Urban methane.

The primary source of methane released into the atmosphere is biological; methane comes from swamps, rice fields and cattle. Cities, too, contribute to methane emission, mainly through the leakage of natural gas. In the past, researchers have estimated that this nonbiological source accounts for 25 to 100 megatons of methane per year. However, these estimates were made with little quantitative measurement of atmospheric methane levels.

So Donald Blake and co-workers at the University of California at Irvine analyzed urban air samples for methane during a six-year study of methane concentrations worldwide. By tempering their calculations with data collected on trichloromethane, a gas emitted only in urban environments, the researchers estimate that the urban release of methane is about 30 to 60 megatons per year. They note in GEOPHYSICAL RESEARCH LETTERS (Vol. II, No. 12) that this amounts to about 8 to 15 percent of the total methane emitted into the atmosphere.