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
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)
H2CO3 ⇔ 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?
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 - http://commons.wikimedia.org/wiki/File:Carbonate_system_of_seawater.svg#/media/File:Carbonate_system_of_seawater.svg
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:-
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:-
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.
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