Ocean acidification describes the decrease in pH of the oceans due to increased concentration of dissolved carbon dioxide (CO2). CO2 is a trace gas found in the atmosphere. The concentration of dissolved oceanic CO2 is in equilibrium with the gas's atmospheric concentration, such that an increase in the atmospheric concentration leads to an increase in the dissolved concentration, and vice versa.

Throughout geologic time, atmospheric CO2 reflected a balance between various sources and sinks found on land and in the oceans--such as photosynthesis, respiration, chemical weathering of rocks and burial of organic and inorganic carbon. Since the onset of the industrial age, human activities, such as fossil fuel combustion and intensification of agriculture, represent a new source of atmospheric CO2.

These activities are thought to cause the observed increases in atmospheric CO2, which in turn lead to increased dissolved CO2 concentrations in the oceans. Dissolved CO2 combines with water to form carbonic acid (H2CO3), which subsequently dissociates to produce bicarbonate (HCO3 -) and carbonate ions (CO3 2-) and protons (H+). The balance among these different reactions is such that increased concentrations of dissolved CO2 lead to increased proton concentrations (acidity). Since pH is an inverse scale of the concentration of dissolved H+, the higher the proton concentrations, the lower the pH and the greater the acidity. Modeling experiments using different scenarios to project future atmospheric CO2 concentrations indicate that by 2100, the ocean's pH will drop by up to 0.45 unit.

The reactions by which CO2 and water form H2CO3, HCO3 - and CO3 2- control the pH of the world's oceans. Oceanic pH has important ecological consequences, because many plankton (including the Coccolithophoridae, which are marine algae, and the Foraminiferida, which are planktic protists) and macrofauna (corals, mollusks, brachiopods, and so on) precipitate the minerals calcite or aragonite (which both have the chemical formula CaCO3) to make exoskeletons, shells, and tests in a process called calcification. Under normal conditions, calcite and aragonite are stable minerals in surface waters, because CO3 2- concentrations are naturally maintained at levels that prevent their dissolution. However, increasing atmospheric CO2 concentrations lead to decreased oceanic pH values and a concomitant decrease in carbonate ion concentration.

This effect is currently observed in a vertical profile of the oceans, where carbonate ion concentrations decrease with increasing depth (Doney, 2006). At a depth known as the lysocline, the rate of carbonate mineral dissolution increases rapidly with decreasing carbonate ion concentrations. Thus, in surface and intermediate waters, carbonate minerals are not dissolved. Below the lysocline, carbonate minerals are readily dissolved. Many laboratory experiments seem to indicate that ocean acidification will prove detrimental to many marine ecosystems (Hall-Spencer, 2008). This finding has been recently validated by a study of a benthic ecosystem located near a naturally occurring volcanic vent that delivers CO2 to the surrounding waters. The full-scale consequences of acidification of the global ocean are likely to be numerous, and could include extinctions as food webs collapse due to a loss of calcareous planktonic primary producers and consequences for the strength of the biological pump.

The biological pump is the process by which CO2 is actively removed from the atmosphere by primary producing phytoplankton who convert the CO2 to organic matter via photosynthesis. When these organisms die, their hard parts act as ballast to help the organic matter sink to the ocean floor, where it is buried in sediments. Ultimately, this sequesters the CO2 in rocks for geologic time scales.

However, if ocean acidification leads to dissolution of calcareous tests, organic matter will not be effectively buried because it is not dense enough to settle to the ocean floor. Not all scientists agree with the negative predicted consequences of ocean acidification. They argue that carbonate minerals present in sedimentary rocks on the ocean floor should, over time, consume the excess H+ produced from increased atmospheric CO2 concentrations, causing ocean acidification to slow or even stop. The increase in atmospheric CO2 concentrations appears to be linked to an increase in global temperatures.

Temperature is a strong control on the conversion of atmospheric CO2 to dissolved CO2 in the ocean. Henry's Law states that the dissolved concentration of a gas is proportional to the partial pressure of that gas in the atmosphere in contact with that liquid. Gases are characterized by different Henry's Law constants that give the proportionality of dissolved gas that will be in equilibrium with the overlying atmosphere. For most gases, including CO2, the value of the Henry's Law constant decreases with increasing temperature--that is, at higher temperatures, the amount of CO2 that can be dissolved in the ocean will decrease. This temperature dependence provides a negative feedback on the amount of ocean acidification that could occur. However, this also means that a greater proportion of anthropogenic CO2 would remain in the atmosphere as global temperature rises.

References

1. Doney, Scott C. "The Dangers of Ocean Acidification." Scientific American 294, no. 3 (March, 2006): 58-65.

2. Hall-Spencer, Jason M., et al. "Volcanic Carbon Dioxide Vents Show Ecosystem Effects of Ocean Acidification." Nature 454, no. 7200 (July 3, 2008): 96-99.

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