A global assessment warns that nearly all shallow-water coral reef ecosystems face an extremely high risk of collapse by 2075 as climate change increases pressure on the oceans.

The International Union for Conservation of Nature’s findings classify 96 per cent of assessed reef systems as critically endangered when future climate threats are included.

That figure does not mean 96 per cent of reefs have already disappeared. AFP reported that the share classified as critically endangered would be 24 per cent if the assessment excluded future climate projections.

The analysis examined 127 tropical and subtropical reef systems. It used a scenario projecting approximately 2.7 degrees Celsius of warming above pre-industrial levels by 2100.

Heat stress is the most consistent threat, while overfishing, pollution and coastal development also contribute.

Ecosystem collapse occurs when reef-building hard corals largely disappear and other organisms or rubble take their place.

The findings cover internationally recognised systems including the Great Barrier Reef and Coral Triangle. Despite the warning, the IUCN said reefs in most regions are not yet lost.