The Great Barrier Reef has been hit with a sixth mass coral bleaching event, the marine park’s authority has confirmed, with aerial surveys showing almost no reefs across a 1,200km stretch escaping the heat.
The Guardian understands a United Nations mission currently under way to check the health and management of the reef will be briefed on the initial findings of the surveys as early as Friday in Townsville.
The confirmation from the Great Barrier Reef Marine Park Authority (GBRMPA) marks an alarming milestone for the ocean icon, with 2022 going down as the first time mass bleaching has happened in a cooler La Niña year which scientists had hoped would be a period of recovery for corals.
Government scientists said the confirmation showed the urgency of cutting greenhouse gas emissions that were driving the repeated mass bleachings.
Dr David Wachenfeld, GBRMPA chief scientist, told the Guardian bleaching wasn’t expected in a La Niña.
“But having said that, the climate is changing and the planet and the reef is about 1.5 degrees centigrade warmer than it was 150 years ago. Because of that, the weather is changing. Unexpected events are now to be expected. Nothing surprises me any more.”
Aerial surveys, mostly using helicopters, were completed by the Australian Institute of Marine Science (Aims) and GBRMPA on Wednesday along the entire 2,300km length of the marine park covering about 750 individual reefs.
The marine park is divided into four management areas, and Wachenfeld said there was widespread bleaching in all four zones. “Therefore, we can confirm this is the fourth mass bleaching event since 2016 and also the first under La Niña condition,” he said.
Dr Neal Cantin, an Aims research scientist, led one of two observing teams and personally observed reefs from helicopters across 1,800km.
Between the Whitsunday Islands and Cooktown he said he “did not fly over a reef and score it as ‘no bleaching’”.
Reefs closer to the shore between the Whitsunday Islands to Cooktown had seen the most extreme bleaching, but the “spatial footprint of severe bleaching is very wide”.
Most reefs along that stretch were recorded as “severe”, which means at least 60% of an individual reef’s corals had bleached. Some of those reefs also had corals that had bleached and then died in the last few weeks.
But even within this, there was variability between reefs and across individual coral colonies. Bleaching in the southern section of the marine park was much milder than elsewhere, and there were some reefs there with no bleaching at all.
Coral bleaching happens when the animal becomes stressed from above-average water temperatures. The coral animal expels the algae that lives inside them and provides the coral with food and colour.
Corals can survive bleaching and Cantin said between now and the end of the year, scientists at Aims and elsewhere would be carrying out in-water checks to see how many corals survived and regained their colour.
Widespread mass bleaching of corals on the reef was first seen in 1998, and happened again in 2002, 2016, 2017, 2020 and now 2022.
Scientists started raising the alarm for this year’s event as early as December, when ocean temperatures over the reef hit a record high for that month.
Studies have shown that heat stress can have several “sublethal” effects on corals, including making them more susceptible to disease, slowing their growth and limiting their ability to spawn.
Cantin said it appeared this year’s bleaching had covered a wider area than back-to-back outbreaks in 2016 and 2017, but the intensity of the heat was broadly lower than those years.
“The fact that we are seeing a mass bleaching event in a La Niña year is concerning and a clear sign of the increasing intensity of climate change and ocean warming,” he said.
“I’m extremely concerned – beyond concerned. We need action immediately. We are on bleaching watch every year. The spacial footprint of severe heat is increasing at an alarming rate. It’s also happening faster than was predicted 20 years ago. The severity and frequency is very alarming.”
North of Cooktown many reefs are dominated by a family of boulder-shaped corals called porites, some of which can grow several metres wide and live for centuries. Some porites had bleached, Cantin said.
He said the bleaching along the whole reef strongly overlapped satellite observations of heat stress and ocean temperature from the US government’s National Oceanic and Atmospheric Administration.
Wachenfeld said climatologists would study the factors that drove this year’s bleaching to establish statistical probabilities of it happening under a changed climate.
“But in my experience, this could not have happened without climate change,” he said. “We need to see [these events’] as the reef ringing a very loud alarm bell about the impacts of climate change.
“The reef remains a vast and resilient ecosystem. Despite the concerns that we have when we see a climate-driven impact like this, we have to maintain determination and hope for the future.
“But that determination and hope has to be based on the strongest and fastest possible action to reduce greenhouse gas emissions globally and the ongoing commitments of governments, the community and industry to protect the system.”
Two scientists – one from Unesco and another from the International Union for Conservation of Nature – will be briefed on the findings of the aerial surveys in the coming days as part of their 10-day monitoring mission.
The mission, which started with briefings in Brisbane on Monday, will compile a report to go before the next world heritage meeting currently scheduled for June.
Last year, scientific advisors at Unesco recommended the reef be placed on a list of world heritage sites “in danger” because of impacts from climate change and slow progress on improving water quality.
But fierce and sustained lobbying from the Morrison government saw the 21-country committee go against that advice. The committee said it wanted to see Australia accelerate efforts on climate change through its reef policy.
Australia requested Unesco carry out the monitoring mission, but the UN agency is keeping details of the mission confidential.
Conservation groups and the government’s own reef envoy, Cairns-based MP Warren Entsch, have said the UN mission should be taken to see the bleaching.