The world's most important coral region is in danger of being wiped out by the end of this century unless fast action is taken, says a new report.
The international conservation group WWF warns that 40% of reefs in the Coral Triangle have already been lost.
The area is shared between Indonesia and five other South East Asian nations and is thought to contain 75% of the world's coral species.
It is likened to the Amazon rainforest in terms of its biodiversity.
The WWF report paints a bleak picture. If the world's richest coral reef is destroyed, the fish that people rely on for food could be gone.
By the end of the century, 100 million people across South East Asia could be on the march, looking for something to eat. Communities might be breaking down and economies destroyed.
It's billed as a worst-case scenario, but the report's chief author, Professor Ove Hoegh-Guldberg, says it is not as bad as the future we're currently headed towards.
"Up until now we haven't realized how quickly this system is changing," says Professor Hoegh-Guldberg.
"In the last 40 years in the Coral Triangle, we've lost 40% of coral reefs and mangroves - and that's probably an underestimate. We've fundamentally changed the way the planet works in terms of currents and this is only with a 0.7 degree change in terms of temperature.
"What's going to happen when we exceed two or four or six?"
Climate change consequences
Avoiding a worst-case scenario would need significant reductions in greenhouse gas emissions and better controls on fishing and coastal areas, says the report.
The Coral Triangle covers 1% of the earth's surface but contains a third of all the world's coral, and three-quarters of its coral reef species.
If it goes, an entire eco-system goes with it - and that, says Prof Hoegh-Gudberg, has serious consequences for its ability to tackle climate change.
"Pollution, the inappropriate use of coastal areas, these are destroying the productivity of ocean which is plummeting right now. That is the system that traps CO2 - 40% of CO2 goes into the ocean.
"Now if we interrupt that, the problems on planet earth become even greater," says Prof Hoegh-Gudberg.
Indonesia is hosting the World Ocean Conference this week because, it says, oceans have been neglected so far in global discussions on climate change.
It wants the issue to have a bigger profile at UN climate talks later this year.