Guest essay by James R. Barrante, Ph.D.
Emeritus Professor of Physical Chemistry, Southern Connecticut State University, New Haven, CT
It is well-known that ocean pH has decreased from approximately 8.2 pH units to 8.1 pH units in the last 150 years. One explanation is that the increase in atmospheric carbon dioxide from the burning of fossil fuels is responsible. This is based on a law of physical chemistry, known as Henry’s Law, that states: if the partial pressure of a gas over a solution is increased, the concentration of the dissolved gas in the solution will increase. Since dissolved CO2 is a weak acid, one would expect the pH of the oceans to decrease.
Describing Henry’s Law in this simple manner, however, is a very narrow interpretation of the boundaries of the system. For example, the above statement of Henry’s Law is only valid, if the temperature of the solution is constant. Moreover, using pH to describe the acidity or alkalinity of a solution can be misleading. The pH of a pure water solution (neither acidic nor alkaline) is 7.00 at 298.2 K. Increase the temperature of the water and the pH will drop to below 7.00. Does this mean that simply raising the temperature of water will cause water to become “acidic?” That would be a ridiculous interpretation of pH.
Thermodynamic equilibrium constants are a sensitive function of temperature. We know that if the temperature of an aqueous solution of a gas increases, the solubility of that gas has to decrease, not increase. Moreover, if the ionic strength of the solution is high, describing equilibrium constant equations in terms of concentration will produce, at best, only approximate results. Activities must be used. Ocean chemistry is complex, involving a number of important equilibria, that include the dissociation of carbonic acid (dissolved CO2), the buffering equilibria due to the presents of dissolved salts of bicarbonate and carbonate, the solubility of the sparingly soluble salt CaCO3, and the equilibrium between dissolved CO2 and the partial pressure of carbon dioxide in the atmosphere. Luckily, the temperature dependence of these equilibrium constants has been highly studied. A typical example of each is given below.
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; 
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Ksp (weighted average for calcite and aragonite) 
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The pH of seawater (ionic strength of approximately 0.7) can be determined at any temperature a modified form of the Henderson-Hasselbalch equation for buffer systems (derived in my blog: climaterx.wordpress.com., post OAII). For reasons described in the post, the activity of calcium ion in the oceans is assumed to be constant.
To test this equation, consider the following:
Preindustrial pH: PCO2 = 0.000280 atm
aCa++ = 0.00123 (described in blog)
T = 288.2 K
pK1 = 6.4149; pK2 = 10.4202
kH = 22.24 atm/C
Ksp = 6.05 x 10-9 (average calcite/aragonite)
pH = 8.214 (very close to published value)
Post-industrial pH: PCO2 = 0.000380 atm
aCa++ = 0.00123 (described in blog)
T = 290.2 K
pK1 = 6.4105; pK2 = 10.4063
kH = 23.56 atm/C
Ksp = 5.718 x 10-9 (average calcite/aragonite)
pH = 8.138 (very close to published value)
We find from similar calculations that a 2-degree C increase in temperature can lower ocean pH by approximately 0.05 pH units. The temperature range taken in this study was ocean SST at the Equator, approximately 305 K, to ocean SST at the poles, approximately 273 K. This alone with play havoc with pH measurements. Moreover, ocean temperature ranges in bands containing water of different surface areas and land masses running parallel to the Equator. Simply recording ocean temperature at various points over the ocean and averaging them to get a number has no useful scientific meaning, no more than the average diameter of a football tells us anything about the shape of a football. At best, a weighted average must be used. In fact, if the measurement of ocean pH were not so complicated, and we had that data for the last 150 years, I would bet that we could show exactly that the increase in atmospheric CO2 from 280 ppmv to 380 ppmv in the last 150 years is an ocean temperature effect and not at all related to burning fossil fuels.
The graph below is a three-dimensional representation of P-pH-T data, similar to PVT graphs. The two-dimensional dependence of pH on either CO2 pressure or ocean surface temperature can be followed by looking at various isotherms or isobars; however, be sure to follow the graphs back to their original axes. For convenience, pH values at the intersection of these curves are given. It is obvious that to assume the drop in alkalinity of such a highly buffered system as our oceans is due to an uptake of atmospheric CO2 without taking the increase in ocean temperature, particularly in the Northern Hemisphere, into account is a very narrow interpretation of the science.
