Whether Caribbean coral reefs retain their vibrant colors or turn a deathly white depends in part on how much dust there is in the atmosphere. Climate scientists have long known that aerosols cool the atmosphere and that the Pacific warming called El Niño globally influences the climate and warms the Caribbean, but now researchers have shown that these effects influence bleaching of the over a million square miles of Caribbean coral reefs.

Dinoflagellate algae inhabit coral, give it color and provide it with food. During an El Niño summer or other warming condition, the algae vanishes from the coral, rendering it colorless and without anything to eat. "Some coral can recover," says Jennifer Gill, an ecologist at the University of East Anglia in Norwich, England. "If it doesn't die, it has reduced growth rates and reproduction." When, however, a lot of aerosols hang in the air, the water remains cool, the algae stay put and the coral is not bleached.

Gill and her colleagues compiled coral bleaching reports, El Niño indices, and aerosol levels in the atmosphere over the Caribbean reefs to see how these factors interrelated during summers between 1983 and 2000. They also looked at years that the volcanoes El Chichón or Mount Pinatubo erupted and added even more particles to the atmosphere.

Sweeping winds blow millions of tons of dust from the dry Sahara Desert over the Caribbean. These airborne particles, the researchers report in this week's Proceedings of the National Academy of Sciences, USA protect the coral from bleaching by creating a haze that scatters sunlight, preventing it from warming the sea. El Niño years, however, hurt the coral. This warm weather raises the sea surface temperature, resulting in an increase in coral bleaching. But, the volcanic eruptions in Mexico and in the Philippines protected corals from bleaching even during strong El Niño years. "The coral reefs got lucky," Gill says.

To predict coral bleaching in the Caribbean, the researchers created a model based on aerosol levels and El Niño effects; it forecast that in 2005, 38 percent of the reefs would be bleached. The actual bleaching that year was about 33 percent.

Rather than rely on the protective effects of aerosols, however, there may better ways to help coral reefs. "Obviously, cleaning up air pollution is a good thing," Gill says. Instead, to protect the corals, she suggests creating more marine protected zones and properly developing coastal areas. It will not stop coral bleaching, she says, but the coral, without other problems, will be more resilient