Ocean acidification
Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.[2] Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide (CO2) levels exceeding 422 ppm (as of 2024[update]).[3] CO2 from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid (H2CO3) which dissociates into a bicarbonate ion (HCO−3) and a hydrogen ion (H+). The presence of free hydrogen ions (H+) lowers the pH of the ocean, increasing acidity (this does not mean that seawater is acidic yet; It is still alkaline, with a pH higher than 8). Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.[4]
A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more CO2. This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. Several other factors influence the atmosphere-ocean CO2 exchange, and thus local ocean acidification. These include ocean currents and upwelling zones, proximity to large continental rivers, sea ice coverage, and atmospheric exchange with nitrogen and sulfur from fossil fuel burning and agriculture.[5][6][7]
A lower ocean pH has a range of potentially harmful effects for marine organisms. Scientists have observed for example reduced calcification, lowered immune responses, and reduced energy for basic functions such as reproduction.[8] Ocean acidification can impact marine ecosystems that provide food and livelihoods for many people. About one billion people are wholly or partially dependent on the fishing, tourism, and coastal management services provided by coral reefs. Ongoing acidification of the oceans may therefore threaten food chains linked with the oceans.[9][10]
One of the only solutions that would address the root cause of ocean acidification is reducing carbon dioxide emissions. This is one of the main objectives of climate change mitigation measures. The removal of carbon dioxide from the atmosphere would also help to reverse ocean acidification. In addition, there are some specific ocean-based mitigation methods, for example ocean alkalinity enhancement and enhanced weathering. These strategies are under investigation, but generally have a low technology readiness level and many risks.[11][12][13]
Ocean acidification has happened before in Earth's geologic history.[14] The resulting ecological collapse in the oceans had long-lasting effects on the global carbon cycle and climate.
Cause
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In 2021, atmospheric carbon dioxide (CO2) levels of around 415 ppm were around 50% higher than preindustrial concentrations.[16] According to the National Oceanic and Atmospheric Administration in 2023, atmospheric CO2 levels have risen from approximately 280 parts per million (ppm) in the pre-industrial era to over 410 ppm today, primarily due to human activities such as fossil fuel combustion and deforestation.[17] The current elevated levels and rapid growth rates are unprecedented in the past 55 million years of the geological record. The sources of this excess CO2 are clearly established as human driven: they include anthropogenic fossil fuel, industrial, and land-use/land-change emissions. One source of this is fossil fuels, which are burned for energy. When burned, CO2 is released into the atmosphere as a byproduct of combustion, which is a significant contributor to the increasing levels of CO2 in the Earth's atmosphere.[18] The ocean acts as a carbon sink for anthropogenic CO2 and takes up roughly a quarter of total anthropogenic CO2 emissions.[19] However, the additional CO2 in the ocean results in a wholesale shift in seawater acid-base chemistry toward more acidic, lower pH conditions and lower saturation states for carbonate minerals used in many marine organism shells and skeletons.[19]
Accumulated since 1850, the ocean sink holds up to 175±35 gigatons of carbon, with more than two-thirds of this amount (120 Gt C) being taken up by the global ocean since 1960. Over the historical period, the ocean sink increased in pace with the exponential anthropogenic emissions increase. From 1850 until 2022, the ocean has absorbed 26% of total anthropogenic emissions.[16] Emissions during the period 1850–2021 amounted to 670±65 gigatons of carbon and were partitioned among the atmosphere (41%), ocean (26%), and land (31%).[16]
The carbon cycle describes the fluxes of carbon dioxide (CO2) between the oceans, terrestrial biosphere, lithosphere,[20] and atmosphere. The carbon cycle involves both organic compounds such as cellulose and inorganic carbon compounds such as carbon dioxide, carbonate ion, and bicarbonate ion, together referenced as dissolved inorganic carbon (DIC). These inorganic compounds are particularly significant in ocean acidification, as they include many forms of dissolved CO2 present in the Earth's oceans.[21]
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 the 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.
