Corals on the Great Barrier Reef developed a protective mechanism that helped them survive past bleaching events, according to a new study.
But more than a quarter of reefs could lose this protective mechanism within 40 years if sea level temperatures rise as little as 0.5 degrees Celsius above present levels, Australian and US scientists report in the journal Science.
Co-author Dr Juan Ortiz, from the University of Queensland, said their analysis of data from the past three decades showed corals were exposed to a “practice run” of gradually warming waters ahead of each bleaching event.
“We found historically in the past 27 years about 70 per cent of the thermal stress events have been characterised by a temperature profile that helps the coral to be ready for when the stress happens,” Dr Ortiz said.
“The water becomes warm enough to send signals into the coral’s metabolic pathways so the coral has a better ability to withstand the bleaching event.”
They found this reduced coral mortality during bleaching by more than 50 per cent.
For the study, which also included scientists from the ARC Centre of Excellence for Coral Reef Studies at James Cook University (JCU) and the National Oceanic and Atmospheric Administration USA, the team collected coral specimens from the Great Barrier Reef and did experimental work to determine the effect of sea surface temperature change.
They then used remote sensing data from the past 27 years to look at historical sea surface temperature patterns on the Great Barrier Reef and climate change modelling to look at future trends.
The study showed there had been 372 thermal stress events, capable of causing bleaching, that occurred across 115 reef areas.
Future thermal stress ‘far more lethal’ for corals
Dr Ortiz said given sea temperatures were steadily rising, it was possible the temperature of the “practice run” could eventually exceed the threshold at which bleaching occurred, switching these events from being protective to becoming increasingly lethal.
He said their work showed with just a 0.5C temperature increase — predicted to happen within 40 years — about 20-30 per cent of reefs would lose this protective mechanism.
With a 2-degree temperature rise, more than 80 per cent of reefs would be directly exposed to bleaching events, he said.
He said reefs that maintained this protective mechanism were likely to withstand the effect of climate change if emissions were reduced.
If emissions were not reduced, maintaining this mechanism would delay the effect of climate change by about 20 years, he added.
Dr Ortiz said the models would be an important management tool for the Great Barrier Reef as it showed reefs lost this protective mechanism at different rates.
“We know even if we reduce emissions there will be different levels of effect of climate change on the Great Barrier Reef and this approach can help identify the reefs that will maintain their protection and should be given priority for further protection.”