Global warming disrupts recovery of coral reefs

"Dead corals don't make babies," said lead author Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University (JCU). "The number of new corals settling on the Great Barrier Reef declined by 89 percent following the unprecedented loss of adult corals from global warming in 2016 and 2017."

The unique study measured how many adult corals survived along the length of the world's largest reef system following extreme heat stress, and how many new corals they produced to replenish the Great Barrier Reef in 2018. The loss of adults resulted in a crash in coral replenishment compared to levels measured in previous years before mass coral bleaching.

"The number of coral larvae that are produced each year, and where they travel to before settling on a reef, are vital components of the resilience of the Great Barrier Reef. Our study shows that reef resilience is now severely compromised by global warming," said co-author Professor Andrew Baird.

"The biggest decline in replenishment, a 93% drop compared to previous years, occurred in the dominant branching and table coral, Acropora. As adults these corals provide most of the three-dimensional coral habitat that support thousands of other species," he said.

"The mix of baby coral species has shifted, and that in turn will affect the future mix of adults, as a slower than normal recovery unfolds over the next decade or longer."

"The decline in coral recruitment matches the extent of mortality of the adult brood stock in different parts of the Reef," added Professor Hughes. "Areas that lost the most corals had the greatest declines in replenishment."

"We expect coral recruitment will gradually recover over the next five to ten years, as surviving corals grow and more of them reach sexual maturity, assuming of course that we don't see another mass bleaching event in the coming decade," he said.

So far, the Great Barrier Reef has experienced four mass bleaching events due to global warming, in 1998, 2002, and back-to-back in 2016 and 2017. Scientists predict that the gap between pairs of coral bleaching events will 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," he said.

"For example, when one part was damaged by a cyclone, the surrounding reefs provided the larvae for recovery. But now, the scale of severe damage from heat extremes in 2016 and 2017 was nearly 1500km -- vastly larger than a cyclone track."

Professor Pratchett added that the southern reefs that escaped the bleaching are still in very good condition, but they are too far away to replenish reefs further north.

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