Coral reefs, some of the world's most productive and diverse ecosystems, may already be wrecked beyond repair by global warming, according to a new study.

An international team, writing in Proceedings of the National Academy of Sciences, studied how reefs in the Seychelles failed to recover after a huge bleaching event in 1998.

It's the first time researchers have shown that the damage caused may be unrecoverable.

Coral bleaching occurs when increased sea temperatures cause colourful zooxanthellate algae to be expelled from their jellyfish-related hosts. The zooxanthellates form the photosynthetic half of the partnership that coral reefs are based on.

During 1998, the Indian Ocean warmed to unprecedented levels, and as much as 90 per cent of Seychelles' coral reef was bleached. Without the algae, the animal which builds the calcium skeleton dies too, and the reef crumbles away. Sometimes reefs are able to “reseed” themselves, but the marine scientists say the damage done in the Seychelles was too severe and and widespread for local currents to bring the algae back.

The survey also found that four fish species had become extinct locally, and overall species diversity across fish in the worst-hit areas had plummeted by 50 per cent.

Research leader Nick Graham, of the University of Newcastle, said: “Unfortunately it may be too late to save many of these reefs, but that research shows the importance of countries tackling greenhouse gas emissions and trying to reduce global warming and its effect on some of the world's finest and most diverse wildlife.” ®