Arguments
OA not OK part 20: SUMMARY 2/2
Posted on 25 August 2011 by Doug Mackie
This is part 2 of 2 summarising out series about ocean acidification. The 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.
During July and August we posted an 18 part series to introduce the basics of ocean acidification chemistry. In this post we summarise posts 11-18. We have distilled each post to the bare minimum (100 words average). Naturally a lot has been lost; please refer to the original posts.
Part 11: Did we do it? Yes we did!
The International Energy Annual reports that globally, 603 billion tons of CO2 were released from 'consumption and flaring of fossil fuels' between 1980 and 2006 giving an expected change of 77 ppm CO2 in the atmosphere. BUT atmospheric CO2 increased by only 43 ppm. 270 billion tons of CO2 is missing.
Part 12: Christmas present
4 parameters describe the marine CO2 system. If you know any 2 you can calculate the other 2.
Total dissolved inorganic carbon (CO2, H2CO3, HCO3–, and CO32–), Total Alkalinity, pH, and CO2 partial pressure. The partial pressure of CO2 in the atmosphere changes seasonally as plants photosynthesize. Since pH is influenced by the partial pressure of CO2, the pH of seawater also shows seasonal fluctuations. Though the rate of change is not even around the world, all the data agree: ocean pH is decreasing.
Figure 6. pH recorded at the Hawaii Ocean Time Series (HOTS) station. Data here.
Part 13: Polymorphs: The son of Poseidon
Biological calcium carbonate comes in two main forms, or polymorphs: aragonite and calcite.
Aragonite is found in pteropods (small free swimming snails) and corals. Calcite is found coccolithophores and foraminifera. Molluscs can use either or both polymorphs.
Part 14: Going down
Photosynthesis in the ocean is confined to the upper 200 m or so where sunlight reaches. Below that depth falling organic matter gets eaten by bacteria (producing CO2). Making CO2 produces H3O+ via Eq. 7 & 8. Dissolution of calcium carbonate via Eq. 4 increases the pH slightly (by removing this acid).
As depth increases, the pH does not recover to the surface value because there is more respiration (producing acid) than shell dissolution (consuming acid). Thus, total dissolved carbon increases with depth (due to respiration) but carbonate decreases with depth because it is a function of pH which decreases with depth.
Figure 13. pH (at 25 deg C) as a function of depth in the Atlantic and Pacific oceans. Data .
Part 15: No accounting for taste
The common ion effect describes the way the dissolution of a salt is inhibited if the solvent already contains an ion in common with the salt (e.g. the solubility of CaCO3 decreases in water containing lots of Ca2+ ions). Similarly, dissolution is enhanced if the concentration of one of the ions, say carbonate, is low (e.g. the dissolution of CaCO3 increases in water if CO32- has been removed by an external factor).
Part 16: Omega
If Ω < 1 the solution is undersaturated and dissolution occurs. If Ω > 1 the solution is supersaturated and dissolution does not occur. For a line at Ω = 1, the depth at which this line crosses the actual Ω gives the depth below which aragonite and calcite dissolve. This is the saturation depth. A poetic image used is to compare Ω to a mountain snow line.
Figure 15. Depth profile of Ω for calcite and aragonite as a function of ocean basin.
Adding CO2 to the surface ocean will cause overall carbon to increase, but also causes the relative fraction of carbonate to decrease. In turn this decreases Ω so the profiles in Figure 15 to move to the left and the saturation horizon moves towards the surface.
Part 17: Pumping currents
Two processes move CO2 from the air-sea boundary into deep waters:
1. Biological pump: Photosynthesis transforms the CO2 into big organic particles that sink rapidly. As the particles fall to the bottom, they get eaten by bacteria, producing CO2 in deep waters.
2. Ocean circulation: The high volume but slow (100s-1000s of years) circulation called the thermohaline ocean circulation swaps surface water with deep water. Added CO2 remains in surface waters until the slow ocean circulation turns the surface waters over. This means the surface ocean is undergoing acidification to a much greater extent than the deep ocean.
Part 18 : Been this way before
We know CO2 levels have been high in the past. What makes this time different?
1. We are outside normal CO2 levels
During an ice age, CO2 is about 180 ppm; during an interglacial CO2 is about 280 ppm. Atmospheric CO2 is currently 394 ppm - 100 ppm above the normal recent maximum.
2. Changes are happening faster than normal.
At the end of an ice age, the 10oC temperature change and the 100 ppm CO2 change occur over 10,000-15,000 years. CO2 is currently increasing at >2 ppm per year – over 100 times faster than the glacial-interglacial cycle.
The ocean carbonate buffer system will not restore the ocean to a preindustrial state. Instead the ocean, and the climate of the Earth as a whole, will converge towards a new stable state. We still don't know precisely what that state will be.
That's all folks. An index with a 2 line description of each post should now greet you when you click the "OA not OK" button at the top left. The booklet will be available in a few days and will be announced in a very brief post.
Written by Doug Mackie, Christina McGraw, and Keith Hunter. This post is the final summary of 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.
