In the aftermath of a “catastrophic” marine heatwave, the Great Barrier Reef lost as much as half of its coral in some areas. Now, scientists say it’s unlikely to recover.
The report, which is published in Nature, mapped the impact of the 2016 heatwave along the 2,300-kilometer (1,429-foot) reef, chronicling the “mass mortality” that occurred where heat exposure was most extreme. Of the 3,863 reefs that make up the world’s largest coral reef system, 29 percent lost two-thirds or more of their corals. In the northern region, as much as 50 percent of coral is lost.
Mass bleaching has transformed the reef’s ability to sustain full ecological functioning, and prospects for a full recovery to pre-bleaching levels are “poor” as many coral colonies continue to die. Replacement of fast-growing species can take at least a decade while regrowth for longer-lived, slow-growing coral will “almost certainly” take decades longer.
"The coral die-off has caused radical changes in the mix of coral species on hundreds of individual reefs, where mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining," said study co-author Professor Andrew Baird in a statement.
It confirms what scientists already knew about the 2016 Great Barrier Reef bleaching, which was the worst in known history. The findings serve as an autopsy report into just how bad it really was. In the years spanning 2014-2017, in some areas of the reef temperatures rose as much as 6°C (10.8°F), sometimes lasting as long as eight months. In 2017, the reef again suffered severe stress and bleaching from increased water temperatures.
“We’re now at a point where we’ve lost close to half of the corals in shallow-water habitats across the northern two-thirds of the Great Barrier Reef due to back-to-back bleaching over two consecutive years,” said study co-author Professor Sean Connolly.
It serves as a warning that failure to curb climate change could have irreversible effects on the fragile coastal ecosystem, say the authors. If global temperatures continue to climb above 2°C (3.6°F) tropical reefs could be “radically altered” and the benefits they provide to hundreds of millions of people could be lost.
"But, that still leaves a billion or so corals alive, and on average, they are tougher than the ones that died. We need to focus urgently on protecting the glass that's still half full, by helping these survivors to recover," added Professor Terry Hughes, Director of The ARC Centre of Excellence for Coral Reef Studies.
The study is part of an IUCN program that aims to classify vulnerable ecosystems as “safe”, “threatened”, or “endangered”, much like the IUCN currently does with animal species. The scientists say their findings reinforce a need to study the risk of wide-scale collapse of reef ecosystems in the face of global climate change.