Great Barrier Reef at breaking point after worst coral bleaching on record

Posted 20 March 2017

Coral bleachingStopping global warming is the only way to save the Great Barrier Reef and end coral bleaching, an international study authored by 46 scientists concludes. 

The research, published in Nature, disproved the theory that keeping coral healthy would reduce the stress of high temperatures. 

Lead author and Director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University Terry Hughes said global warming was the number one threat to the reef.

“We have now assessed whether past exposure to bleaching in 1998 and 2002 made reefs any more tolerant in 2016. Sadly, we found no evidence that past coral bleaching makes the corals any tougher,” he said. 

Although protecting reefs from fishing and improving water quality was likely to help them recover in the long term, the study revealed that it made no difference to the damage sustained during 2016's coral bleaching – the worst one on record.

During that event, only 7% of the Great Barrier Reef avoided bleaching and in Western Australia up to 80% of coral reefs were bleached, the Australia’s National Coral Bleaching Taskforce found.

The report was informed by aerial and underwater surveys across the Great Barrier Reef and a number of Western Australian reefs, including inshore Kimberley reefs, the Ningaloo Reef and Rottnest Island reefs.

Now, with coral bleaching reappearing for a second year running, Hughes and other coral researchers are mobilising to conduct more surveys.

“With rising temperatures due to global warming, it’s only a matter of time before we see more of these events. A fourth event after only one year is a major blow to the reef,” Hughes said. 

“We’re hoping that the next two to three weeks will cool off quickly, and this year’s coral bleaching won’t be anything like last year. The severity of the 2016 coral bleaching was off the chart.”

Coral bleaching occurs when abnormal environmental conditions, like heightened sea temperatures cause corals to expel tiny photosynthetic algae, called zooxanthellae. The loss of these colourful algae causes the corals to turn white. 

Bleached corals can recover if the temperature drops and zooxanthellae are able to recolonise them – otherwise the coral might die.

Here are other key findings from the study:

  • 2015-16 saw record temperatures that triggered a massive episode of coral bleaching across the tropics.
  • Coral bleaching events should no longer be thought of as individual disturbances to reefs, but as recurring events that threaten the viability of coral reefs globally.
  • The Great Barrier Reef has had three major bleaching episodes: one in 1998, one in 2002 and again 2016. This latest one was the most severe and resulted in catastrophic levels of bleaching occurring in the northern third of the Reef (a region approximately 800km in length).
  • The amount of bleaching on individual reefs in 2016 was tightly linked to local heat exposure.
  • The cumulative, superimposed footprint of the three mass bleaching events on the Great Barrier Reef has now encompassed virtually all of the Great Barrier Reef.
  • Past exposure to bleaching in 1998 and 2002 did not lessen the severity of the bleaching in 2016.