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Ocean acidification (Rev #6)

Ocean acidification


As the concentration of carbon dioxide in the atmosphere increases, it is making the oceans more acidic, in a phenomenon known as ocean acidification. This in turn can hurt calcareous organisms such as shellfish and coral reefs. For more, see:


Apparently the ocean is more acidic now than any time in the last 650,000 years. According to the Wikipedia entry on ocean acidification, the basic numbers are as follows:

Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.179 to 8.104, a change of −0.075 on the logarithmic pH scale which corresponds to an increase of 18.9% in H+ (acid) concentration. By the first decade of the 21st century however, the net change in ocean pH levels relative pre-industrial level was about -0.11, representing an increase of some 30% in “acidity” (ion concentration) in the world’s oceans.

Here is a plot showing the acidification of the oceans:

Measurement of ocean pH

There are three points frequently used by those claiming ocean acidification is not happening, along with some vague commentary:

  1. Measurements of historical pH seem excessively precise.

    This appears to be on the assumption that measurements were made at the time, rather than being back-inferred from ratios of 11B{}^11 B in deposited material. Is the accuracy of this technique confirmed in the literature anywhere?

  2. Ocean pH vaies greatly geographically, diurnally and historically.

    It’s not logically impossible for a system which exhibits fluctuations to nonetheless be sensitive to changes in the mean, particularly if they are rapid. Is there a reference that confirms that mean pH changes matter even in the presence of fluctuations? The historical graphs used on this “The Ocean Acidification Fiction” webpage show signficant local minima, eg, in the 1930s. Are these correct and if so, is there an accepted explanation?

  3. Some recent papers/press releases don’t distinguish clearly between measurements and model predictions of ocean pH.

    It’s always in the eye of the beholder if a paper states what it’s doing in loud enough terms to prevent the reader (or onwards reporter) getting the wrong impression. I don’t know if this criticism is generally valid.

Points 1 and 2 seem analogous to similar points that are raised for CO 2CO_2, and presumably an oceanographer/chemist (which I’m not) could rate their validity and context.

I’ve seen some additional claims that the correlation between decreasing pH and “bad things happening” isn’t there, but let’s figure out the pH issues first. – David Tweed

We need to add information about the chemical mechanism of ocean acidification, which we worked out on the Azimuth blog entry Dying coral reefs. The chemistry is surprisingly subtle and poorly understood! – John Baez


  • Ocean acidification, Wikipedia.

  • AUTHOR???, Climate sensitivity to the carbon cycle modulated by past and future changes in ocean chemistry, Nature Geoscience 2 (2009), 145–150. doi:10.1038/ngeo416

  • AUTHOR???, Past constraints on the vulnerability of marine calcifiers to massive carbon dioxide release, Nature Geoscience 3 (2010), 196–200. doi:10.1038/ngeo755

  • Rollion-Bard C., Blamart D., Trebosc J., Tricot G., Mussi A. & Cuif J.-P., in press. Boron isotopes as pH proxy: a new look at boron speciation in deep-sea corals using 11B MAS NMR and EELS. Geochimica et Cosmochimica Acta.

Abstract: Dissolved boron in modern seawater occurs in the form of two species, trigonal boric acid B(OH)3 and tetrahedral borate ion B(OH)4-. One of the key assumption in the use of boron isotopic compositions of carbonates as pH proxy is that only borate ions, B(OH)4-, are incorporated into the carbonate. Here we investigate the speciation of boron in deep-sea coral microstuctures (Lophelia pertusa specimen) by using high field magic angle spinning nuclear magnetic resonance (11B MAS NMR) and electron energy-loss spectroscopy (EELS). We observe both boron coordination species, but in different proportions depending on the coral microstructure, i.e. centres of calcification versus fibres. These results suggest that careful sampling is necessary before performing boron isotopic measurements in deep-sea corals. By combining the proportions of B(OH)3 and B(OH)4- determined by NMR and our previous ion microprobe boron isotope measurements, we propose a new equation for the relation between seawater pH and boron isotopic composition in deep-sea corals.

  • Ocean acidification blog.

    Apparently “An information outlet on ocean acidification provided by EPOCA, the European Project on Ocean Acidification”.

category: oceans