BONN, Germany, June 1 (Reuters) - Climate change is turning the oceans more acid in a trend that could endanger everything from clams to coral and be irreversible for thousands of years, national science academies said on Monday.

Seventy academies from around the world urged governments meeting in Bonn for climate talks from June 1-12 to take more account of risks to the oceans in a new U.N. treaty for fighting global warming due to be agreed in Copenhagen in December.

"To avoid substantial damage to ocean ecosystems, deep and rapid reductions of carbon dioxide emissions of at least 50 percent (below 1990 levels) by 2050, and much more thereafter, are needed," the academies said in a joint statement.

The academies said rising amounts of carbon dioxide, the main greenhouse gas emitted mainly by human use of fossil fuels, were being absorbed by the oceans and making it harder for creatures to build protective body parts.

The shift disrupts ocean chemistry and attacks the "building blocks needed by many marine organisms, such as corals and shellfish, to produce their skeletons, shells and other hard structures", it said.

On some projections, levels of acidification in 80 percent of Arctic seas would be corrosive to clams that are vital to the food web by 2060, it said.

And "coral reefs may be dissolving globally," it said, if atmospheric levels of carbon dioxide were to rise to 550 parts per million (ppm) from a current 387 ppm. Corals are home to many species of fish.

"These changes in ocean chemistry are irreversible for many thousands of years, and the biological consequences could last much longer," it said.

The warning was issued by the Inter-Academy Panel, representing science academies of countries from Albania to Zimbabwe and including those of Australia, Britain, France, Japan and the United States.

UNDERWATER CATASTROPHE

Martin Rees, president of the Royal Society, the British science academy, said there may be an "underwater catastrophe".

"The effects will be seen worldwide, threatening food security, reducing coastal protection and damaging the local economies that may be least able to tolerate it," he said.

The academies' statement said that, if current rates of carbon emissions continue until 2050, computer models indicate that "the oceans will be more acidic than they have been for tens of millions of years".

It also urged actions to reduce other pressures on the oceans, such as pollution and over-fishing.