A Time-Dependent Two-Dimensional-Model Study of the Trend in Atmospheric Methane

Abstract

The atmospheric concentration of methane (CH4) has increased from about 0.7 to 1.7 ppm in the last two centuries. Causes of the increase are uncertain. Possibilities include increased emissions from the use of fossil fuel, landfills, biomass burning, domestic cattle, and rice paddies. Another possibility is a reduction in the photochemical removal rate of CH4 in the atmosphere due to decreasing concentrations of OH. Such reductions could result from increasing CO emissions and the strong coupling between CO, OH, and CH4. We use a time-dependent two-dimensional photochemical model of the atmosphere to calculate the trend in CH4 emission required to maintain the observed increasing concentrations of CH4 and CO over the past two decades, and illustrate the potential impact of changes in tropospheric OH. The results show that about 40% of the methane trend could be due to decreasing concentrations of tropospheric OH, caused mostly by increasing abundances of CO, with the remaining 60% due to increasing emissions of CH4. However, our calculations only include the direct coupling between CO, CH4, and OH. Uncertainties due to the impact of other factors, such as increases in tropospheric ozone and NOx (NO + NO2), or decreases in stratospheric ozone, could result in different calculated trends for tropospheric OH. This study illustrates that strategies aimed at controlling increases in methane must consider not only methane emissions, but also the anthropogenic emissions for other tropospheric species, particularly CO.