CO2 in the air, CO2 in the seawater

Guest Post by Willis Eschenbach [See updated graph]

Inspired by some comments on another thread, I decided to see what I could find in the way of actual measurements of the amount of CO2 in the surface layer of the ocean. I found the following data on the Scripps Institute web site. What they did was drive around the ocean on four different cruises, measuring both the atmospheric CO2 levels and at the same time, the amount of CO2 in the surface seawater. Figure 1 shows those results:

scatterplot sst vs co2 scripps allFigure 1. All air-ocean simultaneous measurements from four Scripps cruises are shown as blue dots. The horizontal axis shows sea surface temperature. The vertical axis shows the difference between the CO2 in the overlying air, and the CO2 in the water. The red line is a lowess curve through the data. The paper describing the Scripps data and methods is here.

Now, I have to say that those results were a big surprise to me.

The first surprise was that I was under the impression that there was some kind of close relationship between the atmospheric CO2, and the CO2 in the surface seawater. I expected their values to be within maybe 5 ppmv of each other. But in fact, many parts of the ocean are 50 ppmv lower than the CO2 concentration of the overlying air, and many other parts of the ocean have 50 ppmv or more of CO2 than the CO2 in the air above.

The second surprise was the change in not only the size but even in the sign of the trendline connecting temperature and CO2 (red line in Figure 1). Compared to the CO2 level in the air, below about 17°C the seawater CO2 decreases with increasing temperature, at a rate of about -2 ppmv per °C.

Above about 17°C, however, the seawater CO2 content relative to the air increases fairly rapidly with temperature, at about +4 ppmv per °C.

To describe the situation in another way, when the water is cool, it contains less CO2 than the overlying air … but when the water is warm, it has more CO2 than the overlying air.

Say what? I gotta confess, I have little in the way of explanations or comprehension of the reason for that pattern … all suggestions welcome.

w.

[UPDATE] By popular request, here is the same data, but in absolute rather than relative units and without the lowess curve.

scatterplot sst vs co2 scripps all no delta

 

Figure 2. As in Figure 1, but showing the CO2 content of the surface seawater directly. Atmospheric CO2 varied very little during the time of the measurements.

My main question in all of this is, how does the CO2 content of the seawater get to be up to 100 ppmv above the CO2 content of the overlying air? It seems to me that the driver must be biology … but I was born yesterday.

Regards,

w.

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lemiere jacques

CO2 roduced by biomass …rain…evaporation…

ronald

Can it be a agw survey? Cold water absorbes CO2 and warm water let it go by out gassing. It looks to me that someone wants to let look to work the other way to help agw.

Juan Slayton

Are we assuming equilibrium? The air temperature is constantly changing, I would presume the water temperature also, though less so. CO2 levels may also be changing. Would be nice to have a ‘dynamic’ record from a single location, showing response to temperature or CO2 change.

lemiere jacques

oh sorry and wind…some co2 arrived from elsewhere land for instance…so you can see a dynamical process…and some saturation too…quite complicated indeed for sure you can’t see it like an equilibrium…
well and the water at the surface exchange a bit withe the water underneath…
you should try to figure out how co2 concentration vary in the atmospheric surface layer how it varies in the ocean surface layer… temperature paly a role..
another point…temperature of what? air of temperature?
it they are fifferent it is weird to assume that there should be some kind of equilibrium.

johnmarshall

You missed most of the reaction. Sea water is an ionic solution and dissolved CO2 continues reacting to form bicarbonate ions, which increase the pH so maintaining the pH balance, and this bicarbonate ion is used by animals to form shell and skeletal frameworks. Without this further reaction corals would not grow. So the takeup of CO2 depends on planktonic life, the more active this life the more bicarbonate is used then the faster the CO2>bicarb reaction continues. Dissolved CO2 remaining in the water is used bu planktonic and other plants to photosynthesize. These chemical reactions speed up with temperature rise.

Juan Slayton

A lot of photosynthesis goes on in the ocean. Could the dip from 5 to 25 degrees reflect favorable conditions for photosynthetic growth?

Julien

It would seem like the correlation between higher CO2 and higher temperatures would be only verified for seawaters above 25c? Alternatively, another way to look at it, is that the radiative equations only work when the angle formed by the sun and the surface close to perpendicular: Global warmist assume a flat earth as a greenhouse, they may underestimate the fact that without direct sunshine there is no greenhouse effect going and temperatures and sinking rapidly.

