Carbon-dioxide emissions are threatening marine life and human food supplies by making the oceans more acidic, an international group of scientists says.

"Ocean acidification is accelerating and severe damages are imminent," according to the Monaco Declaration, a document signed by 155 marine scientists from 26 countries and issued last Friday.

That could destroy coral reefs, threaten the fishing and tourism industries and affect the food supplies of millions of people unless policy makers work to curb carbon dioxide levels, the declaration said.

Carbon-dioxide levels in the atmosphere have climbed sharply over the last century, largely due to human activity such as the burning of fossil fuels. The gas is one of the main "greenhouse gases" blamed by scientists for helping trap heat in the atmosphere, leading to climate change.

That in itself has been blamed for damaging coral reefs, as warmer temperatures lead to the spread of coral diseases.

But the oceans also absorb about a quarter of the carbon dioxide in the atmosphere, forming carbonic acid.

Acid dissolves corals, shells

That increases the acidity of seawater, making it easier for the water to dissolve the calcium carbonate shells of corals and shellfish.

Measurements show that the acidity at the surface of the ocean has increased by 30 per cent since industrialization began in the 18th century, the declaration said.

The scientists predicted that if nothing is done, most of the world's oceans could be inhospitable to coral reefs by the middle of this century. The reefs currently provide important fish habitat and protect coastal areas from flooding. The rising acidity could also make it more difficult for the oceans to absorb additional carbon dioxide, leaving more in the atmosphere itself and exacerbating climate change.

The declaration was based on conclusions from an international symposium that was held in Monaco in October.