View source for Ocean acidification

{{Short description|Decrease of pH levels in the ocean}}
{{Use dmy dates|date=August 2019}}
[[File:Mean-seawater-ph.png|thumb|upright=1.6|Ocean acidification means that the average [[seawater]] [[pH value]] is dropping over time.<ref name=":15">[[Hannah Ritchie|Ritchie]], [[Max Roser|Roser]], Mispy, Ortiz-Ospina. "[https://sdg-tracker.org/oceans SDG 14 – Measuring progress towards the Sustainable Development Goals] {{Webarchive|url=https://web.archive.org/web/20220122161929/https://sdg-tracker.org/oceans |date=22 January 2022 }}." ''SDG-Tracker.org, website'' (2018).</ref>]]
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'''Ocean acidification''' is the ongoing decrease in the [[pH]] of the Earth's [[ocean]]. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.<ref name="role">{{Cite journal |last1=Terhaar |first1=Jens |last2=Frölicher |first2=Thomas L. |last3=Joos |first3=Fortunat |date=2023 |title=Ocean acidification in emission-driven temperature stabilization scenarios: the role of TCRE and non-{{CO2}} greenhouse gases |url=https://dx.doi.org/10.1088/1748-9326/acaf91 |journal=Environmental Research Letters |language=en |volume=18 |issue=2 |pages=024033 |bibcode=2023ERL....18b4033T |doi=10.1088/1748-9326/acaf91 |issn=1748-9326 |s2cid=255431338 |quote=Figure 1f}}</ref> [[Carbon dioxide emissions]] from human activities are the primary cause of ocean acidification, with [[Carbon dioxide in Earth's atmosphere|atmospheric carbon dioxide ({{CO2}}) levels]] exceeding 422 ppm ({{as of|2024|lc=y}}).<ref>{{Cite web|url=https://www.co2.earth/|title=Earth's CO2 Home Page|last=Oxygen|first=Pro|date=2024-09-21|access-date=2024-09-21}}</ref>  {{CO2}} from the [[atmosphere]] is absorbed by the oceans. This chemical reaction produces [[carbonic acid]] ({{chem2|H2CO3}}) which [[dissociate]]s into a [[bicarbonate ion]] ({{Chem2|HCO3-}}) and a [[hydrogen ion]] ({{chem2|H+}}). The presence of free hydrogen ions ({{chem2|H+}}) lowers the pH of the ocean, increasing [[acid]]ity (this does not mean that [[seawater]] is acidic yet; it is still [[alkali]]ne, with a pH higher than 8). [[Marine biogenic calcification|Marine calcifying organisms]], such as [[Mollusca|mollusks]] and [[coral]]s, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.<ref name="raven05" />

A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more {{CO2}}. This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. There are several other factors that influence the atmosphere-ocean {{CO2}} exchange, and thus local ocean acidification. These include [[ocean current]]s and [[upwelling]] zones, proximity to large continental rivers, [[sea ice]] coverage, and atmospheric exchange with [[nitrogen]] and [[sulfur]] from [[fossil fuel]] burning and [[agriculture]].<ref name="Jian2019" /><ref>{{Cite journal |last1=Zhang |first1=Y. |last2=Yamamoto-Kawai |first2=M. |last3=Williams |first3=W.J. |date=2020-02-16 |title=Two Decades of Ocean Acidification in the Surface Waters of the Beaufort Gyre, Arctic Ocean: Effects of Sea Ice Melt and Retreat From 1997–2016 |journal=Geophysical Research Letters |volume=47 |issue=3 |doi=10.1029/2019GL086421 |s2cid=214271838 |doi-access=free}}</ref><ref>{{Cite journal |last1=Beaupré-Laperrière |first1=Alexis |last2=Mucci |first2=Alfonso |last3=Thomas |first3=Helmuth |date=2020-07-31 |title=The recent state and variability of the carbonate system of the Canadian Arctic Archipelago and adjacent basins in the context of ocean acidification |journal=Biogeosciences |volume=17 |issue=14 |pages=3923–3942 |bibcode=2020BGeo...17.3923B |doi=10.5194/bg-17-3923-2020 |s2cid=221369828 |doi-access=free}}</ref>

A lower ocean pH has a range of potentially harmful effects for [[Marine life|marine organisms]]. Scientists have observed for example reduced calcification, lowered [[immune response]]s, and reduced energy for basic functions such as reproduction.<ref name="bleaching">{{Cite journal |last1=Anthony |first1=K. R. N. |last2=Kline |first2=D. I. |last3=Diaz-Pulido |first3=G. |last4=Dove |first4=S. |last5=Hoegh-Guldberg |first5=O. |date=11 November 2008 |title=Ocean acidification causes bleaching and productivity loss in coral reef builders |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=45 |pages=17442–17446 |bibcode=2008PNAS..10517442A |doi=10.1073/pnas.0804478105 |pmc=2580748 |pmid=18988740 |doi-access=free}}</ref> Ocean acidification can impact [[marine ecosystem]]s that provide food and livelihoods for many people. About one billion people are wholly or partially dependent on the fishing, tourism, and coastal management services provided by [[coral reef]]s. Ongoing acidification of the oceans may therefore threaten [[food chain]]s linked with the oceans.<ref name="auto">{{Cite news |last=Dean |first=Cornelia |date=30 January 2009 |title=Rising Acidity Is Threatening Food Web of Oceans, Science Panel Says |url=https://www.nytimes.com/2009/01/31/science/earth/31ocean.html |work=New York Times}}</ref><ref name=":18">{{Cite journal |last=Service |first=Robert E. |date=13 July 2012 |title=Rising Acidity Brings an Ocean of Trouble |journal=Science |volume=337 |issue=6091 |pages=146–148 |bibcode=2012Sci...337..146S |doi=10.1126/science.337.6091.146 |pmid=22798578}}</ref>

One of the only solutions that would address the root cause of ocean acidification is reducing carbon dioxide emissions. This is one of the main objectives of [[climate change mitigation]] measures. [[carbon dioxide removal|The removal of carbon dioxide]] from the atmosphere would also help to reverse ocean acidification. In addition, there are some specific [[Carbon sequestration#Sequestration techniques in oceans|ocean-based mitigation methods]], for example ocean [[alkalinity]] enhancement and [[enhanced weathering]]. These strategies are under investigation, but generally have a low [[technology readiness level]] and many risks.<ref>IPCC (2022) [https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_Chapter_12.pdf Chapter 12: Cross sectoral perspectives]</ref><ref>{{Webarchive|url=https://web.archive.org/web/20221013122242/https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_Chapter_12.pdf|date=13 October 2022}} in [https://www.ipcc.ch/report/ar6/wg3/ Climate Change 2022: Mitigation of Climate Change] Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change]</ref><ref>{{Webarchive|url=https://web.archive.org/web/20220802125242/https://www.ipcc.ch/report/ar6/wg3/|date=2 August 2022}}, Cambridge University Press, Cambridge, United Kingdom and New York, NY, US{{rp|12–36}}</ref>

Ocean acidification has happened before in Earth's geologic history.<ref name=":9">{{cite Q|Q28261134}}</ref> The resulting ecological collapse in the oceans had long-lasting effects on the [[Carbon cycle|global carbon cycle]] and [[Climate variability and change|climate]].

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==Cause==
[[File:Spatial distribution of global surface ocean pH.webp|thumb|upright=1.2|Spatial distribution of global surface ocean pH (Panel a: the annually-averaged surface ocean pH to be approximate for the year 1770; Panel b: the difference between pH in 2000 and 1770 in the global surface ocean).<ref name="Jian2019" />]]
[[File:Carbon cycle.jpg|thumb|upright=1.2|This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon.<ref>{{Cite web |date=2011-06-16 |title=The Carbon Cycle |url=https://earthobservatory.nasa.gov/features/CarbonCycle |access-date=2022-11-16 |website=earthobservatory.nasa.gov}}</ref>]]
[[File:Impacts of ocean acidification (NOAA EVL).webm|thumb|upright=1.5|right|alt=refer to caption and image description|Video summarizing the impacts of ocean acidification. Source: [[NOAA]] Environmental Visualization Laboratory.]]
{{see also|Oceanic carbon cycle}}In 2021, atmospheric carbon dioxide (CO<sub>2</sub>) levels of around 415 ppm were around 50% higher than preindustrial concentrations.<ref name="carbon budget 2022">{{Cite journal |last1=Friedlingstein |first1=Pierre |last2=O'Sullivan |first2=Michael |last3=Jones |first3=Matthew W. |last4=Andrew |first4=Robbie M. |last5=Gregor |first5=Luke |last6=Hauck |first6=Judith |last7=Le Quéré |first7=Corinne |last8=Luijkx |first8=Ingrid T. |last9=Olsen |first9=Are |last10=Peters |first10=Glen P. |last11=Peters |first11=Wouter |last12=Pongratz |first12=Julia |last13=Schwingshackl |first13=Clemens |last14=Sitch |first14=Stephen |last15=Canadell |first15=Josep G. |date=2022-11-11 |title=Global Carbon Budget 2022 |journal=Earth System Science Data |volume=14 |issue=11 |pages=4811–4900 |bibcode=2022ESSD...14.4811F |doi=10.5194/essd-14-4811-2022 |doi-access=free}} [[File:CC-BY icon.svg|50px]] Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=16 October 2017 }}</ref> According to the National Oceanic and Atmospheric Administration in 2023, atmospheric CO2 levels have risen from approximately 280 parts per million (ppm) in the pre-industrial era to over 410 ppm today, primarily due to human activities such as fossil fuel combustion and deforestation.<ref>{{Cite journal |last1=Çelekli |first1=Abuzer |last2=Zariç |first2=Özgür Eren |date=2024-12-31 |title=Rising tide of ocean acidification |url=http://dergipark.org.tr/en/doi/10.35208/ert.1407588 |journal=Environmental Research and Technology |volume=7 |issue=4 |pages=605–613 |doi=10.35208/ert.1407588 |issn=2636-8498}}</ref> The current elevated levels and rapid growth rates are unprecedented in the past 55 million years of the geological record. The sources of this excess CO<sub>2</sub> are clearly established as human driven: they include anthropogenic fossil fuel, industrial, and land-use/land-change emissions. One source of this is fossil fuels, which are burned for energy. When burned, CO<sub>2</sub> is released into the atmosphere as a byproduct of combustion, which is a significant contributor to the increasing levels of CO<sub>2</sub> in the Earth's atmosphere.<ref>{{Cite journal |last1=Doney |first1=Scott C. |last2=Fabry |first2=Victoria J. |last3=Feely |first3=Richard A. |last4=Kleypas |first4=Joan A. |date=2009-01-01 |title=Ocean Acidification: The Other CO 2 Problem |url=https://www.annualreviews.org/doi/10.1146/annurev.marine.010908.163834 |journal=Annual Review of Marine Science |language=en |volume=1 |issue=1 |pages=169–192 |doi=10.1146/annurev.marine.010908.163834 |issn=1941-1405 |pmid=21141034}}</ref> The ocean acts as a [[carbon sink]] for anthropogenic CO<sub>2</sub> and takes up roughly a quarter of total anthropogenic CO<sub>2</sub> emissions.<ref name=":14">{{Cite journal |last1=Doney |first1=Scott C. |last2=Busch |first2=D. Shallin |last3=Cooley |first3=Sarah R. |last4=Kroeker |first4=Kristy J. |date=2020-10-17 |title=The Impacts of Ocean Acidification on Marine Ecosystems and Reliant Human Communities |journal=Annual Review of Environment and Resources |volume=45 |issue=1 |pages=83–112 |doi=10.1146/annurev-environ-012320-083019 |s2cid=225741986 |doi-access=free}} [[File:CC-BY icon.svg|50x50px]] Text was copied from this source, which is available under a [[creativecommons:by/4.0/|Creative Commons Attribution 4.0 International License]]</ref> However, the additional CO<sub>2</sub> in the ocean results in a wholesale shift in [[seawater]] acid-base chemistry toward more acidic, lower pH conditions and lower saturation states for carbonate minerals used in many marine organism shells and skeletons.<ref name=":14" />

Accumulated since 1850, the ocean sink holds up to {{val|175|35|ul=gigatons}} of carbon, with more than two-thirds of this amount ({{val|120|ul=Gt}} C) being taken up by the global ocean since 1960. Over the historical period, the ocean sink increased in pace with the exponential anthropogenic emissions increase. From 1850 until 2022, the ocean has absorbed 26% of total anthropogenic emissions.<ref name="carbon budget 2022" /> Emissions during the period 1850–2021 amounted to {{val|670|65|u=gigatons}} of carbon and were partitioned among the atmosphere (41%), ocean (26%), and land (31%).<ref name="carbon budget 2022" />

The [[carbon cycle]] describes the fluxes of carbon dioxide ({{chem|CO|2}}) between the oceans, [[Earth|terrestrial]] [[biosphere]], [[lithosphere]],<ref>{{Cite encyclopedia |title=carbon cycle |encyclopedia=[[Encyclopædia Britannica Online]] |url=http://www.search.eb.com/eb/article-9020247 |access-date=11 February 2010}}</ref> and [[atmosphere]]. The carbon cycle involves both [[organic compound]]s such as [[cellulose]] and inorganic carbon compounds such as [[carbon dioxide]], [[carbonate|carbonate ion]], and [[Bicarbonate|bicarbonate ion]], together referenced as dissolved inorganic carbon (DIC). These inorganic compounds are particularly significant in ocean acidification, as they include many forms of dissolved {{chem|CO|2}} present in the Earth's oceans.<ref>{{Cite book |last1=Kump |first1=Lee R. |title=The Earth System |last2=Kasting |first2=James F. |author-link2=James Kasting |last3=Crane |first3=Robert G. |publisher=Upper Saddle River: Prentice Hall |year=2003 |isbn=978-0-613-91814-5 |edition=2nd |pages=162–164}}</ref>

When {{chem|CO|2}} dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide ({{chem|CO|2(aq)}}), [[carbonic acid]] ({{chem|H|2|CO|3}}), [[bicarbonate]] ({{chem|HCO|3|-}}) and carbonate ({{chem|CO|3|2-}}). The ratio of these species depends on factors such as [[seawater]] [[temperature]], pressure and salinity (as shown in a [[Bjerrum plot]]). These different forms of [[Total inorganic carbon|dissolved inorganic carbon]] are transferred from an ocean's surface to its interior by the ocean's [[solubility pump]]. The resistance of an area of ocean to absorbing atmospheric {{chem|CO|2}} is known as the [[Revelle factor]].

== Main effects ==
The [[Marine chemistry|ocean's chemistry]] is changing due to the uptake of anthropogenic carbon dioxide (CO<sub>2</sub>).<ref name="Jian2019">{{Cite journal |last1=Jiang |first1=Li-Qing |last2=Carter |first2=Brendan R. |last3=Feely |first3=Richard A. |last4=Lauvset |first4=Siv K. |last5=Olsen |first5=Are |date=2019 |title=Surface ocean pH and buffer capacity: past, present and future |journal=Scientific Reports |volume=9 |issue=1 |pages=18624 |bibcode=2019NatSR...918624J |doi=10.1038/s41598-019-55039-4 |pmc=6901524 |pmid=31819102 |doi-access=free}} [[File:CC-BY icon.svg|50px]] Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=16 October 2017 }}</ref><ref name=":43">Cooley, S., D. Schoeman, L. Bopp, P. Boyd, S. Donner, D.Y. Ghebrehiwet, S.-I. Ito, W. Kiessling, P. Martinetto, E. Ojea, M.-F. Racault, B. Rost, and M. Skern-Mauritzen, 2022: [https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Chapter03.pdf Chapter 3: Oceans and Coastal Ecosystems and Their Services] {{Webarchive|url=https://web.archive.org/web/20221021104519/https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Chapter03.pdf |date=21 October 2022 }}. In: [https://www.ipcc.ch/report/ar6/wg2/ Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20220228114918/https://www.ipcc.ch/report/ar6/wg2/ |date=28 February 2022 }} [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, US, pp. 379–550.</ref>{{rp|395}} Ocean pH, carbonate ion concentrations ([CO<sub>3</sub><sup>2−</sup>]), and calcium carbonate mineral saturation states (Ω) have been declining as a result of the uptake of approximately 30% of the anthropogenic carbon dioxide emissions over the past 270 years (since around 1750). This process, commonly referred to as "ocean acidification", is making it harder for [[Marine biogenic calcification|marine calcifiers]] to build a shell or skeletal structure, endangering coral reefs and the broader marine ecosystems.<ref name="Jian2019" />

Ocean acidification has been called the "evil twin of [[global warming]]" and "the other CO<sub>2</sub> problem".<ref name="rsc">{{Cite web |last=Notman |first=Nina |date=29 July 2014 |title=The other carbon dioxide problem |url=http://www.rsc.org/chemistryworld/2014/07/ocean-acidification |website=Chemistry World}}</ref><ref>{{Cite web |last=Rogers |first=Alex |date=9 October 2013 |title=Global warming's evil twin: ocean acidification |url=http://theconversation.com/global-warmings-evil-twin-ocean-acidification-19017 |website=The Conversation}}</ref> Increased ocean temperatures and oxygen loss act concurrently with ocean acidification and constitute the "deadly trio" of climate change pressures on the marine environment.<ref>{{Cite web |date=November 2017 |title=Ocean acidification (Issues Brief) |url=https://www.iucn.org/sites/dev/files/ocean_acidification_issues_brief.pdf |access-date=3 November 2020 |website=IUCN (International Union for Conservation of Nature)}}</ref> The impacts of this will be most severe for coral reefs and other shelled marine organisms,<ref>{{Cite journal |last1=Brander |first1=Luke M. |last2=Rehdanz |first2=Katrin |last3=Tol |first3=Richard S. J. |last4=Van Beukering |first4=Pieter J. H. |date=2012-02-01 |title=The economic impact of ocean acidification on coral reefs |journal=Climate Change Economics |volume=03 |issue=1 |pages=1250002 |doi=10.1142/S2010007812500029 |bibcode=2012ClChE...350002B |hdl-access=free |hdl=2262/27779|url=https://research.vu.nl/en/publications/4f6326ec-b0ff-4431-a201-cac1e7a0b7dd }}</ref><ref>{{Cite journal |last1=Allemand |first1=Denis |last2=Osborn |first2=David |date=2019 |title=Ocean acidification impacts on coral reefs: From sciences to solutions |journal=Regional Studies in Marine Science |volume=28 |pages=100558 |bibcode=2019RSMS...2800558A |doi=10.1016/j.rsma.2019.100558 |s2cid=135282569 |doi-access=free}}</ref> as well as those populations that depend on the ecosystem services they provide.