Macha

Relative difference is not the same as absolute. Warmer water can absorb and hold more CO2, than cold. The rate of change I more a question of kinetics.

martin brumby

The vast majority of their dots are for sea surface temperatures greater than 20ºC.
Perhaps the cruises in oceans where this was the case were more popular with the psyentists than those trawling around oceans with temperatures below 10ºC?
Or maybe the latter group just kept warm and cosy below decks?

“To describe the situation in another way, when the water is cool, it contains less CO2 than the overlying air … but when the water is warm, it has more CO2 than the overlying air.”
When water is cool CO2 flows from air to water – net absorption.
When water is warm CO2 flows from water to air – net release.
” many parts of the ocean are 50 ppmv lower than the CO2 concentration of the overlying air, and many other parts of the ocean have 50 ppmv or more of CO2 than the CO2 in the air above.”
Looks like there are mechanisms in play that make the application of Henry’s Law rather hit and miss.
Most likely the CO2 exchange rate varies as many other factors such as internal ocean movements and winds in the air interfere with the exchange process.

TimC

Purely from eye-balling the data, the Lowess curve line at SST 0. Without this cluster a normal linear regression would seem to fit well (don’t have time to do it right now) crossing the OSco2-Aco2 axis at about SST = 18 and with d(OSco2-Aco2)/dSST =1 approx.

TimC

Sorry – the first sentence of my post above should be: Purely from eye-balling the data, the Lowess curve line at SST less than 5 seems to rest essentially on the small data cluster at around SST = 2, OSco2-Aco2 greater than 0. Without this cluster …

The vast majority of CO2 in the oceans is in the form of bicarbonate. Only a tiny fraction is in the form of the dissolved gas, which is what this paper is measuring. The inorganic exchange between dissolved gas (as in soda water) and bicarbonate is governed mostly by the reaction:
CO2 + H2O —> HCO3- + H+
You can see the H+ lowers the pH. There is a huge organic component to this process as well. We escaped the fate of Venus in large part due to the formation of calcium carbonate by marine organisms, that pulled nearly all of the original CO2 out of the atmosphere to create limestone, the abundance of which is evident to anyone who has looked at rocks around the world.

Mike Edwards

The most straightforward observation I can make regarding the scatterplot is that the data looks rather inhomogeneous – clusters here and there and gaps between them. To me, this would indicate that other factors are at play here beyond CO2 and temperature.

dudleyhorscroft

Just possible that at close to zero (0 to 5 C) CO2 in air and water are in equilibrium – not much happening. As the water warms, the amount of CO2 in water decreases, as one would expect. Further, as temperatures approach 10 to 15 C plant life does well and it reduces CO2. However, past 17C, it is better for animal life, and this eats the plants and exhales CO2, so increasing the amount of CO2 in the sea water.
BTW – we “exhale” via our lungs – does a fish “exhale” via its gills – if not, what is the correct term for “a fish getting rid of CO2?”

Bloke down the pub

In areas with lower temps the sea water might be down-welling thereby taking the CO₂ below depths where the measurements were being taken.

Bill Onesty

My only response is – What? You would draw a TREND line? Through THAT? REALLY?

Keitho

It almost looks as though the CO2 rising from deeper down is encountering a choke at the surface slowing down release. Perhaps the surface tension is doing it. But it seems to result in a CO2 rich zone close to the underwater surface with only a slow release to the atmosphere.
Interesting finding W

Claude Harvey

In summary: You can’t cough it up unless you have more of it.

dudleyhorscroft

Unfrozen Caveman said:
“We escaped the fate of Venus in large part due to the formation of calcium carbonate by marine organisms, that pulled nearly all of the original CO2 out of the atmosphere to create limestone”.
Circular reasoning. If the runaway greenhouse effect were valid, with all that original CO2 in the UnfrozenCavemanMD said:
“We escaped the fate of Venus in large part due to the formation of calcium carbonate by marine organisms, that pulled nearly all of the original CO2 out of the atmosphere to create limestone”.
Circular reasoning. If the ‘runaway greenhouse’ effect were true, with the original atmosphere largely of CO2, temperatures would be on average above boiling point of water, there would be no oceans – no liquid water in which marine organizms could exist, and which could turn the CO2 into limestone. These marine organisms would most likely have been very happy with an ocean temperature perhaps 8 to 10 degrees C above present – which would probably be the likely figure if the atmosphere were 60 – 90% CO2.

