Thus far, a wide range of threats to the GBR have been examined. All have been greatly exaggerated. Of these, coral bleaching is perhaps the most famous.
There is a far more plausible threat. It is also probably the least understood by scientists. This is the effect of rising carbon dioxide concentrations caused by burning fossil fuels. Carbon dioxide dissolves in the ocean and is used by corals and many other organisms to make its hard calcium carbonate shell. Changing carbon dioxide levels may, however, change the rate at which corals grow or lay down their calcium carbonate skeleton.
For both marine and land plants, increased carbon dioxide concentration causes an increase in growth rate, a fact often forgotten in debate about carbon dioxide. Plants need plenty of carbon. Because, for plants, carbon dioxide is the second most important chemical after water, food crops are growing faster now than they would be with lower carbon dioxide concentrations. The same is true for marine plants.
Corals are not plants and, although they need the carbon dioxide to build their calcium carbonate shells, there is a peculiar quirk of chemistry that might mean that higher concentrations of carbon dioxide can cause the corals to grow their shells more slowly. This is because carbon dioxide reacts with water to form carbonate and bicarbonate ions. The relative mix of these ions seems to be important to corals, although the precise mechanisms is open to considerable debate. The common assumption is that the carbonate ion is most important for corals, but as the atmospheric carbon dioxide concentration rises, the carbonate concentration decreases, and the hypothesis is that this is what could reduce coral growth rates. The precise chemical mechanisms that corals use to take carbon dioxide from the water into the polyp and then lay down its skeleton is not well understood. It would not be surprising if some very profound discoveries occur in the next decade that will totally alter our views on this potential threat.
There may be a long way to go before this hypothesis can be regarded as a well-founded and reliable scientific fact, but it is plausible to start with – unlike some of the other threats we have considered. Carbon dioxide levels are definitely rising. Contrast that with the mostly unmeasurable increases in sediment and nutrients in the water of the GBR caused by agriculture. At least there is measurable change occurring in the carbon dioxide concentrations whereas there is almost no measurable change in the sedimentary “climate” of the GBR.
There are also many experiments that show that higher concentrations of carbon dioxide slow coral growth rate. Contrast that with the general observation that corals growing in hotter water grow faster than in cold water and the known mechanisms of coral to take advantage of increasing temperature.
It would be nice to know that, as the scientific investigations of the threat of carbon dioxide progresses, scientific institutions are reliable. This is assuredly not so (see chapter 12), but there are encouraging signs that some scientists in this field are not going to jump to conclusions too fast. For example, Carlos Duarte and colleagues have cautioned that this problem needs to be subjected to “organised auditing.”
In addition, the editor of the Journal of Marine Science, Howard Browman, also suggested application of “organised scepticism” and highlighted contradictory evidence on this topic. He even pointed out that publishing a result that found the increasing carbon dioxide concentrations were not a problem was much more difficult than publishing a result that predicted the end of coral reefs. He further asked the science community to contribute work to his journal that demonstrated no effect, or even positive influences, of higher carbon dioxide.
The principled and genuinely scientific approaches of Duarte and Browman are not often reflected in many other fields of environmental research. The question of organised scepticism will be revisited in other parts of this website.
This section will conclude with consideration of some of the evidence about the proposition that corals are affected by increasing carbon dioxide concentrations.
Ocean “acidity” and pH.
The ocean is presently “basic,” the opposite of acidic. One effect of the increasing carbon dioxide concentration is to make the ocean slightly less basic, that is, closer to acidic. It will certainly never become acid for all sorts of chemical reasons. Chemists measure the scale between acids and bases on the pH scale with pH 1 being an extremely strong acid (hydrochloric acid has pH 1.5) and pH 14 being extremely strong base (such as sodium hydroxide which is used to scour drains). Pure water is pH 7 and the ocean is about 8.1, but could reduce to 7.7 with continued use of fossil fuels. It may have already dropped by 0.1 pH units (Santos et al., 2011).
It is important to note that water on coral reefs often has a daily variation pH that are considerably greater than what is likely with rising carbon dioxide concentrations., For example, Santos et al. (2011) found a daily variation from 7.7 at night to 8.4 in the day time, and it is possible that organisms exposed to such large variations are tolerant to long term changes.
