At a Glance

Scientists say the die-off at the Great Barrier Reef following marine heatwaves in 2016 and 2017 is transforming the world's largest reef system.While many corals died immediately from heat exposure, others died more slowly.The lead author says there is still hope, noting that there are "still about a billion corals alive."

A massive coral die-off at Australia's Great Barrier Reef following marine heat waves in 2016 and 2017 has irreparably changed the world's largestreef system, a new study says.

According to a press release sent to weather.com, researchers with Australia's James Cook University say the nearly1,500-mile-long reefexperienced a "catastrophic die-off"which is "transforming the ecological functioning of almost one-third of the 3,863 reefs that comprise the world’s largest reef system."

"We were surprised by the severity of the event(s)," Terry Hughes, lead author of the study and director of the Australia Research Council's Centre of Excellence for Coral Reef Studies, told weather.com in an email. "Thirty percent of the corals died in 2016, and another 20 percent in 2017."

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Bymapping the geographical pattern of heat exposure and the resulting coral death, Hughes and his colleaguesdetermined that while many corals died immediately from heat exposure, others died more slowly. The die-off also changed the composition of hundreds of individual reefs in the system and how the corals work together as an assemblage. Many of the more mature and diverse assemblages were transformed into "degraded" systems.

They also found that the northern third of the reef experienced a higher incidence of coral death, which correlated with the higher water temperatures recorded in that regionof the reef.

Hughes and his colleaguesnoted in the press release that a full recovery of affected reef assemblages is"unlikely" because"many surviving coral colonies continue to die slowly, and the replacement of dead corals will take at least a decade even for fast-growing species."

In 2017, UNESCO's World Heritage Committee said despite mounting evidence that the back-to-back coral bleaching events in 2016 and 2017threatened the survival of the site.

To be, a natural site must show a "serious decline in the population of the endangered species" or "severe deterioration of the natural beauty or scientific value of the property," among other things.

Hughes says UNESCO's decision was the right call.

"I think that was the right decision," Hughes told weather.com. "The reality is, as our study demonstrates, the GBR is in danger from climate change. But putting the GBR on UNESCO’s List would be a mistake. The ongoing dialogue between UNESCO and the Australian government has been very productive, resulting in many recent improvements in GBR governance, such as new legislation to curb dredging near coal ports."

The researcherspoint out that the study published this week in the journal Nature reinforces the "need for ," particularly if global efforts to curb greenhouse emissions fails to limit global warming to 1.5 to 2 degrees Celsius above pre-industrial levels.

"The elephant in the room is climate change," Hughes said. "If we can reach theParis Agreement targets of 1.5- to 2-degree limits to global warming, we will still have a functioning GBR in the future. It will be a highly altered system, with a different mix of species from two years ago, or today, but it will still function as a coral reef. Our study shows we have already begun that transformation. But if we continue with business-as-usual emissions, then severe climate change will destroy coral reefs as we know them today by mid-century."

Despite the dire findings and predictions, Hughes said there is still hope, noting that there are "still about a billion corals alive."

"The glass is still half full, and there is no time to lose to address the root cause of the decline of the GBR," said Hughes. "Australia needs to leave its fossil fuels in the ground."