Ocean Acidification: A Primer

We’ve all at least heard of the greenhouse effect and global warming: trace gasses (primarily carbon dioxide) in the earth’s atmosphere alter its thermal properties, causing it to retain heat. Human activity, primarily the burning of fossil fuels, is increasing the carbon dioxide content of the atmosphere and as a result heating up the earth. However, a less appreciated fact is that in addition to changing the atmosphere’s thermal properties, carbon dioxide can also be a chemical pollutant.

Many people will likely startle at that last line- after all, we all exhale carbon dioxide; plants use it for food; I’m sitting breathing it right now with no ill effects (the oxygen I’m also breathing is arguably a more pressing chemical threat). CO2 certainly doesn’t have the toxic appeal of PCBs or dioxin.

But a pollutants aren’t just nasty oil slicks. A pollutant is a substance which, by virtue of its chemical activity, interferes with the functioning of an ecosystem. A pollutant is an ecophysiological poison. And poisons are situational: whether or not a substance is poisonous depends upon its amount, the rate it’s encountered, and other factors. To draw an analogy with the human body, there are plenty of chemicals which in some situations are essential, but poisonous in others. Nitric oxide, carbon monoxide, and hydrogen sulfide are all essential signalling molecules in our nervous system- but they’re also all quite poisonous. Hydrochloric acid is important in my stomach, where it helps me digest food- but I don’t want to get it in my eyes.

“The other CO2 problem” is ocean acidification. Carbon dioxide, in addition to being a greenhouse gas, is acidic. When carbon dioxide (CO2) dissolves in water (H2O) the carbon atom in CO2 has a slight positive charge, while the oxygen atom in water has a slight negative charge. Just like your hair and a balloon stick to each other after you rub them together, the two molecules stick together. The result is a single molecule called carbonic acid- acid, because the hydrogen atoms are liable to fall off.

Reacting carbon dioxide with water produces carbonic acid.

As we pump more carbon dioxide into the air, we are indirectly pumping more carbon dioxide into the ocean and in the process we’re making it more acidic. This can have several ecological effects; the most obvious is on calcifying organisms like corals. When carbonic acid forms and releases hydrogen atoms, some of those hydrogens recombine with carbonate to form bicarbonate. Since corals’ exoskeletons are made of calcium carbonate, acidification poses a threat to them and the ecosystems that they harbor.

Atmospheric CO2 winds up in the ocean, where it undergoes a series of reactions. As more CO2 is added to the air, more flows into the ocean, where it forms carbonic acid. Some of that carbonic acid in turn loses a hydrogen ion to form bicarbonate. However, as hydrogen ions build up, they react with carbonate to turn it into bicarbonate- which is a problem if you're a sea creature using that carbonate to build shells.

Just as we can already detect changes in the global climate from increased atmospheric carbon dioxide, we are also starting to see changes in ocean chemistry. Below is a graph linking the two – on the top is a graph showing the increase in both atmospheric and oceanic carbon dioxide over time. The other graphs show the associated decrease in pH (pH is a measure of acidity of a solution – the lower the pH, the more acidic. pH is also logarithmic, which means that big changes look smaller than they are: a change of 0.1 pH changes the acidity by a factor of about 125%). The pH graphs, in orange and green, show measurements from successively deeper parts of the ocean, and we can see that the change is greatest closer to the surface, confirming that the change is due to changes in the atmosphere.

Measurements of carbon dioxide in the air and oceans (top) and the accompanying drop in pH. pH is a measure of acidity; keep in mind that more acidity means a smaller pH. Taken from Dore et al. 2009; click for the full paper.

The science of anthropogenic climate change has been politically and economically inconvenient for many people, and a cottage industry has popped up in trying to dismiss it. As I will discuss in the next few posts, we are also starting to see the political campaigns devoted to denying climate change turning their attentions to ocean acidification.