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

=== 5.3.5 Coastal Ocean Acidification and Deoxygenation ===

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The coastal ocean, from the shoreline to the isobath of 200 m, is highly heterogeneous due to the complex interplay between physical, biogeochemical and anthropogenic factors ( [[#Gattuso--1998|Gattuso et al., 1998]] ; [[#Chen--2009|Chen and Borges, 2009]] ; [[#Dürr--2011|Dürr et al., 2011]] ; [[#Laruelle--2014|Laruelle et al., 2014]] ; [[#McCormack--2016|McCormack et al., 2016]] ). These areas, according to SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) are, with ''high confidence'' , already affected by ocean acidification and deoxygenation. This section assesses the drivers and spatial variability of acidification and deoxygenation based on new observations and data products.

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==== 5.3.5.1 Drivers ====

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Observations and data products including models ( [[#Astor--2013|Astor et al., 2013]] ; [[#Bakker--2016|Bakker et al., 2016]] ; [[#Kosugi--2016|Kosugi et al., 2016]] ; [[#Vargas--2016|Vargas et al., 2016]] ; [[#Laruelle--2017|Laruelle et al., 2017]] , 2018; [[#Orselli--2018|Orselli et al., 2018]] ; [[#Roobaert--2019|Roobaert et al., 2019]] ; [[#Cai--2020|Cai et al., 2020]] ; H. [[#Sun--2020|]] [[#Sun--2020|Sun et al., 2020]] ) confirm the strong spatial and temporal variability in the coastal ocean surface carbonate chemistry and sea-air CO <sub>2</sub> fluxes ( ''high agreement'' , ''robust evidence'' ). The anthropogenic CO <sub>2</sub> -induced acidification is either mitigated or enhanced through biological processes; primary production removes dissolved CO <sub>2</sub> from the surface, and respiration adds CO <sub>2</sub> and consumes oxygen in the subsurface layers. The relative intensity of these processes is controlled by natural or anthropogenic eutrophication. Other drivers of variability include biological community composition, freshwater input from rivers or melting ice, sea ice cover and calcium carbonate precipitation/dissolution dynamics, coastal upwelling and regional circulation, and seasonal surface cooling ( [[#Fransson--2015|Fransson et al., 2015]] , 2017; [[#Feely--2018|Feely et al., 2018]] ; [[#Roobaert--2019|Roobaert et al., 2019]] ; [[#Cai--2020|Cai et al., 2020]] ; [[#Hauri--2020|Hauri et al., 2020]] ; [[#Monteiro--2020b|Monteiro et al., 2020b]] ; H. [[#Sun--2020|]] [[#Sun--2020|Sun et al., 2020]] ). Near-shore surface waters are often supersaturated with CO <sub>2</sub> , regardless of the latitude, especially in highly populated areas receiving substantial amounts of domestic and industrial sewage ( [[#Chen--2009|Chen and Borges, 2009]] ). Nevertheless, thermal or haline-stratified eutrophic coastal areas may act as net atmospheric CO <sub>2</sub> sinks ( [[#Chou--2013|Chou et al., 2013]] ; [[#Cotovicz%20Jr.--2015|Cotovicz Jr. et al., 2015]] ). Continental shelves, excluding near-shore areas, act as CO <sub>2</sub> sinks at a rate of 0.2 ± 0.02 PgC yr <sup>–1</sup> ( [[#Laruelle--2014|Laruelle et al., 2014]] ; [[#Roobaert--2019|Roobaert et al., 2019]] ), considering ice-free areas only. Under increasing atmospheric CO <sub>2</sub> and eutrophication, such ecosystems would be more vulnerable to ecological and seawater chemistry changes, impacting the local economy.

Since AR5, ( [[#Ciais--2013|Ciais et al., 2013]] ) and in agreement with SROCC ( [[#IPCC--2019b|IPCC, 2019b]] ), there is now ''high agreement (robust evidence)'' that coastal ocean acidification, whether induced only by increasing atmospheric CO <sub>2</sub> or exacerbated by eutrophication or upwelling, has negative effects on specific groups of marine organisms such as reef-building corals, crabs, pteropods, and sessile fauna (AR6 WGII, Chapter 3; [[#Dupont--2010|Dupont et al., 2010]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Bednaršek--2020|Bednaršek et al., 2020]] ; [[#Osborne--2020|Osborne et al., 2020]] ), especially when combined with stressors such as temperature and deoxygenation, and potentially increased bioavailability of toxic elements such as arsenic and copper ( [[#Millero--2009|Millero et al., 2009]] ; [[#Boyd--2015|Boyd et al., 2015]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ).

