Doing away with fossil fuels is the only hope for most of the world's coral, a US science agency says.
The National Oceanic and Atmospheric Administration said the use of fossil fuels had prompted climate change which was creating hostile ocean environments that were killing or 'bleaching' coral.
This is bad because coral is the foundation of marine ecosystems, much like trees are to forests, it said.
The coordinator of the administration's Coral Reef Watch, Mark Eakin, said this year was the first since 2014 that there has not been a global coral bleaching phenomenon.
But with a new El Nino weather pattern developing, the respite is going to be short-lived, Dr Eakin said.
"Well the big action that we need to be taking is moving to a completely renewable energy economy," he said.
"We need to move off of fossil fuels. That is what is causing the climate change.
"This is a major impact to an important eco-system that supports a half a billion to a billion peoples' primary food source along with being worth about $US9 trillion dollars globally."
However, there is cautious optimism among scientists about signs of resilience being shown by some corals to warmer oceans.
Studies of Australia's Great Barrier Reef show corals that survived a major bleaching event in 2016 also lived through a second bleaching in 2017, Dr Eakin said.
"The corals that were left after the 2016 bleaching event were the survivors, they were the tough ones," he said.
"There certainly is no mutation, we are not dealing with any new genes or anything of that sort.
"The corals that were around in 2017 were the ones that survived 2016."