Southern oceans are facing their fourth mass coral bleaching event.

The Southern Hemisphere's coral reefs, including vast areas of the Great Barrier Reef, are at imminent risk. 

Predictions from the US National Oceanic and Atmospheric Administration (NOAA) highlight the consequences of a changing climate and the El Niño climate pattern on marine ecosystems.

“It's looking like the entirety of the Southern Hemisphere is probably going to bleach this year. We are literally sitting on the cusp of the worst bleaching event in the history of the planet,” says Derek Manzello, an ecologist and coordinator of NOAA's Coral Reef Watch.

Coral bleaching, a stress response to ocean conditions, leads to corals expelling the symbiotic algae that give them colour and sustenance, leaving them vulnerable to disease and starvation. The phenomenon poses significant threats not only to marine biodiversity but also to the economies that rely on vibrant coral ecosystems for tourism and fisheries. 

The repercussions of previous mass bleaching events have already been devastating, with the 2014-2017 episode resulting in the loss of nearly a third of the corals in the Great Barrier Reef.

The current conditions have already triggered widespread bleaching across the Caribbean and the Southern Hemisphere, including reports of extensive damage to the Great Barrier Reef and American Samoa. 

With global temperatures soaring past critical thresholds, scientists fear a dramatic increase in coral mortality worldwide.

Efforts to monitor and assess the extent of the bleaching are underway, with aerial and in-water surveys revealing extensive damage across various regions. 

Prof Bill Leggat from the University of Newcastle has noted the rapid onset of bleaching at Lord Howe Island, describing it as "the start of a major and quite severe bleaching event."

The implications of this impending crisis extend beyond the immediate environmental impact. 

As the situation develops, the global scientific community and conservationists are calling for immediate action to mitigate the effects of climate change and protect these critical marine habitats. 

The upcoming weeks are likely to be crucial in determining the full scope of this potential mass bleaching event.