The world's oceans are slowly getting more acidic, say scientists.

The researchers from California say the change is taking place in response to higher levels of carbon dioxide in the atmosphere.

The lowering of the waters' pH value is not great at the moment but could pose a serious threat to current marine life if it continues, they warn.

Ken Caldeira and Michael Wickett, from the Lawrence Livermore National Laboratory, report their concerns in the journal Nature.

Increasing use of fossil fuels means more carbon dioxide is going into the air, and most of it will eventually be absorbed by seawater. Once in the water, it reacts to form carbonic acid.

Scientists believe that the oceans have already become slightly more acidic over the last century.

But these researchers have tried to predict what will happen in the future by combining what we know about the history of the oceans with computer models of climate change.

"This level of acidity will get much more extreme in the future if we continue releasing CO2 into the atmosphere," said Dr Caldeira.

"And we predicted amounts of future acidity that exceed anything we saw over the last several hundred million years, apart from perhaps after rare catastrophic events such as asteroid impacts."

If carbon dioxide release continues unabated, ocean pH could be reduced by as much as 0.77 units, the authors warn.

It is not absolutely clear what that means for marine life, however.

Most organisms live near the surface, where the greatest pH change would be expected to occur, but deep-ocean lifeforms may be more sensitive to pH changes.

Coral reefs and other organisms whose skeletons or shells contain calcium carbonate may be particularly affected, the team speculate. They could find it much more difficult to build these structures in water with a lower pH.

In recent years some people have suggested deliberately storing carbon dioxide from power stations in the deep ocean as a way of curbing global warming.

But Dr Caldeira said that such a strategy should now be re-considered.

"Previously, most experts had looked at ocean absorption of carbon dioxide as a good thing - because in releasing CO2 into the atmosphere we warm the planet; and when CO2 is absorbed by the ocean, it reduces the amount of greenhouse warming.

(Agencies)