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The Effect of Global Warming on Coral Reefs

Updated: May 12

Writer: Beyza Kaya

Coral reefs are diverse and ecologically valuable ecosystems found in different environments. Warm-water coral reefs are situated in shallow, sunlit, warm, and alkaline waters, allowing them to grow and build their calcium carbonate structures rapidly. Deeper mesophotic coral reefs exist in low-light conditions and accumulate calcium carbonate at slower rates. Cold-water coral reefs are found at even greater depths, where photosynthesis is not possible, and they rely on particle feeding. Coral reefs provide myriad benefits for underwater life, supporting fisheries and offering various ecological goods and services, including coastal protection, cultural value, and potential pharmaceutical discoveries. Despite the precautions, they are under threat from various human activities such as pollution, over-harvesting, physical destruction, and climate change. In this article, we discuss the effects of coral bleaching, decreased calcification, sea level rise, ocean acidification, and storms on coral reefs due to climate change. [1][2]

1. Coral Bleaching

Coral bleaching is an event that occurs in coral reefs when the symbiotic algae that corals depend on for their survival separate from the coral tissues. These algae are important to provide nutrients and their colors for coral reefs. However, some stress factors, such as high water temperatures and acid levels, cause small increases in the body temperature of coral reefs, and this puts coral reefs under stress. When coral reefs are stressed due to increased temperature and acidity, they can’t protect algae, which either die or are expelled from the body. Because of the breakdown of symbiotic relationships, coral reefs lose their color and turn white. This process is called “Bleaching”. Lack of access to food and oxygen results in reduced corals ability to grow, survive, and feed. Bleaching not only harms corals, but also affects coral reef ecosystems. Corals form the foundational structure of the reef ecosystem and provide food and shelter for other marine organisms. If bleaching is prolonged, corals may suffer, and reef ecosystems can be severely affected. The root cause of the bleaching is global warming and rising seawater temperatures. On the other hand, other factors such as water pollution, high levels of sunlight, overfishing, etc. may also be effective in this. Coral bleaching is a serious global threat, and this threat is increasing day by day. [2][3][4]

Photograph: XL Catlin Seaview Survey;

2. Decreased calcification

Decreased calcification refers to the reduction in the ability of marine organisms to produce the limestone compound known as calcium carbonate, particularly corals and other calcium carbonate-producing organisms such as foraminifera and marine snails. These organisms create rigid structures by accumulating calcium carbonate in their shells or skeletons. These structures are fundamental components of coral reefs and other calcareous structures, which makes them a crucial part of marine ecosystems. Calcification is vital for these organisms because calcium carbonate shells protect them from predators and external factors while providing structural integrity and serving various essential functions for nutrition. However, some changes in environmental factors can adversely impact calcification processes. The main reason for decreased calcification is the increasing acidification of water due to rising CO2 levels. Excess CO2 is absorbed into the ocean from the atmosphere, resulting in the acidification of seawater. Acidified seawater hinders the production of calcium carbonate, making it difficult for calcium carbonate-producing organisms to grow and form shells. Decreased calcification poses a significant threat, especially to coral reefs. Corals play a critical role in coral reef formation due to their calcium carbonate production. Therefore, decreased calcification adversely affects the development and growth of coral reefs. Additionally, calcium carbonate-producing marine organisms are vital for the diversity and integrity of marine ecosystems. Reduced calcification can negatively affect the life cycles and populations of these organisms, thereby jeopardizing the health of marine ecosystems. [5]

3. Sea Level Rise

Rising sea level in direct proportion to global warming. Global warming causes an increase in the average temperature worldwide with rising greenhouse gases. As a result of this rising, glacial masses melt, and the melting water contributes to the rising sea levels. Increasing sea levels are also raising the water level in coastal waters, which influences shallow coral reefs. Coral reefs are photosynthetic organisms that are located in coastal or shallow areas and continue their growth and life processes by obtaining energy from sunlight. However, as sea levels rise, shallow coral reefs become partially or completely submerged. In this situation, more seawater covers the shallow coral reefs, submerging them. As a result, corals that receive insufficient sunlight are unable to photosynthesize adequately and consequently reduce their energy production. Photosynthesize also plays a crucial role in the process of calcium carbonate production by corals. Inadequate light slows down or halts the production of calcium carbonate, weakening the coral structures and impeding their growth rate. Furthermore, rising sea levels can affect the salinity and temperature of the seawater surrounding the reefs. Corals can live in specific salinity and temperature conditions, and deviations from these optimal situations can pose challenges to their survival. Submerged corals may encounter lower salinity and higher temperatures that adversely impact their health and resilience. [1][4]

