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Global Warming Blamed for Caribbean Coral Kill
By Hillary Mayell, National Geographic News. May 5, 2000

Global warming has been blamed for the extermination of the coral reefs of central Belize‹an event described by scientists as the first mass coral kill in the Caribbean in at least 3,000 years. A coral bleaching event that took place in 1998 has all but wiped out the reefs of the central part of the Central American country, marine scientists say. The bleaching event was reported two years ago, but bleaching doesn't automatically kill coral, and reefs usually recover from such an event. What surprised the scientists is the catastrophic mortality rate in Belize. "The coral kill in the central barrier reefs of Belize covers at least 375 square kilometers (145 square miles), probably more," says Richard Aronson, a marine ecologist with the Dauphin Sea Island Sea Lab and lead author of a report in the May 4 issue of the journal Nature. "The fossil record shows that this is the first mass coral kill in the Caribbean in at least 3,000 years."

Coral reefs are made up of large colonies of tiny animals known as coral polyps and the skeletons they leave behind after they die. They maintain a symbiotic relationship with zooxanthellae, microscopic, one-celled algae that live within the thin layer of live coral tissue. Zooxanthellae convert sunlight into fuel for their coral hosts, facilitating the formation of the coral skeleton that makes up a reef, and also providing corals with their rich colors. If water temperatures rise even one degree Celsius (about two degrees Fahrenheit) above the maximum temperature that corals are used to, they become stressed. When stress levels get too high, corals expel their zooxanthellae. The loss of color that results is referred to as bleaching. If the water temperature doesn't stay too high for too long and the reef is not stressed in other ways, corals that have been bleached can and usually do recover. Bleaching events that occurred prior to the 1980s were generally attributed to localized phenomena such as major storms or resulted from the hand of man-overfishing, destructive fishing, or an influx of increased nutrients, sediments, or pollution. That pattern has been changing over the past 20 years. "All of the bleaching events in the 1980s and 1990s occurred during El Niño conditions," says Aronson.

The El Niño weather phenomenon occurs approximately every two to seven years and brings significant weather changes-drought in some areas, increased rainfall in others, and unusually warm ocean temperatures along both sides of the equator in the central and eastern Pacific Ocean. Temperatures automatically rise in El Niño years, says Aronson, and global warming is bumping this upward curve even more. The highest sea temperatures ever recorded occurred during the 1997-1998 El Niño, causing a worldwide bleaching event. "In 1982-83 there was a huge wipeout in the eastern Pacific, and we had somewhat lesser events occur in 1987 and 1995," says Aronson. "The bleaching events are happening with increasing intensity and frequency." "The intensity of the 1998 event was augmented by worldwide global warming," says Aronson. "As we move into the future, we will see more of El Niño and global warming conspiring to cause these massive kills."

"The importance of coral reefs to people can't be overestimated-this has become a real human issue, not just a scientific one," he adds. Coral reefs are meccas of biodiversity, home to one-quarter of all marine plants and animals. They act as nurseries for many commercial fisheries. Reef ecosystems are the primary protein source for much of the world's population. They provide tourism dollars and protect against beach erosion, storm surges, and hurricanes. "If people think that global warming is something that will affect us in the future, they need to think again, because it's here now," says Aronson. "This should be a real wake-up call that we need to do something about greenhouse gas emissions now." The National Geographic Society, National Science Foundation, and the Smithsonian Institution provided support for the Belize coral project.