We describe the development of an automated gas chromatography-isotope ratio mass spectrometry (GC-IRMS) system capable of measuring the carbon isotopic composition of atmospheric methane (δ13CH4) with a precision of better than 0.1?. The system requires 200 mL of air and completes a single analysis in 15 min. The combination of small sample size, fast analysis time, and high precision has allowed us to measure background variations in atmospheric δ13CH4 through the NOAA Climate Monitoring and Diagnostics Laboratory Cooperative Air Sampling Network. We then present a record of δ13CH4 obtained from six surface sites of the network between January 1998 and December 1999. The sites are Barrow, Alaska (71¿N); Niwot Ridge, Colorado (40¿N); Mauna Loa, Hawaii (20¿N); American Samoa (14¿S); Cape Grim, Tasmania (41¿S); and the South Pole (90¿S). For the years 1998 and 1999, the globally averaged surface δ13C value was -47.1?, and the average difference between Barrow and the South Pole was 0.6?. Consistent seasonal variations were seen only in the Northern Hemisphere, especially at Barrow, where the average amplitude was 0.5?. Seasonal variations in 1998, however, were evident at all sites, the cause of which is unknown. We also use a two-box model to examine the extent to which annual average δ13C and CH4 mole fraction measurements can constrain broad categories of source emissions. We find that the biggest sources of error are not the atmospheric δ13C measurements but instead the radiocarbon-derived fossil fuel emission estimates, rate coefficients for methane destruction, and isotopic ratios of source emissions.