Monday 1 June 2015

Flawed Arguments on Calcification.

By Pat Hackett.

Not only may life not be able to evolve fast enough in the oceans to keep up with the rate that carbon dioxide is being dissolved into the oceans today, but at a more fundamental level the chemistry of the oceans cannot keep up. Normally the addition of appropriate ions from silicate weathering would keep up with the CO2 dissolved in the oceans through volcanic activity.

By using Le Chatelier’s principle carefully (and a knowledge of the existing chemistry of the oceans) one can see that the addition of calcium ions or carbonate ions or bicarbonate ions through silicate weathering will reduce the effects of CO2 dissolving in the oceans.

The reason that CO2 reduces carbonate ions but increases bicarbonate ions is because the percentage of hydrogen ions is increased at a faster rate than the percentage of bicarbonate ions when CO2 dissolves in water and this in turn is because of the existing chemistry of the oceans.

(A naive use of Le Chatelier’s principle could show that bicarbonate ions could be either reduced or increased by the addition of CO2.)

There is much evidence that increasing levels of CO2 are causing our oceans to become acidic and this is detrimental to coral reefs and some life forms in our oceans such as shell fish certain algae and foraminifera.  In particular organisms that require to produce calcium carbonate,CaCO3, for shell production are at risk. You can find a lot of evidence on this by a simple internet search that will give you information and the risks to life and ultimately to supplies of food from fisheries around the world that are at peril.
Unfortunately there are some that present misinformation on this by using an over simplification of the chemistry of sea water and reach meaningless or false conclusions. The purpose of this post it to show the flaws in these arguments that could lead to the conclusion there is no problem of adding billions of extra tonnes of CO2 to sea water that come from mainly our combustion of fossil fuels. A major consequence of this additional CO2 is the rate at which available carbonate ions are removed from sea water.
First here is some information on ocean acidification:-

“Numerous marine organisms such as corals, mollusks, crustaceans and sea urchins rely on carbonate ions to form their calcareous shells or skeletons in a process known as calcification.”
 Further examples of the causes and impacts of ocean acidification from Denial101x- Making Sense of Climate Science Denial can be viewed here:-

Coral bleaching and Acidification. May 25, 2015

Flawed Arguments
Contrarian viewpoints not only just argue over the semantics of the term acidification but some actually argue that adding CO2 will make more carbonic ions in the oceans making calcification easier for shell fish.

This link is such an example

There are a number of things to observe from this flawed argument.
  • There is the meaningless semantic criticism of the term acidification used instead of the term pH.
  • There is the inference made that if our oceans where moving towards a PH of 7 that would be fine because that is the pH of neutral tap water. Few would argue that life in the oceans would flourish with no consequences if the oceans became or moved towards neutral. However it is not the actual pH value that is of most concern but more the rate of change involved. Not only does the rate of change make life difficult for species at risk to adapt but the rapid rate of change makes it difficult for the chemistry of the oceans to maintain an adequate stability. Undoubtedly as the chemistry of the ocean changes some species will benefit and others will not. There will be changes that will require species to adapt, if they are indeed able at the fast rate our oceans are presently changing. The false sense of security that may be implied with a move towards a neutral pH is complete wishful thinking.
  • This flawed proposal is made:- “any increase in dissolved carbon dioxide simply pushes the reactions towards the production of more bicarbonate and more carbonate ions.”
  •  Finally an oversimplification is made regarding shells being made from bicarbonate ions without considering the effects of the decreasing pH due to the increase in H+ ions. 
Addressing the flaws
In this description I will ignore the quantitative aspects and the energy involved in the reactions by living organisms.{Living organisms can use (otherwise valuable) energy to remove the increased H+ ions and thus make carbonate ions but here I focus on the chemistry alone}

I will consider a simple qualitative explanation of the relevant equations to address the counter arguments that I discussed above, with full realization that a quantitative explanation is far more complicated. I will do this in three stages:-
a). How do the oceans become more acidic with extra CO2?
b. How is calcium carbonate made in the oceans?
c). How can adding extra carbon to the oceans make it harder for organisms to make calcium carbonate or experience reduced calcification or enhanced dissolution when exposed to elevated CO2? This may seem counter-intuitive.

1. Acidifying the oceans.
  So what happens when we have more carbon dioxide in our atmosphere?

CO2(g)  ⇔ CO2(aq),                        (1)

CO2(aq)+ H2O ⇔  H2CO3,             (2)

H2CO⇔ H+ + HCO3-,               (3)

Equation 1 tells us the obvious consequence that more CO2 will dissolve in the sea water. Equation 2 tells us that this will produce carbonic acid and equation 3 tells us that this can dissociate into hydrogen and bicarbonate  ions, H+ and HCO3-. The double arrow tells us that the equations work both ways reaching an equilibrium value. They are in fact reversible.

