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It’s a catastrophe for coral reefs. Sea surface temperatures are so high across much of the tropics that many reefs will suffer severe bleaching for an unprecedented fourth year in a row. Divers in Australia are already reporting new bleaching in the northern part of the Great Barrier Reef, where last year half of corals in the worst-hit areas died.

Corals bleach – and can die – when stresses such as abnormal heat make them expel their symbiotic algae. The ongoing global bleaching is the longest and most widespread ever known.

It began in 2014, when global warming and a developing El Niño heated seas. During El Niños, changes in trade winds spread warm surface waters across the Pacific Ocean. The bleaching became the worst on record when the strong El Niño of 2015 and 2016 hit.

The 2015/16 El Niño was followed by a La Niña, which cools sea surfaces by bringing up deeper waters. But surface waters remain so warm that NOAA’s is now predicting that many reefs will bleach in the next three months despite the recent La Niña conditions.

While , not everyone agrees. “Bleaching occurs in summer, and is not ‘continuous’,” says Terry Hughes of James Cook University in Australia, whose helicopter survey last year documented the extensive bleaching in the Great Barrier Reef.

But whether it’s one event or a series of events, it is still bad news. It takes around a decade for an undisturbed reef to recover from bleaching, says Gareth Williams of Bangor University in the UK. So if bleaching occurs more often, reefs don’t have time to recover.

A suggests that most reefs will suffer annual bleaching by the 2040s. “The projections are terrifying,” he says.

Read more: Face-to-face with Great Barrier Reef’s worst coral bleaching