Globally Coral Bleaching Is Worse Than We Thought

  Last updated June 15, 2017 at 2:04 pm


A new, more-accurate database of coral reef mass bleaching events around the world reveals bad news. 

Australian and Canadian scientists have just published a new way of tracking the geographical and historical extent of coral bleaching episodes as well as assessing their severity and they have determined that the probability of reefs bleaching increased eight-fold after the 1997/1998 El Nino.

Reports of mass coral bleaching in recent decades have raised concerns about the future of coral reefs on a warming planet. But, despite the efforts to improve coral reef monitoring around the world, our understanding of the geographic spread of mass coral bleaching is fragmentary.

Prolonged ocean temperatures of only 1–2 ̊C above the range that coral usually experience can lead to coral bleaching where the colourful dinoflagellate Symbiodinium that live inside the coral are expelled. Episodes of mass coral bleaching have been recorded from around the world since the early 1980s and these events have led to widespread coral death and raised questions about the viability of coral reef ecosystems during a period of rapid climate change.

Previously a voluntary system called ReefBase had been used to track and record coral bleaching events but it was plagued by problems of geographical biases in data collection and variations in the scale of bleaching reports.

The new research builds on the ReefBase data by including new bleaching reports and speaking directly with scientists and divers conducting monitoring in under-reported locations. The researchers also developed a gridded, global-scale historical coral bleaching database.

This new work has increased the number of observed bleaching reports by 79 per cent, from 4,146 to 7,429.

The researchers were able to use this data to develop annual high-resolution maps of the probability of bleaching occurrence from 1985 through 2010. They were then able to analyse the new datasets to test for changes in the frequency of mass bleaching and the heat stress thresholds at which bleaching tends to occur.

Annual number of bleaching observations for 1985–2010. Blue bars represent original ReefBase reports; orange bars represent new reports. Shading reflects bleaching level, from mild (1–10%, lightest colour) to severe ( > 50%, darkest colour) bleaching.

And initial results from the improved database paint a grim picture of the health of the world’s coral reefs. The researchers conclude that the area of coral reefs likely to have been exposed to bleaching was eight times higher in the second half of the assessed time period from 1985 to 2010.

Annual area of moderate and severe bleaching observations for 1985–2010. ReefBase reports (navy blue), new records (orange), and those in both datasets (grey).

They also looked at coral regions around the world. Although, the largest relative increase in observed bleaching occurred in the Caribbean (333% increase), Micronesia (205%), and Western Indian Ocean (172%), the area of observed bleaching in Australia, though only doubled, had increased by over 10,000 km2.

Total area (km2 ) of moderate and severe bleaching by region over the 1985–2010 period. ReefBase reports (blue) and new reports (orange).

The results also show that the annual maximum ‘Degree Heating Weeks’ (a measure of thermal stress) for coral reefs had a high probability of bleaching increased over time.

The researchers note that further work will need to be done to expand the coverage of the database including adding new bleaching observations since 2010 and further back-filling the database with older missing reports. The end results should be a much better understanding of the spread of bleaching events.

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About the Author

Paul Willis
Paul is a respected leader in the science community with an impressive career in science. He has a background in vertebrate palaeontology, studying the fossils of crocodiles and other reptiles. He also has a long history as a science communicator, with a career spanning as Director of The Royal Institution of Australia, presenter and host for Australia’s Science Channel, working for the ABC on TV programs such as Catalyst and Quantum as well as radio and online. He’s written books and articles on dinosaurs, fossils and rocks and is finding new ways to engage the people of Australia with the science that underpins their world. Follow him on Twitter @fossilcrox.