=== Reduction in pH value ===
{{See also|Ocean#pH and alkalinity}}
Dissolving {{chem|CO|2}} in seawater increases the [[hydrogen]] ion ({{chem|H|+}}) concentration in the ocean, and thus decreases ocean pH, as follows:<ref name="IPCC">{{Cite book |last1=Freund |first1=Paul |url=http://www.ipcc.ch/pdf/special-reports/srccs/srccs_wholereport.pdf |title=IPCC Special Report on Carbon Dioxide Capture and Storage |last2=Bachu |first2=Stefan |last3=Simbeck |first3=Dale |last4=Thambimuthu |first4=Kelly (Kailai) |last5=Gupta |first5=Murlidhar |publisher=IPCC |year=2005 |editor-last=Metz |editor-first=Bert |page=390 |chapter=Annex I: Properties of {{CO2}} and carbon-based fuels |access-date=1 November 2014 |editor-last2=Davidson |editor-first2=Ogunlade |editor-last3=de Coninck |editor-first3=Heleen |editor-last4=Loos |editor-first4=Manuela |editor-last5=Meyer |editor-first5=Leo |archive-url=https://web.archive.org/web/20100210022620/http://www.ipcc.ch/pdf/special-reports/srccs/srccs_wholereport.pdf |archive-date=10 February 2010}}</ref>{{center|1={{CO2}} <sub>(aq)</sub> + H<sub>2</sub>O {{eqm}} H<sub>2</sub>CO<sub>3</sub> {{eqm}} HCO<sub>3</sub><sup>−</sup> + H<sup>+</sup> {{eqm}} CO<sub>3</sub><sup>2−</sup> + 2 H<sup>+</sup>.}}

In shallow coastal and shelf regions, a number of factors interplay to affect air-ocean {{CO2}} exchange and resulting pH change.<ref name="Carstensen2019">{{Cite journal |last1=Carstensen |first1=Jacob |last2=Duarte |first2=Carlos M. |date=2019-04-16 |title=Drivers of pH Variability in Coastal Ecosystems |journal=Environmental Science & Technology |volume=53 |issue=8 |pages=4020–4029 |bibcode=2019EnST...53.4020C |doi=10.1021/acs.est.8b03655 |pmid=30892892 |s2cid=84841808}}</ref><ref>{{Cite journal |last1=Duarte |first1=Carlos M. |last2=Hendriks |first2=Iris E. |last3=Moore |first3=Tommy S. |last4=Olsen |first4=Ylva S. |last5=Steckbauer |first5=Alexandra |last6=Ramajo |first6=Laura |last7=Carstensen |first7=Jacob |last8=Trotter |first8=Julie A. |last9=McCulloch |first9=Malcolm |date=2013-03-01 |title=Is Ocean Acidification an Open-Ocean Syndrome? Understanding Anthropogenic Impacts on Seawater pH |journal=Estuaries and Coasts |volume=36 |issue=2 |pages=221–236 |doi=10.1007/s12237-013-9594-3 |doi-access=free|bibcode=2013EstCo..36..221D }}</ref> These include biological processes, such as [[photosynthesis]] and respiration,<ref>{{Cite journal |last1=Lowe |first1=Alexander T. |last2=Bos |first2=Julia |last3=Ruesink |first3=Jennifer |date=2019-01-30 |title=Ecosystem metabolism drives pH variability and modulates long-term ocean acidification in the Northeast Pacific coastal ocean |journal=Scientific Reports |volume=9 |issue=1 |pages=963 |bibcode=2019NatSR...9..963L |doi=10.1038/s41598-018-37764-4 |pmc=6353961 |pmid=30700764}}</ref> as well as water upwelling.<ref>{{Cite journal |last1=Fairchild |first1=William |last2=Hales |first2=Burke |date=2021 |title=High-Resolution Carbonate System Dynamics of Netarts Bay, OR From 2014 to 2019 |journal=Frontiers in Marine Science |volume=7 |doi=10.3389/fmars.2020.590236 |doi-access=free|bibcode=2021FrMaS...790236F }}</ref> Also, [[Stream metabolism|ecosystem metabolism]] in freshwater sources reaching coastal waters can lead to large, but local, pH changes.<ref name="Carstensen2019" />

Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious phenomenon.<ref name="Gies2018">{{Cite web |last=Gies |first=E. |date=11 January 2018 |title=Like Oceans, Freshwater Is Also Acidifying |url=https://www.scientificamerican.com/article/like-oceans-freshwater-is-also-acidifying/ |access-date=2018-01-13 |website=[[Scientific American]]}}</ref><ref name="Weiss2018">{{Cite journal |last1=Weiss |first1=L. C. |last2=Pötter |first2=L. |last3=Steiger |first3=A. |last4=Kruppert |first4=S. |last5=Frost |first5=U. |last6=Tollrian |first6=R. |year=2018 |title=Rising pCO<sub>2</sub> in Freshwater Ecosystems Has the Potential to Negatively Affect Predator-Induced Defenses in ''Daphnia'' |journal=Current Biology |volume=28 |issue=2 |pages=327–332.e3 |doi=10.1016/j.cub.2017.12.022 |pmid=29337079 |doi-access=free|bibcode=2018CBio...28E.327W }}</ref>

The absorption of CO<sub>2</sub> from the atmosphere does not affect the ocean's [[alkalinity]].<ref name="IPCC AR6 WGI Glossary" />{{rp|2252}} This is important to know in this context as alkalinity is the capacity of [[water]] to resist [[Freshwater acidification|acidification]].<ref>{{Cite news |date=2014 |title=What is alkalinity? |url=https://www.water-research.net/index.php/the-role-of-alkalinity-citizen-monitoring |access-date=5 February 2018 |publisher=Water Research Center}}</ref> [[Ocean alkalinity enhancement]] has been proposed as one option to add alkalinity to the ocean and therefore buffer against pH changes.

=== Decreased calcification in marine organisms ===
[[File:Foraminifera_(265_36)_Various.jpg|thumb|Various types of [[foraminifera]] observed through a microscope using differential interference contrast.]]
[[File:Carbonate system of seawater.svg|thumb|upright=1.2|[[Bjerrum plot]]: Change in carbonate system of seawater from ocean acidification]]
Changes in ocean chemistry can have extensive direct and indirect effects on organisms and their habitats. One of the most important repercussions of increasing ocean acidity relates to the production of shells out of [[calcium carbonate]] ({{chem2|CaCO3}}).<ref name="raven05">{{Cite book |url=https://royalsociety.org/-/media/Royal_Society_Content/policy/publications/2005/9634.pdf |title=Ocean acidification due to increasing atmospheric carbon dioxide |publisher=Royal Society |year=2005 |isbn=0-85403-617-2}}</ref> This process is called calcification and is important to the biology and survival of a wide range of marine organisms. Calcification involves the [[precipitation (chemistry)|precipitation]] of dissolved ions into solid {{chem2|CaCO3}} structures, structures for many marine organisms, such as [[coccolithophore]]s, [[foraminifera]], [[crustacea]]ns, [[Mollusca|mollusks]], etc. After they are formed, these {{chem2|CaCO3}} structures are vulnerable to [[Dissolution (chemistry)|dissolution]] unless the surrounding seawater contains [[Saturated solution|saturating]] concentrations of carbonate ions ({{chem2|CO3(2−)}}).

Very little of the extra carbon dioxide that is added into the ocean remains as dissolved carbon dioxide. The majority dissociates into additional bicarbonate and free hydrogen ions. The increase in hydrogen is larger than the increase in bicarbonate,<ref name="Mitchell">{{Cite journal |last1=Mitchell |first1=Mark J. |last2=Jensen |first2=Oliver E. |last3=Cliffe |first3=K. Andrew |last4=Maroto-Valer |first4=M. Mercedes |date=8 May 2010 |title=A model of carbon dioxide dissolution and mineral carbonation kinetics |journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=466 |issue=2117 |pages=1265–1290 |bibcode=2010RSPSA.466.1265M |doi=10.1098/rspa.2009.0349 |doi-access=free}}</ref> creating an imbalance in the reaction:
:{{chem2|HCO3− ⇌ CO3(2−) + H+}}
To maintain chemical equilibrium, some of the carbonate ions already in the ocean combine with some of the hydrogen ions to make further bicarbonate. Thus the ocean's concentration of carbonate ions is reduced, removing an essential building block for marine organisms to build shells, or calcify:
:{{chem2|Ca(2+) + CO3(2−) ⇌ CaCO3}}

The increase in concentrations of dissolved carbon dioxide and bicarbonate, and reduction in carbonate, are shown in the [[Bjerrum plot]].

Disruption of the food chain is also a possible effect as many marine organisms rely on calcium carbonate-based organisms at the base of the food chain for food and habitat. This can potentially have detrimental effects throughout the food web and potentially lead to a decline in availability of fish stocks which would have an impact on human livelihoods.<ref>{{Cite journal |last1=Kroeker |first1=Kristy J. |last2=Kordas |first2=Rebecca L. |last3=Crim |first3=Ryan |last4=Hendriks |first4=Iris E. |last5=Ramajo |first5=Laura |last6=Singh |first6=Gerald S. |last7=Duarte |first7=Carlos M. |last8=Gattuso |first8=Jean-Pierre |date=June 2013 |title=Impacts of ocean acidification on marine organisms: quantifying sensitivities and interaction with warming |journal=Global Change Biology |language=en |volume=19 |issue=6 |pages=1884–1896 |bibcode=2013GCBio..19.1884K |doi=10.1111/gcb.12179 |issn=1354-1013 |pmc=3664023 |pmid=23505245}}</ref>

=== Decrease in saturation state ===
[[File:CalciumAragoniteSaturation.jpg|thumb|upright=1.5|Distribution of (A) aragonite and (B) calcite saturation depth in the global oceans<ref name="Feely04">{{Cite journal |last1=Feely |first1=Richard A. |last2=Sabine |first2=Christopher L. |last3=Lee |first3=Kitack |last4=Berelson |first4=Will |last5=Kleypas |first5=Joanie |last6=Fabry |first6=Victoria J. |last7=Millero |first7=Frank J. |date=16 July 2004 |title=Impact of Anthropogenic CO<sub>2</sub>on the CaCO<sub>3</sub> System in the Oceans |journal=Science |volume=305 |issue=5682 |pages=362–366 |bibcode=2004Sci...305..362F |doi=10.1126/science.1097329 |pmid=15256664 |s2cid=31054160}}</ref>]]
The [[Saturated solution|saturation]] state (known as Ω) of seawater for a mineral is a measure of the thermodynamic potential for the mineral to form or to dissolve, and for calcium carbonate is described by the following equation:

:<math chem="">{\Omega} = \frac{\left[\ce{Ca^2+}\right] \left[\ce{CO3^2-}\right]}{K_{sp}}</math>

Here Ω is the product of the concentrations (or [[Activity (chemistry)|activities]]) of the reacting ions that form the mineral (Ca<sup>2+</sup> and CO<sub>3</sub><sup>2−</sup>), divided by the apparent solubility product at [[Chemical equilibrium|equilibrium]] (K<sub>sp</sub>), that is, when the rates of precipitation and dissolution are equal.<ref>{{Cite journal |last1=Atkinson |first1=M.J. |last2=Cuet |first2=P. |year=2008 |title=Possible effects of ocean acidification on coral reef biogeochemistry: topics for research |journal=[[Marine Ecology Progress Series]] |volume=373 |pages=249–256 |bibcode=2008MEPS..373..249A |doi=10.3354/meps07867 |doi-access=free}}</ref> In seawater, dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon.<ref name="raven05" /> Above this saturation horizon, Ω has a value greater than 1, and {{chem|CaCO|3}} does not readily dissolve. Most calcifying organisms live in such waters.<ref name="raven05" /> Below this depth, Ω has a value less than 1, and {{chem|CaCO|3}} will dissolve. The [[carbonate compensation depth]] is the ocean depth at which carbonate dissolution balances the supply of carbonate to sea floor, therefore sediment below this depth will be void of calcium carbonate.<ref>{{Cite book |last1=Thurman |first1=H.V. |title=Introductory Oceanography |last2=Trujillo |first2=A.P. |publisher=Prentice Hall |year=2004 |isbn=978-0-13-143888-0}}</ref> Increasing {{CO2}} levels, and the resulting lower pH of seawater, decreases the concentration of CO<sub>3</sub><sup>2−</sup> and the saturation state of {{chem|CaCO|3}} therefore increasing {{chem|CaCO|3}} dissolution.

Calcium carbonate most commonly occurs in two common [[Polymorphism (materials science)|polymorphs]] (crystalline forms): [[aragonite]] and [[calcite]]. Aragonite is much more soluble than calcite, so the aragonite saturation horizon, and aragonite compensation depth, is always nearer to the surface than the calcite saturation horizon.<ref name="raven05" /> This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite.<ref name="orr05">{{Cite journal |last1=Orr |first1=James C. |last2=Fabry |first2=Victoria J. |last3=Aumont |first3=Olivier |last4=Bopp |first4=Laurent |last5=Doney |first5=Scott C. |last6=Feely |first6=Richard A. |last7=Gnanadesikan |first7=Anand |last8=Gruber |first8=Nicolas |last9=Ishida |first9=Akio |last10=Joos |first10=Fortunat |last11=Key |first11=Robert M. |last12=Lindsay |first12=Keith |last13=Maier-Reimer |first13=Ernst |last14=Matear |first14=Richard |last15=Monfray |first15=Patrick |date=September 2005 |title=Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms |url=https://epic.awi.de/id/eprint/13479/1/Orr2005a.pdf |journal=Nature |volume=437 |issue=7059 |pages=681–686 |bibcode=2005Natur.437..681O |doi=10.1038/nature04095 |pmid=16193043 |s2cid=4306199 |last16=Mouchet |first16=Anne |last17=Najjar |first17=Raymond G. |last18=Plattner |first18=Gian-Kasper |last19=Rodgers |first19=Keith B. |last20=Sabine |first20=Christopher L. |last21=Sarmiento |first21=Jorge L. |last22=Schlitzer |first22=Reiner |last23=Slater |first23=Richard D. |last24=Totterdell |first24=Ian J. |last25=Weirig |first25=Marie-France |last26=Yamanaka |first26=Yasuhiro |last27=Yool |first27=Andrew}}</ref> Ocean acidification and the resulting decrease in carbonate saturation states raise the saturation horizons of both forms closer to the surface.<ref name="raven05" /> This decrease in saturation state is&nbsp;one of the main factors leading to decreased calcification in marine organisms because the inorganic precipitation of {{chem|CaCO|3}} is directly proportional to its saturation state and calcifying organisms exhibit stress in waters with lower saturation states.<ref>{{Cite journal |last1=Hofmann |first1=Gretchen E. |last2=Barry |first2=James P. |last3=Edmunds |first3=Peter J. |last4=Gates |first4=Ruth D. |last5=Hutchins |first5=David A. |last6=Klinger |first6=Terrie |last7=Sewell |first7=Mary A. |date=2010-12-01 |title=The Effect of Ocean Acidification on Calcifying Organisms in Marine Ecosystems: An Organism-to-Ecosystem Perspective |journal=Annual Review of Ecology, Evolution, and Systematics |volume=41 |issue=1 |pages=127–147 |doi=10.1146/annurev.ecolsys.110308.120227}}</ref>

=== Natural variability and climate feedbacks ===
{{Further|Carbonate compensation depth}}

Already now large quantities of water undersaturated in [[aragonite]] are [[upwelling]] close to the Pacific [[continental shelf]] area of North America, from [[Vancouver]] to [[Northern California]].<ref name="feeley08">{{Cite journal |last1=Feely |first1=Richard A. |last2=Sabine |first2=Christopher L. |last3=Hernandez-Ayon |first3=J. Martin |last4=Ianson |first4=Debby |last5=Hales |first5=Burke |date=13 June 2008 |title=Evidence for Upwelling of Corrosive "Acidified" Water onto the Continental Shelf |journal=Science |volume=320 |issue=5882 |pages=1490–1492 |bibcode=2008Sci...320.1490F |citeseerx=10.1.1.328.3181 |doi=10.1126/science.1155676 |pmid=18497259 |s2cid=35487689}}</ref> These continental shelves play an important role in marine ecosystems, since most [[marine organisms]] live or are [[Spawn (biology)|spawned]] there. Other shelf areas may be experiencing similar effects.<ref name="feeley08" />

At depths of 1000s of meters in the ocean, calcium carbonate shells begin to dissolve as increasing pressure and decreasing temperature shift the chemical equilibria controlling calcium carbonate precipitation.<ref name=":13">{{Cite journal |last1=Boudreau |first1=Bernard P. |last2=Middelburg |first2=Jack J. |last3=Hofmann |first3=Andreas F. |last4=Meysman |first4=Filip J. R. |date=2010 |title=Ongoing transients in carbonate compensation |journal=Global Biogeochemical Cycles |volume=24 |issue=4 |bibcode=2010GBioC..24.4010B |doi=10.1029/2009GB003654 |s2cid=53062358 |bibcode-access=free |doi-access=free |s2cid-access=free}}</ref> The depth at which this occurs is known as the [[carbonate compensation depth]]. Ocean acidification will increase such dissolution and shallow the carbonate compensation depth on timescales of tens to hundreds of years.<ref name=":13" /> Zones of [[downwelling]] are being affected first.<ref>{{Cite journal |last1=Sulpis |first1=Olivier |last2=Boudreau |first2=Bernard P. |last3=Mucci |first3=Alfonso |last4=Jenkins |first4=Chris |last5=Trossman |first5=David S. |last6=Arbic |first6=Brian K. |last7=Key |first7=Robert M. |date=13 November 2018 |title=Current CaCO 3 dissolution at the seafloor caused by anthropogenic CO 2 |journal=Proceedings of the National Academy of Sciences |volume=115 |issue=46 |pages=11700–11705 |bibcode=2018PNAS..11511700S |doi=10.1073/pnas.1804250115 |pmc=6243283 |pmid=30373837 |bibcode-access=free |doi-access=free}}</ref>

In the North Pacific and North Atlantic, saturation states are also decreasing (the depth of saturation is getting more shallow).<ref name=":43" />{{rp|396}} Ocean acidification is progressing in the open ocean as the CO<sub>2</sub> travels to deeper depth as a result of ocean mixing. In the open ocean, this causes carbonate compensation depths to become more shallow, meaning that dissolution of calcium carbonate will occur below those depths. In the North Pacific these carbonate saturations depths are shallowing at a rate of {{val|1|–|2|ul=m|upl=year}}.<ref name=":43" />{{rp|396}}

