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OA not OK part 9: Henry the 8th I was (*)

Posted on 23 July 2011 by Doug Mackie

This post is number 9 in a series about ocean acidification. Other posts: Introduction , 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.


Welcome to the 9th post in our series about ocean acidification. How does CO2 get into the ocean? We saw back in post 5 that the oceans contain 50 times more carbon than the atmosphere. Is it possible that instead the oceans are actually the source of CO2 in the atmosphere, not fossil fuels? The relevant equation (below) is the equilibrium of CO2 in the atmosphere (or gas phase) and CO2 absorbed by the ocean:

Equation 13

In post 5 with equations 7-9 we showed what can happen once CO2 is absorbed, but Eq. 13 (in the left to right direction) is the process of getting the CO2 into the seawater in the first place. The reverse process (right to left) is called degassing. Familiar examples of degassing are carbonated beverages, like beer and fizzy drinks, going flat as they release CO2 to the atmosphere. To follow and predict what happens we need to know some gas laws.

The first is Dalton's law of partial pressures. Dalton's law is that the total pressure exerted by a gas mixture is the sum of the pressure exerted by each component. For example the pressure exerted by a container of compressed air is the sum of the pressure exerted by the oxygen + the pressure exerted by the nitrogen + the pressure exerted by the argon + the pressure exerted by the carbon dioxide + all the other components. The pressure exerted by a given component is called its partial pressure.

The other gas law we need to know is Henry's law of gas solubility. Henry's law is sort of an extension of Dalton's and says that if you have a liquid in equilibrium with the gas above it, then the amount of gas that is absorbed in the liquid is directly proportional to the partial pressure of the gas. That is, if you take a vessel half full of water and double the amount of air by pressurising the air space, then twice as much of each component in the air is absorbed.

Henry's law

Henry's Law. The more molecules of a gas (i.e., the greater the partial pressure) then the more of that gas that dissolves in the water. In the headspace above the water the number of orange dots has increased from left to right. Henry's Law tells us that therefore the number of orange dots in the water also increases from left to right. (See also this clip at YouTube).

A Henry's Law coefficient, KH expresses the equilibrium ratio between the two sides of Eq. 13. However, a critical point to note is that the ocean is NOT in such equilibrium with the atmosphere. (We will return to this in a later post). Calculations are complicated because of biological activity and because; as we have seen in post 5, as soon as CO2 is absorbed then it forms carbonic acid. This disturbs the equilibrium in each of Equations 7-9, which in turn disturbs the equilibrium in Eq. 13. However these gas laws tell us that the concentration (really we should say partial pressure) of CO2 in both the atmosphere and the oceans are directly related. That is, a CO2 increase in the atmosphere causes a CO2 increase in the ocean.

Henry's Law is temperature dependent and warmer liquids hold less gas than colder liquids. This temperature dependence explains why a warm soda is much more likely to fizz out of the bottle than a cold soda and has large implications for regional differences in ocean acidification.

There is a huge range in temperature (and consequently CO2 partial pressure) throughout the ocean. For example, deep ocean water that has recently been in contact with the atmosphere contains more CO2 than surface water for two reasons. Firstly, it is colder so it can hold more CO2. Secondly, falling organic matter – fish poo and the like – gets eaten by bacteria as it falls to the bottom - releasing CO2 by respiration. Exchange of cold, CO2-rich deep ocean water and surface water happens through upwelling and downwelling. This can result in some ocean regions being sources of CO2 as deep water comes to the surface, warms up and degasses. Other regions are sinks of CO2 as warm water with low CO2 cools down, sucks up extra CO2 and sinks.

How can we be sure that ocean acidification is caused by CO2 in the atmosphere? In the next post, we'll discuss how we know that the extra CO2 in the atmosphere is not coming from a natural warming of the ocean.

*This was to be the 8th post in the series but we intercalated one and lost the original title for this post: "Henry the 8th I am".

Written by Doug Mackie, Christina McGraw , and Keith Hunter . This post is number 9 in a series about ocean acidification. Other posts: Introduction , 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.




