Coral Reefs, Temperature and Ocean pH

By Andy May

Georgiou, et al. 2015 have reported that coral reefs in the Australian Great Barrier Reef, near Heron Island, are insensitive to ocean pH changes. The location of Heron Island, about 257 miles (414 km) north of Brisbane, Queensland, Australia, is shown in figure 1 using Google maps.

Figure 1

Figure 2 shows the island and a portion of the reef using Google Earth.

Figure 2

Georgiou and colleagues observed that while the pH of the ocean around the reef may vary dramatically, the pH at the site of calcification remains in a narrow range. This suggests that the animals (coral polyps) can actively modify the pH of the water at the calcification site to maximize their growth rate. To quote the paper:

“This result reflects the capacity of these coral to homeostatically maintain a pHsw [pH of seawater] of ~8.4–8.6 at the site of calcification … and thus near constant up-regulation of pHcf [pH at cite of calcification] during the calcification process. As such, these findings are in marked contrast to earlier laboratory studies in which corals grown under stable and constant pH conditions exhibited a stronger sensitivity to ambient pH, whereby pHcf decreased by up to 0.5 units for each unit decrease in ambient seawater pH.”

Georgiou, et al. did their experiments in situ the open ocean by building enclosures that are open on two sides and on the bottom and raising the level of CO2 in the “treated” enclosures. They compared the results to nearby corals that were in enclosures with no added CO2 (the “controls”). They found no reef growth differences between the two environments. This implies a high degree of tolerance to ocean acidification. The enclosures for injecting CO2 and the control enclosures are called FOCE (Free Ocean Carbon Enrichment) and are described in Kline, et al., 2012. They describe a very ingenious way to measure the effects of sea-water carbon dioxide concentration differences on corals in-situ.

They constructed four FOCE systems near the island. There were two controls and two treatment systems. They were all oriented parallel to the shore with each flume open at both ends and on the bottom. Figure 3, which is a portion of figure 5 in Kline, et al., 2012, shows a photograph of the flumes and a schematic of the system.

Figure 3, source Kline, et al., 2012

The area chosen for the experiment has a lot of diel (daily) variation and a lot of seasonal variation in both pH and temperature. This can be seen in figure 4A and figure 4C below. Figure 4 is a portion of figure 1 in Georgiou, et al., 2015.

It is a little difficult to see the main point of the paper from figure 4, so I downloaded the supplementary data and made figure 5. In figure 5, the orange and yellow curves are the average monthly calcification site pH for the control and treated (added CO2) areas. The blue and gray curves are the average monthly pH for the ambient ocean around the experiment. There are two things to notice about figure 5. First, the calcification site on the corals has a controlled pH of about 8.5, regardless of the ambient ocean pH. Second, the ambient ocean pH, over the reef, varies 0.2 units, without CO2 treatment, over the six months of the experiment.

Figure 5: Monthly average pH for the controls and the CO2 treated areas.

There are two differences between this study and earlier laboratory studies that are significant. First this study was done on an actual dynamic, growing reef. Second and just as important, this study was done in an area where there are natural diel and seasonal changes. Thus, the corals in this environment are used to changes in pH and deal with it routinely and the additional changes due to CO2 injection do not affect them. In the laboratory, the animals adapted to a constant pH, had no access to alternative symbiotic algae and when it is radically changed they are affected. As discussed by Jim Steele here, corals live with a variety of symbiotic algae species, each uniquely adapted to a particular environment and the corals receive up to 90% of their food from their algae. Thus, as their natural environment changes the corals will expel their current symbiotic algae and repopulate with a more suitable species. This is an action they can do in their native environment, but not in the laboratory. The ability to maintain a relatively constant pH at the calcification site varies from species to species, the Porites coral species seems to be particularly resilient to ocean acidification.

It has been reported by Hofmann, et al., 2011, that the natural daily and seasonal range of pH in the oceans is much larger than model predictions of future changes in pH. They illustrate the daily range in their figure 2, shown here as figure 6. Currently, the open ocean pH is between 8.01 and 8.08, as seen in figure 6 (top left). Climate model projections predict that if we continue emitting CO2 on the current trend, ocean pH will drop to 7.8 by the end of this century according to the European Project on Ocean Acidification (EPOCA).

