Ocean acidification

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World map showing varying change to pH across different parts of different oceans
Estimated change in sea water pH caused by human created CO2 between the 1700s and the 1990s, from the Global Ocean Data Analysis Project and the World Ocean Atlas

Ocean acidification is the name given to the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of anthropogenic carbon dioxide (CO2) from the atmosphere.[1] About 30–40% of the carbon dioxide released by humans into the atmosphere dissolves into the oceans, rivers and lakes.[2][3] To maintain chemical equilibrium, some of it reacts with the water to form carbonic acid. Some of these extra carbonic acid molecules react with a water molecule to give a bicarbonate ion and a hydronium ion, thus increasing the ocean's "acidity" (H+ ion concentration). Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14,[4] representing an increase of almost 30% in H+ ion concentration in the world's oceans.[5][6]

This increasing acidity is thought to have a range of direct undesirable consequences such as depressing metabolic rates in jumbo squid[7] and depressing the immune responses of blue mussels.[8]

Other chemical reactions are also triggered which result in an actual net decrease in the amount of carbonate ions available. In the oceans, this makes it more difficult for marine calcifying organisms, such as coral and some plankton, to form biogenic calcium carbonate, and existing such structures become vulnerable to dissolution.[9] Thus, ongoing acidification of the oceans also poses a threat to the food chains connected with the oceans.[10][11]

Contents

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,[12] 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,[13] with some also taken up by terrestrial plants.[14]

The carbon cycle involves both organic compounds as well as inorganic carbon compounds such as carbon dioxide and the carbonates. The inorganic compounds are particularly relevant when discussing ocean acidification for it includes the many forms of dissolved CO2 present in the Earth's oceans.[15]

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 and alkalinity (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:

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

Caldeira and Wickett (2003)[1] placed the rate and magnitude of modern ocean acidification changes in the context of probable historical changes during the last 300 million years.

Average surface ocean pH[9]
Time pH pH change relative
to pre-industrial		 Source H+ concentration change
relative to pre-industrial
Pre-industrial (18th century) 8.179 analysed field[16][not in citation given]
Recent past (1990s) 8.104 −0.075 field[16] + 18.9%
Present levels ~8.069 −0.11 field[5][17][6][18] + 28.8%
2050 (2×CO2 = 560 ppm) 7.949 −0.230 model[9] + 69.8%
2100 (IS92a)[19] 7.824 −0.355 model[9] + 126.5%


Since the industrial revolution began, it is estimated that surface ocean pH has dropped by slightly more than 0.1 units on the logarithmic scale of pH, representing an approximately 29% increase in H+, and it is estimated that it will drop by a further 0.3 to 0.5 pH units (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.[1][9][20] These changes are predicted to continue rapidly as the oceans take up more anthropogenic CO2 from the atmosphere. The degree of change to ocean chemistry, including ocean pH, will depend on the mitigation and emissions pathways[21] society takes.[22]

Although the largest changes are expected in the future,[9] 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.[23] 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.[23]

Rate [edit]

Similarly, one of the first detailed datasets examining temporal variations in pH at a temperate coastal location found that acidification was occurring at a rate much higher than previously predicted, with consequences for near-shore benthic ecosystems.[24][25] 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. 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."[26]

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,[27] 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.[28] 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."[29][30] 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.[31][32]

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:[33]

"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.[34] 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."[35]

Calcification [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).[20] 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. The saturation state of seawater for a mineral (known as Ω) is a measure of the thermodynamic potential for the mineral to form or to dissolve, and is described by the following equation:

{\Omega} = \frac{\left[\textrm{Ca}^{2+}\right] \left[\textrm{CO}_{3}^{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 (Ksp), that is, when the mineral is neither forming nor dissolving.[36] In seawater, a natural horizontal boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon, or lysocline.[20] Above this saturation horizon, Ω has a value greater than 1, and CaCO3 does not readily dissolve. Most calcifying organisms live in such waters.[20] Below this depth, Ω has a value less than 1, and CaCO3 will dissolve. However, if its production rate is high enough to offset dissolution, CaCO3 can 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.[37]

Bjerrum plot: Change in carbonate system of seawater from ocean acidification.

As shown in the Bjerrum plot, along with the change in pH, adding extra CO2 to the oceans also changes the oceans' concentrations of the different forms of dissolved inorganic carbon. There is a decrease in the concentration of CO32−, which decreases Ω, and hence makes CaCO3 dissolution more likely.

