As everybody knows (or should know !), man is now putting significant amounts of CO2 into the atmosphere, though our emissions are taking place into a natural carbon cycle much more complex, which includes exchanges between the atmosphere and the ocean.

Annual flows in 2000, and existing stocks of carbon, in gigatonnes (billion of tonnes) of carbon. Source : IPCC, 1996

The CO2 exchanges between the atmosphere and the surface ocean (roughly 90 billion tonnes of carbon each way per year on the above graph) do not owe much to the existence of a marine life : if we suppressed all fish and all whales, it would definitely generate some inconvenients, but not a slowdown of the CO2 exchanges between the atmosphere and the surface ocean ! These exchanges are essentially a consequence of the existence of large scale marine currents, that cool down water masses in some places (like the water of the Gulf Stream that shifts towards the North Pole), and heats up the surface ocean in some other places (for example the water of the Labrador Current that shifts away from the North Pole).

Indeed, CO2 dissolves better in cold water than in warm water (if one wonders why, the best answer will probably be "because that's how it is" !), so that where water is cooling, some CO2 goes from the atmosphere to the ocean, when where water is heating up, some CO2 goes from water to the air. This explains why, in a globally warming climate, the oceanic sink will have a tendancy to weaken : the absorption of CO2 will be lower with cooling water masses remaining a little warmer than before (on average), while the emissions of the warming water, that will reach temperatures a little higher, will be more important.

But once absorbed by seawater, this CO2 will not remain as such for the most part, no more than the CO2 "absorbed" by the continental ecsystems remains under the form of CO2. Once dissolved in seawater, part of the CO2 reats with water to form hydrogenocarbonate ions (once named bicarbonate ions), HCO3-, then carbonate ions, CO3--.

The successive reactions are thus as follows :

Actually, as any chemical reaction, these can happen either way (it depends on the initial conditions, and for given initial conditions there is of course a way which is privileged). One should then rather write :

Well, old memeories from the chemistry class might allow the reader to remember that a coumpound that "produces" H+ ions is named... an acid. What the above reaction means is that the dissolution of CO2 in sea water makes this water more acid. This property explains why, in old times, the CO2 was called "carbonic acid" (it is for example the expression used by Arrhenius in his premonitory article on how the industrial era would bring a global climate change).

What the chemical reaction above doesn't say, however, is that if we have more CO2 in the air, we will get more in the ocean. We are not facing chemistry here, but thermodynamics (ouch ! here are more and more barbarian expressions !). In other words, if we have more CO2 in the air, it "penetrates" in larger quantities in the underlying water.

From all that preceedes, one can thus deduct that if we increase the atmospheric CO2 concentration, what we are definitely doing right now, not only will we change the climate, but we will also make the ocean more acid.

pH change of the ocean depending on time and the atmospheric CO2 concentration (this concentration corresponds to the upper curve). This CO2 atmospheric concentration is expressed in parts per million, or ppm, knowing that 1 ppm = 1 millilitre per m3 (or 0,0001%) The evolution of this concentration - here it increases from 300 to 2000 ppm in 2,5 centuries - is of course an hypothesis, and one should note that here it jumps very (very !) high compared to the values in the emission scenarios generally used to assess the climatic future. In this last case, the hypotheses explored are rather in the range of 500-1000 ppm of CO2 in 2100. Source : Caldeira & Wickett, Nature, 2003

Intercomparison of the past pH changes of the surface ocean (same colour code than for the graph on the left). The "vertical" curves give the surface ocean pH loss depending on the rate of atmospheric CO2 increase. For example, an increase to 2000 ppm of the CO2 concentration in a couple million years generates a decrease of the pH limited to 0,5 unit, but if it happens in a couple centuries, the ocean will become more acid, losing 0,8 pH unit. A - variation of the acidity of the surface ocean between glacial ages and interglacial ages (such as today) : pH±0,1 in several thousand years B - variation of the acidity of the surface ocean during the last 300 million years : pH ±0,5 C - variation of the acidity of the surface ocean during the last century, as a result of the CO2 emissions already done : pH±0,1 D - possible variation of the pH as a result of future emissions, in the "high" case : with 1000 to 2000 ppm extra of CO2 in a couple of centuries, the ocean would loose 0,5 pH units or more. Such an evolution would be without precedent for million (tens of million ? Hundreds of million ?) years, because the rate of increase of the CO2 in the air would be unprecedented. Such an evolution - in several centuries - is not impossible if the continental CO2 sinks are turned into a source. Source Caldeira & Wickett, Nature, 2003

One can see on the aabove graph that with a CO2 concentration in the air that would reach 1000 ppm, which is definitely in the high end of the bracket of emission scenarios, the ocean acidity could increase by more than 0,5 pH unit (more exactely the pH would decrease by 0,5 unit). This would have significant consequences on the marine life, even with no associated climate change. As usual, some consequences can already be "theoretically" identified, and for others it will be a surprise, because nobody will have thought of them before they happen (which is normal when a situation is new).

Among the marine organisms that will feel a change of the acidity of the surface ocean, one will find corals, because these animals build up a skeletton made of lime, and the chemical reaction leading to the formation of calcite becomes harder to perform when the water gets more acid. May we get back to some simple chemical equations ? Then here is what normally happens when the coral builds up its skeletton :

In other terms, the coral is using calcium ions dissolved in seawater and combines them with carbonate ions to "produce" calcium carbonate, which is nothing else than the "scientific" name of .... lime.

We have seen above that the CO2 that dissolves into seawater generates some "descendants" that are hydrogenocarbonate (HCO3-) and carbonate (CO3--) ions, and that the respective proportions of each compound at equilibirum depends on the conditions at the time. In the tropics (where corals live), hydrogenocarbonate ions represent 85% of the dissolved carbon, and carbonate ions 15%. If the acidity increases, the respective proportions will change, and more precisely the share of the carbonates decreases (graph below).

Well it is only carbonates that are used by marine organisms that need to build up a shell or a limestone skeletton, and we can quite reasonnably consider that if carbonates are less available in seawater, all the concerned life will have a harder time. Indeed, laboratory experiments show that with a doubling of the CO2 compared to today, calcification (the "production" of limestone by living organisms that do so) slows down by 10 to 40%, and this applies both to corals and to the phytoplankton algae that have a shell.

What the above graph suggests is that if the ocean pH decrases by more than 0,8 units (turning the sea almost acid), we would make all marine life that needs to produce limestone very difficult, and that includes, apart from corals, all molluscs (oysters, mussels, etc), all crustaceans, a large fraction of the phytoplankton.... Hopefully, we are not there for the moment, and given the amount of CO2 in the air it requires to obtain such a drop of the pH we can resonnably consider that we will not be there any time soon, but if we cross the threshold of the terrestrial "carbon sink" reversal (and not that of the ocean), what might happen in several decades if we prolongate the trends, it is not impossible that such a drop of the pH happens much later on, making the ocean our remote descendants will face very different from what it is today....