Reports of minor to moderate coral bleaching across the reef could be a sign of another mass bleaching event amid record high March temperatures in Australia. 

The largest and longest reef system in the world, The Great Barrier Reef, is potentially undergoing a sixth mass bleaching event, according to one of the world’s leading coral scientists. 

The reef, which covers about 350,000 square kilometres – larger than the UK and Ireland combined – has previously suffered five mass bleaching events in 1998, 2002, 2016, 2017 and 2020. The events in 2016 and 2017 were so severe that it cost the death of 50% of Australia’s iconic reef.

Great Barrier Reef Marine Park Authority chief scientist David Wachenfeld has received reports of minor to moderate coral bleaching at locations scattered across the reef. Due to the vastness of the reef, the monitoring agencies can’t confirm the extent of the bleaching until they have completed systematic aerial surveys, due by the end of March.

Professor Terry Hughes, a leading expert on coral bleaching at James Cook University, however told the Guardian, that he believes a sixth mass bleaching event is already unfolding, and that it was not mild or local.

Coral bleaching occurs due to rising ocean temperatures, which drives algae away from coral reefs, causing reefs to lose their vibrant colours. While heat stress over the reef tends to peak in early to mid-March each year, ocean temperatures have started to climb as early as December with record-high temperatures reported this month. 

“We all breathed a sigh of relief because corals that were pale in December regained their colour in January and February,’ said Hughes. “But in the last three weeks there have been reports of moderate to strong bleaching all along the reef.” 

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Far North Queensland experienced its hottest start of March on record at 37.2C and saw record high overnight temperature occurring several days in a row. Since the Industrial Revolution, Australia has warmed by 1.4C, faster than the global average, which puts the Great Barrier reef at high risk of bleaching every summer. 

Hughes previously conducted a study that more than 98% of the Great Barrier Reef has been bleached at least once in the past 30 years. Though much of the northern parts of the reef were “halfway to recovery”, significant portions of  “vulnerable corals” are either still susceptible or experiencing bleaching.

The development comes less than a week before the start of a United Nations monitoring mission to the reef ahead of a crucial meeting of the world heritage committee in June. In 2021, UNESCO issued a draft recommendation that the Great Barrier Reef should be listed as an “in danger” World Heritage site, aiming to spur parties into action to save the endangered sites. 

In late January, Australia Prime Minister Scott Morrison had pledged AUD$1 billion, the “largest single investment” into the Great Barrier Reef to combat climate change and other environmental threats.