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Ocean acidification
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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).
he natural carbon cycle, the atmospheric concentration of carbon dioxide (CO2) represents a balance of fluxes between the oceans, terrestrial biosphere, lithosphere, and the atmosphere.
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Encyclopedia
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
In the natural carbon cycle, the atmospheric concentration of carbon dioxide (CO2) represents a balance of fluxes between the oceans, terrestrial biosphere, lithosphere, 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 in the atmosphere (where it is responsible for the rise in atmospheric concentrations), some has been taken up by terrestrial plants, and some has been absorbed by the oceans.
Within the carbon cycle, there are two more specific classifications: 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 - 0.5 units by 2100 as the ocean absorbs 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 or emissions pathway 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 alkaline.
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.
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 use the calcite or aragonite polymorphs of calcium carbonate to construct cell coverings or skeletons. Calcifiers span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs.
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 and
photosynthesis 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. 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
See also
Further reading
, ().
- Kleypas, J.A., R.A. Feely, V.J. Fabry, C. Langdon, C.L. Sabine, and L.L. Robbins. (2006). , report of a workshop held 18-20 April 2005, St. Petersburg, FL, sponsored by NSF, NOAA and the U.S. Geological Survey, 88pp.
- Kolbert, E. (2006). The Darkening Sea: Carbon emissions and the ocean. The New Yorker magazine. 20 November 2006. .
- Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) (2008). ISSN: 1835–7911. Hobart, Tasmania.
- Kump, Lee R., James F. Kasting, and Robert G. Crane. “The Earth System.” Second ed. Pages: 162-164. Upper Saddle River: Prentice Hall, 2003.
External links
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- , David Archer, a RealClimate discussion
- , Pacific Science Association
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- , Washington Post
- - a multimedia, interactive site from
- by Tundi Agardy,
- ,
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- project of Australia's Antarctic Climate and Ecosystems Cooperative Research Centre
- , ABC News
- , an upcoming documentary and related blog about the science and the socio-economic impacts of ocean acidification
- Initiated June 2008. The EPOCA consortium includes more than 100 researchers from 27 institutes and 9 European countries.
- , by Clayton Sandell ABC News
Carbonate system calculators
The following packages calculate the state of the carbonate system in seawater (including pH):
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