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

=== 5.3.2 Historical Trends and Spatial Characteristics in the Upper Ocean ===

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==== 5.3.2.1 Reconstructed Centennial Ocean Acidification Trends ====

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Ocean pH time series are based on the reconstruction of coral boron isotope ratios ( δ <sup>11</sup> B). A majority of coral δ <sup>11</sup> B data have been generated from the western Pacific region with a few records from the Atlantic Ocean. Biweekly resolution paleo-pH records show monsoonal variation of about 0.5 pH unit in the South China Sea ( [[#Liu--2014|Liu et al., 2014]] ). Interannual ocean pH variability in the range of 0.07–0.16 pH unit characterizes southwest Pacific corals that are attributed to El Niño–Southern Oscillation (ENSO) ( [[#Wu--2018|]] [[#Wu--2018|H.C. Wu et al., 2018]] ) and river runoff ( [[#D’Olivo--2015|D’Olivo et al., 2015]] ). Decadal (10-, 22- and 48-year) ocean pH variations in the south-west Pacific have been linked to the Inter-decadal Pacific Oscillation, causing variations of up to 0.30 pH unit in the Great Barrier Reef ( [[#Pelejero--2005|Pelejero et al., 2005]] ; [[#Wei--2009|Wei et al., 2009]] ) but weaker (about 0.08 pH unit) in the open ocean ( [[#Wu--2018|]] [[#Wu--2018|H.C. Wu et al., 2018]] ). Decadal variations in the South China Sea pH changes of 0.10–0.20 have also been associated with the variation in the East Asian monsoon ( [[#Liu--2014|Liu et al., 2014]] ; [[#Wei--2015|Wei et al., 2015]] ), as a weakening of the Asian winter monsoon leads to sluggish water circulation within the reefs, building up localised CO <sub>2</sub> concentration in the water due to calcification and respiration.

Since the beginning of the industrial period in the mid-19th century, coral δ <sup>11</sup> B-derived ocean pH has decreased by 0.06–0.24 pH unit in the South China Sea ( [[#Liu--2014|Liu et al., 2014]] ; [[#Wei--2015|Wei et al., 2015]] ) and 0.12 pH unit in the south-west Pacific ( [[#Wu--2018|]] [[#Wu--2018|H.C. Wu et al., 2018]] ). Since the mid-20th century, a distinct feature of coral δ <sup>11</sup> B records relates to ocean acidification trends, albeit having a wide range of values: 0.12–0.40 pH unit in the Great Barrier Reef ( [[#Wei--2009|Wei et al., 2009]] ; [[#D’Olivo--2015|D’Olivo et al., 2015]] ), 0.05–0.08 pH unit in the north-west Pacific ( [[#Shinjo--2013|Shinjo et al., 2013]] ) and 0.04–0.09 pH unit in the Atlantic Ocean ( [[#Goodkin--2015|Goodkin et al., 2015]] ; [[#Fowell--2018|Fowell et al., 2018]] ). Concurrent coral carbon isotopic ( δ <sup>13</sup> C) measurements infer ocean uptake of anthropogenic CO <sub>2</sub> from the combustion of fossil fuel, based on the lower abundance of <sup>13</sup> C in fossil fuel carbon. Western Pacific coral records show depleted δ <sup>13</sup> C trends since the late 19th century that are more prominent since the mid-20th century ( ''high confidence'' ) (Pelejeroet al., 2005; [[#Wei--2009|Wei et al., 2009]] ; [[#Shinjo--2013|Shinjo et al., 2013]] ; [[#Liu--2014|Liu et al., 2014]] ; [[#Kubota--2017|Kubota et al., 2017]] ; [[#Wu--2018|]] [[#Wu--2018|H.C. Wu et al., 2018]] ).

Overall, many of the records show a highly variable seawater pH underlaying strong imprints of internal climate variability ( ''high confidence'' ) and, in most instances, superimposed on a decreasing δ <sup>11</sup> B trend that is indicative of anthropogenic ocean acidification in recent decades ( ''medium confidence'' ). The robustness of seawater pH reconstructions is currently limited by the uncertainty on the calibration of The δ <sup>11</sup> B proxy in different tropical coral species.

