Ocean acidification

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Current rates of ocean acidification have been compared with the greenhouse event at the Paleocene–Eocene boundary (about 56 million years ago) when surface ocean temperatures rose by 5–6 degrees Celsius. Surface ecosystems experienced stress, yet bottom-dwelling organisms in the deep ocean experienced a major extinction.[43] The rate of carbon addition to the atmosphere-ocean system at present day is about ten times the rate of carbon addition than at  the Paleocene–Eocene boundary.[44] While the current ocean acidification is on a path to reach lower pH levels than any other level recorded in the last 300 million years,[45][46] the rate of carbon addition is unparalleled, therefore the current and projected acidification, namely the decrease in carbonate saturation states has been described as unprecedented in the geological record.[47] A National Research Council study released in April 2010 likewise concluded that "the level of acid in the oceans is increasing at an unprecedented rate".[48][49] A 2012 paper in the journal Science examined the geological record in an attempt to find a historical analog for current global conditions as well as those of the future. The researchers determined that the current rate of ocean acidification is faster than at any time in the past 300 million years.[50][51]

A review by climate scientists at the RealClimate blog, of a 2005 report by the Royal Society of the UK similarly highlighted the centrality of the rates of change in the present anthropogenic acidification process, writing:[52]

The natural pH of the ocean is determined by a need to balance the deposition and burial of CaCO3 on the sea floor against the influx of Ca2+ and CO2−3 into the ocean from dissolving rocks on land, called weathering. These processes stabilize the pH of the ocean, by a mechanism called CaCO3 compensation...The point of bringing it up again is to note that if the CO2 concentration of the atmosphere changes more slowly than this, as it always has throughout the Vostok record, the pH of the ocean will be relatively unaffected because CaCO3 compensation can keep up.

In the 15-year period 1995–2010 alone, acidity has increased 6 percent in the upper 100 meters of the Pacific Ocean from Hawaii to Alaska.[53] 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."[54]

A 2013 study claimed acidity was increasing at a rate 10 times faster than in any of the evolutionary crises in Earth's history.[55] 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 Great Dying, Earth's most severe known extinction event, emphasizing that the 2 °C maximum temperature increase agreed upon by governments reflects too small a cut in emissions to prevent "dramatic impacts" on the world's oceans, with lead author Jean-Pierre Gattuso remarking 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".[56]

The rate at which ocean acidification will occur may be influenced by the rate of surface ocean warming, because warm waters will not absorb as much CO2.[57] Therefore, greater seawater warming could limit CO2 absorption and lead to a smaller change in pH for a given increase in CO2.[57] The difference in changes in temperature between basins is one of the main reasons for the differences in acidification rates in different localities. At present, the surface ocean is acidifying at a rate of 0.003-0.026 units per decade. However this rate is faster in the polar regions (-0.002 to -0.026 per decade) than at the subtropical regions (-0.016 to -0.020 per decade)[58].^: 83

Earth System Models project that, by around 2008, ocean acidity exceeded historical analogues[66][67] and, in combination with other ocean biogeochemical changes, 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.[68][69]

If the 'business as usual' model for human activity persists, model projections estimate that surface ocean pH could decrease by 0.16 to 0.44 units compared to the present day by the end of the century [70]^: 608

A panel of experts who had previously participated in the IPCC reports have determined that it is not yet possible to determine a threshold for ocean acidity that should not be exceeded.[71][72]

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 gas hydrates.[73] Elevated CO2 levels impacted biodiversity,[74], more recently, decreased CaCO3 saturation due to seawater uptake of volcanogenic CO2 was suggested as a possible kill mechanism during the marine mass extinction at the end of the Triassic.[75] 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,[76][77][78][79] and (b) there was pronounced selectivity of the extinction against organisms with thick aragonitic skeletons,[76][80][81] which is predicted from experimental studies.[82] Ocean acidification has also been suggested as a one cause of the end-Permian mass extinction[83][84] and the end-Cretaceous crisis.[85] Overall, multiple climatic stressors, including Ocean Acidification, was likely the cause of geologic extinction events.[86]

The most notable example of ocean acidification is the Paleocene-Eocene Thermal Maximum (PETM),[87] 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.[88] Relatively new geochemical methods of testing for pH in the past indicate the pH dropped 0.3 units across the PETM.[89] [90]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 methods to test for saturation state are needed to assess how much this pH change may have affected calcifying organisms.

