Underwater view of waves breaking over a healthy coral reef, reducing wave energy at the shoreline that can cause flooding. Image: Curt Storlazzi, USGS
In 2015 and 2016, the Chagos Archipelago of the British Indian Ocean Territory experienced unusually high water temperatures in periods just 12 months apart. In 2015, the heatwave lasted eight weeks, with researchers surveying the sea floors and learning that live, healthy coral cover had fallen by 60 percent as a result.
When the same time of year rolled around in 2016, the region was hit with unusually high sea temperatures again, this time lasting for a full four months. The sea floor wasn’t surveyed again immediately following that second ocean heatwave but data from the Peros Banhos Atoll now show that 68 percent of the remaining corals there were bleached and 29 percent died. So in back-to-back ocean heatwaves, more than two-thirds of hard corals, the foundation of one of the ocean’s most important ecosystems, were wiped out.
This feels and sounds like another doom and gloom climate change discovery on that information alone, but it turns out the researchers studying the region have found a positive twist inside a phenomenon that isn’t exclusive to this little area in the middle of the Indian Ocean: While the second heatwave did kill off more coral and the overall impact between 2015 and 2017 was drastic, a far lower rate of existing corals were killed off in the second heatwave of 2016. The researchers actually believe the resilience of the existing corals could be a key to protecting reefs around the world.
“It is encouraging that reefs may have some degree of natural resilience, though further research is needed to understand the mechanisms by which some corals are able to protect themselves,” says Catherine Head at the Zoological Society of London. “This may be our best hope to save these vital habitats from the catastrophic effects of climate change.”
In 1998, similar heatwaves took out coral in the same region and took a full decade to recover. That’s actually considered a fairly rapid recovery, which again suggests there’s something uniquely resilient about the reefs in the archipelago. The 2015-2016 event didn’t offer the same period of time for recovery and 2019 data suggests another series of high temperatures could continue the assault on corals, but researchers still see the area as a tool for learning how to protect them.
“We know it has taken about 10 years for these reefs to recover in the past but, with global temperatures rising, severe heatwaves are becoming a more regular occurrence, which will hinder the reef’s ability to bounce back,” says Head. “Our data shows the event in 2016 was worse than in 2015, but it did less damage. We think this is because the 2015 heatwave killed off the more vulnerable species, and those that survived were more tolerant of hotter temperatures.”
