Coral Bleaching and Global Climate Change

This topic submitted by Lynne Nickol ( at 12:12 AM on 6/10/07.

A view of our boat at Gaulin Reef in the Bahamas

Tropical Field Courses -Western Program-Miami University

Global warming, or more accurately global climate change, has developed into a hot topic in science and politics. Politicians, scientists and lobbyists argue over the many different implications of global climate change and the effects it will render on our planet. Carbon dioxide emissions and the use of fossil fuels have spurred debate and study in many areas, including global weather patterns, glacial melting, and energy resource use and consumption. One of the lesser known, yet equally important effects of global climate change lies within the tropical coral reefs of the world. A phenomenon called coral bleaching is devastating reefs as the symbiotic algae hosted by corals are suffering, causing once colorful reefs to fade, developing a bleached whitish coloration. Besides the loss of color in the coral reef ecosystems, there are many other devastating effects to the surrounding organisms as a result of coral bleaching, caused by Earth’s changing climate.

Corals thrive only under specific conditions, especially to allow the formation of a reef. Waters are ideal for coral reef growth at temperatures around 24° C, but can be sustained in waters ranging from as low as 20° C to as high as 36° C, but growth decreases as temperatures deviate from the ideal (Bulloch 150). Of course, “the temperature threshold for bleaching is not an absolute value, but is relative to other environmental variables…and to the duration and severity of the departure from the normal temperature conditions of a reef (Aronson 16). Salinity of the water is also crucial to corals, growing best in a salinity of thirty-four parts per thousand, and in waters free of silt (Bulloch 150). Corals rely on sunlight for food production, but they are also affected by the amount of ultraviolet radiation they receive. Too much exposure decreases photosynthetic activity and causes DNA damage to the coral (Torregiani). Conditions, in shallow waters allow for the clear, sunlit, warm waters necessary for coral reefs to grow and thrive. When these conditions are not met, however, coral mortality through bleaching is apt to occur. “Several factors can cause bleaching, including increased ultraviolet radiation, excessive shade, disease, sedimentation, pollution, and changes in salinity. Yet none of these is as compelling as the theory that global warming is the primary culprit” (Christianson 214).

Global warming, or global climate change is the phenomenon of rising temperatures in the earth’s climate as a result of an increase in greenhouse gasses. Studies such as the Keeling Curve have established that carbon dioxide levels in the atmosphere are rising at alarming rates. Although the current carbon dioxide level, approximately 350 parts per million, “is not unprecedented in the geologic record, our concern is the speed at which a basic characteristic of the planet has changed to levels not previously experienced during human history or during the evolution of current ecosystems” (The Global Carbon Cycle 316). The buildup of greenhouse gasses in the atmosphere causes ozone depletion, allowing increased ultraviolet radiation to enter the stratosphere and heat to earth. Tropical ecosystems experience the worst of the ultraviolet radiation due to the small solar zenith angle, as they surround the equator (Torregiani). Because such tropical marine waters are so clear, ultraviolet radiation can often penetrate deeper than thirty meters (Torregiani). Coral reefs are struggling all over the planet as a result of climate change. In fact, “many scientists consider the death of the delicate corals to be the equivalent of the canary in the coal mine, the early warning of a rise in the ocean temperatures of the world” ( Christianson 214). To understand the effect global climate change has on coral reefs, it is important to understand the structure of corals.

What appears to the naked eye to be a single coral is really thousands of individual coral animals, or polyps (Kaplan 78). Put succinctly, “each animal resembles a hollow cylinder, closed at the base and connected to its neighbors by the gut cavity” (Christianson 213). The lower end of each coral polyps is attached to a hard floor of calcium carbonate (left from other living and dead corals) and its upper end consists of “a circlet of tentacles surrounding an oral disk containing a slit-like mouth,” (Buchheim, Kaplan 78). Each individual polyp is composed of two cell layers, the outer epidermis and the inner gastrodermis (Davidson 14). Each coral, while perfectly capable of independent survival is connected to surrounding polyps, creating colonies of thousands of individual animals all capable of sharing food through diffusion (Kaplan 79).

Corals get their food from two sources: zooplankton and zooxanthellae. Zooplanktons are microscopic animals carried through ocean waters, and when the pass too closely to a coral the polyp will shoot its tentacles out of the mouth and capture their prey (Davidson 14-15). But the clear tropical waters do not hold nearly enough plankton to support coral growth. Thus, corals rely on their symbiotic relationship with autotrophic microalgaes known as zooxanthellae. “Zooxanthellae are stored inside the cells of the coral polyp; their numbers are in proportion to the amount of light the coral receives (Bulloch 154). In this mutuality symbiotic relationship, both coral and zooxanthellae help each other. The host corals provide a well protected environment to the algae, and provide them with a steady supply of carbon dioxide for photosynthesis (Buchheim). Besides providing nutrient food energy for their host, zooxanthellae also provide the coral with fixed carbon compounds, enhance calcification for skeletal growth, and mediate the corals nutrient flux (Buchheim).
Typically, corals contain about “1-5 x 106 zooxanthellae cm-2 of live surface tissue and 2-10 pg of chlorophyll per zooxanthellae. When corals bleach they commonly lose 60-90% of their zooxanthellae and each zooxanthella may lose 50-80% of its photosynthetic pigments and ability”. If the stressors causing coral bleaching are minor and short lived, the corals may regain their zooxanthellae relationship over time and recover, but often this is not the case. With global climate change, thermal and ultraviolet stressors are often continued for extended periods of time, and the corals eventually die. Even tiny alterations in water temperature can induce bleaching, as little as 1-2ľ C. Because corals live in shallow waters, they are exposed to strong amounts of ultraviolet radiation, especially in areas suffering from ozone depletion. (Buccheim)

