Atmospheric methane (CH4): Trends and seasonal cycles

On the basis of 22 months of almost continuous, automated, GC/FID measurements of atmospheric CH 4 at Cape Meares (45°N), we show that the concentration of CH 4 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 CH 4 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 CH 4 to tropospheric and stratospheric chemical processes is discussed.