Main effects
[edit]The ocean's chemistry is changing due to the uptake of anthropogenic carbon dioxide (CO2).[5][22]: 395 Ocean pH, carbonate ion concentrations ([CO2−3]), and calcium carbonate mineral saturation states (Ω) have been declining as a result of the uptake of approximately 30% of the anthropogenic carbon dioxide emissions over the past 270 years (since around 1750). This process, commonly referred to as "ocean acidification", is making it harder for marine calcifiers to build a shell or skeletal structure, endangering coral reefs and the broader marine ecosystems.[5]
Ocean acidification has been called the "evil twin of global warming" and "the other CO2 problem".[23][24] Increased ocean temperatures and oxygen loss act concurrently with ocean acidification and constitute the "deadly trio" of climate change pressures on the marine environment.[25] The impacts of this will be most severe for coral reefs and other shelled marine organisms,[26][27] as well as those populations that depend on the ecosystem services they provide.
Reduction in pH value
[edit]Dissolving CO2 in seawater increases the hydrogen ion (H+) concentration in the ocean, and thus decreases ocean pH, as follows:[28]
- CO2 (aq) + H2O ⇌ H2CO3 ⇌ HCO−3 + H+ ⇌ CO2−3 + 2 H+.
In shallow coastal and shelf regions, a number of factors interplay to affect air-ocean CO2 exchange and resulting pH change.[29][30] These include biological processes, such as photosynthesis and respiration,[31] as well as water upwelling.[32] Also, ecosystem metabolism in freshwater sources reaching coastal waters can lead to large, but local, pH changes.[29]
Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious phenomenon.[33][34]
The absorption of CO2 from the atmosphere does not affect the ocean's alkalinity.[35]: 2252 This is important to know in this context as alkalinity is the capacity of water to resist acidification.[36] Ocean alkalinity enhancement has been proposed as one option to add alkalinity to the ocean and therefore buffer against pH changes.
Decreased calcification in marine organisms
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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 out of calcium carbonate (CaCO3).[4] 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, structures for many marine organisms, such as coccolithophores, foraminifera, crustaceans, mollusks, etc. After they are formed, these CaCO3 structures are vulnerable to dissolution unless the surrounding seawater contains saturating concentrations of carbonate ions (CO2−3).
Very little of the extra carbon dioxide that is added into the ocean remains as dissolved carbon dioxide. The majority dissociates into additional bicarbonate and free hydrogen ions. The increase in hydrogen is larger than the increase in bicarbonate,[37] creating an imbalance in the reaction:
- HCO−3 ⇌ CO2−3 + H+
To maintain chemical equilibrium, some of the carbonate ions already in the ocean combine with some of the hydrogen ions to make further bicarbonate. Thus the ocean's concentration of carbonate ions is reduced, removing an essential building block for marine organisms to build shells, or calcify:
- Ca2+ + CO2−3 ⇌ CaCO3
The increase in concentrations of dissolved carbon dioxide and bicarbonate, and reduction in carbonate, are shown in the Bjerrum plot.
Disruption of the food chain is also a possible effect as many marine organisms rely on calcium carbonate-based organisms at the base of the food chain for food and habitat. This can potentially have detrimental effects throughout the food web and potentially lead to a decline in availability of fish stocks which would have an impact on human livelihoods.[38]
Decrease in saturation state
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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[{\mathrm {Ca} {\vphantom {A}}^{2+}}\right]\left[{\mathrm {CO} {\vphantom {A}}_{\smash[{t}]{3}}{\vphantom {A}}^{2-}}\right]}{K_{sp}}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/0422ec8bbb280ae8547a20d486b928b0c21846f4)
Here Ω is the product of the concentrations (or activities) of the reacting ions that form the mineral (Ca+2 and CO2−3), divided by the apparent solubility product at equilibrium (Ksp), that is, when the rates of precipitation and dissolution are equal.[40] In seawater, dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon.[4] Above this saturation horizon, Ω has a value greater than 1, and CaCO3 does not readily dissolve. Most calcifying organisms live in such waters.[4] Below this depth, Ω has a value less than 1, and CaCO3 will dissolve. The carbonate compensation depth is the ocean depth at which carbonate dissolution balances the supply of carbonate to sea floor, therefore sediment below this depth will be void of calcium carbonate.[41] Increasing CO2 levels, and the resulting lower pH of seawater, decreases the concentration of CO2−3 and the saturation state of CaCO3 therefore increasing CaCO3 dissolution.
