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

==== 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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