• Gina Lusardi

Coral Bleaching: What's really happening?

Lizard Island from above, The Great Barrier Reef

Think of tropical, shallow water coral reefs as the tropical rainforests of the ocean. They are not only incredibly productive; they are also the most biodiverse amongst marine ecosystems. Unlike rainforests which cover 2.3% of the Earth’s surface, coral reefs cover less than 0.1% (1); that’s about twice the size of Italy. The corals themselves are responsible for the structural composition of reef systems. Stony corals, such as large boulder-like corals and intricate branching corals, are the reef builders. Unlike other types of coral, stony corals produce a calcium carbonate skeleton which enables them to construct large limestone structures (2).

Healthy hard corals at Heron Island, GBR

Corals are invertebrates related to jellyfish and sea anemones in the phylum Cnidaria. When looking at corals it may seem like you are looking at one large individual, but actually a single structure can be made up of hundreds, if not thousands of independent polyps. The polyps themselves are the actual coral animal. Naturally colourless, polyps have a symbiotic relationship with photosynthetic algae called zooxanthellae (2). These algae are not only responsible for the corals’ vibrant colours, but also aid in sustaining them. In fact, 90% of a coral’s food source come from the zooxanthellae living within its tissues (3). When a coral is stressed, it expels the zooxanthellae. As it does so, the coral loses its colour which exposes the white skeleton underneath. This phenomenon has been coined “coral bleaching”. Corals become stressed for a number of reasons including: changes in light intensity, low and high temperatures, freshwater inundation, pollution and disease (4,5). It is possible that a coral can rebound after a bleaching event if the stress is short-term. However, if stressful conditions persist, the coral will die. For instance, just 4 weeks of temperature stress can trigger bleaching (5). If conditions persist for 8 weeks or longer, corals begin to perish (5). Of all the stressors that cause bleaching, unusually high sea temperatures are the primary cause of mass coral bleaching.

What is Mass Coral Bleaching?

Source: Great Barrier Reef Marine Park Authority (5)

Mass coral bleaching was first described in the early 1980’s and was attributed to warmer sea temperatures in conjunction with an El Niño event (6). Between 1982 and 1983, there was a 97% reduction of corals in the Galapagos, as well as, 75% in Panama and 50% in Costa Rica (6). Since then, there have been periodic events world-wide. In most recent years, the time between these events has been shrinking. If warmer sea temperatures are to blame then we need to identify the cause. Of course, the El Niño Southern Oscillation (ENZO) definitely plays a role due to its influence on global sea temperatures (7). However, climate change is also a key player. As the Earth heats up, the ocean absorbs around 90% (8) of that heat energy causing the ocean to warm at an increasingly fast rate (9).

In 2016, the news was flooded with articles about a mass bleaching event occurring on Australia’s Great Barrier Reef (GBR). One of the most shocking articles was Outside Magazine’s “Obituary: Great Barrier Reef (25 million BC-2016)”, which claimed that the reef was dead (10). Of course, it was a complete exaggeration, but there was indeed cause for concern. The Great Barrier reef was being devastated by a mass bleaching event triggered by record sea temperatures along the most Northerly section of the reef (11). In 2017, the bleaching continued to affect around 66% percent of the reef (12). Even in early 2018 a coral reef biologist by the name of Dr Selina Ward has seen some localised bleaching in the time before the “high risk” period (13).

Unfortunately, the GBR is only a small part of a bigger story. In 2016 alone, there was 29% loss of corals worldwide (14). As it stands, that year was the warmest year on record since 1880 (15). It is also important to note that 17 of the 18 warmest years have been since 2001 (15). In an article published in 2017 by Nature, 46 experts concluded that “securing a future for coral reefs, including intensively managed ones such as the Great Barrier Reef, ultimately requires urgent and rapid action to reduce global warming” (16). If things continue along this trend, the outlook will remain quite bleak. Fortunately, there is a lot of work being done around the world to help save coral reefs.

The global extent of mass bleaching of corals in 2015 and 2016

Symbols show 100 reef locations that were assessed: red circles, severe bleaching affecting >30% of corals; orange circles, moderate bleaching affecting <30% of corals; and blue circles, no substantial bleaching recorded. See table S1 for further details.

Source: Hughes, et al. (2018). Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science, 359(6371), [Fig 3]. Available at: http://science.sciencemag.org/content/359/6371/80.full

What is Being Done to Save Coral Reefs?

