Estimated change in sea water pH caused by human created CO2 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.^[1]

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.^[2] 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).^[3] An estimated 30–40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers and lakes.^[4]^[5] 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,^[6] representing an increase of almost 30% in H⁺ ion concentration in the world's oceans.^[7]^[8] Earth System Models project that, by around 2008, ocean acidity exceeded historical analogues^[9] 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.^[10]

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.^[11] 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.^[12] 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.^[13] Ongoing acidification of the oceans may threaten future food chains linked with the oceans.^[14]^[15] As members of the InterAcademy Panel, 105 science academieshave issued a statement on ocean acidification recommending that by 2050, global CO2 emissions be reduced by at least 50% compared to the 1990 level.^[16]

While ongoing ocean acidification is at least partially anthropogenic in origin, it has occurred previously in Earth's history,^[17]and the resulting ecological collapse in the oceans had long-lasting effects for global carbon cycling and climate.^[18]^[19] The most notable example is the Paleocene-Eocene Thermal Maximum (PETM),^[20] 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"^[21]^[22]^[23]^[24]^[25] and "the other CO2 problem".^[22]^[24]^[26] Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious phenomenon.^[27]^[28]

Carbon cycle[edit]

The CO2 cycle between the atmosphere and the ocean

The carbon cycle describes the fluxes of carbon dioxide (CO2) between the oceans, terrestrial biosphere, lithosphere,^[29]and the atmosphere. Human activities such as the combustion of fossil fuels and land use changes have led to a new flux of CO2 into the atmosphere. About 45% has remained in the atmosphere; most of the rest has been taken up by the oceans,^[30] with some taken up by terrestrial plants.^[31]

Distribution of (A) aragonite and (B) calcite saturation depth in the global oceans^[5]

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.^[32]

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 CO2 present in the Earth's oceans.^[33]

When CO2 dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide (CO2(aq)), carbonic acid (H2CO3), 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 CO2 is known as the Revelle factor.

Acidification[edit]

Dissolving CO2 in seawater increases the hydrogen ion (H+) concentration in the ocean, and thus decreases ocean pH, as follows:^[34]

CO₂ _(aq) + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2 H⁺.

Caldeira and Wickett (2003)^[2] 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 CO2 we have produced since then ^[35]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^[10] (an additional doubling to tripling of today's post-industrial acid concentrations) by 2100 as the oceans absorb more anthropogenic CO2, the impacts being most severe for coral reefs and the Southern Ocean.^[2]^[13]^[36] These changes are predicted to accelerate as more anthropogenic CO2 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^[37] taken by society.^[38]

Although the largest changes are expected in the future,^[13] 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.^[1]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.^[1]

Average surface ocean pH^[13]^{[failed verification]}
Time	pH	pH change relative to pre-industrial	Source	H⁺ concentration change relative to pre-industrial
Pre-industrial (18th century)	8.179		analysed field^[39]^{[failed verification]}
Recent past (1990s)	8.104	−0.075	field^[39]	+ 18.9%
Present levels	~8.069	−0.11	field^[7]^[8]^[40]^[41]	+ 28.8%
2050 (2×CO2 = 560 ppm)	7.949	−0.230	model^[13]^{[failed verification]}	+ 69.8%
2100 (IS92a)^[42]	7.824	−0.355	model^[13]^{[failed verification]}	+ 126.5%

Rate[edit]

“

If we continue emitting CO₂ at the same rate, by 2100 ocean acidity will increase by about 150 percent, a rate that has not been experienced for at least 400,000 years.

”

— UK Ocean Acidification Research Programme, 2015^[43]

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.^[44]^[45] 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."^[46] 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.^[10]^[47] 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.^[48]

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,^[49]^[50]^[51] 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.^[52] 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".^[53]^[54] 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.^[55]^[56]

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:^[57]

"The natural pH of the ocean is determined by a need to balance the deposition and burial of CaCO3 on the sea floor against the influx of Ca2+ and CO2−3 into the ocean from dissolving rocks on land, called weathering. These processes stabilize the pH of the ocean, by a mechanism called CaCO3 compensation...The point of bringing it up again is to note that if the CO2 concentration of the atmosphere changes more slowly than this, as it always has throughout the Vostok record, the pH of the ocean will be relatively unaffected because CaCO3 compensation can keep up. 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.^[58] 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."^[21]

A 2013 study claimed acidity was increasing at a rate 10 times faster than in any of the evolutionary crises in Earth's history.^[59] 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".^[60]

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.^[61] Greater seawater warming could lead to a smaller change in pH for a given increase in CO₂.^[61]

Calcification[edit]

Overview[edit]

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 (CaCO3).^[36] 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 CaCO3 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[edit]

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,^[62] 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 CaCO3 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[edit]

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.^[63] In seawater, a natural horizontal boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon.^[36] 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.^[36] 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
3can 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.^[64]

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.^[36] This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite.^[13] 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.^[65] 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.^[66]

Possible impacts[edit]

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,^[67] depressing the immune responses of blue mussels,^[68] and coral bleaching. However it may benefit some species, for example increasing the growth rate of the sea star, Pisaster ochraceus,^[69] while shelled plankton species may flourish in altered oceans.^[70]

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.^[71]^[72]

Impacts on oceanic calcifying organisms[edit]

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 calcifyingorganisms. These span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs.^[10]^[73] 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.^[74]

