SYDNEY, April 10 (Xinhua) -- Australia's Great Barrier Reef has been significantly damaged by back-to-back bleaching events and the devastation caused by tropical Cyclone Debbie, a new aerial survey revealed on Monday.

The Australian Research Council's Center of Excellence for Coral Reef Studies discovered that around two-thirds of the iconic 2,300 km reef is not expected to recover.

"The most common cause of coral bleaching is when water temperatures rise causing coral to expel symbiotic micro algae called zooxanthellae," Director of the Sydney Institute of Marine Science Peter Steinberg told Xinhua.

"Algae gives coral their color and it also provides them with great deal of nutrition, so bleaching is a sign those corals are in significant stress."

There have been four bleaching events over the past 25 years during 1998, 2002, 2016 and 2017.

This is the first time on record the Great Barrier Reef has had two major bleaching events in two successive years.

"These events correlate with a gradual increase in temperatures seen by climate change," Steinberg said.

"2016 was an El Nino year, meaning we saw warmer water, but this year isn't, and yet the water temperature is still very high."

Because coral reefs can take up to 10 years or more to recover from bleaching, Steinberg said, "If it continues to happen every year, there is simply no time for recovery."

To make matters worse, an area that had been spared significant damage from bleaching, the Whitsundays, has been left in tatters by Cyclone Debbie.

"By all accounts the cyclone has caused tremendous damage to the reefs in the Whitsundays," Steinberg said.

"We need to look carefully at whether the management process that we have in place for the Great Barrier Reef is really adequate for the challenges it's currently facing."

"In the last 20 years or so, the percent of the bottom of the ocean that is covered by coral on the Great Barrier Reef has decreased by about 50 percent."