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==== 5.3.2.2 Observations of Ocean Acidification over Recent Decades ====

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The SROCC ( [[#5.2.2.3|Section 5.2.2.3]] ) indicated that it is ''virtually certain'' that the ocean has undergone acidification globally in response to ocean CO <sub>2</sub> uptake, and concluded that pH in open ocean surface water has changed by a ''virtually certain'' range of –0.017 to –0.027 pH units per decade since the late 1980s. Since SROCC, evidence of the progress of acidification across all regions of the oceans has been further strengthened by continued observations of seawater carbonate chemistry at ocean time series stations, and compiled shipboard studies providing temporally resolved and methodologically consistent datasets ( [[#Jiang--2019|Jiang et al., 2019]] ) (Figure 5.20; Supplementary Material Table 5.SM.3; [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.5|Section 2.3.3.5]] ).

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[[File:f8c1048a44db02ea97346655a5cec7e7 IPCC_AR6_WGI_Figure_5_20.png]]

'''Figure 5.20 |''' '''Multi-decadal trends of pH (Total Scale) in surface layer at various sites of the oceans and a global distribution of annual mean pH adjusted to the year 2000''' . Time-series data of pH are from [[#Dore--2009|Dore et al. (2009)]] , [[#Olafsson--2009|Olafsson et al. (2009)]] , [[#González-Dávila--2010|González-Dávila et al. (2010)]] , [[#Bates--2014|Bates et al. (2014)]] , [[#Takahashi--2014|Takahashi et al. (2014)]] , [[#Wakita--2017|Wakita et al. (2017)]] , [[#Merlivat--2018|Merlivat et al. (2018)]] , [[#Ono--2019|Ono et al. (2019)]] , and [[#Bates--2020|Bates and Johnson (2020)]] . Global distribution of annual mean pH have been evaluated from data of surface ocean ''p'' CO <sub>2</sub> <sup></sup> measurements ( [[#Bakker--2016|Bakker et al., 2016]] ; [[#Jiang--2019|Jiang et al., 2019]] ). Acronyms in panels: KNOT and K2 – Western Pacific subarctic gyre time series; HOT – Hawaii Ocean Time-series; BATS – Bermuda Atlantic Time-series Study; DYFAMED – Dynamics of Atmospheric Fluxes in the Mediterranean Sea; ESTOC – European Station for Time-series in the Ocean Canary Islands; CARIACO – Carbon Retention in a Colored Ocean Time-series. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

In the subtropical open oceans, decreases in pH have been reported with a ''very'' ''likely'' rate range from –0.016 to –0.019 pH units per decade since 1980s, which equates to approximately 4 % increase in hydrogen ion concentration ([H <sup>+</sup> ]) per decade. Accordingly, the saturation state Ω (=[Ca <sup>2+</sup> ][CO <sub>3</sub> <sup>2-</sup> ]/ ''K'' <sub>sp</sub> ) of seawater with respect to calcium carbonate mineral aragonite has been declining at rates ranging from –0.07 to –0.12 per decade ( [[#González-Dávila--2010|González-Dávila et al., 2010]] ; [[#Feely--2012|Feely et al., 2012]] ; [[#Bates--2014|Bates et al., 2014]] ; [[#Takahashi--2014|Takahashi et al., 2014]] ; [[#Ono--2019|Ono et al., 2019]] ; [[#Bates--2020|Bates and Johnson, 2020]] ; Supplementary Material Table 5.SM.3). These rates are consistent with the rates expected from the transient equilibration with increasing atmospheric CO <sub>2</sub> concentrations, but the variability of rate in decadal time scale has also been detected with ''robust evidence'' (Ono et al., 2019; [[#Bates--2020|Bates and Johnson, 2020]] ). In the tropical Pacific, its central and eastern upwelling zones exhibited a faster pH decline of –0.022 to –0.026 pH unit per decade due to increased upwelling of CO <sub>2</sub> -rich sub-surface waters in addition to anthropogenic CO <sub>2</sub> uptake ( [[#Sutton--2014|Sutton et al., 2014]] ; [[#Lauvset--2015|Lauvset et al., 2015]] ). By contrast, warm pools in the western tropical Pacific exhibited slower pH decline of –0.010 to –0.013 pH unit per decade (Supplementary Material Table 5.SM.3; Lauvsetet al., 2015; [[#Ishii--2020|Ishii et al., 2020]] ). Observational and modelling studies ( [[#Nakano--2015|Nakano et al., 2015]] ; [[#Ishii--2020|Ishii et al., 2020]] ) consistently suggest that slower acidification in this region is attributable to the anthropogenic CO <sub>2</sub> taken up in the extratropics around a decade ago and transported to the tropics via shallow meridional overturning circulations.