Temperature and Pressure Behavior of Ocean pH
Note: the word “only” was added to the title for clarity – Anthony
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Maybe the alarmists should go look at lake Natron in Tanzania where water gets as hot as 140f. It literally calcifies flamingos and other animals.
The lake is fed by hot springs of mineral rich water, and no doubt oceanic vents also release mineral rich water. We have no real clue about this issue of OA. Oceanic crust adds dolomite and other dissolved minerals to the water as it is filtered, this water transfers heat from the crust too, nice hot mineral absorbing water, another part of the story.
It takes a lot of mineral input to keep ocean seas at 8.1 pH, given there is no single larger buffer substance to regulate ocean pH then one would assume the constant mineral addition keeps pH up, the battle is always to stop water returning to neutral which it is inclined to do if minerals are taken out.
The CO2 cycle may just keep ocean waters at a pH range, and lack of said CO2 could mean pH too high for most ocean life as it has currently evolved.
it could be that ocean carbon capture geoengineering could raise pH to dangerous levels, creating an imbalance between gas and mineral input that life has evolved in.
There is far more mineral in the oceans than CO2, my bet is CO2 has a pH limiting function rather than driving a pH drop.
First, Dr. Barrante, thanks for your essay. Always glad to see new guest authors.
However, you start out very badly when you say:
This is nonsense. We have so few measurements in the last 50 years that the change is not even statistically significant. 150 years ago we had doodley-squat in the way of pH measurements. Heck, the very concept of pH was only developed in 1909.
Sorry, amigo, but when you start out your discussion with what is obviously a very slanted view of your subject, and you are willing to twist the facts to suit your view, I lose interest …
In addition, it is not clear which pH you are discussing. There are three pH scales for seawater—”total” pH, “free” pH, and “seawater scale” pH … which one are you using?
Next, you make an extraordinary claim when you say:
Me, I’d bet that is not true. The temperature of the planet has not warmed anywhere near enough for the change in atmospheric CO2 to be a result of outgassing from ocean warming.
Finally, I busted out laughing when you claimed that the “preindustrial temperature” was 288.2°C, and the “postindustrial temperature” was 290.2°C … how on earth would you know the pre-industrial temperature to a tenth of a degree, and what time spans are you talking about?
And this was not the only howler. You claim to know all kinds of variables about the ocean in pre-industrial times, pK1 and pK2 among others … really? Unless you have a time machine, you’re just pulling numbers out of the air.
Sorry, Dr. B., but there are far too many holes in your claims for them to be valuable.
w.
And this was not the only howler. You claim to know all kinds of variables about the ocean in pre-industrial times, pK1 and pK2 among others … really? Unless you have a time machine, you’re just pulling numbers out of the air.
pK1 and pK2 are measures of the equilibrium constants of the deprotonation reactions, as such they are thermodynamic constants and are the same now as the always were.
Phil. March 21, 2016 at 2:18 pm Edit
Not according to Dr. Barrente,viz:
w.
Willis: ln(K)= -ΔGº/RT
So pK = -ΔGº/RT
ΔGº is the free energy change of the reaction and R is the universal gas constant and T is the absolute temperature.
Consequently the differences are due to the small difference in the assumed temperature, the constants are unchanged.
“The temperature of the planet has not warmed anywhere near enough for the change in atmospheric CO2 to be a result of outgassing from ocean warming.”
If the ocean were a static pool of water. It isn’t.
The very name of the Thermohaline-Circulation tells you it is temperature dependent. And, when you have a continuous transport process which is modulated by temperature, you end up having properties for which the rate of change depends on temperature. Like this:
http://i1136.photobucket.com/albums/n488/Bartemis/temp-CO2-long.jpg_zpsszsfkb5h.png
Bart,
As discussed already last time, the MOC/THC is 70% driven by wind, 30% by temperature, where the main sink rate is caused by freezing, not by a small change in temperature of the waters themselves.