Figure 6, source: Hofmann, et al., 2011

These graphs are for 30-day periods, so they do not reflect seasonal variability which can be significant. Compare these to the blue and gray curves in figure 5, which cover six months on the Great Barrier Reef.

EPOCA claims in an alarming tone:

“Modeling demonstrates that if CO2 continues to be released on current trends, ocean average pH will reach 7.8 by the end of this century, corresponding to 0.5 units below the pre-industrial level, a pH level that has not been experienced for several millions of years. A change of 0.5 units might not sound as a very big change, but the pH scale is logarithmic meaning that such a change is equivalent to a three-fold increase in H+ concentration. All this is happening at a speed 100 times greater than has ever been observed during the geological past. Several marine species, communities and ecosystems might not have the time to acclimate or adapt to these fast changes in ocean chemistry.”

Figures 5 and 6 show changes much larger than EPOCA have forecast occur on both a monthly basis and on a seasonal basis. These changes occur near existing reefs as well as in other parts of the ocean. Joint, et al., 2011, have concluded:

“However, it is important to place these changes within the context of pH in the present-day ocean, which is not constant; it varies systematically with season, depth and along productivity gradients. Yet this natural variability in pH has rarely been considered in assessments of the effect of ocean acidification on marine microbes. Surface pH can change as a consequence of microbial utilization and production of carbon dioxide, and to a lesser extent other microbially mediated processes such as nitrification. Useful comparisons can be made with microbes in other aquatic environments that readily accommodate very large and rapid pH change. For example, in many freshwater lakes, pH changes that are orders of magnitude greater than those projected for the twenty-second century oceans can occur over periods of hours. Marine and freshwater assemblages have always experienced variable pH conditions.”

Georgiou, et al. found that the growth rates of the coral nubbins (coral seedlings) in the CO2 flumes, the control flumes, and in the surrounding area were very similar. More importantly, the density of the low pH FOCE corals was within 3% of the control corals. Thus, the corals were able to grow at normal rates, even when the ambient pH was low (ΔpH = -0.25).

There are many unknowns about how reefs will react to higher CO2 concentrations in sea water and to higher temperatures. It is also apparent from the study that different species of corals will react differently. However, nearly all of the reef-building Scleractinia coral species alive today survived the “PETM” or the Paleocene-Eocene Thermal Maximum, 50 million years ago, when global temperatures were 9°C warmer than today (Paleomap Project) and the atmospheric CO2 was twice what it is today (Black Hills Institute). A temperature history of the Phanerozoic can be seen in figure 7 here. The Scleractinia Coral family tree is shown in figure 7.

Figure 7: The dominant reef building animal today is the Scleractinian coral. The various types of Scleractinian corals are shown in the family tree. Many went extinct at the beginning of the Paleocene or Eocene, but with a few exceptions, if they survived the extinction event at the end of the Cretaceous and the cooling at the end of the PETM (Paleocene-Eocene Thermal Maximum) they still exist today. Source: AIMS.

Discussion and Conclusions

While the study performed by Georgiou, et al. only involves a small area near Heron Island on the Australian Great Barrier Reef and only a few species of corals, it does demonstrate that some corals actively modify their environment and grow in a lower pH environment. Further, the study highlights the diel and seasonal variability in pH in our oceans and shows it is greater than the modeled changes in pH ascribed to increasing atmospheric CO2 concentrations.

We do not contest the idea that additional CO2 in the atmosphere will lower ocean pH, this is easily demonstrated. Although, the amount of change in pH is in question, the ocean is buffered, so the ultimate change in pH may be very small.  We see no evidence that this will affect marine life, as marine organisms have demonstrated an ability to tolerate rapid changes in ocean pH. They demonstrate this on a daily and seasonal basis. Further, existing species of reef-building Scleractinian corals have thrived in much higher temperatures and much higher carbon dioxide concentrations as (geologically) recently as 50 million years ago.

