Understanding the Trends of Atmospheric Methane in the Past Decade

Methane (CH₄) is the second most important anthropogenic greenhouse gas after carbon dioxide. In this study, the GEOS-CHEM global three-dimensional chemical transport model with assimilated meteorology is used to quantify the contributions of individual CH₄ sources and sinks to the trends of CH₄ concentration in the past decade. The model is evaluated with observations from the NOAA CMDL network. The model accounts for interannual variations in meteorology and concentrations of OH radical. Emissions are scaled using yearly country-by-country socioeconomic and meteorological data. The model simulates well the horizontal and vertical distributions, seasonal cycles, trends, and interannual variations of CH₄, capturing over 80% of the variance between 1988 and 1997 at many sites. It appears that the observed decrease in growth rate of CH₄ over the past decade is a result of stabilization of global emissions from livestock, rice paddies, gas venting, and coal mining along with a slight decrease in CH₄ lifetime. The anomalous growth rates during 1991-1992 can be explained by changes in OH concentrations and wetland emissions due to the eruption of Mt. Pinatubo.