Nylo

Thanks a lot for this article Willis, it made me think and I love it when new data or ideas make me think.
I had to reconsider everything I know, even regarding outgassing, as it is at odds with the outgassing of CO2 from the ocean that supposedly happens when temperature increases. Is it really true?
Well, of course it is. What perhaps may be not so true, is that this outgassing affects the atmospheric concentration of CO2. There will be more CO2, yes, but I’m guessing that the oceans will NOT ONLY release CO2. There must be other gasses disolved as well. So it outgasses CO2, and maybe O2, N2, Argon… as well. So concentration of CO2 in the atmosphere MAY NOT change as a result of the outgassing.
But wait! We have paleoclimatic records showing more atmospheric CO2 when warmer! Isn’t that proof of CO2 release by the oceans? Well, it may perhaps be proof of more CO2 PRODUCTION in the ocean, increasing the ammount of CO2 that is dissolved in the water just because of biological productivity (more O2 consummers than CO2 consummers), making the concentration greater than there is in the atmosphere as your calculation here has showm… and then by outgassing, transfering some of this difference in CO2 concentration to the atmosphere.
That’s the only explanation I can think of right now. But it brings interesting conclussions if it happens to be true, doesn’t it? To begin with, all the story about ocean acidification crumbles. Ocean acidifies, yes, but not necessarily because of our emissions, but because of its own biological processes as a result of the warming… And part of the accumulated CO2 in the atmosphere would have been released by the oceans, not us.

Willis,
I don’t think the water measurement reflects concentration of CO2, and I’m sure it isn’t ppmv of water. It’s described in your link as pCO2, which would be the partial pressure of CO2 in equilibrium with the seawater.
In that case, there’s no particular expectation about variation with temperature. With no flux, it would be zero at any temperature. What it does reflect is which way CO2 is moving.
On that basis it should indeed be higher in warm water, because that is thought to be a region of degassing. So pCO2 must be higher below the surface. CO2 moves down a gradient of pCO2.
In fact, though, it would be very seasonally dependent. All sea water loses CO2 toward summer, and takes it back before winter. So timing of the readings would be important.

dudleyhorscroft

As has been pointed out many times, the more the CO2, the less the additional effect of ‘greenhouse warming’. The additional temperatures would be due to the increased internal heat of the earth escaping to the surface, and increased heat as a result of a turbulent atmosphere. Those good with figures can calculate the surface air pressure if all the CO2 in the rocks in the form of chalk, limestone and marble were free in gaseous form, and what the surface air temperature would be if the atmosphere were turbulent, and the temperature on the surface were due to compression of downward moving air. And of course, in those early days, there would be plenty of vulcanism adding heat at ground level.
Remember that much of the CO2 was removed from the atmoshere not so long ago – the chalk was laid down in the Cretaceous. Before that what was the amount of CO2 in the atmosphere, and what was the surface temperature?
I apologize for the previous post – the start of my comment disappeared, so I wrote it in again, and then found that the end had been disappeared. So I chpped it and found that the end was still on my clipboard, and put it here.

François

Five years of measurements, fifty years ago, with the instruments available then. I am impressed.

So, according to that plot when I pop the top on my barley pop I will get more CO2 as it warms up and it shouldn’t go flat faster standing at warmer temperatures? I’m not sure what they were measuring but the concentration of CO2 (all forms) is inversely proportional to temperature. CO2 chemistry in water is a bit more complicated so if they were just measuring dissolved CO2 they might have gotten strange results. You need to know pH and total carbonate also. http://www.soest.hawaii.edu/oceanography/faculty/zeebe_files/Publications/ZeebeWolfEnclp07.pdf

Sera

There is a natural carbon-dioxide cycle during the year- look at the months (of the year) that the data were taken. I would expect higher concentrations during the fall/winter, and lower concentrations during the spring/summer.

geran

Interesting data, for sure. Several have mentioned photosynthesis, and I agree that might be a huge factor. It would be interesting to see the concentrations versus time of day (or even better, sunlight intensity) to look for the correlation to photosynthesis.
The correlation to temperature, demonstrated by the scatter plot, appears to be opposite of what I have always heard. That is, the “outgassing” appears to increase at colder temps. WUWT?

