The Great Barrier Reef will be so degraded by warming waters that it will be unrecognisable within 20 years, an eminent marine scientist has said.

Charlie Veron, former chief scientist of the Australian Institute of Marine Science, told The Times: “There is no way out, no loopholes. The Great Barrier Reef will be over within 20 years or so.”

Once carbon dioxide had hit the levels predicted for between 2030 and 2060, all coral reefs were doomed to extinction, he said. “They would be the world’s first global ecosystem to collapse. I have the backing of every coral reef scientist, every research organisation. I’ve spoken to them all. This is critical. This is reality.”

Dr Veron’s comments came as the Zoological Society of London, the Royal Society and the International Programme on the State of the Ocean (IPSO) held a crucial meeting on the future of coral reefs in London yesterday. In a joint statement they warned that by mid-century extinctions of coral reefs around the world would be inevitable.

Warming water causes coral polyps to eject the symbiotic algae that provide them with nutrients. These “bleaching events” were widespread during the El Niño of 1997-98, and localised occurrences are becoming more frequent. (During an El Niño, much of the tropical Pacific becomes unusually warm.) Reefs take decades to recover but by 2030 to 2050, depending on emissions and feedback effects, bleaching will be occurring annually or biannually.

Although surface sea temperatures are rising fastest in tropical regions the other big threat to coral reefs comes from the higher latitudes. The cold water there absorbs atmospheric carbon dioxide more readily than warm water and acidifies more easily.

When carbon dioxide concentrations reach between 480 and 500 parts per million warm water is no barrier to acidification, and the pH in equatorial regions will have dropped so far, meaning higher acidity, that coral reef growth becomes impossible anywhere in the ocean.

“Coral reefs are the most sensitive of marine ecosystems,” said Alex Rogers, scientific director of IPSO.

“Increased temperature and decreased pH will have a double-whammy effect. Reefs were safe at CO2 levels of 350 parts per million. We are at 387ppm today. Beyond 450 the fate of corals is sealed.”

In the five mass extinction events in geological history, key was the carbon cycle, in which carbon dioxide is the primary currency. Its concentration in the atmosphere is higher than it has been for 20 million years. In the Permian extinction, as in all the big extinctions, tropical marine life was the hardest hit. Reef-building corals took more than ten million years to return.

The Great Barrier Reef, the world’s largest and most diverse marine ecosystem, is worth $4.5 billion (£2.8 billion) a year to Australia. Worldwide, reefs are worth $300 billion. “But that is trivial compared with the costs if coral reefs fail,” Dr Veron said. “Then it won’t be a matter of no income, it will be a matter of damage to livelihoods, economies and ecosystems.”

Yesterday’s meeting renewed calls for networks of marine conservation zones to boost the resilience of reefs.

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