersaturation for at least one month per year by 2100 is projected to be more than 95%. Under RCP2.6, short periods (less than one month) of aragonite undersaturation are expected to be found in less than 2% of the area during this century ( [[#Sasse--2015|Sasse et al., 2015]] ; [[#Hauri--2016|Hauri et al., 2016]] ; [[#Negrete-García--2019|Negrete-García et al., 2019]] ). These long term projections are modified at interannual time scales by large-scale climate modes ( [[#Ríos--2015|Ríos et al., 2015]] ) such as the ENSO and the Southern Annular Mode ( [[#Conrad--2015|Conrad and Lovenduski, 2015]] ). In other regions, acidification trends are influenced by a range of processes such as changes in ocean circulation, temperature, salinity, carbon cycling, and the structure of the marine ecosystem. As, at present, models do not resolve fine-scale variability of these processes, current projections do not fully capture the changes that the marine environment will experience in the future ( [[#Takeshita--2015|Takeshita et al., 2015]] ; [[#Turi--2016|Turi et al., 2016]] ).

Overall, with the rise of atmospheric CO <sub>2</sub> , the physics of CO <sub>2</sub> transfer across the air–sea interface, the carbonate chemistry in seawater, the trends of ocean acidification being observed in the past decades ( [[#5.3.3.2|Section 5.3.3.2]] ) and modelling studies described in this section, it is ''virtually certain'' that ocean acidification will continue to grow. However, the magnitude and sign (direction) of many of ocean carbon–climate feedbacks are still poorly constrained (Matear and Lenton, 2014, 2018), leading to ''low confidence'' in their significant and long-lasting impacts on ocean acidification.

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