Almost 75 per cent of world heritage coral reefs have been exposed to heat stress that causes coral bleaching in the past three years, UNESCO says.
The draft report on climate change released overnight is outlined in the agenda paper ahead of a UNESCO (United Nations Educational, Scientific and Cultural Organisation) meeting in Poland next month.
It states three-quarters of coral reefs over the past three years of the 29 world heritage coral reefs were exposed to conditions that cause bleaching.
"Coral mortality during the latest global bleaching event most likely has been the worst ever observed, including at world heritage property reefs," the paper said.
It states the Great Barrier Reef bleaching event as the most widely reported and it would need to be assessed by the World Heritage Committee.
Other reefs that suffered severe bleaching were in Seychelles, New Caledonia and the United States.
UNESCO expressed its "utmost concern regarding the reported serious impact from coral bleaching that have affected world heritage properties in 2016-17 and that the majority of world heritage coral reefs are expected to be seriously impacts by climate change".
The UNESCO draft paper reiterated the importance of governments to implement and ratify the Paris Agreement as soon as possible.
It urged governments where possible to strengthen all efforts to build resilience of reefs to climate change including the reduction "to the greatest extent possible" of all other pressures and threats.
Scientists will undertake further work and advise UNESCO on what can be done to limit the risks and severity of bleaching in the future.
Not just the Great Barrier Reef at risk
Queensland Environment Minister Steven Miles said the paper showed the extent of the problem across the world.
"Sadly it is not just the Great Barrier Reef that is suffering this catastrophic level of coral bleaching — it is being seen right around the world and it is good that the global community is seeing it as a global issue," Mr Miles said.
He said Queensland was focused on reducing other impacts as stated in the paper, including an additional $63 million per year for reef water quality in the recent state budget.
"There is no doubt we are entering a period where these bleaching events will become more common," he said.
"Early reports from our meteorologists suggest that next summer could be even worse — that's very concerning."
He said his message to the global community was Queensland's reef was still incredible, but it was under threat.
"I think we are going to need to start having discussions about recovery projects — what is it we can do to give certain reefs a better chance of surviving," he said.
He said scientists were turning their minds to that with coral seeding and genomic projects.