Corals,^[75]^[76]^[77]^[78] coccolithophore algae,^[79]^[80]^[81]^[82] coralline algae,^[83] foraminifera,^[84] shellfish^[85] and pteropods^[13]^[86] 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.^[36] However, some studies have found different response to ocean acidification, with coccolithophore calcification and photosynthesis both increasing under elevated atmospheric pCO
2,^[87]^[88]^[89] an equal decline in primary production and calcification in response to elevated CO
2^[90] or the direction of the response varying between species.^[91] 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.^[89] 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.^[92] 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.^[58] 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.^[93] All marine ecosystems on Earth will be exposed to changes in acidification and several other ocean biogeochemical changes.^[10]

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.^[94] Under the current progression of carbon emissions, around 70% of North Atlantic cold-water corals will be living in corrosive waters by 2050–60.^[95]

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.^[96]

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.^[97] 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%.^[98]

Ocean acidification may force some organisms to reallocate resources away from productive endpoints such as growth in order to maintain calcification.^[99]

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.^[7] 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.^[100] However, in Palaucarbon 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.^[101] Seawater acidification could also see Antarctic phytoplanktons smaller and less effective at storing carbon.^[102]

Effect on reef fish[edit]

With the production of CO₂ from the burning of fossil fuels, oceans are becoming more acidic since CO₂ dissolves in water and forms the acidic bicarbonate ion. 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.^[103] 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.^[104]^[103] Spawning in the spring will have a shortened period, while fall spawning will be delayed substantially.^[104] 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.^[103]^[105] 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.^[103]

In addition, ocean acidification will make fish larvae more sensitive to the surrounding pH since they are more sensitive to environmental fluctuations than adults.^[104] 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.^[106]^[107] Also, elevated CO₂ in marine environments can lead to neurotransmitter interference in both predator and prey fish which increases their mortality rate.^[108]^{[full citation needed]} 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.^[105] 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.

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.^[103]

Other biological impacts[edit]

Aside from the slowing and/or reversing of calcification, organisms may suffer other adverse effects, either indirectly through negative impacts on food resources,^[36] 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;^[67] depress the immune responses of blue mussels;^[68] and make it harder for juvenile clownfish to tell apart the smells of non-predators and predators,^[109] or hear the sounds of their predators.^[110]This is possibly because ocean acidification may alter the acoustic properties of seawater, allowing sound to propagate further, and increasing ocean noise.^[111] This impacts all animals that use sound for echolocation or communication.^[112] 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.^[113] However, as with calcification, as yet there is not a full understanding of these processes in marine organisms or ecosystems.^[114]

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.^[115]

Although red tide is harmful, other beneficial photosynthetic organisms may benefit from increased levels of carbon dioxide. Most importantly, seagrasses will benefit.^[116] 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.^[116]

Ecosystem impacts amplified by ocean warming and deoxygenation[edit]

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.^[117]^[118]^[119] 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,^[120]^[121] 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.^[122]^[123]^[124] These meta-analyses have been further tested by mesocosm studies^[125]^[126] 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[edit]

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.^[127] 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.^[128]

Impact on human industry[edit]

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.^[129] 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.^[130] Arctic waters are changing so rapidly that they will become undersaturated with aragonite as early as 2016.^[130] Additionally the brittle star's eggs die within a few days when exposed to expected conditions resulting from Arctic acidification.^[131] 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".^[132] 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.^[133] 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.^[134] 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.^[135] 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.^[136] 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.^[10]

Impact on indigenous peoples[edit]

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.^[137] The rapid decrease or disappearance of marine life could also affect the diet of Indigenous peoples.

Possible responses[edit]

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

Reducing CO
2 emissions[edit]

Members of the InterAcademy Panel recommended that by 2050, global anthropogenic CO
2 emissions be reduced less than 50% of the 1990 level.^[16] The 2009^[16] statement also called on world leaders to:

Acknowledge that ocean acidification is a direct and real consequence of increasing atmospheric CO
2 concentrations, is already having an effect at current concentrations, and is likely to cause grave harm to important marine ecosystems as CO
2 concentrations reach 450 [parts-per-million (ppm)] and above;

... Recognize that reducing the build up of CO
2 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.^[138]

Stabilizing atmospheric CO
2 concentrations at 450 ppm would require near-term emissions reductions, with steeper reductions over time.^[139]

The German Advisory Council on Global Change^[140] 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.^[141] Meeting this target would require substantial reductions in anthropogenic CO
2 emissions.^[142]

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.^[143]

On 25 September 2015, USEPA denied^[144] a 30 June 2015, citizens petition^[145] 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,^[146] 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.^[147] On 1 June 2017 it was announced the US would withdraw from the Paris accords,^[148] and on 12 June 2017 that the US would abstain from the G7 Climate Change Pledge,^[149] two major international efforts to reduce CO
2 emissions.

Geoengineering[edit]

Geoengineering has been proposed as a possible response to ocean acidification. The IAP (2009)^[16] statement said more research is needed to prove that this would be safe, affordable and worthwhile:

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),^[140] the UK's Royal Society (2009),^[150] and the US National Research Council (2011)^[151] warned of the potential risks and difficulties associated with climate engineering.

Iron fertilization[edit]

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.^[152] 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.^[153]

A report by the UK's Royal Society (2009)^[154] 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')".^[155]

The Global Problem of Ocean Acidification

June 07, 2011 Lisa Speer

One year from now, nations from around the world will convene to discuss the future of our planet at the Earth Summit 2012. As NRDC’s President Frances Beinecke says so well, this global meeting must be one of action, and not of lofty promises.

There is precious little time to waste, and the issue of ocean acidification highlights the urgency for action. Carbon dioxide (CO2) from burning fossil fuels is changing the fundamental chemistry of our oceans. CO2 reacts with sea water to form carbonic acid. As atmospheric CO2 has risen, the oceans have become 30% more acidic over the last 150 years. This effect is measurable and undisputed, and affects all of the world’s oceans.

