Coral reefs are having their growth stunted by ocean acidification caused by global warming, new research has confirmed.
For the first time, scientists conducted an experiment on a natural coral reef which involved altering sea water chemistry to mimic the effect of excess carbon dioxide in the atmosphere.
The results provide strong evidence that ocean acidification linked to greenhouse gas emissions is already slowing coral reef growth, the team claims.
Without "deep cuts" in greenhouse gas emissions, the world's coral reefs may not survive into the next century, scientists say.
Carbon dioxide is absorbed by the ocean where it reacts with seawater to increase acidity.
If the water becomes too acid it dissolves away the calcium carbonate corals that molluscs and creatures such as crabs and lobsters need to build their shells and stony skeletons.
Although previous studies have demonstrated large scale declines in coral reefs in recent decades, the reason for the trend has been harder to pinpoint.
Acidification is one possible cause, but others include warming, pollution and over-fishing.
To investigate the role played by greenhouse gas emissions, the US scientists manipulated the acidity of seawater flowing over a section of the Great Barrier Reef off Australia's One Tree Island.
Bringing the reef's pH value - a measurement of acidity or alkalinity - closer to what it would have been in pre-industrial times increased the rate at which calcium carbonate was deposited to grow hard coral exoskeletons.
Lead researcher Dr Rebecca Albright, from the Carnegie Institution in Washington DC, said: "Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already slowing coral reef growth.
"Ocean acidification is already taking its toll on coral reef communities.
"This is no longer a fear for the future; it is the reality of today."
The research is reported in the latest issue of the journal Nature.
Other work by Carnegie colleague Professor Ken Caldeira found that rates of reef calcification in 2008 and 2009 were 40% lower than they were in 1975 and 1976.
He said: "The only real, lasting way to protect coral reefs is to make deep cuts in our carbon dioxide emissions.
"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."