Global Warming: Changing Life on the Coral Reef

This topic submitted by Beth Brownson ( at 1:38 AM on 6/10/07.

Check out this huge M. annularis and accompanying fish at Molasses Reef, Key Largo, Florida.

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Global warming is an issue that has been hotly debated among scientists, politicians, economists, and the general public for years. Because the change in global temperature seems to be happening gradually, many people wish to deny the existence of this phenomenon. They prefer to believe that it is just a natural geological occurrence that will work itself out over time. Though people in the public sector continue to debate the reality of global warming, scientists are no longer divided on the issue. The vast majority of people involved in environmental research have found that global warming is, in fact, occurring and presents an incredibly serious danger to life as we know it.

Global warming is predominantly caused by an increase in carbon dioxide emissions. Gases in the atmosphere, most notably CO2, create what is called the greenhouse effect. The gases function in the same way as greenhouse panes; they allow radiation from the sun to enter to warm the Earth, but they stop all the radiation from leaving. By absorbing and re-reflecting some of the light back toward Earth, the atmosphere acts as an insulating layer for the planet. This phenomenon keeps the Earth’s surface temperature at an average of 15 degrees Celcius. Without the atmosphere working to rereflect some of the sun’s radiation, the Earth would be too cold to sustain life (Union of Concerned Scientists, 2006).

There has been a tremendous spike in CO2 emissions in recent decades, which has increased the amount of gas working to trap and rereflect the sun’s radiation. Other gases, including methane, nitrous oxide, and chlorofluorocarbons, are also contributing to global warming and the greenhouse effect. The United States is a leading cause of this crisis. Atmospheric carbon dioxide is increasing due to activities that are commonplace in the United States, such as driving cars, using electricity, and heating houses. Deforestation is also a substantial source of greenhouse gases. There has been an increase in average surface temperature of 1 degree Celcius since the beginning of the twentieth century. Though this number does not seem to be of great importance, due to the interconnectedness of so many species and environmental conditions, this change in temperature has the power to disrupt entire ecosystems (Union of Concerned Scientists, 2005).

Global warming has already begun to take its toll on the delicate life of the coral reef. Coral reefs are one of the most beautiful, prolific, and dynamic ecosystems on the planet. They attract tourists who marvel at their beauty, protect the shorelines of countries from erosion, provide homes for a great variety of marine life, and have been used in medicinal breakthroughs. However, it is predicted that close to 60% of the current coral reefs may be lost by 2030 (Hughes, 2003).

Corals and zooxanthellae, a type of algae, have a critical symbiotic relationship with one another. The zooxanthellae give the corals their characteristic color and aid in nutrition, while the corals provide protection and nutrients for the zooxanthellae to perform photosynthesis. Increases in sea temperature cause the zooxanthellae to leave the corals. Without their symbiotic algae, the corals lose their color and are considered to be bleached; they will die unless the zooxanthellae return (Fagoonee, 1999).

Coral reefs are able to recover from bleaching if temperatures are only elevated for a short period of time, but sustained high temperatures put great stress on the coral and can cause irreversible damage. An increase of only two degrees for a few weeks has the ability to kill corals (Davidson, 1998). El Nino Southern Oscillation (ENSO) events are cyclical but unpredictable events that cause coral bleachings when trade winds are slack, allowing large amounts of warm water to raise temperatures and sea level. These ENSO events used to occur relatively rarely, but coral reef bleachings are now occurring even in non-ENSO years due to the steady warming of the globe. To make matters worse, greenhouse gases also increase the frequency of ENSO events because of the complexity of the interactions of the sea and the atmosphere (Davidson, 1998).

The sea level is rising due to higher ocean temperatures and glacial melting, which means that when hurricanes and tropical storms occur, the storm surge is higher, leading to more flooding and damage. Oceans have absorbed a great deal of heat in the past few decades, which has increased sea surface temperatures as well as deeper water. This increase in temperature heightens the intensity of tropical storms, which creates a greater threat for coral reefs, marine life, and humans. “A 2005 study published in the journal Nature examined the duration and maximum wind speeds of each tropical cyclone that formed over the last 30 years and found that their destructive power has increased around 70 percent in both the Atlantic and Pacific Oceans” (Union of Concerned Scientists, 2006). Hurricanes gain speed when they hit areas of warm water and they are slowed when they encounter cold water. However, these storms do not always encounter the cold water needed to decrease their speed. Hurricane Katrina intensified greatly when it went through the deep, warm waters in the Gulf of Mexico (Union of Concerned Scientists, 2006).

