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. Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide levels exceeding 422 ppm. from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid which dissociates into a bicarbonate ion and a hydrogen ion. The presence of free hydrogen ions lowers the pH of the ocean, increasing acidity. Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.
A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans. Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more. This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. There are several other factors that influence the atmosphere-ocean 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.
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. 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.
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.
Ocean acidification has happened before in Earth's geologic history. The resulting ecological collapse in the oceans had long-lasting effects on the global carbon cycle and climate.

Cause

In 2021, atmospheric carbon dioxide levels of around 415 ppm were around 50% higher than preindustrial concentrations. According to the National Oceanic and Atmospheric Administration in 2023, atmospheric CO₂ levels have risen from approximately 280 parts per million in the pre-industrial era to over 410 ppm today, primarily due to human activities such as fossil fuel combustion and deforestation. 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. The ocean acts as a carbon sink for anthropogenic CO₂ and takes up roughly a quarter of total anthropogenic CO₂ emissions. 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.
Accumulated since 1850, the ocean sink holds up to of carbon, with more than two-thirds of this amount 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. Emissions during the period 1850–2021 amounted to of carbon and were partitioned among the atmosphere, ocean, and land.
The carbon cycle describes the fluxes of carbon dioxide between the oceans, terrestrial biosphere, lithosphere, 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. These inorganic compounds are particularly significant in ocean acidification, as they include many forms of dissolved present in the Earth's oceans.
When dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide, carbonic acid, bicarbonate and carbonate. The ratio of these species depends on factors such as seawater temperature, pressure and salinity. 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 is known as the Revelle factor.

Main effects

The ocean's chemistry is changing due to the uptake of anthropogenic carbon dioxide. Ocean pH, carbonate ion concentrations, and calcium carbonate mineral saturation states have been declining as a result of the uptake of approximately 30% of the anthropogenic carbon dioxide emissions over the past 270 years. 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.
Ocean acidification has been called the "evil twin of global warming" and "the other CO₂ problem". Increased ocean temperatures and oxygen loss act concurrently with ocean acidification and constitute the "deadly trio" of climate change pressures on the marine environment. The impacts of this will be most severe for coral reefs and other shelled marine organisms, as well as those populations that depend on the ecosystem services they provide.

Reduction in pH value

Dissolving in seawater increases the hydrogen ion concentration in the ocean, and thus decreases ocean pH, as follows:
In shallow coastal and shelf regions, a number of factors interplay to affect air-ocean exchange and resulting pH change. These include biological processes, such as photosynthesis and respiration, as well as water upwelling. Also, ecosystem metabolism in freshwater sources reaching coastal waters can lead to large, but local, pH changes.
Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious phenomenon.
The absorption of CO₂ from the atmosphere does not affect the ocean's alkalinity. This is important to know in this context as alkalinity is the capacity of water to resist acidification. 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

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. 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 structures, structures for many marine organisms, such as coccolithophores, foraminifera, crustaceans, mollusks, etc. After they are formed, these structures are vulnerable to dissolution unless the surrounding seawater contains saturating concentrations of carbonate ions.
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, creating an imbalance in the reaction:
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:
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.

Decrease in saturation state

The saturation state 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:
Here Ω is the product of the concentrations of the reacting ions that form the mineral, divided by the apparent solubility product at equilibrium, that is, when the rates of precipitation and dissolution are equal. In seawater, dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon. Above this saturation horizon, Ω has a value greater than 1, and does not readily dissolve. Most calcifying organisms live in such waters. Below this depth, Ω has a value less than 1, and 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. Increasing levels, and the resulting lower pH of seawater, decreases the concentration of CO₃²⁻ and the saturation state of therefore increasing dissolution.
Calcium carbonate most commonly occurs in two common polymorphs : 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. This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite. Ocean acidification and the resulting decrease in carbonate saturation states raise the saturation horizons of both forms closer to the surface. This decrease in saturation state is one of the main factors leading to decreased calcification in marine organisms because the inorganic precipitation of is directly proportional to its saturation state and calcifying organisms exhibit stress in waters with lower saturation states.