Researchers from James Cook University (JCU) have found that coral bleaching on the Great Barrier Reef is influenced not only by extreme summer heat, but also by winter conditions months earlier.
The study, published in Coral Reefs, tests how winters influence coral sensitivity to bleaching – either priming them to cope with heat stress or leaving them more vulnerable in the subsequent summer.
"Most bleaching prediction tools focus on summer heat and assume that corals start each summer in the same condition," said JCU student Valerie Cornet, who led the study.
"We wanted to see how cooler seasons, particularly winter, influence bleaching in the following summer."
The team analysed in-water and aerial bleaching observations alongside sea surface temperatures across the central GBR, from Townsville to Port Douglas, focusing on the 2016/17 bleaching seasons.
"2016 and 2017 were the first consecutive years of severe bleaching on the GBR," said JCU Professor Scott Heron, who supervised the study.
"The scale and severity of these mass bleaching events were unprecedented, affecting vast areas of the reef.
"This gave us a rare opportunity to study how repeated heat stress and recovery periods interacted, including effects before summer conditions set in."
Their results show that sustained high winter temperatures worsen bleaching the following summer.
In contrast, moderate levels of winter heat may actually reduce bleaching impacts.
"It's a Goldilocks range –– it's not too hot, not too cold, it's just right," said Prof Heron.
"We think that corals living in these conditions might be better prepared for a stressful summer because they are exposed to a little bit of prior heat."
Ms Cornet explained that moderate winter heat could stimulate corals to produce heat-shock proteins, boosting cellular repair and metabolic activity, and can shift their symbiotic algae to more heat-tolerant types.
"Sustained winter heat can deplete energy reserves, damage tissues, and weaken this symbiosis, making corals more vulnerable," she said.
These results provide a new layer of information that can improve early-warning systems and predictive models.
"Incorporating winter conditions into forecasting tools can help managers better anticipate when and where severe bleaching is likely to occur," Ms Cornet said.
"If we know that certain reefs have experienced unfavourable pre-summer conditions, they may be at higher risk during the next heatwave and could be prioritised for monitoring, protection, or local stress-reduction measures."
Prof Heron said the findings are applicable beyond the GBR and underscore the need for continued conservation.
"The influence of non-stressful, moderate winters on the impacts of summertime stress is vital to all Australian and international reefs," he said.
"But we must remember that while these findings can improve our predictive skills, that does not reduce the urgency of reducing greenhouse gas emissions that cause ocean warming.
"Over a billion people rely on coral reefs for their daily livelihoods. This research isn't just about science — it's about people, both here in Queensland and around the world."
