'Avatar' star Weaver fights for oceans on Earth Day

US actress Sigourney Weaver marked Earth Day on Thursday by taking on the role of a real-life environmentalist, urging lawmakers to take action to stop the world's oceans from turning into deadly acid baths.

"I've done enough science fiction to know that our Earth will survive through various nightmare scenarios," Weaver, who played a feisty environmentalist in James Cameron's blockbuster movie "Avatar," said at a Senate hearing on ocean acidification, which is caused by carbon pollution.

"But that's entertainment. This is real and as citizens we have to act now so that we are able to look into our children's eyes in 20 years and say we did the right thing when we had to," she said.

Environmentalists call ocean acidification "the other carbon problem," the 60-year-old actress told the Senate subcommittee on oceans, speaking alongside a fisherman, a diver and two scientists.

"Ocean acidification is changing the chemistry of the oceans at a scale and magnitude greater than thought to occur on Earth for many millions of years and is expected to cause changes in the growth and survival of a wide variety of marine organisms," said James Barry, a senior scientist at the Monterey Bay Aquarium Research Institute in California.

A report released Thursday by the National Academy of Sciences (NAS) said ocean chemistry was changing "at an unprecedented rate and magnitude" because of carbon pollution, and warned that unless manmade CO2 emissions are substantially curbed, the seas will become too acidic for some marine life.

"Scientists fear many organisms may not survive so radical a shift in chemistry," Weaver said.

"And some of those organisms - certain plankton and corals, for instance - form the foundation of ocean food webs. If they perish, what happens to the tens of thousands of species further up the chain?"

Carbon dioxide absorbed from the atmosphere by the oceans dissolves in seawater to form carbonic acid, which decreases the concentration of carbonate ions in the water and reduces the availability of calcium carbonate for corals and other marine life that need calcium to form their shells and skeletons.

"The oceans are 30 percent more acidic today than they were during pre-industrial times," said Weaver.

"If we continue burning fossil fuels as we are now, we will double the ocean's acidity by the end of the century," Weaver warned, urging lawmakers to include ways of addressing the problem in climate and energy legislation.