SYDNEY (Reuters) - When marine scientist Ray Berkelmans went diving at Australia's Great Barrier Reef earlier this year, what he discovered shocked him -- a graveyard of coral stretching as far as he could see
"It's a white desert out there," Berkelmans told Reuters in early March after returning from a dive to survey bleaching -- signs of a mass death of corals caused by a sudden rise in ocean temperatures -- around the Keppel Islands.
Australia has just experienced its warmest year on record and abnormally high sea temperatures during summer have caused massive coral bleaching in the Keppels. Sea temperatures touched 29 degrees Celsius (84 Fahrenheit), the upper limit for coral.
High temperatures are also a condition for the formation of hurricanes, such as Katrina which hit New Orleans in 2005.
"My estimate is in the vicinity of 95 to 98 percent of the coral is bleached in the Keppels," said Berkelmans from the Australian Institute of Marine Science.
Marine scientists say another global bleaching episode cannot be ruled out, citing major bleaching in the Caribbean in the 2005 northern hemisphere summer, which coincided with one of the 20 warmest years on record in the United States.
"In 2002, it would appear the Great Barrier Reef went first and then the global bleaching followed six to 12 months later. Is it the same this time around? No," said Berkelmans.
"The Caribbean beat us to it. We seem to be riding on the back of that event. We don't know what is ahead in six months for the Indian Ocean reefs as they head into their summer."
"This might be part of a global pattern where the warm waters continue to get warmer."
Other threats to coral reefs -- vast ecosystems often called the nurseries of the seas -- include pollution, over-fishing, coastal development and diseases.
CAN CORAL RECOVER?
Corals are vital as spawning grounds for many species of fish, help prevent coastal erosion and also draw tourists.
Bleaching is due to higher than average water temperatures, triggered mainly by global warming, scientists say. Higher temperatures force corals to expel algae living in coral polyps which provide food and color, leaving white calcium skeletons. Coral dies in about a month if the waters do not cool.
Berkelmans said the Keppels had previously bounced back from bleaching once the waters had cooled. But if temperatures remained abnormally high then that would be much more difficult.
Many scientists say global temperatures are rising because fossil fuel emissions from cars, industry and other sources are trapping the earth's heat. Experts worry some coral reefs could be wiped out by the end of the century.
Global warming could also damage corals by raising world sea levels by up to a meter by 2100. That could result in less light reaching deeper corals, threatening the important algae.
The Great Barrier Reef -- the world's largest living reef formation stretching 2,000 km (1,250 miles) north to south along Australia's northeast coast -- was the first to experience what turned out to be global coral bleaching in 1998 and 2002.
The Keppels bleaching is as severe as those two events and scientists say the threat of widespread bleaching is moderate.
"Sea temperatures in all regions of the Great Barrier Reef are at levels capable of causing thermal stress to corals," said the Great Barrier Reef Marine Park Authority's February report.
The U.S. National Oceanic and Atmospheric Administration's Coral Reef Watch said the 2005 Caribbean bleaching centered on the U.S. Virgin Islands, but stretched from the Florida Keys to Tobago and Barbados in the south and Panama and Costa Rica.
Reef Watch said sea temperature stress levels in the Caribbean in 2005 were more than treble the levels that normally cause bleaching and almost double the levels that kill coral.
"Time will tell whether there was large-scale mortality or not," said Professor Robert Van Woesik from the Florida Institute of Technology in a statement issued by Australia's Queensland University. He said corals did have some ability to bounce back but that this was an unusually warm event.
Queensland University's Professor Ove Hoegh-Guldberg, head of a group of 100 scientists monitoring bleaching, said scientists were concerned about how close in time the two severe bleaching episodes were.
"The 2006 Great Barrier Reef event comes soon after the worst incidence of coral bleaching in the Caribbean in October 2005," said Hoegh-Guldberg who also went diving on the Keppels where he said damage was extensive.
"The traces suggest we are tracking the temperature profile of 2001-2002, which led to the worst incidence of coral bleaching ... for the Great Barrier Reef," he said.
In 2002, between 60 and 95 percent of the reefs that make up the Great Barrier Reef were bleached. Most corals survived but in some locations up to 90 percent were killed.
Hoegh-Guldberg said projections from 40 climate models suggested that oceans would warm by as much as three to four degrees Celsius in the next 100 years.
"We're starting to get into very dangerous territory where what we see perhaps this year will become the norm and of course extreme events will become more likely," he said.
"The climate is changing so quickly that coral reefs don't keep up ... the loss of that ecosystem would be tremendous."