Great Barrier Reef waters were hottest in 400 years over the past decade, study finds

WASHINGTON (AP) — Ocean temperatures in the Great Barrier Reef hit their highest level in 400 years over the past decade, according to researchers who warned that the reef likely won’t survive if planetary warming isn’t stopped.

During that time, between 2016 and 2024, the Great Barrier Reef, the world’s largest coral reef ecosystem and one of the most biodiverse, suffered mass coral bleaching events. That’s when water temperatures get too hot and coral expel the algae that provide them with color and food, and sometimes die. Earlier this year, aerial surveys of over 300 reefs in the system off Australia’s northeast coast found bleaching in shallow water areas spanning two-thirds of the reef, according to Great Barrier Reef Marine Park Authority.

Researchers from Melbourne University and other universities in Australia, in a paper published Wednesday in the journal Nature, were able to compare recent ocean temperatures to historical ones by using coral skeleton samples from the Coral Sea to reconstruct sea surface temperature data from 1618 to 1995. They coupled that with sea surface temperature data from 1900 to 2024.

They observed largely stable temperatures before 1900, and steady warming from January to March from 1960 to 2024. And during five years of coral bleaching in the past decade — during 2016, 2017, 2020, 2022 and 2024 — temperatures in January and March were significantly higher than anything dating back to 1618, researchers found. They used climate models to attribute the warming rate after 1900 to human-caused climate change. The only other year nearly as warm as the mass bleaching years of the past decade was 2004.

“The reef is in danger and if we don’t divert from our current course, our generation will likely witness the demise of one of those great natural wonders,” said Benjamin Henley, the study’s lead author and a lecturer of sustainable urban management at the University of Melbourne. “If you put all of the evidence together ... heat extremes are occurring too often for those corals to effectively adapt and evolve.”

Across the world, reefs are key to seafood production and tourism. Scientists have long said additional loss of coral is likely to be a casualty of future warming as the world approaches the 1.5 degrees Celsius (2.7 degrees Fahrenheit) threshold that countries agreed to try and keep warming under in the 2015 Paris climate agreement.

Even if global warming is kept under the Paris Agreement’s goal, which scientists say Earth is almost guaranteed to cross, 70% to 90% of corals across the globe could be threatened, the study’s authors said. As a result, future coral reefs would likely have less diversity in coral species — which has already been happening as the oceans have grown hotter.

Coral reefs have been evolving over the past quarter century in response to bleaching events like the ones the study’s authors highlighted, said Michael McPhaden, a senior climate scientist at the National Oceanic and Atmospheric Administration who was not involved with the study. But even the most robust coral may soon not be able to withstand the elevated temperatures expected under a warming climate with “the relentless rise in greenhouse gas concentrations in the atmosphere,” he said.

The Great Barrier Reef serves as an economic resource for the region and protects against severe tropical storms.

As more heat-tolerant coral replaces the less heat-tolerant species in the colorful underwater rainbow jungle, McPhaden said there’s “real concern” about the expected extreme loss in the number of species and reduction in area that the world’s largest reef covers.

“It’s the canary in the coal mine in terms of climate change,” McPhaden said.

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This story has corrected attribution for the aerial surveys in the 2nd paragraph to the Great Barrier Reef Marine Park Authority, rather than NASA.

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