Ocean Acidification Time Bomb Is Ticking
by Bill Chameides | August 5th, 2009
posted by Erica Rowell (Editor)
While the world argues about the effects of and solutions to carbon dioxide (CO2) on climate, the ocean is slowly but inexorably becoming more acidic.
The climate system is a very complicated thing. While it is clear that rising concentrations of CO2 and other greenhouse gases will warm the planet, exactly how much and how that warming will affect other weather variables like rainfall are uncertain. And despite statements [pdf] like that of the U.S. National Academy of Sciences (along with 12 other national academies) warning of the consequences of global warming if the world’s greenhouse gas pollution is not curbed, some remain unconvinced.
In contrast to the climate, the chemistry of solutions equilibrating with gases is well-characterized — one whose basic principles were largely worked out in the late 19th century, as many of you no doubt learned about in your introductory chemistry class in high school or college. And the effect of adding CO2 to the ocean is indisputable. (Interestingly, Svante Arrhenius, who is credited with hypothesizing that increasing atmospheric CO2 concentrations would cause global warming, won the Nobel Prize for Chemistry in 1903 for his work on the chemistry of solutions.)
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The Good and Bad of CO2 in the Ocean
As atmospheric CO2 concentrations increase, a disequilibrium between CO2 in the atmosphere and CO2 in the ocean is established. In response to that disequilibrium, a portion of the extra CO2 that had been added to the atmosphere dissolves into the surface of ocean. It is estimated that of the 1.8 trillion tons of CO2 that have been emitted to the atmosphere since the Industrial Revolution from fossil fuel and biomass burning, some 530 billion tons have found their way into the ocean.
In one sense this dissolution is a good thing, because it removes a heat-trapping gas from the atmosphere. But there is a very sinister side to this “good thing.” When CO2 dissolves in seawater, it forms carbonic acid, which makes the seawater more acidic. We call that ocean acidification.
A Little Bit of Acidification Goes a Long Way for Some Sea Critters — and the Wrong Way
Now, the amount of acidification expected by 2050 is modest — just a few tenths of pH units, if you are familiar with the pH scale for acidity. No big deal, right?
Wrong. The ocean is filled with creatures whose bodies are made in part of calcium carbonate; they are called calcareous. The reason ocean species use calcium carbonate is that both calcium and carbonate are abundant in the ocean, calcium carbonate is a hard substance, and the calcium carbonate solution chemistry in the ocean is just right.
Calcium and carbonate exist in dissolved form so that calcareous species can easily extract them from seawater to make their bodies. But calcium and carbonate are not too soluble and so the bodies of calcareous species don’t just dissolve away. The key factor in maintaining that balance is the acidity of the ocean. Increase the ocean’s acidity, and you shift the equilibrium in favor of more dissolved calcium and carbonate and less solid calcium carbonate.
The trouble is it only requires a very small change in pH, on the order of magnitude discussed here, to upset that delicate balance. And it’s already happening. In a paper out earlier this spring, researchers documented how acidification of the Southern Ocean is causing some calcifiers to grow thinner, lighter shells compared to pre-industrial times. (See my post on this.) Other research points to regions of the ocean that may reach the point of disequilibrium faster than previously thought.
Acidification Makes It Harder for Calcareous Species to Form Shells
And that is the crux of the problem. As the acidity of the ocean increases, calcareous species find it more and more difficult to form their shells and exoskeletons and their lives become more precarious.
Should we care about the existence of calcareous ocean species? You bet. They include species like plankton, mollusks, coral reefs and crustaceans — all of which play critical roles in the ocean food web which supports the fisheries which in turn provide protein for more than two billion people worldwide. Undermine the lives of calcareous ocean species, and in the long run we undermine one of our most important sources of food. This is not a problem about saving coral reefs for SCUBA divers (which is no small thing); it’s about preserving the livelihood of a billion people or so. Coral reefs along support nearly 500 million people around the globe and provide about $30 billion annually [pdf] in terms of food, tourism revenue, coastal protection, and biodiversity.
In case anyone had any doubt that ocean acidification is real, a recent paper by John Dore of Montana State University and colleagues published in the Proceedings of the National Academy of Sciences should dispel them. The authors report on 20-years of pH-measurements from seawater collected at the ALOHA experiment station near Hawaii. The data show a clear upward trend in ocean acidity over the time period with a magnitude and rate of change in keeping with what has been predicted based on the observed changes in atmospheric CO2.
Some people argue that because of global warming, adding CO2 to the atmosphere is like “playing with fire.” Maybe so. I am convinced. But regardless of what you think about global warming, there is no question that we are also playing with the ocean and we will almost certainly not win that game.filed under: carbon dioxide emissions, climate change, faculty, global warming, oceans
and: calcifier, climate, greenhouse gas emissions, ocean acidification, Svante Arrhenius