- Currently, coral bleaching events are predicted only based on sea surface temperatures increasing beyond a specific threshold.
- Scientists studied bleaching responses in 226 reef sites in the Indian and Pacific Oceans and found that incorporating more factors – including the location of the reef, peak hot temperatures, duration of cool temperatures – into these predictions can make it more accurate.
- These accurate predictions can help scientists not only respond in time but also track bleaching events as they occur.
Off the coast of Red Skin Island in the Andamans, it was a colourful world that marine biologist Vardhan Patankar dived into in the summer of 2016. Here, corals wore colours ranging from fluorescent greens to earthy browns. Fish life – including damselfish, snappers, sturgeons, angelfish, squirrelfish and butterflyfish – matched the corals in colour and diversity.
But when Patankar re-visited the same site a week later, he was shocked. Coral colours had paled visibly, and most of the reef fish had disappeared. In another week, only ghostly white coral skeletons remained. Coral bleaching – the phenomenon where global warming-caused warmer water temperatures stresses corals by expelling polyps (tiny animals whose secretions create the coral structures) that live in it – was happening right before his eyes.
“It was extremely depressing to watch,” recalls Patankar, scientist at the Wildlife Conservation Society-India.
But it was not unexpected: a global alert – based on changes in sea surface water temperatures obtained from satellites – had warned most marine biologists including Patankar about potential coral bleaching worldwide. But while the Andaman islands faced widespread and severe bleaching, the nearby Nicobar islands did not despite the prediction that it would.
While Patankar collected data on this in the Andaman-Nicobar island complexes that year, 19 other scientists across 11 tropical countries were doing the same. Their results, published recently in the journal Nature Climate Change, reveal that contemporary predictions of coral bleaching based on a simple threshold of sea surface temperatures alone may not be accurate for some regions.
Several factors – including the history of bleaching and location of reefs – need to be incorporated into global models to predict future bleaching events effectively, cautions the study.
Over the years, instances of coral bleaching have been numerous and expansive. The 2014-2017 global-scale coral bleaching event that killed several reefs – including large tracts of Australia’s Great Barrier Reef – was widespread and intensive, with bouts occurring repeatedly and for a long duration.
Such events have been linked to coral stress caused by an increase in sea surface water temperatures that remain above a specific threshold for three months. The threshold is usually one degree celsius above the highest summertime mean sea surface temperature for three months, or above four ‘degree heating weeks’ (DHWs), as this metric is called.
Current predictions of global bleaching events are also based on this ‘threshold’ concept. Many scientists, however, have questioned such simplistic reasoning. Recent studies have shown that numerous other factors could change the degree to which coral bleaching affects reefs as well as their resilience.
For instance, while India’s Lakshadweep islands witnessed bleaching events in 1998, 2010 and 2016, the last one was the most severe. But scientist Rohan Arthur (Nature Conservation Foundation, India) and his team who monitored all these events found that coral death decreased from 87 to 31 percent after 2016. That’s because these bleaching events also drastically altered the coral species present there.
Fast-growing, temperature-sensitive corals that dominated the community in 1998 declined while slow-growing but warmer temperature-tolerant species thrived later, finally decreasing overall coral death. The history of bleaching and the changes this caused in species composition played roles in the complex coral death and recovery patterns in this area.
Capturing nuances of coral bleaching
Investigating 26 such factors that could influence coral bleaching (including reef location in terms of latitude and longitude, water temperatures and coral species composition), a large team including Arthur and Patankar studied 226 reef sites in the Indian and Pacific Oceans within 21 days of the predicted changes in sea temperature.
This coordinated effort revealed that though 127 of these sites experienced fewer than four DHWs – well within the ‘threshold’ bleaching limit – they faced substantial bleaching. In their models, longitude or reef location emerged as the strongest predictor of bleaching; it modified coral responses to heat stress immensely.
While peak hot temperatures, duration of cool temperatures and temperature bimodality (“when temperatures rise, then fall and then rise even higher again”, as explained by Arthur; a factor that shows the variability of temperature and how such histories of temperature can influence a reef’s response to bleaching) explained around 50 per cent of the variability in bleaching patterns across sites, DHWs explained only 9 per cent of it.
“There is much talk about DHWs, this ‘threshold’ till which corals can take pressure,” said Patankar. “But our study reveals that’s not true for many sites.”
An index based on a simple threshold value, while clearly appealing, is much too simplistic to capture the reality of how reefs respond across the world, wrote co-author Arthur in an email to Mongabay-India.
“Knowing the past bleaching history of your site is critical, as is knowing if there are mitigating or exacerbating geographical features that may make a reef more or less prone to bleaching,” he wrote.
Towards more accurate predictions
Arthur added that incorporating these factors into coral bleaching predictions can make them more accurate; this accuracy is important not only because it will allow monitoring teams to respond adequately by establishing baselines in time, but also help to track bleaching as it progresses.
“This is certainly important in areas like India, where detailed baselines are still not available for most reefs,” he wrote.
Commenting on this study in the ‘News and Views’ section of Nature Climate Change, independent scientists Matheiu Pernice and David Hughes agree that apart from providing a solution to identify imminent coral bleaching in regions where the DHW metric “historically underperforms”, the study’s models “also highlight the value in identifying additional empirical descriptors that further capture nuances in local environmental conditions.”
In an email to Mongabay-India, Hughes (scientist at the Climate Change Cluster in Sydney’s University of Technology) added that this is an “important and novel study”, because though the DHW metric has long faced criticism for not being an accurate predictor of coral bleaching, there was no systematic, global-scale analysis to test alternative metrics to predict coral bleaching until now.
“The apparent importance of longitude is a very interesting finding and I think future studies will certainly be looking at this in more detail (whereas most studies have previously focussed on latitude),” he wrote.
While the results may not be as implementable as the use of DHWs in the National Oceanic and Atmospheric Administration’s Coral Reef Watch (which is very easy to use and accessible) right how he expects this to change given the wide target audience, commented Hughes.
And Arthur doesn’t rule out the importance of DHWs altogether: though they may not work well at local scales, they continue to be still useful because they are “extremely simple predictors of reef responses at broad regional levels,” he added.
McClanahan, T. R., Darling, E. S., Maina, J. M., Muthiga, N. A., D’agata, S., Jupiter, S. D., … & Ussi, A. M. (2019). Temperature patterns and mechanisms influencing coral bleaching during the 2016 El Niño. Nature Climate Change, 9(11), 845-851.
Banner image: Bleached anemones on the reef of the Andaman Islands. Photo by Vardhan Patankar.