Snow-white coral of once-vibrant Great Barrier Reef a sign urgent action must be taken | Adam Morton

The news is overwhelming and exhausting in a way it has rarely been in most of our lifetimes, but if you have five minutes of energy left this is worth your attention. That it hasn’t been reported in most of Australia’s major news outlets doesn’t make that any less the case.

Across nine days last month, Prof Terry Hughes from James Cook University travelled the length of the Great Barrier Reef in a small plane to survey the health of more than 1,000 individual sites. He was joined by an observer from the Great Barrier Reef Marine Park Authority, a government agency.

Hughes has been here before. It was his work in 2016 and 2017 that told us there had been back-to-back mass bleaching events that ultimately killed an estimated 49% of shallow water corals.

His technique sounds uncomplicated, but it relies on sharp observational skills honed over decades. Reefs are filmed, assessed and scored. If 60% of coral on a reef has been drained of its colour it is considered severely bleached. Where possible, the assessments are checked and confirmed in the water.

Hughes this week released maps that show 25% of reefs falling into this category, with another 35% having recorded modest bleaching. The marine park authority has confirmed it is the third mass bleaching event to hit the global landmark in just five years. It is the most widespread and second most severe episode of its type on record.

Crucially, areas that previously escaped bleaching at the southern end, particularly those near or below Mackay, have been badly hit. The affected reefs include those around the Keppel group of islands, where Pauline Hanson was filmed diving in 2016 as she denied bleaching was a significant problem. Hughes says the coral there is now “snow white”.

The impact of bleaching is now cumulative. Across the three events, in 2016 , 2017 and 2020, reefs along nearly the full length of the 2,300km natural wonder have been severely affected. Some of the staghorn corals in the south this year are expected to die in the months ahead, and turn a scuzzy brown as they are swamped by algae. Others will survive but be less resilient and aren’t likely to make it next time bleaching hits.

Unprecedented is a word that gets thrown around so often now that it has lost meaning but it is a clear understatement when describing what has happened here.

Look at it this way: when the Australian government last reported to the Unesco world heritage centre on the health of the reef back in 2015 – a moment that led to an extended diplomatic dance by the Coalition government to avoid it being labelled world heritage “in danger” – there had been only two mass bleaching events in observed history, and none since 2002.

Since then, the reef has bleached more years than not. The two most recent events have been in both years when there was no El Nino, the climate cycle in the Pacific that inflates water temperatures.

It suggests the underlying global heating is now at a level that an extended spell of warm weather is enough to leave the reef stricken. Next summer is considered a reasonable chance to bring another El Nino on top of that.

Hughes took to Twitter on Tuesday to express his heartbreak at again seeing the natural monument he has dedicated his life to studying irrevocably hurt, and raise doubts whether he would make the trip again.

But the most striking commentary on the reef came from David Wachenfeld, the marine park authority’s chief scientist. It is rare for public officials to speak their minds in ways that may be uncomfortable for their elected overlords but, in an interview with my colleague Graham Readfearn, Wachenfeld set out the reef’s plight in clear, blunt language.

He described the bleaching as a clear signal the reef was calling for urgent help, warned the resilience of the reef was not limitless and said it would require the “strongest action possible” on climate change to save it given the planet is headed for 3C heating – a level at which science tells us coral reefs can’t be protected.

That science was summarised by the UN’s Intergovernmental Panel on Climate Change, which reported a majority of tropical coral reefs would disappear if heating was limited to 1.5C, and would be “at very high risk” at just 1.2C. The globe has already warmed about 1C since the industrial revolution.

The government’s language did not match Wachenfeld’s urgency. The environment minister, Sussan Ley, acknowledged mass bleaching was “deeply concerning” and the importance of coordinated global action to cut emissions but said the focus had to be on programs to reduce the pressure on the reef and strengthen its resilience.

It echoed Scott Morrison’s eventual response to the summer bushfires: conceding the role played by rising emissions but quickly emphasising adaptation over Australia doing more to tackle the problem at its source.

There is another potential path, of course. Written down, it couldn’t sound simpler. It is true that Australia cannot address the climate crisis on its own but it is the world’s 14th biggest emitter and, as Morrison pointed out after he joined a G20 video conference to discuss coronavirus, can be influential in global debates.

A growing number of analysts and policy experts, including ClimateWorks Australia and Ross Garnaut, have suggested in detail that the country could set a jobs-friendly and economically sound course to net-zero emissions. The stimulus program that will be needed to recover from the pandemic offers a one-shot opportunity to speed up a clean transition.

This combination of action at home combined with a new diplomatic effort would give Australia the best chance at saving the reef. At a minimum, Australia’s new role as a climate hawk would point to a future in which the country embraced what will be an inevitable shift to a low-emissions world, rather than be dragged to it. And the prime minister, in his new collaborative mode, could have influenced others on the way.

Unlikely, perhaps. Some would say impossible. But these are unlikely, impossible times.

• Adam Morton is Guardian Australia’s environment editor