At the Earth Summit, NRDC is calling on the international community to develop, on an urgent basis, an integrated, international program aimed at monitoring the chemical and biological changes resulting from ocean acidification that are likely to have socio-economic consequences. Such a monitoring network is essential to provide coastal nations with the information necessary to prepare for the impacts of ocean acidification on fisheries, corals and marine food webs.

As NRDC’s movie ACID TEST so vividly illustrates, rising ocean acidity reduces the availability of carbonate, a critical component of shell-building. If acidity gets high enough, ocean water becomes corrosive and shells literally dissolve. Unchecked, ocean acidification could affect marine food webs and lead to substantial changes in commercial fish stocks, threatening protein supply and food security for millions of people as well as the multi-billion dollar global fishing industry. By mid-century vast ocean regions may be inhospitable to coral growth and reefs will begin to erode faster than they can grow. Regions dependent on healthy coral reefs for fisheries, tourism, and storm protection will be profoundly impacted.

Currently, there are only approximately 30 monitoring stations capable of measuring ocean acidity, and most of these are in developed countries. There is very little monitoring of biological impacts of acidification anywhere in the world. Without better monitoring it will not be possible to identify areas of vulnerability or develop effective mitigation measures and management strategies.

The single most important step we can take to address ocean acidification is to dramatically reduce CO2 emissions. But ocean acidification is already affecting marine life, and States and coastal communities need information that can help them assess risks, plan for impacts and initiate management strategies, including, for example:

Vulnerability Analyses – Based on current research and observations, scientists have identified broad geographic regions and marine species that are vulnerable. High latitudes, regions of upwelling, and coastal estuaries with heavy river input, will experience episodes of corrosive water first. In addition, certain species such as tropical corals and some oysters and other mollusks are particularly sensitive to changes in carbonate chemistry. There may be many other marine animals affected, and a more comprehensive and refined understanding of vulnerabilities is greatly needed.

Early warning systems - Real-time information about ocean chemistry can serve as an early warning system for already affected regions and industries. For example, oyster hatcheries along the west coast of the United States have deployed monitoring systems to alert their operators to episodes of corrosive which are harmful to larval oysters. With the use of these systems, hatchery owners have restored their production by 80% and have rescued their businesses.

Management guidance – Ocean acidification is happening against the backdrop of a rapidly changing ocean. In addition to changes in ocean chemistry, ocean water is getting warmer, oxygen availability is decreasing, and a host of local stressors exacerbate global change. Enhanced ocean observations are critically needed to improve ocean management in a changing world.

The map below illustrates current existing and planned monitoring stations (red circles and yellow triangles). The ovals identify areas likely to experience the impacts of ocean acidification soonest. The entire Arctic Ocean and Southern Ocean are also likely to experience impacts in the near future. Supplementing the existing network is estimated to cost in the neighborhood of $50 million – a small investment that will allow coastal States and communities to plan for the future.

Feely et al., 2009. Red circles represent deployed or planned open-ocean monitoring sites; yellow triangles represent deployed or planned coral reef monitoring sites. Ovals represent areas likely to experience ocean acidification impacts soonest. Arctic and Antarctic marine ecosystems are also likely to experience impacts in the near future.

Oceans absorb a substantial proportion of the CO₂ emitted into the atmosphere by human activities, with potentially negative effects on shell-forming organisms.

Increasing CO₂ in the atmosphere due to human activities not only affects the climate; it also has direct, chemical effects on ocean waters.

The oceans have absorbed between a third and a half of the CO₂ humans have released into the atmosphere since about 1850. This has slowed the rate of climate change.

When CO₂ dissolves in seawater, the water becomes more acidic. The acidity of the oceans has increased by 26 % since about 1850, a rate of change roughly 10 times faster than any time in the last 55 million years.

Associated chemical reactions can make it difficult for marine calcifying organisms, such as coral and some plankton, to form shells and skeletons, and existing shells become vulnerable to dissolution.

The extent to which calcifying organisms are already being affected by acidification is unclear, as this is a very new area of study. Limited evidence suggests that some organisms are more sensitive than others.

The rate at which acidification occurs is a determining factor in the extent to which calcifying organisms will be able to adapt.

The impacts of acidification will extend up the food chain to affect economic activities such as fisheries, aquaculture and tourism. Wherever there are marine calcifying organisms, there are risks from ocean acidification.

What is ocean acidification?

Human activities release CO₂ into the atmosphere, which leads to atmospheric warming and climate change, as explained in Causes of climate change. Around a third to a half of the CO₂released by human activities is absorbed into the oceans. While this helps to reduce the rate of atmospheric warming and climate change, it also has a direct, chemical effect on seawater, which we call ocean acidification (Figure 1).

T2I3_Figure-1.gif

Some of the carbon dioxide emitted to the atmosphere by human activities is absorbed by the oceans. When carbon dioxide combines with water in the ocean it forms carbonic acid, which makes the ocean more acidic.

Figure 1: Some of the carbon dioxide emitted to the atmosphere by human activities is absorbed by the oceans. When carbon dioxide combines with water in the ocean it forms carbonic acid, which makes the ocean more acidic and may reduce the ability of calcifying organisms to form their shells and skeletons. Source: Adapted from J. Cook, skepticalscience.com.

Box 1: What is pH?

Ocean acidification is often expressed in terms of the pH of seawater. pH is a measure of acidity or alkalinity. A pH below 7 is considered acidic, and a pH greater than 7 is considered alkaline, or basic.

Average ocean water pH is currently 8.1. The pH scale is logarithmic, so a one point change on the scale means a tenfold change in concentration.

