Global warming has caused such extensive damage to the Great Barrier Reef that scientists say its coral may never recover. According to a study published Wednesday in the journal Nature, baby coral in Australia's Great Barrier Reef have declined by 89% due to mass bleaching in 2016 and 2017.

The study measured the number of surviving adult corals in the Great Barrier Reef — the world's largest reef system — following extreme heat stress, as well as how many new corals it was able to replenish in 2018. Deadly back-to-back bleaching events devastated the reef, and now its ecosystem is struggling to recover. Not only have ocean heat waves led to a dramatic decrease in new coral, but also a change in the types of coral species being produced.

Researchers studied adult and baby coral from 47 locations in various years from 1996 to 2016, then returned to the reef in 2018 to collect the same data. They found that a majority of the northern region's coral has not been able to recover following mass bleaching events, leading to a decline in new coral as well.

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"Dead corals don't make babies," the study's lead author, Professor Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said in a press release.

Dead staghorn coral killed by bleaching on the northern Great Barrier Reef off the coast of Australia, November 2016. Greg Torda, ARC Centre of Excellence for Coral Reef Studies

According to the study, the biggest decline in replenishment came from the reef's dominant species of adult coral, called Acropora, which supports thousands of other species. It experienced a 93% drop compared to previous years. The change is likely to reduce the reef's overall diversity, which will make it even less resilient to future bleaching events. "Our study shows that reef resilience is now severely compromised by global warming," said co-author Professor Andrew Baird.

Scientists expect the coral may recover over the next five to ten years — but only if another mass bleaching event doesn't occur during that time. That's unlikely given the current trajectory of climate change. The researchers said southern reefs that did not suffer bleaching are still in good condition, they are not close enough to replenish the damaged reefs that are further north.

Bleaching occurs when corals are stressed by unusual environmental changes, such as increased sea temperature. They respond by expelling the symbiotic algae living in their tissues, causing the coral to turn white. Without the algae, the coral loses its major source of food and often can't survive. Widespread coral bleaching during back-to-back summers was particularly toxic, as it did not leave enough time for the corals to fully recover.

The Great Barrier Reef — which stretches for more than 1,400 miles off the coast of Australia — has gone through four mass bleaching events due to above-average sea temperatures in 1998, 2002, 2016 and 2017. Time periods between future bleaching events are expected to continue to shrink as global warming intensifies.

"It's highly unlikely that we could escape a fifth or sixth event in the coming decade," said co-author Professor Morgan Pratchett. "We used to think that the Great Barrier Reef was too big to fail – until now."

UNESCO — the United Nations Educational, Scientific and Cultural Organization — placed the Great Barrier Reef on its list of World Heritage sites in 1981. In recent years it has registered official "concern" about the reef's condition but stopped short of declaring it "in danger."

"There's only one way to fix this problem," said Hughes, "and that's to tackle the root cause of global heating by reducing net greenhouse gas emissions to zero as quickly as possible."