Ocean acidification

Source: Wikipedia, the free encyclopedia.

Ocean acidification means that the average ocean pH value is dropping over time.[1]
Spatial distribution of global surface ocean pH (Panel a: the annually-averaged surface ocean pH to be approximate for the year 1770; Panel b: the difference between pH in 2000 and 1770 in the global surface ocean).[2]

Ocean acidification is the reduction in the

bicarbonate ion (HCO3) and a hydrogen ion (H+). The free hydrogen ions (H+) decrease the ocean pH of the ocean, causing "acidification" (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). The lowered pH causes a decrease in the concentration of carbonate ions, which are the main building block for calcium carbonate (CaCO3) shells and skeletons. It also lowers carbonate mineral saturation states. Marine calcifying organisms, like mollusks, oysters and corals, are particularly affected by this as they rely on calcium carbonate to build shells and skeletons.[4]

The change in pH value from 8.25 to 8.14 represents an increase of almost 30% 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).[5] Sea-surface pH and carbonate saturation states can vary depending on ocean depth and location. Colder and higher latitude waters have the capacity to absorb more CO2. This can increase acidification, lowering the pH and carbonate saturation states in these regions. Other factors that affect the atmosphere-ocean CO2 exchange, and therefore impact local ocean acidification, include: ocean currents (upwelling zones), proximity to large continental rivers, sea ice coverage, and atmospheric exchange with nitrogen and sulfur from fossil fuel burning and agriculture.[2][6][7]

Decreased ocean pH has a range of potentially harmful effects for marine organisms. These include reduced calcification, depressed metabolic rates, lowered immune responses, and reduced energy for basic functions such as reproduction.

food chains linked with the oceans.[9][10] Ocean alkalinity is not changed by ocean acidification, but over long time periods alkalinity may increase due to carbonate dissolution and reduced formation of calcium carbonate shells.[9][11]

The United Nations Sustainable Development Goal 14 ("Life below Water") has a target to "minimize and address the impacts of ocean acidification".[12] Reducing carbon dioxide emissions (i.e. climate change mitigation measures) is the only solution that addresses the root cause of ocean acidification. Other ocean-based mitigation technologies that can achieve carbon dioxide removal from the ocean (e.g. ocean alkalinity enhancement, enhanced weathering) generally have a low technology readiness level and many risks.[13]: 12–36 

Ocean acidification has occurred previously in Earth's history.[14] The resulting ecological collapse in the oceans had long-lasting effects on the global carbon cycle and climate.

Cause

This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon.[15]
NOAA
Environmental Visualization Laboratory.

Present-day (2021) atmospheric carbon dioxide (CO2) levels of around 415 ppm are around 50% higher than preindustrial concentrations.[16]< The current elevated levels and rapid growth rates are unprecedented in the past 55 million years of the geological record. The source for this excess CO2 is clearly established as human driven, reflecting a mix of anthropogenic fossil fuel, industrial, and land-use/land-change emissions. The concept that the ocean acts as a major sink for anthropogenic CO2 has been present in the scientific literature since at least the late 1950s. The ocean takes up roughly a quarter of total anthropogenic CO2 emissions.[17] It is also well understood that 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.[17]

Cumulated since 1850, the ocean sink adds up to 175 ± 35 gigatons of carbon, with more than two-thirds of this amount (120 GtC) 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. Since 1850, the ocean has removed 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 (CO
2
) between the oceans, terrestrial biosphere, lithosphere,[18] 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 CO
2
present in the Earth's oceans.[19]

When CO
2
dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide (CO
2(aq)
), carbonic acid (H
2
CO
3
), bicarbonate (HCO
3
) and carbonate (CO2−
3
). The ratio of these species depends on factors such as seawater temperature, pressure and salinity (as shown in a Bjerrum plot). These different forms of dissolved inorganic carbon are transferred from an ocean's surface to its interior by the ocean's solubility pump. The resistance of an area of ocean to absorbing atmospheric CO
2
is known as the Revelle factor.

