Ocean acidification

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Estimated change in sea water pH caused by human created CO
₂ between the 1700s and the 1990s, from the Global Ocean Data Analysis Project (GLODAP) and the World Ocean Atlas

NOAA provides evidence for upwelling of "acidified" water onto the Continental Shelf. In the figure above, note the vertical sections of (A) temperature, (B) aragonite saturation, (C) pH, (D) DIC, and (E) pCO₂ on transect line 5 off Pt. St. George, California. The potential density surfaces are superimposed on the temperature section. The 26.2 potential density surface delineates the location of the first instance in which the undersaturated water is upwelled from depths of 150 to 200 m onto the shelf and outcropping at the surface near the coast. The red dots represent sample locations.

Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide (CO₂) from the atmosphere. Seawater is slightly basic (meaning the pH is greater than 7), and ocean acidification involves a shift towards pH-neutral conditions rather than a transition to acidic conditions (pH less than 7). An estimated 30–40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers and lakes. To achieve chemical equilibrium, some of it reacts with the water to form carbonic acid. Some of the resulting carbonic acid molecules dissociate into a bicarbonate ion and a hydrogen ion, thus increasing ocean acidity (H⁺ ion concentration). Between 1751 and 1996, surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14, representing an increase of almost 30% in H⁺ ion concentration in the world's oceans. Earth System Models project that, within the last decade, ocean acidity exceeded historical analogues 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.

Increasing acidity is thought to have a range of potentially harmful consequences for marine organisms, such as depressing metabolic rates and immune responses in some organisms, and causing coral bleaching. By increasing the presence of free hydrogen ions, the additional carbonic acid that forms in the oceans ultimately results in the conversion of carbonate ions into bicarbonate ions. Ocean alkalinity (roughly equal to [HCO₃⁻] + 2[CO₃²⁻]) is not changed by the process, or may increase over long time periods due to carbonate dissolution. This net decrease in the amount of carbonate ions available may make it more difficult for marine calcifying organisms, such as coral and some plankton, to form biogenic calcium carbonate, and such structures become vulnerable to dissolution. Ongoing acidification of the oceans may threaten future food chains linked with the oceans. As members of the InterAcademy Panel, 105 science academies have issued a statement on ocean acidification recommending that by 2050, global CO₂ emissions be reduced by at least 50% compared to the 1990 level.

While ongoing ocean acidification is at least partially anthropogenic in origin, it has occurred previously in Earth's history. The most notable example is the 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 in all ocean basins.

Ocean acidification has been compared to anthropogenic climate change and called the "evil twin of global warming" and "the other CO₂ problem". Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious phenomenon.

Carbon cycle

The CO
₂ cycle between the atmosphere and the ocean

The carbon cycle describes the fluxes of carbon dioxide (CO
₂) between the oceans, terrestrial biosphere, lithosphere, and the atmosphere. Human activities such as the combustion of fossil fuels and land use changes have led to a new flux of CO
₂ into the atmosphere. About 45% has remained in the atmosphere; most of the rest has been taken up by the oceans, with some taken up by terrestrial plants.

Distribution of (A) aragonite and (B) calcite saturation depth in the global oceans

The map was created by the National Oceanic and Atmospheric Administration and the Woods Hole Oceanographic Institution using Community Earth System Model data. This map was created by comparing average conditions during the 1880s with average conditions during the most recent 10 years (2003–2012). Aragonite saturation has only been measured at selected locations during the last few decades, but it can be calculated reliably for different times and locations based on the relationships scientists have observed among aragonite saturation, pH, dissolved carbon, water temperature, concentrations of carbon dioxide in the atmosphere, and other factors that can be measured. This map shows changes in the amount of aragonite dissolved in ocean surface waters between the 1880s and the most recent decade (2003–2012). Aragonite saturation is a ratio that compares the amount of aragonite that is actually present with the total amount of aragonite that the water could hold if it were completely saturated. The more negative the change in aragonite saturation, the larger the decrease in aragonite available in the water, and the harder it is for marine creatures to produce their skeletons and shells. The global map shows changes over time in the amount of aragonite dissolved in ocean water, which is called aragonite saturation.