Dodgy Geezer

…The first surprise was that I was under the impression that there was some kind of close relationship between the atmospheric CO2, and the CO2 in the surface seawater. …
Alas, Willis, you have been infected by IPCC reasoning. The idea that there are only a few big variables and they interact with each other in a simple manner is what you say when you are a political advisor hoping to persuade a politician.
“Yes, Mr Prime Minister – if you enact this law you WILL get more votes…”
In reality we have two domains here, the sea and the air. Each has a set of pressures and balances which determine the local CO2 concentration. At the point where they touch – the sea surface, they probably interact with one another. But how important that interaction is compared with their own internal driving variables… who knows?

Richard Graves

I like to make soda water. Thinking very cold water would make bubblier soda that’s what I tried. Results not good! Then I tried water from tap around 20C. Result nice bubbly sodas. Seems the warmer water absorbs more CO2 more easily. Its been bothering me why?

Macha

But consider the Y -axis is difference , so 10ppm ocean CO2 – 15ppm air CO2 can be in cold water when gas is less absorbed, and 40ppm ocean – 35ppm can be for warm water. Yet each has opposite sign. I would also like to see gross CO2 on y axis.

chris moffatt

Where did they do their sampling? Most of the plot points are where SST is >25C (77F). Seems to me to be hardly a representative sampling of the world’s oceans.

Oof, where to start.
first you need to change compatible units before subtracting. Ocean measurements will be partial pressure as someone else comments. ppmv can be converted to atm. part. pressure. Then you can subtract and the difference is physically what drives exchanges.
As for the data I see several concentration of dots that seem to suggest some very difference relationships are being dumped together. You probalby need to do some geographic splitting of the data to separate things before doing and modelling and fitting exercise.
I see:
1) about 27 deg. a huge range of variation that is dependent of SST (straight up and down).
2) Between 17-24 a nice linear section in the middle.
3) from 8 deg down through a min at 20 and back up throught the most dense region a parabolic curve. This may be mental pattern spotting or some real grouping that can be isolated by geography or latitudinal grouping of data.
4) Lots of loops that will probably be clearer if you join the dots rather than doing a scatter plot. Such loops indicate phase relationships ( see Lissajous figures ) that likely point to a dCO2 SST relationship rather than direct CO2 SST linkage.
Does not surprise me that there is such a mix in view of variety of in-flux , out-flux conditions that exist.

when studying time series , don’t be too keen to throw out the time content of the data. In this case it means drawing a line graph , not isolated dots.

Scan my comments and graphs in this thread to see discussion and examples of the significance of phase and joining the dots:
http://euanmearns.com/uk-temperatures-since-1956-physical-models-and-interpretation-of-temperature-change/#comment-294

William Astley

A quick visual scan of the changes of CO2 by year indicates there is a problem with the standard theory (Bern model) and basis of IPCC report Vs observations. http://www.esrl.noaa.gov/gmd/webdata/ccgg/trends/co2_data_mlo_anngr.pdf
Our thoughts concerning the observations are affected by the standard paradigm that has been repeated ad infinitum. To determine what is cause and what is effect the standard analysis technique is lead/lag analysis. It is interesting that we have spent two trillion dollars on green scams prior to doing lead/lag analysis to determine the cause of the CO2 increase in the atmosphere.
The phase relation between atmospheric carbon dioxide and global temperature by Humlum et al, August, 2012.
http://scholar.google.ca/scholar_url?hl=en&q=http://xa.yimg.com/kq/groups/18208928/233408642/name/phase%2Brelation%2Bbetween%2Batmospheric%2Bcarbon%2Band%2Bglobal%2Btemperature.pdf&sa=X&scisig=AAGBfm2_FClsSVBbTLdzlwJJytToRLHpNw&oi=scholarr&ei=N-SVUvKOD9PtoATG6IHgCQ&sqi=2&ved=0CCsQgAMoADAA
“From this, changes in atmospheric CO2 appear to be initiated near or a short distance south of the Equator, and from there spread towards the two poles within a year or so. En route, the signal presumable is modulated by local and regional effects, as is indicated by the much larger annual CO2 variation (not shown here) in the High Arctic, compared to that recorded at the South Pole. There is however no indications of the main signal originating at mid-latitudes in the Northern Hemisphere as would be expected from the release pattern shown in Fig. 12.”
“Summing up, our analysis suggests that changes in atmospheric CO2 appear to occur largely independently of changes in anthropogene emissions. A similar conclusion was reached by Bacastow (1976), suggesting a coupling between atmospheric CO2 and the Southern Oscillation. However, by this we have not demonstrated that CO2 released by burning fossil fuels is without influence on the amount of atmospheric CO2, but merely that the effect is small compared to the effect of other processes. Our previous analyses suggest that such other more important effects are related to temperature, and with ocean surface temperature near or south of the Equator pointing itself out as being of special importance for changes in the global amount of atmospheric CO2.”

