# Ocean acidification

Estimated change in seawater 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
Here is a detailed image of the full carbon cycle
NOAA provides evidence for the 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) pCO
2
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 Infographic

Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide (CO
2
) from the atmosphere. The main cause of ocean acidification is the burning of fossil fuels. Seawater is slightly basic (meaning pH > 7), and ocean acidification involves a shift towards pH-neutral conditions rather than a transition to acidic conditions (pH < 7). The issue of ocean acidification is the decreased production of the shells of shellfish and other aquatic life with calcium carbonate shells. The calcium carbonate shells can not reproduce under high saturated acidotic waters. An estimated 30–40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers and lakes. 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, by around 2008, 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 CO
2
emissions be reduced by at least 50% compared to the 1990 level. To ensure that ocean acidification is minimized, the United Nation's Sustainable Development Goal 14 ("Life below Water") aims to ensure that oceans are conserved and sustainably used.

Latest research 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. Controversially, laboratory experiments in the controlled environment showed CO
2
induced growth of the phytoplankton species. 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, rather than acidification, the main factor for coral reef vulnerability due to global warming.

While ongoing ocean acidification is at least partially anthropogenic in origin, it has occurred previously in Earth's history, and the resulting ecological collapse in the oceans had long-lasting effects for global carbon cycling and climate. 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 CO
2
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
This map shows changes in the aragonite saturation level of ocean surface waters between the 1880s and the most recent decade (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.

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
2
CO
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:

CO2 (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.

Here is detailed diagram of the carbon cycle within the ocean

### 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. An ecological tipping point was projected to occur by the year 2030 and no later than 2038. 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. 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.

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:

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 CO
2
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

Video summarizing the impacts of ocean acidification. Source: NOAA Environmental Visualization Laboratory.

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 reports "Ocean Acidification Summary for Policymakers 2013" and the IPCC approved "Special Report on the Ocean and Cryosphere in a Changing Climate" from 2019 describe research findings and possible impacts.

### Impacts on oceanic calcifying organisms

Shells of pteropods dissolve in increasingly acidic conditions caused by increased amounts of atmospheric CO2

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 also decreases, 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 pCO
2
, an equal decline in primary production and calcification in response to elevated CO
2
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 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. For example, the oyster, Magallana gigas, is recognized to experience metabolic changes alongside altered calcification rates due to energetic tradeoffs resulting from pH imbalances.

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.

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. Seawater acidification could also see Antarctic phytoplanktons smaller and less effective at storing carbon. Such changes are being increasingly studied and synthesized through the use of physiological frameworks, including the Adverse Outcome Pathway (AOP) framework.

### Impact on reef fish

With the production of CO2 from the burning of fossil fuels, oceans are becoming more acidic since CO2 dissolves in water and forms carbonic acid. This results in a pH drop which then causes corals to expel their algae with which they have a symbiotic relationship with, causing the coral to eventually die due to a lack of nutrients.

Since corals reefs are one of the most diverse ecosystems on the planet, coral bleaching due to ocean acidification could result in a major loss of habitat for the many species of reef fish, resulting in increased predation and the eventual endangered classification or extinction of countless species. This will ultimately decrease the overall diversity of fish in marine environments, which will cause many predators of reef fish to die off since their normal supply of food was cut off. Food webs in coral reefs will also be greatly impacted because once a species goes extinct or is less prevalent, their natural predators will lose their primary food source causing the food web to collapse in on itself. If such an extinction event occurred in our oceans, it will greatly affect humans since much of our food supply is reliant on fish or other marine animals.

Ocean acidification due to global warming will also change the reproductive cycles of reef fish who normally spawn during late spring and fall. On top of this, there will be increased mortality rates among the larvae of coral reef fish since the acidic environment slows down their development. The hypothalamo-pituitary-gonadal (HPG) axis is one of the regulatory sequences in fish for reproduction, which is mainly controlled by surrounding water temperature. Once a minimum temperature threshold is reached, the production of hormone synthesis increases significantly, causing the fish to produce mature egg and sperm cells. Spawning in the spring will have a shortened period, while fall spawning will be delayed substantially. Because of the increased CO2 levels in the ocean from coral bleaching, there will be a substantial decrease in the number of young reef fish that survive to maturity. There is also evidence that shows that embryo and larval stage fish have not matured enough to express the appropriate levels of acid/base regulation that is present in adults. These will ultimately lead to hypoxia due to the Bohr effect driving oxygen off of hemoglobin. This will lead to increased mortality as well as impaired growth performance for fish in slightly acidic conditions relative to the normal proportion of acid dissolved in marine water.

