Unesco has launched an emergency plan to save the world’s reefs amid warnings they will all disappear by the end of the century.
Audrey Azoulay, Unesco's director-general, is calling for an international effort to help safeguard the vital reefs.
The threat level has been raised after significant amounts of coral bleaching has occurred as a result of ocean warming, acidification and extreme weather.
The process of coral bleaching occurs when water is too warm and the algae the coral expels from its tissues causes it to turn completely white.
"Reefs are bleaching far more rapidly than the initial science suggested," Unesco, the UN cultural agency, said.
"These bleached corals are highly vulnerable to starvation and disease and have an increasingly high mortality rate. This year, for the first time, mass coral bleaching also occurred in a traditionally cooler period.
"Under the current emissions scenario, all World Heritage-listed reefs are at risk of disappearing by the end of this century."
Coral reefs play a critical role in absorbing carbon emissions and protect coastlines from storms and erosion.
"I call for an international mobilisation to prevent coral reef extinction and announce a strong contribution from Unesco: an emergency plan to boost the resilience of World Heritage-listed reefs, in particular those in developing countries," Ms Azoulay said.
"Unesco is partnering with the Global Fund for Coral Reefs to help finance this commitment."
The partnership between Unesco and the UN-led private/public Global Fund for Coral Reefs will step up investment for climate resilience strategies across World Heritage-listed reefs in developing countries.
The actions will focus on reducing local drivers of degradation, strengthening the sustainable management of marine protected areas, and supporting local communities.
"Global warming means that local reef conservation practices are no longer enough to protect the world's most important reef ecosystems," Fanny Douvere, head of Unesco's Marine Programme, said.
"But a healthy, resilient reef can regenerate after a bleaching incident and survive."
The new large-scale plan will build on the success of the Resilient Reefs Initiative launched in 2018, which has been working on four World Heritage reef pilot sites in Australia, Belize, France and Palau.
The project has shown that local pressures can be reduced through active intervention and that empowering local communities helps them to adapt their income and livelihoods to the changing realities.
In Palau, the Southern Lagoon of the Rock Islands, the initiative involved rangers and communities trained in the latest science and skills in fisheries management, adaptation and resilience.
It led to the design of a fishing permit system to control access; the implementation of fish size limits to increase spawning biomass; and the protection of habitats, ensuring the life history of species to help to create the conditions in which fish stocks can rebound.
The new initiatives are set to include using water quality monitoring to protect coral reefs in Tanzania, and deploying nanotechnology to make reefs more resilient in the Caribbean.
Unesco World Heritage coral reefs cover over half a million square kilometres worldwide – the equivalent of the size of France – exhibiting exceptional biodiversity.
