Great Barrier Reef bleaching made 175 times likelier by human-caused climate change, say scientists

The hot water temperature that drove the devastating bleaching on the Great Barrier Reef this year was made 175 times more likely by human-caused climate change, and could be normal in just 18 years, according to preliminary findings by leading climate and coral reef scientists.

The scientists said they took the unusual step of releasing the work prior to peer-review, because the methods used to reach the findings are now accepted in the climate science community and the alarming results needed to be released as quickly as possible.

“We are confident in the results because these kind of attribution studies are well established but what we found demands urgent action if we are to preserve the reef,” said Andrew King, a lead author of the study from the University of Melbourne.

They found the record warm temperatures in the Coral Sea that drove the bleaching this year were driven by a combination of 1C of warming since 1900 caused by greenhouse gas emissions, and about a 0.5C jump in temperature driven by natural variability.

Using climate model simulations, the team found such an event was incredibly unusual in models that did not include the effects of greenhouse gasses. But when those effects were put back in, temperatures such as those seen this March were 175 times more likely.

Moreover, despite most of the current temperature record being driven by natural changes in temperature, they found that once greenhouse gasses reach levels expected in 2034, temperatures seen this March will be the new average for the Coral Sea.

“In a world without humans, it’s not quite impossible that you’d get March sea surface temperatures as warm as this year but it’s extremely unlikely,” King told Guardian Australia. “Whereas in the current climate it’s unusual but not exceptional. By the mid 2030s it will be average. And beyond that it will be cooler than normal if it was as warm as this year.”

The increase in regularity of bleaching events is worrying news, since reefs can often recover, so long as they are not being repeatedly affected by bleaching or other harms. Other research has shown coral reefs need around 15 years to recover from serious bleaching events, and only then recover if they are protected from fishing and poor water quality.

Ove Hoegh-Guldberg, another author on the paper and director of the Global Change Institute at the University of Queensland, predicted a similar outcome for the reef in a paper from 1999. He said coral dominated reefs will be gone from the Great Barrier Reef by the middle of the century.

“The current bleaching event and this attribution study leads me to believe that my highly controversial predictions in 1999 were actually conservative,” he said. “Being right – in this case – is hardly cause for celebration.”

“Recovery rates are being overwhelmed by more frequent and severe mass coral bleaching.”

Wenju Cai, a world-leading climate scientist at the CSIRO, told Guardian Australia the findings could be very important. “The result is very significant if it turns out to be robust,” he said. “We never thought the Great Barrier Reef is going to die completely by the 2030s. If that’s true it’s a lot faster than we thought.”

Terry Hughes, a coral reef biologist at James Cook University and head of the National Coral Bleaching Taskforce, said the concerning results were not surprising – noting Hoegh-Guldberg had predicted similar results 17 years ago.

Further independent expert views on the research is being sought.