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The Acid Ocean – the Other Problem with CO2 Emission

Filed under: — david @ 2 July 2005

The Royal Society has just issued a summary report on the effects of CO2 on the pH chemistry of seawater and aquatic organisms and ecosystems. In addition to its pivotal role in the atmosphere in the regulation of global climate, CO2 and its sister chemical species, HCO3 and CO32- comprise the carbonate buffer system which regulates the pH of seawater. The new report can be found here. Acidifying the ocean is particularly detrimental to organisms that secrete shell material made of CaCO3, such as coral reefs and a type of phytoplankton called coccolithophorids [Kleypas et al., 1999]. The ocean pH change will persist for thousands of years. Because the fossil fuel CO2 rise is faster than natural CO2 increases in the past, the ocean will be acidified to a much greater extent than has occurred naturally in at least the past 800,000 years [Caldeira and Wicket, 2003].

For those of you who look back on your freshman chemistry days with less than fondness, the acidity or pH of an aqueous solution is a measure of the concentration of H+ ions in the solution, with low pH meaning high H+ concentration. H+ ions are aggressive little guys, and too much H+ in water can burn the skin off your hand or make a coral limestone go fizz. The link between CO2 and H+ arises by the combination of CO2 and water, H2O, to form carbonic acid, H2CO3. An acid is a chemical species that releases H+ ions into solution, as does H2CO3 to form HCO3 and CO32-. Adding CO2 to water causes the pH to drop.

The pH of seawater is buffered by the chemistry of carbon, just as is the chemistry of blood and cellular fluids. The buffering action arises from the fact that the concentrations of the various carbon species are much higher than is the concentration of H+ ions. If some process tries to add or remove H+ ions, the amount of H+ ions required will be determined by the amount of the carbon species that have to be converted from one form to another. This will be an amount much higher than the actual change in H+ concentration itself.

Most of the carbon in seawater is in the form of HCO3, while the concentrations of CO32- and dissolved CO2 are one and two orders of magnitude lower, respectively. The equilibrium reaction for CO2 chemistry in seawater that most cogently captures its behavior is

CO2 + CO32- + H2O == 2 HCO3

where I am using double equal signs as double arrows, denoting chemical equilibrium. Since this is a chemical equilibrium, Le Chatlier’s principal states that a perturbation, by say the addition of CO2, will cause the equilibrium to shift in such a way as to minimize the perturbation. In this case, it moves to the right. The concentration of CO2 goes up, while the concentration of CO32- goes down. The concentration of HCO3 goes up a bit, but there is so much HCO3 that the relative change in HCO3 is smaller than the changes are for CO2 and CO32-. It works out in the end that CO2 and CO32- are very nearly inversely related to each other, as if CO2 times CO32- equaled a constant.

Coral reefs are built from limestone by the reaction Ca2+ + CO32- == CaCO3, where Ca is calcium. Acidifying the ocean decreases the concentration of CO32- ions, which by le Chatlier’s principal shifts the equilibrium toward the left, tending to dissolve CaCO3. Note that this is a sort of counter-intuitive result, that adding CO2 should make reefs dissolve rather than pushing carbon into making more reefs. It’s all because of those H+ ions.

CaCO3 tends to dissolve in the deep ocean, both because of the high pressure and because the waters have been acidified by CO2 from rotting dead plankton. Surface waters, however, are supersaturated with respect to CaCO3, meaning that there is enough Ca2+ and CO32- in surface waters that you could give up some, and still not provoke CaCO3 to dissolve. However, it has been documented that corals produce CaCO3 more slowly as the extent of supersaturation decreases. This is also true for planktonic CaCO3-secreters such as coccolithophorids and foraminifera. We should note that for coral reef communities, the acid ocean is only one problem that they face, and it’s not the worst. Rising temperatures are tightly correlated with coral bleaching events, the expulsion of symbiotic algae, often followed by death of the coral. There is a terrifying time-series of temperature and coral bleaching from Tahiti in Hoegh-Guldberg, 1999]. When you look at the temperatures that killed the coral, and project future temperatures, it looks to be all over for corals. Coral communities are also impacted by water turbidity, resulting from fertilizer runoff, and by overfishing.

Elevated CO2 levels also affect fish and other aquatic organisms, in part because of the decrease in pH, but also because CO2 is what heterotrophic organisms try to exhale. However, we should note that dissolved CO2 levels were substantially higher than today in the geologic past, and organisms were able to cope with this OK, so apparently there can be some acclimation of populations to higher CO2.