The carbon cycle involves both organic compounds such as cellulose and inorganic carbon compounds such as carbon dioxide, carbonate ion, and bicarbonate ion. The inorganic compounds are particularly relevant when discussing ocean acidification for they include many forms of dissolved CO
₂ present in the Earth's oceans.

When CO
₂ dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide (CO
_2(aq)), carbonic acid (H
₂CO
₃), bicarbonate (HCO⁻
₃) and carbonate (CO²⁻
₃). 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 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 CO
₂ is known as the Revelle factor.

Acidification

Dissolving CO
₂ in seawater increases the hydrogen ion (H⁺) concentration in the ocean, and thus decreases ocean pH, as follows:

CO_{2 (aq)} + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2 H⁺.

Caldeira and Wickett (2003) placed the rate and magnitude of modern ocean acidification changes in the context of probable historical changes during the last 300 million years.

Since the industrial revolution began, the ocean has absorbed about a third of the CO
₂ we have produced since then and it is estimated that surface ocean pH has dropped by slightly more than 0.1 units on the logarithmic scale of pH, representing about a 29% increase in H⁺. It is expected to drop by a further 0.3 to 0.5 pH units (an additional doubling to tripling of today's post-industrial acid concentrations) by 2100 as the oceans absorb more anthropogenic CO
₂, the impacts being most severe for coral reefs and the Southern Ocean. These changes are predicted to accelerate as more anthropogenic CO
₂ is released to the atmosphere and taken up by the oceans. The degree of change to ocean chemistry, including ocean pH, will depend on the mitigation and emissions pathways taken by society.

Although the largest changes are expected in the future, a report from NOAA scientists found large quantities of water undersaturated in aragonite are already upwelling close to the Pacific continental shelf area of North America. Continental shelves play an important role in marine ecosystems since most marine organisms live or are spawned there, and though the study only dealt with the area from Vancouver to Northern California, the authors suggest that other shelf areas may be experiencing similar effects.

Average surface ocean pH

Time	pH	pH change relative to pre-industrial	Source	H+ concentration change relative to pre-industrial
Pre-industrial (18th century)	8.179		analyzed field
Recent past (1990s)	8.104	−0.075	field	+ 18.9%
Present levels	~8.069	−0.11	field	+ 28.8%
2050 (2×CO 2 = 560 ppm)	7.949	−0.230	model	+ 69.8%
2100 (IS92a)	7.824	−0.355	model	+ 126.5%

Rate

One of the first detailed datasets to examine how pH varied over 8 years at a specific north temperate coastal location found that acidification had strong links to in situ benthic species dynamics and that the variation in ocean pH may cause calcareous species to perform more poorly than noncalcareous species in years with low pH and predicts consequences for near-shore benthic ecosystems. Thomas Lovejoy, former chief biodiversity advisor to the World Bank, has suggested that "the acidity of the oceans will more than double in the next 40 years. He says this rate is 100 times faster than any changes in ocean acidity in the last 20 million years, making it unlikely that marine life can somehow adapt to the changes." It is predicted that, by the year 2100, If co-occurring biogeochemical changes influence the delivery of ocean goods and services, then they could also have a considerable effect on human welfare for those who rely heavily on the ocean for food, jobs, and revenues.

Current rates of ocean acidification have been compared with the greenhouse event at the Paleocene–Eocene boundary (about 55 million years ago) when surface ocean temperatures rose by 5–6 degrees Celsius. No catastrophe was seen in surface ecosystems, yet bottom-dwelling organisms in the deep ocean experienced a major extinction. The current acidification is on a path to reach levels higher than any seen in the last 65 million years, and the rate of increase is about ten times the rate that preceded the Paleocene–Eocene mass extinction. The current and projected acidification has been described as an almost unprecedented geological event. 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". 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.

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:

The natural pH of the ocean is determined by a need to balance the deposition and burial of CaCO
₃ 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 CaCO
₃ compensation...The point of bringing it up again is to note that if the CO
₂ 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 CaCO
₃ compensation can keep up. The [present] fossil fuel acidification is much faster than natural changes, and so the acid spike will be more intense than the earth has seen in at least 800,000 years.

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. 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."

A 2013 study claimed acidity was increasing at a rate 10 times faster than in any of the evolutionary crises in Earth's history. 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".