It is expected that ocean acidification in the future will lead to a significant decrease in the burial of carbonate sediments for several centuries, and even the dissolution of existing carbonate sediments.<ref name="ridgwell07">{{Cite journal |last1=Ridgwell |first1=A. |last2=Zondervan |first2=I. |last3=Hargreaves |first3=J. C. |last4=Bijma |first4=J. |last5=Lenton |first5=T. M. |year=2007 |title=Assessing the potential long-term increase of oceanic fossil fuel CO<sub>2</sub> uptake due to CO<sub>2</sub>-calcification feedback |journal=[[Biogeosciences]] |volume=4 |issue=4 |pages=481–492 |bibcode=2007BGeo....4..481R |doi=10.5194/bg-4-481-2007 |bibcode-access=free |doi-access=free}}</ref>

== Measured and estimated values ==
=== Present day and recent history ===
[[File:Co2 time series aloha 08-09-2023.jpg|thumb|right|Time series of atmospheric {{CO2}} at Mauna Loa (in parts per million volume, ppmv; red), surface ocean p{{CO2}} (μatm; green) and surface ocean pH (blue) at Ocean Station ALOHA in the subtropical North Pacific Ocean.<ref>{{Cite web |date=2012-09-07 |title=Atmospheric {{CO2}} and Ocean pH |url=https://cleanet.org/details/images/33568.html |access-date=2022-11-17 |website=CLEAN}}</ref><ref>{{Cite web |title=Quality of pH Measurements in the NODC Data Archives |url=https://www.pmel.noaa.gov/co2/story/Quality+of+pH+Measurements+in+the+NODC+Data+Archives |access-date=2023-12-18 |website=NOAA Pacific Marine Environmental Laboratory}}</ref>]]
[[File: Estimated change in annual mean sea surface pH from 1770s-1990s (GLODAP).png|thumb|upright=1.3|Estimated change in seawater [[pH]] caused by [[anthropogenic impact]] on [[carbon dioxide|{{chem|CO|2}}]] levels between the 1700s and the 1990s, from the [[Global Ocean Data Analysis Project]] (GLODAP) and the [[World Ocean Atlas]]|alt=World map showing the varying change to pH across different parts of different oceans]]Between 1950 and 2020, the average pH value of the ocean surface is estimated to have decreased from approximately 8.15 to 8.05.<ref name = role/> This represents an increase of around 26% in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH unit is equivalent to a tenfold change in hydrogen ion concentration).<ref name=":19">{{Cite web |title=The pH Scale |url=https://www.whoi.edu/know-your-ocean/ocean-topics/how-the-ocean-works/ocean-chemistry/ocean-acidification/the-ph-scale/ |access-date=2023-03-29 |website=Woods Hole Oceanographic Institution |language=en-US}}</ref> For example, in the 15-year period 1995–2010 alone, acidity has increased 6 percent in the upper 100&nbsp;meters of the Pacific Ocean from Hawaii to Alaska.<ref name="2010aug_sciam">{{Cite web |last1=Hardt |first1=Marah J. |last2=Safina |first2=Carl |date=9 August 2010 |title=How Acidification Threatens Oceans from the Inside Out |url=http://www.scientificamerican.com/article.cfm?id=threatening-ocean-life |url-status=dead |archive-url=https://web.archive.org/web/20101226161319/http://www.scientificamerican.com/article.cfm?id=threatening-ocean-life |archive-date=2010-12-26 |website=Scientific American}}</ref>

The [[IPCC Sixth Assessment Report]] in 2021 stated that "present-day surface pH values are unprecedented for at least 26,000 years and current rates of pH change are unprecedented since at least that time.<ref name=":82" />{{rp|76}} The pH value of the ocean interior has declined over the last 20–30 years everywhere in the global ocean.<ref name=":82">Arias, P.A., N. Bellouin, E. Coppola, R.G. Jones, G. Krinner, J. Marotzke, V. Naik, M.D. Palmer, G.-K. Plattner, J. Rogelj, M. Rojas, J. Sillmann, T. Storelvmo, P.W. Thorne, B. Trewin, K. Achuta Rao, B. Adhikary, R.P. Allan, K. Armour, G. Bala, R. Barimalala, S. Berger, J.G. Canadell, C. Cassou, A. Cherchi, W. Collins, W.D. Collins, S.L. Connors, S. Corti, F. Cruz, F.J. Dentener, C. Dereczynski, A. Di Luca, A. Diongue Niang, F.J. Doblas-Reyes, A. Dosio, H. Douville, F. Engelbrecht, V.&nbsp; Eyring, E. Fischer, P. Forster, B. Fox-Kemper, J.S. Fuglestvedt, J.C. Fyfe, et al., 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Technical Summary] {{Webarchive|url=https://web.archive.org/web/20220721021347/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf |date=21 July 2022 }}. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20210809131444/https://www.ipcc.ch/report/ar6/wg1/ |date=9 August 2021 }} [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 33–144.</ref>{{rp|76}} The report also found that "pH in open ocean surface water has declined by about&nbsp;0.017 to 0.027 pH units per decade since the late 1980s".<ref name=":2">Canadell, J.G., P.M.S. Monteiro, M.H. Costa, L. Cotrim da Cunha, P.M. Cox, A.V. Eliseev, S. Henson, M. Ishii, S. Jaccard, C. Koven, A. Lohila, P.K. Patra, S. Piao, J. Rogelj, S. Syampungani, S. Zaehle, and K. Zickfeld, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter05.pdf Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks] {{Webarchive|url=https://web.archive.org/web/20220727145459/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter05.pdf |date=27 July 2022 }}. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20210809131444/https://www.ipcc.ch/report/ar6/wg1/ |date=9 August 2021 }} [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 673–816.</ref>{{rp|716}}

The rate of decline differs by region. This is due to complex interactions between different types of forcing mechanisms:<ref name=":2" />{{rp|716}} "In the tropical Pacific, its central and eastern upwelling zones exhibited a faster pH decline of minus 0.022 to minus 0.026 pH unit per decade." This is thought to be "due to increased [[upwelling]] of {{CO2}}-rich sub-surface waters in addition to anthropogenic {{CO2}} uptake."<ref name=":2" />{{rp|716}} Some regions exhibited a slower acidification rate: a pH decline of minus 0.010 to minus 0.013 pH unit per decade has been observed in warm pools in the western tropical Pacific.<ref name=":2" />{{rp|716}}

The rate at which ocean acidification will occur may be influenced by the rate of [[Sea surface temperature|surface ocean warming]], because warm waters will not absorb as much {{CO2}}.<ref name="humphreys20162">{{Cite journal |last=Humphreys |first=M. P. |year=2016 |title=Climate sensitivity and the rate of ocean acidification: future impacts, and implications for experimental design |journal=ICES Journal of Marine Science |volume=74 |issue=4 |pages=934–940 |doi=10.1093/icesjms/fsw189 |doi-access=free}}</ref> Therefore, greater seawater warming could limit CO<sub>2</sub> absorption and lead to a smaller change in pH for a given increase in CO<sub>2</sub>.<ref name="humphreys20162" /> The difference in changes in temperature between basins is one of the main reasons for the differences in acidification rates in different localities.

Current rates of ocean acidification have been likened to the greenhouse event at the [[Paleocene–Eocene Thermal Maximum|Paleocene–Eocene]] boundary (about 56 million years ago), when surface ocean temperatures rose by {{val|5|–|6|ul=degC}}. In that event, surface ecosystems experienced a variety of impacts, but bottom-dwelling organisms in the deep ocean actually experienced a major extinction.<ref>{{Cite journal |last1=McInerney |first1=Francesca A. |last2=Wing |first2=Scott L. |date=2011-05-30 |title=The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future |journal=Annual Review of Earth and Planetary Sciences |volume=39 |issue=1 |pages=489–516 |bibcode=2011AREPS..39..489M |doi=10.1146/annurev-earth-040610-133431}}</ref> Currently, the rate of carbon addition to the atmosphere-ocean system is about ten times the rate that occurred at the Paleocene–Eocene boundary.<ref>{{Cite journal |last=Zeebe |first=Richard E. |date=2012-05-30 |title=History of Seawater Carbonate Chemistry, Atmospheric CO 2 , and Ocean Acidification |journal=Annual Review of Earth and Planetary Sciences |volume=40 |issue=1 |pages=141–165 |bibcode=2012AREPS..40..141Z |doi=10.1146/annurev-earth-042711-105521}}</ref>

Extensive observational systems are now in place or being built for monitoring seawater {{CO2}} chemistry and acidification for both the global open ocean and some coastal systems.<ref name=":14" />

{| class="wikitable sortable"
|+ Rates of increasing acidity in different marine regions
! Location !! Change in pH units<br>per decade !! Period !! Data source
!Year of publication
|-
| Iceland<ref>{{Cite journal |last1=Olafsson |first1=J. |last2=Olafsdottir |first2=S. R. |last3=Benoit-Cattin |first3=A. |last4=Danielsen |first4=M. |last5=Arnarson |first5=T. S. |last6=Takahashi |first6=T. |date=25 November 2009 |title=Rate of Iceland Sea acidification from time series measurements |journal=Biogeosciences |volume=6 |issue=11 |pages=2661–2668 |bibcode=2009BGeo....6.2661O |doi=10.5194/bg-6-2661-2009 |doi-access=free}}</ref> || minus 0.024|| 1984–2009 || Direct measurements
|2009
|-
| Drake Passage<ref>{{Cite journal |last1=Midorikawa |first1=Takashi |last2=Inoue |first2=Hisayuki Y. |last3=Ishii |first3=Masao |last4=Sasano |first4=Daisuke |last5=Kosugi |first5=Naohiro |last6=Hashida |first6=Gen |last7=Nakaoka |first7=Shin-ichiro |last8=Suzuki |first8=Toru |date=March 2012 |title=Decreasing pH trend estimated from 35-year time series of carbonate parameters in the Pacific sector of the Southern Ocean in summer |journal=Deep Sea Research Part I: Oceanographic Research Papers |volume=61 |pages=131–139 |bibcode=2012DSRI...61..131M |doi=10.1016/j.dsr.2011.12.003}}</ref> || minus 0.018|| 2002–2012|| Direct measurements
|2012
|-
| Canary (ESTOC)<ref>{{Cite journal |last1=González-Dávila |first1=M. |last2=Santana-Casiano |first2=J. M. |last3=Rueda |first3=M. J. |last4=Llinás |first4=O. |date=11 October 2010 |title=The water column distribution of carbonate system variables at the ESTOC site from 1995 to 2004 |journal=Biogeosciences |volume=7 |issue=10 |pages=3067–3081 |bibcode=2010BGeo....7.3067G |doi=10.5194/bg-7-3067-2010 |doi-access=free}}</ref> || minus 0.017|| 1995–2004|| Direct measurements
|2010
|-
| Hawaii ([[Hawaii Ocean Time-series|HOT]])<ref>{{Cite journal |last1=Dore |first1=J. E. |last2=Lukas |first2=R. |last3=Sadler |first3=D. W. |last4=Church |first4=M. J. |last5=Karl |first5=D. M. |date=28 July 2009 |title=Physical and biogeochemical modulation of ocean acidification in the central North Pacific |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=30 |pages=12235–12240 |doi=10.1073/pnas.0906044106 |pmc=2716384 |pmid=19666624 |doi-access=free}}</ref> || minus 0.019|| 1989–2007|| Direct measurements
|2009
|-
| Bermuda ([[Bermuda Atlantic Time-series Study|BATS]])<ref>{{Cite journal |last1=Bates |first1=N. R. |last2=Best |first2=M. H. P. |last3=Neely |first3=K. |last4=Garley |first4=R. |last5=Dickson |first5=A. G. |last6=Johnson |first6=R. J. |date=11 July 2012 |title=Detecting anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean |journal=Biogeosciences |volume=9 |issue=7 |pages=2509–2522 |bibcode=2012BGeo....9.2509B |doi=10.5194/bg-9-2509-2012 |doi-access=free}}</ref> || minus 0.017|| 1984–2012|| Direct measurements
|2012
|-
| Coral Sea<ref>{{Cite journal |last1=Pelejero |first1=Carles |last2=Calvo |first2=Eva |last3=McCulloch |first3=Malcolm T. |last4=Marshall |first4=John F. |last5=Gagan |first5=Michael K. |last6=Lough |first6=Janice M. |author-link6=Janice Lough |last7=Opdyke |first7=Bradley N. |date=30 September 2005 |title=Preindustrial to Modern Interdecadal Variability in Coral Reef pH |journal=Science |volume=309 |issue=5744 |pages=2204–2207 |bibcode=2005Sci...309.2204P |doi=10.1126/science.1113692 |pmid=16195458 |s2cid=129883047}}</ref>|| minus 0.002|| ~1700 – ~1990|| Proxy reconstruction
|2005
|-
| Eastern Mediterranean<ref>{{Cite journal |last1=Bialik |first1=Or M. |last2=Sisma-Ventura |first2=Guy |date=December 2016 |title=Proxy-based reconstruction of surface water acidification and carbonate saturation of the Levant Sea during the Anthropocene |journal=Anthropocene |volume=16 |pages=42–53 |bibcode=2016Anthr..16...42B |doi=10.1016/j.ancene.2016.08.001}}</ref> || minus 0.023|| 1964–2005|| Proxy reconstruction
|2016
|}
{| class="wikitable"
|+Rates of pH change for some regions of the world<br>(Many more regions available in source table)<ref>Josep G. Canadell, J. G., P. M. S. Monteiro, M. H. Costa, L. Cotrim da Cunha, P. M. Cox, A. V. Eliseev, S. 3 Henson, M. Ishii, S. Jaccard, C. Koven, A. Lohila, P. K. Patra, S. Piao, J. Rogelj, S. Syampungani, S. Zaehle, 4 K. Zickfeld, 2021, [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_05_Supplementary_Material.pdf Global Carbon and other Biogeochemical Cycles and Feedbacks Supplementary Material. Chapter 5] {{Webarchive|url=https://web.archive.org/web/20221118113000/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_05_Supplementary_Material.pdf |date=18 November 2022 }} In: [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth 6 Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20210809131444/https://www.ipcc.ch/report/ar6/wg1/ |date=9 August 2021 }} [Masson-Delmotte, V., P. Zhai, A. 7 Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. 8 Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. 9 Available from <nowiki>https://ipcc.ch/static/ar6/wg1</nowiki>.</ref>{{rp|Table 5.SM.3}}
!
!Station, region
!Study period
!pH change<br>per decade
|-
|Equatorial Pacific
|TAO
|2004–2011
|−0.026
|-
|[[Indian Ocean]]
|IO-STPS
|1991–2011
|−0.027
|-
|[[Mediterranean Sea|Mediterranean]]
|Dyfamed (43.42°N, 7.87°E)
|1995–2011
|−0.03
|-
|[[North Atlantic]]
|[[Iceland Sea]] (68°N, 12.67°W)
|1985–2008<br>1985–2010
|−0.024<br>−0.014
|-
|North Atlantic
|[[Irminger Sea]] (64.3°N, 28°W)
|1983–2004
|−0.026
|-
|[[North Pacific]]
|NP-STSS
|1991–2011
|−0.01
|-
|[[Southern Ocean]]
|PAL-LTER, west Antarctic Peninsula
|1993–2012
| +0.02
|}

=== Geologic past ===
Ocean acidification has occurred previously in Earth's history.<ref name=":9" /> It happened during the [[Capitanian mass extinction event|Capitanian mass extinction]],<ref name="bond15">Bond, D.P.G., Wignall, P.B., Joachimski, M.M., Sun, Y., Savov, I., Grasby, S.E., Beauchamp, B. and Blomeier, D.P. 2015. [https://pubs.geoscienceworld.org/gsa/gsabulletin/article/127/9-10/1411/126168/an-abrupt-extinction-in-the-middle-permian An abrupt extinction in the Middle Permian (Capitanian) of the Boreal Realm (Spitsbergen) and its link to anoxia and acidification]. ''Geological Society of America Bulletin'', '''127''' (9–10): 1411–1421.</ref><ref name="6th extinction">{{Cite news |last=Hand |first=Eric |date=April 16, 2015 |title=Sixth extinction, rivaling that of the dinosaurs, should join the big five, scientists say |url=https://www.science.org/content/article/sixth-extinction-rivaling-dinosaurs-should-join-big-five-scientists-say |work=Science}}</ref><ref name="bbc15BerezowAlex">{{Cite news |last=Berezow |first=Alex |date=April 21, 2015 |title=New mass extinction event identified by geologists |url=https://www.bbc.com/news/science-environment-32397220 |work=BBC}}</ref> at the [[Permian-Triassic extinction event|end-Permian extinction]],<ref name="ClarksonP-TAcid">{{Cite journal |last1=Clarkson |first1=M. O. |last2=Kasemann |first2=S. A. |last3=Wood |first3=R. A. |last4=Lenton |first4=T. M. |last5=Daines |first5=S. J. |last6=Richoz |first6=S. |last7=Ohnemueller |first7=F. |last8=Meixner |first8=A. |last9=Poulton |first9=S. W. |last10=Tipper |first10=E. T. |date=2015-04-10 |title=Ocean acidification and the Permo-Triassic mass extinction |url=http://eprints.whiterose.ac.uk/85124/1/Clarkson_Boron_final.pdf |journal=Science |volume=348 |issue=6231 |pages=229–232 |bibcode=2015Sci...348..229C |doi=10.1126/science.aaa0193 |pmid=25859043 |s2cid=28891777}}</ref><ref name="PayneOceanAcidPermian">{{Cite journal |last1=Payne |first1=J. L. |last2=Lehrmann |first2=D. J. |last3=Follett |first3=D. |last4=Seibel |first4=M. |last5=Kump |first5=L. R. |last6=Riccardi |first6=A. |last7=Altiner |first7=D. |last8=Sano |first8=H. |last9=Wei |first9=J. |date=1 July 2007 |title=Erosional truncation of uppermost Permian shallow-marine carbonates and implications for Permian-Triassic boundary events |journal=Geological Society of America Bulletin |volume=119 |issue=7–8 |pages=771–784 |bibcode=2007GSAB..119..771P |doi=10.1130/B26091.1 |hdl-access=free |hdl=11511/35436}}</ref><ref name="PayneEtAl2015OceanAcid">{{Cite journal |last1=Payne |first1=J. |last2=Turchyn |first2=A. |last3=Paytan |first3=A. |last4=Depaolo |first4=D. |last5=Lehrmann |first5=D. |last6=Yu |first6=M. |last7=Wei |first7=J. |year=2010 |title=Calcium isotope constraints on the end-Permian mass extinction |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=107 |issue=19 |pages=8543–8548 |bibcode=2010PNAS..107.8543P |doi=10.1073/pnas.0914065107 |pmc=2889361 |pmid=20421502 |doi-access=free}}</ref> during the [[Triassic–Jurassic extinction event|end-Triassic extinction]],<ref name=":20">{{Cite journal |last=Hautmann |first=Michael |date=September 2004 |title=Effect of end-Triassic {{CO2}} maximum on carbonate sedimentation and marine mass extinction |journal=Facies |volume=50 |issue=2 |doi=10.1007/s10347-004-0020-y |s2cid=130658467}}</ref><ref name="GreeneOceanAcidTriassic">{{Cite journal |last1=Greene |first1=Sarah E. |last2=Martindale |first2=Rowan C. |last3=Ritterbush |first3=Kathleen A. |last4=Bottjer |first4=David J. |last5=Corsetti |first5=Frank A. |last6=Berelson |first6=William M. |date=June 2012 |title=Recognising ocean acidification in deep time: An evaluation of the evidence for acidification across the Triassic-Jurassic boundary |journal=Earth-Science Reviews |volume=113 |issue=1–2 |pages=72–93 |bibcode=2012ESRv..113...72G |doi=10.1016/j.earscirev.2012.03.009}}</ref><ref name=":4">{{Cite journal |last1=Hautmann |first1=Michael |last2=Benton |first2=Michael J. |last3=Tomašových |first3=Adam |date=11 July 2008 |title=Catastrophic ocean acidification at the Triassic-Jurassic boundary |journal=Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen |volume=249 |issue=1 |pages=119–127 |doi=10.1127/0077-7749/2008/0249-0119|bibcode=2008NJGPA.249..119H }}</ref> and during the [[Cretaceous–Paleogene extinction event|Cretaceous–Palaeogene extinction event]].<ref name="PNAS-20191021" />