Like other aspects of the climate change scare, or really any environmental scare, someone finds something (anything) that changes due to man’s influence and assumes it changes for the worse. This assumption that change is bad, is proclaimed without checking the geological record to see if it has happened before. Or, in this case, without even checking to see if it happens today on a daily or seasonal basis.

We do not contest the idea that rapidly rising temperatures cause corals to undergo thermal stress, but this is temporary and corals do adapt with time, as discussed by Jim Steele here. The symbiotic algae that corals depend on for most of their food vary with depth, temperature, pH, and other factors. Laboratory experiments conducted in tanks remove corals from a source of alternative symbionts (algae) that will allow them to adapt, essentially freezing their environment. So, removed from their natural state, and their main adaptation mechanism, they are harmed by rapidly rising temperatures and changes in pH. Corals, in the ocean, have no problem with the same changes.

Further, reefs are composed of many different species and they each adapt to changes in pH and temperature (and other environmental factors) in different ways, but they do adapt. A very good discussion of the adaptability of corals, also by Jim Steele, can be found here. As Steele notes, most reef deaths are due to cold temperatures, not warm temperatures, a fact often ignored by NOAA and others with an agenda.


41 thoughts on “Coral Reefs, Temperature and Ocean pH

  1. Blimey. An actual real world experiment!

    Surely a first for ‘climate studies’.

    Expect the usual crop of bedwetters to tell us how the results are clearly wrong as they disagree with the infallible sacrosanct models invented in a darkened room by nerds who found practical science far too hard.

  2. One of the issues is the fact that pH is not a conservative quantity. It can vary with temperature, pressure, salinity, photosynthesis rates and many other factors.

    This claim is abject nonsense:

    Modeling demonstrates that if CO2 continues to be released on current trends, ocean average pH will reach 7.8 by the end of this century, corresponding to 0.5 units below the pre-industrial level, a pH level that has not been experienced for several millions of years.

    This is entirely based on the assumption of a linear relationship between CO2 and pH, with the assumption that all other factors are static.


      This nomogram outlines many of the variables…

      Using the Hawaiian Ocean Time Series (HOTS) at Station Aloha, I crosplotted total alkalinity (TA) vs dissolved organic carbon (DIC) using in situ and constant salinity.

      For a doubling of atmospheric CO2, the constant salinity (poor correlation) indicated a pH drop from 8.40 to 7.96. The in situ salinity (strong correlation) yielded a much smaller pH decline from 8.30 to 8.14.

      • Good analysis. EPOCA is just wrong because of the ocean’s strong pH buffering in the carbonic acid/carbonate system. Essay Shell Games provided detailed calcs and references, as well as the range of average pH in different parts of the Pacific ecosystems.

      • Thanks David. Ronan Connolly noted that the ocean is heavily buffered as well. The buffering may cause the theoretical effect of adding CO2 immeasurable. There are so many reasons why laboratory measurements may not relate to what happens in nature, its amazing anyone has the courage to make a prediction.

      • That’s because most of the predictions, particularly the dire ones, won’t be verified or falsified for decades.

        I wish I could make “predictions” about wells we drill, collect my bonus, and then not see any drilling results until long-after retirement.

        It always makes me think of a scene from Ghostbusters

    • They aren’t testing the ocean ph over time claim, they are marine biologist they are simply following if that is true lets test what would happen to the coral.

    • Well, there are other inherent assumptions in there. For example, the assumption that measurements recorded by ships are in some way homogeneous as to circumstance and accuracy. Suppose measurements were made at noon each day. Is that typical of the 24 hr average? Obviously not, rendering them incompatible with night-time readings.

      The ‘millions of years’ is abject nonsense. It depends where you measure and when. There is nothing like enough data available to make a determination of a ‘global average’ which looks more and more like a postulated ‘global average temperature’. It is an abstract number with no local relevance.

