SYDNEY (Reuters) - Australian scientists managing the Great Barrier Reef have lifted their emergency response to the highest level following the publication of video footage of damage caused by coral bleaching.

A boat carrying tourists floats above an area called the 'Coral Gardens' near Lady Elliot Island, north-east of the town of Bundaberg in Queensland, Australia, June 10, 2015. REUTERS/David Gray/Files

Authorities this month said that areas of the World Heritage site were experiencing the worst bleaching in 15 years, at least partially as a result of the current El Nino, one of the strongest in two decades.

Coral bleaching is a process by which coral expels living algae, causing it to calcify. Coral can only survive within a narrow band of ocean temperature.

The Great Barrier Reef Marine Park Authority on Sunday said it was lifting its response to level three, authority chairman Dr Russell Reichelt said.

“A level-three response level means we’re stepping up surveys in response to the coral mortality to help us better understand the effects of various pressures on the reef and help guide management actions,” Reichelt said in a statement.

The footage, shot on Saturday by the University of Queensland’s CoralWatch group, has raised serious concerns amongst scientists and environmental groups about the growing impact of climate change.

“The new video and stills are very concerning and show large sections of coral drained of all colour and fighting for survival,” World Wildlife Fund spokesman Richard Leck said in a statement.

“This is the worst coral bleaching event ever to hit this most pristine part of the Great Barrier Reef.”

Scientists said the Great Barrier Reef needs a break in El Nino conditions within weeks if some coral areas are to survive, but the Australian Bureau of Meteorology’s most recent forecast calls for a continuation of El Nino conditions.

This year will be the hottest on record and 2016 could be even hotter due to El Nino, the World Meteorological Organization has said.

The Great Barrier Reef stretches 2,000 km (1,200 miles) along Australia’s northeast coast and is the world’s largest living ecosystem. It brings in billions of dollars a year in tourism revenue.

UNESCO’s World Heritage Committee last May stopped short of placing the Great Barrier Reef on an “in danger” list, but the ruling raised long-term concerns about its future.