Last month, a team of Australian researchers used a small airplane to swoop low over the Great Barrier Reef in north Queensland.
As they peered below, their worst fears seemed to be confirmed. Large swathes of the reef - the world's largest living marine structure - were ghastly white and bore the distinct marks of coral bleaching.
Professor Terry Hughes, a marine scientist at James Cook University in Queensland who led the survey, later said in a tweet that it was "without doubt the most confronting research project I've ever done".
After weeks of aerial and underwater surveys, the study findings were released last Monday: Two-thirds of the 2,300km stretch of the Great Barrier Reef had suffered serious bleaching, mainly due to warming water temperatures.
It marked the first time scientists have observed consecutive years of mass bleaching events on the reef.
Saving the reef
Q What is coral bleaching?
A Rising water temperatures are the main cause of bleaching, according to scientists. The heat stress causes the algae that live in the coral's tissue to leave. This deprives the coral of its major food source and can leave it white or pale. If the algae loss is prolonged, the coral eventually dies.
Climate Council, a non-governmental organisation in Australia, said "extreme coral bleaching and the death of reefs will become the new normal unless serious and rapid reductions in greenhouse gas emissions are achieved".
Q What can help revitalise the reef?
A If the temperature cools and the algae return, the coral can recover, but it may take years or decades. Scientists say urgent action is needed to combat climate change in Australia and around the world.
Efforts to address overfishing and pollution can also help.
There have been four severe bleachings in recent times: in 1998, 2002, 2016 and 2017.
Scientists said the duration of the current bleaching means it will be much harder for the coral to recover. It is a development that threatens a lucrative tourism sector that attracts 2.8 million visitors a year and earns an estimated A$7 billion (S$7.4 billion) annually.
"It takes at least a decade for a full recovery of even the fastest-growing corals, so mass bleaching events 12 months apart offers zero prospect of recovery for reefs that were damaged in 2016," said Dr James Kerry, from James Cook University, in a statement last Monday.
According to the survey, the worst of the bleaching was in the middle third of the reef. Only the bottom third was unscathed.
Sea surface temperatures in the Great Barrier Reef last year were the hottest since records began in 1900. Other causes of damage include cyclones, overfishing and pollution.
"Clearly the reef is struggling with multiple impacts," said Prof Hughes. "Ultimately, we need to cut carbon emissions, and the window to do so is rapidly closing."
The bleaching has removed some of the allure from the famed stretch of colourful reefs. Australian non-governmental organisation Climate Council said the bleaching could cost a million tourists a year and up to A$1 billion in lost revenue.
"Over the next two to three decades, bleaching events are likely to have catastrophic impacts on reef health and the economy," it said in a report released last Wednesday.
The federal government has come under domestic and international pressure to protect the reef. The United Nations Educational, Scientific and Cultural Organisation (Unesco) has in recent years considered putting the reef on its "in danger" list, due to the widespread damage and ongoing threats.
The Australian federal authorities have adopted various programmes to protect the reef, including efforts to contain outbreaks of the coral-eating crown-of-thorns starfish and improving the quality of water entering the reef, for example from rivers.
For now, tourists are still flocking to the reef. But the sustained and high-profile damage is worrying local authorities and tour operators.
"Bleaching is a real problem from a marketing perspective," Mr Col McKenzie, from the Association of Marine Park Tourism Operators, told Fairfax Media last month.
Dr Russell Reichelt, chairman of the Great Barrier Reef Marine Park Authority, said some reefs remained largely unaffected and the healthy reefs would help to "seed recovery".
"Reefs and individual coral colonies do have the ability to recover, depending on the severity of the bleaching and whether other pressures are reduced," he said in a statement.
"The international community urgently needs to work together to implement the Paris Agreement (to address climate change).
"Otherwise coral reefs worldwide face a bleak future."