MikeP

If you cannot control for other effects than CO2 directly, then you have nothing. For example, some comments above have pointed out biological effects such as photosynthesis and shell formation. But CO2 is not the only driver of these. Upwelling brings both cooler water temperatures and nutrients. These are the most biologically active regions around. The warmer gyre centers such as the sargasso sea have warmer waters and are largely deserts. It would be useful to know where the exact ship tracks and data collection points were.

Gamecock

Little atmospheric gas will enter into the ocean without wind or waves.

dccowboy

Unfrozen Caveman said:
“We escaped the fate of Venus in large part due to the formation of calcium carbonate by marine organisms, that pulled nearly all of the original CO2 out of the atmosphere to create limestone”.
we escaped the fate of Venus because of several factors, some of which are, earth has a magnetic field whereas Venus does not, Venus is much closer to the sun and therefore receives a LOT more solar radiation than earth, it appears that life never got started on Venus and we are finding out that early life forms contributed a great deal to the modification of earth’s atmosphere, Venus has no Moon.
I weary of the comparisons of earth and Venus simply because they are of similar planetary mass. The two are entirely different.

Richard111
Old England

Willis,
What depth were they measuring CO2 at? I assume there will be a layer at the surface where CO2 exchange with atmosphere occurs – higher water temperatures will mean more out gassing from the ocean from greater depths and thus higher CO2 concentration at the exchange layer ? Lower temperatures and the opposite occurs with presumably a lower concentration through less out gassing ? As to the differences between atmospheric and ocean CO2 levels I wonder if they plotted in wind speeds and surface water speeds and directions of each as this could presumably create huge anomalies in concentrations.

wsbriggs

Purely physical chemistry explanations of biospheres could be poor matches to the real world where life reverses entropy locally. It would help to know the biological composition of the sea water as well. Simple, this planet isn’t, nor is the rest of the Universe.

DHR

Willis,
I am not sure the data mean anything.
The Scripts paper says the “surface” water measurements were taken from a sea chest of unknown location. This class of tug had a draft of 13 feet and sea chests for intake of seawater (primarily for engine cooling) were most always low in the ship to make sure they were underwater as the ship is rolled and pitched – which these ships did a lot of. I don’t know if a depth of a bit less than 13 feet means “surface” for the purpose of your discussion.
Also, the Scripts paper describes how the air intake was switched depending on the relative wind direction to avoid contamination of the samples by the exhaust of the ship’s diesel engines. Air can swirl about a ship in bizarre ways and sea air is, as you know, burdened with sea mist (with its entrained CO2 I suppose) and salt. On windy days more than less. I found no data that would show whether the intake switching was successful or whether differences between windy and calm days were measured and accounted for. Maybe not.

Nick Stokes says:
November 27, 2013 at 4:02 am
I agree. It looks like they were measuring pCO2air and pCO2water. If they were they were trying to get the flux (difference in partial pressures) that’s the way they would do it. Transport phenomenon would be dependent on temperature and surface layer turnover.
Henry’s law describes the prime driver http://chemengineering.wikispaces.com/Henry's+law
These guys have a good summary description of sea water flux http://ocean.mit.edu/~mick/download/Quick-tour-of-carbon-cycle-2011.pdf
And, as usual, John Daly was there with the firstest and mostest
http://www.john-daly.com/co2-conc/ahl-co2.htm

OK, looking at the data files that Willis linked to, it’s not the CO2 in sea water but the “equilbriator” CO2 concentration. ie the level of atm CO2 with which the water is at equilibrium.
It can be taken are representing the partial pressure in the sea water and thus the difference used by Willis is that which would drive out-gassing or absoption processes.