Main effects

The ocean’s chemistry is changing due to the uptake of anthropogenic carbon dioxide (CO2).[2][20]: 395  Ocean pH, carbonate ion concentrations ([CO32−]), 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 is commonly referred to as “ocean acidification”. Ocean acidification is making it harder for marine calcifiers to build a shell or skeletal structure, endangering coral reefs and the broader marine ecosystems.[2]

Ocean acidification has been called the "evil twin of

global warming"and "the other CO2 problem".[21][22] Increased ocean temperatures and oxygen loss act concurrently with ocean acidification and constitute the "deadly trio" of climate change pressures on the marine environment.[23] The impacts of this will be most severe for coral reefs and other shelled marine organisms,[24][25]
as well as those populations that depend on the ecosystem services they provide.

Reduction in pH value

Dissolving CO
2
in seawater increases the
hydrogen ion (H+
) concentration in the ocean, and thus decreases ocean pH, as follows:[26]

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

In shallow coastal and shelf regions, a number of factors interplay to affect air-ocean CO2 exchange and resulting pH change.[27][28] These include biological processes, such as photosynthesis and respiration,[29] as well as water upwelling.[30] Also, ecosystem metabolism in freshwater sources reaching coastal waters can lead to large, but local, pH changes.[27]

Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious phenomenon.[31][32]

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

saturating
concentrations of carbonate ions (CO32−).

Given the current pH of the ocean (around 8.14), of the extra carbon dioxide added into the ocean, very little 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,[34] 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, removing an essential building block for marine organisms to build shells, or calcify: Ca2+ + CO32− ⇌ CaCO3.

The increase in concentrations of dissolved carbon dioxide and bicarbonate, and reduction in carbonate, are shown in the Bjerrum plot.

Decrease in saturation state

The

saturation
state (known as Ω) of seawater for a mineral is a measure of the thermodynamic potential for the mineral to form or to dissolve, and for calcium carbonate is described by the following equation:

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 apparent solubility product at equilibrium (Ksp), that is, when the rates of precipitation and dissolution are equal.[35] In seawater, dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon.[33] 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.[33] Below this depth, Ω has a value less than 1, and CaCO
3
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.[36]
Increasing CO2 levels, and the resulting lower pH of seawater, decreases the concentration of CO32− and the saturation state of CaCO
3
therefore increasing CaCO
3
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.[33] This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite.[37] Ocean acidification and the resulting decrease in carbonate saturation states raise the saturation horizons of both forms closer to the surface.[38] This decrease in saturation state is one of the main factors leading to decreased calcification in marine organisms because the inorganic precipitation of CaCO
3
is directly proportional to its saturation state and calcifying organisms exhibit stress in waters with lower saturation states.[39]

Observations and predictions

Observed pH value changes

Time series of atmospheric CO2 at Mauna Loa (in parts per million volume, ppmv; red), surface ocean pCO2 (µatm; blue) and surface ocean pH (green) at Ocean Station ALOHA in the subtropical North Pacific Ocean.[40]
anthropogenic impact on CO
2
levels between the 1700s and the 1990s, from the Global Ocean Data Analysis Project (GLODAP) and the World Ocean Atlas

Between 1751 (the beginning of the industrial revolution) and 2021, the pH value of the ocean surface is estimated to have decreased from approximately 8.25 to 8.14.[3][41] This represents an increase of almost 30% 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).[5] 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.[42]

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.[43]: 76  The pH value of the ocean interior has declined over the last 20-30 years everywhere in the global ocean.[43]: 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".[44]: 716 

The rate of decline differs by region due to the "complex interplay between physical and biological forcing mechanisms".[44]: 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."[44]: 716 
  • Some regions exhibited a slower acidification rate: a pH decline of minus 0.010 to minus 0.013 pH unit per decade has been observed in warm pools in the western tropical Pacific.[44]: 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.[45] Therefore, greater seawater warming could limit CO2 absorption and lead to a smaller change in pH for a given increase in CO2.[45] 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 degrees Celsius. In that event, surface ecosystems experienced a variety of impacts, but bottom-dwelling organisms in the deep ocean actually experienced a major extinction.[46] Currently, the rate of carbon addition to the atmosphere-ocean system is about ten times the rate that occurred at the Paleocene–Eocene boundary.[47]

