Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Ocean acidification

  1. Apr 25, 2010 #1
    How can the ocean simultaneously release CO2 and decrease in pH?

    Currently the ocean is acidifying, as it absorbs about a third of the fossil-carbon dioxide that we emit, which then in part assumes the form of carbonic acid. But in the future, if the increasing atmospheric greenhouse effect continues to also warm the ocean enough, we expect this absorption will be reversed and vast quantities of CO2 will distil out from the ocean.

    Nonetheless, apparently we do not expect warming to cause any reverse to the acidification. (I asked one paleoceanographer/marine-chemist, and heard there is no contradiction for water to be simultaneously decreasing in pH and liberating CO2.) But naively, if warmed water begins losing carbon, then shouldn't the concentration of carbonic acid fall (and hence the pH start to rise back again)?

    Edit: The topic of this thread is not "global warming or climate change". It is purely an ocean-chemistry question. Regardless of what is actually happening to our ocean (or rather, regardless of what external factors may be controlling the temperature of and the partial pressure of CO2 above a hypothetical test-ocean) the question is simply whether in principle such an ocean hypothetically could ever be driven (by adjusting those two parameters) to release CO2 while simultaneously to decrease in pH, and how exactly? (So this is what self-censorship is like..)
     
    Last edited: Apr 25, 2010
  2. jcsd
  3. Apr 25, 2010 #2

    Xnn

    User Avatar

    Currently, the ocean is absorbing CO2 from the atmosphere and its pH is decreasing.
    As the ocean warms, it ability to absorb CO2 will become diminished.
    Eventually, if it warmed enough, it could begin to release CO2.
    However, that'd be a long time in the future and the pH would have become stable at a much lower value.

    So, we are not currently witnessing a simultaneous release of CO2 and decreasing pH.




    http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-ts.pdf

    See page 48:

     
    Last edited by a moderator: Apr 25, 2010
  4. Apr 25, 2010 #3

    sylas

    User Avatar
    Science Advisor

    [strike]Unfortunately, this topic is one physicsforums feels unable to cope with.[/strike] I stand corrected; my apologies. Thanks, Evo.

    However, just for the record, there is no prospect of the ocean releasing dissolved CO2 unless atmospheric levels are much lower. The only prospect is a reduced capacity to absorb CO2.
     
    Last edited by a moderator: Apr 25, 2010
  5. Apr 25, 2010 #4
    Excuse me?
     
    Last edited by a moderator: Apr 25, 2010
  6. Apr 25, 2010 #5

    Evo

    User Avatar

    Staff: Mentor

    The question about the effect of CO2 on ocean acidity, as posed by the OP, is perfectly fine.
     
    Last edited: Apr 25, 2010
  7. Apr 25, 2010 #6
    The problem I see is one of the sheer size of the ocean.

    To this day there are soundings on the Admiralty charts that were taken by Captain Cook.
    That part of the ocean has not been resounded let alone had its chemical composition measured.

    I just think our sample sizes are way too small to make such claims about the composition of the ocean, with any confidence, let alone changes in composition of several orders of magnitude finer graining.
     
  8. Apr 25, 2010 #7

    sylas

    User Avatar
    Science Advisor

    The measurements are plenty good enough to identify the falling pH levels. The chemistry of how this occurs is also well known. The increase in atmospheric CO2 is very well measured indeed, and the carbon cycle which exchanges carbon between ocean and atmosphere means that the ocean is currently increasing in total carbon content as well. This inevitably leads to a falling pH.

    The carbon cycle also means that there's an equilibrium maintained between the oceans and the atmosphere, and given the rapidly rising atmospheric carbon levels, there is a large excess in atmospheric carbon from the equilibrium state. That's why the ocean is removing carbon from the atmosphere, and also why there is no credible prospect of that reversing. The only issue is whether that rate of ocean take up might slow down as levels in the ocean continue to increase. So far it seems to be holding pretty steady.

    Hence as atmospheric carbon increases, ocean carbon increases, and ocean pH falls. This much is straightforward.

    Of course, that's a pretty broad description, and scientists want to measure the rates of change more precisely, and there's a lot of work and data on this as well.

    See, for example:

    Cheers -- sylas
     
    Last edited by a moderator: Apr 25, 2017
  9. Apr 25, 2010 #8
    Would these be the same measurements that 15/20 years ago lead to the belief/claim that life could not exist at the depths and temperatures of the thermal plumes at the ocean bed?
     
  10. Apr 25, 2010 #9

    sylas

    User Avatar
    Science Advisor

    I guess that is meant to be a rhetorical question, since the answer is so obviously of course not. More importantly, it wasn't measurements that were in question, but finding examples of living things that were able to tolerate those conditions.

