Ocean acidification



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 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 is believed to have been taken up by terrestrial plants, and some has been absorbed by the 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
Dissolving CO2 in seawater also increases the hydrogen ion (H+) concentration in the ocean, and thus decreases ocean pH. The use of the term "ocean acidification" to describe this process was introduced in Caldeira and Wickett (2003).

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), 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.

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.

A report from NOAA scientists published in the journal Science in May 2008 found that large amounts of water that is undersaturated in aragonite (i.e. a proxy measure indicating relatively acidified / low pH water) are upwelling to within four miles of the Pacific continental shelf area of N. America. This area is a critical zone where most local marine life lives or is born. While the paper only dealt with the areas from Vancouver to northern California, other continental shelf areas may be experiencing similar effects. 

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 under-saturated, 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.

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

Versauerung der Meere Acidificación del océano Acidification de l’océan 해양 산성화 Acidificazione degli oceani Merien happamoituminen