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.[17]

Acidification rates in different marine regions
Location Change in pH units per decade Period Data source Year of publication
Iceland[48] minus 0.024 1984 – 2009 Direct measurements 2009
Drake Passage[49] minus 0.018 2002 – 2012 Direct measurements 2012
Canary (ESTOC)[50] minus 0.017 1995 – 2004 Direct measurements 2010
Hawaii (HOT)[51] minus 0.019 1989 – 2007 Direct measurements 2009
Bermuda (BATS)[52] minus 0.017 1984 – 2012 Direct measurements 2012
Coral Sea[53] minus 0.002 ~1700 – ~1990 Proxy reconstruction 2005
Eastern Mediterranean[54] minus 0.023 1964 – 2005 Proxy reconstruction 2016
Rates of pH change for some regions of the world (many more regions available in source table)[55]: Table 5.SM.3 
Station, region Study period pH change (per decade)
Equatorial Pacific TAO 2004-2011 minus 0.026
Indian Ocean IO-STPS 1991-2011 minus 0.027
Mediterranean Dyfamed (43.42°N, 7.87°E) 1995-2011 minus 0.03
North Atlantic
Iceland Sea (68°N, 12.67°W) 1985-2008

1985-2010

minus 0.024

minus 0.014

North Atlantic Irminger Sea (64.3°N, 28°W) 1983-2004 minus 0.026
North Pacific
NP-STSS 1991-2011 minus 0.01
Southern Ocean PAL-LTER, west Antarctic Peninsula 1993-2012 plus 0.02

Predicted future pH value changes

NOAA (AOML
))

biogeochemical changes, this 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.[57] Current ocean acidification is now on a path to reach lower pH levels than at any other point in the last 300 million years.[58][59] The rate of ocean acidification is also estimated to be unprecedented over that same time scale.[60][14] The expected changes are considered unprecedented in the geological record.[61][62][63]

The extent of further chemistry changes, including ocean pH, will depend on climate change mitigation efforts taken by nations and their governments.[43] Different scenarios of projected socioeconomic global changes are modelled by using the Shared Socioeconomic Pathways (SSP) scenarios.

If the 'business as usual' model for human activity persists (where little effort is made to curb greenhouse gas emissions, leading to a very high emission scenario called SSP5-8.5), model projections estimate that surface ocean pH could decrease by as much as 0.44 units compared to the present day by the end of the century.[64]: 608  This would mean a pH as low as about 7.7, and represents a further increase in H+ concentrations of two to four times beyond the increase to date.

Estimated past and future global mean surface pH for different emission scenarios[43]: values estimated from Figure TS.11 (d) 
Time period pH value (approx.)
Pre-industrial (1850) 8.18
Now (2021) (assessed observational change spans 1985–2019) 8.14
Future (2100) with low emission scenario (SSP 1-2.6) 8.0
Future (2100) with very high emission scenario (SSP 5-8.5) 7.7

Ocean acidification in the geologic past

Three of the big five mass extinction events in the geologic past were associated with a rapid increase in atmospheric carbon dioxide, probably due to volcanism and/or thermal dissociation of marine gas hydrates.[65] Elevated CO2 levels impacted biodiversity.[66] 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.[67] 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,[68][69][70] and (b) there was pronounced selectivity of the extinction against organisms with thick aragonitic skeletons,[68][71][72] which is predicted from experimental studies.[73] Ocean acidification has also been suggested as a one cause of the end-Permian mass extinction[74][75] and the end-Cretaceous crisis.[59] Overall, multiple climatic stressors, including ocean acidification, was likely the cause of geologic extinction events.[65]

The most notable example of ocean acidification is the

Distribution of (A) aragonite and (B) calcite saturation depth in the global oceans[79]