Regards, Jos
No. I am sorry, but you have made a common mistake. In freshwater the Ksp values for calcite and aragonite are indeed on the order of 10-9. but in SEAWATER the values are, as given, on the order of 10-7.
Though the currently accepted values are little different, this paper explains things quite well. I like this paper because it has 3 of the biggest names (though I confess that in my opinion Wally's star has shined the brightest) in the business on one paper – sort of like the αβγ paper.
Now, it may be beyond the scope of what you set out to do but are there any plans to explore further implications, particularly the biological and food chain impacts, of what the projected range of increased acidification might produce?
I have heard it said many times that OA is the "sleeping gorilla" of fossil fuel based CO2 emissions and that, in some ways, it is more worrying than increased average temperatures and the associated weather pattern changes.
It is certainly seldom taked about in the press. Would welcome any comments from you or the moderators.
Yes, we have plans to return after a decent break with more.
Yes, it is seldom talked about. It may not have been obvious but most of the posts were written to rebut the "science" behind a particular denialist meme without actually detailing that meme. (Something of a turnaround from usualy SkS approach).
Denialist memes are like viruses (I know PZ Meyers will have at me for mixing evolutionary metaphors) and change their packaging slightly each time. Our science foundation approach should neutralise them all. Given this rationale, we will be focussing on observations more than projections.
do you have a reference for this?
average ocean pH has decreased by 0.11 pH units (from 8.25 to 8.14) since the industrial revolution and is on track to decrease by a further 0.3 units
Thank you
Tony
Orr, J. C., V. J. Fabry, O. Aumont, et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437(7059): 681-686.
But AR4 and the IPCC 2009 Compendium have more recent refernces.
thanks. just downloaded both Orr and IPCC 2009.
good reading for the week end. :+)
Tony
As I said, it's largely semantic, but when arguing with acidification deniers it can be quite pertinent...
If you stand at the South Pole, then walk 100 meters in any direction, you are still in the Southern hemisphere. But, you have indisputably moved 'northward'. And 'acidification' is the term used for a chemical mixture moving towards acidity.
More caffeine needed, I believe...
We had hoped that it was obvious that this meant "...as pure water at the same conditions of temperature etc" but we can see how people might not have appreciated this. Nevertheless we felt it was as concise a definition as we could give without (as we had said we would not do) mentioning activity or delving into the intricacies of the several pH scales used for seawater.
I suspect that most people, even lay folk, would probably easily appreciate that neutrality is the point of hydronium/hydroxide equivalence, and certainly it complicates a discussion by venturing away from standard conditions. I do think however that there is value in pointing out to those who might otherwise take it for granted that pH 7 (and its reflection of negative log of hydronium ion concentration) only coincides with neutrality under certain conditions.
What I probably didn't make as clear as I could have in post #11 is that under other conditions it is quite possible to have a pH value greater than 7, and yet have a solution that has a surfeit of hydronium ions over hydroxide ions. Such a solution will thus technically be 'acidic', and this even with a pH greater than the abritrary 7 that ocean-acidification deniers claim is required to be passed before the solution can be defined as being "acid" or "acidifying".
The overall thrust was to attempt to emphasise that the notions of acidity and acidification are in certain ways contextual, both with respect to the overall chemical conditions involved, and to changes relative to starting conditions. The number "7" is, in and of itself, not an absolute landmark in acid chemistry.
As I said, the point is largely a semantic one, but it does underscore how the whole idea of acidification is abused by those people who find the particular idea of ocean acidification inconvenient.
Perhaps my pedantry stems from the fact that I have dealt with too many chemistry distorters in the past, who have attempted to claim that the oceans aren't becoming more acid, but simply less basic - as if there is a fundamental difference in the context of the changing hydronium ion concentration of seawater...
It's enough to reduce to tears anyone with teritiary training in chemistry or biology...
Funnily enough, "skeptics" have a habit of complaining that uncertainty is not plainly revealed in the litterature.
Oh well. Compared to the recent Erl Happ fiasco, in which he was scolded by contributors for his gross igorance, and by Watts, for writing posts while drunk (and bragging about it), this one is rather tame. Voted best science blog, mind you.
First Goddard, now Happ - how many more before even Watts's friends admit the denial-reality of his website ?
Bernard: Perhaps your pedantry is up to the task? I still don't follow your concern. A definition of 'neutral' is not relevant to a definition of acidification. We thought we were careful in the first post to remind readers that acidification is an absolute increase in [H3O+] and not a relative change with respect to some 'neutral' point. (Yes, yes, actually activity not concentration but we also explained that we would avoid activities to not totally confuse readers).
I completely concur with your definition of 'acidification', and I put essentially the same definition to denialists when I ask them if an increase in the concentration of acid species is in fact acidification.
I emphasised my previous points simply to attempt to add to the over-all effort to stopper any rat-holes through which these acidification deniers might bolt. Frustratingly there are folk on Watts' Magnificent Monument to Ignorance who are this very week going back to square one and using the "not acidifying, just less alkalinity" canard again, and many other laughable notions as well, even after they've been directed to these pages on Skeptical Science. Indeed, there are some who have (apparently) read these posts, and claim them to be nonsense, though they never actually produce any science that explains why...
