Scientists unveiled the first smoking-gun evidence Wednesday that growing ocean acidity caused by global warming is already stifling growth of vital coral reefs.

The decline of shallow water corals, home to a quarter of the ocean's species and a lifeline for a billion people, has long been in evidence.

Earlier studies had shown that the rate at which living coral reefs calcify, or accumulate mass, had dropped by about 40 percent in just over 30 years.

Up to now, however, it was not possible to tease out the impact of acidification from other threats such as pollution, over-fishing and warming water.

The world's oceans are 26 percent more acidic today than at the start of the Industrial Revolution, when mankind started massively burning fossil fuels which give off harmful carbon dioxide (CO2).

About a quarter is absorbed by the oceans, changing their chemical composition, and making the water more acidic and corrosive to corals and shellfish.

"Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already slowing coral reef growth," said Rebecca Albright, a researcher at the Carnegie Institution for Science in Stanford, California.

"This is no longer a fear for the future. It is the reality of today."

The findings were published in the science journal Nature.

Albright and colleague Ken Caldeira led experiments on natural reefs off the coast of Australia's One Tree Island, in the southern Great Barrier Reef.

Deep cuts

Manipulating the chemistry of the seawater flowing over the flat reef, the researchers restored it's pH -- the balance between alkalinity and acidity -- to what it would have been without climate change.

As suspected, the corals became better able to build themselves up.

"By turning back time in this way, they demonstrate that -- all things being equal -- net coral-reef calcification would have been around seven percent higher than at present," Janice Lough, a scientist at the Australian Institute of Marine Science, noted in a comment, also published in Nature.

The novelty of the experiment, she said, is that it "restored the ocean chemistry of a natural reef to that of pre-industrial times, thus factoring out other potentially confounding factors, such as temperature."

Some researchers have proposed artificially reducing the acidity of ocean water around coral reefs -- a form of geo-engineering -- as a means of preserving shallow marine ecosystems.

But even if the experiments underlying the study did exactly that, implementing such a scheme on the required scale would be nigh impossible, the authors caution.

"The only real, lasting way to protect coral reefs is to make deep cuts in our carbon dioxide emissions," Caldeira said.

"If we don't take action on this issue very rapidly, coral reefs -- and everything that depends on them, including both wildlife and local communities -- will not survive into the next century."