The good news is the world's oceans appear to have stored nearly half of the carbon dioxide from human activity; the bad news is the resulting acidity could harm marine life, scientists say.

Researchers collected more than 72,000 samples from the Atlantic, Pacific and Indian oceans between 1989 and 1998.

Ice cores suggest atmospheric concentrations of carbon dioxide were between 200 and 280 parts per million before the industrial revolution began pumping out human-derived or anthropogenic CO2.

Tests suggest when carbon dioxide dissolves in sea water, it may harm marine life like coral.

"If the ocean had not removed 118 billion metric tons of anthropogenic carbon between 1800 and 1994, the CO2 level in the atmosphere would be about 55 parts per million greater than currently observed," said Christopher Sabine of the National Oceanic and Atmospheric Administration in Seattle.

Today, atmospheric levels of carbon dioxide are about 380 parts per million.

Most of the CO2 is in near-surface waters, Sabine's team said in Friday's issue of the journal Science.

Richard Feely, a marine chemist at NOAA, and his team said if current scenarios continue, the pH of ocean surfaces could drop for more than five million years.

In a companion study, Feely's team said laboratory tests suggest the high acidity may dissolve the calcium carbonate shells of marine animals such as corals, snails and plankton that other animals feed on.

Scientists have observed coral have less ability to produce shells when marine levels of carbon dioxide are high.

Having better estimates of the amount of extra carbon in the ocean should help researchers to make more accurate predictions about the effects of greenhouse gases.

Researchers from the U.S., South Korea, Australia, Canada, Japan, Spain and Germany also participated in the studies.