T2I3_Figure B1.png

Figure B1: The pH scale. Source: Feely et. al 2006.

What are the observed changes?

Since around 1850, the oceans have absorbed between a third and a half of the CO₂ emitted to the atmosphere. As a result, the average pH of ocean surface waters has fallen by about 0.1 units, from 8.2 to 8.1 (Figure 2). This corresponds to a 26 % increase in ocean acidity, a rate of change roughly 10 times faster than any time in the last 55 million years.

T2I3_Figure-2.gif

Figure 2: Global mean ocean surface pH from 1850 to 2100, from climate models. The modelled historical trend shows an overall decrease of about 0.1 pH units (black). Projections up to 2100 are shown for high emission scenarios (RCP8.5, red) and low emission scenarios (RCP2.6, blue). Source: Gattuso et al. 2014, Fig. OA1b.

What can we expect in the future?

The degree of future ocean acidification will be very closely linked to future increases in atmospheric CO₂(Figure 3). If greenhouse gas emissions continue as they are doing at present (the RCP8.5 trajectory, see Causes of climate change), seawater could increase its acidity by 0.4 units (see Box 1) by the end of the century.

The acidification of the oceans will not be uniform worldwide. Polar seas, and upwelling regions, often found along the west coasts of continents, are expected to acidify faster than temperate or tropical regions. The pH will vary significantly depending on the ecosystem. In some parts of the Arctic the water is acidic enough to corrode some types of shells and in California occasional corrosive events have already occurred. Most surface waters will be continually corrosive within decades.

T2I3_Figure 3.jpg

Atmospheric CO2 concentrations and ocean pH values.

Figure 3: Atmospheric CO₂ concentrations and ocean pH values. Atmospheric CO₂, shown in blue (seasonal variations) and red (long-term smoothed trend), is measured at Mauna Loa, Hawai’i. Ocean pH values (green and orange) are from the ocean to the north of Hawa’ii (Station Aloha). As CO₂ accumulates in the ocean, the water becomes more acidic (the pH declines). Source: Modified from Feely et al. 2009.

What are the effects of ocean acidification on marine organisms and ecosystems?

Ocean acidification reduces the amount of carbonate, a key building block in seawater. This makes it more difficult for marine organisms, such as coral and some plankton, to form their shells and skeletons, and existing shells may begin to dissolve.

The present-day pH of seawater is highly variable, and a single organism can cope with fluctuations of different pH levels during its lifetime. The problem with ocean acidification is the sustained nature of the change, as the risk comes from the lifetime exposure to lower pH levels. The rapid pace of acidification will influence the extent to which calcifying organisms will be able to adapt.

The impacts of ocean acidification are not uniform across all species. Some algae and seagrass may benefit from higher CO₂concentrations in the ocean, as they may increase their photosynthetic and growth rates. However, a more acidic environment will harm other marine species such as molluscs, corals and some varieties of plankton (Figure 4). The shells and skeletons of these animals may become less dense or strong. In the case of coral reefs this may make them more vulnerable to storm damage and slow the recovery rate.

T2I3_Figure-4 low res.gif

Figure 4: A mollusc shell dissolves under acidic conditions. The shell almost completely dissolves after 45 days when placed in seawater with pH and carbonate levels projected by models for the year 2100. Source: © David Liittschwager/National Geographic Creative.

Marine organisms could also experience changes in growth, development, abundance, and survival in response to ocean acidification (Figure 5). Most species seem to be more vulnerable in their early life stages. Juvenile fish for example, may have trouble locating suitable habitat to live.

Despite the different responses within and between marine groups, positive or negative, research suggests that ocean acidification will be a driver for substantial changes in ocean ecosystems this century. These changes may be made worse by the combined effect with other emerging climate-related hazards, such as the decrease of ocean oxygen levels – a condition known as ocean deoxygenation –that is already affecting marine life in some regions (Long et al. 2016).

T2I3_Figure 5.jpg

Figure 5: Summary of effects of ocean acidification among key taxonomic groups. The main responses are represented in percent changes, which could be either positive (green) or negative (red). Source: Adapted from Kroeker et al. 2013.

What are the effects on human societies?

Changes in marine ecosystems will have consequences for human societies, which depend on the goods and services these ecosystems provide. The implications for society could include substantial revenue declines, loss of employment and livelihoods, and other indirect economic costs.

Socioeconomic impacts associated with the decline of the following ecosystem services are expected:

Food: Ocean acidification has the potential to affect food security. Commercially and ecologically important marine species will be impacted, although they may respond in different ways. Molluscs such as oysters and mussels are among the most sensitive groups. By 2100, the global annual costs of mollusc loss from ocean acidification could be over US$100 billion for a business-as-usual (RCP8.5) CO₂ emissions pathway.

Coastal protection: Marine ecosystems such as coral reefs protect shorelines from the destructive action of storm surges and cyclones, sheltering the only habitable land for several island nations. This protective function of reefs prevents loss of life, property damage, and erosion, and has been valued at US$9 billion per year.

Tourism: This industry could be severely affected by the impacts of ocean acidification on marine ecosystems (e.g. coral reefs). In Australia, the Great Barrier Reef Marine Park attracts about 1.9 million visits each year and generates more than A$5.4 billion to the Australian economy.

Carbon storage and climate regulation: The capacity of the ocean to absorb CO₂decreases as ocean acidification increases. More acidic oceans are less effective in moderating climate change.

What can coastal decision makers do?

While reducing global greenhouse gas emissions (mitigation) is the ultimate solution to ocean acidification, undertaking some challenging decisions and actions can help us prepare for the adverse effects of ocean acidification. This is the adaptation approach.