Australia’s GBR may be one the best managed reefs in the world. The Great Barrier Reef Marine Park Authority (GBRMPA), who is in charge of managing the reef, recently launched a Coral Reef Initiative campaign called “The International Year of the Reef 2018” to try and spark world-wide public engagement in order to help save coral reefs (17). The Australian government also announced $2 million (Australian dollars) in funding for an international competition for research proposals to increase coral abundance (18). On the other side of the world, the United States of America has its own tools for protecting reefs. The National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Conservation Program was initiated in 2000 to help protect reefs from climate change, pollution and unsustainable fishing (19). In Central America, 60 organisations are collaborating across international borders in order to keep track of the health of the Mesoamerican Reef (20). Globally, a network of marine protected areas (MPAs) have been established to protect vulnerable ecosystems including: coral reefs. Some of the largest such as the Marae Moana Marine Park in the Cook Islands (21), the Papahānaumokuākea Marine National Monument in Hawaii (22) and the Natural Park of the Coral Sea in New Caledonia (23), include vast expanses of coral reef ecosystems.

There are even a few people willing to take on more extreme measures. Some scientists are working on ways to genetically engineer zooxanthellae (24) and corals (25) to be more resilient to climate change. An American engineer, Mo Ehsani, has conceptualised a pipeline that pumps cold water from the seafloor up to the surface to keep surface temperatures on reefs down (26). Whether these ideas are plausible or not, people are urgently taking drastic steps to help save coral reefs for a reason. At this point it may seem obvious that coral reefs are important but if you are not convinced that the world needs them, let us explore just how important they are.

Why are Coral Reefs So Important?

Coral reefs are important for a number of reasons including: local and global economies, storm protection, sustainable fisheries and medical advances. In tourism alone, they are worth $36 billion (US dollars) a year (27). Scaling up to the total value of ecosystem services, coral reefs are valued at $172 billion (US dollars) (28). They also provide natural breakwaters to protect coastlines and islands from wave and flood damage. There are up to 200 million people around the world living in high risk regions that rely on coral reefs for storm protection (29).

Homes along the ocean's edge in Malaysia

With around 25% of all life in the oceans living in coral reefs (4), a single square kilometre of reef can provide over 15 tons of seafood annually (30). In fact, half of the United States’ commercial fisheries rely on healthy coral reefs; without them these fisheries would not exist (28). In countries such as Bangladesh, Cambodia, Ghana, Indonesia, Sierra Leone and Sri Lanka, fish makeup over 50% of their annual animal protein consumption (31). The future of sustainable fishing may just rely on healthy coral reef ecosystems.

Coral reefs, just like tropical rainforest are important for medical advances. Species that live on reefs are being used in the development of cancer and Alzheimer’s disease treatments as well as antiviral drugs. In fact, it is “300 to 400 times more likely” to find new drug sourced from the ocean than from land (32).

Our reliance on reefs for protection, livelihoods, food and medicine, is extensive. If coral reefs were to disappear, the world would be a very different place. This may come as a surprise to you, but at the current rate in which the oceans are warming and acidifying, coral reefs may not be around much longer. According the United Nation’s 5th Global Environment Lookout Report, most of the world’ reefs will be dead by 2050 if current trends continue (33). Fortunately, you do not need to be a scientist, policy maker or a politician to help save the reefs.

What can you do to Help Save Coral Reefs?

Reefs around the world are relying on us to change the way we live. It doesn’t matter if you live thousands of kilometres inland, in a city or on the coast, the choices you make everyone can affect the coral reefs around the world. Small changes can make a big difference.

Here are some everyday things that you can do to help save coral reefs:

  • Conserve water

  • Reduce waste

  • Dispose of trash properly

  • Use eco-friendly household products

  • Skip the fertilizer this season

  • Plant trees

  • Use public transport, walk or ride a bike instead of driving

  • Support eco-friendly businesses

  • Choose seafood that is sustainable

  • Be a responsible boater, snorkeler and diver

  • Volunteer for coral surveys, beach and underwater clean-ups

  • Vote in favour of the environment

  • Share your knowledge with others

  • Be a steward for the environment

If you want to learn more please visit:

  • www.coral.org (Coral Reef Alliance Non-Profit Organisation)

  • www.reefcheck.org (Reef Check Non-Governmental Organisation)

  • www.gbrmpa.gov.au (Great Barrier Reef Marine Park Authority)

  • www.coralreef.noaa.gov (Coral Reef Conservation Program USA)

  • www.50reefs.org (50 Reefs Initiative)

* I am not an affiliate nor am I sponsored by any of the organizations listed.


1. Reaka-Kudla, M.L. 1997. The global biodiversity of coral reefs: A comparison with rainforests. In. Reaka-Kudla, M.L., D.E. Wilson, and E.O. Wilson (eds) Biodiversity II: Understanding and Protecting Our Natural Resources, pp 83-108 Joseph Henrey/National Academy Press, Washington, D.C

2. "NOAA's Coral Reef Conservation Program: Coral Anatomy and Structure", Coralreef.noaa.gov, 2018. [Online]. Available: https://coralreef.noaa.gov/aboutcorals/coral101/anatomy/. [Accessed: 24- Feb- 2018].

3. "NOAA National Ocean Service Education: Corals", Oceanservice.noaa.gov, 2018. [Online]. Available: https://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html. [Accessed: 24- Feb- 2018].

4. L. Fujise, H. Yamashita, G. Suzuki, K. Sasaki, L. Liao and K. Koike, "Moderate Thermal Stress Causes Active and Immediate Expulsion of Photosynthetically Damaged Zooxanthellae (Symbiodinium) from Corals", PLoS ONE, vol. 9, no. 12, p. e114321, 2014.

5. "Coral bleaching - GBRMPA", Gbrmpa.gov.au, 2018. [Online]. Available: http://www.gbrmpa.gov.au/managing-the-reef/threats-to-the-reef/climate- change/what-does-this-mean-for-species/corals/what-is-coral-bleaching. [Accessed: 24- Feb- 2018].

6. P. Glynn, "Coral reef bleaching in the 1980s and possible connections with global warming", Trends in Ecology & Evolution, vol. 6, no. 6, pp. 175-179, 1991.

7. "What is El Niño?", Met Office, 2018. [Online]. Available: https://www.metoffice.gov.uk/learning/learn-about-the-weather/what-is-el-nino-la- nina. [Accessed: 24- Feb- 2018].

8. P. Durack, P. Gleckler, F. Landerer and K. Taylor, "Quantifying underestimates of long-term upper-ocean warming", Nature Climate Change, vol. 4, no. 11, pp. 999-1005, 2014.

9. G. Wang, L. Cheng, J. Abraham and C. Li, "Consensuses and discrepancies of basin-scale ocean heat content changes in different ocean analyses", Climate Dynamics, 2017.

10. R. Jacobsen, "Obituary: Great Barrier Reef (25 Million BC-2016)", Outside Online, 2016. [Online]. Available: https://www.outsideonline.com/2112086/obituary-great-barrier- reef-25-million-bc-2016. [Accessed: 24- Feb- 2018].

11. "Bleaching Events - AIMS", Aims.gov.au, 2016. [Online]. Available: https://www.aims.gov.au/docs/research/climate-change/coral-bleaching/bleaching- events.html. [Accessed: 24- Feb- 2018].

12. Heathcote, "Breakdown: the state of the Great Barrier Reef in 2017", Australian Geographic, 2017. [Online]. Available: http://www.australiangeographic.com.au/topics/science- environment/2017/09/the-state-of-the-great-barrier-reef-in-2017. [Accessed: 24- Feb- 2018].

13. M. Slezak, "Great Barrier Reef coral bleaching has started early, biologist says", the Guardian, 2018. [Online]. Available: https://www.theguardian.com/environment/2018/jan/20/great-barrier-reef-coral- bleaching-has-started-early-biologist-says. [Accessed: 24- Feb- 2018].

14. "Reef health - GBRMPA", Gbrmpa.gov.au, 2017. [Online]. Available: http://www.gbrmpa.gov.au/about-the-reef/reef-health. [Accessed: 24- Feb- 2018].

15. "Global Temperature", Climate.nasa.gov, 2018. [Online]. Available: https://climate.nasa.gov/vital-signs/global-temperature/. [Accessed: 24- Feb- 2018].