Calcium carbonate most commonly occurs in two common polymorphs (crystalline forms): aragonite and calcite. Aragonite is much more soluble than calcite, so the aragonite saturation horizon, and aragonite compensation depth, is always nearer to the surface than the calcite saturation horizon.[4] This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite.[42] Ocean acidification and the resulting decrease in carbonate saturation states raise the saturation horizons of both forms closer to the surface.[4] This decrease in saturation state is one of the main factors leading to decreased calcification in marine organisms because the inorganic precipitation of CaCO3 is directly proportional to its saturation state and calcifying organisms exhibit stress in waters with lower saturation states.[43]
Natural variability and climate feedbacks
[edit]Already now large quantities of water undersaturated in aragonite are upwelling close to the Pacific continental shelf area of North America, from Vancouver to Northern California.[44] These continental shelves play an important role in marine ecosystems, since most marine organisms live or are spawned there. Other shelf areas may be experiencing similar effects.[44]
At depths of 1000s of meters in the ocean, calcium carbonate shells begin to dissolve as increasing pressure and decreasing temperature shift the chemical equilibria controlling calcium carbonate precipitation.[45] The depth at which this occurs is known as the carbonate compensation depth. Ocean acidification will increase such dissolution and shallow the carbonate compensation depth on timescales of tens to hundreds of years.[45] Zones of downwelling are being affected first.[46]
In the North Pacific and North Atlantic, saturation states are also decreasing (the depth of saturation is getting more shallow).[22]: 396 Ocean acidification is progressing in the open ocean as the CO2 travels to deeper depth as a result of ocean mixing. In the open ocean, this causes carbonate compensation depths to become more shallow, meaning that dissolution of calcium carbonate will occur below those depths. In the North Pacific these carbonate saturations depths are shallowing at a rate of 1–2 m/year.[22]: 396
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.[47]
Measured and estimated values
[edit]Present-day and recent history
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Between 1950 and 2020, the average pH value of the ocean surface is estimated to have decreased from approximately 8.15 to 8.05.[2] This represents an increase of around 26% in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH unit is equivalent to a tenfold change in hydrogen ion concentration).[50] For example, 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.[51]
The IPCC Sixth Assessment Report in 2021 stated that "present-day surface pH values are unprecedented for at least 26,000 years and current rates of pH change are unprecedented since at least that time.[52]: 76 The pH value of the ocean interior has declined over the last 20–30 years everywhere in the global ocean.[52]: 76 The report also found that "pH in open ocean surface water has declined by about 0.017 to 0.027 pH units per decade since the late 1980s".[53]: 716
The rate of decline differs by region. This is due to complex interactions between different types of forcing mechanisms:[53]: 716 "In the tropical Pacific, its central and eastern upwelling zones exhibited a faster pH decline of minus 0.022 to minus 0.026 pH unit per decade." This is thought to be "due to increased upwelling of CO2-rich sub-surface waters in addition to anthropogenic CO2 uptake".[53]: 716 Some regions exhibited a slower acidification rate: a pH decline of minus 0.010 to minus 0.013 pH units per decade has been observed in warm pools in the western tropical Pacific.[53]: 716
The rate at which ocean acidification will occur may be influenced by the rate of surface ocean warming, because warm waters will not absorb as much CO2.[54] Therefore, greater seawater warming could limit CO2 absorption and lead to a smaller change in pH for a given increase in CO2.[54] The difference in changes in temperature between basins is one of the main reasons for the differences in acidification rates in different localities.
