"Beyond comprehension: In the summer of 2015, more than 2 billion corals lived in the Great Barrier Reef. Half of them are now dead."
Between March and November of 2016, a "record-breaking" marine heatwave caused rampant coral bleaching around the globe, and the Great Barrier Reef, located off the coast of northeastern Australia, lost nearly a third of its corals.
"When corals bleach from a heatwave, they can either survive and regain their color slowly as the temperature drops, or they can die," explained Terry P. Hughes, the report's lead author and director of the ARC Center of Excellence for Coral Reef Studies.
While the team of researchers focused on the 2016 heatwave, Hughes shared with The Atlantic early results from a follow-up wave last year:
Combined, he said, the back-to-back bleaching events killed one in every two corals in the Great Barrier Reef. It is a fact almost beyond comprehension: In the summer of 2015, more than 2 billion corals lived in the Great Barrier Reef. Half of them are now dead.
What caused the devastation? Hughes was clear: human-caused global warming. The accumulation of heat-trapping pollution in the atmosphere has raised the world's average temperature, making the oceans hotter and less hospitable to fragile tropical corals.
"People often ask me, 'Will we have a Great Barrier Reef in 50 years, or 100 years?'" Hughes said. "And my answer is, yes, I certainly hope so—but it's completely contingent on the near-future trajectory of greenhouse gas emissions."
Following the 2016 heatwave, Hughes's team found that across the Great Barrier Reef, "fast-growing staghorn and tabular corals suffered a catastrophic die-off, transforming the three-dimensionality and ecological functioning of 29 percent of the 3,863 reefs comprising the world's largest coral reef system."
The severity of that heatwave's impact surprised even experts, as it was far more powerful than past bleaching events, which have caused five-to-ten percent of corals to die off, Hughes told the Guardian.
Coral bleaching is highly selective, affecting branching and tabular corals the most. The greater the heat exposure, the larger the transformation in the mix of coral species. https://t.co/YeW6ymNDok pic.twitter.com/ZHPs6STCiG
— Terry Hughes (@ProfTerryHughes) April 19, 2018
Bleaching occurs when the coral expels algae that lives within it and provides food. For past mass bleaching events, corals either recovered when the water cooled down, or died slowly of "starvation." However, with the 2016 heatwave, Hughes said, "That's not what we found."
"About half of the mortality we measured occurred very quickly," he explained. Rather than starving, "temperature-sensitive species of corals began to die almost immediately in locations that were exposed to heat stress," which radically altered the mix of coral species that now live in the sprawling 1,400-mile system.
Global warming, the report concludes, "is rapidly emerging as a universal threat to ecological integrity and function, highlighting the urgent need for a better understanding of the impact of heat exposure on the resilience of ecosystems and the people who depend on them."