At the local level, the following policy and management options can help to minimise the adverse effects of other local stressors and, as a result, help marine ecosystems to cope better with changing environmental conditions.

Improvements in water quality: Monitoring and regulating localised sources of acidification from runoff and pollutants such as fertilisers.

Development of sustainable fisheries management practices: Regulating catches to reduce overfishing and creating long-term bycatch[1] reduction plans.

Implementation of new technologies: Different techniques can be applied depending on the industry. For example, in the aquaculture industry, new forecasting systems have been developed to account for seasonal upwellings that bring low pH seawaters to the ocean surface and cause massive shellfish die-offs.

Sustainable management of habitats: Increasing coastal protection, reducing sediment loading and applying marine spatial planning.

Establishment and maintenance of Marine Protected Areas: Protecting highly vulnerable and endangered marine ecosystems.

[1] Bycatch: a fish or other marine species that is caught unintentionally. Bycatch is either of a different species, the wrong sex, or is undersized or juvenile individuals of the target species

Ocean acidification

Coastal acidification

Ocean Acidification

The Industrial Revolution’s Effect on the Global Carbon Cycle

Until recently, the amount of carbon dioxide in the atmosphere has fluctuated slightly and slowly during the past 10,000 years. However, the Industrial Revolution of the 1700s started a global adoption of fossil fuels to power human activity. The rate at which fossil fuels like coal, oil, and natural gas are burned has increased up until the present day. Burning fossil fuels releases carbon dioxide gas to the atmosphere, and the ever-increasing global use of fossil fuels has caused the amount of carbon dioxide in the atmosphere to increase to a concentration that is higher than any time in the past 800,000 years. The cutting of forests for fuel or to clear land for agriculture over the past 200 years has contributed to higher carbon dioxide in the atmosphere because trees capture and store carbon dioxide.

Not only do higher atmospheric concentrations of carbon dioxide alter the Earth’s climate, they also shift ocean chemistry. This phenomenon is due to the fact that carbon dioxide in the air can dissolve into bodies of water.

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Dissolved Carbon Dioxide: Gases in Liquid?

Time series plots of ocean pCO2 and pH for Bermuda, the Canary Islands, and Hawaii from 1983 to 2015

As the amount of dissolved carbon dioxide in seawater increases, declining pH indicates increasing acidity. Just as solids like sugar can dissolve in water, gases like carbon dioxide do as well. This idea is easily demonstrated in a bottle of soda. The manufacturer dissolves carbon dioxide in the beverage. The dissolved carbon dioxide is invisible to the naked eye, but once the bottle is opened carbon dioxide escapes as bubbles that tickle your nose. The extra carbon dioxide in soda water imparts more acidity to the liquid than would be found in uncarbonated water. Similarly, about one third of the carbon dioxide gas in Earth’s atmosphere dissolves in the oceans.

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Carbon Dioxide Imparts Acidity: Transformations of Carbon Dioxide in Water

Once carbon dioxide dissolves in water, carbon dioxide molecules react with water molecules to form . Carbonic acid can be further transformed to and ions. These four different forms of carbon (dissolved carbon dioxide, carbonic acid, bicarbonate, and carbonate) exist in balanced proportions in seawater. As more carbon dioxide is added to seawater, the balance shifts and carbonate is lost as it is transformed to bicarbonate due to increasing acidity.

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How Acidity is Measured: pH

Simple conceptual diagram showing carbon dioxide molecules from the air reacting with water molecules to form carbonic acid

Carbon dioxide molecules from the air react with water molecules to form carbonic acid. Graphic developed by our partner the National Environmental Education Foundation (NEEF).The acidity of a liquid is reported as . The lower the pH value, the higher the acidity of a liquid. Solutions with low pH are acidic and solutions with high pH are basic (also known as alkaline).

Prior to the Industrial Revolution, average ocean pH was about 8.2. Today, average ocean pH is about 8.1. This might not seem like much of a difference, but the relationship between pH and acidity is not direct. Each decrease of one pH unit is a ten-fold increase in acidity. This means that the acidity of the ocean today, on average, is about 25% higher than it was during preindustrial times.

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Acidity and Availability of Shell-forming Calcium

Carbonic acid that forms in water decreases the avialability of carbonate that marine life needs to build shells and skeletons. Graphic developed by our partner the National Environmental Education Foundation (NEEF).

Carbonate

Marine life uses carbonate from the water to build shells and skeletons. As seawater becomes more acidified, carbonate is less available for animals to build shells and skeletons. Under conditions of severe acidification, shells and skeletons can dissolve.

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Coastal Acidification

Closer to Home: Coastal Acidification

Human activities also contribute to acidification in coastal waters. Besides carbon dioxide gas, acid-forming compounds are released Illustrated pH scale, the hydrogen ion concentrations each pH value represents, examples of solutions at each pH, and a sketch of relative hydrogen ion or hydroxide concentrations under acidic, neutral, and basic conditions

Illustrated pH scale, the hydrogen ion concentrations each pH value represents, examples of solutions at each pH, and a sketch of relative hydrogen ion or hydroxide concentrations under acidic, neutral, and basic conditions

The acidity of a liquid results from its concentration of hydrogen ions, which is typically measured and reported as pH. Courtesy of WHOI.to the atmosphere when fossil fuels are burned, and excess nutrients contribute to acidification in coastal waters when peak and die.

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Acid Rain

Burning fossil fuels for energy releases water and carbon dioxide as the main byproducts, but nitrogen oxides and sulfur dioxide are also released in smaller amounts. These two acid-forming compounds fall back to Earth’s surface. They can land in coastal waters directly, or more often mix with water in the atmosphere before falling as acid precipitation, like acid rain. Acid rain typically has a pH between 4.2 and 4.4.