Bjerrum plot: Change in carbonate system of seawater from ocean acidification

Given the current pH of the ocean (~8.1), of the extra carbon dioxide added into the ocean, very little 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,[92] creating an imbalance in the reaction HCO3− ⇌ CO32− + 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: Ca2+ + CO32− ⇌ CaCO3.

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

The 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:

${\displaystyle {\Omega }={\frac {\left[{\ce {Ca^2+}}\right]\left[{\ce {CO3^2-}}\right]}{K_{sp}}}}$

Here Ω is the product of the concentrations (or activities) of the reacting ions that form the mineral (Ca2+ and CO2−3), divided by the apparent solubility product at equilibrium (Ksp), that is, when the rates of precipitation and dissolution are equal..[93] In seawater, a natural horizontal boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon.[91] Above this saturation horizon, Ω has a value greater than 1, and CaCO
₃ does not readily dissolve. Most calcifying organisms live in such waters.[91] Below this depth, Ω has a value less than 1, and 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.[94] Increasing CO2 levels, and the resulting lower pH of seawater, decreases the concentration of CO32− and the saturation state of CaCO3 therefore increasing CaCO3 dissolution.

Calcium carbonate most commonly occurs in two common 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.[91] This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite.[34] Ocean acidification and the resulting decrease in carbonate saturation states raise the saturation horizons of both forms closer to the surface.[95] This decrease in saturation state is  one of the main factors leading to decreased calcification in marine organisms because the inorganic precipitation of CaCO3 is directly proportional to its saturation state and calcifying organisms exhibit stress in waters with lower saturation states.[96][97]

The phenomenon of coral bleaching or coral whitening and the degeneration of coralline reef ecosystems is one consequence of increasing ocean acidity. The tropical and sub-tropical environments, including areas such as the Caribbean and surrounding regions, tropical Asia (e.g. Indonesia, Philippines, Thailand, Maldives) and the tropical Pacific (e.g. Australian Barrier Reef, Pacific Islands, Papua New Guinea) are mostly affected by coral bleaching, as these are the regions of the world that contain the largest and most extensive coral reef systems.[102]

Increasing ocean acidification makes it more difficult for shell-accreting organisms to access carbonate ions, essential for the production of their hard exoskeletal shell.[27] Oceanic calcifying organism span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs.[68][57] As described above, under normal conditions, calcite and aragonite are stable in surface waters since the carbonate ions are supersaturated with respect to seawater. However, as ocean pH falls, the concentration of carbonate ions also decreases, and when calcium carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to calcification stress[103] and dissolution. Therefore, even if there is no change in the rate of calcification, the rate of dissolution of calcareous material increases.[39] In particular, studies show that corals,[98] [99] [104][105] coccolithophores,[57][39] [106]coralline algae,[107] foraminifera,[108] shellfish[87] and pteropods[109] experience reduced calcification or enhanced dissolution when exposed to elevated CO2. A 2010 study from Stony Brook University suggested that even with active marine conservation practices it may be impossible to bring back many previous shellfish populations.[110] Similarly, when exposed in experiments to pH reduced by 0.2 to 0.4, larvae of a temperate brittlestar, a relative of the common sea star, fewer than 0.1 percent survived more than eight days.[68]

A normally-protective shell made thin, fragile and transparent by acidification

The Royal Society published a comprehensive overview of ocean acidification, and its potential consequences, in June 2005.[91] However, some studies have found different responses to ocean acidification, with coccolithophore calcification and photosynthesis both increasing under elevated atmospheric pCO₂,[111][112][113] an equal decline in primary production and calcification in response to elevated CO₂[114] or the direction of the response varying between species.[115] A study in 2008 examining a sediment core from the North Atlantic found that while the species composition of coccolithophorids has remained unchanged for the industrial period 1780 to 2004, the calcification of coccoliths has increased by up to 40% during the same time.[113] Understanding calcification changes in coccolithophores may have secondary importance because a decline in the coccolithophores may have secondary effects on climate, contributing to global warming by decreasing the Earth's albedo via their effects on oceanic cloud cover.[116] Similarly, the sea star, Pisaster ochraceus, shows enhanced growth in waters with increased acidity[117] Overall, all marine ecosystems on Earth will be exposed to changes in acidification and several other ocean biogeochemical changes.[118] When exposed in experiments to pH reduced by 0.2 to 0.4, larvae of a temperate brittlestar, a relative of the common sea star, fewer than 0.1 percent survived more than eight days.[53] There is also a suggestion that a decline in the coccolithophores may have secondary effects on climate, contributing to global warming by decreasing the Earth's albedo via their effects on oceanic cloud cover.[119] All marine ecosystems on Earth will be exposed to changes in acidification and several other ocean biogeochemical changes.[68]

