THE CORALS ON the northern Great Barrier Reef experienced a catastrophic die-off following the extended marine heatwave of 2016, a new study has found.

The study, published online in the Nature journal today, examined the link between heat exposure and coral survival along the 2,300km length of the Great Barrier Reef following last year’s heatwave.

“When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperatures drop, or they can die,” Professor Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies said.

Averaged across the whole Great Barrier Reef, Hughes said 30% of the corals were lost in the nine-month period between March and November 2016.

The amount of coral death they measured was closely linked to the amount of bleaching and level of heat exposure, with the northern third of the Great Barrier Reef being the most severely affected.

The study found that 29% of the 3,863 reefs comprising the world’s largest reef system lost two-thirds or more of their corals, transforming the ability of these reefs to sustain full ecological functioning.

“The coral die-off has caused radical changes in the mix of coral species on hundreds of individual reefs, where mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining,” co-author Professor Andrew Baird of James Cook University said.

Professor Sean Connolly of James Cook University added:

We’re now at a point where we’ve lost close to half of the corals in shallow-water habitats across the northern two-thirds of the Great Barrier Reef due to the back-to-back bleaching over two consecutive years.

“But this still leaves a billion or so corals alive, and on average, they are tougher than the ones that died. We need to focus urgently on protecting the glass that’s still half full, by helping these survivors to recover.”

Risk assessment

The scientists say these findings reinforce the need for assessing the risk of a wide-scale collapse of reef ecosystems, especially if global action on climate change fails to limit warming to 1.5-2 degrees Celsius above pre-industrial levels.

The study is unique because it tests the emerging framework for the International Union for Conservation of Nature’s (IUCN) Red List of Ecosystems, which seeks to classify vulnerable ecosystems as “safe”, “threatened” or “endangered”.

“The Great Barrier Reef is certainly threatened by climate change, but it is not doomed if we deal very quickly with greenhouse gas emissions. Our study shows that coral reefs are already shifting radically in response to unprecedented heatwaves,” Professor Hughes said.

The researchers have warned that failure to curb climate change, causing global temperatures to rise far above 2 degrees, will radically alter tropical reef ecosystems and undermine the benefits they provide to hundreds of millions of people, mostly in poor, rapidly-developing countries.