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Largest ever coral die-off confirmed on Great Barrier Reef

Scientists say 2016 saw the largest recorded die-off of coral in the Great Barrier Reef. But it's not all bad news.

Red Zone Bleaching Mortality (Coral Reef Studies/Arc Centre of Exellence)

Scientists assess coral mortality on Zenith Reef, in the Northern Great Barrier Reef, following the bleaching event

Scientists confirmed the largest die-off of coral ever recorded in Australia's renowned Great Barrier Reef on Monday, saying 67 percent of shallow-water corals in its northern region have been lost over the past eight to nine months. 

"Most of the losses in 2016 have occurred in the northern, most-pristine part of the Great Barrier Reef," said Terry Hughes, director of the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies, which has led aerial surveys of the damage, in a statement. "This region escaped with minor damage in two earlier bleaching events in 1998 and 2002, but this time around it has been badly affected." 

It's the third time in 18 years that the 2300-kilometer reef system has undergone mass bleaching as a result of global warming and strong El Ninos. But the current event is said to be the worst on record, partly triggered by warmer underwater temperatures over the past year.

Corals can only survive within a narrow temperature range. As the oceans warm, they expel the symbiotic algae that grow in their tissues, which causes bleaching and coral death. An unhealthy coral reef is also much more vulnerable to predators such as crown-of-thorns starfish. The marine invertebrates can recover if temperatures drop and the algae recolonize them. But scientists are concerned that a fourth bleaching event could happen soon and interrupt the recovery process, which could take at least 10-15 years. 

Good news 

Still, the scientists from ARC, which is based at James Cook University, were relieved to find a much lower coral death toll in the central and southern regions of the biodiversity-rich reef system. In 2016, about 6 percent of bleached corals died in the central region and 1 percent in the south.

"The good news is the southern two-thirds of the Reef has escaped with minor damage," said the ARC's Andrew Baird, who led teams of divers to re-survey reefs in October and November. "The corals have now regained their vibrant color, and these reefs are in good condition." 

Coral in the northern offshore corner of the Great Barrier Marine Park also fared better than the other parts of the northern reef. "We suspect these reefs are partially protected from heat stress by upwelling of cooler water from the Coral Sea," said Hughes.

That comes as good news after an article went viral in October declaring the death of what is often described as the Earth's largest organism. "The Great Barrier Reef of Australia passed away in 2016 after a long illness. It was 25 million years old," read the obituary in Outside Online. 


Scientists and environmentalists quickly took to Twitter to criticize the article, saying the reef was still very much alive and it was counterproductive to say otherwise as it could cause people to lose hope and not take action to save it. 

Vital for millions of people 

Korallbleiche (XL Catlin Seaview Survey)

A before and after picture of bleaching in American Samoa

Coral reefs are not only host to a wide range of marine plants and animals; some half a billion people around the globe are dependent upon them for food or through tourism. The Great Barrier Reef alone employs around 70,000 people and generates around $5 billion (4.7 billion euros) in income each year, according to the ARC statement.

But it is not the only system under threat. A recent study by a research team from the University of Edinburgh found that climate change could also threaten North Atlantic coral populations.
 

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