||This article may be too technical for most readers to understand. (June 2011)|
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. About 30–40% of the carbon dioxide released by humans into the atmosphere dissolves into the oceans, rivers and lakes. 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, representing an increase of almost 30% in H+ ion concentration in the world's oceans.
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. Thus, ongoing acidification of the oceans also poses a threat to the food chains connected with the oceans.
Carbon cycle 
The carbon cycle describes the fluxes of carbon dioxide (CO2) between the oceans, terrestrial biosphere, lithosphere, 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, with some also taken up by terrestrial plants.
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.
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.
Dissolving CO2 in seawater increases the hydrogen ion (H+) concentration in the ocean, and thus decreases ocean pH, as follows:
|Time||pH||pH change relative
|Source||H+ concentration change
relative to pre-industrial
|Pre-industrial (18th century)||8.179||analysed field[not in citation given]|
|Recent past (1990s)||8.104||−0.075||field||+ 18.9%|
|Present levels||~8.069||−0.11||field||+ 28.8%|
|2050 (2×CO2 = 560 ppm)||7.949||−0.230||model||+ 69.8%|
|2100 (IS92a)||7.824||−0.355||model||+ 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. 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 society takes.
Although the largest changes are expected in the future, 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. 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.
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. 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."
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, 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. 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." 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.
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:
"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. 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."
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). 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:
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. 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. Above this saturation horizon, Ω has a value greater than 1, and CaCO3 does not readily dissolve. Most calcifying organisms live in such waters. 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.
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. This also means that those organisms that produce aragonite may possibly be more vulnerable to changes in ocean acidity than those that produce calcite. 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. 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.
Possible impacts 
Impacts on oceanic calcifying organisms 
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. 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.
Research has already found that corals, coccolithophore algae, coralline algae, foraminifera, shellfish and pteropods 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. However, some studies have found different response to ocean acidification, with coccolithophore calcification and photosynthesis both increasing under elevated atmospheric pCO2, an equal decline in primary production and calcification in response to elevated CO2 or the direction of the response varying between species. 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. 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. 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. 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.
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.
Ocean acidification may also force some organisms to reallocate resources away from productive endpoints such as growth in order to maintain calcification.
Other biological impacts 
Aside from the slowing and/or reversing of calcification, organisms may suffer other adverse effects, either indirectly through negative impacts on food resources, 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; depress the immune responses of blue mussels; and make it harder for juvenile clownfish to tell apart the smells of non-predators and predators, or hear the sounds of their predators. 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.
Shelled plankton species may flourish in altered oceans.
Nonbiological impacts 
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. 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.
Possible solutions 
Carbon negative fuels 
Carbonic acid can be extracted from seawater as carbon dioxide for use in making synthetic fuel. 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.
Iron fertilization 
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. 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.
See also 
- Biological pump
- Carbon dioxide sinks
- Carbonate compensation depth
- Continental shelf pump
- Greenhouse Gas
- Global Ocean Data Analysis Project
- Paleocene–Eocene Thermal Maximum
- Seawater pH
- Solubility pump
- Environmental impact of the coal industry
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Further reading 
- Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) (2008). Position analysis: CO2 emissions and climate change: Ocean impacts and adaptation issues. ISSN: 1835–7911. Hobart, Tasmania.
- Cicerone, R.; J. Orr, P. Brewer et al. (2004). "The Ocean in a High CO2 World". EOS, Transactions American Geophysical Union 85 (37): 351–353. Bibcode:2004EOSTr..85R.351C. doi:10.1029/2004EO370007.
- Doney, S. C. (2006). "The Dangers of Ocean Acidification". Scientific American 294 (3): 58–65. doi:10.1038/scientificamerican0306-58. ISSN 0036-8733. PMID 16502612., (Article preview only).
- Feely, R. A.; Sabine, Christopher L.; Lee, Kitack; Berelson, Will; Kleypas, Joanie; Fabry, Victoria J.; Millero, Frank J. (2004). "Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans". Science 305 (5682): 362–366. Bibcode:2004Sci...305..362F. doi:10.1126/science.1097329. PMID 15256664.
- Harrould-Kolieb, E.; Savitz, J. (2008). "Acid Test: Can We Save Our Oceans From CO2?". Oceana.
- Henderson, Caspar (2006-08-05). "Ocean acidification: the other CO2 problem". NewScientist.com news service.
- Jacobson, M. Z. (2005). "Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry". Journal of Geophysical Research – Atmospheres 110: D07302. Bibcode:2005JGRD..11007302J. doi:10.1029/2004JD005220.
- Kleypas, J. A., R. A. Feely, V. J. Fabry, C. Langdon, C. L. Sabine, and L. L. Robbins. (2006). Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Further Research, report of a workshop held 18–20 April 2005, St. Petersburg, FL, sponsored by National Science Foundation, NOAA and the U.S. Geological Survey, 88pp.
- Kolbert, E. (2006-11-20). "The Darkening Sea: Carbon emissions and the ocean". The New Yorker.
