Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.3.5 Ocean pH ====

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The AR5 assessed with ''high confidence'' that the pH of the ocean surface had decreased since preindustrial times, primarily as a result of ocean uptake of CO <sub>2</sub> . SROCC concluded that the global ocean absorbed 20–30% of total CO <sub>2</sub> emissions since the 1980s, with ''virtually certain'' ocean surface pH decline. The SROCC assessed a rate of surface pH decline of 0.017–0.027 pH units per decade across a range of time series of pH observations longer than 15 years. The decline in surface open ocean pH was assessed by SROCC as having ''very likely'' already emerged from background natural variability for more than 95% of the global surface open ocean.

Understanding of changes in surface pH at paleo time-scales has increased since AR5 ( [[#Clarkson--2015|Clarkson et al., 2015]] ; [[#Foster--2016|Foster and Rae, 2016]] ; [[#Zeebe--2016|Zeebe et al., 2016]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Sosdian--2018|Sosdian et al., 2018]] ; [[#Henehan--2019|Henehan et al., 2019]] ; [[#Anagnostou--2020|Anagnostou et al., 2020]] ; [[#Harper--2020|Harper et al., 2020]] ; [[#Müller--2020|Müller et al., 2020]] ). Over the last 65 million years there have been several intervals when the pH of surface waters varied concurrently with climate change such as during the PETM, EECO, and MCO (Figure 2.29a and Section 5.3.1.1). However, only during the PETM is the change sufficiently well-constrained to allow for a direct comparison with recent and current trends ( [[#Kirtland%20Turner--2018|Kirtland]] [[#Turner--2018|Turner, 2018]] ). This event was associated with profound perturbations of the global carbon cycle, ocean warming, deoxygenation and a surface ocean pH decrease ''likely'' ranging from 0.15 to 0.30 units ( [[#Penman--2014|Penman et al., 2014]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Babila--2018|Babila et al., 2018]] ) – a rate that was ''likely'' at least an order of magnitude slower than today ( [[#Cui--2011|Cui et al., 2011]] ; [[#Bowen--2015|Bowen et al., 2015]] ; [[#Frieling--2016|Frieling et al., 2016]] ; [[#Zeebe--2016|Zeebe et al., 2016]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Kirtland%20Turner--2018|Kirtland]] [[#Turner--2018|Turner, 2018]] ; [[#Gingerich--2019|Gingerich, 2019]] ).

Paleo evidence suggests that surface ocean pH has gradually increased over the last 50 Myr ( [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020; [[#Sosdian--2018|Sosdian et al., 2018]] ) (Figure 2.29a). Global mean surface pH values as low as observed during recent decades are uncommon in the last 2 Myr (Figure 2.29b) ( [[#Martínez-Botí--2015a|Martínez-Botí et al., 2015a]] ; [[#Chalk--2017|Chalk et al., 2017]] ; [[#Dyez--2018|Dyez et al., 2018]] ; [[#Sosdian--2018|Sosdian et al., 2018]] ), and have not been experienced in at least the last 25 kyr (Figure 2.29c; [[#Palmer--2003|Palmer and Pearson, 2003]] ; [[#Foster--2008|Foster, 2008]] ; [[#Palmer--2010|Palmer et al., 2010]] ; [[#Henehan--2013|Henehan et al., 2013]] ; [[#Kirschke--2013|Kirschke et al., 2013]] ; [[#Martínez-Botí--2015a|Martínez-Botí et al., 2015a]] ; [[#Naik--2015|Naik et al., 2015]] ; [[#Ezat--2017|Ezat et al., 2017]] ; [[#Gray--2018|Gray et al., 2018]] ; [[#Shao--2019|Shao et al., 2019]] ). The magnitude of pH change during the Pleistocene glacial–interglacial cycles was 0.1–0.15 pH units – similar to recent changes in the modern era (Figure 2.29c and Section 5.3.1.2; [[#Hönisch--2009|Hönisch et al., 2009]] ; [[#Chalk--2017|Chalk et al., 2017]] ; [[#Shao--2019|Shao et al., 2019]] ). Maximum rates of pH change during the LDT, inferred from changes in atmospheric CO <sub>2</sub> recorded in ice cores ( [[#Marcott--2014|Marcott et al., 2014]] ) and the established relationships between pH and CO <sub>2</sub> changes and the boron isotope proxy ( [[#Hain--2018|Hain et al., 2018]] ), reached –0.02 pH units per century at about 11.7 ka, about 14.8 ka and about 16.3 ka, as previously sequestered CO <sub>2</sub> was transferred from the ocean interior to the subsurface ocean ( [[#Martínez-Botí--2015a|Martínez-Botí et al., 2015a]] ; [[#Jaccard--2016|Jaccard et al., 2016]] ; [[#Rae--2018|Rae et al., 2018]] ).

