Scientists have chronicled the “mass mortality” of corals on the Great Barrier Reef, in a new report that says 30% of the reef’s corals died in a catastrophic nine-month marine heatwave.
The study, published in Nature and led by Prof Terry Hughes, the director of the ARC Centre of Excellence for Coral Reef Studies, examined the link between the level of heat exposure, subsequent coral bleaching and ultimately coral death.
The extent and severity of the coral die-off recorded in the Great Barrier Reef surprised even the researchers. Hughes told Guardian Australia the 2016 marine heatwave had been far more harmful than historical bleaching events, where an estimated 5% to 10% of corals died.
“When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperature drops, or they can die,” Hughes said. “Averaged across the whole Great Barrier Reef, we lost 30% of the corals in the nine-month period between March and November 2016.”
The scientists set out to map the impact of the 2016 marine heatwave on coral along the 2,300km length of the Great Barrier Reef. They established a close link between the coral die-off and areas where heat exposure was most extreme. The northern third of the reef was the most severely affected.
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The study found that 29% of the 3,863 reefs that make up the Great Barrier Reef lost two-thirds or more of their corals.
Hughes said researchers were also surprised at how quickly some corals died in the extreme marine temperatures.
“The conventional thinking is that after bleaching corals died slowly of ... starvation. That’s not what we found. We were surprised that about half of the mortality we measured occurred very quickly.”
The study found that “initially, at the peak of temperature extremes in March 2016, many millions of corals died quickly in the northern third of the Great Barrier Reef over a period of only two to three weeks”.
“These widespread losses were not due to the attrition of corals that slowly starved because they failed to regain their symbionts. Rather, temperature-sensitive species of corals began to die almost immediately in locations that were exposed to heat stress.”
The research team observed “markedly divergent responses to heat stress”. Some corals, such as staghorn and tabular corals, suffered a “catastrophic die-off”. Others proved more resilient.
Report co-author Prof Andrew Baird said the coral die-off had caused “radical changes in the mix of coral species on hundreds of individual reefs”.
“Mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining,” he said.
The researchers estimate half of the corals in shallow-water habitats in the northern Great Barrier Reef have been 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,” Hughes said.
“The Great Barrier Reef is certainly threatened by climate change, but it is not doomed if we deal very quickly with greenhouse gas emissions. Our study shows that coral reefs are already shifting radically in response to unprecedented heatwaves.”
The scientists said their research underscores the need for further risk assessment into the collapse of reef ecosystems, especially if global action on climate change fails to limit warming to a 1.5C to 2C increase on pre-industrial levels.
They also warn that a failure to curb climate change, resulting in an increase above 2C, will radically alter tropical reef ecosystems and undermine benefits they provide to hundreds of millions of people.
Hughes said that left the reef in “uncharted territory”, its future dependent on how quickly emissions peak and come down.
If the targets in the Paris agreement are met, the reef will survive as “a mixture of heat-tolerant [corals], and the ones that can bounce back”.
“Biodiversity will likely be less, coral cover will likely be less,” Hughes said.
If warming continues apace: “Then it’s game over.”