Some of the experimental measurements of reef calcification rates (the rate at which the calcium carbonate skeleton builds) indicate that rising carbon dioxide concentrations has a detrimental effect. Albright et al. (2018) did an experiment on a coral reef at One Tree Island on the GBR where she pumped large amounts of carbon dioxide into the water to simulate levels that will be reached in the middle of the twenty-first century and found a 30 percent reduction in calcification rate for the entire reef area. It is not practical to measure directly the calcification rates of the corals (by perhaps weighing them), so relatively complicated chemical methods are used to infer the growth (calcification). As the Reef is far from entirely coral this does not necessarily mean that the corals slowed down growth by 30 percent and Albright cautioned that the crustose coralline algae may have been responsible for much of the change. Many follow-up experiments need to be done to confirm this result, and scientists familiar with the subject need to scrutinise it carefully, but it was a good experiment, and it is a cause for concern.
In previous work, Albright also measured the reverse effect – what would happen if there were a reversion to pre-industrial levels of carbon dioxide. In this experiment she added sodium hydroxide to the water which increases the pH. This would be expected to show an increasing calcification rate and, indeed, it did, but only by about seven percent. Nevertheless, this is consistent with her later work.
Albright’s work deserves attention because it was carried out on a real coral reef. There are hundreds of other experiments that have been done, in laboratory conditions, which are fraught with problems as it is very difficult to simulate a real coral reef in a small tank. Although Albright avoids this problem, there is always the difficulty of using an experiment in which a sudden shock was given to the coral by quickly changing the carbon dioxide concentration to infer what would occur if the same change were to occur gradually over 50 years.
Measurements of coral growth rates in the last hundred years do not seem to have fallen by the seven percent that we might infer from Albright’s work, but have probably increased by about 10 percent., This may be because the 0.5- 1o increase in temperature of the ocean, which should certainly increase growth rates (possibly by 10 – 20 percent), may have counteracted the effect of rising carbon dioxide.
The safe conclusion on this topic is that there is some distance to go before it is understood properly but there is certainly a credible case that must be answered that carbon dioxide is a problem.
 Kimball, B.A. and Idso, S.B. (1983). Increasing atmospheric CO2: effects on crop yield, water use and climate. Agricultural Water Management, 7(1–3), pp.55–72.
 Jokiel, P.L. (2015). Predicting the impact of ocean acidification on coral reefs: evaluating the assumptions involved. ICES Journal of Marine Science, 73(3), pp.550–557.
 Duarte, C.M., Fulweiler, R.W., Lovelock, C.E., Martinetto, P., Saunders, M.I., Pandolfi, J.M., Gelcich, S. and Nixon, S.W. (2014). Reconsidering Ocean Calamities. BioScience, 65(2), pp.130–139.
 Browman, H.I. (2016). Applying organized scepticism to ocean acidification research. ICES Journal of Marine Science, 73(3), pp.529–536.
 Santos, I.R., Glud, R.N., Maher, D., Erler, D. and Eyre, B.D. (2011). Diel coral reef acidification driven by porewater advection in permeable carbonate sands, Heron Island, Great Barrier Reef. Geophysical Research Letters, 38(3).
 Albright, R., Takeshita, Y., Koweek, D.A., Ninokawa, A., Wolfe, K., Rivlin, T., Nebuchina, Y., Young, J. and Caldeira, K. (2018). Carbon dioxide addition to coral reef waters suppresses net community calcification. Nature, 555(7697), pp.516–519.
 Albright, R., Caldeira, L., Hosfelt, J., Kwiatkowski, L., Maclaren, J.K., Mason, B.M., Nebuchina, Y., Ninokawa, A., Pongratz, J., Ricke, K.L., Rivlin, T., Schneider, K., Sesboüé, M., Shamberger, K., Silverman, J., Wolfe, K., Zhu, K. and Caldeira, K. (2016). Reversal of ocean acidification enhances net coral reef calcification. Nature, 531(7594), pp.362–365.
 D’Olivo, JP, McCulloch, NT, an Judd, K (2013) Long-term records of coral calcification across the central Great Barrier Reef: assessing the impacts of river runoff and climate change. Coral Reefs (2013) 32:999–1012
 Ridd, PV, DaSilva, ET, Stieglitz, TC (2013) Have coral calcification rates slowed in the last twenty years. Marine Geology. 346 (2013) 392–399