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Excess Nutrients Delivered Via Streams

The elements nitrogen and phosphorous are essential nutrients for living things. For this reason farmers, homeowners, and gardeners supply nitrogen and phosphorous to crops, lawns, and gardens to stimulate plant growth. However, water can carry excess nutrients down streams and into coastal waters. Agricultural activities are a major source of nutrients to coastal waters, but other sources include Aerial photo of excessive algal growth

Excessive algal growth increases acidity when it dies and decomposes, releasing carbon dioxide into coastal waters. sewage, wastewater treatment plant effluent, and nitrogen oxide air pollution. In coastal waters excess nutrients stimulate the growth of algae. Algae multiply rapidly under ideal growing conditions and algal blooms can impair water quality by causing hypoxia, foul odors and even toxins. A less well-known fact is that algal blooms can contribute to acidification. When algae die their decomposing tissue releases carbon dioxide directly into the water, resulting in acidification.

Have you started hearing the term "ocean acidification"? This unit will explain what ocean acidification means, so that you will be able to explain it to others! It is split into two lessons. Lesson one is a refresher about the pH scale, acids and bases. Lesson two focuses on the chemical process of ocean acidification, and how the pH of the ocean is being lowered by an excess of carbon dioxide in the water.

pH Balance

The science behind ocean acidification is not as complicated as it may sound, but it will help to refresh your memory about the basic concepts of the pH scale and how it relates to the chemistry of the world's ocean. Many people will forget this information as soon as they pass their chemistry exam, and will never think about pH again unless it relates to their fish tanks, swimming pools, or deodorant for women. pH is a measurement of the amount of hydrogen ions in a solution, and the scale goes from 0 to 14. pH values between 0 and 7 are considered acidic. Values between 7 and 14 are basic. A solution with a pH of 7 is considered neutral. It is important to remember that being labeled an acid or a base is not a bad thing, many things that we consume regularly (like orange juice and soda) are acids. What is important is that living things have adapted to cope with specific pH ranges, and the pH of ocean water is decreasing faster than many animals can adapt.

The Chemistry Of Ocean Acidification

Ocean acidification represents a direct chemical change to global ocean chemistry in response to rising levels of atmospheric carbon dioxide (CO2).

Carbon dioxide is responsible for many of today's climate change-related issues. Because CO2 gets blamed for global warming and ocean acidification, this essential gas gets a bad reputation. Remember, plants need CO2 for photosynthesis, so it is a valuable component to our atmosphere. Historically, there has been a balance between CO2 being generated, and CO2 being taken in. The problem now is that CO2 is being created faster than it can be absorbed through natural processes. It is excess carbon dioxide that is the problem.

Ocean acidification occurs when CO2 is absorbed into the water at a high rate. It reacts with water molecules (H2O) to form carbonic acid (H2CO3). This compound then breaks down into a hydrogen ion (H+) and bicarbonate (HCO3-). The presence of all these hydrogen ions is what decreases the pH, or acidifies the ocean. This can be summed up with a nifty chemical equation:

CO2 + H2O -> (H+) + (HCO3-)

The saga does not end here, unfortunately. That carbonate molecule (HCO3-) is going to go on to cause trouble for marine organisms. This will be discussed in unit 2.

The Impact of Ocean Acidification

As our one ocean becomes more acidic, what will the impacts be? This unit is split into three lessons. Lesson one focuses on the marine food chain, and on the fact that many creatures that form the base of this food chain will be negatively impacted by ocean acidification. The resources in lesson two are focused specifically on coral reefs, and on their precarious future in a more acidic ocean. Lesson three ties the first two lessons to human beings. How will ocean acidification affect our daily lives?

The Effects of Ocean Acidification on the Marine Food Chain

Many organisms that form the basis for the marine food chain are going to be affected by ocean acidification. It turns out that changing the pH of the ocean is not the only impact from this phenomenon. There is another, equally impactful side effect. When carbon dioxide (CO2) mixes with water molecule (H2O) it forms carbonic acid (H2CO3) that then breaks down easily into hydrogen ions (H+) and bicarbonate (HCO3-), those available hydrogen ions bond with other carbonate ions to form more bicarbonate. The problem here is that marine organisms possessing shells (many mollusks, crustaceans, corals, coralline algae, foramaniferans) need available carbonate ions to form the calcium carbonate (CaCO3) that comprises their shells. In essence, ocean acidification is robbing these organisms of their necessary building blocks.

There have been scientific experiments focusing on how the projected acidity of the oceans will affect different organisms. Marine pteropods already have thin shells, and these shells literally dissolve over 30 days in seawater with a 7.8 pH. Studies on sea urchins and mollusks show similar results.

There are many resources included in this lesson, and many of them are going to say the same things, but each resource does a good job of explaining a certain part of the ocean acidification story.

The Effects of Ocean Acidification on Coral Reefs

Most people are familiar with the concept that compares coral reefs to underwater rainforests. Coral reefs form the most biodiverse habitats in the ocean, and their presence is essential to the survival of thousands of other marine species - many of which we rely on for food.

Hard corals are the reef-building corals, and their stonelike structures are composed of calcium carbonate, the same substance found in the shells of many marine organisms including oysters, clams and snails. Like these mollusks, corals must have access to available calcium in the seawater in order to build their hard skeleton. This is especially important in the early stages of a coral polyps's life, when it settles onto a hard substance and starts "building" its skeleton. Some studies have shown a 52-73% decline in larval settlement on reefs that are experiencing lower pH levels. Scientists can also measure the calcification rates of hard corals, and ocean acidification has had a negative impact on the rate at which corals calcify. This means that coral colonies in the future may be more brittle and less resilient to other factors influencing their survival.

