Clownfish, seen swimming among anemones in Indonesia, are highly dependent on coral reef ecosystems. (Canadian Press)

Climate change threatens to snuff out the world's coral reefs within the next half-century, a group of scientists led by David Attenborough warned on Monday.

Several prominent scientists and marine experts gathered at the Royal Society in London, England, to discuss the future of the world's coral reefs. After the meeting, they called on world leaders to make greater cuts in carbon emissions.

"A coral reef is the canary in the cage as far as the oceans are concerned," said Attenborough, speaking after the meeting.

"They are the places where the damage is most easily and quickly seen. It is more difficult for us to see what is happening in, for example, the deep ocean or the central expanses of ocean," the renowned British naturalist and broadcaster said.

The reefs provide important fish habitat and protect coastal areas from flooding. They support a vast array of marine ecosystems, which humans in turn depend on for sustenance — be it for food or the economic benefits yielded by tourism.

The scientists at the meeting warned that if carbon emissions continue to grow at current rates, levels of carbon dioxide in the atmosphere could reach 450 parts per million by 2050 — up from around 387 ppm today.

Acidity threatens reefs

Oceans absorb around a quarter of the world's carbon dioxide. This process creates carbonic acid, and seawater that acidifies creates an inhospitable environment for coral — existing coral can bleach and die, while new coral is unable to grow.

"When we get up to and above 450 ppm, that really means we're into the realms of catastrophic destruction of coral reefs and we'll be moving into a planetary-wide global extinction," said Alex Rogers, one of the scientists in attendance. He said carbon dioxide levels below 350 ppm were necessary to ensure healthy reefs.

"The only way to get to 350 ppm or below is not only to have major cuts in CO2 emissions but also to draw CO2 out of the atmosphere through measures such as geo-engineering," said Rogers, who is the scientific director of the International Program on the State of the Ocean.

Rogers said the vulnerability of the corals should spur decisive action by world leaders when they meet in Copenhagen at the end of the year to strike a climate pact to succeed the Kyoto Protocol.

"Essentially coral reefs are on death row and Copenhagen is one of the last opportunities for a reprieve," said Rogers. "Because if we carry on business as usual collapse is inevitable, whereas if we decide to do something about it we can make a difference to the current trajectory."