'Extremely unusual': Hottest ocean temperature in 400 years threatens the Great Barrier Reef

The Great Barrier Reef is now facing the hottest sea surface temperatures in four centuries, a new study finds. The rapid warming is causing massive coral bleaching which threatens the marine ecosystem and biodiversity, the scientists warned. 

"The world is losing one of its icons," study lead author Benjamin Henley, a paleoclimatologist at the University of Melbourne, Australia, said at a news conference Tuesday (Aug. 6). "We will sadly see the demise of one of Earth's most spectacular natural wonders."  

The Great Barrier Reef, off the coast of Queensland, is home to the world's largest collection of coral reefs, stretching for more than 1,400 miles (2,253 kilometers), and covering an area greater than 134,000 square miles (348,000 square kilometers). 

In a new study published Wednesday (Aug. 7) in the journal Nature, scientists revealed that the sea surface temperature in the first three months of 2024 is the warmest ever recorded in 400 years, reaching 0.34 degrees Fahrenheit (0.19 degrees Celsius) above the previous record high. 

This unprecedented high temperature is the key driver of coral bleaching, the scientists said. 

Related: Great Barrier Reef Again Hit by Severe Coral Bleaching

Coral bleaching occurs when environmental stress, such as heat and pollution, causes a coral to expel the colorful algae — its primary food source — that live inside it. When the algae leave, the coral loses its vibrant colors and turns white. Bleached corals are more susceptible to disease and death. 

The study authors reconstructed sea surface temperature from 1618 to 2024 using temperature records from several locations on the eastern part of the reef. The temperature records came from ship and satellite data as well as coral cores, which are drilled from coral skeletons and contain light and dark bands denoting the years. It's "a bit like tree rings that we can count," study co-author Helen McGregor, a paleoclimatologist at the University of Wollongong in Australia, said at the news conference. 

By measuring the strontium and calcium ratio in these coral cores, the scientists could infer the water temperature at the time of the corals' growth — the higher the temperature, the lower the strontium-to-calcium ratio. Similarly, the scientists also measured levels of one version, or isotope, of oxygen left behind in the coral. The higher the oxygen-18 content, the cooler the water temperature.

The scientists focused on the period between January to March, which marks the Australian summer. They found that in the area they studied, the sea surface temperatures between 2016 and 2024 were 1.38 F (0.77 C) higher than they were between 1970 to 1990, and nearly 3 F (1.7 C) higher than the coolest summer in the four-century dataset.

The new study shows the danger climate change poses to the Great Barrier Reef, McGregor said. In a June 25 decision, UNESCO’s World Heritage Committee declined to change the reef's status from under "serious threat" to the more urgent category, "in danger," in part because conservationists have done a good job protecting the reef. 

But McGregor said the new study clearly shows the problems with the UNESCO decision. 

"The science is pointing very clearly to the Reef being in danger," she said. 

While the findings highlight how urgently emissions need to be reduced, "there is a glimmer of hope," Henley said. If the sea surface temperature can be kept from increasing further, there is a possibility of restoring the ecosystem."We have everything we need to solve this problem. We just aren't doing it. We need to act so much quicker."