Man-made climate change is also causing the world’s oceans to warm up. In and around the Great Barrier Reef in Australia, water temperatures have reached new record levels in the last ten years. The water on the famous coral reef was warmer than ever before in the last 400 years, as researchers have determined using coral core samples. The sad peak is therefore the current year, 2024. Ocean warming increases the risk of repeated mass bleaching and thus permanent coral death. This has consequences for biodiversity in the sea, warn the researchers.
The Great Barrier Reef is the largest coral reef and one of the most diverse marine ecosystems in the world. However, due to global and ocean warming, the famous reef in northeast Australia has experienced a series of extensive coral bleaching events in recent years, resulting in the death of a large proportion of the coral and the degradation of the entire reef habitat. During such bleaching, the heat stress causes the corals to shed the symbiotic microalgae that live in their tissues and give them their bright colors. Without the algae, the white skeleton of the coral is exposed and no longer supplied with nutrients and energy; the corals starve. These coral bleaching events are becoming more and more frequent.
But how much has the sea temperature in the Coral Sea, where the Great Barrier Reef is located, actually changed and is it really the reason for the mass bleaching? Until now, only measurement data from the recent past have been available to answer this question.
A look into the past of the coral reef
A team led by Benjamin Henley from the University of Melbourne has now reconstructed the temperatures on the sea surface over the last 400 years – from 1618 to 1995. To do this, the researchers took samples from the skeletons of corals at various locations on the Great Barrier Reef and its surroundings. Geochemical analyses of the samples can be used to determine summer growth and thus annual sea temperatures. Henley and his colleagues compared the data obtained with the actual measured temperatures on the sea surface from 1900 to 2024.
It was found that temperatures before 1900 were relatively stable and hardly fluctuated. After that, however, the ocean warmed slowly at first and then increasingly faster, as the models showed. In addition to natural climate fluctuations such as El Niño, man-made climate change was the main contributor to this: “Our climate model analysis confirms that human influence on the climate system is responsible for the rapid warming in recent decades,” says Henley. From 1960 to 2024, for example, the researchers observed an average warming of the Coral Sea during the coral growth phase from January to March of 0.12 degrees Celsius per decade.
In the years with particularly severe coral bleaching – 2004, 2016, 2017, 2020, 2022 and 2024 – the sea temperature was significantly higher and even reached the highest values in the last 400 years, as the team reports. 2024 was by far the warmest year since records began: It was another 0.19 degrees above the previous record year of 2017 and a total of 1.73 degrees above the pre-industrial period before 1900.
Marine biodiversity will continue to decline
Climate change is having catastrophic consequences for the Great Barrier Reef and other coral reefs, warn Henley and his colleagues. Even if global warming is kept below the Paris Agreement’s target of 1.5 degrees above pre-industrial levels, 70 to 90 percent of corals around the world could be lost in the long term, they stress. Global warming is currently just under 1.5 degrees, and measurements have temporarily exceeded this value.
“If there is not rapid, coordinated and ambitious global action to combat climate change, we are likely to see the demise of one of the Earth’s most spectacular natural wonders,” says Henley, referring to the Great Barrier Reef. The few surviving coral reefs will also almost certainly have lower biodiversity and a lower variety of coral species in the future. Nevertheless, further climate protection is worthwhile: “Every fraction of a degree of warming that we avoid will lead to a better future for humanity and the nature of our planet,” says Henley.
Source: Benjamin Henley (University of Melbourne) et al.; Nature, doi: 10.1038/s41586-024-07672-x