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Northern Hemisphere's carbon dioxide emissions. "That's much more than any terrestrial plant ecologist would have considered likely," Tans said. Conserving tree and plant populations, he added, might raise their carbon dioxide uptake even higher. Understanding how carbon cycles through plants is vital to decreasing global warming. By burning fuel and clearing forests, human activity generates 7 billion tons of carbon dioxide into the atmosphere every year. About half of it remains there. The rest is believed to be absorbed by oceans and plants, though no one knows exactly how the gas is distributed. In the atmosphere, carbon dioxide molecules allow sunlight to reach the earth's surface. At night, reradiated energy from earth surfaces is absorbed, trapping heat temporarily like glass in a greenhouse. According to the Intergovernmental Panel on Climate Change, increasing concentrations of carbon dioxide in the atmosphere may cause the world's average surface temperature to rise by up to 6.30F over the next century. Some subtropical areas will become tropical, disrupting ecosystems and allowing animal and microbe species that thrive in a hot dimate to migrate into more northern and southem latitudes. Global warming may also increase rainfall and cause the sea level to rise. The best way to slow global warming, scientists say, is to cut back on burning fossil fuels like oil, coal, and gas. Some have suggested disposing of excess carbon dioxide by pumping it into the ocean, which can absorb vast amounts of the gas. But pumping would be expensive, and concentrated streams of carbon dioxide-rich water would kill many ocean organisms. The Tans study is one of the first to suggest that saving trees might help keep the earth cool. In the study, glass flasks were used to collect 8,000 air samples at 43 sites throughout the Northern Hemisphere, on the oceans, and in Australia. Researchers checked the air samples for an isotopic "fingerprint" unique to plants. When accumulating carbon dioxide, oceans absorb the carbon-12 and carbon-13 isotopes in roughly equal proportion. Plants, however, absorb far more carbon-12, allowing leftover carbon-13 to build up in the air. "When we looked at our air samples," Tans explains, "the carbon was enriched with C13." Measuring isotope concentrations and the carbon-12/carbon-13 ratio, the researchers found plants had accumulated up to half the fossil fuel emissions for 1992 and 1993. Paul Quay, a professor of oceanography at the University of Washington, said that although the study was well designed, its findings are preliminary. "One thing you have to be aware of is that the study only looks at two years of data," Quay said. "As the records become longer, we'll see if those two years are anomalous or representative." The study offers two potential reasons for its unusually high rate of plant carbon dioxide uptake: regrowth of American forest once used for farms, and "fertilization" of forests worldwide by increased environmental nitrogen and carbon dioxide, which may enhance plant growth. Many carbon dioxide cyde questions still lack answers, such as where, and for how long, do plants store atmospheric carbon. Tree trunks are one storage spot. But researchers worry that some carbon dioxide simply enters leaves of trees and decays back into the atmosphere within a year. "It's very important to pin down how the carbon cyde is working in order to make policy decisions on things like conservation," noted Mike MacCracken, director of the Office of the U.S. Global Change Program. Meanwhile, James White, a geology professor at the University of Colorado and coauthor of the Tans study, cautioned that simply planting trees won't solve the global warming problem. "Neither will dumping all the carbon dioxide in the ocean," White said. "We've got to fix the energy problem. Or else this will be something serious for our great, great grandchildren to deal with."