The world authority on threatened wildlife classified 96 percent of all coral reef systems as “critically endangered”.

Almost all of the world’s shallow-water coral reefs face an extremely high risk of collapse in the next 50 years, the International Union for Conservation of Nature (IUCN) has warned in a new global assessment.

In a report released on Thursday, the world authority on threatened wildlife classified 96 percent of the planet’s coral reef systems as “critically endangered”, its highest risk category, with total extinction possible by 2075.

“The future threat of climate change is looming and severe, and that poses a very high risk to the survival of these ecosystems,” said Mishal Gudka, scientific lead of the IUCN Red List of Coral Reef Ecosystems assessment.

Corals are heat-sensitive and particularly vulnerable to human-driven climate change, which is warming the world’s oceans and increasing the frequency and intensity of damaging bleaching events.

When a reef collapses, hard corals disappear and are replaced by algae, rubble and sponges. As a result, the reef no longer supports a diverse ecosystem that can include everything from tiny invertebrates to reef fish like grouper.

The emergence this year of a “super” El Nino, the natural warming phenomenon, also presents a risk to corals, Gudka said at a news conference at the UN’s COP17 biodiversity summit in Yerevan, Armenia.

It’s the latest dose of bad news for coral reefs, which are critical sources for seafood and tourism and often serve as buffers against coastal erosion and storms.

Sometimes dubbed “rainforests of the sea”, they have some of the world’s richest biodiversity and support a quarter of marine species as well as coastal communities and businesses.

Most of the world’s famous reef systems – including the Great Barrier Reef in Australia, the Coral Triangle in the Western Pacific Ocean and Southeast Asia, the Mesoamerican Barrier Reef System and the Red Sea coral reefs – are critically endangered, according to the assessment.

Elevated risk of extinction

For its report, the IUCN used the “middle-of-the-road” scenario from the UN’s panel of climate experts that projects global temperatures rising 2.7C above pre-industrial levels by 2100.

Its analysis examined 127 tropical and subtropical coral systems covering 188,362 square kilometres.

A week ago, another IUCN study in the journal Nature found that nearly 44 percent of reef-building coral species face an elevated risk of extinction due to climate change. A year ago, a survey found that harmful bleaching of the world’s coral had grown to include 84 percent of the ocean’s reefs, the most intense of its kind in recorded history.

Despite its warning, the IUCN said coral reefs in most parts of the world “are not yet lost”.

“Coral reefs have shown a remarkable ability to bounce back from devastating bleaching events,” said Gudka. “Pockets of surviving, resilient coral reefs can still remain in the future, so we must do everything within our reach to set them up for success.”