The natural pH of the ocean is determined by a need to balance the deposition and burial of CaCO3 on the sea floor against the influx of Ca2+ and CO32- into the ocean from dissolving rocks on land, called weathering. These processes stabilize the pH of the ocean, by a mechanism called CaCO3 compensation. CaCO3 compensation works on time scales of thousands of years or so. Because of CaCO3 compensation, the oceans were probably at close to their present pH of around 8 even millions of years ago when atmospheric CO2 was 10 times the present value or whatever it was. The CaCO3 cycle was discussed briefly in regards to the uptake of fossil fuel by the ocean, here. The point of bringing it up again is to note that if the CO2 concentration of the atmosphere changes more slowly than this, as it always has throughout the Vostok record, the pH of the ocean will be relatively unaffected because CaCO3 compensation can keep up. The fossil fuel acidification is much faster than natural changes, and so the acid spike will be more intense than the earth has seen in at least 800,000 years.

There are several feedbacks between decreasing the rate of calcification that organisms do in the ocean, and the carbon cycle. Removing CaCO3 from surface waters tends to raise the CO2 concentration of the waters (it should be possible for you to work that out for yourself based on the chemical reactions above). This is a negative feedback, tending to remove excess CO2 from the atmosphere, but it is a small effect. Decreasing the flux of CaCO3 to the sea floor tends to diminish the amount of CaCO3 that gets buried in sediments, which hastens the pH-recovery from the CaCO3 compensation mechanism. This may not be a small effect at all, but it is a slow effect: thousands of years.

Caldeira, K., and Wickett, M.E. Anthropogenic carbon and ocean pH. Nature: 425, 365, 2003.

Hoegh-Guldberg, O. Climate change, coral bleaching and the future of the world’s coral reefs. Mar. Freshwater Res.: 50, 839-8–66, 1999.

Kleypas, J., R.W. Buddemeier, D. Archer, J.-P. Gattuso, C. Langdon, and B. Opdyke (1999) Geochemical consequences of increased atmospheric CO2 on coral reefs. Science 284: 118-120.

66 Responses to “The Acid Ocean – the Other Problem with CO2 Emission”

  1. 51
    Steve Latham says:
    This article describes some potential food web changes in the NE Pacific this year.

    With respect to evolution, and this comment from #47:
    “Complex marine organisms like fish usually have many offspring at each spawning event and because of this it is sometimes assumed that their rate of reproduction is rapid. This is not necessarily so. Most marine animals have large amounts of small and poorly developed offspring and do not exercise any parental care. Very few offspring survive but this is balanced out by having large numbers of offspring. The net result is under natural conditions these animals do not reproduce rapidly. Because they do not reproduce rapidly they are less likely to adapt to rapid environmental changes.”
    I think there are a couple of issues here that could be clarified.
    First, it is a short generation time relative to the speed of environmental change that makes “reproducing rapidly” relevant to the ability of a species/population to track environmental changes in an adaptive manner. In sexually reproducing organisms it means the replacement of the parental generation so that the next round of recombination involves more successful alleles (genetic variants) than were present in the previous generation. Note that in the case of directional environmental change, parental care will be associated with slower generation turn-over and slower adaptation in general (parental care is great for fluctuating environments to help offspring survive until more favorable conditions return).
    Second, many algae reproduce primarily by asexual fission, but in times of stress will be more likely to reproduce sexually.
    Third, the intrinsic rate of increase (proxied by the output of offspring over time regardless of survival) actually helps natural selection to operate. By increasing the sample size of allelic variants and combinations, it is less likely that those which are best suited to prevailing environmental conditions will be lost by chance. That is, the stochastic nature of genetic drift becomes less important as selection can more thoroughly choose among offspring. The same idea applies to the total number of individuals in a population or species.

  2. 52
    Joseph O'Sullivan says:

    A clarification of the clarification
    It’s a little off topic but at least its not one of my long comments about the politics. There are other issues concerning ecosystems, evolution, climate change and acidification.

    Phytoplankton reproduce rapidly asexually (bloom is the term used), but when they reproduce sexually the reproduction rate is slower, making it less likely to adapt to rapid environmental change. Sexual reproduction leads to offspring that differ in small ways from their parents, but in asexual reproduction offspring are essentially clones and identical to their parents. These small variations could be traits that allow adaptation; this is again basic evolution theory.

    Rapid reproduction is in part a function of how long it takes offspring to mature and have offspring themselves. If they mature rapidly reproduction can be rapid.