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4. Ocean Acidification

Ocean acidification is directly related to the rising CO2 in the atmosphere. Excess CO2 released into the atmosphere from fossil fuel usage, industrial activities, and other human activities is absorbed by the oceans, leading to the acidification of seawater. This process is known as "ocean acidification." Seawater reacts with carbon dioxide to form carbonic acid, which releases hydrogen ions, thereby increasing the acidity of seawater. This increase in acidity leads to a decrease in the pH level of water, making it more acidic. In normal conditions, seawater is slightly basic (with a pH of 8.1-8.3), but increased CO2 levels cause a drop in pH, making the seawater more acidic. Rising ocean acidification poses an important threat, especially to organisms that produce calcium carbonate. Calcium carbonate is used by coral reefs and other marine organisms to form shells and skeletons. Acidification makes it more difficult for organisms to produce calcium carbonate, which in turn affects their growth and the formation of their shells. Coral reefs, for instance, use calcium carbonate shells to form their foundational structure. As seawater becomes more acidic, the coral shells become less resilient, weakening their structure. Furthermore, acidification adversely influences coral calcification processes, reducing their growth rates and overall reef health. Coral bleaching events occur when corals expel the symbiotic algae (dinoflagellates) they live in. Acidified seawater can stress corals, leading them to expel their symbiotic algae. These algae provide corals with their color and nutrients. When corals expel these algae, they turn white and experience health issues. If the stressful conditions persist, bleached corals can die, severely impacting the structure and health of coral reefs. [1][2][4][5]

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5. Storm Damages

Rising temperatures cause severe storms by increasing energy transfer over the sea. These storms are called hurricanes, typhoons, or cyclones and are characterized by heavy rain, strong winds, high waves, and strong currents that carry large amounts of energy around them. These violent storms can damage coastal marine ecosystems and coral reefs by creating strong currents or waving the sea surface. This type of damage causes dead spaces. Moreover, violent storms can create heavy rain that deteriorates the water quality on coral reefs. Excess water runoff can carry away soil and land particles from the environment and muddy the reefs, blocking access to sunlight and photosynthesis. Photosynthesis is a vital process for reefs because, in the process, algae produce energy and nutrients that contribute to the growth and development of the reefs. [1]

As a result, global warming directly affects the health of coral reefs and marine ecosystems. If this danger cannot be avoided, there may be significant and irreversible destruction for marine life in the future. We must take a few basic precautions to prevent extinction. Primarily, we should avert to use unneceserly fossil fuel and give priority to renewable energy sources. Additionaly, To mitigate the adverse effects of overexploitation of marine resources and pollution on coral reefs, strict monitoring and regulations are necessary. Besides, climate change and the conservation of coral reefs require international cooperation and agreements. Inter-country coordination is a critical factor in protecting these vital ecosystems. As always, the important action what we need to do is raise the awareness of public. Conscious communities play a key role in agreements and regulations to control climate change. We are not the only species living on this earth, and we are not the only ones affected by our behavior. Our behavior as humans leads to the extinction of both ourselves and other species. We must live with this awareness and organize our actions accordingly.


1- The Ocean Foundation. (7.09.2023]). Ocean and Climate Change.

2- Hoey, A.S., Howells, E., Johansen, J.L., Hobbs, J.-P.A., Messmer, V., McCowan, D.M., Wilson, S.K., & Pratchett, M.S. (2016). Recent Advances in Understanding the Effects of Climate Change on Coral Reefs. Diversity, 8, 12.

3-Hoegh-Guldberg, O. (1999). Climate change, coral bleaching and the future of the world's coral reefs. Marine and Freshwater Research, 50, 839-866.

4- Hoegh-Guldberg, O., Poloczanska, E.S., Skirving, W., Dove, S. (2017). Coral Reef Ecosystems under Climate Change and Ocean Acidification. Frontiers in Marine Science, Global Change and the Future Ocean, Volume 4, 29 May 2017.

5- Anthony, K.R.N., Kline, D.I., Diaz-Pulido, G., Dove, S., & Hoegh-Guldberg, O. (2008). Ocean acidification causes bleaching and productivity loss in coral reef builders. Proceedings of the National Academy of Sciences, 105(45).

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