Measurements of atmospheric methane and its carbon isotopic composition from aircraft samples collected across a broad latitude range during August 1991 and 1993 show both latitudinal and temporal structure. Latitudinal gradients in August 1993 are generally consistent with three dimensional model simulations using the atmospheric transport model TM2 and previously developed methane budgets. However, there is a significant enrichment in Δ13C values in 1991 compared to 1993, and this difference is more pronounced with increasing latitude in the Southern Hemisphere (SH). In the extratropical SH, Δ13C values are about 0.25? larger in August 1991 than in August 1993, consistent with previously published results. The observed 13C enrichment decreases with northerly latitude and reaches zero in the subtropical Northern Hemisphere. Sensitivity tests using transient sources in the TM2 model suggest that a regional source of isotopically enriched methane located in the SH (e.g., savannah burning from southern Africa) cannot explain the observed spatial and temporal pattern of enrichment. We conclude that the anomalous behavior of methane between 1991 and 1993 is likely due to several concurrent factors involving both isotopically light and heavy sources. ¿ 2000 American Geophysical Union