Image: Bleached coral
A NOAA diver surveys a bleached reef colony in St. Croix, U.S. Virgin Islands, in October 2005.
updated 11/17/2010 4:54:42 PM ET 2010-11-17T21:54:42

A coral bleaching event in 2005 that killed or stressed many Caribbean reefs was the worst on record, said experts who also warned that bleaching this last summer is likely to have proven even worse in some areas.

"As this paper went to press in 2010, major bleaching was again striking reefs in the Caribbean, in some places worse than in 2005," the researchers wrote in their study, published this week in the peer-reviewed online journal PLoS ONE.

"Major bleaching events have returned to the Caribbean every five years or less, and with growing intensity," they added, tying the events to warming ocean temperatures. "With no real sign of recovery after bleaching in Caribbean reefs, these repeated events are likely to have caused reef decline that will extend beyond our lifetimes."

Bleaching happens when corals are stressed and then expel the algae living inside — turning what were colorful reefs into tracts of white. If prolonged, that bleaching can kill coral directly or by allowing diseases to take over.

This brain coral off St. John, U.S. Virgin Islands, is seen on Aug. 28 (left) when it had started to bleach. On Sept. 6 (center), it was very bleached. By Sept. 25 (right), it had started to recover as temperatures cooled. The symbiotic algae that give the corals their color recover more quickly on the sides of the colony, which are not as exposed to direct sunlight.

In the study, experts from 22 countries reported that more than 80 percent of surveyed corals bleached in 2005, and more than 40 percent of the total surveyed died.

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That damage made it "the most severe bleaching event ever recorded in the basin," according to the National Oceanic and Atmospheric Administration, which helped fund the study.

"This severe, widespread bleaching ... suggests a troubled future for tropical marine ecosystems under a warming climate," the experts wrote.

The researchers noted that other testing had found that the Caribbean's reefs had been stable for at least 200,000 years — until the 1980s when bleaching started to happen more often.

Bleaching has also hit reefs in other parts of the world where waters have warmed, in particularly off Indonesia and Australia.

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