Great Barrier Reef
Scientists predict widespread coral bleaching may damage over a third of the world’s coral reefsReuters

Mass coral bleaching on the Great Barrier Reef caused by an El Nino weather event will take place early next year, marine biologists have warned. Coral reefs around the world have already been severely damaged by unusually warm ocean temperatures.

When corals are put under stress by changes in conditions such as temperature, light or nutrients, they expel the symbiotic algae living in their tissues, which causes them to turn completely white, according to the National Oceanic and Atmospheric Administration.

Senior research scientist Line Bay of the Australian Institute of Marine Science said the forecasts were very concerning. Earlier this year, scientists predicted over a third of the world's coral reefs could be damaged by the widespread coral bleaching.

"The El Nino is as strong as it was in 1998 so the likelihood of marine heatwaves is high," she told ABC News.

Coral may be able to recover if the stress-caused bleaching is not severe and if water temperatures return to normal, but prolonged algae loss and continued stress may eventually kill the coral. Some coral may be more resilient than others, however.

"Lots of people are surprised to hear that corals are animals but what we're learning is that in fact they're individuals too and some coral individuals are more tolerant than others," Bay said.

"What we're looking at is whether tolerant individuals will have tolerant babies and if so, if we can identify genetic markers."

Coral reefs are fragile ecosystems that can only tolerate a narrow temperature range. The mass coral bleaching event of 1998 is considered to be the most severe on record, when around one-sixth of the world's coral colonies died. In 2005, the US lost half of its coral reefs in the Caribbean in a single year, due to a massive bleaching event.

This year has been provisionally declared the warmest on record thanks to a combination of global warming and a strong El Nino, a climate cycle in the Pacific Ocean with a global impact on weather patterns.