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.
Although the natural absorption of CO2 by the world's oceans helps mitigate the climatic effects of anthropogenic emissions of CO2, 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 CO2.
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 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.
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.
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 CO2 may produce CO2-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.
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.
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.
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.
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.
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.
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:
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:
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 25 September 2015, USEPA denied a 30 June 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 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 CO2 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 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 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')".
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.
"Present day" (1990s) sea surface pH
Present day alkalinity
"Present day" (1990s) sea surface anthropogenic CO2
Vertical inventory of "present day" (1990s) anthropogenic CO2
Change in surface CO2−3 ion from the 1700s to the 1990s
Present day DIC
A NOAA (AOML) in situ CO2 concentration sensor (SAMI-CO2), attached to a Coral Reef Early Warning System station, utilized in conducting ocean acidification studies near coral reef areas
A NOAA (PMEL) moored autonomous CO2 buoy used for measuring CO2 concentration and ocean acidification studies