    If an organism has a large number of offspring it is more likely that some will have traits that could aid adaptation and it is more likely some offspring with these advantageous traits will survive. This is basic evolution theory and statistics. As an example for the purpose of this discussion 1% offspring have a trait. If one species has 1000 offspring and another 100,000 offspring per spawning event there will be 10 and 1000 offspring respectively with the trait. This goes to the logical conclusion that it will be much more likely that some of 1000 to survive into adulthood then the 10.

    As Dave noted biological systems are messy, and there are additional considerations that determine the ability of a species to adapt to rapid environmental change. There are species that will be able to adapt to climate change and acidification in the oceans. Organisms that have large numbers of offspring, these offspring mature rapidly and the third factor, the ability to live in a wide range of environmental conditions, are the ones most likely to survive rapid environmental change. This adaptability makes it more likely for large numbers of offspring to survive when environmental conditions change. These types of animals and plants are called technically r-selected and less technical but fitting “weedy”.

    Wikipedia has a good short explanation of this here

    There are some interesting papers in press.

    First, in the 125th anniversary issue of Science there was a section about what we don’t know, and one of the things listed was the ecological effects of global warming, but didn’t go into any detail. My educated guess would be that to understand ecological changes it is necessary to accurately predict regional climate changes, which as far as I know we can’t, and predicting biological systems is difficult because of their chaotic nature and our incomplete understanding of ecological processes.

    There is a very interesting recent paper in Science. It states that fish are shifting their ranges as a consequence of warming temperatures, and notes that species with a more rapid reproduction rate are shifting more then fish with slower reproduction rates.
    Climate change and distribution shifts in marine fishes Science June 05
    Its subscription only but the abstract is here on a non-subscription site.

  3. 53
    Michael Jankowski says:

    Then again…

    Tasmanian coral reef ‘proof of global warming’

    ***Scientists believe they have discovered proof that global warming has altered Tasmania’s marine environment.

    A group of biologists from the Tasmanian Aquaculture and Fisheries Institute has found a shallow reef extensively covered by coral at the Kent Group Marine Protected Area near Flinders Island off the north-east of Tasmania.

    Coral reefs only survive in warmer waters and are usually found in tropical areas such as Queensland.

    The senior biologist who discovered the reef, Neville Barrett, believes it is evidence that rising water temperatures are having an impact on the marine environment.

    “A lot of these corals are occasionally found in very small clumps in Bass Strait,” Dr Barrett said.

    “It’s exceptionally rare to find very extensive, pretty much 100 per cent cover areas of it south of Sydney, so it’s something completely unexpected.”…***

  4. 54
    Joseph O'Sullivan says:

    A final clarification

    I now understand what the scientists at RealClimate have to go through to when explaining complex scientific topics.

    The paragraph in Science about the effect of climate change on ecosystems did not state that we do not know if there will be effects on the ecosystem. Climate change is already starting to have an effect on ecosystems. What is not known is the details and degree of disturbance that will be caused by climate change.

    There have been reports that have tracked smaller and short-term changes like ENSO and NAO and how ocean ecosystems have reacted. See
    Shifts in deep-sea community structure linked to climate and food supply.
    Climatic influence on a marine fish assemblage
    So climate change and its effects on ecosystems, while many details remain uncertain, is something that has been studied and there have been direct observations in the field.
    The appearance of corals outside their normal range (#53) is another piece of evidence that the climate is changing and is consistent with global warming.

    Acidification of the oceans (to get closer to the topic of the post) is different. As a Pew report noted “our knowledge of this influence and these interactions is rudimentary, making it difficult to predict the consequences of any chemical changes”. There has not been a lot of work on this issue and there have not been episodes equivalent to ENSO and NAO that allow real world observation. Most of the work has been laboratory studies and modeling. I want to thank Ken Caldeira for his comments. It’s always good to get comments on RealClimate from the authors of the cited papers.

    The evolution topic (brought up by #16) is kind of a red herring. When anthropogenic disturbances of ecosystems are considered some people counter that the plants and animals will evolve so will not be harmed. Some species do evolve in response to environmental change but many others do not and become extinct. In the past glacial cycles organisms and ecosystems responded to climate change by shifting geographical ranges and when unable to shift local populations died and at times entire species became extinct. Some species underwent micro-evolutionary changes that enhanced their ability to adapt but large-scale evolutionary changes that allowed adaptation were rare.
    Both acidification and climate change are now occurring at rates that may out pace any evolutionary changes. The ability of entire ecosystems and individual species to change distribution is now limited by antropogenic habitat fragmentation.

