Warm water has already begun bleaching coral on the Great Barrier Reef, weeks ahead of the period with highest forecast risk. Satellite data suggest widespread bleaching is possible by March.

Selina Ward, a coral reef biologist from the University of Queensland, has photographed the bleaching, which she said appeared to be very localised so far, but was concerning because of how early in the season it was.

“It was quite a large stretch and there were some very recently dead corals,” Ward said. “Hopefully it isn’t a sign of more to come.”

“It is the earliness and the early death that worries me,” Ward said, noting that it wasn’t yet an indication that there would be severe or widespread bleaching.

The US National Oceanographic and Atmospheric Administration (Noaa) recently updated its bleaching outlook, finding that the entire Great Barrier Reef would face a status of “possible” or “bleaching likely” by February.

Bleaching outlook map for Pacific Ocean corals including the Great Barrier Reef. Photograph: Noaa

The world’s coral reefs are reeling from the worst recorded global coral bleaching event, in which the Great Barrier Reef was hit in both 2016 and 2017. Between the two events, half the reef’s coral is thought to have been killed.

Heron Island escaped significant bleaching throughout the two bleaching events.

In a Facebook post, Noaa’s Coral Reef Watch program posted: “Currently not expecting anything as bad as the last two years but these corals don’t need repeated heat stress.”



The latest results firm up uncertain forecasts from November that suggested the same thing.

Photos taken by Ward around Heron Island show a large stretch of bleached coral, with some already dead and covered in algae.

Coral bleaches when the water around it is too hot for too long. When that happens, it expels the colorful symbiotic algae that lives inside it, leaving transparent flesh exposing the white skeleton. Since the algae provides most of the energy the coral needs to survive, unless water temperatures quickly return to normal, the coral dies.

According to a 2013 paper published in Nature, a 2C rise in global surface air temperatures will result in the loss of more than 95% of coral around the world.



If the world limits warming to 1.5C, it might save 10%, the paper finds. To save 50% of coral reefs, global warming needs to be halted at 1.2C. The world may already have warmed by about 1C.