The fluid in the internal compartments (the coelenteron) where corals grow their exoskeleton is also extremely important for calcification growth. When the saturation rate 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 how much aragonite is in the surrounding water, the corals may even stop growing because the levels of aragonite are too low to pump into the internal compartment. Depending on the aragonite saturation state  in the surrounding water, the corals may halt growth because pumping aragonite  into the internal compartment will not be energetically favorable.[120] Under the current progression of carbon emissions, around 70% of North Atlantic cold-water corals will be living in corrosive waters by 2050–60.[121]

A study conducted by the Woods Hole Oceanographic Institution in January 2018 showed that acidified conditions primarily reduce the coral’s capacity to build dense exoskeletons, rather than affecting the linear extension of the exoskeleton. Using Global Climate Models, they show that the density of some species of corals could be reduced by over 20% by the end of this century.[122]

An in situ experiment on a 400 m² patch of the Great Barrier Reef to decrease seawater CO₂ level (raise pH) to close to the preindustrial value showed a 7% increase in net calcification.[123] A similar experiment to raise in situ seawater CO₂ level (lower pH) to a level expected soon after the middle of this century found that net calcification decreased 34%.[124] However, a field study of the coral reef in Queensland and Western Australia from 2007 to 2012 argues that corals are more resistant to the environmental pH changes than previously thought, due to internal homeostasis regulation; this makes thermal change, which leads to coral bleaching, rather than acidification, the main factor for coral reef vulnerability due to climate change.[125]

Ocean acidification may force some organisms to reallocate resources away from productive endpoints such as growth in order to maintain calcification.[126] For example, the oyster Magallana gigas is recognized to experience metabolic changes alongside altered calcification rates due to energetic tradeoffs resulting from pH imbalances.[127] Therefore, while the full ecological consequences of these changes in calcification are complex, it appears likely that many calcifying species will be adversely affected by ocean acidification.[128]

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.[2] 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.[129] 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.

Ocean acidification 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.[130] Seawater acidification could also reduce the size of Antarctic phytoplankton, making them less effective at storing carbon.[131] Such changes are being increasingly studied and synthesized through the use of physiological frameworks, including the Adverse Outcome Pathway (AOP) framework.[127]

Aside from the slowing and/or reversal of calcification, organisms may suffer other adverse effects, either indirectly through negative impacts on food resources,[91] or directly as reproductive or physiological effects. For example, the elevated oceanic levels of CO₂ may produce CO
₂-induced acidification of body fluids, known as hypercapnia. Also, increasing ocean acidity is believed to have a range of direct consequences. For example, increasing acidity has been observed to: reduce metabolic rates in jumbo squid;[98] depress the immune responses of blue mussels.[99] This is possibly because ocean acidification may alter the acoustic properties of seawater, allowing sound to propagate further, and increasing ocean noise.[132] This impacts all animals that use sound for echolocation or communication.[133] Atlantic longfin squid eggs took longer to hatch in acidified water, and the squid's statolith was smaller and malformed in animals placed in sea water with a lower pH.[134] However, these studies are ongoing and there is not a full understanding of these processes in marine organisms or ecosystems.[135]

Another possible effect would be an increase in red tide 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.[136]

Although red tide is harmful, other beneficial photosynthetic organisms may benefit from increased levels of carbon dioxide. Most importantly, seagrasses will benefit.[137] An experiment done in 2018 concluded that as seagrasses increased their photosynthetic activity, calcifying algae's calcification rates rose. This could be a potential mitigation technique in the face of increasing acidity.[137]