Thank you for this survey. It makes sense to me. The CO2 cycle is about 10 years and the oceans contain 60 times more CO2 than the atmosphere. The rain forests and the Gobi desert has the highest concentrations of CO2 on land and parts of Siberia the least.
Blame it on the clouds. In the north snow and rain absorb CO2 from the air, and it finds its way to the ocean. It is cold so it can absorb a lot which goes into the icy ocean. In mid temperate regions less clouds, less rain. In the tropics the ocean evaporates CO2 even more than the water and the thunderstorms reabsorbs CO2 in the higher elevations which rains down and re-evaporates.
In the Gobi desert all the cold water coming from the mountains evaporates, raising the CO2 level.

F.A.H.

Looking at the linked paper I note first that the data for CO2 in air across all latitudes was generally quite flat (i.e +- 5 ppmv or so) around 320 ppmv. In contrast, the values for water varied about an order of magnitude more about +-50 ppm. The water measurement techniques (“equilibrator”) varied from cruise to cruise and appeared significantly more correction dependent and systematic error prone than the air measurements. Before doing any interpretation of the data I would go through the experimental details a bit more, particularly the water measurements and corrections.
Secondly, the atmosphere might be much better mixed than the ocean. Atmospheric wind speeds can average 10 or 20 m/s and upwards (near the surface) and turbulent mixing due to diurnal variations and turbulent boundary layer flow should exceed that of similar phenomena in the ocean (based on Reynolds number arguments). Ocean currents seem to run 1 m/s or less and the mixed layer ( from a few to a few 10s of meters) seems to depend on the relative calmness of the sea surface and varies greatly with atmospheric temperature (i.e weather) and the seasons as well as parts of the ocean. It looks to me that there is a lot more opportunity for variation in the water measurements. I would have liked to see them simultaneously measuring salinity or some other chemical markers to scope mixing and general homogeneity of measurements. It would have been interesting to see whether salinity or some other constituent (nitrogen or whatever) varied in concert with CO2. Salinity in the upper ocean layer seems to vary between about 30-40 parts per thousand over the oceans (on a large scale). http://www.shorstmeyer.com/msj/geo130/salinitypatterns.pdf
Anyway, I am no expert in any of these things but I would spend a bit more time understanding the measurement details before expending a lot of energy trying to interpret the plot above.

Nick Stokes: “All sea water loses CO2 toward summer, and takes it back before winter.”
Not necessarily. It may out-gas quicker in summer and slower in winter, or absorb slowly in summer and quicker in winter.
Then you may need to comment on the tropics which don’t have winter and summer.
However, time of year will be relevant factor.

MattE

I’m curious if the cold water/CO2 effect is confounded by WHERE the cold water/CO2 measurements were taken. For example, the 5C water air/water CO2 temps are more likely taken near Anchorage Alaska or Greenland and the 25C+ measures are tropical. Your curve fit implies ‘all other things equal.’ I submit that that is almost certainly not the case. Imagination can run wild with many reasons why, differences in ocean fauna, CO2 sinks, etc, air/water mixing differences, seasonal changes… It would be one thing if all these temps were from one place, so that would be controled for, but they are clearly not.

phlogiston

According to the NASA earth fact-sheet,
http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
the masses of ocean and atmosphere are:
atmosphere: 5.1 E18 kg
ocean: 1.4 E21 kg
With atmospheric CO2 at 400 ppm, the mass of CO2 in the atmosphere is 2 E15 kg
Therefore, if ALL the CO2 in the atmosphere were to be instantly sucked into the ocean (and evenly mixed), that would result in an increase in the concentration of CO2 in seawater of about 1.5 ppm.
How can this represent a threat of acidification of the ocean?
So it only mixes at the surface? OK, then how does heat from AGW get down to below 2000m depth?
Can they really be as wrong as this?

Retired Engineer John

Willis, I like your open mind. Remember your Argo float graphs that you posted, I believe it was in early 2012, where you showed how ocean temperature is limited to 31C. There is another piece to this puzzle, the PH of the tropical – subtropical oceans. The warm layer is approximately one PH number lower than the cold ocean. The reason generally given for this difference is the effect of photosynthesis on PH; however, if you check the world wide distribution for photosythesis, you will find that it is most active in the high latitudes. This is because the layering of the tropical ocean in inversion layers limits the flow of nutrients and thus limits the reaction. The amount of photosysthesis in cold regions is much higher. This means that some other reaction is responsible for the change of the PH in the warm ocean.