Ocean acidification is just one more threat to the success of hard corals. Coral reefs are already being affected by many other pressures, some human-related and some natural. Warming ocean temperatures are contributing to coral bleaching and making them more susceptible to diseases. Nutrient and chemical pollution coming into the oceans from rivers is also making suitable coral habitat very scarce. Natural threats impacting coral reefs include predation from urchins and a variety of fishes, and also tropical storms. Coral reefs are naturally very resilient to many of these threats, but now their ability to recolonize and grow sturdy structures is being compromised by ocean acidification. Sometimes, coral habitat is gradually being replaced by non-calcifying organisms, like seagrass, once the coral has been killed off.

The resources included in this lesson will describe many of these processes, and will also provide suggestions for how to protect coral habitats. Protecting oceans as a whole will help corals maintain resiliency in the face of these threats.

Ocean acidification is happening right now, and it will definitely have a noticeable impact on our lives. But ocean acidification is not just affecting us, we are affecting it. In essence, humans are the problem and the solution when it comes to climate change and ocean acidification. This unit is divided into two lessons. Lesson one demonstrates how human actions have been contributing to and speeding up ocean acidification since the industrial revolution. Lesson two focuses on solutions. We started it, but can we stop it? Only if we act now, and encourage others to take action and change some of their behaviors regarding energy use!

How are humans causing ocean acidification?

Ocean acidification is occurring because too much carbon dioxide is being released into the atmosphere. Carbon dioxide is nothing new, and its presence in moderate quantities is not a concern. The rate at which we are pumping it into our atmosphere is a concern, however. There are two major sources for this influx of atmospheric CO2: fossil fuel emissions and deforestation.

Fossil fuel emissions are the gases that are spewed out of most cars, airplanes, power plants, and factories that are burning fossil fuels (coal, oil or gas). Since the industrial revolution, fossil fuel consumption has risen exponentially to create many climate change-related issues, including ocean acidification.

Deforestation is a two-fold issue. Burning down forests is similar to burning fossil fuels, it emits a lot of carbon dioxide into the atmosphere. Forests are important because large expanses of plantlife (even in the ocean) are known to be "carbon sinks", taking in carbon dioxide for photosynthesis. Historically, carbon dioxide levels have been balanced; the CO2 being produced was in turn being absorbed. Deforestation not only creates more CO2, but it also destroys one of the very things that helps absorb it!

The silver lining here is that ocean acidification is just one more symptom of ailments that people are already aware of. Most people know that deforestation is bad for the environment. And most people know that they should try to drive less, and consume less energy. Ocean acidification brings the problem into the ocean, and to a world many people are not familiar with.

Humans can take action to slow the process of ocean acidification

Now that we know some of the anthropogenic sources of CO2 in the atmosphere, what can we do about it? All of the things we have been doing since the industrial revolution to put CO2 into the atmosphere should be examined to find more efficient uses. There are many solutions available, and there are things that people can do on almost any level to make an impact. People should understand that some of these changes need to be made on an individual basis, some on the community level, and all the way up to corporations, governments and global organizations.

The burning of fossil fuels is the major contributor to ocean acidification. Fossil fuels are burned to produce energy, and to make vehicles run. One of the easiest ways for people to have a positive impact on an individual level is to use less energy. Many people already think of energy as electricity, so convince people that conserving electricity will save them money and will also reduce the amount of energy their power plant needs to produce. But fossil fuels are also burned in factories to make products that we use every day. The saying "reduce, re-use, recycle" also applies to the ocean acidification crisis. Using less products will lead to a decreased demand to create new product out of new materials. Every person should be able to think of a way they can consume less in their daily lives. Transportation is a huge concern, and is one that is difficult for people to make adjustments to. Driving less and using public transportation may not be a realistic option for everyone, but people can make sure their automobiles run efficiently by keeping the tires properly inflated, getting their cars serviced regularly, and by choosing fuel efficient vehicles. There are many resources listed below which offer some tips, and also offer ways to engage people in learning about their daily energy choices.

Protecting wildlife has many benefits, but most people don't know that it's an important factor in how the Earth responds to climate change. Natural places are very resilient to change, but now there are far fewer natural places in most areas. It is important to preserve existing habitats and to identify more areas that need protection. Visiting natural parks is a great way to experience nature, but also provides funding for protecting those areas. Monitoring pollution and nutrient run-off helps protect coral reefs, so they can be healthy enough to withstand global warming and ocean acidification. Purchasing products that are grown in coexistence with forests and rainforests decreases the need for deforestation for agriculture. Even eating sustainable seafood can make a difference, because healthy fish populations are essential to the overall success of the coral reefs and the ocean.

Since ocean acidification is yet another side effect of excess CO2, there are many things that people are already doing that help make a difference. In short, most things that are considered "green" options, or are environmentally friendly, will also help fight the effects of ocean acidification.

Fossil fuel consumption and greenhouse gas cause increase in atmospheric CO2 gas and reduce in pH of the ocean. This leads to collapse of the global carbon cycle. Ocean acidification inhibits calcium carbonate minerals and it gives rise to unsaturation of oceans in terms of calcium carbonate minerals. Calcium carbonate minerals are vital for the shell formation and creation of aragonite and calcite. Many of the calcifying species show low rate of growing in the high level carbon dioxide environment as it is more acidic. One of these organisms is phytoplankton. Some of the phytoplankton such as coccolitophorids show developmental disorders due to its inability to merge with carbonates and hydrogen ions. It is well known that pH levels of the seas have changed in certain proportions from geological eras till today. Carbon dioxide levels have been increasing since the industrial revolution. If it continues within these conditions, biological diversity will be under threat in the near future. Photosynthetic algae and seagrass can benefit from extra carbon dioxide for photosynthesis, however, this extra carbon dioxide has adverse effects on calcifying species like coccolithoporids. If calcifying organisms, mainly photosynthetics, are in danger, whole food web might also be in danger. People rely on food from ocean as their primary protein and fatty acid source which are beneficial and have high quality.

