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Great Barrier Reef coral bleaching on the rise

Coral bleaching across Australia’s Great Barrier Reef has been increasing since the late 18th century, and may threaten its long-term survival, new research shows.

Using cores taken from long-lived corals, scientists show that incidences of bleaching – when coral expel the tiny algae that live in their tissues – has steadily affected more corals.

This is happening more frequently than in the past, the study shows.

Warm waters

Bleaching occurs when water temperatures rise too high, disrupting the relationship between the coral and the algae that live inside it.

This leads the coral to expel the algae, leaving the coral a stark white.

If the water temperature cools, algae can re-join the coral, but corals will die if this separation period is too long.

About one in six people in the world relies on coral reefs for food, shelter and livelihoods.

Ecosystem threat

Recent and repeated mass bleaching events have raised concerns about the future of these key ecosystems and the implications that their loss could have for biodiversity, the economy, and human health.

Large-scale observations of the Great Barrier Reef first began in the late 1970s.

However, until now little has been known about the frequency and extent of bleaching events prior to this.

Researchers from the Universities of Glasgow and Edinburgh have for the first time extended the record of bleaching events in the Great Barrier Reef by four centuries.

Tracing history

Scientists took advantage of coral’s ability to grow slowly and live for hundreds of years. Over time, corals lay down their limestone skeleton in lines of annual growth, similar to tree rings.

By investigating cores extracted from corals hundreds of years old, researchers were able to reconstruct the history of bleaching for each coral.

Their study, published in the journal Frontiers, was supported by the Natural Environment Research Council (NERC), the Royal Society of Edinburgh and the Scottish Government.

The increase in bleaching frequency and the numbers of corals affected since temperatures started consistently increasing in the modern era raises serious concerns about whether corals are approaching a critical threshold beyond which their long-term survival is uncertain.

Dr Nick KamenosSchool of Geographical and Earth Sciences, University of Glasgow

For this study we used the most conservative methods we could in some of the toughest corals out there today. The fact that we are seeing an increase in bleaching even in these tough corals highlights just how real the threat of coral bleaching is, and how important it is that we take urgent action now to reduce this threat.

Dr Sebastian HennigeSchool of Geosciences, University of Edinburgh

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