Calcium carbonate occurs in two common polymorphs: aragonite and calcite. Aragonite is much more soluble than calcite, with the result that the aragonite saturation horizon is always nearer to the surface than the calcite saturation horizon.[20] This also means that those organisms that produce aragonite may possibly be more vulnerable to changes in ocean acidity than those that produce calcite.[9] Increasing CO2 levels and the resulting lower pH of seawater decreases the saturation state of CaCO3 and raises the saturation horizons of both forms closer to the surface.[38] This decrease in saturation state is believed to be one of the main factors leading to decreased calcification in marine organisms, as it has been found that the inorganic precipitation of CaCO3 is directly proportional to its saturation state.[39]

Possible impacts [edit]

Impacts on oceanic calcifying organisms [edit]

Although the natural absorption of CO2 by the world's oceans helps mitigate the climatic effects of anthropogenic emissions of CO2, it is believed that the resulting decrease in pH will have negative consequences, primarily for oceanic calcifying organisms. These span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs.[40] 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.[41]

Research has already found that corals,[42][43][44] coccolithophore algae,[45][46][47][48] coralline algae,[49] foraminifera,[50] shellfish[51] and pteropods[9][52] experience reduced calcification or enhanced dissolution when exposed to elevated CO2.

The Royal Society published a comprehensive overview of ocean acidification, and its potential consequences, in June 2005.[20] However, some studies have found different response to ocean acidification, with coccolithophore calcification and photosynthesis both increasing under elevated atmospheric pCO2,[53][54][55] an equal decline in primary production and calcification in response to elevated CO2[56] or the direction of the response varying between species.[57] 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.[55] And another study in 2010 from Stony Brook University drew a dismal conclusion 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.[58] 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.[34] 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.[59]

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 in to 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.[60]

Ocean acidification may also force some organisms to reallocate resources away from productive endpoints such as growth in order to maintain calcification.[61]

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,[20] or directly as reproductive or physiological effects. For example, the elevated oceanic levels of CO2 may produce CO2-induced acidification of body fluids, known as hypercapnia. Also, increasing ocean acidity is believed to have a range of direct consequences. For example, increasing acidity has been observed to: reduce metabolic rates in jumbo squid;[7] depress the immune responses of blue mussels;[8] and make it harder for juvenile clownfish to tell apart the smells of non-predators and predators,[62] or hear the sounds of their predators.[63] This is possibly because ocean acidification may alter the acoustic properties of seawater, allowing sound to propagate further, and increasing ocean noise. This impacts all animals that use sound for echolocation or communication.[64]

However, as with calcification, as yet there is not a full understanding of these processes in marine organisms or ecosystems.[65]

Shelled plankton species may flourish in altered oceans.[66]

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.[67] This will cause an elevation of ocean alkalinity, leading to the enhancement of the ocean as a reservoir for CO2 with implications for climate change as more CO2 leaves the atmosphere for the ocean.[68]

Possible solutions [edit]

Carbon negative fuels [edit]

Main article: Carbon neutral fuel

Carbonic acid can be extracted from seawater as carbon dioxide for use in making synthetic fuel.[69][70] If the resulting flue exhaust gas is subject to carbon capture, then the process is carbon negative over time, resulting in permanent extraction of inorganic carbon from seawater and the atmosphere with which it is in equilibrium. Based on the energy requirements, this process is expected to cost about $50 per tonne of CO2.[71]

Iron fertilization [edit]

It has been proposed that 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 other open-sea experiments confirmed that adding iron to the ocean increases photosynthesis in phytoplankton by up to 30 times.[72] While this approach has been proposed as a potential solution to the ocean acidification problem, it could potentially mitigate some amount of surface ocean acidification at the cost of increasing acidification in the deep ocean.[73]

Gallery [edit]

  • "Present day" (1990s) sea surface pH

  • "Present day" (1990s) sea surface anthropogenic CO2

  • Vertical inventory of "present day" (1990s) anthropogenic CO2

  • Change in surface CO2−
    3     ion from the 1700s to the 1990s

  • A NOAA (AOML) in situ CO2 concentration sensor, attached to a Coral Reef Early Warning System station, utilized in conducting ocean acidification studies near coral reef areas

  • A NOAA (PMEL) moored autonomous CO2 buoy used for measuring CO2 concentration and ocean acidification studies

See also [edit]

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References [edit]

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Further reading [edit]

External links [edit]

Scientific sources:

Educational sites:

Scientific projects:

Popular media sources:

Videos on Ocean Acidification:

Carbonate system calculators [edit]

The following packages calculate the state of the carbonate system in seawater (including pH):
General
Planetary boundaries
Causes
Effects
Mitigation

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