Record-breaking Coral Sea temperatures threaten the Great Barrier Reef

Australia’s Great Barrier Reef faces critical danger from back-to-back bouts of extreme ocean heat.

Ocean heat in the Coral Sea is at its highest in four centuries, scientists report in the Aug. 8 Nature. The researchers drilled into coral skeletons from in and around the region and analyzed the chemical makeup of those samples to reconstruct sea surface temperatures from 1618 to 1995, alongside modern instrumental sea surface measurements spanning 1900 to 2024.

Before 1900, ocean temperatures in the region were relatively stable. But from 1960 to 2024, those temperatures have climbed relentlessly. That upward climb is linked to humans’ greenhouse gas emissions, the team found.

Five of the six hottest years in the record were in the last decade: 2016, 2017, 2020, 2022 and 2024, with temperatures as much as 1 degree Celsius hotter than average. Each year had a mass bleaching event during the warmest months, from January to March (SN: 4/29/24).

A photograph of scientists drilling into a coral in the Great Barrier Reef. — Scientists drilled into corals in the Great Barrier Reef (shown) to collect core samples. The chemical makeup of the corals reveals changing water conditions, including temperature, over the corals’ lifetime.Tane Sinclair-Taylor

Researchers have long sounded the alarm about mass bleaching, in which corals stressed by extreme heat or pollution expel symbiotic algae living in their tissues, leaving them stark white (SN: 8/9/23). Corals can bounce back, given time. But back-to-back bleaching can ultimately kill a reef.

“The Great Barrier Reef is iconic,” climate scientist Benjamin Henley of the University of Melbourne in Australia said at an Aug. 6 news conference. UNESCO designated the reef as a World Heritage Site in 1981.

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The U.N. organization has considered adding the reef to its World Heritage in Danger list, noting mounting threats from rising ocean temperatures as well as pollution (SN: 4/7/20). In 2023, UNESCO held off, in part due to the Australian government’s pledge to improve safeguards.

But that was before these new findings, Henley said. “The sense that we could lose [it] in our lifetimes … I hope that spurs action.”

Global warming threatens up to 90 percent of global coral reefs, said Helen McGregor, a paleoclimatologist at Australia’s University of Wollongong. “The more [emissions] we cut now, the better off not just the Great Barrier Reef will be, but society is going to be. It’s the coral in the coal mine.”