Mass bleaching has killed more than a third of the coral in the northern and central parts of Australia’s Great Barrier Reef, though corals to the south have escaped with little damage, scientists said.

Researchers who conducted months of aerial and underwater surveys of the 2,300-kilometre reef off Australia’s east coast found that around 35 per cent of the coral in the northern and central sections of the reef are dead or dying, said Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland state.

And some parts of the reef had lost more than half of the coral to bleaching. The extent of the damage has serious implications, Hughes said. Though bleached corals that haven’t died can recover if the water temperature drops, older corals take longer to bounce back and likely won’t have a chance to recover before the next bleaching event occurs, he said. Coral that has died is gone for good, which affects other creatures that rely on it for food and shelter. “Is it surprising Not anymore. Is it significant Absolutely,” said Mark Eakin, the coral reef watch coordinator for the US National Oceanic and Atmospheric Administration.

“We’re talking about losing 35 per cent of the population of coral in some of these reefs that’s huge.” The damage is part of a massive bleaching event that has been impacting reefs around the world for the past two years. Experts say the bleaching has been triggered by global warming and El Nino, a warming of parts of the Pacific Ocean that changes weather worldwide. Hot water puts stress on coral, causing it to turn white and become vulnerable to disease. Other reefs have suffered even more severely from the recent bleaching; Some Pacific islands, for example, have reported over 80 per cent coral death rates, Eakin said.

This is the third and most extreme mass bleaching event in 18 years to strike the Great Barrier Reef, and in each case, the areas that suffered the worst bleaching were the areas where the water was hottest for the longest period of time, Hughes said.

This time, the southern half of the reef was spared largely due to a lucky break that arrived in the form of a tropical cyclone. The remnants of the storm which had lashed the South Pacific brought cloud cover and heavy rains to the region, cooling the ocean enough to stop bleaching that had just begun in the south. About 95 per cent of the coral in the southern portion of the reef has survived.