Ocean acidification can also have affects on marine fish larvae. It internally affects their olfactory systems, which is a crucial part of their development, especially in the beginning stage of their life. Orange clownfish larvae mostly live on oceanic reefs that are surrounded by vegetative islands.[138] With the use of their sense of smell, larvae are known to be able to detect the differences between reefs surrounded by vegetative islands and reefs not surrounded by vegetative islands.[138] Clownfish larvae need to be able to distinguish between these two destinations to have the ability to locate an area that is satisfactory for their growth. Another use for marine fish olfactory systems is to help in determining the difference between their parents and other adult fish in order to avoid inbreeding.

At James Cook University's experimental aquarium facility, clownfish were sustained in non-manipulated seawater that obtained a pH of 8.15 ± 0.07 which is similar to our current ocean's pH. To test for effects of different pH levels, seawater was manipulated to three different pH levels, including the non-manipulated pH. The two opposing pH levels correspond with climate change models that predict future atmospheric CO₂ levels.[138] In the year 2100 the model predicts that we could potentially acquire CO₂ levels at 1,000 ppm, which correlates with the pH of 7.8 ± 0.05. Results of this experiment show that when larvae is exposed to a pH of 7.8 ± 0.05 their reaction to environmental cues differs drastically to larvae's reaction to cues in a non-manipulated pH. At the pH of 7.6 ± 0.05 larvae had no reaction to any type of cue. However, a  2020 study challenges the potential negative impact of end-of-century ocean acidification level on the coral fish behavior and suggests that the effect could be negligible.[139]

Multiple studies published in 2009 and 2010 reported drastic effects of ocean acidification on the behavior of coral fish.[140][141][142] This led to more than a decade of research regarding the effects of ocean acidification on animal behavior, including fish and invertebrates.[143] However, a study in 2020 challenged the potential negative impact of end-of-century ocean acidification level on the coral fish behavior, reporting that the results of the aforementioned studies from 2009 to 2010 were not replicable and suggesting that the effect of acidification on fish behavior could be negligible.[144] Furthermore, a meta-analysis published in 2022 found that the effect sizes 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, constituting a textbook example of the decline effect in science.[145]

Drivers of 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.[146][147]

While the full implications of elevated CO₂ on marine ecosystems are still being documented, there is a substantial body of research showing that a combination of ocean acidification and elevated ocean temperature, driven mainly by CO₂ and other greenhouse gas emissions, have a compounded effect on marine life and the ocean environment. This effect far exceeds the individual harmful impact of either.[148][149][150] In addition, ocean warming, along with increased productivity of phytoplankton from higher CO2 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.[151][152]

Meta analyses have quantified the direction and magnitude of the harmful effects of ocean acidification, warming and deoxygenation on the ocean.[153][154][155] These meta-analyses have been further tested by mesocosm studies[156][157] that simulated the interaction of these stressors and found a catastrophic effect on the marine food web, i.e. that the increases in consumption from thermal stress more than negates any primary producer to herbivore increase from elevated CO₂.

Leaving aside direct biological effects, 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.[158] This will cause an elevation of ocean alkalinity, leading to the enhancement of the ocean as a reservoir for CO_2, which would cause further invasion of e CO2 from  the atmosphere to the ocean.[159]

The threat of acidification includes a decline in commercial fisheries and in the Arctic tourism industry and economy. Commercial fisheries are threatened because acidification harms calcifying organisms which form the base of the Arctic food webs.

Pteropods and brittle stars both form the base of the Arctic food webs and are both seriously damaged from acidification. Pteropods shells dissolve with increasing acidification and the brittle stars lose muscle mass when re-growing appendages.[160] 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.[161] The degradation of organic matter in Arctic waters has amplified ocean acidification; some Arctic waters are already undersaturated with respect to aragonite.[161] In the North Pacific and North Atlantic, saturation states are also decreasing (the depth of saturation is getting more shallow). Additionally the brittle star's eggs die within a few days when exposed to expected conditions resulting from Arctic acidification.[162]

