Climate crisis: Australia’s Great Barrier Reef has lost half its corals since 1995, study finds

High ocean temperatures have impacted large and small corals in deep and shallow water, scientists say

The once brightly hued coral has turned a pale grey due to coral bleaching caused by high temperatures (Getty Images)

The vibrant corals which once made up Australia’s Great Barrier Reef are dying so quickly due to the climate crisis that more than 50 per cent have died over the last 25 years, a new assessment has found.

A research team examined coral communities and their size all along the length of the reef between 1995 and 2017.

They found declines across small, medium and large corals, in both deep and shallow water, across “virtually all species”.

The reef – a Unesco World Heritage site –  was particularly badly hit by two years of record-breaking temperatures which caused mass coral bleaching in 2016 and 2017.

Approximately one-third of the world's coral reefs were estimated to have been affected by bleaching in 2016, according to separate studies.

That year, on the Great Barrier Reef, less than 10 per cent of corals escaped with no bleaching, compared with more than 40 per cent in previous bleaching events.

Severe bleaching also occurred the following year.

Coral bleaching occurs when conditions such as overheating or too much light cause the corals to be put under stress. As a result, they expel the symbiotic algae living in their pores. This turns the coral completely white. This doesn’t necessarily kill the corals, which can recover over long periods of time, but it does leave them much more vulnerable.

Lead author of the new research, Dr Andy Dietzel, from the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland, said there is a lack in long-term history detailing the changes in coral populations.

“We measured changes in colony sizes because population studies are important for understanding demography and the corals' capacity to breed,” he said.

He and his co-authors’ results show a depletion of coral populations, and earlier this year, a further coral-bleaching event occurred on the reef.

“We found the number of small, medium and large corals on the Great Barrier Reef has declined by more than 50 per cent since the 1990s,’ said co-author Professor Terry Hughes, also from CoralCoE.

“The decline occurred in both shallow and deeper water, and across virtually all species – but especially in branching and table-shaped corals. These were the worst affected by record-breaking temperatures that triggered mass bleaching in 2016 and 2017,” Professor Hughes said.

The branching and table-shaped corals provide the structures important for reef inhabitants such as fish. The loss of these corals means a loss of habitat, which in turn diminishes fish abundance and the productivity of coral reef fisheries.

Dr Dietzel said one of the major implications of coral size is its effect on survival and breeding.

“A vibrant coral population has millions of small, baby corals, as well as many large ones – the big mamas who produce most of the larvae,” he said.

“Our results show the ability of the Great Barrier Reef to recover – its resilience – is compromised compared to the past, because there are fewer babies, and fewer large breeding adults.”

The authors of the study say better data on the demographic trends of corals is urgently needed.

“If we want to understand how coral populations are changing and whether or not they can recover between disturbances, we need more detailed demographic data: on recruitment, on reproduction and on colony size structure,” Dr Dietzel said.

“We used to think the Great Barrier Reef is protected by its sheer size, but our results show that even the world's largest and relatively well-protected reef system is increasingly compromised and in decline,” Professor Hughes said.

The authors said the climate crisis is driving an increase in the frequency of reef disturbances such as marine heatwaves.

The study records steeper deteriorations of coral colonies in the northern and central Great Barrier Reef after the mass coral bleaching events in 2016 and 2017. And the southern part of the reef was also exposed to record-breaking temperatures in early 2020.

“There is no time to lose – we must sharply decrease greenhouse gas emissions asap,” the authors said.

Despite the evidence of global climate breakdown, Australia’s president Scott Morrison has pledged a “gas-led recovery” following the coronavirus pandemic.

Last month he said there was “no credible energy transition plan for an economy like Australia that does not involve the greater use of gas”.

The study is published in the journal Proceedings of the Royal Society B.