Ocean acidification by carbon dioxide

We often read in the media that increasing carbon dioxide in the atmosphere will make the oceans too acidic, and dissolve or otherwise harm 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) Ironically, a recent New York Times article was fretting that the ocean is not absorbing enough carbon dioxide to act as a good carbon “sink.”

The metric for acidity/alkalinity is called pH. 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.

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 Interdecadal Pacific 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)

Ocean acidification is just another scary scenario, a phantom menace.

Jonathan DuHamel, Tucson Citizen, 14 December 2009. Article.

3 Responses to “Ocean acidification by carbon dioxide”

  1. 1 Lina Hansson 18 December 2009 at 10:15

    This article does not summarize the current understanding. Make sure that you read the comments to this article.

  2. 2 Robert Kernodle 7 January 2010 at 22:01

    Dear Lina Hansson,

    You did not leave a link to any article that summarizes the “current understanding” you mention. And at the time of this writing, there are NO other comments below mine (Jan 7, 2010).

    A general claim without a specific reference seems a bit weak and suspect.

  3. 3 Jean-Pierre Gattuso 8 January 2010 at 10:20

    Lina Hansson referred to the comments on the original article, not to the comments on her blog post. You can also check the present blog to have a comprehensive overview of the current understanding of ocean acidification and of its impacts on marine organisms and ecosystems.

    Thank you for your interest.
    Jean-Pierre Gattuso

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