The Great Barrier Reef Marine Park Authority has said its recent aerial surveys show widespread bleaching across the reef has been detected. They revealed a mixed picture, with some severe bleaching on reefs closer to shore, but outer "ribbon reefs" in the far north escaping damage.
"Some southern areas of the Reef that had little or no bleaching in 2016 and 2017 have now experienced moderate or severe bleaching", the government agency said about the impact of the intense heat of Australia's summer on the reef.
Warmer sea temperatures - particularly in February - are feared to have caused huge coral loss across the world's largest reef system.
The Great Barrier Reef has suffered a massive coral bleaching event, its third in a period of just five years, according to a scientist who conducts aerial surveys of hundreds of individual reefs, speaking to the Guardian Australia on Wednesday. Marine heatwaves are the main cause, but other threats such as farming pollution, coastal development and illegal fishing were also cited.
That conditions exacerbated a years-long drought in much of eastern Australia, contributing to the summer's devastating wildfires that burnt out an area nearly the size of England.
"We could see that some of those corals were big enough that they must have survived the 2017 bleaching and now they re-bleached", he said.
The researchers expect this year's event to drive a high coral mortality rate on the severely bleached sections of the reef.
Terry Hughes, director of the Australian Research Council's Centre of Excellence for Coral Reef Studies at James Cook University in Australia, decribed the coral damage he viewed as "heartbreaking".
There are current programs to plant new corals in parts of the reef that desperately need rejuvenation, but Dr Heron explains as mass bleaching spreads further, the recovery efforts become more hard.
"Climate change remains the single greatest challenge to the Reef", it said on its website, calling efforts to reduce global emissions "essential" to saving the reef.
"In order to protect the Great Barrier Reef", she continued, "we need real solutions to address the climate crisis which is being driven by the burning of coal, oil and gas".
Last August, the GBRMPA downgraded its outlook for the corals' health from "poor" to "very poor" and said the target set by the government's Reef 2050 plan to improve water quality had not been achieved.