Nearly half the excess carbon dioxide spilled into the air by humans over the past two centuries has been taken up by the ocean, a study says. If that continues, it could damage the ability of many ocean creatures to make their shells, says an accompanying report.
Carbon dioxide, produced by burning fossil fuels and other industrial processes, is one of the most important "greenhouse" gasses that many scientists fear may be causing global warming by trapping heat in the Earth's atmosphere.
The atmosphere currently includes about 380 parts per million of carbon dioxide, up from 280 parts per million in 1800, according to scientists.
But that accounts for only about half the CO2 released into the air in that period, causing researchers to speculate about what had happened to the rest.
"The ocean has removed 48 percent of the CO2 we have released to the atmosphere from burning fossil fuels and cement manufacturing," Sabine said after reviewing data gathered between 1989 and 1998 from three major studies of the Atlantic, Pacific and
Indian oceans. The studies collected more than 72,000 ocean samples.
Overall, Sabine said, between 1800 and 1994 the oceans absorbed 118 billion metric tons of carbon that had been released into the air. A metric ton is 2,205 pounds, indicating that during that period carbon dissolved in the oceans about equaled the weight of 118 billion small cars.
While some researchers have raised the possibility that increasing forests and other plants could take up CO2, that appears not to have been the case until recent years.
Over the past two centuries, land plants appear to have contributed CO2 to the air as forests were cut for farming, Sabine said. Only in the last few decades, as reforestation has gotten under way, has that been reversed with plants taking in more carbon dioxide than they release.
Taro Takahashi of Columbia University's Lamont-Daugherty Earth Observatory notes in an accompanying commentary in Science that over time, the amount of CO2 taken up by plants has been nearly balanced by CO2 released by changes in land use patterns.
The oceans could continue absorbing the gas for centuries, Sabine said, because ocean waters mix slowly and most of the CO2 is in near-surface water.
An accompanying study by Richard A. Feely, also of NOAA's Pacific Marine Environmental Laboratory in Seattle, notes that dissolving CO2 in water forms an acid and that process can affect ocean life.
Feely and his research team found in laboratory tests that the water near the ocean surface with added CO2 can cause shells of marine animals, including corals, snails and plankton, to dissolve.
Carbon dioxide levels that may occur in the seas by the end of the century could reduce the amount of calcium in shells by 25 percent to 45 percent, the researchers said.
That process hasn't yet been studied in the oceans, he noted, but the lab findings indicate a need for concern.
The increasing CO2 could "compromise the fitness or the success" of these animals, said Victoria J. Fabry of California State University at San Marcos.
That might mean a change in the structure of the food chain, she said, but not enough is known about the effects yet to say what that change would be.
Data for the ocean CO2 study was collected in three research efforts: the National Science Foundation-led World Ocean Circulation Experiment, the Joint Global Ocean Flux Study, and NOAA's Ocean-Atmosphere Carbon Exchange Study. The data from these studies were analyzed for the two papers, by Sabine and Feely.
Sabine and Feely worked together on the studies and each is listed as a co-author on the paper led by the other. Other researchers on their teams came from the United States, South Korea, Australia, Canada, Japan, Spain and Germany.
Funding for the studies came from NOAA, National Science Foundation, Department of Energy and Pohang University of Science and Technology in South Korea.
By Randolph E. Schmid