Three Years of Bleaching is at an End - But Scientists Say Much Worse May Follow

NOAA Bleached Acropora

Bleached table coral (acropora) in American Samoa (photo NOAA)

According to a press release from the National Oceanic and Atmospheric Administration (NOAA) dated 19 June, the global mass bleaching event that has plagued the world's coral reefs since 2015, may finally be at an end.

More than 70 per cent of the world's tropical reefs have seen elevated water temperatures – the primary cause of coral bleaching – over the last three years. 

This does not, however, mean that coral reefs are in the clear. A separate report from the UNESCO World Heritage Centre on 23 June confirms that 21 of the 29 coral reefs listed as World Heritage Sites, including 'iconic sites like the Great Barrier Reef (Australia), Papahānaumokuākea (USA), the Lagoons of New Caledonia (France) and Aldabra Atoll (Seychelles)' have been badly hit by the sustained temperatures.

Bleaching occurs when elevated water temperatures cause a symbiotic algae called zooxanthelea – repsonsible for providing coral polyps with both nutrients and vibrant colours – to be ejected from the coral. Once the sea temperature returns to normal, the algae are re-absorbed and the coral can recover within a few months. 

If the surrounding water remains too warm, however, the coral will eventually die. The problem with the last few years of bleaching events is that sea temperatures have taken a long time to cool, and the coral has been hit by repeated mass bleaching events, from which it has been difficult to recover.

'This global coral bleaching event has been the most widespread, longest and perhaps the most damaging on record,' said C Mark Eakin, NOAA’s Coral Reef Watch Coordinator. 'NOAA is working with scientists, resource managers and communities around the world to determine what the true impacts of this event will be on coral reefs.'

UNESCO estimates that coral colonies may take between 15-25 years to recover completely, meaning the repeated mass bleaching events of recent years are a serious threat to the future of the world's reefs. As the frequency and intensity of bleaching events continues to rise in line with warming ocean temperatures, the situation is expected to get worse.

Great Barrier Reef Mass Bleaching

The Great Barrier Reef has been hit particularly hard since 2015 (Photo: Ed Roberts)

As stated in the report: the social, cultural and economic value of coral reefs is estimated at US$1 trillion. Recent projections indicate that climate-related loss of reef ecosystem services will total US$500 billion per year or more by 2100, with the greatest impacts felt by people who rely on reefs for day-to-day subsistence.

'The 29 globally significant coral reefs on UNESCO’s World Heritage List are facing existential threats, and their loss would be devastating ecologically and economically,' said Dr Mechtild Rossler, director of the World Heritage Centre. 'These rainforests of the sea protect coastal communities from flooding and erosion, sustain fishing and tourism businesses, and host a stunning array of marine life.'

There may, however, be a silver lining to the damaging events of recent years. Despite the challenging conditions, NOAA reports that some reef systems did not show any signs of bleaching, and study of these reefs may help to provide scientists with a better idea of how to protect the world's coral as global temperatures continue to rise.

'Coral reefs are not beyond help,' said Jennifer Koss, director of the NOAA Coral Reef Conservation Program. 'Many proactive steps to make coral reef ecosystems more resilient are being taken around the world. We are reducing local threats to coral, and are looking into innovative ways to increase coral populations and species that are more resilient to rising ocean temperatures and acidified waters.'

The full report from the UNESCO World Heritage Centre can be found on their website as a PDF





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