Your graph shows that most of the variability is caused by temperature. As that is mainly a proven transient response of vegetation, but vegetation is a net sink over periods longer than 3 years, it is not the cause of the trend, which is from a separate process. Oceans are a minor cause, as these can supply not more than 16 ppmv/°C. Human emissions were twice as high as the measured increase. Thus the most probable cause. Here the same graph where all variability is caused by temperature and ~90% of the slope by human emissions, ~10% by temperature:
http://www.ferdinand-engelbeen.be/klimaat/klim_img/had_co2_emiss_nat_deriv.jpg
This is just a contrived similarity, which requires a great many assumptions and fine tuning. Occam’s Razor comes firmly down on the side that the rate of change of CO2 is simply proportional to appropriately baselined temperature anomaly.
Bart,
The only assumptions I have made is that most short term CO2 rate of change variability is temperature driven, what you assume too. The second assumption is that the sink rate is linearly proportional to the increased pressure in the atmosphere above the steady state level for the average surface temperature. Which is proven by the similar average ΔpCO2 / net sink ratio over the full 55 years. The third is that natural CO2 levels follow Henry’s law for the solubility of CO2 in seawater. No “fine tuning” necessary at all, only three very basic assumptions. The combination of these three gives the same fit as your completely arbitrary match of the two slopes, which either gives a mismatch in amplitudes or a mismatch in slopes…
Your “match” violates:
– Henry’s law for the solubility of CO2 in seawater for the current area weighted ocean surface temperature.
– The observed area weighted ΔpCO2 which is 7 μatm higher in the atmosphere than in the ocean surface.
– The increase of CO2/derivatives in the ocean surface with decreasing pH everywhere longer time series were taken.
– The 13C/12C ratio decline in atmosphere and ocean surface.
– The oxygen and 13C/12C balances which prove that the short term variability is caused by the reaction of the biosphere (mainly the Amazon) on short term temperature variability, but not the cause of the long term increase in CO2: vegetation is a net, growing sink for CO2. The earth is greening.
Violating only one observation destroys the finest theory. Violating every available observation doesn’t even convince you that your theory may be wrong…
The short term transient response of CO2 to temperature is 4-5 ppmv/°C. The long-term process that increases the CO2 levels in the atmosphere according to you gets over 100 ppmv/°C (while Henry’s law says 16 ppmv/°C). If you think that is from the same process, then you have a remarkable view on transient processes…
Willis Eschenbach – March 21, 2016 at 12:43 pm
The “temperature of the planet”, ….. HUH, …… as if someone actually knew what that was?
A better question is, ……. how many degrees has the ocean waters “warmed” after being subjected to 500+ years (from about 1300 to about 1850) of the extremely cold near-surface temperatures of the Little Ice Age?
Anyone who would base their argument on the accuracy of the Historical Temperature Record(s) …… has no basis for a scientific argument to begin with.
Anyone that thinks or truly believes that the UAH near-surface global average temperatures have anything to do with the yearly increase in atmospheric CO2 ppm …… really needs to study this graph and explain the reasoning for their belief, to wit:
http://i1019.photobucket.com/albums/af315/SamC_40/1979-2013UAHsatelliteglobalaveragetemperatures.png
Ocean dead zones, do they generally have more towards acidic conditions due to the absence of oxygen?
Another thing that I often wondered about, oxygen rich surface water is not great at taking on CO2, or am I mistaken?
Mark,
Uptake and release of any gas in solution is a matter of partial pressure of each gas on itself, no matter the rest of what is in the atmosphere.
Sure, but what about the Anderson-Hasselhof equation?
Willis Your reaction is understandable, but if you read Barrante’s last paragaphs and the first few comments, you’ll find that Barrante seems to have no more faith in conventional wisdom about ocean pH measurement and pH change than do you (or I). He just doesn’t seem to think it’s important. I’m about 90% sure that what he is doing here is to apply the principles of Physical Chemistry to “Ocean Acidification”. That results in the horrifying equations he presents. In my opinion that’s a useful contribution, and not one that we’re likely to encounter elsewhere because applying physical laws to ocean pH is extremely complex, quite difficult, and tedious as well.
I think, not 100% sure, that his conclusion is that “ocean pH” (no matter what we, he, or anyone else thinks that is) is going to be dictated by ocean temperature, not partial pressure of CO2. And it’s going to be small for any realistic temperature change.
I have some doubts that his conclusions match what passes for data, but OA really is not a subject I wish to bother my pretty little head about. The smart kids can (and I predict will) argue about that one for all eternity
I could be all wrong on this.