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Comments 1 to 13:

  1. Correct me if I am wrong but I would have thought that the fizzing of carbonated water relates to the solubility (maximum dissolvable amount) of CO2 in water and not to the Henry's law equilibrium. The temperature dependence would be in the same direction but the actual relationship would be different.
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  2. So the next thing to find out is whether seawater is above or below the equilibrium amount in CO2. A nice global map would be nice. Seems to me that some in climate sceptic land have seized on the idea that the ocean is emitting CO2, but I suspect this idea is a matter of faith for them and not based on evidence.
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  3. LT @1: they amount to the same thing. Henry's Law describes the equilibrium solubility of a gas for a given atmospheric concentration. A fizzy drink is made by pressurizing the bottle with a high pressure of pure CO2. hence it has a high solubility. When the bottle is opened to the air, Henry's Law dictates that the concentration of CO2 is now too high too be in equilibrium with the new air space, so CO2 must escape from solution, hence bubbles. A related factor is that fizzy drinks are pressurized when cold to give a higher solubility. This is because the Henry's Law constant changes with temperature (as do all equilibrium constants).
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  4. Lazy teenager @2: Degree of disequilibrium depends on the set of K values - see previous post. Ocean as a source: see next post.
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  5. Doug Mackie Original Post #9 "How can we be sure that ocean acidification is caused by CO2 in the atmosphere? That is, how do we know that the extra CO2 in the atmosphere is not coming from a natural warming of the ocean?" Well - how can we?? Forgive my ignorance of higher chemistry - but do we really need 9+ posts to get a result on the subject of ocean acidification? I have tried to follow through some of your earlier posts -and they seem to be a set of draft lecture notes for a graduate course. Now that is fine in itself - but some editing and a set of conclusions would be in order for the interested layman. As a separate point - oceanic biological processes are mentioned above. It has been a criticism of others - Freeman Dyson for example - that biological and plant processes are not part of any of Hansen's AGW modelling (and most if not all other GCM) and are probably very significant players.
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  6. Ken wrote "Well - how can we??" (i) why not wait for the next article in the series (ii) the mass balance argument shows beyond doubt that the natural environment is a net carbon sink, and has been discussed many times on SkS this article also written by Doug Your comment on the style of Doug et al.'s contribution is rather churlish. A lot of effort has gone into these posts, and just becuase they are not tailored specifically to your requirements does not mean they are not suitable for the interested layman. Sadly it is a shame when eminent scientists like Dyson make such criticisms of science beyond their area of genuine expertise. It is generally an indication of Dunning-Kruger syndrome. Do you agree with Dyson that GCMs do a good job of modelling the fluid motion of the atmosphere and oceans?
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  7. Keith Hunter at 13:35 PM on 23 July, 2011 LT @1: they amount to the same thing. Henry's Law describes the equilibrium solubility of a gas for a given atmospheric concentration. --------- Are you sure? Henry's law only applies at low partial pressures. So at high partial pressure of CO2 what happens? Does the amount of CO2 dissolved in water reach a maximim limit in the same way that a dissolved solid reaches a limit?
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    Moderator Response: Given the current atmospheric concentration of CO2, your line of questioning is not relevant to ocean acidification. Please stay on topic. Thank you for your help in this. Doug
  8. Oh dear Ken. Haven't you been paying attention? The series will run to about 20 parts by the time we are done. Sit back and enjoy.
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  9. Ken Lambert @5. Skeptical Science - and other blogs - have plenty of good short posts on the impacts of OA. However, as we stated in the introduction to the series (and have mentioned several times since), the purpose of these posts is to give readers interested in the chemical and physical processes of OA the background information they need to understand the commentary on the web. We are certainly not introducing graduate-level concepts, but high school and 1st year university chemistry is helpful. Perhaps it was not clear that we would answer the How can we... question in the next post. The post has been modified to state this clearly.
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  10. I think perhaps Ken, like myself, is falling victim to the instant gratification that is so prevalent in modern society, so having to wait for the whole story to be revealed is almost excruciating... On the other hand, I quite like the pacing of the series of articles, and the bite-sized nature of each post. Are you planning to post an omnibus version when all the installments have been published, or let it stand as linked posts? Re the overall question - well, I think there have been enough studies published on the matter that it's pretty clear the oceans are a net sink of CO2 - and that this series is about explain just why that is so, rather than merely asserting it.
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  11. Bern, I hope you are not too much like Ken - take a minute to search out some previous comments by Ken (those that moderators did not delete). At the end a booklet with added bonus material, a centrefold widescreen version of figures 13 and 14, and discount coupons will be published here as a free download. Yes, as we said at the outset, this series is intended to allow readers to understand some of the more technical posts here and elsewhere. We saw phrases like "carbonate compensation depth" bandied around and we felt that few would have been able to actually explain or derive it.
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  12. Doug, no, my opinions on climate change are almost diametrically opposed to Ken's. :-)
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  13. second summary post

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