    I hope I have explained this clearly. If not or for those who want further information the Pew Foundation has put together several reviews and summaries of the scientific literature addressing global warming and ecosystems. See
    Environmental reports

    Some of the interesting reports include
    Observed impacts of global warming in the U.S. Nov. 2004
    Coastal and Marine Ecosystems and Global Change Aug. 2002

  5. 55
    Michael Jankowski says:

    “The appearance of corals outside their normal range (#53) is another piece of evidence that the climate is changing and is consistent with global warming.”

    Thriving coral in the area mentioned in #53 may be consistent with global warming, but I would argue that decreasing coral in the area mentioned in #53 would also be considered consistent with global warming.

    #53 seems to be potentially conflicting with the woes discussed in “the Acid Ocean.”

  6. 56
    Georgia says:

    In your response to question #10
    “The acidification of the ocean is a much longer-term issue than acid rain, which goes away about two weeks after you reduce sulfur and nitrogen emissions from smokestacks.” is a bit misleading.
    While acidic rain fall would indeed go away if emissions were reduced significantly the impact from acid rain is thought to be much longer term with recovery of soils taking some time to occur due to sulfur and nitrogen storage. Check the Hubbard Brook Research Foundation site and papers…
    It may still be a quicker recovery than ocean acidification but it would take longer than a few weeks for terrestrial soils to rebound from many decades of acidification.

    [Response:You are absolutely right. What was I thinking? Acidification of soils is a process that takes centuries, and it takes centuries to recover. I stand corrected. Thanks! David]

    btw- thanks to all for this great site.

  7. 57
    Joseph O'Sullivan says:

    Re #55 “Thriving coral in the area mentioned in #53 may be consistent with global warming”

    It is ecosystem disturbance not just individual corals thriving. The temperate reef ecosystems with scattered corals are being displaced by a tropical coral-dominated ecosystem. This is a local example of a shift in an ecosystem caused by a climate change. Geographical shifts like this one are known to be the result of climate changes. The scientists interviewed did say that while this is clear evidence of warming they said that more research is needed to directly attribute it to anthropogenic global warming.

    “I would argue that decreasing coral in the area mentioned in #53 would also be considered consistent with global warming.”

    I think you might be confused with another issue, coral bleaching. In some of the tropical oceans where reef-building corals live temperatures rose above average tropical ocean temperatures primarily due to the ENSO. These higher temperatures caused large-scale coral mortality. These temperature increases, only a few degrees C, are less then the predicted warming that will be caused by anthropogenic climate change.

    The #53 article described a different situation. The waters in Tasmania were temperate where there is limited coral survival and have warmed to become in some areas tropical, which has allowed coral-dominated ecosystems to displace the naturally occurring temperate ecosystems. It is not a thermal maximum for coral being exceeded it is temperatures rising above a thermal minimum for coral.

    “#53 seems to be potentially conflicting with woes discussed in ‘the Acid Ocean'”

    The effect of acidification on living things is something that has only recently been examined. There have been laboratory studies that have shown a reduction in growth of stony corals due to CO2. It is probably premature to make definitive claims about the effects of acidification on individual examples without further research.
    Also, all species of coral will not react the same to acidification. Different species of coral will have different levels of resiliency. It is possible that the corals in question are species that are more tolerant of acidification. #16 stated fish live in a wide range of pH. The 24,000 species of fish live in pHs that range approximately from 5 to 10 but individual species, especially marine fish, have a much narrower range of livable pH and these ranges vary from species to species.
    Finally the acidification has only recently started and even though there might be no harm now, it is increasingly likely that harm will occur as the pH drops further.

  8. 58
    Steve Latham says:

    Hi Joseph (#57),
    Mike Jankowski doesn’t need any defense from me but I don’t think he is confused at all. I think he is just saying that whatever changes were happening to the coral, they would have been blamed on anthropogenic global warming. Your point that it’s the changing species compositions of these ecosystems that is informative, etc, is one with which I agree very very much. I hope someone is studying which species should be the winners and losers in acidifying and warming habitats so that trends can be predicted and, when observed, be attributed properly to cause. Otherwise we’ll just be noticing that things are changing and wanting to blame AGW whilst the skeptics will say, “Change ain’t new!”

  9. 59
    Steve Latham says:

    Judging by the feasibility scores, it looks like the warming part of the problem is solved, and for most of these the acidification is taken care of also:,20967,1075786-4,00.html

  10. 60
    Michael Jankowski says:

    RE#59 Steve,

    Is the “John Latham” in your link by any chance related? Just curious (no other reason).