    Back to the actual question of the thread.... measurements show without any doubt that carbon levels in the atmosphere and in the ocean are increasing, and that pH is decreasing as well. You can't have one without the other; the chemistry means more carbon in the ocean corresponds to a lower pH.

    Cheers -- sylas

    PS. And on the side issue of the fascinating history of hydrothermal plumes. The plumes were discovered in 1977, and life in those plumes was discovered at the same time.
     
    Last edited: Apr 25, 2010
  11. Apr 25, 2010 #10

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    1. Beliefs/claims are relevant to any discussion only if there are printed in peer reviewed publications or standard textbooks. All other claims are irrelevant.

    2. Any assertions about claims need to be accompanied by citations to the papers/texts that they appear in. All other assertions are irrelevant.
     
  12. Apr 25, 2010 #11

    Evo

    User Avatar

    Staff: Mentor

  13. Apr 26, 2010 #12
    Thank you for the link, Evo.

    I was not thinking of micro-organisms. I was really thinking about the BBC videos showing the 'fish' swimming near these spots.

    But you are right, any life is still life.

    I have just been reading Sylas' excellent blog on debate and communication; I am just trying to make the point how little information we really know about the deeps.
    A peer review of 3, 30 or 3000 measurements in an ocean the size of the Pacific is pointless.

    Can anyone say how many measurements we take of the composition of the bottom 50%of the Pacific water per decade?
    Divide this by the volume of that water and you will have a figure of measurements per cubic mile or whatever. The figure will not be large enough in my opinion to statistically state that the composition of the Pacific is changing in any direction.

    It may be (probably is) that some locations are sampled more frequently so such a statement could be made confidently about such a location. But it would be a statistical fallacy to use this to represent the ocean as a whole.
     
  14. Apr 26, 2010 #13

    sylas

    User Avatar
    Science Advisor

    Thanks for the comment on my blog article! It's something I've often thought about, and how discussions like this can be productive even with disagreements and even when they don't seem to be actually resolved.

    I expect the pH of the deep ocean is uncertain; but what is quite certain is the following.

    (A) The ocean is soaking up one heck of a lot of carbon, at the rate of about 2 Gton per year. This is absorbed mainly at the surface, from the atmosphere, and then circulates through the rest of the water column; although the rate at which carbon is accumulating at different depths is not completely clear. Roughly speaking, new carbon is being added to the carbon cycle at about 8 Gt per year from geological sources (fossil fuels). Almost half remains in the atmosphere (giving a well measured increase) and the rest is about half each into oceanic and terrestrial carbon sinks.

    Details of how carbon is increasing in the reservoirs of the carbon cycle is considered in (for example)

    (B) Adding carbon in the ocean is chemically fairly straightforward; it ends up as carbonic acid, and reduced the pH of the ocean. This is measured mainly in the upper ocean, where we also can see its effects most clearly, and where the impacts are likely to be greatest.

    You don't need to know all the details of all the ocean to know that it is absorbing one heck of a lot of carbon, and that this lowers the pH. Details of how it is distributed are a perfectly good question, but the answers to the original thread question are still pretty straightforward. Specifically, there is no prospect of changing ocean temperatures reversing the currently measured trends of ocean water absorbing carbon and reducing in pH.

    Cheers -- sylas
     
  15. Apr 26, 2010 #14
    Sylas I don't disagree with your assesment of the atmousphere - ocean interaction.

    But the ocean has a bottom as well as a top. And as I understand the current theories of geological activity, movement of most of the carbon in the world occurs in carbonate rock in the bottom part. I also understand that the disposition of the continents plays a bigger role than is generally recognised in the generation of (extreme) weather.

    We have seen at least one earth shattering (no pun intended) discovery in geology every decade in the second half of the 20th century which has stood conventional thinking on its head.

    The outcropping of these dicoveries seems unabated as we enter the 21st century,
    for instance the work on the PT bounday and the mass extinction at that time.
     
  16. Apr 26, 2010 #15

    sylas

    User Avatar
    Science Advisor

    OK... but I would have thought that was the underlying basis of the question of the thread.

    Quite so. But geological activity occurs at much too slow a rate to have much of an impact on anything in the time frames for the question being asked here.

    Most of the carbon may be bound up in carbonate rock; but because this exchanges only very slowly, such rock has little impact on the carbon cycle except over extremely long time scales. There is very little actual movement of carbon in and out of rock, by comparisons with the enormous fluxes of carbon dominating the carbon cycle.

    The shape of land forms certainly has an impact on weather in all kinds of ways; and this means weather patterns in the past might be radically different. But the disposition of continents changes slowly, and continents are going to remain arranged pretty much as we have them now for a long time to come. I had taken the question to be about the impact of the ocean on carbon cycles during the coming century or millennium.

