Looking to the past to better predict coral bleaching

Research published recently in the Biological Conservation journal found analysing a range of historical coral reef data can improve early warning systems for bleaching.

The researchers used a ‘vulnerability framework’ from the Intergovernmental Panel on Climate Change to incorporate factors of location-specific sensitivity and a reef’s capacity to adapt to increasing temperatures, together with the traditional use of exposure to coral heat stress.

Lead author and AIMS@JCU PhD candidate Valerie Cornet said assessing how corals previously responded to periods of increased temperature could help researchers determine a reef’s sensitivity and capacity to adapt.

“Current prediction models are only assessing summertime heat stress that has accumulated in an area,” she said.

“What we’re developing are new metrics, such as winter heat or prior year heat stress, so they can be incorporated into the predictions.

“We’re hoping that by using these historical temperature metrics, we’ll be able to achieve more accurate predictions of coral bleaching.”

Paper co-author and JCU Professor of Physical Sciences Scott Heron said applying the new framework would help researchers understand the variability in how corals respond to heat stress, based on their temperature history.

“Past temperature variability and past exposure to heat stress events are certainly only one part of the coral sensitivity story but we think those factors are going to become a very important part as we dig deeper into understanding how they influence coral response,” he said.

Ms Cornet said she had already applied the model to previous back-to-back bleaching events over 2016 and 2017 in her PhD thesis and found “hot winters” prior to each event correlated with exacerbated bleaching of coral communities.

“Our analyses found that mild heat stress during the winter could potentially lead to lower coral sensitivity,” she said.

“But too much heat experienced during the winter prior to summer appeared to exacerbate bleaching.

“We also tested how heat stress in a prior summer affected bleaching sensitivity and found that high heat in the prior year exacerbated coral sensitivity the following year.

“In both cases of prior winter and prior summer, a moderate amount of heat appears to help acclimatise the corals and decrease the bleaching sensitivity; however, too much heat increased the sensitivity and led to an even greater bleaching response.”

Ms Cornet said the winters and summers of the past two years were unusually hot for the Great Barrier Reef.

“It is getting to the point where the sensitivity could be increased, resulting in more intense bleaching,” she said.

Prof Heron said these insights would serve as a clear warning to the public, industry and stakeholders of the dangers posed by rising carbon emissions.

“Understanding coral sensitivity can help us understand the factors that are most important for managers to take action in preparation for, and during, heat stress events in the future,” he said.

“But the single most important thing we need to do to protect coral reefs is to minimise their exposure to heat stress, which means significant and immediate reductions in the causes of climate change.

“Without that, these incremental pieces of understanding are not going to help coral reefs decades into the future. We must act now to reduce greenhouse gas emissions.”

Ms Cornet said there was no reason why the model could not be applied to any coral reef in the world.

“While we focused on the Great Barrier Reef, it’s just the beginning,” she said.

“It would definitely have to be tested location by location, but we believe it will open doors for people to test these metrics or use their own metrics within this framework.”