Higher than normal water temperatures have caused the largest die-off of corals ever recorded on the Great Barrier Reef, scientists said on Tuesday.
Two-thirds of the corals living in shallow waters in a 700km swath of reef in the northern region have been killed by a massive coral bleaching event over the past eight to nine months, according to the Australian Research Council of Excellence for Coral Reef Studies.
“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 council, which is based at James Cook University.
“This region escaped with minor damage in two earlier bleaching events in 1998 and 2002, but this time around it has been badly affected.”
The council’s study found two-thirds of the reef in more southerly areas had escaped with minor damage with on average 6 per cent of bleached corals dying in the central region and 1 per cent in the south.
The coral die-off follows one of the world’s worst ever coral bleaching events, which began last year and has devastated reefs stretching from the Caribbean in the northern hemisphere to the waters of the South Pacific. Scientists say a combination of global warming and this year’s El Niño weather event is responsible for the rise in sea temperatures that cause bleaching.
The Great Barrier Reef is made by trillions of tiny invertebrate creatures known as coral polyps, which have built it over the past 600,000 years. The polyps, which excrete calcium carbonate to make reefs, are extraordinarily sensitive to changes in water temperature. When it rises by two to three degrees Celsius above normal levels many species of coral are forced to expel the multicoloured algae that live within its tissues, an effect known as “bleaching”.
The white coral skeletons that remain can regenerate if temperatures fall and water quality conditions are good. But in many instances entire reef systems can be destroyed if water temperatures remain elevated for several months.
Scientists said they expect that the northern region will take at least 10-15 years to regain the lost corals. But they are concerned that the continuing increase in global temperatures could cause a fourth bleaching event before then and interrupt the slow recovery.
Tourism on the Great Barrier Reef employs 70,000 people and generates A$5bn ($3.7bn) in income every year. Operators welcomed news that the coral die-off had not extended as far south as first thought.
“The patchiness of the bleaching means that we can still provide our customers with a world class coral reef experience by taking them to reefs that are still in top condition,” said Craig Stephen, who manages Mike Ball Dive Expeditions — one of the reef’s largest tourist operations.
Scientists warn climate change, water pollution and outbreaks of coral eating predators such as the Crown of Thorns starfish are causing severe challenges to the health of the reef. A 2012 study found that half the coral had died over the previous 27 years.
Environmentalists are pressing Canberra to stop the development of new coal mines in Australia, which they say would contribute to increasing global warming and threats to the health of the reef.
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