Ocean acidification: Map

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Ocean acidification is the name given to the ongoing decrease in the pH of the Earth's oceans, caused by their uptake of anthropogenic carbon dioxide from the atmosphere. Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.179 to 8.104 (a change of −0.075).

Carbon cycle

The carbon cycle describes the fluxes of carbon dioxide (CO₂) between the oceans, terrestrial biosphere, lithosphere"carbon cycle." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 11 Feb. 2009 /www.search.eb.com/eb/article-9020247>., and the atmosphere. Human activities such as land-use changes, the combustion of fossil fuels, and the production of cement have led to a new flux of CO₂ into the atmosphere. Some of this has remained there; some has been taken up by terrestrial plants, and some has been absorbed by the oceans.

The carbon cycle comes in two forms: the organic carbon cycle and the inorganic carbon cycle. The inorganic carbon cycle is 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 (HCO₃⁻) and carbonate (CO₃²⁻). The ratio of these species depends on factors such as seawater temperature and alkalinity (see the article on the ocean's solubility pump for more detail).

Acidification

Average surface ocean pH
Time	pH	pH change	Source
Pre-industrial (1700s)	8.179	0.000	analysed field
Recent past (1990s)	8.104	−0.075	field
2050 (2×CO₂ = 560 ppm)	7.949	−0.230	model
2100 (IS92a)	7.824	−0.355	model

Dissolving CO₂ in seawater also increases the hydrogen ion (H⁺) concentration in the ocean, and thus decreases ocean pH. Caldeira and Wickett (2003) placed the rate and magnitude of modern ocean acidification changes in the context of probable historical changes during the last 300 million years.

Since the industrial revolution began, it is estimated that surface ocean pH has dropped by slightly less than 0.1 units (on the logarithmic scale of pH; approximately a 25% increase in H⁺), and it is estimated that it will drop by a further 0.3 to 0.5 units by 2100 as the oceans absorb more anthropogenic CO₂. 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, for example ocean pH, will depend on the mitigation and emissions pathways society takes. Note that, although the ocean is acidifying, its pH is still greater than 7 (that of neutral water), so the ocean could also be described as becoming less basic.

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 that previously predicted, with consequences for near-shore benthic ecosystems.

Calcification

Changes in ocean chemistry can have extensive direct and indirect effects on organisms and the habitats in which they live. 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:

{\Omega} = \frac{\left[Ca^{2+}\right] \left[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 CO₃^2-), divided by the product of the concentrations of those ions when the mineral is at equilibrium (K_sp), 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.

Calcium carbonate occurs in 2 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 which 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

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, so does the concentration of this ion, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution.

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 of London 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 andphotosynthesis both increasing under elevated atmospheric p , an equal decline in primary production and calcification in response to elevated or the direction of the response varying between species. Recent work 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. While the full ecological consequences of these changes in calcification are still uncertain, it appears likely that many calcifying species will be adversely affected. There is also a suggestion that a decline in the coccolithophores may have secondary effects on climate change, by decreasing the Earth's albedo via their effects on oceanic cloud cover.

Aside from calcification, organisms may suffer other adverse effects, either directly as reproductive or physiological effects (e.g. CO₂-induced acidification of body fluids, known as hypercapnia), or indirectly through negative impacts on food resources. Ocean acidification may also force some organisms to reallocate resources away from feeding and reproduction in order to maintain internal cell pH (i.e. expenditure of extra energy to run proton pumps). It has even been suggested that ocean acidification will alter the acoustic properties of seawater, allowing sound to propagate further, increasing ocean noise and impacting animals that use sound for echolocation or communication. However, as with calcification, as yet there is not a full understanding of these processes in marine organisms or ecosystems.

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 with moderate (and potentially beneficial) implications for climate change as more leaves the atmosphere for the ocean.

Gallery

File:WOA05 GLODAP pd pH AYool.png | "Present day" (1990s) sea surface pHFile:WOA05 GLODAP pd aco2 AYool.png | "Present day" (1990s) sea surface anthropogenic CO₂File:WOA05 GLODAP invt aco2 AYool.png | Vertical inventory of "present day" (1990s) anthropogenic CO₂File:WOA05 GLODAP del co3 AYool.png | Change in surface CO₃^2- ion from the 1700s to the 1990sImage:oa-sami.jpg | A NOAA (AOML) in situ pCO₂ sensor, attached to a Coral Reef Early Warning System station, utilized in conducting ocean acidification studies near coral reef areasImage:Oa-buoy-enrique-reef.jpg | A NOAA (PMEL) moored autonomous pCO₂ buoy used for measuring pCO₂ and ocean acidification studies

References

Kump, Lee R., James F. Kasting, and Robert G. Crane. “The Earth System.” Second ed. Pages: 162-164. Upper Saddle River: Prentice Hall, 2003.
Review of Past IPCC Emissions Scenarios, IPCC Special Report on Emissions Scenarios (ISBN 0521804930).
Raven, J. A. et al. (2005). Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society, London, UK.
The Royal Society. Ocean Acidification Due To Increasing Atmospheric Carbon Dioxide, The Clyvedon Press Ltd. (2005): 11.
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Acid In The Oceans: A Growing Threat To Sea Life by Richard Harris. All Things Considered, 12 August 2009.