Atmospheric methane (CH4): Trends and seasonal cycles


  • R. A. Rasmussen,

  • M. A. K. Khalil


On the basis of 22 months of almost continuous, automated, GC/FID measurements of atmospheric CH4 at Cape Meares (45°N), we show that the concentration of CH4 is increasing at about 2% per yr (±0.5% yr−1). The data also revealed stable seasonal cycles with peak concentrations in October and minimum concentrations in July. The magnitude of the seasonal variations during these months is about ±20 ppbv from the average (∼±1.2%). If the current trend continues, the increased CH4 concentration may result in a 0.2°K to 0.4°K average increase in earth's surface temperature over the next 40 years or so (based on calculations of Wang et al. (1976)). The coupling of CH4 to tropospheric and stratospheric chemical processes is discussed.