Great Barrier Reef lost 29% of its corals in 2016 as ocean temperatures rose

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The impact of rising ocean temperatures on the Australian Great Barrier Reef has been worse than expected in the past year, with 29% of corals dying in 2016, surveys show.

Coral "bleaching", which has been seen across the world in the past two years, has led to widespread coral decline and habitat loss on the Great Barrier Reef, experts said.

Bleaching happens when algae that lives in the coral is expelled due to stress caused by extreme and sustained changes in temperatures, turning the coral white and putting it at risk of dying if conditions do not return to normal.

The Great Barrier Reef is the world's largest collection of coral reefs, with 400 types of coral, 1,500 species of fish and 4,000 mollusc species, and is the habitat of wildlife such as the dugong, or sea cow, and the large green turtle.

Aerial and in-water surveys of the reef revealed an estimated 29% of shallow water corals died from bleaching in 2016, with most of the dead coral occurring in the north of the reef.

The figure is up from the 22% estimated in mid-2016, according to the surveys by the Marine Park Authority, Queensland Parks and Wildlife Service, Australian Institute of Marine Science and the ARC Centre of Excellence for Coral Reef Studies.

The worst-hit area was to the north of Port Douglas, Queensland, where some 70% of shallow water corals died, but there was strong recovery of corals in the south of the reef which did not suffer bleaching and other impacts.

In 2017, further coral loss is expected from the second year of bleaching in a row, and the impacts of tropical cyclone Debbie, the experts said.

Great Barrier Reef Marine Park Authority chairman Russell Reichelt said: "As has been the case with reefs across the world, the Great Barrier Reef has experienced significant and widespread impacts over the last two years.

"We're very concerned about what this means for the Great Barrier Reef itself and what it means for the communities and industries that depend on it.

"The amount of coral that died from bleaching in 2016 is up from our original estimates and, at this stage, although reports are still being finalised, it's expected we'll also see an overall further coral cover decline by the end of 2017."

The greatest coral loss before 2016 was caused by cyclones and coral-eating crown-of-thorns starfish.

Combined with bleaching that is expected to become more frequent and severe as ocean temperatures continue to rise, the impacts are set to cause ongoing and accelerated coral declines on the reef, the experts said.

The back-to-back bleaching is being driven by climate change and the window is closing fast to cut the greenhouse gas emissions pushing up temperatures and harming the reef, they have warned.