Le Chatelier’s principle:-
If a change is made to the concentration of a quantity the reactions move in the direction to reduce that change. Hence if more CO2 is added to the atmosphere equation1 moves to the right (meaning the amount of molecules moving into the oceans increase and /or the amount of molecules moving out of the oceans decreases) which means equations 2 and 3 move to the right also.
An important point here as I will discuss later is that we have produced an equal number of hydrogen and bicarbonate ions (H+ and HCO3- ions) (from these reactions alone). Now the acidity of the ocean has been increased, or the pH has been reduced, by the addition of H+ ions. The concentration of H+ ions quantifies how much the acidity has been increased (or the alkalinity reduced).

2. Making Calcium Carbonate.
As well as hydrogen and bicarbonate ions the ocean contains dissolved calcium and carbonate ions obtained from natural rock weathering

Ca2+ + CO32- ⇔ CaCO3(aq)           (4)

CaCO3(aq) ⇔ CaCO3(s)                             (5)

These simple equations tell us that to make CaCO3 we need carbonate ions. Now these equations are also reversible. Appling Le Chatelier’s principle a reduction in carbonate ions in the oceans would mean equation 4 would move to the left decreasing CaCO3(aq) and thus equation 5 would move to the left enhancing dissolution of CaCO3(s). This is the overall effect of adding extra CO2 it decreases the number of carbonate ions in the ocean (as will be explained).

3. Why does adding CO2 reduce the carbonate ions?
We have to consider the relevant concentration of ions that are in the sea water. See figure 1.

Figure 1
"Carbonate system of seawater" by Karbonatsystem_Meerwasser_de.svg: User:BeArderivative work: Meiyuchang (talk) - Karbonatsystem_Meerwasser_de.svg. Licensed under Public Domain via Wikimedia Commons -
Figure 1 shows that at the current pH of sea water we have more bicarbonate ions than carbonate ions and if the pH were to reduce the bicarbonate ions would increase further but the carbonate ions would decrease. Why should this happen?
 Consider the following possible reaction between bicarbonate and carbonate ions,
  which is also reversible:-

HCO3-  ⇔ H+ + CO32-                (6)

The important question here is which way does this equation move in sea water as CO2 is added to our current oceans? Does it move to the left as stated by the Royal Society, Ocean acidification and virtually all oceanography scientific organizations or does it move to the right as stated by the contrarian viewpoint above?
If the H+ ions alone are increased it will move to the left but if the bicarbonate ions alone are increased it will move to the right. However both these ions are increased in equal numbers when CO2 initially dissolves in sea water as seen from equations 1 to 3 above.
Sea water is alkaline with a pH a little over 8 which means it has relatively few H+ ions compared to bicarbonate ions. The dissolved CO2 added extra H+ and bicarbonate in equal numbers. However the concentration of H+ ions will have increased in a much larger % due to the relatively small concentration in alkaline waters. This is the crucial factor.

When CO2 dissolved in seawater it produced hydrogen ions and bicarbonate ions. The bicarbonate ions will not(on average) dissociate into the ions as described in equation 6 because the percentage concentration of H+ ions have been increased from equation 3 more than the bicarbonate concentration. Instead the equation moves to the left removing carbonate ions, removing some of the extra H+ ions from the existing oceans (but not all the extra H+ ions otherwise the pH would not be declining) and increasing the bicarbonate ions. This will mean in turn equations 4 and 5 move to the left decreasing the CaCO3.

The fault made by the contrarian is assuming that because there is a lot of bicarbonate ions in the ocean this will move equation 6 to the right. On its own this may be true but this oversimplification ignores the fact that H+ ions have increased more in terms of percentage. However these bicarbonate ions are already in the ocean so the addition of CO2 does not affect the percentage much, and on the other hand, because the oceans are alkaline with a low concentration of H+ ions it is the addition of extra H+ ions that pushes the reaction to the left.

Adding about 30 G tonnes per year of CO2 to the atmosphere results in nearly 10G tonnes per year of CO2 going into the ocean. (This is extra CO2 over and above the naturally produced CO2 from volcanic activity of around 0.5 G tonnes per year).
 To reverse the effects from this we can see from equation 4 that this could be achieved by adding calcium or carbonate ions to the oceans. (Or indeed bicarbonate ions from weathering that come along with calcium ions, not H+ ions when CO2 dissolves in water) This is naturally achieved by weathering or rocks which roughly keeps up with the volcanic activity of the production as in the long term carbon cycle. In other words, the natural weathering in our present atmosphere can only balance somewhere around 0.5Gtonnes/year of CO2 that is naturally produced. We can see the removal of carbonate ions from the oceans, due to the rapid increase in CO2 and consequential rapid increase in H+ ions, far exceeds the addition of carbonate ions achieved via weathering. This means that to restore the oceans from any further imbalance may take 1000’s of years.

Further viewing:-


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