Understanding the trends of atmospheric methane in the past century

Methane (CH₄), an important greenhouse gas and reactive species, has exhibited a rapid increase in concentration since pre-industrial times with a slowdown in its growth rate beginning in the 1980s. The budget of CH₄ and the causes of observed trends and interannual variability are not well understood.; To examine the contribution of increases in stratospheric Cl to the observed increase in δ13C of CH₄ over the past century, we conducted time-dependent simulations using the Harvard 2-D stratospheric-tropospheric model of chemistry and dynamics. We present a simple framework to explain features of the atmospheric distribution of δ13C in which there are two branches of stratospheric circulation and distinct regions of high and low fractionation efficiency. We constrain the contribution of anthropogenic Cl to the 2‰ increase in δ13C over the past century to be at most 0.54‰, less than the 0.7‰ estimated by Gupta et al. [1996]. It follows that growth in the relative importance of heavier sources of CH₄ (fossil fuels and/or biomass burning) has been largely responsible for the observed trend in δ13C.; We utilized the 3-D GEOS-CHEM chemical transport model accounting for interannually varying emissions, transport, and sinks to analyze trends in CH₄ from 1988 to 1997. We carried out simulations of 85 Kr to verify the accuracy of the treatment of interhemispheric mixing, which is crucial for studies of long-lived tracers such as CH₄. Interannual variations in CH₄ sources were based on meteorological and country-level socioeconomic data. An inverse method was used to optimize the strengths of sources and sinks for the base year, 1994. Our best-guess budget suggests that the sum of emissions from animals, fossil fuels, landfills, and waste water estimated using IPCC default methodology is too high. Recent bottom-up estimates of the source from rice paddies appear to be too low. Previous top-down estimates of emissions from wetlands may be around a factor of two higher than bottom-up estimates because of possible overestimates of OH. The slowdown in the CH₄ growth rate is attributed to a combination of slower growth of sources and increases in OH. The economic downturn in the former Soviet Union and Eastern Europe made a significant contribution to the decrease in growth rate of emissions. The trend in OH was driven mainly by decreases in column ozone during the period of our investigation. The 1992–1993 anomalies in CH₄ growth rate can be explained by fluctuations in wetland emissions and OH in the aftermath of the eruption of Mt. Pinatubo. The results of this study suggest that the recent slowdown of CH₄ may be temporary and that CH₄ is likely to increase significantly in the future in the absence of systematic efforts to moderate emissions.