In the past, I have responded to questions at talks I have given on ocean acidification with an “I don’t know.”  I hadn’t studied the theory and didn’t want to knee-jerk respond with skepticism just because the theory came from people who propounded a number of other theories I knew to be BS.

The theory is that increased atmospheric CO2 will result in increasing amounts of CO2 being dissolved .  That CO2 when in solution with water forms carbonic acid.  And that acidic water can dissolve the shells of shellfish.  They have tested this by dumping acid in sea water and doing so has had a negative effect on shellfish.

This is one of those logic chains that seems logical on its face, and is certainly scientific enough sounding to fool the typical journalist or concerned Hollywood star.  But the chemistry just doesn’t work this way.   This is the simplest explanation I have found, but I will take a shot at summarizing the key problem.

It is helpful to work backwards through this proposition.  First, what is it about acidic water  — actually not acidic, but “more neutral” water, since sea water is alkaline  — that causes harm to the shells of sea critters?   H⁺ ions in solution from the acid combine with calcium carbonate in the shells, removing mass from the shell and “dissolving” the shall.  When we say an acid “eats” or “etches” something, a similar reaction is occurring between H⁺ ion and the item being “dissolved”.

So pouring a beaker of acid into a bucket of sea water increases the free H⁺ ions and hurts the shells.  And if you do exactly that – put acid in seawater in an experiment – I am sure you would get exactly that result.

Now, you may be expecting me to argue that there is a lot of sea water and the net effect of trace CO2 in the atmosphere would not affect the pH much, especially since seawater starts pretty alkaline.  And I probably could argue this, but there is a better argument and I am embarrassed that I never saw it before.

Here is the key:  When CO2 dissolves in water, we are NOT adding acid to the water.  The analog of pouring acid into the water is a false one.  What we are doing is adding CO2 to the water, which combines with water molecules to form carbonic acid.  This is not the same as adding acid to the water, because the H+ ions we are worried about are already there in the water.  We are not adding any more.  In fact, one can argue that increasing the CO2 in the water “soaks up” H+ ions into carbonic acid and by doing so shifts the balance  so that in fact less calcium carbonate will be removed from shells.    As a result, as the link above cites,

As a matter of fact, calcium carbonate dissolves in alkaline seawater (pH 8.2) 15 times faster than in pure water (pH 7.0), so it is silly, meaningless nonsense to focus on pH.

Unsurprisingly, for those familiar with  climate, the chemistry of sea water is really complex and it is not entirely accurate to isolate these chemistries absent other effects, but the net finding is that CO2 induced thinning of sea shells seems to be based on a silly view of chemistry.

Am I missing something?  I am new to this area of the CO2 question, and would welcome feedback.