The rate at which ocean acidification will occur may be influenced by the rate of surface ocean warming, because the chemical equilibria that govern seawater pH are temperature-dependent. Greater seawater warming could lead to a smaller change in pH for a given increase in CO₂.

Calcification

Overview

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 and plates out of calcium carbonate (CaCO
₃). This process is called calcification and is important to the biology and survival of a wide range of marine organisms. Calcification involves the precipitation of dissolved ions into solid CaCO
₃ structures, such as coccoliths. After they are formed, such structures are vulnerable to dissolution unless the surrounding seawater contains saturating concentrations of carbonate ions (CO₃²⁻).

Mechanism

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

Of the extra carbon dioxide added into the oceans, some remains as dissolved carbon dioxide, while the rest contributes towards making additional bicarbonate (and additional carbonic acid). This also increases the concentration of hydrogen ions, and the percentage increase in hydrogen is larger than the percentage increase in bicarbonate, 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, creating an imbalance in the reaction Ca²⁺ + CO₃²⁻ ⇌ CaCO₃, and making the dissolution of formed CaCO
₃ structures more likely.

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

Saturation state

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 product of the concentrations of those ions when the mineral is at equilibrium (K
_sp), that is, when the mineral is neither forming nor dissolving. In seawater, a natural horizontal boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon. 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. Below this depth, Ω has a value less than 1, and CaCO
₃ will dissolve. However, if its production rate is high enough to offset dissolution, CaCO
₃ can still occur where Ω is less than 1. The carbonate compensation depth occurs at the depth in the ocean where production is exceeded by dissolution.

The decrease in the concentration of CO₃²⁻ decreases Ω, and hence makes CaCO
₃ dissolution more likely.

Calcium carbonate occurs in two common polymorphs (crystalline forms): aragonite and calcite. Aragonite is much more soluble than calcite, so the aragonite saturation horizon is always nearer to the surface than the calcite saturation horizon. This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite. Increasing CO
₂ levels and the resulting lower pH of seawater decreases the saturation state of CaCO
₃ and raises the saturation horizons of both forms closer to the surface. This decrease in saturation state is believed to be one of the main factors leading to decreased calcification in marine organisms, as the inorganic precipitation of CaCO
₃ is directly proportional to its saturation state.

Possible impacts

Increasing acidity has possibly harmful consequences, such as depressing metabolic rates in jumbo squid, depressing the immune responses of blue mussels, and coral bleaching. However it may benefit some species, for example increasing the growth rate of the sea star, Pisaster ochraceus, while shelled plankton species may flourish in altered oceans.

The report "Ocean Acidification Summary for Policymakers 2013" describes research findings and possible impacts.

Impacts on oceanic calcifying organisms

Although the natural absorption of CO
₂ by the world's oceans helps mitigate the climatic effects of anthropogenic emissions of CO
₂, it is believed that the resulting decrease in pH will have negative consequences, primarily for oceanic calcifying organisms. These span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs. As described above, under normal conditions, calcite and aragonite are stable in surface waters since the carbonate ion is at supersaturating concentrations. However, as ocean pH falls, the concentration of carbonate ions required for saturation to occur increases, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. Therefore, even if there is no change in the rate of calcification, the rate of dissolution of calcareous material increases.

Corals, coccolithophore algae, coralline algae, foraminifera, shellfish, and pteropods experience reduced calcification or enhanced dissolution when exposed to elevated CO
₂.

The Royal Society published a comprehensive overview of ocean acidification, and its potential consequences, in June 2005. However, some studies have found different response to ocean acidification, with coccolithophore calcification and photosynthesis both increasing under elevated atmospheric pCO₂, an equal decline in primary production and calcification in response to elevated CO₂ or the direction of the response varying between species. 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. A 2010 study from Stony Brook University suggested that while some areas are overharvested and other fishing grounds are being restored, because of ocean acidification it may be impossible to bring back many previous shellfish populations. While the full ecological consequences of these changes in calcification are still uncertain, it appears likely that many calcifying species will be adversely affected.

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. 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. All marine ecosystems on Earth will be exposed to changes in acidification and several other ocean biogeochemical changes.