In addition, ocean acidification will make fish larvae more sensitive to the surrounding pH since they are more sensitive to environmental fluctuations than adults. In addition, larvae of common prey species will have lower survival rates, which in turn will eventually cause the species to become endangered or extinct. Also, elevated CO2 in marine environments can lead to neurotransmitter interference in both predator and prey fish which increases their mortality rate. It has also been shown that when fish spend considerable time in high concentrations of dissolved CO2 up to 50,000 micro-atmospheres (μatm) of CO2 in marine environments, cardiac failure leading to death is much more common than in normal CO2 environments. In addition, fish that live in high CO2 environments are required to spend more of their energy to keep their acid/base regulation in check. This diverts precious energy resources from important parts of their life cycle such as feeding and mating to keep their osmoregulatory functions in check. However, a more recent study found that acidification has had no significant impact on the behavior of reef fish.

Recent experiments on sensory systems in tropical coral reef fishes have suggested other ways in which fishes can be affected by ocean acidification. Simulations with near-future levels of carbon dioxide (800 µatm) have shown olfactory cue disruption for orange clownfish larvae. Furthermore, sight and hearing are also affected by increased carbon dioxide concentrations in the ocean. An experiment conducted on juvenile ambon damselfish in which a possible predator was introduced into their habitat in a watertight plastic bag showed that the damselfish were less wary of the predatory threat at higher CO2 concentrations. Another experiment conducted on clownfish examined hearing responsiveness under heightened CO2 concentrations. When reef noises were played from one side of an aquarium under current CO2 levels, the clownfish generally avoided that side. However, under higher CO2 conditions, the clownfish would congregate closer to the reef noises. These experiments suggest that ocean acidification affects brain functioning in reef fishes.

Another important consequence of ocean acidification is that endangered species will have fewer places where their eggs are laid. For species with poor larval dispersal, it puts them at a greater risk of extinction because natural egg predators will find their nests or hiding places and eat the next generation.

### 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. For example, the elevated oceanic levels of CO
2
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 CO
2
. 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.

Although red tide is harmful, other beneficial photosynthetic organisms may benefit from increased levels of carbon dioxide. Most importantly, seagrasses will benefit. 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.

### Ecosystem impacts amplified by ocean warming and deoxygenation

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.

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 CO
2
with implications for climate change as more CO
2
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.

## 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 CO
2
emissions be reduced less than 50% of the 1990 level. The 2009 statement also called on world leaders to:

Stabilizing atmospheric CO
2
concentrations at 450 ppm would require near-term emissions reductions, with steeper reductions over time.

The German Advisory Council on Global Change stated:

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 CO
2
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 25 September 2015, USEPA denied a 30 June 2015, citizens petition that asked EPA to regulate CO
2
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 28 March 2017 the US by executive order rescinded the Climate Action Plan. On 1 June 2017 it was announced the US would withdraw from the Paris accords, and on 12 June 2017 that the US would abstain from the G7 Climate Change Pledge, two major international efforts to reduce CO
2
emissions.

The prevention and significant reduction of all kinds of marine pollution including ocean acidification is part of the targets of the United Nations' Sustainable Development Goal 14.

### Geoengineering

Geoengineering has been proposed as a possible response to ocean acidification. The IAP (2009) statement said more research is needed to prove that this would be safe, affordable and worthwhile:

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 (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. 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')".

## Ocean acidification and mass extinction events in the geologic past

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. Early research focused on the climatic effects of the elevated CO2 levels on biodiversity, but in 2004, 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. The end-Triassic biotic crisis is still the most well-established example of a marine mass extinction due to ocean acidification, because (a) volcanic activity, changes in carbon isotopes, decrease of carbonate sedimentation, and marine extinction coincided precisely in the stratigraphic record, and (b) there was pronounced selectivity of the extinction against organisms with thick aragonitic skeletons, which is predicted from experimental studies. Ocean acidification has also been suggested as a cause of the end-Permian mass extinction and the end-Cretaceous crisis.