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Earth's Coral Reefs in Danger

bleached coral
Bleached coral
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Flooding, mud slides and destructive storms of the great El Niño of 1997-98 made the headlines, but some of the worst devastation took place quietly out-of-sight under water on the world's coral reefs. With their amazing diversity, they are often called the rainforests of the oceans. While reefs cover only a tiny fraction of the ocean floor, they are home to millions of plants and animals, representing tens of thousands of species.

Reefs face a double-whammy -- mother nature and human nature. When nature dished out unusually warm ocean temperatures and high sea levels as it did in 1982-83 and again in 1997-98, the result was a massive die-off of corals around the globe. Meanwhile, humans continue stressing the fragile ecosystem with pollution, over-fishing and development.

Reefs off the Philippines
Reefs off the Philippines seen by JPL's ASTER instrument on NASA's Terra satellite
"With our ability today to monitor global sea-surface temperatures and global sea levels with the National Oceanic and Atmospheric Administration's polar-orbiting satellites and NASA's Topex/Poseidon, we are in a position to track how heat -- corals' friend and foe -- moves through the world's oceans," said Dr. William Patzert, JPL oceanographer.

"Knowing how this most recent coral die-off developed in time and space will help us understand how sensitive coral reefs are to environmental changes," he said.

While corals have survived for hundreds of millions of years by adapting to and recovering from major climate changes -- such as ice ages, hurricanes and El Niños -- the question is now whether they are resilient enough to survive the additional stresses caused by humans.

Researchers estimate that nearly 60 percent of the world's reefs are seriously threatened. Racing against the destruction clock, scientists are looking for ways to improve their understanding of these complex systems and find ways to help.

Coral itself is a fragile animal, a polyp that creates a shell around itself of calcium carbonate. Within its individual compartment, it lives in perfect harmony with algae called zooxanthellae. These plantlike algae share the food they produce through photosynthesis with the coral. Although some corals live as solitary individuals, most live in dense colonies of tiny polyps. Century after century, reefs grow as living coral colonies build new structures atop older ones. They can be thousands of years old.

healthy coral
Healthy coral
Corals depend on clean, clear, relatively shallow waters to survive. When stressed, they can lose their symbiotic zooxanthelle -- a process called "bleaching" since most of the coral's color comes from the algae. Without its food source, the coral often dies. Along polluted coastlines or on reefs where plant-eating fish have been severely reduced by fishing, the reefs are quickly overrun with seaweed. The abundant sea life the reef supported loses its home.

The climate events of 1997-98 struck a major blow to the world's corals. "Global ocean and land temperatures averaged about four degrees Fahrenheit above normal, and sea level rose at least one inch above average," said Patzert.

"When the water gets hot, you get massive coral mortality," said Dr. John McManus, director of the National Center for Caribbean Coral Reef Research at the University of Miami's Rosenstiel School of Marine and Atmospheric Research. "We saw areas where for tens or hundreds of kilometers, there was 90 percent death. Total global coral mortality may have been as high as 20 percent."

This became the best-documented global coral bleaching event in history. "For the first time," said Patzert, "scientists were watching all the tropical oceans and seas from space, while an army of researchers and citizen scuba divers recorded the underwater devastation -- the locations and intensity of coral bleaching. This was definitely a first. These data will provide a quantum leap in our understanding of coral reefs."

In the past, corals have managed to survive destructive events like the 1997-98 El Niño. "The question now is whether coral reefs will continue to have the same resilience now that they face additional man-made stresses," said McManus.

Red Sea coral
Coral reefs in the Red Sea as seen by ASTER
Using satellite data on sea-surface temperature, scientists are developing maps of "hot spots," places where unusually warm sea-surface temperatures may be stressing coral. These may provide an early warning of reefs in distress and, possibly, another worldwide coral bleaching event. Remote sensing from satellites and aircraft are providing new ways to survey and monitor reefs.

"We are trying to figure out how we can help manage reefs better -- determine just how much pollution they can tolerate, how much fishing," said McManus. "More than 30 million people depend on coral reefs for their livelihoods. Most coral reefs are also in developing countries without the financial resources for coral reef research. Satellite remote sensing is very cost-effective and is often the only recourse for broad area assessment in these countries. There are satellite sensors on the drawing board with high resolution and good depth penetration, which promise to revolutionize coral reef assessment and management."

"We know that climate events like El Niño, La Niña and the larger longer-lasting Pacific Decadal Oscillation, the large fluctuation of heat around the tropics, have immense impacts on corals," said Patzert. "While we can't change these natural events, the more we know about them and how they affect corals, the more we can do to help reefs survive."

Related Sites:

National Center for Caribbean Coral Reef Research

TOPEX/Poseidon home page

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)

Rosemary Sullivant
NASA Jet Propulsion Laboratory