  11. 61
    Steve Latham says:

    Hi Mike, evolutionarily-speaking we’re all related but the direct answer is no. There are no (other) scientists in my family which is why I spend too much time conversing about science stuff on the internet.

    I have wanted to respond to your question regarding your analogy on the now defunct ‘bet’ thread: I guess you would have to know whether or not it was possible to have a better driveway. You couldn’t very well complain if everybody else’s driveways were as crappy as yours, and if the driveway was up to the standards advertised by the contractor. So maybe you need an example or you need to provide some expertise if you want to demand better than what’s available. Ah, but here’s a rebuttal you could have to my ‘logic’: “If the only products for some service suck, then I won’t subscribe to that service, yet I’m forced to buy into the climate models via taxes, government regulation, etc.” If we just stick to the science, though, and you’re not forced to buy, then you might just have to admit that the models are the best there is and you have very little basis for demonstrating that they are worse than they should be.

  12. 62
    Tom Benson says:

    Hi, just stumbled over this forum. Good comments!

    Here’s a question…it seemed from my read of the Royal Society report that a significant number of plankton species might be adversely affected by the ocean acidification. Since plankton account for 50% or so of the carbon recycling on the planet (plus permanent carbon sequestration if the iron fertilization hypothesis is right)doesn’t this mean that CO2-induced acidification would accelerate the rate of CO2 increase in the atmosphere, which would then accelerate the rate of acidification? Would this qualify as a runaway cycle?

    Or am I missing something here?

  13. 63
    Steve Latham says:

    Re: #62, Yep, Tom, I think you’re missing something. You’re saying that with less plankton to sequester CO2 to the bottom of the ocean we will have more CO2 in the atmosphere. Okay, that’s sort of true — let’s take it to its logical conclusion where the ocean is highly acidic and there are no phytoplankton. It means that less CO2 will be sequestered, and the near-surface CO2 levels in the ocean will saturate more easily (because there is no transport to the depths by plankton), and therefore LESS CO2 will be taken up by the ocean to reduce the burden on the atmosphere. It does not mean that MORE CO2 is going to go to the atmosphere. Therefore this is not a runaway cycle.

    Another problem with your suggestion (again this is just my opinion) is that you say “significant number of plankton species”. The plankton species that are less affected will likely replace the others and continue their role as carbon recyclers (perhaps at reduced efficiency but still doing it).

  14. 64
    Steve Latham says:

    This is about changing fish diversity in response to fishing and temperature changes. Perhaps because it’s a news article no trend is very well described, yet conclusions are made.

  15. 65
    Joseph O'Sullivan says:

    I did write that I was done with the evolution and climate change discussion, but I am back again.

    I did a quick review of some of the literature and I found a lot of interesting info. In conservation biology evolution was not really considered, but evolution really is a factor. Small evolutionary changes (microevolution) do occur at rapid rates and have occurred in response to various anthropogenic changes. There will be microevolutionary changes that will be a factor when ecosystems react to anthropogenic climate change, but this does not mean that there is no need to worry because plants and animals will just evolve and not be harmed.

    Some interesting papers:
    Keeping Pace with Fast Climate Change- Can Arctic life Count on Evolution?
    Evolutionary Enlightened Management
    Contemporary Evolution meets Conservation Biology,%20Hendry,%20and%20Kinnison%202003.pdf
    (cut and paste this one, the commas in the address screw up the link)

    Re #62 Could Acidification be a run-away cycle? In theory I guess it could be. 1 increased CO2 causes acidification of the oceans, which kills phytoplankton; 2 less phytoplankton means less CO2 is sequestered; 3 this increases unsequestered CO2 and then go back to 1. I am unsure if this is a possible real world outcome, but I don’t think so.

    There doesn’t seem to be a lot of scientific literature on the effect of acidification on marine ecosystems.
    A short outline of what needs to be examined about CO2 and the oceans is here

    A few months ago the scientists at RealClimate recommended a paper (Feely et al 2004, Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans) and said Dr. Feely thinks that acidification of the oceans is a serious issue that needs more attention from the scientific community. From what I have seen I have to agree.

    Even environmental groups who have a reputation for alarmism have really not mentioned acidification until the Royal Society Report came out. The only time I heard about it was on the NRDC site but only a paragraph in a summary of climate change science papers, but not really anywhere else.

  16. 66

    […] ongoing problem of ocean acidification, triggered by the oceans absorbing CO2. For a primer, see this article on RealClimate. [Categories: Global warming, Hydrogen] [] [link] […]