Three of the big five mass extinction events in the geologic past were associated with a rapid increase in atmospheric carbon dioxide, probably due to [[volcanism]] and/or thermal dissociation of marine [[Clathrate hydrate|gas hydrates]].<ref name=":8">{{Cite journal |last1=Clapham |first1=Matthew E. |last2=Renne |first2=Paul R. |date=2019-05-30 |title=Flood Basalts and Mass Extinctions |url=https://escholarship.org/uc/item/3q4714cg |journal=Annual Review of Earth and Planetary Sciences |volume=47 |issue=1 |pages=275–303 |bibcode=2019AREPS..47..275C |doi=10.1146/annurev-earth-053018-060136 |s2cid=133715470}}</ref> Elevated CO<sub>2</sub> levels impacted biodiversity.<ref name="Zachos2005">{{Cite journal |last1=Zachos |first1=James C. |last2=Röhl |first2=Ursula |last3=Schellenberg |first3=Stephen A. |last4=Sluijs |first4=Appy |last5=Hodell |first5=David A. |last6=Kelly |first6=Daniel C. |last7=Thomas |first7=Ellen |last8=Nicolo |first8=Micah |last9=Raffi |first9=Isabella |last10=Lourens |first10=Lucas J. |last11=McCarren |first11=Heather |last12=Kroon |first12=Dick |date=10 June 2005 |title=Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum |journal=Science |volume=308 |issue=5728 |pages=1611–1615 |bibcode=2005Sci...308.1611Z |doi=10.1126/science.1109004 |pmid=15947184 |s2cid=26909706 |hdl-access=free |hdl=1874/385806}}</ref> Decreased {{chem2|CaCO3}} saturation due to seawater uptake of volcanogenic CO<sub>2</sub> has been suggested as a possible kill mechanism during the marine mass extinction at the end of the [[Triassic]].<ref name=":20" /> The end-Triassic biotic crisis is still the most well-established example of a marine mass extinction due to ocean acidification, because (a) carbon isotope records suggest enhanced volcanic activity that decreased the carbonate sedimentation which reduced the carbonate compensation depth and the carbonate saturation state, and a marine extinction coincided precisely in the stratigraphic record,<ref name=":4" /><ref name="GreeneOceanAcidTriassic" /><ref>{{Cite journal |last1=Lindström |first1=Sofie |last2=van de Schootbrugge |first2=Bas |last3=Hansen |first3=Katrine H. |last4=Pedersen |first4=Gunver K. |last5=Alsen |first5=Peter |last6=Thibault |first6=Nicolas |last7=Dybkjær |first7=Karen |last8=Bjerrum |first8=Christian J. |last9=Nielsen |first9=Lars Henrik |date=July 2017 |title=A new correlation of Triassic–Jurassic boundary successions in NW Europe, Nevada and Peru, and the Central Atlantic Magmatic Province: A time-line for the end-Triassic mass extinction |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=478 |pages=80–102 |bibcode=2017PPP...478...80L |doi=10.1016/j.palaeo.2016.12.025 |s2cid=133353132 |hdl-access=free |hdl=1874/351998}}</ref> and (b) there was pronounced selectivity of the extinction against organisms with thick aragonitic skeletons,<ref name=":4" /><ref>{{Cite journal |last1=Hautmann |first1=M. |last2=Stiller |first2=F. |last3=Huawei |first3=C. |last4=Jingeng |first4=S. |date=1 October 2008 |title=Extinction-Recovery Pattern of Level-Bottom Faunas Across the Triassic-Jurassic Boundary in Tibet: Implications for Potential Killing Mechanisms |url=https://www.zora.uzh.ch/id/eprint/9775/10/Hautmann_Extinction_2008.pdf |journal=PALAIOS |volume=23 |issue=10 |pages=711–718 |bibcode=2008Palai..23..711H |doi=10.2110/palo.2008.p08-005r |s2cid=42675849}}</ref><ref>{{Citation |last=Hautmann |first=Michael |title=eLS |date=2012-08-15 |pages=a0001655.pub3 |chapter=Extinction: End-Triassic Mass Extinction |publisher=John Wiley & Sons |doi=10.1002/9780470015902.a0001655.pub3 |isbn=978-0-470-01617-6 |s2cid=130434497}}</ref> which is predicted from experimental studies.<ref>{{Cite journal |last1=Fine |first1=Maoz |last2=Tchernov |first2=Dan |date=30 March 2007 |title=Scleractinian Coral Species Survive and Recover from Decalcification |journal=Science |volume=315 |issue=5820 |pages=1811 |bibcode=2007Sci...315.1811F |doi=10.1126/science.1137094 |pmid=17395821 |s2cid=28535145}}</ref> Ocean acidification has also been suggested as a one cause of the end-Permian mass extinction<ref name="PayneOceanAcidPermian" /><ref name="ClarksonP-TAcid" /> and the end-Cretaceous crisis.<ref name="PNAS-20191021" /> Overall, multiple climatic stressors, including ocean acidification, was likely the cause of geologic extinction events.<ref name=":8" />

The most notable example of ocean acidification is the [[Paleocene–Eocene Thermal Maximum|Paleocene-Eocene Thermal Maximum]] (PETM), which occurred approximately 56 million years ago when massive amounts of carbon entered the ocean and atmosphere, and led to the dissolution of carbonate sediments across many ocean basins.<ref name="Zachos2005" /> Relatively new geochemical methods of testing for pH in the past indicate the pH dropped 0.3 units across the PETM.<ref>{{Cite journal |last1=Penman |first1=Donald E. |last2=Hönisch |first2=Bärbel |last3=Zeebe |first3=Richard E. |last4=Thomas |first4=Ellen |last5=Zachos |first5=James C. |date=May 2014 |title=Rapid and sustained surface ocean acidification during the Paleocene-Eocene Thermal Maximum |url=https://academiccommons.columbia.edu/doi/10.7916/D8X92P2F/download |journal=Paleoceanography |volume=29 |issue=5 |pages=357–369 |bibcode=2014PalOc..29..357P |doi=10.1002/2014PA002621}}</ref><ref>{{Cite journal |last1=Gutjahr |first1=Marcus |last2=Ridgwell |first2=Andy |last3=Sexton |first3=Philip F. |last4=Anagnostou |first4=Eleni |last5=Pearson |first5=Paul N. |last6=Pälike |first6=Heiko |last7=Norris |first7=Richard D. |last8=Thomas |first8=Ellen |last9=Foster |first9=Gavin L. |date=August 2017 |title=Very large release of mostly volcanic carbon during the Palaeocene–Eocene Thermal Maximum |journal=Nature |volume=548 |issue=7669 |pages=573–577 |bibcode=2017Natur.548..573G |doi=10.1038/nature23646 |pmc=5582631 |pmid=28858305}}</ref> One study that solves the marine carbonate system for saturation state shows that it may not change much over the PETM, suggesting the rate of carbon release at our best geological analogy was much slower than human-induced carbon emissions. However, stronger [[Proxy (climate)|proxy]] methods to test for saturation state are needed to assess how much this pH change may have affected calcifying organisms.

== Predicted future values ==
[[File:Oa-sami.jpg|thumb|upright|''In situ'' {{chem|CO|2}} concentration sensor (SAMI-{{CO2}}), attached to a Coral Reef Early Warning System station, utilized in conducting ocean acidification studies near coral reef areas (by [[NOAA]] ([[Atlantic Oceanographic and Meteorological Laboratory|AOML]]))]]
[[File:Oa-buoy-enrique-reef.jpg|thumb|A moored autonomous {{chem|CO|2}} buoy used for measuring {{chem|CO|2}} concentration and ocean acidification studies ([[NOAA]] (by [[Pacific Marine Environmental Laboratory|PMEL]]))]]
Importantly, the rate of change in ocean acidification is much higher than in the geological past. This faster change prevents organisms from gradually adapting, and prevents climate cycle feedbacks from kicking in to mitigate ocean acidification. Ocean acidification is now on a path to reach lower pH levels than at any other point in the last 300 million years.<ref name="NYT-20191021">{{Cite news |last=Joel |first=Lucas |date=21 October 2019 |title=The Dinosaur-Killing Asteroid Acidified the Ocean in a Flash – The Chicxulub event was as damaging to life in the oceans as it was to creatures on land, a study shows. |url=https://www.nytimes.com/2019/10/21/science/chicxulub-asteroid-ocean-acid.html |access-date=22 October 2019 |work=[[The New York Times]]}}</ref><ref name="PNAS-20191021">{{Cite journal |last1=Henehan |first1=Michael J. |last2=Ridgwell |first2=Andy |last3=Thomas |first3=Ellen |last4=Zhang |first4=Shuang |last5=Alegret |first5=Laia |last6=Schmidt |first6=Daniela N. |last7=Rae |first7=James W. B. |last8=Witts |first8=James D. |last9=Landman |first9=Neil H. |last10=Greene |first10=Sarah E. |last11=Huber |first11=Brian T. |last12=Super |first12=James R. |last13=Planavsky |first13=Noah J. |last14=Hull |first14=Pincelli M. |date=5 November 2019 |title=Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact |journal=Proceedings of the National Academy of Sciences |volume=116 |issue=45 |pages=22500–22504 |bibcode=2019PNAS..11622500H |doi=10.1073/pnas.1905989116 |pmc=6842625 |pmid=31636204 |doi-access=free}}</ref> The rate of ocean acidification (i.e. the rate of change in pH value) is also estimated to be unprecedented over that same time scale.<ref>{{Cite news |title=The Geological Record of Ocean Acidification |url=http://journalistsresource.org/studies/environment/climate-change/geological-record-ocean-acidification/ |access-date=14 March 2012 |work=The Journalist's Resource}}</ref><ref name=":9" /> These expected changes are considered unprecedented in the geological record.<ref name="yale_ridgwell_acidrate">{{Cite web |title=An Ominous Warning on the Effects of Ocean Acidification by Carl Zimmer: Yale Environment 360 |url=http://e360.yale.edu/content/feature.msp?id=2241 |url-status=dead |archive-url=https://web.archive.org/web/20140216034313/http://e360.yale.edu/content/feature.msp?id=2241 |archive-date=16 February 2014 |access-date=2014-01-25 |publisher=e360.yale.edu}}</ref><ref>{{Cite web |last=Newspapers |first=Les Blumenthal-McClatchy |date=2010-04-22 |title=Report: Ocean acidification rising at unprecedented rate |url=http://www.mcclatchydc.com/news/nation-world/national/article24580576.html |website=mcclatchydc}}</ref><ref name=":10" /> In combination with other ocean [[biogeochemical]] changes, this drop in pH value could undermine the functioning of marine ecosystems and disrupt the provision of many goods and services associated with the ocean, beginning as early as 2100.<ref name=":3">{{Cite journal |last1=Mora |first1=Camilo |last2=Wei |first2=Chih-Lin |last3=Rollo |first3=Audrey |last4=Amaro |first4=Teresa |last5=Baco |first5=Amy R. |last6=Billett |first6=David |last7=Bopp |first7=Laurent |last8=Chen |first8=Qi |last9=Collier |first9=Mark |last10=Danovaro |first10=Roberto |last11=Gooday |first11=Andrew J. |last12=Grupe |first12=Benjamin M. |last13=Halloran |first13=Paul R. |last14=Ingels |first14=Jeroen |last15=Jones |first15=Daniel O. B. |date=2013-10-15 |editor-last=Mace |editor-first=Georgina M. |title=Biotic and Human Vulnerability to Projected Changes in Ocean Biogeochemistry over the 21st Century |journal=PLOS Biology |volume=11 |issue=10 |pages=e1001682 |doi=10.1371/journal.pbio.1001682 |pmc=3797030 |pmid=24143135 |doi-access=free}}</ref>

The extent of further ocean chemistry changes, including ocean pH, will depend on [[climate change mitigation]] efforts taken by nations and their governments.<ref name=":82" /> Different scenarios of projected [[Socioeconomics|socioeconomic]] global changes are modelled by using the [[Shared Socioeconomic Pathways]] (SSP) scenarios.

Under a [[Shared Socioeconomic Pathways|very high emission scenario (SSP5-8.5)]], model projections estimate that surface ocean pH could decrease by as much as 0.44 units by the end of this century, compared to the end of the 19th century.<ref>Lee, J.-Y., J. Marotzke, G. Bala, L. Cao, S. Corti, J.P. Dunne, F. Engelbrecht, E. Fischer, J.C. Fyfe, C. Jones, A. Maycock, J. Mutemi, O. Ndiaye, S. Panickal, and T. Zhou, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter04.pdf Chapter 4: Future Global Climate: Scenario-Based Projections and Near- Term Information] {{Webarchive|url=https://web.archive.org/web/20221109133340/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter04.pdf |date=9 November 2022 }}. In [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FullReport.pdf Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20221018055338/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FullReport.pdf |date=18 October 2022 }} [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 553–672.</ref>{{rp|608}} This would mean a pH as low as about 7.7, and represents a further increase in H+ concentrations of two to four times beyond the increase to date.
{| class="wikitable"
|+Estimated past and future global mean surface pH for different emission scenarios<ref name=":82" />{{rp|values estimated from Figure TS.11 (d)}}
!Time period
!Ocean surface<br>pH value (approx.)
|-
|Pre-industrial (1850)
|8.17
|-
|Current (2021)
|8.08
|-
|Future (2100) with low emission scenario ([[Shared Socioeconomic Pathways|SSP]] 1–2.6)
|8.0
|-
|Future (2100) with very high emission scenario ([[Shared Socioeconomic Pathways|SSP]] 5–8.5)
|7.7
|}

== Impacts on oceanic calcifying organisms ==
{{See also|Marine biogenic calcification}}
[[File:A pteropod shell is shown dissolving over time.jpg|thumb|A [[Pteropoda|pteropod]] shell is shown dissolving over time in seawater with a lower pH. When carbon dioxide is absorbed by the ocean from the atmosphere, the chemistry of the seawater is changed (source: [[National Oceanic and Atmospheric Administration|NOAA]])]]
[[File:Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100.jpg|thumb|Pterapod shell dissolved in seawater adjusted to an [[ocean chemistry]] projected for the year 2100 (source: [[National Oceanic and Atmospheric Administration|NOAA]]).]]
[[File:Fis01026 (27555118153).jpg|thumb|Unhealthy pteropod showing effects of ocean acidification including ragged, dissolving shell ridges on upper surface, a cloudy shell in lower right quadrant, and severe abrasions and weak spots at 6:30 position on lower whorl of shell (source: [[National Oceanic and Atmospheric Administration|NOAA]]).]]