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[[File:ff55a94c2abf2085ce29b2e2e0c13e49 IPCC_AR6_WGI_Figure_2_29.png]]

'''Figure 2.29 | Low-latitude surface ocean pH over the last 65 million years (65 Myr). (a)''' Low-latitude (30°N–30°S) surface ocean pH over the last 65 Myr, reconstructed using boron isotopes in foraminifera. '''(b)''' as (a) but for the last 3.5 Myr. Double headed arrow shows the approximate magnitude of glacial-interglacial pH changes. '''(c)''' Multisite composite of surface pH. In (a, b, c) uncertainty is shown at 95% confidence as a shaded band. Relevant paleoclimate reference periods (CCB2.1) have been labelled. Period windows for succeeding panels are shown as horizontal black lines in (a) and (b). '''(d)''' Estimated low-latitude surface pH from direct observations (BATS, HOT) and global mean pH (65°S–65°N) from two indirect estimates (CMEMS, OCEAN-SODA). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Since the 1980s, the global ocean has experienced a decline in surface pH of 0.016 ± 0.006 pH units per decade based on indirect pH products (Figure 2.29d; [[#Lauvset--2015|Lauvset et al., 2015]] ; [[#Hurd--2018|Hurd et al., 2018]] ; [[#IPCC--2019|IPCC, 2019]] ; [[#Gehlen--2020|Gehlen et al., 2020]] ) that agrees with the decline of 0.017–0.025 pH units per decade assessed in SROCC from direct time-series measurements of pH. Section 5.3.2.2 assesses a decline that ranges from 0.01 to 0.026 pH units per decade for the tropical and subtropical open ocean areas, and 0.003–0.026 pH units per decade for the polar and subpolar open ocean regions by using time series and ship-based datasets from the surface ocean CO <sub>2</sub> measurement network ( [[#Bakker--2016|Bakker et al., 2016]] ; [[#Gehlen--2020|Gehlen et al., 2020]] ; [[#Gregor--2021|Gregor and Gruber, 2021]] ). There is general consensus that global surface ocean pH trends over the past two decades have exceeded the natural background variability ( [[#Lauvset--2015|Lauvset et al., 2015]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Gehlen--2020|Gehlen et al., 2020]] ). However, for some areas sparse data coverage, and large year-to-year variations hinders the detection of long-term surface ocean pH trends; for example in the Southern Ocean ( [[#Lauvset--2015|Lauvset et al., 2015]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ) and in the Arctic Ocean ( [[#Lauvset--2015|Lauvset et al., 2015]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ).

For subsurface pH changes, estimates arise from direct ship measurements from repeated hydrography programs ( [[#Carter--2019|Carter et al., 2019]] ), indirect estimates of pH through calcite and aragonite saturation horizons ( [[#Osborne--2020|Osborne et al., 2020]] ; [[#Ross--2020|Ross et al., 2020]] ), and the very recent biogeochemical Argo floats equipped with pH sensors ( [[#Claustre--2020|Claustre et al., 2020]] ). Global subsurface pH has decreased over the past 20 to 30 years, with signals observed to at least 1000 m depths ( [[#Lauvset--2020|Lauvset et al., 2020]] ). Global findings are supplemented by regional findings from the Pacific Ocean ( [[#Carter--2019|Carter et al., 2019]] ; [[#Ross--2020|Ross et al., 2020]] ); the South Atlantic ( [[#Salt--2015|Salt et al., 2015]] ) and Southern Ocean ( [[#Jones--2017|Jones et al., 2017]] ); the North Atlantic Ocean and along the AMOC ( [[#Woosley--2016|Woosley et al., 2016]] ; [[#Perez--2018|Perez et al., 2018]] ), the Arctic Ocean ( [[#Qi--2017|Qi et al., 2017]] ) and marginal seas ( [[#Chen--2017|]] [[#Chen--2017|C.-T.A. Chen et al., 2017]] ). Further details are given in Section 5.3.3.1.

To conclude, it is ''virtually certain'' that surface open ocean pH has declined globally over the last 40 years by 0.003–0.026 pH per decade, and a decline in the ocean interior has been observed in all ocean basins over the past 2–3 decades ( ''high confidence'' ). A long-term increase in surface open ocean pH occurred over the past 50 Myr ( ''high confidence'' ), and surface open ocean pH as low as recent times is uncommon in the last 2 Myr ( ''medium confidence'' ). There is ''very high confidence'' that open ocean surface pH is now the lowest it has been for at least 26 kyr and current rates of pH change are unprecedented since at least that time.

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