New evidence shows that the ocean is the primary repository for excess carbon dioxide produced during the past 200 years of industrialization. The change in the concentration of CO2 in ocean waters could threaten shell formation in certain plankton, corals and mollusks.

The amount of CO2 in the atmosphere has risen from about 280 parts per million in 1800 to 380 parts per million today. But this increase accounts for only about half of the expected contribution from human activity. In a study published today in the journal Science, Christopher L. Sabine of the National Oceanic and Atmospheric Administration and his collaborators determined from a worldwide survey of water samples that 48 percent of the estimated CO2 from the burning of fossil fuels and cement manufacturing has ended up in the ocean. This implies that the terrestrial biosphere, which includes soil and plant life, has not been a long-term carbon sink for industrial emissions and may in fact be a net source for CO2 over the given time frame, owing to forest clearings and other land-use changes.

The scientists estimate that the seas have sequestered 130 billion tons of anthropogenic carbon and that they have the potential to hold three times that amount. But the increase of CO2 alters ocean chemistry. In a companion article, Richard A. Freely, also of NOAA, and his colleagues investigated the effect of rising CO2 levels on calcium carbonate, which certain organisms rely on to create their shells. Elevated concentrations of CO2 make the ocean more acidic and reduce the number of carbonate ions. If the ions are too few, the water will dissolve calcium carbonate. The researchers measured the dissolution rate of calcium carbonate at several sites and determined that parts of the ocean, starting in the high latitudes, could become inhospitable for many shell-forming creatures if current trends in fossil fuel consumption continue. The authors comment that the predicted CO2 levels for the end of the century could "potentially have significant impacts on the biological systems in the ocean in ways we are only beginning to understand."