    As a general principle in science, everything we think we know might be wrong. But its unlikely for it ALL to be wrong, and the fact that we do indeed make discoveries shows that we there must be some kind of a basis for answering scientific questions such as the one in this thread.

    The original question was:
    How can the ocean simultaneously release CO2 and decrease in pH?

    The short answer is... only if CO2 is not actually coming from the ocean itself, but from something else. For example, a large subterranean volcano might suddenly go off, and release a lot of carbon, some of which will remain in the water (and hence lower the pH) and some of which might be released into the atmosphere. Same for melting calthrate deposits, or any other such unexpected process.

    The original post of the thread includes a statement which I believe is incorrect.
    I do not think that second sentence is true. There is no expectation that lots of CO2 will distill out of the ocean; what may happen is that the rate at which it is being absorbed will slow down. As water temperatures increase, solubility of carbon reduces. This is a likely cause of the large changes in atmospheric carbon dioxide during the ice ages. However, with the present equilibirum having so large an excess of atmospheric carbon dioxide, the natural equilibrium is for the ocean to continue to remove atmospheric carbon. Increasing temperatures might reduce the total amount that could be absorbed, but not reverse the direction of net absorption.

    Reference, on changing solubility of carbon dioxide with temperature and the consequent feedback loop between temperature and atmospheric CO2:

    As matters stand, there is one heck of a lot of carbon is being absorbed by the ocean, as CO2 is dissolved at the surface and becomes carbonic acid... lowering the pH.

    For the ocean to actually emit more carbon than it is absorbing, something radically new and different would have to occur -- not merely release of dissolved carbon. Something unexpected. Not merely something we haven't noticed in existed measurements. For now, the ocean is absorbing a bit over 2 Gt of carbon per year, and geological outgassing is negligible by comparison with that. The absorbed carbon reduces pH.

    Cheers -- sylas
     
  17. Apr 26, 2010 #16

    Evo

    User Avatar

    Staff: Mentor

    Actually a study was published in the journal Science January 16, 2009, stating that if the oceans warmed as predicted that they expect ocean alkalinity to increase, but this definitely crosses the boundary into predictions of climate change, but since it addresses the topic, I will post a link to the article discussing the study, but this is purely for information on the study, not to start a discussion on CC. We can defnitely discuss the effects of fish on ocean alkalinity.

    http://www.sciencedaily.com/releases/2009/01/090115164607.htm

    Link to abstract in Science. http://www.sciencemag.org/cgi/conte...09&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT

    Also, to address studiot's question about acidity levels at various places, shallower water will tend to be more alkaline.
     
    Last edited: Apr 26, 2010
  18. Apr 26, 2010 #17

    sylas

    User Avatar
    Science Advisor

    I'm quite happy to stick with chemistry here. The biggest issue here is a potential confusion of alkalinity and pH. They are defined differently, in terms of different ions.

    Thanks for the link! I checked it out. The reference discussed in that press release is:
    Abstract:
    Oceanic production of calcium carbonate is conventionally attributed to marine plankton (coccolithophores and foraminifera). Here we report that marine fish produce precipitated carbonates within their intestines and excrete these at high rates. When combined with estimates of global fish biomass, this suggests that marine fish contribute 3 to 15% of total oceanic carbonate production. Fish carbonates have a higher magnesium content and solubility than traditional sources, yielding faster dissolution with depth. This may explain up to a quarter of the increase in titratable alkalinity within 1000 meters of the ocean surface, a controversial phenomenon that has puzzled oceanographers for decades. We also predict that fish carbonate production may rise in response to future environmental changes in carbon dioxide, and thus become an increasingly important component of the inorganic carbon cycle.

    As the press release seemed to indicate, this research is dealing mainly with alkalinity rather than pH. I don't see that this research conflicts with any of what has been said already in the thread. The bearing on the original questions seems to quite limited, unless I am missing something drastic.

    Cheers -- sylas
     
  19. Apr 26, 2010 #18

    Evo

    User Avatar

    Staff: Mentor

    pH measures both acidity and alkalinity. A low pH number indicates acidity and a high pH rating indicates alkalinity. Technically, you could say pOH for alkalinity, but the article the OP was referring to just uses a scale with high pH = alkaline, low = acid.

    To get more specific, there are different ways to measure ocean water.

    http://en.wikipedia.org/wiki/PH#Mathematical_Definition
     
    Last edited: Apr 26, 2010
  20. Apr 26, 2010 #19

    Xnn

    User Avatar

    That's a good point, but I don't claim to know if that is true for sure.

    What I do know is that as water warms, its ability to hold dissolved gases, such as CO2 is diminished. So, theoretically, atmospheric CO2 levels do not need to fall for the oceans to release CO2. If the oceans were to warm enough, they could begin to release CO2 even if atmospheric CO2 levels were to continue to rise. It's all a matter of the relative response of each characteristic.