16. T. Hughes, J. Kerry, M. Álvarez-Noriega, J. Álvarez-Romero, K. Anderson, A. Baird, R. Babcock, M. Beger, D. Bellwood, R. Berkelmans, T. Bridge, I. Butler, M. Byrne, N. Cantin, S. Comeau, S. Connolly, G. Cumming, S. Dalton, G. Diaz-Pulido, C. Eakin, W. Figueira, J. Gilmour, H. Harrison, S. Heron, A. Hoey, J. Hobbs, M. Hoogenboom, E. Kennedy, C. Kuo, J. Lough, R. Lowe, G. Liu, M. McCulloch, H. Malcolm, M. McWilliam, J. Pandolfi, R. Pears, M. Pratchett, V. Schoepf, T. Simpson, W. Skirving, B. Sommer, G. Torda, D. Wachenfeld, B. Willis and S. Wilson, “Global warming and recurrent mass bleaching of corals", Nature, vol. 543, no. 7645, pp. 373-377, 2017.

17. "International Year of the Reef 2018 - GBRMPA", Gbrmpa.gov.au, 2018. [Online]. Available: http://www.gbrmpa.gov.au/about-the-reef/international-year-of-the-reef-2018. [Accessed: 24- Feb- 2018].

18. E. Zachos, "Your Bright Idea Could Save the Biggest Reef on Earth", News.nationalgeographic.com, 2018. [Online]. Available: https://news.nationalgeographic.com/2018/01/australia-great-barrier-reef-coral- bleaching-conservation-competition-spd/. [Accessed: 24- Feb- 2018].

19. "NOAA's Coral Reef Conservation Program (CRCP) - Who We Are", Coralreef.noaa.gov. [Online]. Available: https://coralreef.noaa.gov/about/welcome.html. [Accessed: 24- Feb- 2018].

20. Srinivas, "Celebrating a Decade of Conservation and Collaboration in the Mesoamerican Reef", Coral.org, 2018. [Online]. Available: https://coral.org/blog/celebratingmar/. [Accessed: 24- Feb- 2018].

21. "What is Marae Moana", Maraemoana.gov.ck, 2014. [Online]. Available: http://www.maraemoana.gov.ck/index.php/about-marae-moana/what-is-marae- moana. [Accessed: 24- Feb- 2018].

22. "Papahānaumokuākea Marine National Monument", Papahanaumokuakea.gov, 2018. [Online]. Available: https://www.papahanaumokuakea.gov/. [Accessed: 24- Feb- 2018].

23. "New Caledonia: Home of the World’s Largest Marine Park", Conservation International, 2017. [Online]. Available: https://www.conservation.org/projects/Pages/New-Caledonia- Home-of-the-Worlds-Largest-Marine-Park.aspx. [Accessed: 24- Feb- 2018].

24. R. Levin, C. Voolstra, S. Agrawal, P. Steinberg, D. Suggett and M. van Oppen, "Engineering Strategies to Decode and Enhance the Genomes of Coral Symbionts", Frontiers in Microbiology, vol. 8, 2017.

25. "Behind the Scenes: Breeding super coral | Coral Health Atlas", Coralhealth.uhh.hawaii.edu, 2018. [Online]. Available: http://coralhealth.uhh.hawaii.edu/node/190. [Accessed: 24- Feb- 2018].

26. M. Cimons, "This engineer has a wild plan to save coral reefs from climate change", Popular Science, 2017. [Online]. Available: https://www.popsci.com/coral-bleaching-water- pipeline. [Accessed: 24- Feb- 2018].

27. M. Spalding, L. Burke, S. Wood, J. Ashpole, J. Hutchison and P. zu Ermgassen, "Mapping the global value and distribution of coral reef tourism", Marine Policy, vol. 82, pp. 104-113, 2017.

28. "Shallow Coral Reef Habitat | NOAA Fisheries", Fisheries.noaa.gov. [Online]. Available: https://www.fisheries.noaa.gov/national/habitat-conservation/shallow-coral-reef- habitat. [Accessed: 24- Feb- 2018].

29. B. CAREY, "Coral reefs provide protection from storms and rising sea levels, Stanford research finds", Stanford University, 2014. [Online]. Available: https://news.stanford.edu/news/2014/may/coral-reef-protect-051314.html. [Accessed: 24- Feb- 2018].

30. "Coral Reefs", Coral.unep.ch. [Online]. Available: http://coral.unep.ch/Coral_Reefs.html. [Accessed: 24- Feb- 2018].

31. FAO. 2016. The State of World Fisheries and Aquaculture 2016. Contributing to food security and nutrition for all. Rome. 200 pp

32. Bruckner, Andrew W. "Life-Saving Products from Coral Reefs." Issues in Science and Technology 18, no. 3 (Spring 2002).

33. Global Environment Lookout 5: Environment for the future we want, 5th ed. Malta: United Nations Environment Programme, 2012, p. 120.

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