Editing IPCC:AR6/WGI/Chapter-4 (section)

=== 4.5.2 Ocean ===

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==== 4.5.2.1 Ocean Temperature ====

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Projections of long-term ocean thermal properties are assessed comprehensively in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Sections 9.2.1.1 and 9.2.2.1) and are not covered here to avoid unnecessary overlap.

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==== 4.5.2.2 Ocean Acidification ====

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The model-simulated, long-term trend of ocean acidification is assessed in [[#4.3.2.5|Section 4.3.2.5]] and [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] (Section 5.3.4.1). It is ''virtually certain'' that surface ocean acidification will continue in response to the rise in atmospheric CO <sub>2</sub> , and continued penetration of anthropogenic CO <sub>2</sub> from the surface to the deep ocean will acidify the ocean interior (Figure 4.29). By the end of this century, under SSP3-7.0, a pH reduction of about 0.3 is found at a few hundred metres depth of the global ocean, with stronger acidification in the interior North Atlantic and the mid- to high-latitude Southern Ocean. At a depth of about 1 km, a pH reduction of about 0.1 is found.

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[[File:9153197c74c572b32e0b9f6e44248665 IPCC_AR6_WGI_Figure_4_29.png]]

'''Figure 4.2''' '''9 |''' '''Long-term change of annual and zonal ocean pH.''' Displayed are multi-model mean change in annual and zonal ocean pH in 2081–2100 relative to themean of 1995–2014 for SSP1-2.6 and SSP3-7.0, respectively. Eleven CMIP6 model results are used. Diagonal lines indicate regions where fewer than 80% of the models agree on the sign of the change and no overlay where at least 80% of the models agree on the sign of change. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Projections with CMIP6 ESMs ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ) show a surface pH decline of –0.16 ± 0.002 (±1 standard deviation) under SSP1-2.6 and –0.44 ± 0.005 under SSP5-8.5 from 1870–1899 to 2080–2099. The high-latitude oceans, in particular the Arctic, show greater decline in pH and accelerated acidification ( [[#Terhaar--2020|Terhaar et al., 2020]] ). For the same period, model-projected bottom-water pH decline is –0.018 ± 0.001 under SSP1-2.6 and –0.030 ± 0.002 under SSP5-8.5. The projected large-scale surface ocean acidification will be primarily determined by the pathway of atmospheric CO <sub>2</sub> , with weak dependence on change in climate ( ''high confidence'' ) (Section 5.3.4.1; [[#Hurd--2018|Hurd et al., 2018]] ). However, for a given atmospheric CO <sub>2</sub> scenario, uncertainty in projected ocean acidification increases with ocean depth because of model-simulated differences in ocean circulation that transports anthropogenic CO <sub>2</sub> from the surface to bottom ocean ( ''high confidence'' ) ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). For example, projected surface pH fully separates between SSPs scenarios before 2050, but some overlap across SSPs is still found for projected bottom-water pH in 2080 ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ).

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