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

=== Ocean Acidification and Ocean Deoxygenation ===

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'''Ocean acidification is strengthening as a result of the ocean continuing to take up CO''' <sub>2</sub> '''from human-caused emissions''' ( ''very high confidence'' ''').''' This CO <sub>2</sub> uptake is driving changes in seawater chemistry that result in the decrease of pH and associated reductions in the saturation state of calcium carbonate, which is a constituent of skeletons or shells of a variety of marine organisms. These trends of ocean acidification are becoming clearer globally, with a ''very likely'' rate of decrease in pH in the ocean surface layer of 0.016 to 0.020 per decade in the subtropics and 0.002 to 0.026 per decade in subpolar and polar zones since the 1980s. Ocean acidification has spread deeper in the ocean, surpassing 2000 m depth in the northern North Atlantic and in the Southern Ocean. The greater projected pH declines in Coupled Model Intercomparison Project Phase 6 (CMIP6) models are primarily a consequence of higher atmospheric CO <sub>2</sub> concentrations in the Shared Socio-economic Pathways (SSPs) scenarios than their Coupled Model Intercomparison Project Phase 5 (CMIP5) Representative Concentration Pathway (RCP) analogues. {5.3.2.2, 5.3.3.1; 5.3.4.1; Figures 5.20, 5.21}

'''Ocean deoxygenation is projected to continue to increase with ocean warming''' ( ''high confidence'' ''').''' Earth system models (ESMs) project a 32–71% greater subsurface (100–600 m) oxygen decline, depending on the scenario, than reported in the Special Report on the Ocean and Cryosphere (SROCC) for the period 2080–2099. This is attributed to the effect of larger surface warming in CMIP6 models, which increases ocean stratification and reduces ventilation ( ''medium confidence'' ). There is ''low confidence'' in the projected reduction of oceanic N <sub>2</sub> O emissions under high-emissions scenarios because of greater oxygen losses simulated in ESMs in CMIP6, uncertainties in the process of oceanic N <sub>2</sub> O emissions, and a limited number of modelling studies available. {5.3.3.2; 7.5}

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