The Great Barrier Reef in Danger of Decline

By Hannah Johnson

Figure 1: An image of coral and fish from the Great Barrier Reef9

When it comes to the 7 natural wonders of the world, it probably comes as no surprise that the Great Barrier Reef falls under this classification. Located off the coast of Australia and considered to be the largest living structure in the world, the Great Barrier Reef is the biggest coral reef in the world. Spanning over 214,000 miles it supports a vast consortium of coral and underwater species2. As such, it is understandably prone to environmental strains that affect the stability of this natural wonder. In the last decade, events such as coral bleaching, starfish outbreaks and cyclones have all contributed to the decline of the Great Barrier Reef, explaining why it is so close to “in danger” status from the United Nations’ World Heritage Committee, UNESCO. Just this year the Australian government successfully lobbied against the Great Barrier Reef’s entry onto this list for the second time since 20151. Since the first recommendation for “in danger” status in 2015, there have been three major coral bleaching events, occurring in 2016, 2017, and just recently in 20201,3. The loss of coral reefs, which can occur as a result of major coral bleaching events, can vastly affect the marine ecosystem as they provide food and shelter to many marine organisms. It is predicted that 25% of marine life would lose their habitats with the loss of coral reefs13, leading to a loss of species diversity, which not only alters marine life, but human industries as well such as tourism, fishing, and medicine13. Australia’s lobbying could have both positive and negative effects for the reef, depending on the Australian government’s response towards conservation and preservation. In an effort to curb the environmental stress, the Australian government started a Reef Restoration and Adaptation Program (RRAP) in 2020 for $220 million USD with the aim of establishing relief efforts to improve the health of the Great Barrier Reef3.

One of the major contributors towards the Great Barrier Reef’s approach towards “in danger” status are the outbreaks of deadly starfish known for their consumption of coral. These starfish are known as crown-of-thorn starfish (COTS), pictured in Figure 2.

Figure 2: Crown-of-thorn starfish8

The term ‘outbreak’ in this case is defined as when the consumption of coral outpaces the growth4. On a small scale, these COTS are helpful in the population control of coral, where enough of the fast-growing coral is consumed, allowing for the slow-growing coral to have space to develop2. However, COTS outbreaks are the result of great quantities of COTS converging on these reefs and eating through them at a substantial rate. The COTS can consume their body weight in coral every day and just one of these starfish can eat 20-32 feet of coral in a year2. The next COTS outbreak is projected to occur sometime within 2025-2027 showcasing the need to control the population immediately4. Mitigation efforts in the population control for these starfish have been put into place, such as divers injecting starfish with lethal injections2,5. While the starfish outbreaks can have deadly consequences, they are not the leading cause of damage seen in the Great Barrier Reef.

The most damaging contributors to the Great Barrier Reef are the five major coral bleaching events that have occurred throughout the past two decades. Normally, healthy coral will form a symbiotic relationship with algae called zooxanthellae6. Not only do these algae provide the color coral are known for, but they are also the primary food source for the coral via photosynthesis11. The zooxanthellae are sensitive to high temperatures and intense sunlight; thus, the coral’s role in the symbiotic relationship under normal conditions is to provide cover from sunlight by producing protein pigments, which are sometimes fluorescent12. Hence, while the brown color of coral is from the zooxanthellae, the vibrant pink, green, and blue colors are the result of sunlight protection for the algae12. Coral bleaching, as shown in Figure 3, is a result of environmental stress causing the algae living in the coral to be expelled due to the harmful release of reactive oxygen species from the zooxanthellae11.

Figure 3: Left) Healthy coral reef Right) The same coral reef after a bleaching event.7

Environmental stressors resulting in these bleaching events are largely associated with climate change such as increased ocean temperature, overexposure to sunlight, and extreme low tides6. The loss of algae in these bleaching events can result in the coral losing their primary source of food, their color, and can increase their vulnerability to disease. While the coral can recover, these major bleaching events occurring in relatively close periods of time can halt any recovery the surviving coral has undergone. Mitigation efforts under the RRAP program Australia started last year have begun, such as breeding heat-tolerant corals and cloud brightening3. Marine cloud brightening, which is essentially “brightening” clouds to better reflect the sun, is a geoengineering technique proposed to increase the albedo, or reflectivity, of clouds by seeding clouds with sub-micrometer droplets of sea water with the purpose of providing a cooling effect on marine environments by reflecting the sunlight10. In 2020, Australia started the first field trial of marine cloud brightening using mist machines to lessen the severity of the increasing ocean temperature on the coral3.

While the Great Barrier Reef is one of the world’s natural wonders it might not always be. The continued withdrawal from adding the Great Barrier Reef to the World Heritage “in danger” list may contribute to its further decline; however, the recent efforts by the Australian government to mitigate these damages caused by harmful events such as COTS outbreaks and coral bleaching may provide for better reef health. The decline in coral reefs is another example for the necessity behind climate control and the need for increasing preservation efforts in the world to keep these natural phenomena in place and healthy. Optimistically, the recent efforts into preserving the reef will result favorably. If not, then you might soon be seeing the Great Barrier Reef as one of the World Heritage sites that are in danger of being lost.


  • The Great Barrier Reef has escaped “In danger” status twice in the last 6 years by the World Heritage Committee due to lobbying by Australia.
  • Crown-of-thorn starfish (COTS) outbreaks can eat through vast amounts of coral and the next outbreak is projected to hit 2025-2027.
  • There have been 3 major coral bleaching events in the last five years due to rising ocean temperatures leading to a Reef Restoration and Adaptation Program (RRAP) started last year in Australia to mitigate the damage to the Great Barrier Reef.


  4. Babcock, R.C., Plagányi, É.E., Condie, S.A. et al. Suppressing the next crown-of-thorns outbreak on the Great Barrier Reef. Coral Reefs 39, 1233–1244 (2020).
  5. Condie, Scott A., et al. “Large-scale interventions may delay decline of the Great Barrier Reef.” Royal Society Open Science 8.4 (2021): 201296.
  7. Li, Alberto Quattrini, et al. “Data correlation and comparison from multiple sensors over a coral reef with a team of heterogeneous aquatic robots.” International Symposium on Experimental Robotics. Springer, Cham, 2016.
  10. Latham, John, et al. “Marine cloud brightening.” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370.1974 (2012): 4217-4262.

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