WRECKING THE REEFS

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But what has brought about the epidemics of "bleaching" that have turned coral reefs white across the Pacific Ocean and parts of the Caribbean? Scientists have known for some time that various kinds of stress can cause corals to expel their zooxanthellae. Since it is the zooxanthellae that give coral colonies their rich coloration, their loss causes entire reefs to turn white. The stress that caused the recent bleachings, scientists say, was a seasonal spike in seawater temperatures. But other sources of stress, such as overfishing and nutrient overload, may have made the corals and their symbiotic friends unusually sensitive to heat.

What if several decades from now global warming causes such swings in temperature to occur more often? That possibility alarms marine scientists, because bleaching--the coral equivalent of running a fever--can be fatal. In 1983 a particularly severe bleaching episode killed 95% of the corals off the Galapagos Islands. Global warming could also trigger more intense hurricanes, scientists fear. And while healthy reefs would no doubt recuperate from the pummeling, sick reefs might not. "What we worry about," says Smithsonian marine biologist Nancy Knowlton, "is a threshold effect, when so much stress piles up that all of a sudden the floor falls through."

The dwindling of reefs in the world's oceans, scientists acknowledge, will not immediately destroy the organisms that build them. Many corals spawn en masse, releasing a vast pinkish slick of fertilized eggs that ride ocean currents for hundreds of miles. In the natural cycle, one reef rises as another declines. This cycle is what humans are now disrupting, however, and no one can foresee what the consequences will be. Creating more marine preserves can help, but even if the reefs are patrolled by armed guards, they may not be able to withstand the twin juggernauts of exploding population and the economic desperation that accompanies it. In the next five decades, the number of people on earth may nearly double, to more than 10 billion, and the pressure that will place on reefs is almost too enormous to contemplate.

--With reporting by Sandra Burton/Hong Kong