Acidification threatens to destroy 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".[163] 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. The value of fish caught from US commercial fisheries in 2007 was valued at $3.8 billion and of that 73% was derived from calcifiers and their direct predators.[164] Other organisms are directly harmed as a result of acidification. For example, decrease in the growth of marine calcifiers such as the American lobster, ocean quahog, and scallops means there is less shellfish meat available for sale and consumption.[165] Red king crab fisheries are also at a serious threat because crabs are calcifiers and rely on carbonate ions for shell development. Baby red king crab when exposed to increased acidification levels experienced 100% mortality after 95 days.[166] 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.[167] 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 emission scenario.[68]

Acidification will affect the way of life of indigenous peoples. Sport fishing and hunting are both culturally important to Arctic Indigenous peoples.The sport fishing industry is threatened by collapsing food webs which provide food for the prized fish. The rapid decrease or disappearance of marine life could also affect the diet of Indigenous peoples. For example, in Washington State and California, USA  Indigenous communities report  potential damage to shellfish resources due to sea level rise and ocean acidification.[168]

Given that modern ocean acidification is caused by anthropogenic emissions, the number one ocean acidification mitigation strategy is to reduce CO2 emissions. Members of the InterAcademy Panel recommended that by 2050, global anthropogenic CO₂ emissions be reduced less than 50% of the 1990 level.[14] The 2009[14] statement also called on world leaders to:

• Acknowledge that ocean acidification is a direct and real consequence of increasing atmospheric CO₂ concentrations, is already having an effect at current concentrations, and is likely to cause grave harm to important marine ecosystems as CO₂ concentrations reach 450 [parts-per-million (ppm)] and above;
• ... Recognize that reducing the build up of CO₂ in the atmosphere is the only practicable solution to mitigating ocean acidification;
• ... Reinvigorate action to reduce stressors, such as overfishing and pollution, on marine ecosystems to increase resilience to ocean acidification.[169]

Stabilizing atmospheric CO₂ concentrations at 450 ppm would require near-term emissions reductions, with steeper reductions over time.[170]

The German Advisory Council on Global Change[171] stated:

In order to prevent disruption of the calcification of marine organisms and the resultant risk of fundamentally altering marine food webs, the following guard rail should be obeyed: the pH of near surface waters should not drop more than 0.2 units below the pre-industrial average value in any larger ocean region (nor in the global mean).

One policy target related to ocean acidity is the magnitude of future global warming. Parties to the United Nations Framework Convention on Climate Change (UNFCCC) adopted a target of limiting warming to below 2 °C, relative to the pre-industrial level.[172] Meeting this target would require substantial reductions in anthropogenic CO₂ emissions.[173]

Limiting global warming to below 2 °C would imply a reduction in surface ocean pH of 0.16 from pre-industrial levels. This would represent a substantial decline in surface ocean pH.[174]

On 25 September 2015, USEPA denied[175] a 30 June 2015, citizens petition[176] that asked EPA to regulate CO₂ under TSCA in order to mitigate ocean acidification. In the denial, 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,[177] 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.

On 28 March 2017 the US by executive order rescinded the Climate Action Plan.[178] On 1 June 2017 it was announced the US would withdraw from the Paris accords,[179] and on 12 June 2017 that the US would abstain from the G7 Climate Change Pledge,[180] two major international efforts to reduce CO₂ emissions. However, on January 20, 2021 the US re-joined the Paris Accord by means of executive action.

Other solutions such as increasing the land devoted to forests and encouraging the growth of CO₂-breathing sea plants can mitigate ocean acidification.[181]

Intervention and mitigation approaches that remove carbon dioxide from the ocean, known as carbon dioxide removal (CDR), include ocean nutrient fertilization, artificial upwelling/downwelling, seaweed cultivation, ecosystem recovery, ocean alkalinity enhancement, and electrochemical processes. All of these methods mitigate climate change by storing carbon in the ocean. A number of the methods have an additional positive effect, or a co-benefit, of mitigating ocean acidification, which are highlighted in this section. The research field for all CDR methods has grown tremendously since 2019.

