The frequency of severe coral bleaching events has increased fivefold in four decades because of climate change, a pace already exceeding the time needed for some species to recover, a new global study has found.

Of 100 reefs examined worldwide, just six have escaped severe bleaching since 1980.

Coral reefs are bleaching at shorter intervals around the world. Credit:AAP

The bleaching, which was initially restricted to years with El Nino events in the Pacific, can now occur in any year. It may become an annual event "in coming decades", according to the study, published in the journal Science on Friday.

"Before 1982-83 [a year with a strong El Nino], there was no bleaching on a regional or global scale," said Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, and the paper's lead author.