Don K March 21, 2016 at 4:07 pm
Thanks, Don, but when a man says:
… it sure sounds like he has faith in conventional wisdom.
You also say:
Oddly enough, I thought his conclusion was that causation went the other way, viz his claim that:
He’s not just saying that pH is a function of temperature. He’s also saying that atmospheric CO2 is a function of temperature.
With all that said, I did like very much his 3-D plot of temperature, pH, and CO2 levels. I’d never thought of that. I’ve been able to reproduce it closely in R, using the “seacarb” package, although I get slightly greater slopes than he gets.
An examination of the data shows that for each 10 ppmv of increased atmospheric CO2, the pH at a constant temperature should drop at about -0.08 pH units. It also shows that at a constant CO2 level, the pH should increase by about .016 pH unit per degree of warming.
SO … let’s assume for the sake of argument that the temperature warmed by 2°C and the atmospheric CO2 went from 180 to 400. The warming of two degrees would increase the pH by about .03 pH units.
On the other hand, the change in atmospheric CO2 of 220 ppmv would cause a theoretical decrease in pH of about 22 times 0.08 = -0.17 pH units. The net of these is a slight neutralization of the ocean waters, of about -0.14 pH units.
HOWEVER … and it is a big however, this assumes that the salinity, the phosphate concentration, the total silicate concentration, and the several equilibrium “constants” do not vary … and in the real world, this is always a suspect assumption. To mangle Shakespeare, there are more buffers in heaven and earth (and the ocean) than are dreamt of in the usual philosophy …
Best to all, thanks to Dr. B,
w.
Willis Eschenbach March 21, 2016 at 7:10 pm
HOWEVER … and it is a big however, this assumes that the salinity, the phosphate concentration, the total silicate concentration, and the several equilibrium “constants” do not vary … and in the real world, this is always a suspect assumption. To mangle Shakespeare, there are more buffers in heaven and earth (and the ocean) than are dreamt of in the usual philosophy …
It’s not so much an assumption as a measurement Willis. The constancy of the sea water composition of the major elements was first measured in 1819 and has been confirmed multiple times since. The residence time of major species such as Na+, Mg2+, K+, Sr2+,, Cl-…….. is over 3 million years, Ca2+ is about 800,00 years. The equilibrium constants, as I told you yesterday depend on the free energy of the products and reactants in there standard states, they don’t change.
Phil. March 22, 2016 at 6:59 pm
Thanks, Phil. Well, to start with the salinity certainly varies and is far from constant in either space or time. Similarly, the silicate concentration of the ocean not only varies from the surface to the deeps, it also varies from one ocean to another. And the phosphate concentrations vary from about zero in the tropics to 2 mmol/m^3 around Antarctica.
So I’m sorry, Phil, but your claims are simply not true. As I said, salinity, phosphate, and silicate all vary over time and space. Not only that, but each of these is temperature dependent in some measure, so there are subtle interactions between the various variables.
In addition, in theory you are correct when you say that “The equilibrium constants, as I told you yesterday depend on the free energy of the products and reactants in there [sic] standard states, they don’t change.” But in the real ocean, reactions affect each other, reactants may or may not be removed by either chemical or physical processes, various substances move both into and out of solution, and buffers of various kinds come into play. All of these can affect the rate at which a reaction proceeds, regardless of the theoretical equilibrium constants.
Finally, life intervenes always and everywhere in the ocean. In the ocean, chemistry doesn’t rule life—instead, life rules chemistry.
My best to you,
w.
Willis Eschenbach March 22, 2016 at 9:28 pm
Phil. March 22, 2016 at 6:59 pm
So I’m sorry, Phil, but your claims are simply not true. As I said, salinity, phosphate, and silicate all vary over time and space. Not only that, but each of these is temperature dependent in some measure, so there are subtle interactions between the various variables.
Willis I would ask you to grant me the courtesy that you expect for yourself, please quote the exact words you disagree with.
In addition, in theory you are correct when you say that “The equilibrium constants, as I told you yesterday depend on the free energy of the products and reactants in there [sic] standard states, they don’t change.” But in the real ocean, reactions affect each other, reactants may or may not be removed by either chemical or physical processes, various substances move both into and out of solution, and buffers of various kinds come into play. All of these can affect the rate at which a reaction proceeds, regardless of the theoretical equilibrium constants.