=== Complexity of research findings ===
The full ecological consequences of the changes in calcification due to ocean acidification are complex but it appears likely that many calcifying species will be adversely affected by ocean acidification.<ref name=":14" /><ref name=":43" />{{rp|413}} Increasing ocean acidification makes it more difficult for shell-accreting organisms to access carbonate ions, essential for the production of their hard exoskeletal shell.<ref name=":11">{{Cite web |title=PMEL {{CO2}} – Carbon Dioxide Program |url=https://www.pmel.noaa.gov/co2 |access-date=2021-09-06 |website=NOAA Pacific Marine Environmental Laboratory}}</ref> Oceanic [[calcification|calcifying]] organism span the [[food chain]] from [[autotroph]]s to [[heterotroph]]s and include organisms such as [[coccolithophore]]s, [[coral]]s, [[foraminifera]], [[echinoderm]]s, [[crustacea]]ns and [[mollusca|molluscs]].<ref name=":3" /><ref name=":0">National Research Council. ''[http://www.nap.edu/openbook.php?record_id=12877&page=R1 Overview of Climate Changes and Illustrative Impacts. Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia] {{Webarchive|url=https://web.archive.org/web/20150906174354/http://www.nap.edu/openbook.php?record_id=12877&page=R1 |date=6 September 2015 }}.'' Washington, DC: The National Academies Press, 2011. 1. Print.</ref>

Overall, all marine ecosystems on Earth will be exposed to changes in acidification and several other ocean biogeochemical changes.<ref>{{Cite journal |last1=Fairchild |first1=William |last2=Hales |first2=Burke |date=14 January 2021 |title=High-Resolution Carbonate System Dynamics of Netarts Bay, OR From 2014 to 2019 |journal=Frontiers in Marine Science |volume=7 |pages=590236 |doi=10.3389/fmars.2020.590236 |doi-access=free|bibcode=2021FrMaS...790236F }}</ref> Ocean acidification may force some organisms to reallocate resources away from productive endpoints in order to maintain calcification.<ref>{{Cite journal |last1=Wood |first1=Hannah L. |last2=Spicer |first2=John I. |last3=Widdicombe |first3=Stephen |year=2008 |title=Ocean acidification may increase calcification rates, but at a cost |journal=[[Proceedings of the Royal Society B]] |volume=275 |issue=1644 |pages=1767–1773 |doi=10.1098/rspb.2008.0343 |pmc=2587798 |pmid=18460426}}</ref> For example, the oyster ''[[Pacific oyster|Magallana gigas]]'' is recognized to experience metabolic changes alongside altered [[calcification]] rates due to energetic tradeoffs resulting from pH imbalances.<ref name=":6">{{Cite journal |last1=Ducker |first1=James |last2=Falkenberg |first2=Laura J. |date=12 November 2020 |title=How the Pacific Oyster Responds to Ocean Acidification: Development and Application of a Meta-Analysis Based Adverse Outcome Pathway |journal=Frontiers in Marine Science |volume=7 |pages=597441 |doi=10.3389/fmars.2020.597441 |doi-access=free|bibcode=2020FrMaS...797441D }}</ref>

Under normal conditions, calcite and aragonite are stable in surface waters since the carbonate ions are [[supersaturation|supersaturated]] with respect to seawater. However, as ocean pH falls, the concentration of carbonate ions also decreases. Calcium carbonate thus becomes undersaturated, and structures made of calcium carbonate are vulnerable to calcification stress and dissolution.<ref>{{Cite journal |last1=Spalding |first1=Christopher |last2=Finnegan |first2=Seth |last3=Fischer |first3=Woodward W. |date=May 2017 |title=Energetic costs of calcification under ocean acidification |url=https://authors.library.caltech.edu/77249/ |journal=Global Biogeochemical Cycles |volume=31 |issue=5 |pages=866–877 |bibcode=2017GBioC..31..866S |doi=10.1002/2016GB005597 |s2cid=133783884}}</ref> In particular, studies show that corals,<ref name="ocean">{{Cite news |date=November 14, 2013 |title=Ocean Acidification Summary for Policymakers |url=http://www.igbp.net/publications/summariesforpolicymakers/summariesforpolicymakers/oceanacidificationsummaryforpolicymakers2013.5.30566fc6142425d6c9111f4.html |publisher=IGBP}}</ref><ref name=":7">{{Cite web |date=2019-09-25 |title=Special Report on the Ocean and Cryosphere in a Changing Climate — Special Report on the Ocean and Cryosphere in a Changing Climate |url=https://www.ipcc.ch/srocc/home/ |access-date=2019-11-12 |website=IPCC}}</ref> coccolithophores,<ref name=":0" /><ref name="Carstensen2019" /><ref name="gatt98">{{Cite journal |last=Gattuso |first=J |date=July 1998 |title=Effect of calcium carbonate saturation of seawater on coral calcification |journal=Global and Planetary Change |volume=18 |issue=1–2 |pages=37–46 |bibcode=1998GPC....18...37G |doi=10.1016/S0921-8181(98)00035-6}}</ref> coralline algae,<ref name="gatt99">{{Cite journal |last1=Gattuso |first1=Jean-Pierre |last2=Allemand |first2=Denis |last3=Frankignoulle |first3=Michel |date=February 1999 |title=Photosynthesis and Calcification at Cellular, Organismal and Community Levels in Coral Reefs: A Review on Interactions and Control by Carbonate Chemistry |journal=American Zoologist |volume=39 |issue=1 |pages=160–183 |citeseerx=10.1.1.321.3042 |doi=10.1093/icb/39.1.160}}</ref> foraminifera,<ref name="lan05">{{Cite journal |last1=Langdon |first1=C. |last2=Atkinson |first2=M. J. |year=2005 |title=Effect of elevated pCO<sub>2</sub> on photosynthesis and calcification of corals and interactions with seasonal change in temperature/irradiance and nutrient enrichment |journal=[[Journal of Geophysical Research]] |volume=110 |issue=C09S07 |pages=C09S07 |doi=10.1029/2004JC002576 |doi-access=free|bibcode=2005JGRC..110.9S07L }}</ref> shellfish and [[Pteropoda|pteropods]]<ref name="rieb00">{{Cite journal |last1=Riebesell |first1=Ulf |last2=Zondervan |first2=Ingrid |last3=Rost |first3=Björn |last4=Tortell |first4=Philippe D. |last5=Zeebe |first5=Richard E. |last6=Morel |first6=François M. M. |date=September 2000 |title=Reduced calcification of marine plankton in response to increased atmospheric {{CO2}} |url=https://epic.awi.de/id/eprint/3784/1/Rie2000a.pdf |journal=Nature |volume=407 |issue=6802 |pages=364–367 |bibcode=2000Natur.407..364R |doi=10.1038/35030078 |pmid=11014189 |s2cid=4426501}}</ref> experience reduced calcification or enhanced dissolution when exposed to elevated {{CO2}}. Even with active [[marine conservation]] practices it may be impossible to bring back many previous shellfish populations.<ref name="gaz07">{{Cite journal |last1=Gazeau |first1=Frédéric |last2=Quiblier |first2=Christophe |last3=Jansen |first3=Jeroen M. |last4=Gattuso |first4=Jean-Pierre |last5=Middelburg |first5=Jack J. |last6=Heip |first6=Carlo H. R. |date=6 April 2007 |title=Impact of elevated CO 2 on shellfish calcification |url=https://hal.archives-ouvertes.fr/hal-03334206/file/2006GL028554.pdf |journal=Geophysical Research Letters |volume=34 |issue=7 |pages=L07603 |bibcode=2007GeoRL..34.7603G |doi=10.1029/2006GL028554 |s2cid=130190489 |hdl-access=free |hdl=20.500.11755/a8941c6a-6d0b-43d5-ba0d-157a7aa05668}}</ref>

Some studies have found different responses to ocean acidification, with coccolithophore calcification and photosynthesis both increasing under elevated atmospheric [[pCO2|p{{CO2}}]],<ref name="Iglesias2008">{{Cite journal |last1=Iglesias-Rodriguez |first1=M.D. |last2=Halloran, P.R. |last3=Rickaby, R.E.M. |last4=Hall, I.R. |last5=Colmenero-Hidalgo, E. |last6=Gittins, J.R. |last7=Green, D.R.H. |last8=Tyrrell, T. |last9=Gibbs, S.J. |last10=von Dassow, P. |last11=Rehm, E. |last12=Armbrust, E.V. |last13=Boessenkool, K.P. |year=2008 |title=Phytoplankton Calcification in a High-CO<sub>2</sub> World |journal=[[Science (journal)|Science]] |volume=320 |issue=5874 |pages=336–340 |bibcode=2008Sci...320..336I |doi=10.1126/science.1154122 |pmid=18420926 |s2cid=206511068}}</ref> and an equal decline in [[primary production]] and calcification in response to elevated {{CO2}},<ref name="sciandra03">{{Cite journal |last1=Sciandra |first1=Antoine |last2=Harlay |first2=Jérome |last3=Lefèvre |first3=Dominique |last4=Lemée |first4=Rodolphe |last5=Rimmelin |first5=Peguy |last6=Denis |first6=Michel |last7=Gattuso |first7=Jean-Pierre |date=17 October 2003 |title=Response of coccolithophorid Emiliania huxleyi to elevated partial pressure of {{CO2}} under nitrogen limitation |journal=Marine Ecology Progress Series |volume=261 |pages=111–122 |bibcode=2003MEPS..261..111S |doi=10.3354/meps261111 |doi-access=free}}</ref> or the direction of the response varying between species.<ref name="langer06">{{Cite journal |last1=Langer |first1=Gerald |last2=Geisen |first2=Markus |last3=Baumann |first3=Karl-Heinz |last4=Kläs |first4=Jessica |last5=Riebesell |first5=Ulf |last6=Thoms |first6=Silke |last7=Young |first7=Jeremy R. |date=September 2006 |title=Species-specific responses of calcifying algae to changing seawater carbonate chemistry |url=https://epic.awi.de/id/eprint/14731/1/Lan2006e.pdf |journal=Geochemistry, Geophysics, Geosystems |volume=7 |issue=9 |bibcode=2006GGG.....7.9006L |doi=10.1029/2005GC001227 |s2cid=14774230}}</ref>

Similarly, the sea star, [[Pisaster ochraceus]], shows enhanced growth in waters with increased acidity.<ref name="Turley 278–2862">{{Cite journal |last1=Turley |first1=Carol |last2=Gattuso |first2=Jean-Pierre |date=July 2012 |title=Future biological and ecosystem impacts of ocean acidification and their socioeconomic-policy implications |journal=Current Opinion in Environmental Sustainability |volume=4 |issue=3 |pages=278–286 |bibcode=2012COES....4..278T |doi=10.1016/j.cosust.2012.05.007 |doi-access=free}}</ref>

Reduced calcification from ocean acidification&nbsp;may affect the ocean's biologically driven sequestration of [[carbon]] from the atmosphere to the ocean interior and [[seafloor sediment]], weakening the so-called [[biological pump]].<ref name="PNAS-20191021" /> Seawater acidification could also reduce the size of Antarctic phytoplankton, making them less effective at storing carbon.<ref>{{Cite web |last1=Petrou |first1=Katherina |last2=Nielsen |first2=Daniel |date=2019-08-27 |title=Acid oceans are shrinking plankton, fueling faster climate change |url=https://phys.org/news/2019-08-acid-oceans-plankton-fueling-faster.html |access-date=2019-11-12 |website=phys.org}}</ref> Such changes are being increasingly studied and synthesized through the use of physiological frameworks, including the [[Adverse outcome pathway|Adverse Outcome Pathway (AOP)]] framework.<ref name=":6" />
[[File:Coccolithus_pelagicus.jpg|thumb|''Coccolithus pelagicus,'' a species of [[coccolithophore]] sampled from the North Atlantic Ocean.]]

=== Coccolithophores ===
{{Further|Coccolithophore}}
A coccolithophore is a [[Unicellular organism|unicellular]], [[eukaryotic]] [[phytoplankton]] ([[alga]]). Understanding calcification changes in coccolithophores may be particularly important because a decline in the coccolithophores may have secondary effects on climate: it could contribute to global warming by decreasing the Earth's [[albedo]] via their effects on oceanic cloud cover.<ref name="Stony2010">{{Cite press release |title=Acidification Of Oceans May Contribute To Global Declines Of Shellfish, Study By Stony Brook Scientists Concludes |date=27 September 2010 |publisher=School of Marine and Atmospheric Sciences at Stony Brook University |url=http://commcgi.cc.stonybrook.edu/am2/publish/General_University_News_2/Acidification_Of_Oceans_May_Contribute_To_Global_Declines_Of_Shellfish_Study_By_Stony_Brook_Scientists_Concludes.shtml |access-date=4 June 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120903222130/http://commcgi.cc.stonybrook.edu/am2/publish/General_University_News_2/Acidification_Of_Oceans_May_Contribute_To_Global_Declines_Of_Shellfish_Study_By_Stony_Brook_Scientists_Concludes.shtml |archive-date=3 September 2012}}</ref> A study in 2008 examined a [[sedimentology|sediment core]] from the [[Atlantic Ocean|North Atlantic]] and found that the species composition of coccolithophorids remained unchanged over the past 224 years (1780 to 2004). But the average coccolith mass had increased by 40% during the same period.<ref name="Iglesias2008" />

=== Corals ===
{{Further|Coral|Coral reef}}

Warm water corals are clearly in decline, with losses of 50% over the last 30–50 years due to multiple threats from ocean warming, ocean acidification, [[Marine pollution|pollution]] and physical damage from activities such as fishing, and these pressures are expected to intensify.<ref name=":32">{{Cite journal |last1=Hoegh-Guldberg |first1=Ove |last2=Poloczanska |first2=Elvira S. |last3=Skirving |first3=William |last4=Dove |first4=Sophie |date=2017 |title=Coral Reef Ecosystems under Climate Change and Ocean Acidification |journal=Frontiers in Marine Science |volume=4 |pages=158 |doi=10.3389/fmars.2017.00158 |doi-access=free|bibcode=2017FrMaS...4..158H }}</ref><ref name=":43" />{{rp|416}}

The fluid in the internal compartments (the coelenteron) where corals grow their [[exoskeleton]] is also extremely important for calcification growth. When the saturation state of aragonite in the external seawater is at ambient levels, the corals will grow their aragonite crystals rapidly in their internal compartments, hence their exoskeleton grows rapidly. If the saturation state of aragonite in the external seawater is lower than the ambient level, the corals have to work harder to maintain the right balance in the internal compartment. When that happens, the process of growing the crystals slows down, and this slows down the rate of how much their exoskeleton is growing. Depending on the aragonite saturation state&nbsp;in the surrounding water, the corals may halt growth because pumping aragonite&nbsp;into the internal compartment will not be energetically favorable.<ref>{{Cite journal |last1=Cohen |first1=A. |last2=Holcomb |first2=M. |year=2009 |title=Why Corals Care About Ocean Acidification: Uncovering the Mechanism |journal=Oceanography |volume=24 |issue=4 |pages=118–127 |doi=10.5670/oceanog.2009.102 |doi-access=free|bibcode=2009Ocgpy..22d.118C }}</ref> Under the current progression of carbon emissions, around 70% of North Atlantic cold-water corals will be living in corrosive waters by 2050–60.<ref name="Perez2018">{{Cite journal |last1=Pérez |first1=F. |last2=Fontela |first2=M. |last3=García-Ibañez |first3=M. |last4=Mercier |first4=H. |last5=Velo |first5=A. |last6=Lherminier |first6=P. |last7=Zunino |first7=P. |last8=de la Paz |first8=M. |last9=Alonso |first9=F. |last10=Guallart |first10=E. |last11=Padín |first11=T. |date=22 February 2018 |title=Meridional overturning circulation conveys fast acidification to the deep Atlantic Ocean |url=https://archimer.ifremer.fr/doc/00426/53783/ |journal=Nature |volume=554 |issue=7693 |pages=515–518 |bibcode=2018Natur.554..515P |doi=10.1038/nature25493 |pmid=29433125 |s2cid=3497477 |hdl=10261/162241}}</ref>

Acidified conditions primarily reduce the coral's capacity to build dense exoskeletons, rather than affecting the linear extension of the exoskeleton. The density of some species of corals could be reduced by over 20% by the end of this century.<ref>{{Cite journal |last1=Mollica |first1=Nathaniel R. |last2=Guo |first2=Weifu |last3=Cohen |first3=Anne L. |last4=Huang |first4=Kuo-Fang |last5=Foster |first5=Gavin L. |last6=Donald |first6=Hannah K. |last7=Solow |first7=Andrew R. |date=20 February 2018 |title=Ocean acidification affects coral growth by reducing skeletal density |journal=Proceedings of the National Academy of Sciences |volume=115 |issue=8 |pages=1754–1759 |bibcode=2018PNAS..115.1754M |doi=10.1073/pnas.1712806115 |pmc=5828584 |pmid=29378969 |doi-access=free}}</ref>

An ''in situ'' experiment, conducted on a 400 m2 patch of the [[Great Barrier Reef]], to decrease seawater CO<sub>2</sub> level (raise pH) to near the preindustrial value showed a 7% increase in net calcification.<ref name="Albright2016">{{Cite journal |last1=Albright |first1=R. |last2=Caldeira |first2=L. |last3=Hosfelt |first3=J. |last4=Kwiatkowski |first4=L. |last5=Maclaren |first5=J. K. |last6=Mason |first6=B. M. |last7=Nebuchina |first7=Y. |last8=Ninokawa |first8=A. |last9=Pongratz |first9=J. |last10=Ricke |first10=K. L. |last11=Rivlin |first11=T. |last12=Schneider |first12=K. |last13=Sesboüé |first13=M. |last14=Shamberger |first14=K. |last15=Silverman |first15=J. |date=24 February 2016 |title=Reversal of ocean acidification enhances net coral reef calcification |journal=Nature |volume=531 |issue=7594 |pages=362–365 |bibcode=2016Natur.531..362A |doi=10.1038/nature17155 |pmid=26909578 |s2cid=205247928 |last16=Wolfe |first16=K. |last17=Zhu |first17=K. |last18=Caldeira |first18=K.}}</ref> A similar experiment to raise in situ seawater CO<sub>2</sub> level (lower pH) to a level expected soon after the 2050 found that net calcification decreased 34%.<ref name="Albright2018">{{Cite journal |last1=Albright |first1=R. |last2=Takeshita |first2=T. |last3=Koweek |first3=D. A. |last4=Ninokawa |first4=A. |last5=Wolfe |first5=K. |last6=Rivlin |first6=T. |last7=Nebuchina |first7=Y. |last8=Young |first8=J. |last9=Caldeira |first9=K. |date=14 March 2018 |title=Carbon dioxide addition to coral reef waters suppresses net community calcification |journal=Nature |volume=555 |issue=7697 |pages=516–519 |bibcode=2018Natur.555..516A |doi=10.1038/nature25968 |pmid=29539634 |s2cid=3935534}}</ref>

However, a field study of the coral reef in [[Queensland]] and [[Western Australia]] from 2007 to 2012 found that corals are more resistant to the environmental pH changes than previously thought, due to internal [[homeostasis]] regulation; this makes thermal change ([[marine heatwave]]s), which leads to [[coral bleaching]], rather than acidification, the main factor for coral reef vulnerability due to climate change.<ref name="MUNDAY" />

==== Studies at carbon dioxide seep sites ====
In some places carbon dioxide bubbles out from the sea floor, locally changing the pH and other aspects of the chemistry of the seawater. Studies of these carbon dioxide seeps have documented a variety of responses by different organisms.<ref name="pmid18536730">{{Cite journal |last1=Hall-Spencer |first1=Jason M. |last2=Rodolfo-Metalpa |first2=Riccardo |last3=Martin |first3=Sophie |last4=Ransome |first4=Emma |last5=Fine |first5=Maoz |last6=Turner |first6=Suzanne M. |last7=Rowley |first7=Sonia J. |last8=Tedesco |first8=Dario |last9=Buia |first9=Maria-Cristina |date=July 2008 |title=Volcanic carbon dioxide vents show ecosystem effects of ocean acidification |journal=Nature |volume=454 |issue=7200 |pages=96–99 |bibcode=2008Natur.454...96H |doi=10.1038/nature07051 |pmid=18536730 |s2cid=9375062 |hdl-access=free |hdl=10026.1/1345}}</ref> Coral reef communities located near carbon dioxide seeps are of particular interest because of the sensitivity of some corals species to acidification. In [[Papua New Guinea]], declining pH caused by carbon dioxide seeps is associated with declines in coral species diversity.<ref>{{Cite journal |last=Fabricius |first=Katharina |date=2011 |title=Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations |journal=Nature Climate Change |volume=1 |issue=3 |pages=165–169 |bibcode=2011NatCC...1..165F |doi=10.1038/nclimate1122 |s2cid=85749253}}</ref> However, in [[Palau]] carbon dioxide seeps are not associated with reduced species diversity of corals, although [[bioerosion]] of coral skeletons is much higher at low pH sites.