In open subpolar and polar zones, the ''very likely'' range (–0.003 to –0.026 pH unit per decade) and uncertainty (up to 0.010) observed in pH decline are larger than in the subtropics, reflecting the complex interplay between physical and biological forcing mechanisms ( [[#Olafsson--2009|Olafsson et al., 2009]] ; [[#Midorikawa--2012|Midorikawa et al., 2012]] ; [[#Bates--2014|Bates et al., 2014]] ; [[#Takahashi--2014|Takahashi et al., 2014]] ; [[#Lauvset--2015|Lauvset et al., 2015]] ; [[#Wakita--2017|Wakita et al., 2017]] ; [[#Merlivat--2018|Merlivat et al., 2018]] ). Nevertheless, the ''high agreement'' of pH decline among these available time-series studies leads to ''high confidence'' in the trend of acidification in these zones. In the Arctic Ocean, a temporally limited time series of carbonate chemistry measurements prevents drawing robust conclusions on ocean acidification trends. However, the carbonate saturation state (Ω) is generally low, and observational studies show with ''robust evidence'' that the recent extensive melting of sea ice leading to enhanced air–sea CO <sub>2</sub> exchange, large freshwater inputs, together with river discharge and glacial drainage, as well as the degradation of terrestrial organic matter in seawater, result in the decline of Ω of aragonite to undersaturation ( [[#Bates--2009|Bates et al., 2009]] ; [[#Chierici--2009|Chierici and Fransson, 2009]] ; [[#Yamamoto-Kawai--2009|Yamamoto-Kawai et al., 2009]] ; [[#Azetsu-Scott--2010|Azetsu-Scott et al., 2010]] ; [[#Robbins--2013|Robbins et al., 2013]] ; [[#Fransson--2015|Fransson et al., 2015]] ; [[#Semiletov--2016|Semiletov et al., 2016]] ; [[#Anderson--2017|Anderson et al., 2017]] ; [[#Qi--2017|Qi et al., 2017]] ; [[#Beaupré-Laperrière--2020|Beaupré-Laperrière et al., 2020]] ; Y. [[#Zhang--2020|]] [[#Zhang--2020|]] [[#Zhang--2020|Zhang et al., 2020]] ; SROCC [[IPCC:Wg1:Chapter:Chapter-3#3.2.1.2.4|Section 3.2.1.2.4]] , [[#IPCC--2019b|IPCC, 2019b]] ). The low saturation state of aragonite (Ω about 1) has also been observed in surface waters of the Antarctic coastal zone associated with freshwater input from glaciers ( [[#Mattsdotter%20Björk--2014|Mattsdotter Björk et al., 2014]] ) and upwelling of deep water ( [[#Hauri--2015|Hauri et al., 2015]] ) as well as along eastern boundary upwelling systems ( [[#Feely--2016|Feely et al., 2016]] ).

Overall, in agreement with SROCC, it is ''virtually certain'' from these observational studies that ocean surface waters undergo acidification globally with the CO <sub>2</sub> increase in the atmosphere. These sustained measurements over the past decades, and campaign studies of ocean carbonate chemistry, also highlight with ''robust evidence'' that trends of acidification have been modulated by the variability and changes in physical and chemical states of ocean, including those affected by the warming of the cryosphere, and need to be better understood.

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