      I just heard that the Climate & Clean Air Coalition (CCAC) wrapped up their 21st working session in Paris. Their announcement includes this:

      “On the 30th anniversary of the Montreal Protocol, leaders called for the world’s nations to quickly ratify an international agreement that could prevent up to half a degree of global warming this century.”

      Leaders? So now it is “half a degree”. We can expect, then, that the terrible consequences of a change in pH will also be tightened until the ‘target’ we should achieve is 1/4 of the natural range.

      Altering the performance requirements to suit the circumstances so you always get a ‘win’ is the stuff of legend:

      How many Microsoft programmers does it take to change a lightbulb?

      None. When the lightbulb fails to performance as expected, they rewrite the performance standard to, “Dark”.

    • Indeed, and stated with usual unwarranted certainty that it “will” happen, which is baseless.

      For me the real take away from this very good article is this:

      However, nearly all of the reef-building Scleractinia coral species alive today survived the “PETM” or the Paleocene-Eocene Thermal Maximum, 50 million years ago, when global temperatures were 9°C warmer than today (Paleomap Project) and the atmospheric CO2 was twice what it is today (Black Hills Institute).

      • To some extent, “it will happen”… There’s just no evidence that it will be significant.

        PETM is an oddity. Atmospheric CO2 could have been anywhere from 400 to 2,000 ppm. Proxy data are all over the place. It was hot. And there was a definitive shoaling of the lysocline (shallowing of the carbonate compensation depth). Ocean acidification is a misnomer for shoaling of the lysocline.

        It was probably related to the formation of the North Atlantic Large Igneous Province… but, who knows?

  3. Note the total lack of correlation between atmospheric CO2 and paleo pH at Flinders Reef.

    And the relatively strong correlation between pH and the low-frequency signal of the PDO…

  4. So another hypothesis about the Reef hangs by a thread. I leave open the possibility there is some species of coral that is sensitive to ph. This is what makes me really concerned with all the Eco-activists that they have no idea how the reef eco system works and yet they are so keen to do things under the guise of helping the reef. I wonder why I get concerned at the recent human assault on the crown of thorn starfish.

    • While there exists the possibility that there are corals that are sensitive to ocean pH levels, they would have necessarily had to recently develop this trait as their ancestors would not have survived the eons of varying ocean pH levels.

  5. We do not contest the idea that additional CO2 in the atmosphere will lower ocean pH, this is easily demonstrated.

    How? Seawater is a buffer solution. The buffer properties are easily demonstrated e.g. at

    • Not only is the ocean a buffer solution, it is a complex buffer solution containing hundreds to thousands of buffering agents and is practically an infinite buffer due to the constant cycling of sources and sinks in addition to its very large volume.

      • And not only that, but it also has a virtually unlimited source of salts to add to the solution at any given moment in order to maintain that buffer.

        Sediment advection, ground water discharge, and the tidal pump, can someone find me an ocean acidification focused paper that even acknowledges that these processes even exist?

    • I agree and others have said I overstated my agreement. It is kind a like stating you agree man has some influence on climate. I do agree, but how much? Is the amount even measureable? The oceans have a very strong buffering system indeed.

    • pH in muddy Louisiana bays, some paved with mollusks which live in reduced sediments, can rarely reach actual acid levels. This occurs most often near the bottom early in the morning from sediment oxygen demand. There are a number of mechanisms used to overcome this, especially for those buried in difficult sediments. Buffering capacity, even in much diluted ocean water, is impressive. However, it is not a good place for corals. Maybe not even for those studying pH.

  6. Hello McFly, we have known biocalcification in marine organisms is mediated with biofilms/mucus for decades, but it’s nice to see the climastrology crowd slowly catching up.

    And if the climatologists want to demonstrate some empirical evidence of CO2 induced acidification effects on biocalcification, they should probably start with fresh water organisms that actually do live in acidic water instead of a highly buffered alkaline solution where abiotic precipitation of CaCO3 (let alone biomediated) is still occurring to this day.

  7. Rapid sea-level change caused by ENSO oscillation, as I think reported previously at wuwt, will make a significant difference to the type of symbiont algae that are best suited to the coral.

