Scientists in Australia have recorded "severe" coral bleaching across large parts of the Great Barrier Reef for the second time in 12 months.
In a news release on Monday, James Cook University (JCU) in Australia said that aerial surveys had shown that the middle third of the reef had seen the most intense bleaching, while in 2016 bleaching had hit the northern third the hardest.
"The combined impact of this back-to-back bleaching stretches for 1,500 km (900 miles), leaving only the southern third unscathed," Terry Hughes, director of the Australian Research Council's (ARC) Centre of Excellence for Coral Reef Studies, said in a statement. The Centre is headquartered at JCU.
Hughes, who was involved with the aerial surveys in 2016 and 2017, went on to add that the bleaching had been caused "by record-breaking temperatures driven by global warming. This year, 2017, we are seeing mass bleaching, even without the assistance of El Niño conditions."
Bleaching takes place when "abnormal" conditions – such as increased sea temperatures – result in coral getting rid of very small photosynthetic algae, according to the ARC and JCU.
When this happens, the corals "bleach" and turn white. While they can recover if the sea temperature falls again, they can die if it doesn't.
"This is the fourth time the Great Barrier Reef has bleached severely – in 1998, 2002, 2016, and now in 2017," James Kerry, who was also involved in the aerial surveys, said.
"Bleached corals are not necessarily dead corals, but in the severe central region we anticipate high levels of coral loss," Kerry added. "It takes at least a decade for a full recovery of even the fastest growing corals, so mass bleaching events 12 months apart offers zero prospect of recovery for reefs that were damaged in 2016."
As well as the mass bleaching, a corridor of the Great Barrier Reef had to contend with the impact of Tropical Cyclone Debbie towards the end of March. This, JCU said, "was likely to have caused varying levels of damage along a path up to 100 km in width."
Hughes said that the reef was struggling with multiple impacts. "Without a doubt the most pressing of these is global warming," he added.
"As temperatures continue to rise the corals will experience more and more of these events: 1°C of warming so far has already caused four events in the past 19 years."