The Great Barrier Reef is facing one of the most widespread coral bleaching events on record, as water temperatures in the Coral Sea spike.

Key points: A senior US National Oceanographic and Atmospheric Administration scientist says the "vast majority of reefs along the GBR will have some bleaching by now"

A senior US National Oceanographic and Atmospheric Administration scientist says the "vast majority of reefs along the GBR will have some bleaching by now" The southern portion of the reef, which largely escaped bleaching events of 2016 and 2017, is unlikely to be spared this time

The southern portion of the reef, which largely escaped bleaching events of 2016 and 2017, is unlikely to be spared this time Reef expert Ove Hoegh-Guldberg stressed bleaching did not necessarily mean the coral had died, and in fact it may recover

"We've been at stressful sea surface temperature levels for some time," said Dr William Skirving, a senior scientist with the US National Oceanographic and Atmospheric Administration's Coral Reef Watch project.

"The vast majority of reefs along the GBR will have some bleaching by now," wrote Dr Skirving in a briefing note sent to top scientists on Wednesday.

He stressed that while the bleaching may have left a larger footprint than the record events of 2016 and 2017, the severity of bleaching was milder. However, forecasts indicated the bleaching would become more intense.

In this forecast, red shows areas that have a high likelihood of coral bleaching on the Great Barrier Reef in mid-March. ( Supplied: NOAA )

"Over the next few weeks, most of the reef will move to a level that we would expect significant bleaching," said Dr Skirving.

"And by that I mean the sensitive corals would have all pretty well turned white. Most of the hardy corals would still be looking OK."

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Reef may recover

The southern portion of the Great Barrier Reef, which largely escaped the major coral bleaching events of 2016 and 2017, is unlikely to be spared this time.

Professor Ove Hoegh-Guldberg from the ARC Centre for Excellence in Coral Reef Studies said his colleagues on Heron Island, in the south of the marine park, had already witnessed extensive bleaching.

"Our team working on the reef crest have noticed that now there's very significant bleaching. Hectares of coral reef have turned white," he said.

"There's a whole bunch of corals that grow on the reef slope and they're also starting to turn white, as well as take on some very vibrant colours, which is usually a sign of stress."

Marine scientist Jesse Bergman witnessed coral bleaching at Heron Island recently. ( Supplied: Jesse Bergman )

He said Heron Island only suffered at most 2 per cent bleaching in 2016 and 2017.

"We're now seeing up to 100 per cent of corals being bleached," he said.

But Professor Hoegh-Guldberg stressed bleaching did not necessarily mean the coral had died, and in fact it may recover.

"Significant mortality has yet to begin. And of course that will depend on how much this region continues to warm up," he said.

Coral reefs around Heron Island on the Great Barrier Reef are experiencing bleaching. ( Supplied: Aaron Chai )

Bleaching rare in non-El Nino years

Speaking from Heron Island, marine biologist Aaron Chai said what concerned him most about the latest bleaching event was that it hadn't occurred during an El Nino year, which normally brought warmer water.

"This means that the background temperature of the water is becoming hot enough to breach temperature thresholds without being associated with natural climate cycles," he said.

"This means that we are well and truly experiencing the effects of anthropogenic climate change."

Tourism operators report 'ghosting'

The CEO of Tourism Tropical North Queensland, Mark Olsen, said his operators weren't reporting big impacts, yet.

The waters around Heron Island, in the southern are of the reef, had previously escaped major coral bleaching. ( Supplied: Heron Island Resort )

"We are seeing some heat stress on some of the visited reefs, what's called ghosting, and there are some small patches of bleaching on our more remote groups but we are not seeing widespread bleaching on the reefs that our visitors go to every day," he said.

"We will be keeping a really close eye on that. We'd encourage anyone who is interested in the health and wellbeing of the Great Barrier Reef to do exactly the same.

"But if people wanted to do more than that, the single best thing that anyone can do is actually visit the reef and know that by visiting you're actually directly contributing to the conservation of the Great Barrier Reef."

Marine Scientist Ove Hoegh-Guldberg agreed.

"I think this is a time when we should be rallying around the ecosystem, the people, the industries that are so dependent on the health of the Great Barrier Reef," he said.

"Traveling with low-carbon footprint to see the reef, even though it may cost that little bit more, to go and see it is really important. It's such a beautiful place."