  8. Ocean acidification was an important discussion factor in scientific circles a few years ago, but it has all but disappeared under the radar due to lack of evidence and some embarrassing bad science being exposed.

    But the scare just changes from one factor to another and refuses to die like in a bad movie. From polar bears to walruses. From corals dying from acidification to dying from bleaching. People are so conditioned to the climate scare that they will just swallow anything from the climate change franchise.

  9. Consider these items.

    1. Corals evolved millions of years ago when atmospheric pH and temperatures were a LOT higher than it is now, and yet they survived and even thrived.
    2. The corals in my saltwater tank routinely see a pH of 7.8 (and sometimes even a little lower) and yet somehow survive and even thrive.

    Therefore, I would suggest that the supposed conclusion of this paper is not groundbreaking at all, but is obvious to a casual observer. Oh, and, yeah, I think the corals will most likely outlast us unless the sun ceases to shine.

  10. The interesting thing to me is that corals first appeared on earth when the atmospheric CO2 concentration was around 7,000 ppm. They seem to like lots of CO2.

  11. scientists discover something…..that everyone else has known for decades
    …film at 11

    Of course corals modify their pH…they have to!

  12. Its not the coral modifying their pH. Its the zooxanthellate algae that live within the coral polyp that are the consumers of CO2.

    • nope…zoox have nothing to do with it…..the corals actively increase the pH of the calcification fluid creating a diffusion gradient that favors the transport of molecular CO2 from the tissues to the calcification site…high pH and saturation cause precipitation of aragonite…

  13. Georgiou and colleagues observed that while the pH of the ocean around the reef may vary dramatically, the pH at the site of calcification remains in a narrow range. This suggests that the animals (coral polyps) can actively modify the pH of the water at the calcification site to maximise their growth rate.

    This part seems to invert the cause and the consequence.

    Anything stirring enough the water kicks in the natural ocean buffering properties. If ocean dwelling animals do something to the ocean pH, it’s just that. They maintain the pH within 8.2 and 8.4 by stirring it. Whether they want it or not. And despite of their various excretions, which can deteriorate their environment up to killing them e.g. in a poorly maintained aquarium.

    Based also on Darwin’s theory on evolution I suggest any animal having to actively alter ocean buffered pH for maximum growth is unlikely to survive a generation, let alone 500+ million years.

  14. Pleasant news indeed from Georgiou and colleagues. Gosh, an experiment out in actual field conditions. I’ll guarantee one thing, though: these results will not be reported by the taxpayer-funded Australian Broadcasting Corporation (ABC). It ruins their constant barrage of propaganda about “climate change” (the sort that started in the 1950’s doncha know).

  15. Good job on the post! However, we all know cherry picking ≠ solid evidence.
    From Georgiou., et al, (2015), they also stated “Porites appears to be a genus whose adult colonies are highly resilient to ocean acidification, as also shown from their occurrence around CO2 seeps (39), yet their skeletal densities and calcification rates can nevertheless decline when exposed to particularly caustic seawater chemistries (Ωsw < 2) (40)". So they investigated Porites, the strongest coral species for the FOCE experiment, and that does not apply to all of the coral population in Australia, nor the rest of the corals in the world. What they intended to do was to investigate on a particular species which appears to be resilient to CO2 changes, not to give a general outlook for the outcome of acidification on coral reef.
    Also from another credible source you used, Hofmann et al (2011), they stated "In contrast to more stochastic changes in pH that were observed in some sites, our coral reef locations displayed a strikingly consistent pattern of diel fluctuations over the 30-day recording period". Maybe citing just the abstract is not a good idea considering your argument is already contradicted in their discussion. They also stated that some organisms may already have been reaching for their physiological limit within those fluctuating environments, acidification of the ocean would just make it worse.
    You also cited Jim Steeles, a person who seems to have questionable authority on this subject. Zero peer-review paper and no scientific credibility. He may have said what you want to say, but citing him would just make your blog post less credible than it already is.

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