Coral Reefs: How much time do they have?

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Written by: Olivia Mark

Figure 1: On the left the coral is its regular healthy colour, on the right coral bleaching has occurred and the coral is left colourless (Frostenson, 2017).

The past 250 years have seen the human race ruthlessly engaged in industrial activities, with atmospheric CO2 dramatically increased. The preindustrial level of 280ppm has been rising exponentially to the current level of 407ppm, a rate over 100 times faster than what occurred when the last ice age ended![1] The ocean takes up  1/3 of anthropogenic CO2 emissions, however this has dire consequences on ocean health and the marine life within.[2]

A close up of a map

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Figure 2: As the concentration of CO2 in the oceans increase, pH decreases (Feel et al. 2006)

CO2 concentration is related to pH levels, shown in figure 2. When CO2 dissolves in the ocean it forms a mild acid, called carbonic acid, which dissociates to reduce the availability of carbonate ions[3]. Corals are made up of tiny creatures called polyps, which secret a calcium carbonate shell, using these carbonate ions. If the availability of these ions is reduced, coral assemblage by polyps is restricted[4]

Carbonic acid dissociation produces hydrogen ions which increase ocean acidity, decreasing the pH. Since pre industrial times, pH has decreased by 0.1 units equivalent to a 30% increase in hydrogen ions[5]. If CO2 emissions continue to rise at this rate, it is likely a decrease of 0.5 pH may occur by 2100[6]. This has the  potential to  have devastating consequences on the entire marine ecosystem with coral reefs providing habitats for a staggering ¼ of marine species[7].

In addition to inhibiting reef building, ocean acidification causes coral bleaching, these being highly sensitive to environmental changes. When the coral polyps become stressed, the algae which provide them with 90% of their energy migrate away, known as bleaching [8]. The coral then turns a pale colour and is more susceptible to disease and starvation. If the perturbated conditions persist, the algae will not return to the coral and it will die. Once coral dies, it is virtually impossible for the reefs to regenerate.[9] We are now seeing dramatic reductions in coral growth as a result of increased CO2 levels. The past 16 years have seen the growth rate of corals in the Great Barrier Reef decreased by a shocking 21%[10]. Projections of coral reef activity in the future suggest that if we exceed 500ppm atmospheric CO2 levels, coral reef building rates may decrease by 40%[11]. If emissions continue to rise, we may see a total, catastrophic, breakdown in marine ecosystems.

In the past, geological records show a significant gap in the presence of reef building corals when CO2 levels were 5 times higher than today, indicating that high levels of CO2 could eradicate corals all together[12]. A grim prospect for above and below water.

We have the power to decrease our carbon emissions, this needs to addressed by all parts of society and the sooner the better.  We only have one planet and now is the time to do everything in our power to keep it healthy. So next time you jump in the car to take a short trip, think of the corals and walk instead. The corals need your help!


  1. Blunden, J. & D. S. Arndt. (2019). State of the Climate in 2018, Bull. Amer. Meteor. Soc, 100 (9), Si–S305, doi:10.1175/2019BAMSStateoftheClimate.1. 
  2. Ocean and Climate Platform, (n.d.). The interactions between ocean and climate.
  3. Doney, S. C., Fabry, V. J., Feely, R. A., & Kleypas, J. A. (2009). Ocean Acidification: The Other CO2 Problem. Annual Review of Marine Science. 1. 169-191.
  4. NOAA, (2020). Are corals animals or plants? National Ocean Service website.
  5. NOAA, (n.d.). What is Ocean Acidification? National Ocean Service website.
  6. Raven, J., Caldeira, K., Elderfield, H., Hoegh-Guldberg, O., Liss, P., Riebesell, U., … & Watson, A. (2005). Ocean acidification due to increasing atmospheric carbon dioxide. The Royal Society.
  7. Knowlton, N. (2018). Corals and Coral Reefs. Ocean Find Your Blue.
  8. Slezak, M. (2016). The Great Barrier Reef: A catastrophe laid bare. The Guardian.
  9. -11. Frostenson, S. (2017). Experts: The Great Barrier Reef cannot not be saved. Vox.

12. Idso, C. D. (2009). Co2, global warming and coral reefs: prospects for the future. Centre for the Study of Carbon Dioxide and Global Change.