Warmest ocean temperatures in 400 years threaten Great Barrier Reef
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Bleached coral, Great Barrier Reef, 2024. Photo: Ove Hoegh-Guldberg
Extreme ocean temperatures in the Coral Sea surrounding Australia's Great Barrier Reef hit their highest levels in at least four centuries this year, scientists reported on Wednesday.
Why it matters: Extreme ocean heat can trigger mass coral bleaching events, making coral more vulnerable to heat and other stress.
- Corals can recover but if there are back-to-back events that don't allow them enough time to do so, they can die, with potentially devastating consequences for other marine life and people who depend on it for their livelihood.
- The team of researchers said their models show greenhouse gas emissions and changes in land-use as well as ozone changes are responsible for the warming trend.
- "The heat extremes are occurring too often for those corals to effectively adapt and evolve," study co-author Benjamin Henley, a paleoclimatologist at the University of Melbourne, said in a press briefing about the new research published Wednesday in the journal Nature.
What they did: As corals grow over centuries, chemical signatures of their environment are recorded in their skeleton, leaving time-stamped markers of the sea surface temperature (SST) and other conditions.
- In the current study, researchers analyzed the ratio of strontium to calcium and the ratio of oxygen isotopes in coral skeletons to reconstruct SSTs between 1618 and 1995.
- They then combined that data with direct measurements spanning 1900 and 2024.
What they found: Before 1900, sea surface temperatures in the Coral Sea and the Great Barrier Reef were relatively stable but there has been a strong warming trend since then, they report.
- Mass coral bleaching occurred on the reef in the southern hemisphere summers of 2016, 2017, 2020, 2022 and 2024 — years when the sea surface temperatures were up to 1 degree Celsius above the estimated average. January to March of "2024 is the most extreme in that full 400-year record," said Henley, who conducted the study when he was a researcher at Wollongong University.
- "The recent events are extreme in nature, in terms of the last four centuries," he said, describing them as "unprecedented events."
What they're saying: "If we don't divert from our current course, our generation will likely witness the demise of one of Earth's great natural wonders, the Great Barrier Reef," Henley said.
- But he said there are glimmers of hope for the reef if global warming can be restricted. Still, limiting "global warming to the Paris Agreement's ambitious 1.5 °C level would be likely to lead to the loss of 70–90% of corals that are on reefs today," the team writes.
- Future coral reefs are likely to have less diverse coral species, they add.
What to watch: Scientists have suggested breeding heat-resistant coral and rescuing corals and then returning them to reefs after deadly marine heat waves subside. But there isn't agreement about whether these are viable conservation solutions.
