Ocean science questions ANSWERED! post 2 - Ocean Acidification

      I wanted a little exercise to help me study for my comprehensive exams in my PhD program in Marine Science.  So i asked you all to write to me with your burning ocean science question, and i will answer right away, in practice for the exam.  Here are some of the q's and a's.

    Q:   What's the deal with ocean acidification?  

      (original question: How has human-caused greenhouse gas build-up affected the acidification of the seas--or is it vice-versa? or separate events convergent? - Qayyum)

A:  Ocean acidification is the process whereby the ocean becomes more acidic with increasing carbon dioxide (CO2) in the atmosphere.  Ocean acidification is an impact to the carbon system, or carbon cycle.  CO2 is the most prevalent greenhouse gas; that is, it’s the one found in highest concentrations in the atmosphere as compared to the others.  The ocean is the largest sink (sequestration site) of CO2, for any entity in continual contact with the atmosphere; the ocean captures and stores about 1/3 of the human-produced carbon dioxide today. This is due to phytoplankton taking CO2 in and eventually sinking (in one form or another) to the bottom and becoming sediment.  There is this magnificent cycle of carbon going on – CO2 exchanges with seawater due to diffusion and photosynthesis via phytoplankton (algae).  In order to understand the details, you need to understand some chemistry.  First lets discuss pH:  pH is a measure of hydrogen ions.  Hydrogen ions are sort of unusual, because they are positively charged and “want” to bond with other things.  The activity of the hydrogen ions in the water is pH, or acidity.  More hydrogen, more acidity.  Now lets get back to CO2.  Once in the seawater, CO2 goes through several different reactions interacting with water (H2O), and in each reaction hydrogen ions are produced.  So on a very basic level, more CO2 coming into the water will mean more acidity.  Its complicated by the remainder of the carbon cycle and the sequestration of carbon mentioned earlier.  In order for pH to stay the same, with increased CO2 in the water, there would need to be increased sequestration (input = output).  Sequestration is accomplished by plants and animals incorporating the CO2 into their bodies in the form of calcium carbonate.  The plants/animals die, and the calcium carbonate sinks and becomes sediment, sequestering that carbon in the sediment.  SO, one problem is that some of the organisms that create calcium carbonate structures in their bodies have a harder time doing that when there’s more CO2 in the water.  Furthermore, there is less deposition of calcium carbonate to the bottom, if there is more CO2 in the water.  This is all to say that there are several feedback loops that make more CO2 in the water feedback on itself and fuel more and more acidity.  There is much more we need to know about this process, and how increased acidity might affect organisms, AND the process of changing pH over the long term is VERY slow.  It makes the fact that we are seeing a change in pH at all in the ocean very troubling, and urgent in terms of understanding more about what’s going on.