    There is also the question of how the oceans will warm. Will they stagnate or not (and if so, then to what degree)?

    If the oceans were to stagnate significantly, then there would be an exceptionally warm surface layer with very low solubility for CO2 atop a colder layer that could be under saturated.
     
  21. Apr 26, 2010 #20

    Borek

    User Avatar

    Staff: Mentor

    Chemically I can't think of a way of decreasing amount of CO2 in water AND decreasing pH. As Sylas pointed out, more dissolved carbon dioxide means more carbonic acid means lower pH, period. But I guess we can be talking apples and oranges, that is, paleoceanographer/marine-chemist asked by OP was probably not thinking about simple model (bucket of water and air with CO2 above), but much more complicated system with additional sources and sinks of CO2 (biomass, precipitation and so on).
     
  22. Apr 26, 2010 #21

    Evo

    User Avatar

    Staff: Mentor

    Read the Science article Borek, it's in my post. It's not about decreasing CO2.
     
  23. Apr 26, 2010 #22

    Borek

    User Avatar

    Staff: Mentor

    I think that's incorrect. Two solutions with different pH can have the same alkalinity, two solutions with the same pH can have different alkalinity. There is no simple dependency between both.

    Please remember that alkalinity measures (more or less) buffering capacity of the solution, that is it measures how the concentration of H+ changes when acids and bases are added, while pH measures just concentration of H+.
     
  24. Apr 26, 2010 #23

    Evo

    User Avatar

    Staff: Mentor

    Yes, and I edited my post
     
    Last edited: Apr 26, 2010
  25. Apr 26, 2010 #24
    I have now had a chance to read the refence from Wooton et al linked to by Sylas.

    So the authors admit that there is a paucity of data and commendably set about gathering some.

    Their results certainly demonstrate changeability and a significant difference from the above extract from their opening statement compared with conventional wisdom about buffering in the pH range 7 - 9.

    Although their results are taken over 8 years, rather better than the other results they refer to, this is a very short time to distinguish between a genuine downward trend and the catching the downward part of a cyclic phenomenon.

    Further they attribute the lowering of pH to increasing atmospheric concentration of CO2 but I could not find any matching measurements to substantiate or deny this theory.

    Conventional chemical wisdom is that there is buffering action on pH, via the bicarbonate ion, when you add CO2 to seawater, even in substatial quantities. This is because the bicarbonate ion can participate in two different reactions one producing pH reducing protons and the other producing pH raising hydroxyl ions.
    So the conventional expectation to change the pH is by the introduction of another protonic acid.

    I will post all the reactions if anyone is sufficiently interested, they are quite a complex set.

    The authors confirm this chemistry later in the article but do not appear to have studied whether any of the species ( or other input) may have produced such an acid.

    I agree the figures are a bit of a puzzle and beg considerable further investigation, especially to see if they can be reproduced elsewhere in the world.

    In answer to the original question

    Conventional chemistry suggests that CO2 will only be released if the pH falls below about 4. Interestingly this is the pH at which concrete loses its ability to protect reinforcement from corrosion.
     
  26. Apr 26, 2010 #25

    sylas

    User Avatar
    Science Advisor

    No; a high pH indicates basicity, which is not the same as alkalinity. They are different things to measure, not two ways to measure the same thing.

    If we are using wikipedia, then look up the definition of alkalinity, not the definition of pH! You are looking up the wrong thing to understand what the paper is actually talking about. From Alkalinity:
    Alkalinity is sometimes incorrectly used interchangeably with basicity. For example, the pH of a solution can be lowered by the addition of CO2. This will reduce the basicity; however, the alkalinity will remain unchanged (see example below).

    So there is precedent for your usage, but it is not actually technically correct and in particular you should use the more correct definitions when reading the paper cited.

    A more comprehensive reference (which is given in the wikipedia article) is http://132.239.122.17/co2qc/handbook.html, which gives more formal definitions. Alkalinity in seawater is defined there as a sum of concentrations of proton acceptors over proton donors and free hydrogen ions. There is an equation in SOP 3 at the above link which is basically listing the relevant ions.

    This research really isn't conflicting at all with the information I have given previously of more direct relevance to the original question; although it certainly is an interesting addition to understanding of the inorganic carbon cycle.

    Cheers -- sylas

    PS. Missed a flurry of posts while writing. I think we are almost on the same page now with respect to pH and alkalinity -- except that pOH is not alkalinity either... it is still basicity. (I'll defer to Borek with any chemistry questions, by the way!) But the important point is that the research cited on fish and carbonates isn't in fact in any conflict with answers given for the original question, about the relationship between pH and CO2 being absorbed or emitted from seawater.
     
    Last edited by a moderator: Apr 25, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook