The latest mass bleaching event on the Great Barrier Reef comes at an embarrassing time for the Morrison government.

Scientists on Friday announced that aerial drone surveillance has confirmed the Reef is in the grip of its fourth climate-change-induced mass bleaching event since 2016.

The Great Barrier Reef Marine Park Authority issued an update to confirm the Reef is experiencing mass bleaching at multiple Reefs in all four management areas (the Far Northern, Cairns–Cooktown, Townsville–Whitsunday and Mackay–Capricorn).

The declaration occurred in the same week the United Nations secretary general called the Morrison government a climate change “holdout”, and as a UN monitoring mission is in Australia to evaluate the government efforts to protect the Reef.

It also comes as hundreds of school students protested outside the Prime Minister’s official Sydney residence in Kirribilli.

Flights over the length of the 2300km World Heritage-listed Reef confirmed bleaching in all parts of the marine park. Some of it is severe and coral mortality is evident from the air.

It is expected to be months before the full extent of mortality is known because it is possible for bleached corals to recover, although their immune and reproductive systems inevitably suffer.

Dr Neal Cantin, from the Australian Institute of Marine Science, led the aerial surveillance work, in conjunction with the Great Barrier Reef Marine Park Authority.

“Yes, it’s crystal clear now,” he told AAP when asked if the Reef is experiencing a mass bleaching.

“There are clear signs of bleaching through all four regions of the marine park, and this is the fourth mass bleaching event to occur in the last seven summers. And it’s the sixth to hit the Great Barrier Reef since 1998.”

The stretch of the Reef from the Whitsundays north to about Cooktown is the worst hit, with severe bleaching and coral deaths observed.

Further north and into the Torres Strait, bleaching is variable from minor to severe, while the southern part of the Reef, where heat stress is lower, is less affected.

The bleaching event is the result of the reef’s hottest December on record, followed by further heat stress in January and then a late summer heatwave in recent weeks.

John Brewer Reef near Townsville was badly bleached. Photo: Grumpy Turtle Films

While conditions have cooled in the past week or so, the Bureau of Meteorology is still forecasting above average temperatures.

Dr Cantin said the scale of bleaching in a La Niña year, when ocean temperatures are typically lower, is particularly concerning.

“The frequency of concerning heat stress is happening faster than we ever thought it would.

“The fact it’s impacting such a large area of the marine park, during a La Niña year, is a clear sign of climate-change-driven ocean warming. We expect these trends to continue and only accelerate in the future.”

It’s reasonable to expect that in the seven summers ahead, the Reef could see another four bleaching events, Dr Cantin said.

Given it takes about 10 years for decent recovery of fast-growing corals, and longer for slow-growing species, the Reef will not have enough time to recover between hits.

UN secretary-general António Guterres on Monday called Australia a “holdout” after Scott Morrison refused to strengthen the nation’s 2030 emissions reduction target.

Mr Guterres used an address to a sustainability summit to take an extraordinary public swipe at Australia’s climate change efforts.

“A growing number of G20 developed economies have announced meaningful emissions reductions by 2030 – with a handful of holdouts, such as Australia,” he said.

Climate protests

Climate protests by Blockade Australia disrupted trains and port operations on Friday while, separately, hundreds of school students protested in Kirribilli.

Prime Minister Scott Morrison reportedly left Kirribilli House before the protesters arrived.

The student-led protests took place in cities, suburbs and regional towns across Australia, and aimed to call on the federal government to divert funding away from coal and gas projects and to clean, renewable energy.

Much of the attention of Friday’s action was directed at Mr Morrison, with a large crowd of protesters arriving at Kirribilli just before lunchtime.

Organisers estimate the crowd swelled to about 3000, while NSW Police said there were about a thousand demonstrators there.

“It was great to see so many people turn up to Scott Morrison’s house, to really bring home the message that we need climate action,” student Owen Magee said.

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Evaluation mission

A UN mission is currently in Australia to speak with scientists, conservation groups, and government authorities before reporting to the World Heritage Committee, which may decide to list the Reef as in danger.

The marine park authority’s chief scientist David Wachenfeld said the mission’s two experts have been shown photos of the latest bleaching and will be taken to bleached sites in the north next week.

“Keeping them fully informed of what’s happening with the current situation on the Reef is a critical part of what we are doing,” he said.

“Every time we see a climate change-driven impact to the Reef it is extremely concerning. We need to act as strongly as we possibly can to reduce global greenhouse gas emissions and protect the resilience of the Reef …”

Dr Wachenfeld urged people to travel to the Reef and connect with it so they would feel energised to be part of efforts to protect it.

“My greatest concern is that people read about events like this and, as concerning as they are, lose hope. That worries me because people who lose hope don’t act. And what we need now is action.”

Dire prediction

The Intergovernmental Panel on Climate Change earlier this month made dire forecasts for the Reef, warning it could experiencing bleaching every year by 2044 potentially costing Australia 10,000 jobs and $1 billion annually.

The IPCC’s latest climate report report bluntly said the Great Barrier Reef was in “crisis” and at very high risk of crossing a critical threshold where further warming might cause irreversible damage.

It also noted the Reef’s limited scope for adaption and warns of the potential for a dramatic escalation in the frequency of bleaching events like the one in 2016, which affected more than 90 per cent of the Reef.

-with AAP