The Great Barrier Reef is losing its ability to recover from bleaching

https://www.newscientist.com/article/2198318-the-great-barrier-reef-is-losing-its-ability-to-recover-from-bleaching/

Great barrier reef

The Great Barrier Reef is struggling to recover from bleaching

imageBROKER / Alamy Stock Photo

Global warming is destroying the Great Barrier Reef’s ability to recover from disasters and reducing its biodiversity by changing the species that live there.

Around half of Australia’s Great Barrier Reef died off in 2016 and 2017 after ocean temperatures warmed enough to cause mass bleaching, where heat stresses coral to the point that it expels the colourful algae living inside it.

Now a study has found that the amount of coral larvae on the reef in 2018 was down by 89 per cent on historical levels.

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“There’s fewer adults after the back-to-back bleaching because of the high rates of mortality, and dead coral doesn’t make babies,” says Terry Hughes of James Cook University in Australia, who led the work.

Such a big number shows the impact of the bleaching was severe, says team member Joerg Wiedenmann at the University of Southampton, UK.

The decline is bad news for the reef’s long-term future. It is also changing the mix of coral species that replenish the reef, which will reduce the amount of suitable habitats for marine life.

For the first time, recruitment of a group of weedy corals, known as brooding pocilloporids, outstripped spawning acroporids, a type of coral that is vital for giving a reef the three-dimensional complexity that many animals rely on.

Losing some of that three-dimensionality means a loss of biodiversity, in coral species and other marine life.

More frequent bleaches

“Instead of oaks, you’ve got brambles. They provide a lot less habitat,” says Jason Hall-Spencer at the University of Plymouth, UK, who wasn’t involved in the research.

The decline in coral rebound was steepest in the north and centre of the Great Barrier Reef, which experienced the worst die-offs. The south, which largely escaped the bleaching, rebounded at higher than historical levels.

We shouldn’t bank on the south reef replenishing the north, though, because the distances involved are too great and the ocean current runs in the wrong direction, says Hughes.

Overall, the team says it is uncertain whether the Great Barrier Reef will make a full recovery, in large part because of the increasing frequency of mass bleaching episodes.

The gap between such events has narrowed from once every 25 years in the 1980s to almost once every six years since 2010.

If global greenhouse emissions carry on rising at their current rate, bleaching events are expected every year from 2044.

One of the key messages of the UN’s landmark climate science report last year was that almost all of the world’s coral would vanish with 2°C of warming, but 10 to 30 per cent could survive if temperatures were checked at 1.5°C.

Historically, bleaching events have been associated with the El Niño climate phenomenon, but the 2017 bleaching took place without one.

Journal reference: Nature, DOI: 10.1038/s41586-019-1081-y

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