The world’s largest coral reef system under heat stress despite the La Nina weather phenomenon associated with cooler temperatures.

Australia’s Great Barrier Reef, the world’s most extensive coral reef system, is in the midst of a massive bleaching event.

“Coral bleaching has been observed on multiple reefs across all four management areas, confirming a massive bleaching event, the fourth since 2016 and despite La Nina conditions,” said the Great Barrier Reef Marine Park Authority (GBRMPA). in a statement posted on its website. Friday after carrying out a series of aerial surveys of the 2,300 km (1,400 mile) long reef.

Coral bleaching occurs when the sea temperature rises, destroying the algae the coral feeds on and causing it to bleach. La Nina is a weather phenomenon believed to bring cooler temperatures.

Mass bleaching was first observed on the reef in 1998 – at the time – the hottest year on record, but such events have occurred more often in recent years. The last mass bleaching dates back to 2020.

A study published in October found that half of the reef’s corals had died in the past 25 years.

The International Union for Conservation of Nature has previously warned of the effect of climate change on the World Heritage-listed reef, which it described as being in “critical condition” in a December 2020 report.

The Reefs Authority said aerial surveys showed the current bleaching was most severe in the northern and central regions of the reef where some coral had died. The situation was “moderate to severe” in the far north while those in the south were less badly affected, with some of the high coral cover reefs showing no signs of bleaching.

It also conducts underwater surveys to assess the extent of bleaching across the reef, as aerial surveys can only reliably observe corals at a depth of around five meters (16.4 ft).

The GBRMPA said that although sea surface temperatures have cooled over the past two weeks, the Bureau of Meteorology expected sea surface temperatures to remain near 1-1.5 degrees. Celsius (1.8 to 2.7 degrees Fahrenheit) above average for the rest of the month.

The U.S. National Oceanic and Atmospheric Administration (NOAA) has predicted heat stress will dissipate across most of the marine park by the end of April.