The Great Barrier Reef faces its biggest threat in centuries

The Great Barrier Reef and surrounding waters have reached warmer temperatures in the last decade than they have in at least 400 years due to climate change, according to new research. published in the magazine Nature.

It’s a stern warning. The survival of the Great Barrier Reef is now essentially in human hands. Corals around the world face almost extinction if temperatures continue to rise with the world running primarily on fossil fuels.

“The reef is in danger, and if we do not deviate from our current course, our generation will likely witness the disappearance of one of Earth’s great natural wonders, the Great Barrier Reef,” said Benjamin Henley, lead author and researcher. . professor at the University of Melbourne and honorary fellow at the University of Wollongong, said at a press conference on Tuesday.

Corals are animals that secrete calcium carbonate skeletons that form reefs. Coral skeletons that can grow for centuries allowed Henley and his co-authors to look into the past. As they grow, they form bands that scientists can study, like tree rings. They contain chemical signatures that show heat stress in certain years. The strontium to calcium ratio and the oxygen isotope ratio, in particular, are related to the water temperature at the time when the corals were still growing.

There is no consistent data on sea surface temperatures from direct measurements before 1900. But researchers were able to combine the data available from direct measurements with data from skeletal samples to build a statistical model that they then used. to reconstruct temperatures. back to 1600. They focused on temperatures between January and March, when the waters around Australia’s Great Barrier Reef tend to be warmer.

This is how they discovered that these periods of 2024, 2020 and 2017 reached the highest temperatures in four centuries. The year 2024 was the most notable, about 1.73 degrees Celsius above the reconstructed average between 1618 and 1899.

Under heat stress, corals will expel algae that give them nutrients and color. It is a phenomenon called bleaching, and corals can die over time. If temperatures remain about 1 degree Celsius above “normal” for summer for more than two months, they could trigger mass coral bleaching events, according to a study. related article about research published in Nature.

Last year marked the world fourth global coral bleaching event already recorded – wreaking havoc on reefs around the world and even triggering a rush to temporarily relocate coral nurseries on dry land to try and save them. Global bleaching events also occurred in 1998, 2010 and between 2014 and 2017.

The coral skeletal cores evaluated by the researchers barely showed any stress bands before 1980, a sign that mass bleaching events were much less likely before then. Using climate models to analyze potential outcomes with and without human influence, the paper’s authors conclude that “human influence on the climate system is responsible for rapid warming in recent decades.”

Policies currently in place to reduce greenhouse gas emissions from fossil fuels are not yet sufficient to prevent the situation from worsening further. Global average temperatures are still on the way to rising between 2 to 3 degrees Celsius higher than they were before the industrial revolution. Previous to look for estimated that even a two-degree rise could be enough to destroy 99% of the world’s coral reefs.

The Great Barrier Reef in particular faces the prospect of bleaching every year for the foreseeable future, co-author and University of Queensland marine studies professor Ove Hoegh-Guldberg said at the press conference. “It is very, very worrying to know that we are very close to [that] point, and I think that will happen in the next 10 years,” said Hoegh-Guldberg. Although corals can survive bleaching, reefs typically need a decade or two to recover from severe mass bleaching, according to the related article.

With all this in mind, every fraction of a degree of global warming we can avoid by turning to clean energy more quickly can make a difference to the world’s coral reefs. Some corals may be better at adapting to a warmer world than others, ensuring the reef’s survival even if it is less biologically diverse than it was in the past.

“We need to believe this if we want to continue,” Hoegh-Guldberg said. “It is possible… if we take the right level of action on greenhouse gases and look for and protect the corals that have a good chance of surviving this type of scenario, then we will be prepared for a coral reef future. ”