We often read in the media that increasing carbon dioxide in the atmosphere is making the oceans too acidic, and dissolving or otherwise harming carbonate-shelled marine fauna. These writers or reporters seem ignorant of the fact that these marine fauna evolved when the atmospheric CO2 concentration was more than 10 times higher than the current level.(1) And, in fact, the oceans have never been acidic. [Note, the numbers in parentheses at the end of sentences refer to references cited at the end of this article.]
The metric for acidity/alkalinity is called pH. It is a mathematical measure of how much hydrogen ion (H+) is in solution. The pH is defined as the negative logarithm of the hydrogen ion concentration (or the logarithm of 1 divided by the hydrogen ion concentration). The pH scale goes from zero to 14 with pH of 7 being neutral, lower than 7 is acidic, and higher than 7 alkaline. A table of common acids is shown below.
Of concern with regard to ocean acidity/alkalinity is carbonic acid (H2CO3), the solution of carbon dioxide in water. Two factors control the amount of carbon dioxide in the ocean: ocean temperature and amount of carbon dioxide in the atmosphere, i.e., its partial pressure. Cooler oceans and higher atmospheric CO2 should result in more carbon dioxide in the oceans.
Henry’s law states that the concentration of a gas in a liquid is proportional to the partial pressure of the gas in equilibrium above the liquid. It stands to reason that more CO2 in the atmosphere would translate to more in the ocean. However, Henry’s law assumes constant temperature. If the temperature changes, then the absorption changes. If the oceans warm, CO2 will leave the ocean and return to the atmosphere. Cold liquids can hold more dissolved gas than warm liquids. Just think of what happens to a carbonated beverage left to warm to room temperature.
It has been estimated that current ocean pH is 0.1 pH unit less alkaline than it was in recent pre-industrial time, and some climate models predict a further decrease of 0.7 pH units by 2300.(2) However, proxy reconstructions of ocean acidity, based on fossil and modern corals, show that ocean pH has oscillated between pH of 7.91 and 8.29 during the past seven thousand years.(3) That cyclic variation is nearly four times larger than the 0.1 decrease alarmists are whining about, and even if the model predicted decrease of 0.7 units occurs, the water will still be alkaline.
An independent reconstruction, again based on corals, shows that between 1708 and 1988, there was a clear interdecadal oscillation of pH, (between 7.9 and 8.2 pH units) which is synchronous with the Pacific Decadal Oscillation of water temperature.(4) During this time, atmospheric CO2 concentration increased by about 100 parts per million. If more CO2 is dissolved in the ocean, the added carbonate (to build the calcium carbonate shells) will more than offset the decreasing alkalinity.(5) The effect of increased CO2 seems benign to other small sea creatures, including corals. (6) For many more studies on the effects of changing alkalinity on sea creatures, see the papers summarized at the Center for the Study of Carbon Dioxide and Global Change here.
The specter of acidification seems irrelevant to carbonate-shelled animals, according to the evidence. What of fish and fish larvae? A study by Munday et al. (7) found CO2 acidification had no detectable effect on embryonic duration, egg survival and size at hatching. As for adult fish, they found that most shallow-water fish tested to date appear to compensate fully their acid-base balance within several days of exposure to elevated CO2 concentrations.
Recent claims by climate alarmists have raised the possibility that terrestrial ecosystems and particularly the oceans have started losing part of their ability to absorb a large proportion of man-made CO2 emissions. However, a study combining data from ice cores, direct atmospheric measurements, and emission inventories shows that the fraction of human emitted CO2 that remains in the atmosphere has stayed constant over the past 160 years, at least within the limits of measurement uncertainty.(8) In other words, Nature compensates for our emissions. By the way, according to the Energy Information Administration, human CO2 emissions comprise less than 3 percent of total carbon dioxide in the atmosphere.
Can the oceans ever become actually acidic? There is no evidence that the oceans were ever acidic during the past 500 million years, even when atmospheric concentration of carbon dioxide was more than 10 times current levels. This implies that besides temperature and partial-pressure, there is a third controlling factor. That factor is the buffering effect of the reaction between basaltic oceanic crustal rocks with carbon dioxide (carbonic acid). This reaction uses up excess carbonic acid thereby keeping the pH of ocean water within strict limits.
Ocean acidification is just another scary scenario, a phantom menace.
1. Berner, R.A. and Kothavala, Z., 2001, A revised model of atmospheric CO2 over Phanerozoic time: Am. J. Sci., v. 301, p. 182-204.
2. Caldeira, K. and Wickett, M.E. 2003. Anthropogenic carbon and ocean pH. Nature 425: 365.
3. Liu, Y., Liu, W., Peng, Z., Xiao, Y., Wei, G., Sun, W., He, J. Liu, G. and Chou, C.-L. 2009. Instability of seawater pH in the South China Sea during the mid-late Holocene: Evidence from boron isotopic composition of corals. Geochimica et Cosmochimica Acta 73: 1264-1272.
4. Pelejero, C., Calvo, E., McCulloch, M.T., Marshall, J.F., Gagan, M.K., Lough, J.M. and Opdyke, B.N. 2005. Preindustrial to modern interdecadal variability in coral reef pH. Science 309: 2204-2207.
5. Gutowska, M.A., Pörtner, H.O. and Melzner, F. 2008. Growth and calcification in the cephalopod Sepia officinalis under elevated seawater pCO2. Marine Ecology Progress Series 373: 303-309
6. Kurihara, H., Ishimatsu, A. and Shirayama, Y. 2007. Effects of elevated seawater CO2 concentration of the meiofauna. Journal of Marine Science and Technology 15: 17-22
7. Munday, P.L., Donelson, J.M., Dixson, D.L. and Endo, G.G.K. 2009. Effects of ocean acidification on the early life history of a tropical marine fish. Proceedings of the Royal Society B 276: 3275-3283.
8. Knorr, W. (2009), Is the airborne fraction of anthropogenic CO2 emissions increasing?, Geophys. Res. Lett., 36, L21710, doi:10.1029/2009GL040613.