Methane, methane, where from art thou?

Methane, methane, where from art thou?

Concerned with rising global temperatures and the threat of a substantial "greenhouse" effect in the near future, scientists have watched carefully during the last decade as the atmospheric levels of methane grew each year by 1.5 percent. Because it efficiently traps energy from the earth, methane is an important player in the greenhouse scenario. But scientists are having trouble determining how much the different sources of methane contribute, which is the first step in understanding why this molecule, the chief component of natural gas, is accumulating in the atmosphere. Now, researchers from the Institute of Nuclear Science in Lower Hutt, New Zealand, report that they have cracked part of the methane source problem. About one-third of the methane in the atmosphere comes from so-called fossil sources, they announce in the April 7 NATURE.

Scientists believe that almost all methane comes from two types of biological sources: organisms that were alive a long time ago, and those alive today or recently dead. Fossil methane, formed millions of years ago from decaying plants and animals, is found in pockets of porous rock deep within the earth. When brought to the surface, most fossil methane is burned as a fuel and never enters the atmosphere. However, some escapes into the air during mining operations and natural-gas production. Sources of modern methane are wet-lands, burning vegetation and the bowels of ruminants.

In the 1950s, researchers measured methane (CH4) for the ratio of two isotopes of carbon: C-12 and the radioactive form, C-14. Fossil methane contains little C-14 because this isotope decays away in thousands of years. The 1950s analysis showed that 10 to 15 percent of the methane in the atmosphere then came from fossil sources. However, above-ground tests of nuclear weapons in the late 1950s littered the atmosphere with extra C-14, and scientists have only now been able to resume these types of analysis in conjunction with studies of C-13.

The New Zealand analysis is by no means a final solution to the problem. Indeed, other atmospheric chemists currently working on similar budgets for the sources of methane are coming up with preliminary figures in the realm of 15 percent to 20 percent for the fossil contribution. According to Ralph Cicerone of the National Center for Atmospheric Research in Boulder, Colo., the New Zealand work "is not perfect, but it's a real step forward." The percentage of fossil methane in the atmosphere has grown, he says, because of increased mining operations and use of natural gas.