They are the actual equilibrium constants, as such they define where the composition will end up, regardless of the rates.
Phil. March 22, 2016 at 6:59 pm
Phil, I already did exactly that in the comment, viz:
So your claim was that the the levels of phosphate, silicate, and salinity do not vary.
Sorry, but they do vary, and widely, both in time and in space.
w.
Phil. March 23, 2016 at 7:12 am
No, they are not actual equilibrium constants. They are theoretical equilibrium constants, and are only valid if the reaction is going on in pure water and no other reactions are going on that might interfere, and no other buffers exist and no life forms interfere with the process … and as I said above, in the ocean none of those are true.
And not only are they theoretical equilibrium constants … but different authors calculate them slightly differently. More to the point, rather than being constant, they vary with both temperature and salinity. Here are the flags for the choices of constants K1, K2, Kf, and Ks in the software I use:
and here are the comments on those choices:
My best to you,
w.
Reblogged this on gottadobetterthanthis and commented:
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Not in-depth, but its thermodynamics, mathematics, and chemistry (in other words, you can figure it out if you want to). So, the short version, the ocean is entirely too complex to suppose driving your SUV matters to it.
So the so called ‘Professor’ guesstimates the PH of the Oceans 150 years ago.
It seems that ‘Guestimates’ is the new science of today.
It’s very sad that so called ‘Science’ in some fields today is becoming ‘on par’ with Astrology.
I see a lot of detailed analysis in this thread from people who obviously know their chemistry. Is it really impossible to put a paper together for publication and demand that any respectable outlet that has carried ocean acidification nonsense do the responsible thing and publish it.
One problem is Barrante’s speculation:
“In fact, if the measurement of ocean pH were not so complicated, and we had that data for the last 150 years, I would bet that we could show exactly that the increase in atmospheric CO2 from 280 ppmv to 380 ppmv in the last 150 years is an ocean temperature effect and not at all related to burning fossil fuels.”
That would never make it past peer review.
Sorry, I had some trouble posting for the first time. I accidentally posted twice. Please remove this and leave the first one in. Thank you, J. Paul
Well that was a lively discussion. Thank you!
Dr. Barante,
While pH is difficult to measure with sufficient accuracy – forget all glass electrode measurements of the past oceans – the current new colorimetric methods of seawater pH are of a much better accuracy, but these series are much too short. An alternative is that pH can be calculated out of other measurements (total alkality and DIC). See the trends in Hawaii, Fig. 1 in:
http://www.pnas.org/content/106/30/12235.full.pdf
Fig. 3 shows the influence of temperature on the local pH.
Further, if pH was the cause of the increase of CO2 in the atmosphere, the total inorganic carbon content (DIC) of the ocean surface would decrease with pH. If the pH decrease is caused by increased CO2 in the atmosphere, then DIC increases. The latter is what is observed. See the discussion in the above essay.
Dr Barrante,
A few remarks:
pH measurements with glass electrodes were by far not accurate enough to measure the changes in the oceans. Colorimetric measurements are far better but their use is only quite recently. There is an alternative: older measurements were made for total alkalinity and DIC (dissolved inorganic carbon) the combination of both gives a quite good alternative.
See: Fig. 1 in:
http://www.pnas.org/content/106/30/12235.full.pdf
Further, if a lower pH / higher temperatures were the cause of the CO2 increase in the atmosphere, that would result in a reduction of DIC in the ocean surface. If the lower pH is caused by the increase in the atmosphere, then DIC increases. The latter is what is observed, as mentioned in the discussion section.
BTW, Henry’s law + the resulting dissociations gives not more than 16 ppm/°C in steady state equilibrium between oceanic sources and sinks at one side and the atmosphere at the other side…
The influence of temperature on pH at Hawaii is shown in Fig 3.
In FIg.1 one can also see that the pCO2 of the ocean increases together with that of the atmosphere, but in average the pCO2 of the atmosphere is higher than of the oceans. That is the case, not only in Hawaii, but also ain Bermuda and other places where is measured over longer periods. Even worldwide. See the compilation by Feely e.a. at:
http://www.pmel.noaa.gov/pubs/outstand/feel2331/exchange.shtml
With an area-weighted average 7 μatm higher CO2 pressure in the atmosphere than in the ocean surface…