=== Pteropods and brittle stars ===
[[Pteropoda|Pteropods]] and [[brittle star]]s both form the base of the Arctic [[food web]]s and are both seriously damaged from acidification. Pteropods shells dissolve with increasing acidification and the brittle stars lose muscle mass when re-growing [[appendage]]s.<ref>{{Cite web |title=Effects of Ocean Acidification on Marine Species & Ecosystems |url=http://oceana.org/en/our-work/climate-energy/ocean-acidification/learn-act/effects-of-ocean-acidification-on-marine-species-ecosystems |access-date=13 October 2013 |website=Report |publisher=OCEANA}}</ref> For pteropods to create shells they require aragonite which is produced through carbonate ions and dissolved calcium and strontium. Pteropods are severely affected because increasing acidification levels have steadily decreased the amount of water supersaturated with carbonate.<ref name="biogeosciences">{{Cite journal |last1=Lischka |first1=S. |last2=Büdenbender |first2=J. |last3=Boxhammer |first3=T. |last4=Riebesell |first4=U. |date=15 April 2011 |title=Impact of ocean acidification and elevated temperatures on early juveniles of the polar shelled pteropod ''Limacina helicina'': mortality, shell degradation, and shell growth |journal=Biogeosciences |volume=8 |issue=4 |pages=919–932 |bibcode=2011BGeo....8..919L |doi=10.5194/bg-8-919-2011 |doi-access=free}}</ref> The degradation of organic matter in Arctic waters has amplified ocean acidification; some Arctic waters are already undersaturated with respect to aragonite.<ref>{{Cite journal |last1=Mathis |first1=Jeremy T. |last2=Cross |first2=Jessica N. |last3=Evans |first3=Wiley |last4=Doney |first4=Scott C. |date=2015 |title=Ocean Acidification in the Surface Waters of the Pacific-Arctic Boundary Regions |journal=Oceanography |volume=28 |issue=2 |pages=122–135 |doi=10.5670/oceanog.2015.36 |jstor=24861875 |doi-access=free|bibcode=2015Ocgpy..25b.122M }}</ref><ref>{{Cite journal |last1=Semiletov |first1=Igor |last2=Pipko |first2=Irina |last3=Gustafsson |first3=Örjan |last4=Anderson |first4=Leif G. |last5=Sergienko |first5=Valentin |last6=Pugach |first6=Svetlana |last7=Dudarev |first7=Oleg |last8=Charkin |first8=Alexander |last9=Gukov |first9=Alexander |last10=Bröder |first10=Lisa |last11=Andersson |first11=August |last12=Spivak |first12=Eduard |last13=Shakhova |first13=Natalia |date=2016 |title=Acidification of East Siberian Arctic Shelf waters through addition of freshwater and terrestrial carbon |journal=Nature Geoscience |volume=9 |issue=5 |pages=361–365 |bibcode=2016NatGe...9..361S |doi=10.1038/ngeo2695 |s2cid=131084581}}</ref><ref>{{Cite journal |last1=Terhaar |first1=Jens |last2=Kwiatkowski |first2=Lester |last3=Bopp |first3=Laurent |date=2020-06-18 |title=Emergent constraint on Arctic Ocean acidification in the twenty-first century |url=https://hal.archives-ouvertes.fr/hal-02884111/file/Terhaar_Nature_2020_preprint.pdf |journal=Nature |volume=582 |issue=7812 |pages=379–383 |bibcode=2020Natur.582..379T |doi=10.1038/s41586-020-2360-3 |pmid=32555488 |s2cid=219729997}}</ref>

The brittle star's eggs die within a few days when exposed to expected conditions resulting from Arctic acidification.<ref name=":02">{{Cite web |title=Comprehensive study of Arctic Ocean acidification |url=http://www.cicero.uio.no/webnews/index_e.aspx?id=11909 |url-status=dead |archive-url=https://web.archive.org/web/20131210231159/http://www.cicero.uio.no/webnews/index_e.aspx?id=11909 |archive-date=10 December 2013 |access-date=14 November 2013 |website=Study |publisher=CICERO}}</ref> Similarly, when exposed in experiments to pH reduced by 0.2 to 0.4, larvae of a temperate [[brittle star]], a relative of the common [[sea star]], fewer than 0.1 percent survived more than eight days.<ref name=":3" />

== Other impacts on ecosystems ==
[[File:Changes in aragonite saturation of the world's oceans, 1880-2012 (US EPA).png|thumb|upright=1.5|This map shows changes in the aragonite saturation level of ocean surface waters between the 1880s and 2006–2015. Aragonite is a form of calcium carbonate that many marine animals use to build their skeletons and shells. The lower the saturation level, the more difficult it is for organisms to build and maintain their skeletons and shells. A negative change represents a decrease in saturation.<ref>{{Cite web |last=Woods Hole Oceanographic Institution |date=August 2016 |title=Changes in Aragonite Saturation of the World's Oceans, 1880–2015 |url=https://www.epa.gov/climate-indicators/climate-change-indicators-ocean-acidity |website=epa.gov}}</ref>]]

=== Other biological impacts ===
Aside from the slowing and/or reversal of calcification, organisms may suffer other adverse effects, either indirectly through negative impacts on food resources, or directly as reproductive or physiological effects.<ref name="raven05" /> For example, the elevated oceanic levels of {{CO2}} may produce {{chem|CO|2}}-induced acidification of body fluids, known as [[hypercapnia]].<ref>{{Cite journal |last1=Onthank |first1=Kirt |last2=Trueblood |first2=Lloyd |last3=Schrock-Duff |first3=Taylir |last4=Kore |first4=Lydia |date=2021 |title=Impact of short- and long-term exposure to elevated seawater pCO<sub>2</sub> on metabolic rate and hypoxia tolerance in Octopus rubescens [Dataset] |journal=Physiological and Biochemical Zoology |publisher=Dryad Data Platform |volume=94 |issue=1 |pages=1–11 |doi=10.5061/dryad.pg4f4qrj8}}</ref>
Increasing acidity has been observed to reduce metabolic rates in [[jumbo squid]]<ref name="Rosa">{{Cite journal |last1=Rosa |first1=R. |last2=Seibel |first2=B. |year=2008 |title=Synergistic effects of climate-related variables suggest future physiological impairment in a top oceanic predator |journal=PNAS |volume=105 |issue=52 |pages=20776–20780 |bibcode=2008PNAS..10520776R |doi=10.1073/pnas.0806886105 |pmc=2634909 |pmid=19075232 |doi-access=free}}</ref> and depress the immune responses of blue mussels.<ref name="Bibby">{{Cite journal |last1=Bibby |first1=R |last2=Widdicombe |first2=S |last3=Parry |first3=H |last4=Spicer |first4=J |last5=Pipe |first5=R |date=27 March 2008 |title=Effects of ocean acidification on the immune response of the blue mussel Mytilus edulis |journal=Aquatic Biology |volume=2 |pages=67–74 |doi=10.3354/ab00037 |doi-access=free|bibcode=2008AquaB...2...67B }}</ref>
[[Atlantic longfin squid]] eggs took longer to hatch in acidified water, and the squid's [[Statocyst|statolith]] was smaller and malformed in animals placed in sea water with a lower pH.<ref>{{Cite web |last=Kwok |first=Roberta |date=4 June 2013 |title=Ocean acidification could make squid develop abnormally |url=http://conservationmagazine.org/2013/06/stunted-growth/ |access-date=2013-08-24 |publisher=University of Washington}}</ref> However, these studies are ongoing and there is not yet a full understanding of these processes in marine organisms or [[ecosystem]]s.<ref name="Ocean Acidification and Krill">{{Cite news |year=2008 |title=Swiss marine researcher moving in for the krill |url=http://www.theaustralian.news.com.au/story/0,25197,24392216-27703,00.html |url-status=dead |archive-url=https://web.archive.org/web/20081211233233/http://www.theaustralian.news.com.au/story/0,25197,24392216-27703,00.html |archive-date=11 December 2008 |access-date=28 September 2008 |work=The Australian}}</ref>

==== Acoustic properties ====
Another potential route to ecosystem impacts is through [[bioacoustics]]. This may occur as ocean acidification can alter the [[Acoustics|acoustic]] properties of seawater, allowing sound to propagate further, and increasing ocean noise.<ref name="Hester">{{Cite journal |last1=Hester |first1=Keith C. |last2=Peltzer |first2=Edward T. |last3=Kirkwood |first3=William J. |last4=Brewer |first4=Peter G. |date=1 October 2008 |title=Unanticipated consequences of ocean acidification: A noisier ocean at lower pH |journal=Geophysical Research Letters |volume=35 |issue=19 |pages=L19601 |bibcode=2008GeoRL..3519601H |doi=10.1029/2008GL034913 |doi-access=free}}</ref> This impacts all animals that use sound for [[animal echolocation|echolocation]] or [[whale song|communication]].<ref name="npr-threat">[https://www.npr.org/templates/transcript/transcript.php?storyId=111807469 Acid In The Oceans: A Growing Threat To Sea Life] {{Webarchive|url=https://web.archive.org/web/20180512043714/https://www.npr.org/templates/transcript/transcript.php?storyId=111807469 |date=12 May 2018 }} by Richard Harris. All Things Considered, 12 August 2009.</ref>

==== Algae and seagrasses ====
{{Further|Eutrophication}}
Another possible effect would be an increase in [[harmful algal bloom]] events, which could contribute to the accumulation of toxins ([[domoic acid]], [[brevetoxin]], [[saxitoxin]]) in small organisms such as [[anchovies]] and [[shellfish]], in turn increasing occurrences of [[amnesic shellfish poisoning]], [[neurotoxic shellfish poisoning]] and [[paralytic shellfish poisoning]].<ref name="Ocean Acidification Promotes Disruptive and Harmful Algal Blooms on Our Coasts">{{Cite web |year=2014 |title=Ocean Acidification Promotes Disruptive and Harmful Algal Blooms on Our Coasts |url=http://coastalscience.noaa.gov/news/climate/ocean-acidification-promotes-disruptive-and-harmful-algal-blooms-on-our-coasts/}}</ref> Although algal blooms can be harmful, other beneficial photosynthetic organisms may benefit from increased levels of carbon dioxide. Most importantly, seagrasses will benefit.<ref name="Turley 278–2862"/> Research found that as seagrasses increased their photosynthetic activity, calcifying algae's calcification rates rose, likely because localized photosynthetic activity absorbed carbon dioxide and elevated local pH.<ref name="Turley 278–2862"/>

====Fish larvae====

Ocean acidification can also have effects on marine [[fish larvae]]. It internally affects their olfactory systems, which is a crucial part of their early development. [[Orange clownfish]] larvae mostly live on oceanic reefs that are surrounded by vegetative islands{{clarify|date=October 2023}}.<ref name="MUNDAY">{{Cite journal |last=Munday |first=Philip L. |year=2009 |title=Ocean Acidification Impairs Olfactory Discrimination and Homing Ability of a Marine Fish |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=6 |pages=1848–52 |bibcode=2009PNAS..106.1848M |doi=10.1073/pnas.0809996106 |pmc=2644126 |pmid=19188596 |doi-access=free}}</ref> Larvae are known to use their sense of smell to detect the differences between reefs surrounded by vegetative islands and reefs not surrounded by vegetative islands.<ref name="MUNDAY" /> Clownfish larvae need to be able to distinguish between these two destinations to be able to find a suitable area for their growth. Another use for marine fish olfactory systems is to distinguish between their parents and other adult fish, in order to avoid inbreeding.

In an experimental aquarium facility, clownfish were sustained in non-manipulated seawater with pH 8.15&nbsp;± 0.07, which is similar to our current ocean's pH.<ref name="MUNDAY" /> To test for effects of different pH levels, the seawater was modified to two other pH levels, which corresponded with climate change models that predict future atmospheric {{CO2}} levels.<ref name="MUNDAY" /> In the year 2100 the model projects possible {{CO2}} levels of 1,000&nbsp;ppm, which correlates with the pH of 7.8&nbsp;± 0.05.

This experiment showed that when larvae are exposed to a pH of 7.8&nbsp;± 0.05 their reaction to environmental cues differs drastically from their reaction to cues at pH equal to current ocean levels.<ref name="MUNDAY" /> At pH 7.6&nbsp;± 0.05 larvae had no reaction to any type of cue. However, a [[meta-analysis]] published in 2022 found that the [[effect size]]s of published studies testing for ocean acidification effects on fish behavior have declined by an order of magnitude over the past decade, and have been negligible for the past five years.<ref name="Clements e3001511">{{Cite journal |last1=Clements |first1=Jeff C. |last2=Sundin |first2=Josefin |last3=Clark |first3=Timothy D. |last4=Jutfelt |first4=Fredrik |date=2022-02-03 |title=Meta-analysis reveals an extreme 'decline effect' in the impacts of ocean acidification on fish behavior |journal=PLOS Biology |volume=20 |issue=2 |pages=e3001511 |doi=10.1371/journal.pbio.3001511 |pmc=8812914 |pmid=35113875 |doi-access=free}}</ref>

Eel embryos, a "critically endangered" species<ref>Pike C, Crook V, GollockM. Anguilla anguilla. The IUCN Red List ofThreatened Species 2020: e. T60344A152845178. International Union for Conservation ofNature; 2020. https://doi.org/10.2305/ IUCN.UK.2020-2.RLTS.T60344A152845178.en</ref> yet profound{{clarify|date=October 2023}} in aquaculture, are also being affected by ocean acidification, specifically the [[European eel]]. Although they spend most of their lives in fresh water, usually in rivers, streams, or estuaries, they go to spawn and die in the [[Sargasso Sea]]. Here is where European eels are experiencing the effects of acidification in one of their key life stages.

Fish embryos and larvae are usually more sensitive to pH changes than adults, as organs for pH regulation are not full developed.<ref>{{Cite book |last1=Brauner |first1=Colin J. |url=https://www.sciencedirect.com/science/article/pii/S1546509819300093 |title=Acid-base physiology and {{CO2}} homeostasis: Regulation and compensation in response to elevated environmental {{CO2}} |last2=Shartau |first2=Ryan B. |last3=Damsgaard |first3=Christian |last4=Esbaugh |first4=Andrew J. |last5=Wilson |first5=Rod W. |last6=Grosell |first6=Martin |date=January 2019 |publisher=[[Academic Press]] |editor-last=Grosell |editor-first=Martin |volume=37 |pages=69–132 |doi=10.1016/bs.fp.2019.08.003 |eissn=1546-5098 |editor-last2=Munday |editor-first2=Philip L. |editor-last3=Farrell |editor-first3=Anthony P. |editor-last4=Brauner |editor-first4=Colin J.}}</ref> Because of this, European eel embryos are more vulnerable to changes in pH in the Sargasso Sea. A study of the European Eel in the Sargasso Sea was conducted in 2021 to analyze the specific effects of ocean acidification on embryos. The study found that exposure to predicted end-of-century ocean pCO<sub>2</sub> conditions may affect normal development of this species in nature during sensitive early life history stages with limited physiological response capacities, while extreme acidification would negatively influence embryonic survival and development under hatchery conditions.<ref>Sganga, Daniela E., et al. "CO<sub>2</sub> Induced Seawater Acidification Impacts Survival and Development of European Eel Embryos." PLOS ONE, edited by Hans G. Dam, vol. 17, no. 4, Apr. 2022, p. e0267228. DOI.org (Crossref), https://doi.org/10.1371/journal.pone.0267228.</ref>

===Compounded effects of acidification, warming and deoxygenation===
{{Further|Effects of climate change on oceans|Ocean deoxygenation}}
[[File:Drivers of hypoxia and acidification in upwelling shelf systems.svg|thumb|upright=1.7| Drivers of [[Hypoxia (environmental)|hypoxia]] and ocean acidification intensification in [[upwelling]] shelf systems. Equatorward winds drive the upwelling of low [[dissolved oxygen]] (DO), high nutrient, and high [[dissolved inorganic carbon]] (DIC) water from above the [[oxygen minimum zone]]. Cross-shelf gradients in productivity and bottom water residence times drive the strength of DO (DIC) decrease (increase) as water transits across a productive [[continental shelf]].<ref name="Chan2019">{{Cite journal |last1=Chan |first1=Francis |last2=Barth |first2=John |last3=Kroeker |first3=Kristy |last4=Lubchenco |first4=Jane |last5=Menge |first5=Bruce |date=1 September 2019 |title=The Dynamics and Impact of Ocean Acidification and Hypoxia: Insights from Sustained Investigations in the Northern California Current Large Marine Ecosystem |journal=Oceanography |volume=32 |issue=3 |pages=62–71 |doi=10.5670/oceanog.2019.312 |s2cid=202922296 |doi-access=free|bibcode=2019Ocgpy..32c..62C }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=16 October 2017 }}.</ref><ref>{{Cite journal |last=Gewin |first=Virginia |date=August 2010 |title=Oceanography: Dead in the water |journal=Nature |volume=466 |issue=7308 |pages=812–814 |doi=10.1038/466812a |pmid=20703282 |s2cid=4358903 |doi-access=free}}</ref>]]

There is a substantial body of research showing that a combination of ocean acidification and elevated [[ocean temperature]] have a compounded effect on marine life and the ocean environment. This effect far exceeds the individual harmful impact of either.<ref>{{Cite journal |last1=Beusen |first1=Arthur |last2=Breitburg |first2=Denise |last3=Comeau |first3=Steve |last4=Dupont |first4=Sam |last5=Isensee |first5=Kirsten |last6=Kudela |first6=Raphael M. |last7=Lundholm |first7=Nina |last8=Otto |first8=Saskia |last9=Schwing |first9=Franklin |last10=Tilbrook |first10=Bronte |last11=Hansen |first11=Per Juel |last12=Boyd |first12=Philip W. |date=2022 |others=UNESCO/IOC, UNESCO/IOC, Philip W. Boyd, Kirsten Isensee |title=Multiple Ocean Stressors: A scientific Summary for Policy Makers. |journal=IOC Information Series |publisher=Unesco-Ioc |doi=10.25607/OBP-1724 |number=1404 |hdl=11329/1892}}</ref> In addition, ocean warming, along with increased productivity of phytoplankton from higher CO<sub>2</sub> levels exacerbates [[ocean deoxygenation]]. Deoxygenation of ocean waters is an additional stressor on marine organisms that increases [[ocean stratification]] therefore limiting nutrients over time and reducing biological gradients.<ref>{{Cite journal |last1=Bednaršek |first1=N. |last2=Harvey |first2=C.J. |last3=Kaplan |first3=I.C. |last4=Feely |first4=R.A. |last5=Možina |first5=J. |year=2016 |title=Pteropods on the edge: Cumulative effects of ocean acidification, warming, and deoxygenation |journal=Progress in Oceanography |volume=145 |pages=1–24 |bibcode=2016PrOce.145....1B |doi=10.1016/j.pocean.2016.04.002}}</ref><ref>{{Cite journal |last1=Keeling |first1=Ralph F. |last2=Garcia |first2=Hernan E. |year=2002 |title=The change in oceanic O2 inventory associated with recent global warming |journal=Proceedings of the National Academy of Sciences |volume=99 |issue=12 |pages=7848–7853 |bibcode=2002PNAS...99.7848K |doi=10.1073/pnas.122154899 |pmc=122983 |pmid=12048249 |doi-access=free}}</ref>