Current rates of ocean acidification have been likened to the greenhouse event at the Paleocene–Eocene boundary (about 56 million years ago), when surface ocean temperatures rose by 5–6 °C. In that event, surface ecosystems experienced a variety of impacts, but bottom-dwelling organisms in the deep ocean actually experienced a major extinction.[55] Currently, the rate of carbon addition to the atmosphere-ocean system is about ten times the rate that occurred at the Paleocene–Eocene boundary.[56]
Extensive observational systems are now in place or being built for monitoring seawater CO2 chemistry and acidification for both the global open ocean and some coastal systems.[19]
| Location | Change in pH units per decade | Period | Data source | Year of publication |
|---|---|---|---|---|
| Iceland[57] | minus 0.024 | 1984–2009 | Direct measurements | 2009 |
| Drake Passage[58] | minus 0.018 | 2002–2012 | Direct measurements | 2012 |
| Canary (ESTOC)[59] | minus 0.017 | 1995–2004 | Direct measurements | 2010 |
| Hawaii (HOT)[60] | minus 0.019 | 1989–2007 | Direct measurements | 2009 |
| Bermuda (BATS)[61] | minus 0.017 | 1984–2012 | Direct measurements | 2012 |
| Coral Sea[62] | minus 0.002 | ~1700 – ~1990 | Proxy reconstruction | 2005 |
| Eastern Mediterranean[63] | minus 0.023 | 1964–2005 | Proxy reconstruction | 2016 |
| Station, region | Study period | pH change per decade | |
|---|---|---|---|
| Equatorial Pacific | TAO | 2004–2011 | −0.026 |
| Indian Ocean | IO-STPS | 1991–2011 | −0.027 |
| Mediterranean | Dyfamed (43.42°N, 7.87°E) | 1995–2011 | −0.03 |
| North Atlantic | Iceland Sea (68°N, 12.67°W) | 1985–2008 1985–2010 |
−0.024 −0.014 |
| North Atlantic | Irminger Sea (64.3°N, 28°W) | 1983–2004 | −0.026 |
| North Pacific | NP-STSS | 1991–2011 | −0.01 |
| Southern Ocean | PAL-LTER, west Antarctic Peninsula | 1993–2012 | +0.02 |
Geologic past
[edit]Ocean acidification has occurred previously in Earth's history.[14] It happened during the Capitanian mass extinction,[65][66][67] at the end-Permian extinction,[68][69][70] during the end-Triassic extinction,[71][72][73] and during the Cretaceous–Paleogene extinction event.[74]
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.[75] Elevated CO2 levels impacted biodiversity.[76] Decreased CaCO3 saturation due to seawater uptake of volcanogenic CO2 has been suggested as a possible kill mechanism during the marine mass extinction at the end of the Triassic.[71] The end-Triassic biotic crisis is still the most well-established example of a marine mass extinction due to ocean acidification, because (a) carbon isotope records suggest enhanced volcanic activity that decreased the carbonate sedimentation which reduced the carbonate compensation depth and the carbonate saturation state, and a marine extinction coincided precisely in the stratigraphic record,[73][72][77] and (b) there was pronounced selectivity of the extinction against organisms with thick aragonitic skeletons,[73][78][79] which is predicted from experimental studies.[80] Ocean acidification has also been suggested as a one cause of the end-Permian mass extinction[69][68] and the end-Cretaceous crisis.[74] Overall, multiple climatic stressors, including ocean acidification, was likely the cause of geologic extinction events.[75]
The most notable example of ocean acidification is the Paleocene–Eocene Thermal Maximum (PETM), which occurred approximately 56 million years ago when massive amounts of carbon entered the ocean and atmosphere, and led to the dissolution of carbonate sediments across many ocean basins.[76] Relatively new geochemical methods of testing for pH in the past indicate the pH dropped 0.3 units across the PETM.[81][82] One study that solves the marine carbonate system for saturation state shows that it may not change much over the PETM, suggesting the rate of carbon release at our best geological analogy was much slower than human-induced carbon emissions. However, stronger proxy methods to test for saturation state are needed to assess how much this pH change may have affected calcifying organisms.
Predicted future values
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