- Riebesell, U., V. J. Fabry, L. Hansson and J.-P. Gattuso (Eds.). (2010). Guide to best practices for ocean acidification research and data reporting, 260 p. Luxembourg: Publications Office of the European Union.
- Sabine, C. L.; Feely, Richard A.; Gruber, Nicolas; Key, Robert M.; Lee, Kitack; Bullister, John L. et al. (2004). "The Oceanic Sink for Anthropogenic CO2". Science 305 (5682): 367–371. Bibcode:2004Sci...305..367S. doi:10.1126/science.1097403. PMID 15256665.
- Stone, R. (2007). "A World Without Corals?". Science 316 (5825): 678–681. doi:10.1126/science.316.5825.678. PMID 17478692.
- How Acidification Threatens Oceans from the Inside Out Scientific American August 9, 2010 by Marah J. Hardt and Carl Safina
- Ocean acidification due to increasing atmospheric carbon dioxide, report by the Royal Society (UK)
- AR4 WG1 Chapter 5: Oceanic Climate Change and Sea Level, IPCC
- State of the Science FACT SHEET: Ocean acidification, NOAA
- Carbon Dioxide Information Analysis Center (CDIAC), the primary data analysis center of the U.S. Department of Energy (located at Oak Ridge National Laboratory)
- Ocean acidification introduction, USGS
- Climate change threatening the Southern Ocean, report by CSIRO
- The Ocean in a High CO2 World, an international science symposium series
- The Acid Ocean – the Other Problem with CO2 Emission, David Archer (scientist), a RealClimate discussion
- Regularly updated "blog" of ocean acidification publications and news, Jean-Pierre Gattuso
- Task Force on Ocean Acidification in the Pacific, including recent presentations on ocean acidification, Pacific Science Association
- Ocean Acidification, a multimedia, interactive site from The World Ocean Observatory
- Acidic Oceans: Why should we care? Perspectives in ocean science, Andrew Dickson, Scripps Institution of Oceanography
- Climate Change: Coral Reefs on the Edge A video presentation by Prof. Ove Hoegh-Guldberg on impact of ocean acidification on coral reefs
- Life in the Sea Found Its Fate in a Paroxysm of Extinction April 30, 2012
- Ocean acidification virtual lab
- Ocean Acidification: Starting with the Science, a booklet from the Division on Earth & Life Studies of the United States National Research Council (released April 2011)
- Ocean Acidification, a United States National Academy of Sciences/ National Research Council website that includes downloadable figures and interviews with ocean scientists
- Ancient Ocean Acidification Intimates Long Recovery from Climate Change, July 22, 2010
- Acidification alters fish behavior: higher carbon dioxide in oceans may affect brain chemistry Feb 25, 2012 Science News
- Understanding Ocean Acidification -- Educational Site from Channel Islands National Marine Sanctuary -- Education Team
- Dr. Francisco Chavez on Ocean Acidification – Smithsonian Ocean Portal
- European Project of Ocean Acidification (EPOCA), a 4-year-long EU initiative to investigate ocean acidification (initiated June 2008)
- Biological Impacts of Ocean Acidification (BIOACID), a German initiative funded by BMBF
- Ocean Acidification Research Programme (UKOARP), a 5-year-long UK initiative funded by NERC, Defra and DECC
- Research Program on Ocean Acidification at the Cluster of Excellence "Future Ocean", Kiel
- Ocean Acidification Research Center at University of Alaska Fairbanks
Popular media sources:
- Threatening Oceans from the Inside Out: How Acidification Affects Marine Life, Scientific American
- "The Darkening Sea, article in The New Yorker magazine, Nov. 20, 2006 (requires registration)
- "Growing Acidity of Oceans May Kill Corals", Washington Post
- "Scientists Grapple with Ocean Acidification", ABC News
- "Ocean Acidification & Climate", by Clayton Sandell ABC News
- A World Without Whales? by Philippe Cousteau, The Huffington Post
- Acid Test: Can we save our oceans from CO2?, Oceana
- The Acid Ocean, Stanford University
Videos on Ocean Acidification:
- The Other CO2 Problem, an EPOCA-commissioned educational animation created by students from Ridgeway School, Plymouth
- Acid Test: The Global Challenge of Ocean Acidification, by Natural Resources Defense Council
- A Sea Change: Imagine a world without fish, an award-winning documentary and related blog about ocean acidification
- Ocean Acidification in a Nutshell, by Greenpeace Aotearoa New Zealand
- Ocean Acidification: An Ecosystem Facing Dissolution by GEOMAR I Helmholtz-Centre for Ocean Research Kiel
Carbonate system calculators 
The following packages calculate the state of the carbonate system in seawater (including pH):
- CO2SYS, a stand-alone executable (also available in a version for Microsoft Excel/VBA)
- seacarb, a R package for Windows, Mac OS X and Linux (also available here)
- CSYS, a Matlab script