Climate change leading to rising sea temperatures has resulted in Australia's Great Barrier Reef losing more than half its corals over the last 25 years, a new study has found.

Work done by the Australian Research Council's Centre of Excellence for Coral Reef Studies in Queensland found that repeated episodes of so-called bleaching were leading to significant coral decline right across the world's largest reef system.

"There is no time to lose－we must sharply decrease greenhouse gas emissions ASAP," its report said.

Scientists compared the health and size of coral colonies in 1995 and 2017, and noticed drops of more than 50 percent across all species and sizes of coral, most particularly types known as branching and table-shaped corals, which are habitats for marine life.

Bleaching is the process whereby zooxanthellae algae, which give corals their color, are driven out of the living organism by stress. It can be reversed, but takes decades.

"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," said the report's lead author, Andy Dietzel.

The reef, which stretches more than 2,300 kilometers, was designated a World Heritage site in 1981 because of its "enormous scientific and intrinsic importance", but altered sea conditions mean it has struggled to survive, particularly in recent years.

The reef was worth an estimated $4 billion a year in tourism revenue for the Australian economy before the coronavirus pandemic.

"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," said report co-author Terry Hughes.

Global warming

Globally, since preindustrial times, temperatures have risen by around 1 C, and the United Nations has warned that should that figure reach 1.5 C, as much as 90 percent of the world's coral could be destroyed.

Hughes said countries sticking to the terms of the 2015 Paris Agreement on climate change would play a huge role in what happened next.

"It takes about a decade for a half-decent recovery for the fastest-growing species, so the chances of us getting decades between the future sixth, seventh and eighth bleaching events is close to zero because temperatures are going up and up and up," he said.

Even if corals did recover, he added, they were unlikely to return to how things were before.

"We don't think they'll rebuild into the mix of species that we've known historically," he added.

"The trajectory is changing very, very quickly. We're shocked and surprised by how quickly these changes are happening, and there's further change ahead."