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Sources, sinks, and seasonal cycles of atmospheric methane


  • M. A. K. Khalil,

  • R. A. Rasmussen


It is shown that a long lifetime of about 8 years is most consistent with the observed latitudinal variation of atmospheric methane, requiring the current global emissions of methane to be around 550 teragrams per year (Tg = 1012 gm). On average there is 25–34 ppbv less methane in the atmosphere of the northern hemisphere during summer when compared with the rest of the year. Methane concentrations rise rapidly to their yearly maximums in fall. Seasonal cycles of CH4 concentration in the southern hemisphere include lowest concentrations during the late Australian summer and fall, being about 14 ppbv less than during the rest of the year. The repeating pattern of a rapid rise of CH4 concentrations during fall in the northern hemisphere suggests a large fall source at latitudes above 30°N. The remaining observed seasonal variations are consistent with the seasonal cycle of OH, which removes methane from the atmosphere. The extensive set of self consistent measurements of methane are reported and analyzed showing that methane has increased during the last 3–4 years at rates of 1–1.9% per year all over the world at sites ranging from inside the arctic circle to the south pole. Observational results are used to estimate the sources, sinks, seasonal cycles of CH4, and the effects of human activities on its atmospheric abundance.