"In a recent interview with Yale Environment 360, Jane Lubchenco, the head of NOAA, called oceanic acidification global warming's "equally evil twin." Ocean acidification occurs because carbon dioxide, when dissolved in water, forms a weak acid. Put more CO2 into the atmosphere, and more will inevitably dissolve into the oceans, lowering their pH. A landmark report published in 2005 by the British Royal Society urged the global community to invest more time and effort into studying this critical issue, warning that "Marine ecosystems are likely to become less robust as a result of the changes to the ocean chemistry and these will be more vulnerable to other environmental impacts."

While research efforts have since taken off, what has been sorely lacking are datasets that could help scientists document the long-term rate of acidification and understand the underlying physical and chemical processes. That is, until now. In the latest issue of the Proceedings of the National Academy of Sciences, John E. Dore of Montana State University and colleagues from the University of Hawaii, Honolulu, present the results of the first set of longitudinal time-series measurements of seawater pH, spanning an almost twenty-year period (from 1988 to 2007). The data were recorded at ALOHA, a research station off Hawaii.

When CO2 enters the ocean, a fraction of it reacts with water to form carbonic acid (H2CO3), a weak acid, and the rest remains in dissolved form. Some of the carbonic acid dissociates, releasing hydrogen ions that further react to produce either bicarbonate (HCO3-) or carbonate ions (CO3-2). These three forms of dissolved inorganic carbon (DIC) make up the carbonic acid system, a natural buffer that handles slight variations in CO2 and maintains seawater pH around 8.1-8.2. The loss of this buffer—even its weakening—could have a significant impact on corals and other organisms that build their shells out of calcium carbonate (CaCO3).

The Hawaiian record shows a long-term decline in surface pH of 0.0019 ± 0.0002 per year—which may not seem like much until you recall that pH units are expressed on a logarithmic scale. This means that a one-unit drop in pH corresponds to a ten-fold increase in the hydrogen ion concentration. To put that into context, the 0.1 decrease in globally averaged pH—from 8.2 to 8.1—over the last 250 years is roughly equivalent to a 30 percent increase in hydrogen ions.

This trend, they emphasize, is "indistinguishable from the rate of acidification expected from equilibration from the atmosphere"—confirming the basic theory that as atmospheric CO2 increases, more and more of it will be absorbed by the oceans, where it will alter the chemistry. Scientists are worried that this sudden influx of CO2—they estimate that the oceans have taken up nearly half of all carbon emitted since the beginning of the industrial era—risks overwhelming the delicate buffering system that has kept the ocean's pH in check for millennia.

Dore and his colleagues also found that the pH cycle displays a strong seasonality, typically reaching a maximum during the winter and a minimum during the summer. There is also interannual variability. The authors attribute these to a combination of photosynthetic activity, air-sea exchange, and the mixing of different layers of water. The surface pH varied by as much as 0.01, largely as a result of diurnal heating and cooling. " (Ars Technica post)