REDUCE YOUR CARBON FOOTPRINT

Eat less meat. Livestock farming produces more greenhouse gasses than all forms of transport combined. This is the biggest cause of climate change, and therefore ocean acidification. The lower on the food chain you eat, the less energy is used. Grain production to feed the livestock we eat requires significant quantities of fertilizer, fuel, pesticides, water and land. Fertilizer applied to soil generates nitrous oxide (N₂0), which has 300 times the warming effect of carbon dioxide. Animal waste releases nitrogen and methane and pollutes our water and air, especially when it is concentrated. Finding healthy, protein-packed veggie sources of nutrition – like beans and lentils – saves water, land, energy, and reduces greenhouse gases. This also limits animal suffering, is typically healthier and it reduces your environmental footprint. Do your research. Want to learn more about the impacts associated with meat production? Check out UNEP’s article. Also try Meatless Mondays.

Use less energy at home. Make sure your home is well insulated, especially in the roof and around windows. Use ENERGY STAR® qualified products for appliances, light bulbs, etc. that conserve energy and save money. Turn off the lights, unplug power sources not in use, and use shorter cycles on your dishwasher and washing machine. Switch to green power such as Bullfrog Power, a Canadian company which provides power to the grid using more sustainable sources of natural gas and green energy like wind power. Compost your waste; grow some of your own food and recycle.

Conserve water. It takes lots of energy to pump, treat and heat water so saving water reduces greenhouse gas emissions. Watch what you pour down the drain and use eco-friendly products that break down. Remember often what goes down the drain ends up in rivers and lakes, which all filter into our oceans.

Reduce your plastic addiction. Don’t use bottled water. Refuse straws when you eat out. Bring your own reusable containers around with you. Drink from the tap, filter it and use water canteens to refill. Don’t use single use plastic bags.

Drive and fly less, carpool, ride bikes and take public transit.

Buy less stuff. Manufacturing products and the transportation required to get it to you burns a lot of carbon. Stop the buying madness! If you’ve got to buy something, buy smarter, local and more environmentally friendly products that are good for your body (toxic-free/organic) and safe for the environment.

Reduce, reuse, recycle and refuse! This helps conserve energy and reduces pollution and greenhouse gas emissions from resource extraction, manufacturing, and disposal.

Assess your life, career and lifestyle choices. Is the industry you’re in destructive, carbon intensive, polluting or unsustainable? Suggest changes in the work place, and do what feels right for you and the environment.

Calculate your carbon footprint. Liv Clean is just one of the many online calculators available

Some marine habitats could face biodiversity loss due to ocean acidification. Research from the University of British Columbia and colleagues from Europe, USA, Australia, Japan and China, combines dozens of existing studies to discover exactly what the impact of ocean acidification will be.

This new research, published in Nature Climate Change, differs from most studies on ocean acidification that examine its impact on individual species. Instead, this study predicts how acidification will affect living habitats, such as coral reefs, seagrass meadows and kelp forests, which support a wealth of ocean species.

The researchers combined data and observations from ten field studies that measured the impact of underwater volcanic vents on the density of habitat-forming species. The vents release carbon dioxide and mimic the conditions of future ocean acidification. They combined that data with 15 studies focusing on how changes in habitat typically impact local species to make their predictions.

“Not too surprisingly, species diversity in calcium carbonate-based habitats like coral reefs and mussel beds were projected to decline with increased ocean acidification,” said UBC zoologist and biodiversity researcher Jennifer Sunday, who led the study. Species that use calcium carbonate to build their shells and skeletons, like mussels and corals, are expected to be particularly vulnerable to acidification. “The more complex responses are those of seagrass beds that are vital to many fisheries species. These showed the potential to increase the number of species they can support, but the real-world evidence so far shows that they’re not reaching this potential. This highlights a need to focus not only on individual species, but on how the supportive habitat that sets nature’s stage responds and interacts to climate change.”

The research was focused on the impact of acidification on specific ocean habitats – coral reefs, mussel beds, kelp forests and seagrass meadows. These habitats support thousands of marine species. Observations of altered habitats around the world were used to project how changes in these habitats brought on by ocean acidification will affect the number of species that each habitat supports. These predictions could be tested against real-world data from two sites – a coral reef near Papua New Guinea and a group of seagrass beds in the Mediterranean. Despite predictions that seagrass beds would fare well under increased levels of carbon dioxide, no increase in biodiversity was observed. And, worryingly, in the case of the coral reefs, the diversity and complexity of marine life in the area decreased as acidification increased.

“We’ve known for a while that there will be big losers and some winners with climate change,” said UBC marine ecologist Christopher Harley, senior author on the paper. “We don’t have time to measure the impact of climate change on each individual species, but using this approach allows us to make reasonable predictions. Now we have a much clearer picture of how some losers can drag biodiversity down with them, and how some other species might be able to help their habitat mediate a response to acidification. For example, in the Pacific Northwest, the number of medium to large-sized edible saltwater mussels is likely to decrease as the chemistry of our oceans changes, and this is bad news for the hundreds of species that use them for habitat.”