Posted by jlwile on April 5, 2012Ocean acidification has been called the “evil twin” of global warming. That’s because rising carbon dioxide levels are not just supposed to result in an overly-warm world. They are also supposed to result in an overly-acidic ocean. How does carbon dioxide in the air affect the acidity of the ocean? Well, the ocean absorbs a large fraction of the carbon dioxide that is in the atmosphere. Some of that carbon dioxide then reacts with the water in the ocean, producing carbonic acid. The more carbon dioxide there is in the atmosphere, the more carbon dioxide the ocean will absorb. As the ocean absorbs more carbon dioxide, more carbonic acid will be made. Thus, rising carbon dioxide levels will lead to a more acidic ocean.
Unlike the link between rising carbon dioxide levels and global warming, the link between rising carbon dioxide levels and ocean acidification is straightforward and has already been seen to some extent. As you may remember from high school chemistry, acidity can be measured using the pH scale. A pH of 7 is neutral. A pH greater than 7 indicates a basic solution, while a pH lower than 7 indicates an acidic solution. The key for this discussion is as follows: the lower the pH, the more acidic the solution. Well, according to the Ocean Studies Board of the National Research council:1
The average pH of ocean surface waters has decreased by about 0.1 unit—from about 8.2 to 8.1—since the beginning of the industrial revolution, with model projections showing an additional 0.2-0.3 drop by the end of the century, even under optimistic scenarios
Now this might not sound like a big change, but the pH scale is logarithmic. That means if the pH decreases by 1 unit, the acidity has increased by a factor of 10! Thus, a drop in pH of 0.1 is actually a change of 26%. This means the ocean is 26% more acidic than it was before the beginning of the industrial revolution. If the models are correct (and who knows if they are), the ocean will increase in acidity by an additional 58 to 100 percent by the end of the century!
So what? The ocean’s pH is above 7, indicating it is slightly basic. Wouldn’t it be better for the ocean to have a pH of 7 and be neutral? Probably not. There are a lot of organisms in the ocean, and they are designed to live in slightly basic conditions. There is a worry that as the pH of the ocean lowers, these organisms might have trouble surviving. As Marah J. Hardt and Carl Safina note in Scientific American:2
As the water becomes more acidic, corals and animals such as clams and mussels have trouble building their skeletons and shells. But even more sinister, the acidity can interfere with basic bodily functions for all marine animals, shelled or not. By disrupting processes as fundamental as growth and reproduction, ocean acidification threatens the animals’ health and even the survival of species.
How do Hardt and Safina know this? Because lots of studies have been done in which ocean organisms are placed in seawater that has been artificially exposed to enough carbon dioxide to make it much more acidic than normal, and the response of the organisms has been studied. While some of these studies indicate that the organisms either were not affected or flourished in the overly-acidic waters, many studies showed that the organisms were negatively impacted by the increased acidity. In particular, many species of coral respond negatively to acidic waters. A good overview of these studies (and others) can be found in the Royal Society’s special report, Ocean Acidification Due to Increasing Atmospheric Carbon Dioxide.
Since I try to keep up with the scientific literature, I have come across such studies in the past. For example, while I was traveling to the Christian Home Educators of New Hampshire convention, I read a report in Science News about a study that indicated clown fish raised in overly acidic waters exhibit confused behaviors. For example, they tend to swim towards the scent of a predator instead of away from that scent, as they would in the wild.3 The problem I have with such studies is that they (1) raise the carbon dioxide level immediately and (2) are incredibly short term. Since I believe that the ocean organisms were created by God with an incredible ability to adapt to their surroundings, I think that the only realistic way to study how ocean acidification will affect ocean organisms is to do long-term experiments in which the acidity is raised slowly over time.
Well, someone has finally done a study that is at least more long term than most of the ocean acidification studies that I have seen. Armin Form and Ulf Riebesell looked at a cold-water coral species called Lophelia pertusa, which is pictured above. This coral is very common over a wide range of ocean environments, so it represents a good species to study. Also, the authors state that cold-water corals should be the most vulnerable to ocean acidification. In their study, they started by doing what everyone else has done. They put the coral in seawater with artificially low pH’s that came from exposure to artificially high carbon dioxide levels. They then watched the coral over a period of eight days. Like everyone else, they showed that the more acidic the water was, the slower the growth rate of the coral became.
However, they then did a different study. They chose three pH levels that represent what might be expected by the end of the century (7.95, 7.85, and 7.75) and watched the coral over a period of six months. For the first several days, the results were the same as what they saw in the short-term study. However, after six months, the corals were all growing at rates comparable to growth rates seen in the wild! Here is what the authors say:4
Growth rates in the long-term experiment (LTE) did not follow the negative trend with increasing pCO2 [decreasing pH] observed in the short-term incubation. Instead, growth rate, which was comparable to that of the control treatment in the short-term experiment, stayed high at elevated CO2 levels… Although not statistically signiﬁcant, a linear regression analysis reveals an increasing trend of coral growth with rising pCO2 concentration [decreasing pH].
In the end, then, a more realistic study produces a conclusion quite opposite the conclusions of most of the other studies that have been done. Given time, this species of cold-water coral ends up doing just fine (and perhaps a bit better) in ocean water that is significantly more acidic than today’s ocean water.
Now please note that this still isn’t an incredibly realistic experiment. After all, the corals were not given a chance to experience a slow decrease in pH. Instead, the pH of the water was instantly lowered to the value the researchers wanted to test. An ideal experiment would not only give the corals time to adjust to the decreased pH, it would produce that decrease very slowly over a long time period. That way, corals would experience the kind of pH change that will actually result from rising carbon dioxide levels. Nevertheless, this experiment is clearly more realistic than most other experiments that have investigated the effects of ocean acidification.
So perhaps the “evil twin” of global warming is not as evil as was once considered. If nothing else, this study shows that if we are to learn exactly how “evil” this twin is, we will need to conduct significantly more realistic experiments!
1. Committee on the Development of an Integrated Science Strategy for Ocean Acidification Monitoring, Research, and Impacts Assessment, Ocean Studies Board, Division on Earth and Life Studies, National Research Council of the National Academies, Ocean Acidification: A National Strategy to Meet the Challenges of a Changing Ocean, National Academies Press 2010, page 1, (Available online).
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3. Janet Raloff, “Acidification Alters Fish Behavior,” Science News, February 25, 2012, p. 14.
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4. Armin U. Form, and Ulf Riebesell, “Acclimation to ocean acidification during long-term CO2 exposure in the cold-water coral Lophela pertusa,” Global Change Biology, 18, 843-853, 2012.
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