New research reveals coral bleaching on Great Barrier Reef has happened for hundreds of years but it’s on the rise

Coral bleaching on the Great Barrier Reef, Port Douglas, Far North Queensland.

Coral bleaching across Australia’s Great Barrier Reef has been occurring since the late 18th century, new research shows.

Using cores taken from long-lived corals, scientists show that bleaching events have steadily affected more and more corals, and are happening more frequently than in the past, adding to existing concerns about the future of coral reefs.

The bleaching occurs when water temperatures rise too high and disrupt the symbiotic relationship between the animal coral and the tiny algae which live inside it. leading the coral to expel the algae. The coral then expel the algae, leaving the coral a stark white. While algae can be reabsorbed when the water temperature drops once more, corals will die if this separation is too long.

Coral bleaching is of great global concern, as around one in six people globally rely on coral reefs for food, shelter and livelihoods. Recent and repeated mass bleaching events have raised widespread concern 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, little has been known until now about the frequency and extent of bleaching events prior to those modern observations.

In a new paper published today (Thursday 16 August) in the journal Frontiers, researchers from the Universities of Glasgow and Edinburgh describe how they have for the first time managed to extend the record of bleaching events in the Great Barrier Reef back by four centuries.

To do so, they took advantage of coral’s ability to live for a long time – hundreds of years – and grow slowly. As corals grow, they lay down their limestone skeleton in lines of annual growth, similar to tree rings.

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

Dr Nick Kamenos of the University of Glasgow’s School of Geographical and Earth Sciences is one of the paper’s co-authors.

Dr Kamenos said: “What we found was fascinating and provides vital additional context for the observations carried out over the last half-century.

“It’s clear in the core data we examined that bleaching has been occurring on the Great Barrier Reef for at least 400 years, but the frequency of bleaching events has increased markedly since the early 1800s and those events have affected 10% more corals since the late 1700s.

“We can see that corals have been able to acclimate and recover from past bleaching events. However, 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 Sebastian Hennige from the University of Edinburgh’s School of Geosciences is a co-author on the paper. He said: “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 serious the threat of coral bleaching is, and how important it is that we take action now to reduce this threat.”

The paper, titled ‘Reconstructing four centuries of temperature-induced coral bleaching on the Great Barrier Reef’, is published in Frontiers. The work was supported by funding from the Natural Environment Research Council (NERC), the Royal Society of Edinburgh, and the Scottish Government.