Great Barrier Reef: the worst bleaching event ever hits the northern region | Photo: Arc

Marine scientists have concluded that the worst bleaching event ever recorded on Australia's Great Barrier Reef killed 67 percent of its shallow-water corals in the past nine months.

The extreme underwater heatwave of 2015/2016 affected a 435-mile (700 kilometers) swath of corals in the northern third of the Reef, especially around Lizard Island. The researchers believe that if climate change continues, the affected area will increase.

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

The team of scientists and divers who analyzed the Great Barrier Reef underlines that the region will need between 10 and 15 years to regain the lost corals, despite fears of a fourth bleaching event.

Great Barrier Reef: coral bleaching mortality varies enormously from north to south | Map: Arc

"The good news is the southern two-thirds of the Reef has escaped with minor damage. On average, 6 percent of bleached corals died in the central region in 2016, and only one percent in the south. The corals have now regained their vibrant color, and these reefs are in good condition," adds Andrew Baird, also from the ARC Centre.

Coral bleaching is the result of warmer-than-normal water temperatures. Algae are the coral's primary food source and give them their color. If the ocean temperature increases, the algae leave the coral, the coral loses its nutrients and turns white or very pale.

Overexposure to sunlight, runoff, and water pollution, and extreme low tides can also contribute to coral bleaching. With 133,000 square miles (344,400 square kilometers), the Great Barrier Reef is the world's largest coral reef system.