I guess it just goes to show that no matter in how many ways one tries to draw simple outlines for the ignorant, if they refuse to open their eyes they’re never going to get the picture...
I did take the time--ugh--to wadethrough that link w/Happ and am now even more delighted to be a part of this resource.Re: the Happ link and his website? Talk about a 'Dunning-Kruger bingo game!"
Thanks to all here.
I think I've just bumped into one of the concepts this series "skips lightly over", and I'm hoping you will kindly help me to crawl around it. The concept is buffer theory, and the reason I only think it's my obstacle is that I encountered it in this "simple description" of the ocean's importance for climate, written by an oceanographer:
"...it is also clear that the ocean’s capacity for carbon dioxide is limited, as indicated by its increasing acidity. This is most extraordinary since the ocean has always been considered to be strongly buffered against acids and alkalis alike;"
To me, a buffer is a space that separates two things for safety, as in "buffer zone" or a buffer in a schedule. After a bit of googling around I'm guessing in chemistry it's a process rather than a space and that buffer theory spells out how buffering mechanisms (mechanisms?) work. I also suspect it's such a fundamental concept that even a simple explanation requires a boatload of technical terminology, but I hope I'm wrong about that.
FYI the simple description is being written for a regular "climate column" we're doing for our local newspaper. Our readers' assumed level of knowledge is about year 8 school science 40 years ago. It's a great opportunity, but it would be very easy to scare the editor, never mind the readers.
I'm not asking you to write to the level we're aiming for, but I'd be very grateful for an explanation that makes sense to me when I'm translating for my oceanographer. Can you help?
Thanks for the series and for this site.
Susanne - I'll reply without using equations, but you really need to follow the equations to understand this in greater depth:
1. The notion of akali and acid, as it pertains to ocean acidification, is irrelevant. Ocean acidification is the process of increasing the hydronium ion concentration in seawater (lowering pH). It has nothing to do with the concept of neutral pH. The oceans will very likely remain above a pH of 7 (neutral), but can still be highly corrosive to marine life that make their shells/skeletons (calcifiers) from calcium carbonate (chalk). If someone drags up the issue of neutrality, or acids and akali, you can be certain they don't understand the fundamentals and are probably playing the troll.
2. The oceans become corrosive to marine calcifiers because increasing the atmospheric CO2 partial pressure forces more CO2 to dissove into the oceans and leads to a series of reactions which diminish the concentration (activity actually) of carbonate ions in seawater. These carbonate ions are one of the key building blocks of the calcium carbonate shell/skeleton. So, although pH (the concentration of hydronium ions) is critical for a number of biological chemical reactions, it's not lower pH per se that makes the oceans corrosive to marine calcifiers - it's the decline in carbonate ion activity.
3. Now on to buffering. The carbon chemistry of the oceans (the series of reactions between the various forms of carbon dissolved in seawater) means that the oceans buffer (act against) the lowering of pH. So yes the oceans are buffered to some extent. The equation in which a hydronium ion combines with carbonate to form bicarbonate is the one which buffers against ocean pH being even lower than it is, because the "extra" hydronium is no longer free in solution - it is now bound with the carbonate ion to form bicarbonate. If this reaction did not take place the hydronium concentration of seawater would be higher and, therefore pH lower. But this is the very same reaction which lowers the carbonate ion concentration in the oceans and makes them corrosive to marine calcifiers!
4. Two other buffering mechanisms exist, but they operate on such long timescales that they are of no use to humanity, or the marine life put at threat by ocean acidification. One is the supply of alkalinity back to the oceans through the chemical weathering of silicate rocks through rainfall. And the other is the dissolution of carbonate sediments on the ocean floor when the oceans become corrosive. These both operate on 10-100,000-year timescales, so won't be of any use to us.
5. This slow buffering is why the oceans became corrosive in Earth's past when CO2 increased in a geologically abrupt manner, but did not when the CO2 content of the atmosphere increased slowly, or was sustained at high levels for long geologic intervals - the system is able to readjust and move itself back toward an ocean condition which is suitable for marine life through these buffering processes. Perhaps the best example of this are the White Cliffs of Dover. These are huge chalk deposits formed by coccoliths (microscopic marine life that build their shells from calcium carbonate) during the Cretaceous - a time of very high atmospheric CO2 and much warmer global temperature. Because there was no geologically-abrupt spike in CO2 the oceans did not become corrosive, despite the low pH.
6. Perhaps the best way to skewer bogus claims about ocean acidification is show what is actually going on today - marine life is beginning to be eaten away by corrosive seawater. The pic below is of a pteropod (sea butterfly) captured from waters around Antarctica recently:
The pic is from this peer-reviewed paper: Extensive dissolution of live pteropods in the Southern Ocean - Bednarsek (2012).
Rob, Thank you very much. I didn't expect such a quick or detailed response, especially as I'm sure that explaining without equations is uncomfortable.
Thanks again.
You're welcome Susanne. I'm actually very comfortable explaining things without equations, I just thought if you remembered a couple of the basics it would stick in your memory, so that you would have the confidence to explain it to others.
I have quite a few posts on ocean acidification coming up over the next few months too, by the way.