Ocean nutrient fertilization, including Iron fertilization, of the ocean could stimulate photosynthesis in phytoplankton (see Iron hypothesis). The phytoplankton would convert the ocean's dissolved carbon dioxide into carbohydrate and oxygen gas, some of which would sink into the deeper ocean before oxidizing. More than a dozen open-sea experiments confirmed that adding iron to the ocean increases photosynthesis in phytoplankton by up to 30 times.[182]   This is one of the more well-researched CDR approaches, however this approach would only sequester carbon on a timescale of 10-100 years dependent on ocean mixing times. While surface ocean acidity may decrease as a result of nutrient fertilization, when the sinking organic matter remineralizes, the deep ocean acidity will increase. A 2021 National Academies report on CDR indicates that there is medium-high confidence that the technique could be efficient and scalable at low cost, with medium environmental risks. [183] One of the key risks of nutrient fertilization is nutrient robbing, a process by which excess nutrients used in one location for enhanced primary productivity, as in a fertilization context, are then unavailable for normal productivity downstream. This could result in ecosystem impacts far outside the original site of fertilization.

Ocean alkalinity enhancement (OAE) is the process of accelerating Earth’s geologic carbon regulator. The process involves increasing the amount of bicarbonate (HCO3-) through accelerated weathering of rocks (silicate, limestone and quicklime). This process mimics the silicate-carbonate cycle, and will ultimately draw down CO2 from the atmosphere, into the ocean. The CO2 will either become bicarbonate, and be stored in the ocean in that form for >100 years, or may precipitate into CaCO3, which when buried in the deep ocean, can store the carbon for ~1 million years when utilizing silicate rocks as the means to increase alkalinity. In addition to sequestering CO2, alkalinity addition buffers the pH of the ocean therefore mitigating ocean acidification. However, little is known about how organisms will respond to added alkalinity, even from natural sources. For example, weathering of some silicate rocks could release a large amount of potentially trace metals into the ocean at the site of enhanced weathering. In addition, the cost and the energy consumed by implementing ocean alkalinity enhancement (mining, pulverizing, transport) is high compared to other CDR techniques. Overall, OAE is scalable, and highly efficient at removing carbon dioxide.[183]

Electrochemical methods, or electrolysis, can strip CO2 directly from seawater. 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. Electrolysis is a classic chemical technique that dates back to the 19th century. However, implementation of electrolysis for carbon capture is expensive and the energy consumed for the process is high compared to other CDR techniques. 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. Similar to OAE, recent reports show electrochemical processes are scalable and highly efficient at removing carbon dioxide.[183]

Ocean acidification: mean seawater pH. Mean seawater pH is shown based on in-situ measurements of pH from the Aloha station.[184]

The importance of ocean acidification is reflected in its inclusion as one of seven Global Climate Indicators.[185] These Indicators are a set of parameters that describe the changing climate without reducing climate change to only 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 GCOS.[186] The Indicators have been endorsed by the World Meteorological Organization (WMO).World Meteorological Organization They form the basis of the annual WMO Statement of the State of the Global Climate, which is submitted to the 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".

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[15], 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".[187] 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".[188] 

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[189], the annual collection of data towards the SDG 14.3.1 Indicator[190] and the reporting of progress to the United Nations.  

The  UN Ocean Decade Action ‘OARS: Ocean Acidification Research for Sustainability”,[191] proposed by the Global Ocean Acidification Observing network (GOA-ON)[192] and its partners has been formally endorsed as a programme of the UN Decade of Ocean Science[193] for Sustainable Development. The OARS programme builds on the work of GOA-ON to further develop the science of ocean acidification by enhancing ocean acidification capacity, increasing observations of ocean chemistry changes, identifying the impacts on marine ecosystems on local and global scales, and providing society and decision makers with the information needed to mitigate and adapt to ocean acidification. OARS encompasses seven specific outcomes tackling increasing ocean acidification monitoring capacity, increasing observations of ocean chemistry changes ocean acidification, identifying the impacts on marine ecosystems, and providing society and decision makers with the information needed to adapt to ocean acidification.

As awareness about ocean acidification grows, policies geared towards increasing monitoring efforts of ocean acidification have been drafted.[194] International efforts, such as the UN Cartagena Convention,[195] are critical to enhance the support provided by regional governments to highly vulnerable areas to ocean acidification. 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 SDG 14; ocean acidification is now starting to be considered within those frameworks.[196] In the United States, robust ocean acidification policy[197] supports sustained government coordination, such as the National Oceanic Atmospheric Administration’s Ocean Acidification Program.[198]

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