Corals rely on zooxanthellae to survive, and without this relationship bleaching occurs. Nearly all coral tissue is transparent; it is the zooxanthellae that give corals their color (Davidson 16). When the zooxanthellae die, corals lose their color becoming whitish. Without the additional nutrition from the photosynthetic zooxanthellae, the corals eventually die. Currently, approximately ten percent of the worlds coral reefs have already been destroyed beyond all hope of repair, and it is predicted that within the next twenty years another thirty percent will decline as well (Davidson 11). The global warming trend indicates that “we may expect an increase in the frequency, severity, and scale of coral reef bleaching” (Buccheim). Coral mortality due to bleaching has severe effects, and influences more than just the corals themselves.

Coral reefs attract many tourists and SCUBA divers to surrounding land, creating business and work for local inhabitants. With the declining health of corals around the world, many people are concerned with the impact coral bleaching will have on tourism. A recent study conducted by researchers at Gothenburg University in Sweden repeated a previous study on the value of access and quality of coral reef tourism; the original study was conducted before coral bleaching became widespread, and the new study followed the same format and was conducted in 2006. They did not find any conclusive results that tourism was suffering as a result of coral mortality, however upon asking interviewees whether coral bleaching would influence their choice of destination, the overwhelming response was yes. Many stated that they did not think it was worth diving on a damaged reef. They also found that while the number of fish living in bleached reefs “remained the same or increased, the composition had changed to more herbivores species and fewer coral reef fish.” Besides the environmental damages, increased coral bleaching may eventually create problems for tourism business in the future. (Andersson)

“Coral reefs are among the most biologically varied ecosystems on Earth and are home to incredibly diverse kinds of plants and animals” (Panse). Due to the intense biodiversity and productivity of coral reef ecosystems, they are often refereed to as “the tropical rainforests of the oceans” (Buchheim). There are many chemicals that can be found in the different plants and animals that thrive in the coral reef ecosystem, and many have been found to have important medical properties, and are used in antibiotics, anti-inflammatory medications, and cancer treatments (Panse). Currently such chemicals found in coral reefs are used in drugs such as AZT, Ara-A, Ara-C, and Dolostatin 10, which combat AIDS, cancer, asthma, arthritis, and inflammatory Disorders, respectively (Panse). “They are also proving beneficial for people with heart, kidney, and liver transplants. Corals have been used in human bone-grafting and Cone Snail poison has been used to produce a painkiller”(Panse). The impressive list of medical uses and cures found in coral reef ecosystems is continually growing as more species are discovered and studied, but coral mortality due to bleaching creates a threat to future discoveries. With the devastation to reefs, we are missing out on chances to discover new cures and medications.

Coral bleaching, caused primarily by warming waters and increasing ultraviolet radiation is posing a dangerous threat to the beautiful marine tropical coral reef ecosystems of the world. Greenhouse gasses are depleting the ozone layer, allowing more ultraviolet radiation to reach the earth and warming the global climate. These changes to the global climate cause the zooxanthellae to die, which in turn causes the corals to die. The loss of coral reefs due to bleaching is devastating in many ways, including the ecosystem itself and its inhabitants, the economy of local towns, and medical research. Reef mortality is increasing, due to the earth’s changing climate, and the repercussions of coral bleaching are severe.

Andersson, J. E.C. (2007). The recreational cost of coral bleaching—A stated and revealed preference study of international tourists. Ecological Economics 62.3-4. 2007.

Aronson, R. B. (2004). Coral Reefs and Global Climate Change. Pew Center on Global Climate Change. Retrieved May 4 2007 from

Buchheim, J. Coral Reef Bleaching. (1998). Odyssey Expeditions. Retrieved May 4 2007 from

Bulloch, D.K. (1991). The Underwater Naturalist. New York: Lyons & Burford, Publishers.

Christianson, G.E. (1999). Greenhouse: The 200-Year Story of Global Warming. New York: Walker Publishing Company.

Davidson, O.G. (1998). The Enchanted Braid: Coming to terms with nature on the coral reef. New York: John Wiley & Sons, Inc.

Panse, S. (2004). Medicines from the Coral Reef Ecosystems. Retrieved May 4 2007 from

Scopélitis, J. (2007). Modelling coral reef habitat trajectories: Evaluation of an integrated timed automata and remote sensing approach. Ecological Modelling, 205(1-2), 59.

“The Global Carbon Cycle.” WCP 334.E A: Environmental Chemistry and Geochemistry Course Reader. 2007. 308-321.*

Torregiani, J. H. (2007). The effects of short-term exposures to ultraviolet radiation in the Hawaiian Coral Montipora verrucosa. Journal of Experimental Marine Biology and Ecology, 340(2), 194.

* This article came from a course reader, and did not have the author, publisher, etc. I asked my professor if he could give me the information to correctly cite it, and he was unable to.

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