By Sid Perkins
Scientists have developed a way to use corn plants to monitor and map the human-generated emissions of carbon dioxide.
Only a small fraction of Earth’s atmosphere is carbon dioxide. In summer 2004, that share averaged about 378 parts per million (ppm), says James T. Randerson, a biogeochemist at the University of California, Irvine. Within that component, about one in a trillion of the carbon atoms is carbon-14 (C-14), a radioactive isotope produced by cosmic rays at high altitudes.
However, the carbon dioxide produced by burning fossil fuels has no C-14. That’s because the isotope decays to undetectable concentrations after about 50,000 years, and fossil fuels derive from organic material much older than that. By measuring the proportion of C-14 in corn plants, Randerson and his colleagues can determine the mix of naturally occurring and fossil fuel–generated carbon dioxide that the plants absorbed as they grew.
Some of the highest concentrations of C-14 in North America—and therefore the least carbon from fossil fuels—appeared in corn grown at a site in northern California where prevailing winds blow relatively unpolluted air off the ocean. The team’s analysis suggests that the air in Brentwood, Calif., downwind of the San Francisco area, had about 11 ppm excess carbon dioxide attributable to fossil fuels, the researchers report in the Jan. 28 Geophysical Research Letters.
Surprisingly, says Randerson, the atmosphere at sites in the eastern portions of the Rocky Mountains didn’t contain much carbon dioxide from fossil fuels. However, 34 sites east of the Mississippi River averaged about 2.7 ppm extra carbon dioxide.
Researchers could use the team’s results, as well as similar studies, to improve their models of large-scale air movements across North America, Randerson’s team asserts.
