"The same heat that's causing the hurricanes is causing the bleaching of corals," Lara Hansen, chief climate change scientist at the WWF, told Reuters.

Bleaching happens when warm temperatures kill algae that live alongside corals, tiny marine creatures protected by a limestone skeleton. Deprived of algae that are a source of food, the whitened corals can die if the waters stay hot.

Temperatures in the Caribbean had been around 29-33 Celsius (84-91 Fahrenheit), high enough to cause damage to corals and well above the 26.5C needed to create hurricanes.

Hurricane Rita was expected to make landfall early on Saturday near the Texas and Louisiana border three weeks after Katrina struck the Gulf coast. Katrina killed at least 1,069 people.

Hurricanes can reduce bleaching of corals by stirring up cooler, deeper waters. "Thankfully for corals off Florida, Katrina cooled things down," Hansen said.

CHURN UP SEABED

But storms can also throw up damaging waves. Cyclone Heta in the Pacific Ocean last year, for instance, caused wide damage to corals of Tutuila island, part of Samoa, with waves that smashed up the ocean floor, Hansen said.

According to reports from government and university scientists around the Caribbean, about 89 percent of corals in parts of Florida's Biscayne national park were affected by bleaching.

Off Puerto Rico, about 50-75 percent had suffered bleaching. And off Barbados, temperatures of 31-32 Celsius (88-90F) had been recorded at depths down to 60 feet (20 metres), causing widespread damage.

Causes of warmer Caribbean temperatures this year are unknown. Many scientists say that a build-up of gases from burning fossil fuels in cars, factories and power plants are blanketing the world and are slowly driving up temperatures.

The scientific panel that advises the United Nations has estimated that average world temperatures could rise by 1.4-5.8 C by the year 2100, triggering ever more powerful storms.