The fluid in the internal compartments 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 level 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. They could even dissolve faster than they can make the crystals to their skeleton, depending on the aragonite levels in the surrounding water. Under the current progression of carbon emissions, around 70% of North Atlantic cold-water corals will be living in corrosive waters by 2050-60.

A study conducted by the Woods Hole Oceanographic Institution in January 2018 showed that the skeletal growth of corals under acidified conditions is primarily affected by a reduced 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.

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. A similar experiment to raise in situ seawater seawater CO₂ level (lower pH) to a level expected soon after the middle of this century found that net calcification decreased 34%.

Ocean acidification may force some organisms to reallocate resources away from productive endpoints such as growth in order to maintain calcification.

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. 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. 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.

Other biological impacts

Aside from the slowing and/or reversing of calcification, organisms may suffer other adverse effects, either indirectly through negative impacts on food resources, 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; depress the immune responses of blue mussels; and make it harder for juvenile clownfish to tell apart the smells of non-predators and predators, or hear the sounds of their predators. This is possibly because ocean acidification may alter the acoustic properties of seawater, allowing sound to propagate further, and increasing ocean noise. This impacts all animals that use sound for echolocation or communication. 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. The lower PH was simulated with 20-30 times the normal amount of CO₂. However, as with calcification, as yet there is not a full understanding of these processes in marine organisms or ecosystems.

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.

Ecosystem impacts amplified by ocean warming and deoxygenation

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. In addition, ocean warming exacerbates ocean deoxygenation, which is an additional stressor on marine organisms, by increasing ocean stratification, through density and solubility effects, thus limiting nutrients, while at the same time increasing metabolic demand.

Meta analyses have quantified the direction and magnitude of the harmful effects of ocean acidification, warming and deoxygenation on the ocean. 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, i.e. that the increases in consumption from thermal stress more than negates any primary producer to herbivore increase from elevated CO₂.

Nonbiological impacts

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. This will cause an elevation of ocean alkalinity, leading to the enhancement of the ocean as a reservoir for CO₂ with implications for climate change as more CO₂ leaves the atmosphere for the ocean.

Impact on human industry

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. For pteropods to create shells they require aragonite which is produced through carbonate ions and dissolved calcium. Pteropods are severely affected because increasing acidification levels have steadily decreased the amount of water supersaturated with carbonate which is needed for aragonite creation. Arctic waters are changing so rapidly that they will become undersaturated with aragonite as early as 2016. Additionally the brittle star's eggs die within a few days when exposed to expected conditions resulting from Arctic acidification. 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". 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. 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. 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. 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. 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.

Impact on indigenous peoples

Acidification could damage the Arctic tourism economy and affect the way of life of indigenous peoples. A major pillar of Arctic tourism is the sport fishing and hunting industry. The sport fishing industry is threatened by collapsing food webs which provide food for the prized fish. A decline in tourism lowers revenue input in the area, and threatens the economies that are increasingly dependent on tourism. The rapid decrease or disappearance of marine life could also affect the diet of Indigenous peoples.

Ocean Acidification in the Arctic Ocean

Annual Arctic Sea Ice Minimum

The Arctic Ocean has experienced drastic change over the years due to global warming. It has been known that the Arctic Ocean acidity levels have been increasing and at twice the rate compared to the Pacific and Atlantic oceans. The loss of sea ice has been connected to a decrease in pH levels in the ocean water. Sea ice has experienced an extreme reduction over the past 30 years, forming a minimum area of 2.9×106 km² at the end of the boreal summer of 2007, 47%, less than in 1980. Sea ice limits the air-sea gas exchange with carbon dioxide. With less water completely exposed to the atmosphere, the levels of carbon dioxide gas in the water remain low. The Arctic Ocean should have low carbon dioxide levels due to intense cooling, run off of fresh water and photosynthesis from marine organisms. However, the decrease of sea ice over the years due to global warming has limited freshwater runoff and has exposed a higher percentage of the ocean surface to the atmosphere. The increase of carbon dioxide in the water decreases the pH of the ocean causing ocean acidification. The decrease in sea ice has also allowed more Pacific water to flow into in the Arctic Ocean during the winter, this is called Pacific winter water. The Pacific water flows into the Arctic Ocean carrying additional amounts of carbon dioxide by being exposed to the atmosphere and absorbing carbon dioxide from decaying organic matter and from sediments.

