ericgrimsrud

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.