Changes in the amount of coral species on a reef also significantly affect the marine fish that rely on the reefs for nutrition and protection. As coral bleaching occurs, marine fish are forced to look for coral elsewhere either temporarily until the zooxanthellae come back or permanently if temperatures remain elevated. As long as bleaching is not widespread, the fish can move to a neighboring reef with relative ease; however, if mass bleachings occur, as many scientists predict will become increasingly common, species of marine fish will face habitat loss and extinction.

Coral reefs in Kimbe Bay in Papua New Guinea have experienced a great deal of coral death as a result of bleaching. There has also been logging, agricultural development, and vegetation loss in these areas in recent years. Due to these factors and the proximity of the reefs to the shore, there are greater amounts of sedimentation, which increases instances of bleaching and makes it more difficult for new corals to grow. Coral reefs that are close to other reefs and experience high gene flow are more likely to recover from bleaching due to greater instances of varied reproduction. However, the reefs on isolated islands recover at a much slower rate, which has devastating effects for both the coral and marine life (Hughes, 2003).

Habitat loss clearly leads to a decrease in the amount of marine fish present in an area. Fish that are specialists and colonize only a particular type of coral are hardest hit by instances of habitat loss. Since specialists often have smaller populations than generalist fish who colonize several different types of coral, they are particularly susceptible to extinction. The Gobiodon species has become locally extinct in Kimbe Bay due to the death of the coral they colonize, but it has not become globally extinct because it still lives on reefs in other areas. However, fish with small geographic ranges, especially specialists, will become rapidly endangered if coral bleaching continues at a high rate (Munday, 2004).

Thriving reefs need fish as much as the fish need them. Marine protected areas, especially no-take areas, are an effort to increase the resilience of coral reefs. Though these areas obviously cannot directly stop coral reef bleaching by decreasing water temperatures, they decrease stress on the reefs, which in turn allows them to better recover from bleaching. “Many reefs are being fished out. Raising the specter of a pending food crisis, a recent study found that 27 of 49 island countries are exploiting their reef fisheries in an unsustainable way” (Stone, 2007). When reefs are overfished, there are not enough herbivores to keep the algae in check, so they can become dominated by algae. Even more devastating is the practice of cyanide fishing, in which poison is used to stun and capture prey. This poison also indiscriminately kills coral and all other marine species living on the reef (Davidson, 1998).

There is current research being done in an attempt to curb the negative effects of global warming and coral bleaching. Scientists are monitoring changes in the current climate and using coral cores that are hundreds of years old in an attempt to track the differing responses of corals to changing environmental conditions in the past. Scientists want to figure out what makes a type of coral either susceptible or resilient to bleaching and gain knowledge about the relationship of coral to the zooxanthellae to determine if something can be done to prevent to algae from leaving when environmental conditions decline (Australian Institute of Marine Science, 2007).

The most direct impact that the public can have on curbing global warming is to decrease carbon dioxide emissions. People can do this directly by driving less often and purchasing more fuel efficient cars, using energy efficient light bulbs and appliances, and conserving electricity whenever possible. People can encourage governments and businesses to raise energy efficiency standards and also encourage the use of renewable energy sources, such as wind and solar power. Protecting and restoring forests will also help curb global warming (Union of Concerned Scientists, 2006).

The implications of global warming are far-reaching and have the potential to be tremendously devastating. Coral reefs must be kept in a very delicate balance with their zooxanthellae, the marine animals around them, and abiotic factors such as temperature in order to thrive. The human race is disturbing this balance by increasing carbon dioxide emissions and must substantially change their behaviors in order to prevent beautiful ecosystems like the coral reefs from being destroyed. There is no better example than the impact of global warming to illustrate the concept that everything in this world is intricately connected.

Australian Institute of Marine Science. (2007). Coral Reefs and Climate Change 2007.

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

Fagoonee, I. (1999). The Dynamics of Zooxanthellae Populations: A Long-Term Study in the Field. Science, 283(5403), 843-845

Hughes, T. P. et al. (2003). Climate Change, Human Impacts, and the Resilience of Coral Reefs. Science, Vol. 301 Issue 5635, p929-933.

Munday. (2004). Habitat loss, resource specialization, and extinction on coral reefs. Global Change Biology, 10(10), 1642.

Stone, R. (2007). A World Without Corals? Science, 316(5825), 678-681.

Union of Concerned Scientists. (2006). Early Warning Signs: Coral Reef Bleaching.

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