The Arctic Ocean pH levels are rapidly decreasing because not only is the ocean water absorbing more carbon dioxide due to increased surface area exposure as a result of a decrease in sea ice. It also has large amounts of carbon dioxide being transferred to the Arctic from the Pacific ocean.

Cold water is able to absorb higher amounts of carbon dioxide compared to warm water. The solubility of gases decreases in relation to increasing temperature. Cold water bodies are absorbing the increasing amount of carbon dioxide in the atmosphere and becoming known as carbon sinks. The increasing amount of carbon dioxide in the water is putting many organisms at risk as they are affected by the increase of acidity in the ocean water.

Effects of Ocean Acidification on Arctic Organisms

Organisms in Arctic waters are already challenged with stressors of living in the Arctic Ocean, such as dealing with cold temperatures, and it is thought that because of this, additional stressors such as ocean acidification, will cause ocean acidification effects on marine organisms to appear first in the Arctic. There exists a significant variation in the sensitivity of marine organisms to increased ocean acidification. Calcifying organisms generally exhibit larger negative responses from ocean acidification than non‐calcifying organisms across numerous response variables, with the exception of crustaceans, which calcify but were not negatively affected. The acidification of the Arctic Ocean will impact these marine calcifiers in several different ways.

The uptake of CO₂ by seawater increases the concentration of hydrogen ions, which lowers pH and, in changing the chemical equilibrium of the inorganic carbon system, reduces the concentration of carbonate ions (CO₃²⁻). Carbonate ions are required by marine calcifying organisms such as plankton, shellfish, and fish to produce calcium carbonate (CaCO₃) shells and skeletons.

Arctic Council map

For either aragonite or calcite, the two polymorphs of CaCO₃ produced by marine organisms, the saturation state of CaCO₃ in ocean water is expressed by the product of the concentrations of CO₂²⁻ and Ca²⁺ in seawater relative to the stoichiometric solubility product at a given temperature, salinity, and pressure. Waters which are saturated in CaCO₃ are favorable to precipitation and formation of CaCO₃ shells and skeletons, but waters which are undersaturated are corrosive to CaCO₃ shells, and in the absence of protective mechanisms, dissolution of calcium carbonate will occur. Because colder arctic water absorbs more CO₂, the concentration of CO₃²⁻ is reduced, therefore the saturation of calcium carbonate is lower in high-latitude oceans than it is in tropical or temperate oceans. In model simulations of the Arctic Ocean, it is predicted that aragonite saturation will decrease, because of an increased amount of freshwater input from melting sea ice and increased carbon uptake as a result of sea ice retreat. This simulation predicts that Arctic surface waters will become undersaturated with aragonite within a decade. The undersaturation of aragonite will cause the shells of organisms which are constructed from aragonite to dissolve. This would have a profound effect on a large variety of marine organisms and has the potential to do devastating damage to keystone species and to the marine food web in the Arctic Ocean. Laboratory experiments on various marine biota in an elevated CO₂ environment show that changes in aragonite saturation cause substantial changes in overall calcification rates for many species of marine organisms, including coccolithophore, foraminifera, pteropods, mussels, and clams.

Although the undersaturation of arctic water has been proven to have an effect on the ability of organisms to precipitate their shells, recent studies have shown that the calcification rate of calcifiers, such as corals, coccolithophores, foraminiferans and bivalves, decrease with increasing pCO₂, even in seawater supersaturated with respect to CaCO₃. Additionally, increased pCO₂ has been found to have complex effects on the physiology, growth and reproductive success of various marine calcifiers. CO₂ tolerance seems to differ between various marine organisms, as well as differences in CO₂ tolerance at different life cycle stages (e.g. larva and adult). The first stage in the life cycle of marine calcifiers which are at a serious risk by high CO2 content is the planktonic larval stage. The larval development of several marine species, primarily sea urchins and bivalves, are highly affected by elevations of seawater pCO₂. In laboratory tests, numerous sea urchin embryos were reared under different CO₂ concentrations until they developed to the larval stage. It was found that once reaching this stage, larval and arm sizes were significantly smaller, as well as abnormal skeleton morphology was noted with increasing pCO₂.

Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100

Similar findings have been found in CO₂ treated-mussel larvae, which showed a larval size decrease of about 20% and showed morphological abnormalities such as convex hinges, weaker and thinner shells and protrusion of mantle. The larval body size also impacts the encounter and clearance rates of food particles, and if larval shells are smaller or deformed, these larvae are more prone to starvation. In addition, CaCO₃ structures also serve vital functions for calcified larvae, such as defence against predation, as well as roles in feeding, buoyancy control and pH regulation. Another example of a species which may be seriously impacted by ocean acidification is Pteropods, which are shelled pelagic molluscs which play an important role in the food-web of various ecosystems. Since they harbor an aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO₂ emissions.Laboratory tests showed that calcification exhibits a 28% decrease at the pH value of the Arctic Ocean expected for the year 2100, compared to the present pH value. This 28% decline of calcification in the lower pH condition is within the range reported also for other calcifying organisms such as corals. In contrast with sea urchin and bivalve larvae, corals and marine shrimps are more severely impacted by ocean acidification after settlement, while they developed into the polyp stage. From laboratory tests, the morphology of the CO₂-treated polyp endoskeleton of corals was disturbed and malformed compared to the radial pattern of control polyps.

This variability in the impact of ocean acidification on different life cycle stages of different organisms can be partially explained by the fact that most echinoderms and mollusks start shell and skeleton synthesis at their larval stage, whereas corals start at the settlement stage. Hence, these stages are highly susceptible to the potential effects of ocean acidification. Most calcifiers, such as corals, echinoderms, bivalves and crustaceans, play important roles in coastal ecosystems as keystone species, bioturbators and ecosystem engineers. The food web in the Arctic Ocean is somewhat truncated, meaning it is short and simple. Any impacts to key species in the food web can cause exponentially devastating effects on the rest on the food chain as a whole, as they will no longer have a reliable food source. If these larger organisms no longer have any source of nutrients, they too will eventually die off, and the entire Arctic Ocean ecosystem will be affected. This would have a huge impact on the Arctic people who catch Arctic fish for a living, as well as the economic repercussions which would follow such a major shortage of food and living income for these families.

Possible responses

Demonstrator calling for action against ocean acidification at the People's Climate March (2017).

Reducing CO₂ emissions

Members of the InterAcademy Panel recommended that by 2050, global anthropogenic CO₂ emissions be reduced less than 50% of the 1990 level. The 2009 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;
• ... Recognise 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.

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

The German Advisory Council on Global Change 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. Meeting this target would require substantial reductions in anthropogenic CO₂ emissions.

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.

On September 25, 2015, USEPA denied a June 30, 2015, citizens petition 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, 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 March 28, 2017 the US by executive order rescinded the Climate Action Plan. On June 1, 2017 it was announced the US would withdraw from the Paris accords, and on June 12, 2017 that the US would abstain from the G7 Climate Change Pledge, two major international efforts to reduce CO₂ emissions.

Climate engineering

Climate engineering (mitigating temperature or pH effects of emissions) has been proposed as a possible response to ocean acidification. The IAP (2009) statement cautioned against climate engineering as a policy response:

Mitigation approaches such as adding chemicals to counter the effects of acidification are likely to be expensive, only partly effective and only at a very local scale, and may pose additional unanticipated risks to the marine environment. There has been very little research on the feasibility and impacts of these approaches. Substantial research is needed before these techniques could be applied.

Reports by the WGBU (2006), the UK's Royal Society (2009), and the US National Research Council (2011) warned of the potential risks and difficulties associated with climate engineering.

Iron fertilization

Iron fertilization of the ocean could stimulate photosynthesis in phytoplankton. 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. While this approach has been proposed as a potential solution to the ocean acidification problem, mitigation of surface ocean acidification might increase acidification in the less-inhabited deep ocean.

A report by the UK's Royal Society (2009) reviewed the approach for effectiveness, affordability, timeliness and safety. The rating for affordability was "medium", or "not expected to be very cost-effective". For the other three criteria, the ratings ranged from "low" to "very low" (i.e., not good). For example, in regards to safety, the report found a "[high] potential for undesirable ecological side effects", and that ocean fertilization "may increase anoxic regions of ocean ('dead zones')".