A new international study found escalating impacts of ocean warming on coral reefs after assembling the largest global dataset on the severe 2014-17 coral bleaching event.
James Cook University (JCU) researchers were among the lead authors of the study, which combined data from more than 15,000 coral surveys with global satellite temperature data.
This approach allowed the scientists to better assess the true impact of the 2014-2017 Third Global Coral Bleaching Event and to predict the scale of damage expected during the current Fourth Event.
The study found that heat stress, bleaching and mortality during this three-year event were more severe than previously recorded.
"More than half of the world's coral reefs suffered at least moderate bleaching (10-100% of colonies bleached), while around one-in-six reefs lost over 10% of their corals," said lead researcher Dr Mark Eakin, Director at Corals and Climate.
"The most vulnerable corals on the bleached reefs were lost in the first two years."
Coral bleaching results from the breakdown of the partnership between coral animals and their symbiotic algae due to stress, often due to unusually high summertime temperatures. Under high levels of heat stress, corals can die - and the health of surviving corals remains impacted.
The central Pacific saw the most severe bleaching and mortality, with corals exposed to heat stress for over a year. In Fiji and the Great Barrier Reef, severe heat caused some corals to rapidly die without bleaching first.
Because heat stress during this event was unprecedented, the team at the National Oceanic Atmospheric Administration's Coral Reef Watch program created new satellite bleaching alert levels to warn reef managers and communities about increasing damage to corals from higher heat stress.
"These new alerts make it easier to track and understand the impacts of longer, more intense marine heatwaves, both during the Third Global Bleaching Event and the current Fourth Event," said JCU Professor and co-lead author Scott Heron.
According to the researchers, while corals can recover, the speed of their bounce back may be compromised by more frequent bleaching events.
"After the back-to-back bleaching events on the Great Barrier Reef in 2016 and 2017, coral cover began to rebound, only to be hit again by mass bleaching in 2020, 2024 and 2025," said Prof Heron.
"This creates a downward ratchet effect, where reefs only partially recover before the next bleaching event impacts them even further."
The researchers also warn that this event is an early sign of what's to come.
"Over one billion people depend upon healthy coral reefs for their lives and livelihoods," said research co-lead Dr Sean Connolly, a Senior Scientist at the Smithsonian Tropical Research Institute and Adjunct Professor at James Cook University.
"The world's coral reefs are also among the ecosystems most vulnerable to warming.
"We know corals are evolving in response to warming ocean temperatures and increasingly severe marine heatwaves, but the fact that bleaching is getting more severe across successive events indicates that evolution is not keeping up with the pace of environmental change."
Prof Heron adds, "If countries choose to not reduce greenhouse gas emissions in line with previous Paris climate agreement commitments, we can expect to lose many of the ecosystem services coral reefs provide."
