These findings could pose a long-term risk for marine organisms, such as corals, which have greater difficulty in forming their outer shells as carbon dioxide levels increase, researchers found.

"There is a price to pay in this process, and that is with living organisms," said Richard Feely, a marine chemist with the National Oceanic and Atmospheric Administration, the lead author on one of the studies.

The research was published in the July issue of "Science."

Oceans, which cover about 75 percent of the Earth's surface, have seen the amount of carbon dioxide they absorb fall to 30 percent as trees and plants soak up more of the gas before it reaches the water. Currently, 20 percent is taken in by foliage, with the remaining 50 percent staying in the atmosphere.

The 15-year study, conducted and analyzed with the help of several researchers around the world, looked at nearly 72,000 samples taken in the Atlantic, Pacific and Indian oceans.

Analysis of carbon dioxide since the industrial age has shown that concentration levels in the atmosphere have increased to about 380 parts per million (ppm) from 280 ppm two centuries ago. Without ocean absorption, carbon dioxide levels in the atmosphere would be about 55 ppm higher.

The data found that even though the oceans continue to absorb more carbon dioxide, they are far from being saturated. Currents stir the ocean very slowly by pulling deep ocean water to the surface, where it is able to absorb more carbon dioxide.

"The oceans have a capacity to continue to take in CO2 for thousands of years with the slow mixing time," said Christopher Sabine, NOAA oceanographer and lead author of the other study.

The greatest threat to increasing levels of carbon dioxide is to species that live in the upper 10 percent of the ocean.

As ocean surfaces capture and store carbon, the slow circulation of water keeps the gas more highly concentrated where these creatures live.

The change in ocean chemistry reduces the level of carbonate ions needed by corals and other organisms to generate their shells. In areas where the ion level has fallen too low, calcium carbonate shells can begin to dissolve.

Researchers said that while the long-term impact on these creatures and other species that depend on them for food is uncertain, they will closely monitor how carbon dioxide absorption is affecting the food chain.