Since SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ), there is further evidence that anthropogenic eutrophication via continental runoff and atmospheric nutrient deposition, and ocean warming are ''very likely'' the main drivers of deoxygenation in coastal areas ( [[#Levin--2015|Levin and Breitburg, 2015]] ; [[#Levin--2015|Levin et al., 2015]] ; [[#Royer--2016|Royer et al., 2016]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ; [[#Cocquempot--2019|Cocquempot et al., 2019]] ; [[#Fagundes--2020|Fagundes et al., 2020]] ; [[#Limburg--2020|Limburg et al., 2020]] ). Increasing intensity and frequency of wind-driven upwelling is responsible for longer and more intense coastal hypoxia, fuelled by organic matter degradation from primary production ''(medium to high agreement, medium evidence)'' ( [[#Rabalais--2010|Rabalais et al., 2010]] ; [[#Bakun--2015|Bakun et al., 2015]] ; [[#Varela--2015|Varela et al., 2015]] ; [[#Fennel--2019|Fennel and Testa, 2019]] ; [[#Limburg--2020|Limburg et al., 2020]] ). Locally, submarine groundwater discharge may enhance the eutrophication state ( ''low agreement'' , ''limited evidence'' , [[#Luijendijk--2020|Luijendijk et al., 2020]] ). Since AR5 ( [[#Ciais--2013|Ciais et al., 2013]] ) and SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) new observations and model studies confirm the trends in increasing coastal hypoxia caused by eutrophication, ocean warming and changes in circulation ( [[#Claret--2018|Claret et al., 2018]] ; [[#Dussin--2019|Dussin et al., 2019]] ; [[#Limburg--2020|Limburg et al., 2020]] ), as well as the ubiquitous impacts on marine organisms and fisheries (AR6 WGII Chapter 3; [[#Carstensen--2019|Carstensen and Conley, 2019]] ; [[#Fennel--2019|Fennel and Testa, 2019]] ; [[#Osma--2020|Osma et al., 2020]] ). Following open ocean deoxygenation trends since the 1950s, more than 700 coastal regions are being reported as hypoxic (dissolved oxygen concentration &lt;2 mg O <sub>2</sub> L <sup>–1</sup> ) ( [[#Limburg--2020|Limburg et al., 2020]] ). Additionally, deoxygenation or increasing severe hypoxic periods in coastal areas may enhance the sea-to-air fluxes of N <sub>2</sub> O and CH <sub>4</sub> especially through microbial-mediated processes in the water column–sediment interface ( ''medium agreement'' ) ( [[#Middelburg--2009|Middelburg and Levin, 2009]] ; [[#Naqvi--2010|Naqvi et al., 2010]] ; [[#Farías--2015|Farías et al., 2015]] ; [[#Limburg--2020|Limburg et al., 2020]] ).

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==== 5.3.5.2 Spatial Characteristics ====

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There is ''high agreement'' ( ''robust evidence'' ) that heterogeneity implies different responses of coastal regions to increasing atmospheric CO <sub>2</sub> , decreasing seawater pH and calcium carbonate saturation state, and deoxygenation ( [[#Duarte--2013|Duarte et al., 2013]] ; [[#Regnier--2013|Regnier et al., 2013]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ; [[#Laruelle--2018|Laruelle et al., 2018]] ; [[#Carstensen--2019|Carstensen and Duarte, 2019]] ).

