The Acid Ocean – the Other Problem with CO2 Emission

The Royal Society has just issued a summary report on the effects of CO2 on the pH chemistry of seawater and aquatic organisms and ecosystems. In addition to its pivotal role in the atmosphere in the regulation of global climate, CO2 and its sister chemical species, HCO3 and CO32- comprise the carbonate buffer system which regulates the pH of seawater. The new report can be found here. Acidifying the ocean is particularly detrimental to organisms that secrete shell material made of CaCO3, such as coral reefs and a type of phytoplankton called coccolithophorids [Kleypas et al., 1999]. The ocean pH change will persist for thousands of years. Because the fossil fuel CO2 rise is faster than natural CO2 increases in the past, the ocean will be acidified to a much greater extent than has occurred naturally in at least the past 800,000 years [Caldeira and Wicket, 2003].

For those of you who look back on your freshman chemistry days with less than fondness, the acidity or pH of an aqueous solution is a measure of the concentration of H+ ions in the solution, with low pH meaning high H+ concentration. H+ ions are aggressive little guys, and too much H+ in water can burn the skin off your hand or make a coral limestone go fizz. The link between CO2 and H+ arises by the combination of CO2 and water, H2O, to form carbonic acid, H2CO3. An acid is a chemical species that releases H+ ions into solution, as does H2CO3 to form HCO3 and CO32-. Adding CO2 to water causes the pH to drop.

The pH of seawater is buffered by the chemistry of carbon, just as is the chemistry of blood and cellular fluids. The buffering action arises from the fact that the concentrations of the various carbon species are much higher than is the concentration of H+ ions. If some process tries to add or remove H+ ions, the amount of H+ ions required will be determined by the amount of the carbon species that have to be converted from one form to another. This will be an amount much higher than the actual change in H+ concentration itself.

Most of the carbon in seawater is in the form of HCO3, while the concentrations of CO32- and dissolved CO2 are one and two orders of magnitude lower, respectively. The equilibrium reaction for CO2 chemistry in seawater that most cogently captures its behavior is

CO2 + CO32- + H2O == 2 HCO3

where I am using double equal signs as double arrows, denoting chemical equilibrium. Since this is a chemical equilibrium, Le Chatlier’s principal states that a perturbation, by say the addition of CO2, will cause the equilibrium to shift in such a way as to minimize the perturbation. In this case, it moves to the right. The concentration of CO2 goes up, while the concentration of CO32- goes down. The concentration of HCO3 goes up a bit, but there is so much HCO3 that the relative change in HCO3 is smaller than the changes are for CO2 and CO32-. It works out in the end that CO2 and CO32- are very nearly inversely related to each other, as if CO2 times CO32- equaled a constant.

Coral reefs are built from limestone by the reaction Ca2+ + CO32- == CaCO3, where Ca is calcium. Acidifying the ocean decreases the concentration of CO32- ions, which by le Chatlier’s principal shifts the equilibrium toward the left, tending to dissolve CaCO3. Note that this is a sort of counter-intuitive result, that adding CO2 should make reefs dissolve rather than pushing carbon into making more reefs. It’s all because of those H+ ions.

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