‘In the 1980s, the gap was 25 years. Before then, mass bleaching didn’t occur,’ says professor Terry Hughes, director of the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies. ‘The average gap between two consecutive bleaching events since 2010 (up to 2016) is 5.9 years. But some reefs have bleached three times in that period.’
Coral bleaching occurs when stressful conditions result in the expulsion of the algae from the coral. These stressful conditions usually relate to above average sea water temperature caused by global warming.
Before human driven climate change, these events were relatively rare, which allowed the reef time to recover between events, but as Hughes’ has discovered, the interval between damaging events is dropping at an alarming rate.
‘Since mass bleaching began in the 1980s, the Caribbean region has accumulated the most events, mainly because it warmed up sooner,’ he says. ‘In the Indo-Pacific, locations that have experienced more than five severe bleaching events are well scattered.’
The ARC study looked at 100 reefs globally and found that the average interval between bleaching events is now less than half of what it was before. In addition, warming events such as El Niño are becoming more severe than previously recorded.
These changes are likely to make it more and more difficult for reefs to recover between stressful events, and Hughes believes the situation is becoming critical. ‘If global warming and business-as-usual greenhouse gas emissions are allowed to continue, reefs as we know them will be destroyed,’ he warns. ‘The COP21 Paris Agreement provides a way forward that could save reefs and other vulnerable ecosystems, but there’s no time to lose.’
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