Meta analyses have quantified the direction and magnitude of the harmful effects of combined ocean acidification, warming and deoxygenation on the ocean.<ref>{{Cite journal |last=Gruber |first=Nicolas |date=2011-05-28 |title=Warming up, turning sour, losing breath: ocean biogeochemistry under global change |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=369 |issue=1943 |pages=1980–1996 |bibcode=2011RSPTA.369.1980G |doi=10.1098/rsta.2011.0003 |pmid=21502171 |s2cid=836308}}</ref><ref>{{Cite journal |last1=Anthony |first1=Kenneth R. N. |last2=Maynard |first2=Jeffrey A. |last3=Diaz-Pulido |first3=Guillermo |last4=Mumby |first4=Peter J. |last5=Marshall |first5=Paul A. |last6=Cao |first6=Long |last7=Hoegh-Guldberg |first7=Ove |date=2011 |title=Ocean acidification and warming will lower coral reef resilience |journal=Global Change Biology |volume=17 |issue=5 |pages=1798–1808 |bibcode=2011GCBio..17.1798A |doi=10.1111/j.1365-2486.2010.02364.x |pmc=3597261}}</ref> These meta-analyses have been further tested by [[mesocosm]] studies that simulated the interaction of these stressors and found a catastrophic effect on the marine food web: thermal stress more than negates any primary producer to herbivore increase in productivity from elevated {{CO2}}.<ref>{{Cite journal |last1=Goldenberg |first1=Silvan U. |last2=Nagelkerken |first2=Ivan |last3=Ferreira |first3=Camilo M. |last4=Ullah |first4=Hadayet |last5=Connell |first5=Sean D. |date=2017 |title=Boosted food web productivity through ocean acidification collapses under warming |journal=Global Change Biology |volume=23 |issue=10 |pages=4177–4184 |doi=10.1111/gcb.13699 |pmid=28447365 |bibcode=2017GCBio..23.4177G |s2cid=8015698}}</ref><ref>{{Cite journal |last1=Pistevos |first1=Jennifer C. A. |last2=Nagelkerken |first2=Ivan |last3=Rossi |first3=Tullio |last4=Olmos |first4=Maxime |last5=Connell |first5=Sean D. |date=2015 |title=Ocean acidification and global warming impair shark hunting behaviour and growth |journal=Scientific Reports |volume=5 |issue=1 |pages=16293 |bibcode=2015NatSR...516293P |doi=10.1038/srep16293 |pmc=4642292 |pmid=26559327}}</ref>

== Impacts on the economy and societies ==
The increase of ocean acidity decelerates the rate of calcification in salt water, leading to smaller and slower growing [[coral reef]]s which supports approximately 25% of marine life.<ref>{{Cite web |title=Coral reefs |url=http://wwf.panda.org/our_work/oceans/coasts/coral_reefs/ |access-date=2019-05-06 |website=WWF}}</ref><ref name=":05">{{Cite journal |last1=Fabry |first1=Victoria J. |last2=Seibel |first2=Brad A. |last3=Feely |first3=Richard A. |last4=Orr |first4=James C. |date=April 2008 |title=Impacts of ocean acidification on marine fauna and ecosystem processes |journal=ICES Journal of Marine Science |volume=65 |issue=3 |pages=414–432 |doi=10.1093/icesjms/fsn048}}</ref> Impacts are far-reaching from fisheries and coastal environments down to the deepest depths of the ocean.<ref name=":14" /> The increase in ocean acidity is not only killing the coral, but also the wildly diverse population of marine inhabitants which coral reefs support.<ref name=":72">{{Cite journal |last1=Hoegh-Guldberg |first1=O. |last2=Mumby |first2=P. J. |last3=Hooten |first3=A. J. |last4=Steneck |first4=R. S. |last5=Greenfield |first5=P. |last6=Gomez |first6=E. |last7=Harvell |first7=C. D. |last8=Sale |first8=P. F. |last9=Edwards |first9=A. J. |last10=Caldeira |first10=K. |last11=Knowlton |first11=N. |last12=Eakin |first12=C. M. |last13=Iglesias-Prieto |first13=R. |last14=Muthiga |first14=N. |last15=Bradbury |first15=R. H. |date=14 December 2007 |title=Coral Reefs Under Rapid Climate Change and Ocean Acidification |journal=Science |volume=318 |issue=5857 |pages=1737–1742 |bibcode=2007Sci...318.1737H |doi=10.1126/science.1152509 |pmid=18079392 |s2cid=12607336 |hdl-access=free |last17=Hatziolos |last16=Dubi |first16=A. |first17=M. E. |hdl=1885/28834}}</ref>

=== Fishing and tourism industry ===
The threat of acidification includes a decline in [[Commercial fishing|commercial fisheries]] and the coast-based [[tourism industry]]. Several ocean goods and services are likely to be undermined by future ocean acidification potentially affecting the livelihoods of some 400 to 800 million people, depending upon the [[Shared Socioeconomic Pathways|greenhouse gas emission scenario]].<ref name=":3" />

Some 1 billion people are completely or partially dependent on the fishing, tourism, and coastal management services provided by coral reefs. Ongoing acidification of the oceans may therefore threaten future [[food chains]] linked with the oceans.<ref name="auto" /><ref name=":18" />

==== Arctic ====
In the Arctic, commercial fisheries are threatened because acidification harms calcifying organisms which form the base of the Arctic food webs (pteropods and brittle stars, see above). &nbsp;Acidification threatens Arctic food webs from the base up. Arctic food webs are considered simple, meaning there are few steps in the food chain from small organisms to larger predators. For example, pteropods are "a key prey item of a number of higher predators – larger plankton, fish, seabirds, whales".<ref>{{Cite web |title=Antarctic marine wildlife is under threat, study finds |work=BBC Nature |url=https://www.bbc.co.uk/nature/20461646 |access-date=13 October 2013 }}</ref> Both pteropods and sea stars serve as a substantial food source and their removal from the simple food web would pose a serious threat to the whole ecosystem. The effects on the calcifying organisms at the base of the food webs could potentially destroy fisheries.

==== United Kingdom commercial fisheries ====

The shellfish industry is an important part of the United Kingdom economy.<ref name=":21">{{Cite journal |last=Mangi |first=Stephen |date=2018-05-25 |title=The economic impacts of ocean acidification on shellfish fisheries and aquaculture in the United Kingdom |url=https://doi.org/10.31230/osf.io/d4wh7 |access-date=2025-02-15 |website=doi.org|doi=10.31230/osf.io/d4wh7 }}</ref> In 2013, the shellfish industry contributed 37% of total landings by value.<ref name=":21" /> England and Scotland are the highest producers of shellfish within the United Kingdom.<ref name=":21" /> It has been found that annually fishers catch 66,000 t and 61,000 t.<ref name=":21" /> In terms of value, the wild-captured shellfish are worth 203 million pounds per year.<ref name=":21" /> However, ocean acidification is causing a decrease in the growth of many shellfish species.<ref name=":21" /> This is causing a drastic economic loss in the United Kingdom economy.<ref name=":21" />

It is predicted that by 2100 there will be an economy-wide economic loss of shellfish production in the United Kingdom.<ref name=":21" /> The direct potential loss ranges from 14 to 28 percent of fishery output.<ref name=":21" /> That is a total loss of about 23 to 88 million pounds.<ref name=":21" /> The financial losses vary regionally due to different patterns of wild-caught shellfish and the exploitation of species with differing sensitivities to ocean acidification.<ref name=":21" /> Shellfish resources in the United Kingdom will require regional, national, or international solutions to reduce the impacts of ocean acidification on shellfish species and stabilize the economy.<ref name=":21" />

==== US commercial fisheries ====
[[File:R0023623_-_Flickr_-_NOAA_Photo_Library.jpg|thumb|An adult [[American lobster]] rests on the sea floor. Rhode Island, Dutch Island, Newport County.]]
The value of fish caught from US commercial fisheries in 2007 was valued at $3.8&nbsp;billion and of that 73% was derived from calcifiers and their direct predators.<ref>{{Cite journal |last1=V. J. Fabry |last2=C. Langdon |last3=W. M. Balch |last4=A. G. Dickson |last5=R. A. Feely |last6=B. Hales |last7=D. A. Hutchins |last8=J. A. Kleypas |last9=C. L. Sabine |name-list-style=amp |title=Present and Future Impacts of Ocean Acidification on Marine Ecosystems and Biogeochemical Cycles |url=http://www.us-ocb.org/publications/OCB_OA_rept.pdf |url-status=dead |journal=Report of the Ocean Carbon and Biogeochemistry Scoping Workshop on Ocean Acidification Research |archive-url=https://web.archive.org/web/20101117232337/http://www.us-ocb.org/publications/OCB_OA_rept.pdf |archive-date=17 November 2010 |access-date=14 November 2013}}</ref> Other organisms are directly harmed as a result of acidification. For example, decrease in the growth of marine calcifiers such as the [[American lobster]], [[Arctica islandica|ocean quahog]], and [[scallop]]s means there is less shellfish meat available for sale and consumption.<ref>{{Cite web |year=2012 |title=Canada's State of the Oceans Report, 2012 |url=http://dfo-mpo.gc.ca/science/coe-cde/soto/report-rapport-2012/index-eng.asp |url-status=dead |archive-url=https://web.archive.org/web/20131106083015/http://www.dfo-mpo.gc.ca/science/coe-cde/soto/report-rapport-2012/index-eng.asp |archive-date=6 November 2013 |access-date=21 October 2013 |website=Report |publisher=Fisheries and Oceans Canada |df=dmy-all}}</ref> Red king crab fisheries are also at a serious threat because crabs are also calcifiers. Baby red king crab when exposed to increased acidification levels experienced 100% mortality after 95&nbsp;days.<ref>{{Cite magazine |last1=Foy |first1=Robert J. |last2=Carls |first2=Mark |last3=Dalton |first3=Michael |last4=Hurst |first4=Tom |last5=Long |first5=W. Christopher |last6=Poljak |first6=Dusanka |last7=Punt |first7=André E. |last8=Sigler |first8=Michael F. |last9=Stone |first9=Robert P. |date=Winter 2013 |title=CO 2 , pH, and Anticipating a Future under Ocean Acidification |url=http://www.afs-alaska.org/wp-content/uploads/Onco331.pdf |access-date=14 November 2013 |magazine=Oncorhynchus |volume=XXXIII |issue=1 |author10=Swiney, Katherine M.}}</ref> In 2006, red king crab accounted for 23% of the total guideline harvest levels and a serious decline in red crab population would threaten the crab harvesting industry.<ref>{{Cite web |date=November 2005 |title=Bering Sea Crab Fishery |url=http://www.alaskaseafood.org/fishingprocessing/seafoodweb/stories/crab.html |url-status=dead |archive-url=https://web.archive.org/web/20131211122841/http://www.alaskaseafood.org/fishingprocessing/seafoodweb/stories/crab.html |archive-date=11 December 2013 |access-date=10 November 2013 |website=Report |publisher=Seafood Market Bulletin |df=dmy-all}}</ref>

==Possible responses==
=== Climate change mitigation ===
{{Main|Climate change mitigation|Carbon dioxide removal|Carbon sequestration#Sequestration techniques in oceans}}

Reducing carbon dioxide emissions (i.e. [[climate change mitigation]] measures) is the only solution that addresses the root cause of ocean acidification. For example, some mitigation measures focus on [[carbon dioxide removal]] (CDR) from the atmosphere (e.g. [[direct air capture]] (DAC), [[bioenergy with carbon capture and storage]] (BECCS)). These would also slow the rate of acidification.

Approaches that remove carbon dioxide from the ocean include [[Ocean fertilization|ocean nutrient fertilization]], [[artificial upwelling]]/downwelling, [[seaweed farming]], ecosystem recovery, ocean alkalinity enhancement, [[enhanced weathering]] and electrochemical processes.<ref name="AR6 WGIII Ch 12">IPCC (2022) [https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_Chapter_12.pdf Chapter 12: Cross sectoral perspectives] {{Webarchive|url=https://web.archive.org/web/20221013122242/https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_Chapter_12.pdf|date=13 October 2022}} in [https://www.ipcc.ch/report/ar6/wg3/ Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20220802125242/https://www.ipcc.ch/report/ar6/wg3/|date=2 August 2022}}, Cambridge University Press, Cambridge, United Kingdom and New York, NY, US</ref>{{rp|12–36}} All of these methods use the ocean to remove {{CO2}} from the atmosphere to store it in the ocean. These methods could assist with mitigation but they can have side-effects on marine life. The research field for all CDR methods has grown a lot since 2019.<ref name=":10" />

In total, "ocean-based methods have a combined potential to remove 1–100 gigatons of {{CO2}} per year".<ref name=":12">IPCC (2022) [https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_TS.pdf Technical Summary] {{Webarchive|url=https://web.archive.org/web/20220923091131/https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_TS.pdf|date=23 September 2022}}. In [https://www.ipcc.ch/report/ar6/wg3/ Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20220802125242/https://www.ipcc.ch/report/ar6/wg3/|date=2 August 2022}}, Cambridge University Press, Cambridge, United Kingdom and New York, NY, US</ref>{{rp|TS-94}} Their costs are in the order of {{val|p={{US$}}|40|–|500}} per ton of {{CO2}}. For example, [[enhanced weathering]] could remove {{val|2|–|4|ul=gigatons}} of {{CO2}} per year. This technology comes with a cost of {{val|50|–|200|p={{USD}}}} per ton of {{CO2}}.<ref name=":12" />{{rp|TS-94}}

===Carbon removal technologies which add alkalinity===
{{See also|Carbon sequestration#Adding bases to neutralize acids}}
Some [[carbon removal]] techniques add alkalinity to the ocean and therefore immediately buffer pH changes which might help the organisms in the region that the extra alkalinity is added to. The two technologies that fall into this category are ocean alkalinity enhancement and electrochemical methods.<ref name=":10" /> Eventually, due to diffusion, that alkalinity addition will be quite small to distant waters. This is why the term ''local ocean acidification mitigation'' is used. Both of these technologies have the potential to operate on a large scale and to be efficient at removing carbon dioxide.<ref name=":10" />{{rp|Table 9.1}} However, they are expensive, have many risks and side effects and currently have a low [[technology readiness level]].<ref name="AR6 WGIII Ch 12" />{{rp|12–36}}

====Ocean alkalinity enhancement====
Ocean alkalinity enhancement (OAE) is a proposed "carbon dioxide removal (CDR) method that involves deposition of alkaline minerals or their dissociation products at the ocean surface".<ref name="IPCC AR6 WGI Glossary">IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf Annex VII: Glossary] {{Webarchive|url=https://web.archive.org/web/20220605175306/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf|date=5 June 2022}} [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.&nbsp; Méndez, S. Semenov, A. Reisinger (eds.)]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20210809131444/https://www.ipcc.ch/report/ar6/wg1/|date=9 August 2021}} [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US</ref>{{rp|2241}} The process would increase surface total alkalinity. It would work to increase ocean absorption of {{CO2}}. The process involves increasing the amount of [[bicarbonate]] (HCO<sub>3</sub>-) through accelerated weathering ([[enhanced weathering]]) of rocks ([[silicate]], [[limestone]] and [[quicklime]]).<ref name=":10">{{Cite book |title=A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration |year=2022 |isbn=978-0-309-08761-2 |doi=10.17226/26278 |pmid=35533244 |s2cid=245089649}}</ref>{{rp|181}} This process mimics the silicate-carbonate cycle. The {{CO2}} either becomes bicarbonate, remaining in that form for more than 100 years, or may precipitate into calcium carbonate (CaCO<sub>3</sub>). When calcium carbonate is buried in the deep ocean, it can hold the carbon indefinitely when utilizing silicate rocks.

Enhanced weathering is one type of ocean alkalinity enhancement. Enhanced weathering increases alkalinity by scattering fine rock particles. This can happen on land and in the ocean (even though the outcome eventually affects the ocean).

In addition to sequestering {{CO2}}, alkalinity addition buffers the pH of the ocean therefore reducing ocean acidification. However, little is known about how organisms respond to added alkalinity, even from natural sources.<ref name=":10" /> For example, weathering of some silicate rocks could release a large amount of trace metals at the weathering site.

Cost and energy consumed by ocean alkalinity enhancement (mining, pulverizing, transport) is high compared to other CDR techniques.<ref name=":10" /> The cost is estimated to be {{val|20|–|50|p={{USD}}}} per ton of CO<sub>2</sub> (for "direct addition of alkaline minerals to the ocean").<ref name="AR6 WGIII Ch 12" />{{rp|12–50}}

Carbon sequestered as bicarbonate in the ocean amounts to about 30% of carbon emissions since the [[Industrial Revolution]].

Experimental materials include limestone, [[brucite]], [[olivine]] and alkaline solutions. Another approach is to use electricity to raise alkalinity during desalination to capture waterborne CO<sub>2</sub>.<ref>{{Cite journal |last=Cornwall |first=Warren |date=30 November 2023 |title=An Alkaline Solution |url=https://www.science.org/content/article/climate-crisis-sparks-effort-coax-oceans-suck-carbon-dioxide?et_cid=5008204&et_rid=49310172 |journal=Science Magazine |volume=382 |issue=6674 |pages=988–992 |doi=10.1126/science.adn1880 |pmid=38033057|bibcode=2023Sci...382..988C }}</ref>

====Electrochemical methods====
Electrochemical methods, or [[electrolysis]], can strip carbon dioxide directly from seawater.<ref name=":10" /> Electrochemical process are a type of ocean alkalinity enhancement, too. Some methods focus on direct {{CO2}} removal (in the form of carbonate and {{CO2}} gas) while others increase the alkalinity of seawater by precipitating metal hydroxide residues, which absorbs {{CO2}} in a matter described in the ocean alkalinity enhancement section. The hydrogen produced during direct carbon capture can then be upcycled to form hydrogen for energy consumption, or other manufactured laboratory reagents such as [[hydrochloric acid]].