There is ''high agreement'' that long-time series of observations utilizing standard methods are needed to distinguish the climate change signal in seawater carbonate chemistry from the natural variability of coastal sites ( [[#Duarte--2013|Duarte et al., 2013]] ; [[#Salisbury--2018|Salisbury and Jönsson, 2018]] ; [[#IOC--2019|IOC, 2019]] ; [[#Sutton--2019|Sutton et al., 2019]] ; [[#Tilbrook--2019|Tilbrook et al., 2019]] ; [[#Turk--2019|Turk et al., 2019]] ). Despite the increasing availability of data and sea–air CO <sub>2</sub> flux budgets for the coastal ocean (Sections 5.3.5.1 and 5.2.3.1), additional long-term observations are required to constrain the global time of emergence of coastal acidification. There is ''high agreement'' ( ''medium evidence'' ) that, for the coastal subtropical to temperate north-east Pacific and north-west Atlantic, the mean time of emergence for acidification is above two decades ( [[#Sutton--2019|Sutton et al., 2019]] ; [[#Turk--2019|Turk et al., 2019]] ).

Observations and models predict an expansion and intensification of low-pH deep water intrusions for the north-east Pacific coastal upwelling area ( ''high agreement'' , ''robust evidence'' ) ( [[#Hauri--2013|Hauri et al., 2013]] ; [[#Feely--2016|Feely et al., 2016]] ; [[#Cai--2020|Cai et al., 2020]] ). Areas such as the California Current System are naturally exposed to intrusions of low‐pH, high pCO <sub>2sea</sub> deep waters from remineralization processes and anthropogenic CO <sub>2</sub> intrusion ( [[#Feely--2008|Feely et al., 2008]] , 2010, 2018; [[#Chan--2019|Chan et al., 2019]] ; [[#Lilly--2019|Lilly et al., 2019]] ; [[#Cai--2020|Cai et al., 2020]] ).The eastern Pacific coastal upwelling displays seasonality in subsurface aragonite undersaturation as a consequence of the interplay between anthropogenic CO <sub>2</sub> , respiration and intrusion of upwelling waters ( [[#Feely--2008|Feely et al., 2008]] , 2010, 2016, 2018; [[#Hauri--2013|Hauri et al., 2013]] ; [[#Vargas--2016|Vargas et al., 2016]] ; [[#Chan--2019|Chan et al., 2019]] ; [[#Lilly--2019|Lilly et al., 2019]] ). The coastal south-east Pacific upwelling combined with low-pH, low-alkalinity, organic matter-rich river inputs display extreme temporal variability in surface seawater ''p'' CO <sub>2</sub> and low aragonite saturation ( [[#Vargas--2016|Vargas et al., 2016]] ; [[#Osma--2020|Osma et al., 2020]] ).

Temperate, non-upwelling coastal areas along the north-west Atlantic display a trend of decreasing seawater pH, mainly attributed to the combined effects of eutrophication and decreasing seawater buffering capacity ( ''high agreement'' , ''robust evidence'' ). Observations show an increasing north to south gradient of aragonite saturation state ( [[#Sutton--2016|Sutton et al., 2016]] ; [[#Fennel--2019|Fennel et al., 2019]] ; [[#Cai--2020|Cai et al., 2020]] ). Low alkalinity and total inorganic carbon concentration, combined with an ocean signal of acidification, diminishes the buffering capacity along the decreasing salinity gradient from the ocean to the coast. Models suggest that, in this area, the aragonite saturation is seasonally controlled by nutrient availability and primary production, supporting the finding that eutrophication is the main driver for exacerbating acidification ( [[#Cai--2017|Cai et al., 2017]] , 2020). The coastal Gulf of Mexico is facing a parallel increase in bottom water acidification and deoxygenation off the Mississippi Delta driven by eutrophication ( [[#Cai--2011|Cai et al., 2011]] ; [[#Laurent--2017|Laurent et al., 2017]] ; [[#Fennel--2019|Fennel et al., 2019]] ).

Many coastal tropical areas are under heavy anthropogenic eutrophication induced by the effluents from large cities, or receive large riverine inputs of freshwater, nutrients, and organic matter (such as Amazon, Mississippi, Orinoco, Congo, Mekong, or Changjiang rivers). Under strong eutrophication, often sub-surface and bottom waters present pH values lower than average surface open ocean (about 8.0) because increased respiration decreases pH ( ''high agreement'' , ''robust evidence'' ), despite a net atmospheric CO <sub>2</sub> sink in shallow and vertically stratified coastal areas ( [[#Koné--2009|Koné et al., 2009]] ; [[#Wallace--2014|Wallace et al., 2014]] ; [[#Cotovicz%20Jr.--2015|Cotovicz Jr. et al., 2015]] , 2018; [[#Fennel--2019|Fennel and Testa, 2019]] ; [[#Lowe--2019|Lowe et al., 2019]] ; [[#5.3.5.1|Section 5.3.5.1]] ).

