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Great Barrier Reef corals have more than halved in past 25 years, study shows

<span>Photograph: Lucas Jackson/Reuters</span>
Photograph: Lucas Jackson/Reuters

Corals on the Great Barrier Reef have more than halved over the past 25 years, according to a study that prompted scientists to again warn the world-famous landmark will become unrecognisable without a sharp reduction in greenhouse gas emissions.

Researchers from the Townsville-based ARC Centre of Excellence for Coral Reef Studies assessed coral communities and size between 1995 and 2017 and found the number of small, medium and large corals had fallen more than 50%.

Related: Scientists claim Coalition misrepresented their evidence to Great Barrier Reef inquiry

The study’s co-author, James Cook University professor Terry Hughes, said it found mass bleaching events triggered by record-breaking water temperatures in 2016 and 2017 had the most significant impact on coral depletion.

The research, published in the journal Proceedings of the Royal Society, did not take into account another major bleaching event earlier this year that affected the southern part of the reef “very severely”, suggesting total coral depletion may be greater than estimated.

“I began surveying the reefs in 1995, and what subsequently unfolded certainly wasn’t planned for. There have been five major bleaching events since then, including three in just the past five years,” Hughes said, adding he was “very concerned” about the “shrinking gap” between bleaching events.

While small, medium and large coral had each been depleted, Hughes said the decline in larger corals was the greatest threat to the reef’s ability to repair as they “spawn more babies”.

Hughes said the species of corals to have suffered the most significant decline were staghorn corals, also known as branching corals, and table corals.

“Those two types of corals are the most three-dimensional – they form habitats,” he said. The loss of habitat affected fish numbers and the productivity of coral reef fisheries. “The reef is flatter and less three dimensional now,” he said.

Related: Unsustainable fishing worsens threats to Great Barrier Reef

Global heating caused by escalating atmospheric greenhouse gases is a major threat to the world’s coral reef ecosystems. Hughes said the only way to fix the problem facing the Great Barrier Reef was to reduce emissions.

“There’s not much time to lose,” he said. “I think if we can control warming somewhere between 1.5-2C [above pre-industrial levels], as per the Paris agreement, then we’ll still have a reef. But if we get to 3-4C because of unrestrained emissions then we won’t have a recognisable Great Barrier Reef.”

Last year, the Great Barrier Reef Marine Park Authority, in its five-yearly reef health report, downgraded the outlook for the world’s biggest reef system to “very poor”. It has repeatedly reiterated that climate change was the “single greatest challenge” facing the 2,300km reef system.

Concern has also been raised for the future of fish and agricultural activities that rely on the southern end of the Murray River, with a University of Sydney study published in The Holocene journal raising concern about the water’s increasing vulnerability to acidification as a result of human impacts.

Dr Thomas Job, who studied sediments revealing 7,000 years of geological records in the lower lakes and estuary of the Murray River in South Australia, said recent infrastructure construction and water use during droughts had led to “historically unprecedented acidification”.

Specifically, Job found the construction of the Goolwa Barrages in 1940, which cut the estuary off from the ocean in an attempt to reduce the salinity of the lower lakes, triggered “widespread oxidation of exposed sulphide minerals” and “caused surface waters to become acidic” in Lake Albert during the millennium drought from 1996 to 2010.

Related: Is it too late to save the Great Barrier Reef?

Job said the barrages meant the lower lakes of the Murray River are “100% reliant on water coming from upstream”.

“There isn’t sufficient water allocated to this estuary to stop acidification to occur during droughts,” he said.

Job said acidification of the water led to fish kills and corrosion of infrastructure.

Communities near the river mouth were fortunate to avoid seeing the effects of acidification during the recent drought, and that it would have likely occurred had the drought extended beyond the first half of this year,” he said.

“Half of Australia’s farming relies on the management of the waters within this system,,” he said. “This issue is only going to become more difficult to manage.”