However, implementation of electrolysis for carbon capture is expensive and the energy consumed for the process is high compared to other CDR techniques.<ref name=":10" /> In addition, research to assess the environmental impact of this process is ongoing. Some complications include toxic chemicals in wastewaters, and reduced DIC in effluents; both of these may negatively impact marine life.<ref name=":10" />

== Policies and goals ==
[[File:Save our ocean, People's Climate March, 29 April 2017 (cropped).jpg|thumb|upright|Demonstrator calling for action against ocean acidification at the [[People's Climate March (2017)]]]]

===Global policies===
As awareness about ocean acidification grows, policies geared towards increasing monitoring efforts of ocean acidification have been drafted.<ref>{{Cite web |title=Action Plans |url=https://www.oaalliance.org/action-plans |access-date=2022-11-04 |website=OA Alliance |language=en-US}}</ref> Previously in 2015, ocean scientist [[Jean-Pierre Gattuso]] had remarked that "The ocean has been minimally considered at previous climate negotiations. Our study provides compelling arguments for a radical change at the UN conference (in Paris) on climate change".<ref name=":1">{{Cite news |last=Harrabin |first=Roger |date=3 July 2015 |title=CO<sub>2</sub> emissions threaten ocean crisis |url=https://www.bbc.com/news/science-environment-33369024 |work=BBC News}}</ref>

International efforts, such as the Wider Caribbean's [[Convention for the Protection and Development of the Marine Environment of the Wider Caribbean Region|Cartagena Convention]] (entered into force in 1986),<ref>{{Cite web |title=Cartagena Convention {{!}} The Caribbean Environment Programme (CEP) |url=https://www.unep.org/cep/who-we-are/cartagena-convention |access-date=2022-11-04 |website=www.unep.org}}</ref> may enhance the support provided by regional governments to highly vulnerable areas in response to ocean acidification.<ref>{{cite web|url=https://www.unep.org/cep/news/editorial/cartagena-convention-secretariat-and-ocean-foundation-cooperate-priority-marine|title=Cartagena Convention Secretariat and The Ocean Foundation to cooperate on priority marine biodiversity and pollution issues|website=Caribbean Environment Programme|publisher=United Nations Environment Programme|access-date=4 September 2024|date=12 November 2019}}</ref>  Many countries, for example in the Pacific Islands and Territories, have constructed regional policies, or National Ocean Policies, National Action Plans, National Adaptation Plans of Action and Joint National Action Plans on Climate Change and Disaster Risk Reduction, to help work towards [[Sustainable Development Goal 14|SDG 14]]. Ocean acidification is now starting to be considered within those frameworks.<ref>{{Cite book |last1=Turner |first1=J |url=https://www.sprep.org/sites/default/files/documents/publications/MainstreamingOAintoNationalPolicies_PacifcIslandsHandbook_0.pdf |title=Mainstreaming Ocean Acidification into National Policies: A Handbook for Pacific Islands |last2=McIntosh |first2=R. Duncan |publisher=Secretariat of the Pacific Regional Environment Programme. |year=2019 |location=Apia, Samoa}}</ref>

==== UN Ocean Decade ====
{{See also|UN Decade of Ocean Science for Sustainable Development}}
The UN Ocean Decade has a program called "Ocean acidification research for sustainability". It was proposed by the Global Ocean Acidification Observing Network (GOA-ON) and its partners, and has been formally endorsed as a program of the [[UN Decade of Ocean Science for Sustainable Development]].<ref>{{Cite web |title=The Ocean Decade – The Science we need for the Ocean we want |url=https://www.oceandecade.org/ |access-date=2022-11-04 |website=Ocean Decade |language=en-US}}</ref><ref>{{Cite web |title=Overview |url=http://goa-on.org/oars/overview.php |website=OARS: Ocean Acidification Research for Sustainability |publisher=GOA-ON}}</ref> The OARS program builds on the work of GOA-ON and has the following aims: to further develop the science of ocean acidification; to increase observations of ocean chemistry changes; to identify the impacts on marine ecosystems on local and global scales; and to provide decision makers with the information needed to mitigate and adapt to ocean acidification.

==== Global Climate Indicators ====
The importance of ocean acidification is reflected in its inclusion as one of seven Global Climate Indicators.<ref>{{Cite web |title=Global Climate Indicators |url=https://gcos.wmo.int/en/global-climate-indicators |website=GCOS |date=19 June 2024 |publisher=WMO}}</ref> These Indicators are a set of parameters that describe the changing climate without reducing climate change to only [[Instrumental temperature record|rising temperature]]. The Indicators include key information for the most relevant domains of climate change: temperature and energy, atmospheric composition, ocean and water as well as the cryosphere. The Global Climate Indicators have been identified by scientists and communication specialists in a process led by [[Global Climate Observing System]] (GCOS).<ref>{{Cite web |title=Home {{!}} GCOS |url=https://gcos.wmo.int/en/home |access-date=2022-11-04 |website=gcos.wmo.int}}</ref> The Indicators have been endorsed by the [[World Meteorological Organization]] (WMO). They form the basis of the annual WMO Statement of the State of the Global Climate, which is submitted to the [[Conferences of the Parties of UNFCCC and the Kyoto Protocol|Conference of Parties]] (COP) of the [[United Nations Framework Convention on Climate Change]] (UNFCCC). Additionally, the [[Copernicus Climate Change Service]] (C3S) of the European Commission uses the Indicators for their annual "European State of the Climate".

==== Sustainable Development Goal 14 ====
In 2015, the United Nations adopted the 2030 Agenda and a set of 17 Sustainable Development Goals (SDG), including a goal dedicated to the ocean, [[Sustainable Development Goal 14]],<ref name=":5">{{Cite web |title=Goal 14 targets |url=https://www.un.org/sustainabledevelopment/oceans/ |access-date=2020-09-24 |website=UNDP}}</ref> which calls to "conserve and sustainably use the oceans, seas and marine resources for sustainable development". Ocean acidification is directly addressed by the target SDG 14.3. The full title of Target 14.3 is: "Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels".<ref name=":17">United Nations (2017) Resolution adopted by the General Assembly on 6 July 2017, [[:File:A RES 71 313 E.pdf|Work of the Statistical Commission pertaining to the 2030 Agenda for Sustainable Development]] ([https://undocs.org/A/RES/71/313 A/RES/71/313] {{Webarchive|url=https://web.archive.org/web/20201023121826/https://undocs.org/A/RES/71/313 |date=23 October 2020 }})</ref> This target has one indicator: Indicator 14.3.1 which calls for the "Average marine acidity ([[pH]]) measured at agreed suite of representative sampling stations".<ref name=":42">{{Cite web |title=Goal 14: Sustainable Development Knowledge Platform |url=https://sustainabledevelopment.un.org/sdg14 |access-date=2020-09-05 |website=sustainabledevelopment.un.org}}</ref>&nbsp;

The [[Intergovernmental Oceanographic Commission]] (IOC) of [[UNESCO]] was identified as the custodian agency for the SDG 14.3.1 Indicator. In this role, IOC-UNESCO is tasked with developing the SDG 14.3.1 Indicator Methodology, the annual collection of data towards the SDG 14.3.1 Indicator and the reporting of progress to the United Nations.<ref>{{Cite web |title=Update on IOC Custodianship Role in Relation to SDG 14 Indicators |url=https://unesdoc.unesco.org/ark:/48223/pf0000265127.locale=en |access-date=2022-11-04 |website=unesdoc.unesco.org}}</ref><ref>{{Cite web |title=SDG 14.3.1 data portal |url=https://oa.iode.org/ |access-date=2022-11-04 |website=oa.iode.org}}</ref>

=== Policies at country level ===
==== United States ====
In the United States, the Federal Ocean Acidification Research And Monitoring Act of 2009 supports government coordination, such as the [[National Oceanic and Atmospheric Administration|National Oceanic Atmospheric Administration]]'s (NOAA) "Ocean Acidification Program".<ref>{{Cite journal |last1=Galdies |first1=Charles |last2=Bellerby |first2=Richard |last3=Canu |first3=Donata |last4=Chen |first4=Wenting |last5=Garcia-Luque |first5=Enrique |last6=Gašparović |first6=Blaženka |last7=Godrijan |first7=Jelena |last8=Lawlor |first8=Paul J. |last9=Maes |first9=Frank |last10=Malej |first10=Alenka |last11=Panagiotaras |first11=Dionisios |last12=Romera |first12=Beatriz Martinez |last13=Reymond |first13=Claire E. |last14=Rochette |first14=Julien |last15=Solidoro |first15=Cosimo |date=2020-08-01 |title=European policies and legislation targeting ocean acidification in european waters – Current state |journal=Marine Policy |volume=118 |pages=103947 |doi=10.1016/j.marpol.2020.103947 |s2cid=218961473 |doi-access=free|bibcode=2020MarPo.11803947G }}</ref><ref>{{Cite web |title=OAP Home |url=https://oceanacidification.noaa.gov/ |access-date=2022-11-04 |website=oceanacidification.noaa.gov}}</ref> In 2015, USEPA denied a citizens petition that asked EPA to regulate {{CO2}} under the [[Toxic Substances Control Act of 1976]] in order to mitigate ocean acidification.<ref>{{Cite web |author=Center for Biological Diversity |author2=Donn J. Viviani |title=TSCA Section 21 Petition Requesting EPA to Regulate Anthropogenic Emissions Carbon Dioxide |url=https://www.epa.gov/sites/production/files/2015-09/documents/petition_oa_tsca_2014_final_2.pdf |website=US EPA}}</ref><ref>{{Cite web |date=2015-10-07 |title=Carbon Dioxide Emissions and Ocean Acidification; TSCA Section 21 Petition; Reasons for Agency Response |url=https://www.federalregister.gov/documents/2015/10/07/2015-25164/carbon-dioxide-emissions-and-ocean-acidification-tsca-section-21-petition-reasons-for-agency#page-60577 |publisher=Environmental Protection Agency (EPA) |pages=60577–60584}}</ref> In the denial, the EPA said that risks from ocean acidification were being "more efficiently and effectively addressed" under domestic actions, e.g., under the [[Presidential Climate Action Plan]], and that multiple avenues are being pursued to work with and in other nations to reduce emissions and deforestation and promote clean energy and energy efficiency.<ref>{{Cite web |date=June 2013 |title=The President's Climate Action Plan |url=https://obamawhitehouse.archives.gov/sites/default/files/image/president27sclimateactionplan.pdf |publisher=obamawhitehouse.archives.gov}}</ref>

== History ==
Research into the phenomenon of ocean acidification, as well as awareness raising about the problem, has been going on for several decades. The fundamental research really began with the creation of the [[pH scale]] by [[Danish people|Danish]] chemist [[Søren Peder Lauritz Sørensen]] in 1909.<ref name="Sørensen">{{Cite journal |last=Sørensen |first=S. P. L. |year=1909 |title=Über die Messung und die Bedeutung der Wasserstoffionenkonzentration bei enzymatischen Prozessen |url=https://core.ac.uk/download/pdf/14517358.pdf |url-status=live |journal=Biochem. Z. |volume=21 |pages=131–304 |archive-url=https://web.archive.org/web/20210415205740/https://core.ac.uk/download/pdf/14517358.pdf |archive-date=15 April 2021 |access-date=22 March 2021 |quote=Original German: Für die Zahl p schlage ich den Namen Wasserstoffionenexponent und die Schreibweise p<sub>H</sub>• vor. Unter dem Wasserstoffionexponenten (p<sub>H</sub>•) einer Lösungwird dann der Briggsche Logarithmus des reziproken Wertes des auf Wasserstoffionenbezagenen Normalitäts faktors de Lösungverstanden.}} Two other publications appeared in 1909, one in French and one in Danish.</ref> By around the 1950s the massive role of the ocean in absorbing fossil fuel CO<sub>2</sub> was known to specialists, but not appreciated by the greater scientific community.<ref name=":16" /> Throughout much of the 20th century, the dominant focus has been the beneficial process of oceanic CO<sub>2</sub> uptake, which has enormously ameliorated climate change. The concept of "too much of a good thing" has been late in developing and was triggered only by some key events, and the [[Carbon sink|oceanic sink]] for heat and CO<sub>2</sub> is still critical as the primary buffer against climate change.<ref name=":16">{{Cite journal |last=Brewer |first=P. G. |date=2013 |title=A short history of ocean acidification science in the 20th century: a chemist's view |url=https://bg.copernicus.org/articles/10/7411/2013/ |journal=Biogeosciences |language=English |volume=10 |issue=11 |pages=7411–7422 |bibcode=2013BGeo...10.7411B |doi=10.5194/bg-10-7411-2013 |issn=1726-4170 |doi-access=free}} [[File:CC-BY icon.svg|50x50px]] Text was copied from this source, which is available under a [[creativecommons:by/3.0/|Creative Commons Attribution 3.0 Unported License]]</ref>

In the early 1970s questions over the long-term impact of the accumulation of fossil fuel CO<sub>2</sub> in the sea were already arising around the world and causing strong debate. Researchers commented on the accumulation of fossil CO<sub>2</sub> in the atmosphere and sea and drew attention to the possible impacts on marine life. By the mid-1990s, the likely impact of CO<sub>2</sub> levels rising so high with the inevitable changes in pH and carbonate ion became a concern of scientists studying the fate of coral reefs.<ref name=":16" />

By the end of the 20th century the trade-offs between the beneficial role of the ocean in absorbing some 90% of all heat created, and the accumulation of some 50% of all fossil fuel CO<sub>2</sub> emitted, and the impacts on marine life were becoming more clear. By 2003, the time of planning for the "First Symposium on the Ocean in a High-CO<sub>2</sub> World" meeting to be held in Paris in 2004, many new research results on ocean acidification were published.<ref name=":16" />

In 2009, members of the [[InterAcademy Panel]] called on world leaders to "Recognize that reducing the build up of {{CO2}} in the atmosphere is the only practicable solution to mitigating ocean acidification".<ref name="iap statement">{{Cite web |author=IAP |date=June 2009 |title=Interacademy Panel (IAP) Member Academies Statement on Ocean Acidification |url=https://www.interacademies.org/statement/iap-statement-ocean-acidification}}, Secretariat: TWAS (the Academy of Sciences for the Developing World), Trieste, Italy.</ref> The statement also stressed the importance to "Reinvigorate action to reduce stressors, such as [[overfishing]] and [[marine pollution|pollution]], on [[marine ecosystem]]s to increase resilience to ocean acidification".<ref>{{Cite news |last=Harvey |first=Fiona |author-link=Fiona Harvey |date=2019-12-04 |title=Tackling degraded oceans could mitigate climate crisis – report |url=https://www.theguardian.com/environment/2019/dec/04/tackling-ocean |access-date=2019-12-07 |work=The Guardian |language=en-GB}}</ref>

For example, research in 2010 found that in the 15-year period 1995–2010 alone, acidity had increased 6 percent in the upper 100&nbsp;meters of the Pacific Ocean from Hawaii to Alaska.<ref name="2010aug_sciam" />

According to a statement in July 2012 by [[Jane Lubchenco]], head of the U.S. [[National Oceanic and Atmospheric Administration]] "surface waters are changing much more rapidly than initial calculations have suggested. It's yet another reason to be very seriously concerned about the amount of carbon dioxide that is in the atmosphere now and the additional amount we continue to put out."<ref name="HPost">{{Cite web |date=9 July 2012 |title=Ocean Acidification Is Climate Change's 'Equally Evil Twin,' NOAA Chief Says |url=http://www.huffingtonpost.com/2012/07/09/ocean-acidification-reefs-climate-change_n_1658081.html |url-status=dead |archive-url=https://web.archive.org/web/20120712003629/http://www.huffingtonpost.com/2012/07/09/ocean-acidification-reefs-climate-change_n_1658081.html |archive-date=12 July 2012 |access-date=2012-07-09 |website=[[Huffington Post]]}}</ref>

A 2013 study found acidity was increasing at a rate 10 times faster than in any of the evolutionary crises in Earth's history.<ref>{{Cite web |last=Harvey |first=Fiona |author-link=Fiona Harvey |date=2013-08-25 |title=Rising levels of acids in seas may endanger marine life, says study |url=https://www.theguardian.com/environment/2013/aug/25/rising-acid-levels-seas-endanger-marine-poertner |access-date=2013-08-29 |website=The Guardian}}</ref>

The "Third Symposium on the Ocean in a High-{{CO2}} World" took place in Monterey, California, in 2012. The summary for policy makers from the conference stated that "Ocean acidification research is growing rapidly".<ref name="ocean" />

In a synthesis report published in ''Science'' in 2015, 22 leading marine scientists stated that {{CO2}} from burning fossil fuels is changing the oceans' chemistry more rapidly than at any time since the [[Permian–Triassic extinction event|Great Dying]] (Earth's most severe known extinction event).<ref name=":1" /> Their report emphasized that the 2&nbsp;°C maximum temperature increase agreed upon by governments reflects too small a cut in emissions to prevent "dramatic impacts" on the world's oceans.<ref name=":1" />

A study done in 2020 argues that ocean acidification is not only negatively affecting marine life, but also human health. Food quality, respiratory issues, and human health are all negatively affected by ocean acidification.<ref>{{Cite journal |last1=Falkenberg |first1=Laura J. |last2=Bellerby |first2=Richard G. J. |last3=Connell |first3=Sean D. |last4=Fleming |first4=Lora E. |last5=Maycock |first5=Bruce |last6=Russell |first6=Bayden D. |last7=Sullivan |first7=Francis J. |last8=Dupont |first8=Sam |date=2020-06-24 |title=Ocean Acidification and Human Health |journal=International Journal of Environmental Research and Public Health |language=en |volume=17 |issue=12 |pages=4563 |doi=10.3390/ijerph17124563 |issn=1660-4601 |pmc=7344635 |pmid=32599924 |doi-access=free}}</ref>

==See also==
{{Portal|Oceans|Environment|Marine life|Global warming|Water}}
* {{annotated link|Estuarine acidification}}
* {{annotated link|Ocean acidification in the Arctic Ocean}}
* {{annotated link|Ocean acidification in the Great Barrier Reef}}
* {{annotated link|Ocean deoxygenation}}
* {{annotated link|Marine pollution}}

==References==
{{Reflist|colwidth=35em}}
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==External links==
* [http://www.goa-on.org/home.php Global Ocean Acidification Observing Network (GOA-ON)]
* [https://www.oceandecade.org/ United Nations Decade of Ocean Science for Sustainable Development (2021–2030)]
* [https://oceanacidification.noaa.gov/ US NOAA Ocean Acidification Program]

{{Human impact on the environment|state=expanded}}
{{Global warming|state=collapsed}}
{{Marine pollution}}
{{Doomsday}}

{{DEFAULTSORT:Ocean Acidification}}
[[Category:Aquatic ecology]]
[[Category:Biological oceanography]]
[[Category:Carbon]]
[[Category:Chemical oceanography]]
[[Category:Fisheries science]]
[[Category:Geochemistry]]
[[Category:Oceanography]]
[[Category:Effects of climate change]]
[[Category:Environmental impact by effect]]
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[[Category:Oceanographical terminology]]