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Extreme Heatwaves Killed Half of the Great Barrier Reef’s Coral in 2 Years, Study Says

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Global warming’s rapid desiccation of the Great Barrier Reef has taken an even more substantial — and permanent— toll than previously understood, a new study has found. Nearly half of the coral in the northern, most pristine part of the reef system has died after two successive heat waves.

Writing in the journal Nature on Wednesday, researchers warned of possible wide-scale collapse of reef ecosystems if global temperatures continue to rise.

In the world’s largest coral system off the Australian coast, the scientists found that a marine heat wave in 2016 prompted catastrophic levels of bleaching and, when the reefs didn’t recover, die-offs.

“We lost 30 percent of the corals in the nine-month period between March and November 2016,” Professor Terry Hughes, director of the coral-reef center at James Cook University in Australia and lead author of the new research, said in a statement.

Bleached coral occurs when extreme temperatures, increased UV rays, disease, chemicals, silinity and exposure to air and rain at extreme low tides occur - Great Barrier Reef, Queensland
Extreme temperatures, increased UV rays, disease, chemicals, silinity and exposure to air and rain at extreme low tides can cause bleached coral.Len Zell—Lonely Planet/Getty Images

The northern third of the 1,400-mile reef suffered the brunt of the damage, according to the researchers.

Then another heat wave in 2017 exacerbated the effect.

“We’ve seen half of the corals on the Great Barrier Reef killed by climate change in just two years,” Mark Eakin, a study author and coordinator for the U.S. National Oceanic and Atmospheric Administration’s Coral Reef Watch, told CNN.

Bleaching episodes, when stressed coral begins to starve, were previously observed in the reef in 1998 and 2002, but the damage sustained was comparably minor.

The new study called the current extent of coral bleaching and mass deaths unprecedented. The casualties endured have already forever altered the Great Barrier Reef, compromising the diversity of fish that can rely on the ecosystem, the researchers said. They called for urgent protection of the remaining reefs, which, among other benefits, protect shorelines from erosion, provide a habitat for millions of fish species and fuel tourism and fishing incomes.

“If we fail to curb climate change, and global temperatures rise far above 2 °C [above the pre-industrial level], we will lose the benefits they provide to hundreds of millions of people,” said Hughes.

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Write to Laignee Barron at Laignee.Barron@time.com