The Acid Ocean the Other Problem with CO2 Emission


Filed under: Climate Science Greenhouse gases Oceans— david @ 2 July 2005


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.

CaCO3 tends to dissolve in the deep ocean, both because of the high pressure and because the waters have been acidified by CO2 from rotting dead plankton. Surface waters, however, are supersaturated with respect to CaCO3, meaning that there is enough Ca2+ and CO32- in surface waters that you could give up some, and still not provoke CaCO3 to dissolve. However, it has been documented that corals produce CaCO3 more slowly as the extent of supersaturation decreases. This is also true for planktonic CaCO3-secreters such as coccolithophorids and foraminifera. We should note that for coral reef communities, the acid ocean is only one problem that they face, and it’s not the worst. Rising temperatures are tightly correlated with coral bleaching events, the expulsion of symbiotic algae, often followed by death of the coral. There is a terrifying time-series of temperature and coral bleaching from Tahiti in Hoegh-Guldberg, 1999]. When you look at the temperatures that killed the coral, and project future temperatures, it looks to be all over for corals. Coral communities are also impacted by water turbidity, resulting from fertilizer runoff, and by overfishing.

Elevated CO2 levels also affect fish and other aquatic organisms, in part because of the decrease in pH, but also because CO2 is what heterotrophic organisms try to exhale. However, we should note that dissolved CO2 levels were substantially higher than today in the geologic past, and organisms were able to cope with this OK, so apparently there can be some acclimation of populations to higher CO2.

The natural pH of the ocean is determined by a need to balance the deposition and burial of CaCO3 on the sea floor against the influx of Ca2+ and CO32- into the ocean from dissolving rocks on land, called weathering. These processes stabilize the pH of the ocean, by a mechanism called CaCO3 compensation. CaCO3 compensation works on time scales of thousands of years or so. Because of CaCO3 compensation, the oceans were probably at close to their present pH of around 8 even millions of years ago when atmospheric CO2 was 10 times the present value or whatever it was. The CaCO3 cycle was discussed briefly in regards to the uptake of fossil fuel by the ocean, here. The point of bringing it up again is to note that if the CO2 concentration of the atmosphere changes more slowly than this, as it always has throughout the Vostok record, the pH of the ocean will be relatively unaffected because CaCO3 compensation can keep up. The fossil fuel acidification is much faster than natural changes, and so the acid spike will be more intense than the earth has seen in at least 800,000 years.

There are several feedbacks between decreasing the rate of calcification that organisms do in the ocean, and the carbon cycle. Removing CaCO3 from surface waters tends to raise the CO2 concentration of the waters (it should be possible for you to work that out for yourself based on the chemical reactions above). This is a negative feedback, tending to remove excess CO2 from the atmosphere, but it is a small effect. Decreasing the flux of CaCO3 to the sea floor tends to diminish the amount of CaCO3 that gets buried in sediments, which hastens the pH-recovery from the CaCO3 compensation mechanism. This may not be a small effect at all, but it is a slow effect: thousands of years.

Caldeira, K., and Wickett, M.E. Anthropogenic carbon and ocean pH. Nature: 425, 365, 2003.

Hoegh-Guldberg, O. Climate change, coral bleaching and the future of the world's coral reefs. Mar. Freshwater Res.: 50, 839-866, 1999.

Kleypas, J., R.W. Buddemeier, D. Archer, J.-P. Gattuso, C. Langdon, and B. Opdyke (1999) Geochemical consequences of increased atmospheric CO2 on coral reefs. Science 284: 118-120.