There is ''medium evidence'' from observations and models that the coastal north-western Antarctic Peninsula (Southern Ocean) will experience calcium carbonate undersaturation by 2060, considering that anthropogenic emissions reach an atmospheric CO <sub>2</sub> concentration of about 500 pm at that date ( [[#Lencina-Avila--2018|Lencina-Avila et al., 2018]] ; [[#Monteiro--2020a|Monteiro et al., 2020a]] ). The synergies among warming, melt water, sea-air CO <sub>2</sub> equilibrium and circulation may, to some extent, offset the coastal ocean acidification trends in Antarctica ( [[#Henley--2020|Henley et al., 2020]] ). In the coastal western Arctic Ocean, there is increasing ''robust evidence'' that ocean acidification is driven by sea-air CO <sub>2</sub> fluxes and sea-ice melt, and increasing intrusions since the 1990s of low-alkalinity Pacific water, lowering aragonite saturation state ( [[#Qi--2017|Qi et al., 2017]] , 2020; [[#Cross--2018|Cross et al., 2018]] ). The Bering Sea (north-eastern Pacific) shows decreasing trends in calcium carbonate saturation, associated to the increasing atmospheric CO <sub>2</sub> uptake combined with riverine freshwater and carbon inputs ( ''high agreement'' , ''robust evidence'' ) ( [[#Pilcher--2019|Pilcher et al., 2019]] ; H. [[#Sun--2020|]] [[#Sun--2020|Sun et al., 2020]] ).

The spatial distribution of hypoxic areas is highly heterogeneous in the coastal ocean, and there is ''high agreement, robust evidence'' that more severe hypoxia or anoxia is often associated with highly populated coastal areas,or local circulation and upwelling, and seasonal stratification leading to an accumulation of organic matter in subsurface waters (Ciais et al., 2013; [[#Rabalais--2014|Rabalais et al., 2014]] ; M. [[#Li--2016|]] [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; SROCC Chapter 5). The causes and trends of coastal deoxygenation can only be assessed by making available long-term time series combined with regional modelling ( [[#Fennel--2019|Fennel and Testa, 2019]] ), as in the California current system ( [[#Wang--2017|Wang et al., 2017]] ), the East China Sea ( [[#Chen--2007|Chen et al., 2007]] ; [[#Qian--2017|Qian et al., 2017]] ), the Namibian or along the north-western Atlantic shelves ( [[#Claret--2018|Claret et al., 2018]] ). Other coastal upwelling sites such as the Arabian Sea display seasonal hypoxia but no worsening trends ( [[#Gupta--2016|Gupta et al., 2016]] ).

The Baltic Sea is the largest semi-enclosed sea where hypoxia is reported to have happened before the 1950s ( [[#Carstensen--2014|Carstensen et al., 2014]] ; [[#Rabalais--2014|Rabalais et al., 2014]] ; [[#Łukawska-Matuszewska--2019|Łukawska-Matuszewska et al., 2019]] ). The frequency and volume of seawater inflow from the North Sea decreased after 1950, leading to an expansion of hypoxic areas from 40,000 to 60,000 km² in combination with increasing eutrophication ( [[#Carstensen--2014|Carstensen et al., 2014]] ). From the available observations, there is ''robust evidence'' that many areas in the Baltic Sea are experiencing deoxygenation despite efforts to reduce nutrient loads ( [[#Lennartz--2014|Lennartz et al., 2014]] ; [[#Jokinen--2018|Jokinen et al., 2018]] ).

There is ''medium agreement'' ( ''medium evidence'' ) that simply reducing anthropogenic nutrient inputs may lead to less severe coastal hypoxic conditions, as observed in the coastal north-western Adriatic Sea ( [[#Djakovac--2015|Djakovac et al., 2015]] ). However, low-oxygen sediments may remain a long-term source of phosphorus and ammonium to the water column, and in this way fuelling primary production ( [[#Jokinen--2018|Jokinen et al., 2018]] ; [[#Fennel--2019|Fennel and Testa, 2019]] ; [[#Limburg--2020|Limburg et al., 2020]] ).

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