Yes, indeed, that statement is true. About 50 million years ago, for example, CO2 levels were about 1,500 ppm, almost four times higher than today, and as a result our oceans were much more acidic.
Since the Deniers of AGW repeat these statements often, it is important to know what the appropriate responses are. That response is relatively easy to provide if the Denier is referring to the effect of CO2 on our climate. One simply has to point out that the world was very much warmer 50 million years ago precisely because of that higher level of CO2 in the atmosphere. It was so warm, in fact, that the world had no ice on it and sea levels were about 70 meters higher than today – thereby flooding a large fraction of today’s land masses on which billions of people live.
But there is another closely related claim that the Deniers of CO2’s detrimental effects make today that is more difficult to respond to. Environmentalists now correctly claim that our increased atmospheric CO2 levels are making our oceans more acidic and that this will be very detrimental to critters in the sea that have shells made of calcium carbonate (CaCO3). This is because CaCO3 is more soluble in water of higher acidity and this will put greater stress on critters with CaCO3 shells. If this claim is correct (which it is) how then does one respond to the Denier’s retort that “if CO2 levels were four times higher 50 million years ago and the oceans were therefore much more acidic than today, why then were ocean critters, including those with CaCO3 shells, so prolific back then (as we also know from the fossil record)? That question is a tougher one to answer and is more likely to produce a “gotcha” moment if the issue is not well understood. Therefore, the rest of this post will provide a response to that potential show-stopper that will hopefully be useful whenever the question comes up.
First, let’s consider the three equilibrium reactions shown below involving atmospheric CO2 and the solid CaCO3(s) of critter shells. Note that the term “equilibrium reactions” means that the reactions shown go both ways, forward and backward. The rates of these three reactions are relatively fast in both directions resulting in an equilibrium state that responds quickly to any changes in the system – such as the addition or removal of any of the species shown.
CO2 (atm) + H2O = H2CO3 (1)
H2CO3 + CO3 = = 2 HCO3– (2)
Ca++ + CO3= = CaCO3(s) (3)
In accordance with Reaction 1, as the level of CO2 rises in our atmosphere, more of it will be dissolved into the oceans – where it is instantly changed to H2CO3 by the addition of one water molecule to that CO2 molecule. This makes the oceans more acidic because H2CO3 is a weak acid.
The ocean naturally contains H2CO3 and the two other acid-base forms of this species shown in Reaction 2. The most abundant of these is the bicarbonate ion HCO3– which is an acid-base neutral species. Another is the weak base, carbonate ion CO3=. Upon being added to the ocean, H2CO3 will react rapidly with the CO3= ion to form more HCO3– at the expense of CO3= as shown by Reaction 2.
Reaction 3 shows how solid CaCO3 is formed by the combination of calcium ions, Ca++, and CO3= ions. As the concentration of CO3= is reduced by Reaction 2, the formation and retention of a calcium carbonate shell (CaCO3) by Reaction 3 will be made less favorable and, therefore, will stress any critter that requires such a shell. Thus, the combination of Reactions 1 – 3 supports the notion that shell-bearing critters should not have existed in the very acidic oceans of 50 million years ago, as claimed by the Deniers.
But wait! There is more and the Deniers are also wrong on this one. Events that occur over a relatively short period of time (such as over a several years or decades) can differ greatly from those occurring over a relatively long period of time (such as a few millennia) because in the natural world other much slower reactions have determined the final equilibrium states.
The most important of these slow processes is called CO2 “weathering” which occurs in raindrops as they fall from the sky and land on rocky surfaces that contain various inorganic calcium and silicon compounds including solid CaCO3 (s). In a raindrop, Reaction 4 is the same process as shown by Reaction 1 above except that in this case, CO2 is being absorbed into a rain drop consisting of pure water instead of into the pH-buffered ocean. Therefore, the H2CO3 molecules thereby formed are not substantially changed by a Reaction 2.
CO2 (atm) + H2O = H2CO3 (4)
H2CO3 + CaCO3 (s) = Ca++ + 2HCO3– (very slow) (5)
Then, by the very slow process shown as Reaction 5, H2CO3 in the rain water reacts with inorganic components of the rock (such as CaCO3) to form Ca++ and bicarbonate ions which are eventually carried to the oceans. Given enough time, this process adds enough calcium ions to facilitate the formation of CaCO3 by Reaction 3 previously shown above. In addition, the amount of CO3= is then also increased somewhat by the reverse of Reaction 2 – also facilitating the formation of CaCO3(s). It should also be noted that Reaction 5 would have been much faster in the warmer world of 50 million years ago when much more water vapor was in the atmosphere resulting in much more rainfall than today.
All of this taken together shows that, yes, increased acidification of the oceans will stress CaCO3-shelled critters in the sea – but only if a change in atmospheric CO2 levels occurs relatively rapidly – as it now is. On the other hand if those changes occur over a long period of time – such as over several millennia or millions of years – the oceans will still get more acidic but the formation of CaCO3 shells will then be facilitated by the slow weathering process shown by Reactions 4 and 5. These two slow reactions will then become the dominant means of providing both Ca++ and CO3= ions to the sea, instead of the dissolution of CaCO3 (s), thereby providing sea critters with an efficient means of making their shells.
Hopefully, this post helps explain why sea critters prospered in geologically ancient times when the Earth did not have forces acting on it that changed background CO2 levels so quickly as mankind is presently doing. Therefore, one simple response to the second question of the Denier posed above is “we are doing some things to both our atmosphere and our oceans much too rapidly as to allow Mother Nature’s natural corrective processes to keep up”. Therefore, the chemical system determining CaCO3 solubility in our oceans is momentarily out of balance and our shell-bearing critters are paying the price.